HomeMy WebLinkAbout9- Final Geotechnical ReportFinal Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 2
SUMMARY OF CONDITIONS AND RECOMMENDATIONS
The project site is blanketed by 6 to 12 inches of topsoil (in most locations) that overlies fine-grained,
fluvial/alluvial deposits consisting of silt/clay and loose, clean sand w/ small gravels. Near the creek, the
topsoil was thickener at 2.0 feet. Beginning at 5.0 to 6.5 feet is alluvial, cobbly, sandy gravel. The gravel
is identifiable based on the large gravels and cobbles and is defined as the “target” bearing material for
all foundation support under buildings, retaining walls, and the pedestrian bridge. In December, the
groundwater was at depths of 4.0 to 6.5 feet and was either at the top of the sandy gravel or 0.5 to 1.0-
foot above it. Based on the fact the groundwater conditions will rise in the spring, it should be assumed
that groundwater levels are always above the “target” bearing gravels.
Given the shallow gravels, the site has no foundation bearing issues. All footings need to bear directly
on the sandy gravel or on compacted, granular structural fill that in turn is supported on the “target”
gravel. Since the site is impacted by shallow groundwater, the buildings cannot be underlain by crawl
spaces or basements. The main building will be underlain by a slab-on-grade, while we understand that
the small ADU building could be an elevated structure. Due the shallow groundwater conditions, some
level of dewatering should be expected during foundation earthwork to lower the water levels as much
as possible. To get above wet subgrade or shallow standing water, we have provided recommendations
for an initial, structural fill lift of clean crushed rock covered with geotextile fabric in the foundation
excavations for the main building and tall retaining wall. Due to more groundwater expected near the
creek, any structural fill required under the ADU building and bridge foundations can consist exclusively
of clean crushed rock.
The most unique architectural element on the project will be the tall retaining wall on the east side of
the main building. The potential issue with the wall is its height and the risk of inward rotation caused
by the backfill conditions. With this tall of a wall, any top of wall deflection will be very noticeable. At a
minimum, the wall must be supported on the “target” sandy gravel (or on structural fill that in turn is
supported on the gravel); and the wall must be backfilled with a drain system (or have weep holes) to
prevent the build-up of hydrostatic pressures behind the wall. We have provided three options for the
design and backfill of the wall. Option 1 includes designing the wall for higher lateral earth pressures (to
make it thicker and more heavily reinforced) and backfilling it with well compacted, clean crushed rock.
Option 2 includes backfilling the wall with light-weight geofoam blocks with an added consideration to
also design the wall for higher earth pressures. Option 3 includes designing the wall with concrete dead-
man and tie-backs and backfilling it with well compacted, clean crushed rock.
PROJECT PLANS
We received the concept plans for the project in early November from Matthew Baird Architects. These
were identified as the pricing set and dated October 20. The geotechnical recommendations provided in
this report are based on our understanding of the project elements from our review of the plans as well
as an on-site meeting with Mr. Baird in November.
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 3
SITE LOCATION AND EXISTING CONDITIONS
The project site is located at 705 S. Church Avenue. The subject property lies on the west side of the
street, encompasses about 1.2 acres, and is about halfway between E. Story Street (to the north) and Ice
Pond Road (to the south). An existing multi-family building lies directly to south of the site, along with
several other homes to the south and north. Most of the properties along the west side of S. Church
Ave. lie in the Bozeman Creek drainage corridor. Across the street to the west are the uplands and hills
that comprise the Peets Hill open space area. The legal description for the property is the SW1/4, SE1/4
of Section 7, T2S, R6E, Gallatin County; and the latitude/longitudinal coordinates (near the center of the
property) are 45.671762°N and -111.030766°W. See Figures 1 and 2 for images of the site location and
existing site conditions. The base maps for Figures 1 and 2 are a Google Earth aerial photo and a site
topographic map, respectively.
Currently, the property contains an old house near the northeast corner and a small shed behind and to
the west of the house. The driveway access off of S. Church Ave. enters near the southeast corner and
loops around to the back of the house. In the yard area between the house and the driveway is the on-
site septic drainfield. All of the existing improvements are in the east half of the property. The west half
of the property is undeveloped and much of it contains Bozeman Creek and the water course setbacks.
The very west side for the site is in its natural “creek corridor/wooded” state; but is outside the required
setbacks along the west side of the creek.
PROPOSED PROJECT IMPROVEMENTS
This project will be a multi-faceted, re-development project and will contain a number of elements and
components. Prior to any new construction, the old house will be demolished and removed since this is
the general location for the new multi-story, multi-family building. It is unclear if the old shed that is
located to the west of the old house will be removed as well.
The primary element of the project is a three-story building that will be located on the east side of the
property. The building will be accessed by a southside driveway that leads to an at-grade garage or
covered canopy area (under the south end of the building). On the east side of the building will be an
elevated parking area that accesses the second floor of the building. The most unique feature of the
project will be a tall retaining wall between the east side of the building and west side of the parking
area. Other components of the project will include a new pedestrian bridge across Bozeman Creek and
a possible small ADU structure on the far west side of the property. As currently shown, the only access
to the ADU will be the bridge crossing and a walking path. Provided below is additional discussion on
the principal items of the project.
• Main Building: The main building will a three-story structure that has all three stories exposed
on the west; but from the street side (east), only appears to be two stories. The ground floor of
the building will contain a studio area (north), mechanical and storage areas (middle), and a
garage or covered canopy area (south). The second and third floors will contain four residential
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 4
units, one for the Owner and three for tenants. The ground floor building elevation will be set
close to existing site/driveway grades. The building will be underlain by a concrete slab-on-
grade and be supported on a standard shallow foundation consisting of perimeter footings/frost
walls and interior footings. If the southside garage area will be more like an open, covered
canopy area, the second and third stories over the south end will be supported on concrete
columns and pad footings. As we understand it, crawl space or basement foundations are not
planned, which is good since the site’s shallow groundwater conditions will not permit these
types of foundation configurations.
• Tall Retaining Wall: There will be a tall, concrete retaining wall between the east side of the
main building and the eastside parking lot area. The total height of the wall from top of footing
to top of wall could be on the order of 25 to 30 feet. Behind the wall on the west side will be a
ground floor courtyard with vegetation. The lower half of the wall along the east side will be
buried and will “retain” the elevated eastside, parking lot area. The most interesting aspect of
the wall is that it will extend up from the parking lot grade by another 12 to 15 feet to hide the
second floor of the building. Walkway openings will be formed in the upper half of the wall to
allow access to the second floor of the building via sky bridges. In profile view, the west side of
the wall would be about 20 to 25 feet tall (from ground floor grade); and the east side of the
wall would be 12 to 15 feet tall (from east side parking lot grade). One of the biggest issues on
the project will be designing and backfilling the wall in a way that will minimize any east to west
rotation at the top of the wall (which will cause it to lean inward and become out of plume). In
this report, we provide three options for wall design and backfill.
• Pedestrian Bridge: A new pedestrian bridge will be constructed over Bozeman Creek. We
expect typical bridge foundation support consisting of pad footings and abutment walls on the
east and west sides of the creek channel.
• ADU Building: There is a possibility that a small ADU building will be erected on the far west side
of the site. Due to no current vehicle access to this side of the creek, we understand that the
building will be very small with a limited concrete foundation. As we understand it, the building
could be elevated (above existing site grades) and supported on some concrete pad footings and
columns (at the corners and a few in between). Due to the shallow groundwater conditions in
the areas adjacent to the creek, we do not recommend crawl space foundations. Therefore, if
the building will not be elevated and supported as described above, then it will need to be
underlain by a concrete slab-on-grade with a standard perimeter footing/frost wall (same as the
main building). One possibility for gaining temporary access to this side of the site with the
construction equipment (incl. excavators, gravel trucks, concrete trucks, etc.) is to use an old
railroad car for a temporary bridge over Bozeman Creek. We have seen this done before on
past projects with access issues relating to creek crossings.
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 5
EXPLORATIONS, TESTING, AND SUBSURFACE CONDITIONS
Subsurface Explorations
Subsurface conditions were investigated at the site on December 3, 2021 by Lee Evans, a professional
geotechnical engineer with Allied Engineering. Five test pits were dug throughout the east half of the
property with a mini-excavator provided by Walker Excavation. The pits were identified as TP-1 through
TP-5 and were all dug to depths of 7.0 to 9.0 feet. Due to the existing house and septic drainfield, the
possible locations for test pits were limited. Four of the test pits were dug in the driveway area to the
west, southwest, south, and east of the old house; while a fifth pit was dug in the central part of the lot
between the old shed and Bozeman Creek. The four eastern pits will provide the representative soil
conditions in the proposed area of the main multi-family building; while the test pit near the creek will
provide insight as to what can/should be expected in the west half of the property. Due to no vehicle
access to the west side of the site, a test pit could not be dug in the proposed ADU building location.
See Figure 1 for a map showing the test pit locations. The base map for this exhibit is a Google Earth
aerial photo and the test pit locations are based on cell phone GPS coordinates.
See Figure 2 for another map showing the test pit locations. In addition to the test pits, the depths to
gravel and groundwater that were measured in each pit are listed. This figure provides a good summary
of these site conditions across the property. The base map for this exhibit is a site topographic map.
Note: TP-5 was dug in the driveway near the southeast corner of the property. At this location, the
driveway is elevated above existing site grades and is in a 2.0-foot fill section. As a result, the upper 2.0
feet in this pit consisted of gravel surfacing and driveway fill material. The consequence of the elevated
condition of the road is that the measured depth to native gravel is 2.0 feet deeper than what it is in the
areas to the north and south of the driveway. Therefore, rather than the native gravels being 8.0 feet
deep in this area of the property (as measured under the elevated driveway), they are instead closer to
6.0 feet deep (from actual native ground grade).
During the explorations, soil and groundwater conditions were visually characterized, measured, and
logged. The relative densities of the soils were estimated based on the ease/difficulty of digging, the
side wall stability of the test pit excavation (vertical wall vs. caving/sloughing), and pocket penetrometer
measurements (which are a field identifier of soil consistency/strength). Our test pit logs are attached.
Each log provides pertinent field information, such as soil depths, thicknesses, and descriptions,
groundwater measurements (at the time of exploration), relative density data, soil sample information,
and a sketch of the soil stratigraphy. Please be aware the detail provided on the logs cannot be
accurately summarized in a paragraph; therefore, it is very important to review the logs in conjunction
with the report. Following completion of the fieldwork, the excavations were fully backfilled, staked
with identifying lath, and cleaned up to the best extent possible.
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 6
To better illustrate the on-site soils, four photos from each test pit are included as part of this report.
The first photo (of each test pit set) shows the sidewall of the excavation, while the latter three photos
are of the excavated spoil piles (at increasing depths). The spoil pile photos illustrate the differences
between the silt/clay, loose/soft sand, and cobbly sandy gravel materials. All photos have been marked
up to call out the soil layers/materials described on the logs as well as identifying characteristics.
Note: Please be aware that no compaction of test pit backfill soils was done; therefore, these areas will
be susceptible to future settlement. As discussed in a later section of the report, all old test pit locations
should be re-excavated to their original depth and properly backfilled and compacted if they will under-
lie any of the home site improvements, including the building foundation footprint areas, exterior slabs,
underground utilities, or exterior concrete and asphalt pavement areas.
Laboratory Testing
Since the native sandy gravel (beginning at depths of 5.0 to 6.5 feet) is the “target” bearing material for
all foundation elements, no laboratory testing was warranted on the project.
Soil Conditions
As expected based on the Bozeman Creek drainage environment, we found the site to be underlain by a
variety of shallow fine-grained, fluvial/alluvial silt, clay, and loose sand materials (at varying thicknesses),
which in turn overlie good, cobbly sandy gravel beginning at depths of 5.0 to 6.5 feet. The one curiosity
we had going into this project was whether the older Tertiary-aged bedrock materials (consolidated
layers of silt/clay and sand/gravel) that comprise the hills along the east side of S. Church Ave. extend
into and under the east side of the project site. We dug TP-5 in the far southeast corner of the property
and to the east of the proposed locations of the main building and tall retaining wall. At this location,
similar fluvial and alluvial materials were found (same as TP-1 through TP-4) with no sign of any bedrock.
Based on this, we do not anticipate that any bedrock will be found in any foundation excavation (for the
main house or tall retaining wall).
Due to our inability to cross Bozeman Creek with the excavator, we could not dig a test pit on the far
west side of the site in the proposed ADU building location. The best we could do was a test pit (TP-1)
on the east side of the creek. As this location, the native sandy gravels and groundwater were shallow
(at 5.0 feet and 4.0 feet, respectively). Since the east side and west side creek environments are the
same, we expect similar conditions to TP-1 on the west side of the property. As stated in the following
groundwater section of the report, we fully expect shallower groundwater depths (< 4.0 feet) during the
spring season.
In general, the entire property is underlain by cobbly sandy gravel beginning at depths of 5.0 to 6.5 feet.
This material, which is identifiable based on its sandy gravel composition with big gravels and cobbles, is
the “target” foundation bearing material for all foundation support of buildings, retaining walls, and the
pedestrian bridge. In the area of TP-5 (in southeast corner), the driveway is in an elevated fill section
that is about 2.0 feet above existing site grades (to the north and south of the driveway). As a result,
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 7
even though gravel depth was measured at 8.0 feet in this pit; in reality, it is closer to 6.0 feet deep in
the areas outside of driveway. See Figure 2 for a map showing the gravel depth at each of the test pits.
In most pits, the “target” cobbly sandy gravel is overlain by a 1.0 to 1.5-foot layer of “clean” sand with
some small gravels. The sand is so clean and loose that where it is either close to the groundwater table
or below the groundwater table it is very soft/wet and sloughs/flows into the excavation. The only pit
that was a little different was TP-3 (southwest side of building site). As this location, the sand was still
clean and loose, but it was much thicker at about 4.0 feet. Due to the site’s fluvial/creek environment
(which results in variable soil conditions), there will undoubtedly be other areas that are similar to TP-3
where the clean sandy soils are thicker.
Overlying the sand layer in all test pits was a 1.0 to 5.0-foot layer of silt/clay (depending on location).
The upper silt/clay is stiff; but with increasing depth, the moisture content increases and the stiffness
decreases. Black, organic topsoil covers the silt/clay and has a thickness of 0.5 to 2.0 feet. Since most of
of the test pits were dug in the existing driveway, the upper soils in most pits consisted of about 0.5-foot
of gravel surfacing material. The only test pit that contained any fill material was TP-5. At this location,
the driveway fill section was comprised of 1.0-foot of gravel surfacing overlying 1.0-foot of driveway fill
material (dirty gravel with silt/clay).
Provided in Table 1 is a summary of the soil conditions observed in TP-1 through TP-5. This terminology
matches the attached test pit logs.
Table 1. Summary of Soil Conditions in Test Pits 1 through 5
TP
#
TP
LOCATION
GRAVEL
SURFACING
DRIVEWAY
FILL
NATIVE
TOPSOIL
NATIVE
SILT/CLAY
NATIVE
SAND
NATIVE
SANDY GRAVEL
1 Near Creek -------- -------- 0.0’ - 2.0’ 2.0’ - 4.0’ 4.0’ - 5.0’ 5.0’ - 7.0’
2 West Side 0.0’ - 0.5’ -------- 0.5’ - 1.5’ 1.5’ - 5.0’ 5.0’ - 6.0’ 6.0’ - 8.0’
3 SW Side 0.0’ - 0.5’ -------- 0.5’ - 1.0’ 1.0’ - 2.0’ 2.0’ - 6.0’ 6.0’ - 8.0’
4 South Side 0.0’ - 0.5’ -------- 0.5’ - 1.0’ 1.0’ - 5.0’ 5.0’ - 6.5’ 6.5’ - 8.0’
5 SE Side 0.0’ - 1.0’ 1.0’ - 2.0’ 2.0’ - 3.0’ 3.0’ - 6.5’ 6.5’ - 8.0’ 8.0’ - 9.0’ (6.0’)
Notes: 1) All soil measurements are depths below existing ground.
2) TP-1 was located in west ½ of site and in the area between Bozeman Creek and the old shed.
3) TP-2 was located in east ½ of site and in the existing driveway area to the west of the main building site.
4) TP-3 was located in east ½ of site and in the existing driveway area to the southwest of the main building site.
5) TP-4 was located in east ½ of site and in the existing driveway area to the south of the main building site.
6) TP-5 was located in east ½ of site and in the existing driveway area to the southeast of the main building site.
7) In the area of TP-5, the driveway is in a fill section and is elevated about 2.0’ above existing site grades.
8) In the area of TP-5, the depth to native sandy gravel to the north/south of the elevated driveway fill section is 6.0’.
9) The gravel surfacing layer consists of 1”-minus roadmix gravel.
10) The driveway fill layer consists of black, dirty gravel with some silt/clay.
11) The topsoil layer consists of black, silty clay w/ roots.
12) The silt/clay layer consists of dark brown/brown, sandy silt to sandy lean clay. Less stiff/more moist w/ depth.
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 8
13) The sand layer consists of loose/soft, dark brown to brown, clean sand w/ small gravels. Flows into pit when wet.
14) The sandy gravel layer consists of brown, sandy gravel w/ big gravels and cobbles. Identifiable based on cobbles.
15) The “target” bearing material for all footings is the native sandy gravel at depths of 5.0’ to 6.5’.
Groundwater Conditions
During our test pits in early December, the groundwater depth in the central and east parts of the site
ranged from 4.0 feet near Bozeman Creek to 5.0 to 6.5 feet on the east side. In all test pits, the level of
the groundwater was either found at the top of the sandy gravel or 0.5 to 1.0-foot above it in the loose
sandy soils. As expected, the shallower groundwater conditions were in the central and east-central
areas of the site (closer to Bozeman Creek). In TP-1, TP-2, and TP-3, the water was at a depth of 4.0 to
5.5 feet and in the sands above the sandy gravel. Due to the loose and saturated condition of the sand
in these test pits, this material caved/flowed/sloughed in the pit during excavation.
See Figure 2 for a map showing the test pit locations and the depth to groundwater that was measured
in each pit. This figure also lists the depth to sandy gravel in each pit as well (so it is easy to see where
the water level is relative to the top of the gravel).
Since the test pits were dug in December, the groundwater levels that were measured are likely close to
seasonal low elevations. During the spring/summer of the year (as a result of snowpack melting/runoff
and spring rains), we expect groundwater levels will be higher. Depending on the location, the seasonal
groundwater fluctuation in Bozeman can be 1.0 to 3.0 feet. Based on the site location (in the Bozeman
Creek drainage) and the fact the groundwater will rise in the spring, we do not recommend crawl space
or basement foundation configurations on this project site. The main building needs to be designed as a
slab-on-grade (as currently planned).
Provided in Table 2 is a summary of the groundwater conditions observed in TP-1 through TP-5. Also
included in the table is the groundwater depth relative to the top of the native sandy gravel.
Table 2. Summary of Groundwater Conditions in Test Pits 1 through 5
TP
#
TP
LOCATION
GROUNDWATER
DEPTH
GW DEPTH RELATIVE TO TOP OF
NATIVE SANDY GRAVEL
1 Near Creek 4.0’ 1.0’ above top of gravel
2 West Side 5.0’ 1.0’ above top of gravel
3 SW Side 5.5’ 0.5’ above top of gravel
4 South Side 6.5’ At top of gravel
5 SE Side 8.0’ (6.0’) At top of gravel
Notes: 1) All groundwater measurements are depths below existing ground.
2) TP-1 was located in west ½ of site and in the area between Bozeman Creek and the old shed.
3) TP-2 was located in east ½ of site and in the existing driveway area to the west of the main building site.
4) TP-3 was located in east ½ of site and in the existing driveway area to the southwest of the main building site.
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 9
5) TP-4 was located in east ½ of site and in the existing driveway area to the south of the main building site.
6) TP-5 was located in east ½ of site and in the existing driveway area to the southeast of the main building site.
7) In the area of TP-5, the driveway is in a fill section and is elevated about 2.0’ above existing site grades.
8) In the area of TP-5, the depth to groundwater to the north/south of the elevated driveway fill section is 6.0’.
GEOTECHNICAL ISSUES
Given the shallow sandy gravels at 5.0 to 6.5 feet, which are the “target” bearing materials for building
foundation support, the site has pretty limited geotechnical issues for foundation design and bearing.
All footings will need to bear on native sandy gravel or on granular structural fill that in turn is supported
on the native gravel. Standard foundation over-excavation and structural fill placement is expected.
Due to the loose/wet sands (which will cave/slough into the excavation), the over-excavation will need
to be wide enough to prevent any caving materials from getting into/contaminating the minimum width
of structural fill that is required under footings. Due to these site conditions, coupled with the fact that
the main building will contain an array of interior footings, we are recommending that the main building
be mass over-excavated down to native gravel and refilled with structural fill under to perimeter footing,
interior footing, and interior slab grades. Given the shallow groundwater conditions, the building can-
not be supported on a crawl space or basement foundation configuration.
Provided below are some other design and construction issues that we foresee on the project:
• Groundwater Dewatering: Groundwater is expected to be at or above the top of the “target”
bearing, sandy gravel during all times of the year. As a result, groundwater dewatering will likely
be required during foundation excavation to lower the water level closer to the top of gravel.
• Structural Fill Placement in Wet Excavations: Structural fill materials consist of imported, 3”-
minus sandy gravel. This material can not be placed directly on wet subgrade or in standing
water. Instead, our recommendation is lower the groundwater table as much as possible and
place an initial layer of 1”-minus, clean crushed rock over wet subgrade or in shallow standing
water (as the first lift of structural fill). The rock layer must be vibratory compacted and covered
with a medium-weight, 8 oz. non-woven geotextile fabric before continuing with placement of
the 3”-minus sandy gravel. See Figures 3, 6, 7, and 8, which show the foundation earthwork
under the main building and tall retaining wall, for an illustration of the fabric-covered, crushed
rock layer that will be required in wet excavations.
• Excessive Groundwater in the Bridge and ADU Building Excavations: Due to the amount of
groundwater, it will most likely be impossible to adequately dewater the foundation excavations
for the pedestrian bridge and ADU building (since they are located adjacent to or near Bozeman
Creek). For this reason, if any structural fill is required to build up from “target” sandy gravel up
to footing grade, we do not recommend trying to use 3”-minus sandy gravel at these locations
(which need dry conditions for proper placement and compaction). Instead, all structural fill
materials under the bridge and ADU building foundations should exclusively consist of 1”-minus,
clean crushed rock. The material can be placed and vibratory compacted in wet conditions and
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 10
in standing water. See Figures 4 and 5, which show the foundation earthwork under the ADU
building and pedestrian bridge, for an illustration of the use of clean crushed rock as structural
fill in overly wet excavations.
• Design and Backfill of Retaining Wall to Limit Top Rotation: In our opinion, one of the biggest
design challenges on the project is how to design and backfill the tall retaining wall on the east
side of the main building such that “top of wall” inward rotation is limited (due to the backfilled
height and overall height of the wall). With such a tall wall, any displacement will be magnified
at the top of the wall and it will be leaning and out of plume. At a minimum, the retaining wall
footing shall bear on “target” sandy gravel or on granular structural fill that in turn is supported
in the native gravel; and the back of the retaining wall shall contain a drain system (to prevent
hydrostatic loading of the wall). We have devised three ways to design and backfill the wall.
These are presented as Options 1, 2, and 3 (later in the report). Option 1 consists of designing
the wall for higher earth pressures (to make it thicker/robust and more heavily reinforced) and
backfilling the wall with 1”-minus, clean crushed rock (which is a light weight, but high strength
material). Option 2 consists of using light-weight, geofoam blocks as backfill. For this option,
the wall could also be designed for higher earth pressures, which will provide for an increased
conservancy in the design. Option 3 consists of designing the wall with concrete deadman/tie-
backs and backfilling the wall with 1”-minus, clean crushed rock (same as Option 1).
GENERAL CONSTRUCTION RECOMMENDATIONS
Re-Excavation of Test Pits
Due to the location of the test pits (in the existing gravel driveway area), we do not expect that they will
be encroached upon during building, retaining wall, and bridge foundation construction. With that said,
some of the pits will underlie the new site access and driveway areas. During backfilling of the pits, the
spoils were not placed in lifts nor compacted; as a result, they will undergo significant soil settlement
over time. If any of the site and building improvements, including foundations, interior and exterior
concrete slabs, underground utilities, and driveways and parking areas, will overlie any of the test pits,
we recommend that they be re-excavated back down to their original depth and properly backfilled and
compacted with suitable material. All of our test pits were dug to depths of 7.0 to 9.0 feet.
Topsoil Stripping and Re-Use
The east side of the property is generally blanketed by 6 to 12 inches of organic topsoil. The thickness of
topsoil is suspected to be greater in the areas closer to Bozeman Creek (and perhaps throughout the
west side of the site) as suggested by our findings in TP-1, which had a topsoil thickness of 2.0 feet. All
topsoil must be completely removed from within building foundation footprint areas, including from
under footings and interior/exterior slabs, and under exterior concrete areas (patios, decks, sidewalks,
and garage aprons) and under asphalt/gravel areas (driveways and parking lot areas). Final site grading
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 11
(in landscape areas) and the reclamation of disturbed construction areas are the only recommended
uses of this material.
Groundwater Dewatering
Given that groundwater is at the top of the native sandy gravel or above it, we expect that some level of
dewatering will be required for the foundation over-excavation (down to “target” gravel) for the main
building and the tall retaining wall. Due to the amount of water that is expected in the area closer to
Bozeman Creek, it may be hard to fully dewater the foundation excavations for the pedestrian bridge
and ADU building. Knowing this, we are recommending the exclusive use of 1”-minus, clean crushed
rock for the structural fill material under the bridge and ADU building foundations (in situations where
structural fill is needed to build up to footing grade).
Construction Equipment Access Over Bozeman Creek
To access the west side of the site for the construction of the west side bridge abutment and the ADU
building, some means will need to be incorporated that allows for construction equipment to cross the
creek channel. What we have seen done on other projects is to use an old, flat, railroad car bed for a
temporary crossing over the creek. The longer span of the railroad car should prevent or minimize any
impacts or disturbance to the creek channel and banks.
Excavation and Re-Use of On-Site Soils
The soils that will be predominantly excavated during site and building construction will include topsoil,
silt/clay, and clean sand with some small gravels. Due to the 5.0 to 6.5-foot depth to “target” bearing,
sandy gravel, very little of this material will be excavated. In most situations, only the top of the sandy
gravel will be exposed in the bottom of the foundation over-excavations. Provided below are the
allowable re-uses of the on-site materials:
• Organic topsoil materials shall only be used for final site grading in landscape areas.
• None of the on-site soils shall be used for granular structural fill under footings and slabs.
• None of the on-site soils shall be used for interior foundation wall backfill under interior slabs.
• None of the on-site soils shall be used for retaining wall backfill above the finished grade on the
front of the wall (see Figures 6, 7, and 8).
• For the main building, only native soils that are relatively dry and that can be compacted to a
dense and unyielding condition shall be used for exterior foundation wall backfill.
• For the ADU building and pedestrian bridge, native soils that are relatively dry and that can be
compacted to a dense and unyielding condition can be used for all foundation backfill.
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• For the tall retaining wall, native soils that are relatively dry and that can be compacted to a
dense and unyielding condition can be used for the front-side and back-side foundation backfill
up to the elevation of the finished grade on the front of the wall.
STRUCTURAL DESIGN PARAMETERS AND CONSIDERATIONS
Foundation Configuration (Main Building)
Due to the shallow groundwater depths, crawl space and basement foundation configurations are not
allowed at this site. The main building shall be underlain by a concrete slab-on-grade.
Foundation Configuration (ADU Building)
We understand the ADU building will be likely be an elevated structure than is supported on concrete
columns and pad footings (at the building corners and possibly some in between). If the elevated design
is eliminated, the building will need to be underlain by a slab-on-grade (same as the main building). The
high water conditions do not allow for a crawl space foundation.
Foundation Configuration (Pedestrian Bridge)
The bridge will be supported on standard concrete footings and abutment walls on the east and west
sides of the Bozeman Creek channel.
Foundation Design
All buildings can be supported on shallow conventional foundation systems consisting of perimeter,
interior, and exterior footings and perimeter frost walls. As stated above, the ADU building may not
have a perimeter foundation wall; but instead be supported on concrete columns. The shallow gravel
conditions due not warrant the use of a deep, pile-supported foundation system.
Seismic Design Factors
A main requirement of the Structural Engineer’s seismic analysis will be a determination of the site class.
Based on our on-site explorations and knowledge of the underlying geology, the site class for the project
site will be Site Class D (as per criteria presented in the 2021 IBC). This site class designation is valid as
long as our foundation recommendations are followed.
To obtain site-specific seismic loading and response spectrum parameters, a web-based application from
the USGS Earthquake Hazards Program can be used. The link to their web page is as follows:
https://earthquake.usgs.gov/hazards/designmaps/. Upon entering this page, there are links to three third-
party interfaces that can be used to obtain the seismic information. The user needs to enter the design
code reference document, site soil classification, risk category, site latitude, and site longitude.
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Foundation Bearing Pressure
As long as our shallow foundation support recommendations are followed (as presented later in the
report), the allowable bearing pressure for all perimeter, interior, and exterior footings and any other
foundation component is 2,500 pounds per square foot (psf). Allowable bearing pressures from
transient loading (due to wind or seismic forces) may be increased by 50 percent. We estimate that the
above-referenced bearing pressure will result in total settlements of one inch or less, with only minor
differential settlements.
Note: All building, retaining wall, and bridge footings must bear on “target” sandy gravel or on imported
granular structural fill that in turn is supported on the “target” gravel materials.
Lateral Earth Pressures
All foundation walls that will be fixed at the top prior to the placement of backfill should be designed for
an “at rest” equivalent fluid pressure of 60 pounds per cubic foot (pcf). Cantilevered retaining walls
may be designed for a lower, “active” equivalent fluid pressure of 45 pcf, provided either some slight
outward rotation of the wall is acceptable upon backfilling or the wall is constructed in such a way that
accommodates the expected rotation. These “at rest” and “active” design values are only applicable for
walls that will have backfill slopes of less than ten percent; and which will not be externally loaded by
surface pressures applied above and/or behind the wall.
Note: For design and backfill Option 1 for the tall retaining wall (on the east side of the main building),
we recommend increasing the design “at rest” and “active” equivalent fluid pressures to a minimum of
70 pcf and 55 pcf, respectively. If increasing the pressures even a little further to 75 pcf and 60 pcf,
respectively, will not overrun the project cost, this should be considered and undertaken to provide for a
stouter wall element. By designing the wall to a higher earth pressure, the wall will be thicker and more
heavily reinforced, which will make it more robust and reduce the likelihood for top of wall deformation.
The use of increased design pressures is in combination with the use of 1”-minus, clean crushed rock for
the backfill behind the wall. The rock backfill is free-draining, lighter weight than 3”-minus sandy gravel,
and a high strength backfill material. For Option 2, which utilizes light-weight, geofoam blocks for the
wall backfill, the use of higher design earth pressures can be considered, but it is not required. Quite
simply, a thicker wall with more reinforcing will provide more conservancy in the design and long-term
performance of the wall. As discussed below, one reason to consider designing the wall to the higher
design pressures for all three backfill options (Options 1, 2, and 3) is due to the fact that the backside of
the wall will be used for vehicle parking (and could be laterally loaded by vehicles, depending on the site
design and the location of the parking stalls relative to the back side of the wall).
Note: The increased earth pressures for the tall retaining wall (given above) assume that the backside of
the wall will be “buffered” from the vehicle loading in the parking stalls by the landscape strip/bed and
sidewalk that are shown on the pricing set of the plan drawings. These design values do not account for
vehicle loading of the wall. In our opinion, the front of the vehicle parking stalls should be a minimum of
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8 to 10 feet off the back side of the wall. The farther vehicles are kept away from the wall, the better.
We recommend installing the sidewalk and landscape strip (behind the wall) that is shown on the plans.
Lateral forces from wind, earthquakes, and earth pressures on the opposite side of the structure will be
resisted by passive earth pressure against the buried portion of the foundation wall and by friction at
the bottom of the footing. Passive earth pressures in compacted, fine-grained backfill (silt/clay) should
be assumed to have an equivalent fluid pressure of 280 pcf; while a coefficient of friction of 0.5 is
estimated between cast-in-place concrete and the “target” sandy gravel (or granular structural fill that is
placed to build back up to footing grade from the “target” gravel). Actual footing loads (not factored or
allowable loads) should be used for calculating frictional resistance to sliding along the base of the
footing. Please be aware that the friction coefficient has no built-in factor of safety; therefore, an
appropriate safety factor should be selected and used in all subsequent calculations for each load case.
The above-referenced, equivalent fluid pressures (for at rest, active, and passive conditions) assume
that the wall will be backfilled with a suitable material that is compacted to an unyielding condition and
it will lie above the groundwater table and/or be well drained; thereby, preventing the backfill from
becoming saturated and the wall from experiencing hydrostatic pressure. Each of these design
pressures is for static conditions and will need to be factored accordingly to represent seismic loading.
We recommend that we be retained to evaluate lateral earth pressures for geometries and/or loading
conditions that do not meet the previously mentioned criteria.
FOUNDATION AND DRAINAGE RECOMMENDATIONS
General
Five, detailed illustrations that show our foundation earthwork, wall backfill, subsurface drainage, and
moisture protection recommendations are included as Figures 3 through 8. Please refer to the figures
during the review of this report. Each is summarized below.
• Figure 3: This figure is for the main building and shows a slab-on-grade configuration.
• Figure 4: This figure is for the ADU building and shows an elevated structure configuration.
• Figure 5: This figure is for the pedestrian bridge and shows a standard footing/abutment wall
configuration.
• Figure 6: This figure is for the tall retaining wall and shows Option 1, which includes designing
the wall for higher earth pressures and backfilling the wall with 1”-minus, clean crushed rock.
• Figure 7: This figure is for the tall retaining wall and shows Option 2, which includes backfilling
the wall with light-weight, geofoam blocks and a consideration to design the wall for higher
earth pressures (for more conservancy).
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• Figure 8: This figure is for the tall retaining wall and shows Option 3, which includes concrete
deadman/tie-backs and backfilling the wall with 1”-minus, clean crushed rock.
Foundation Excavation (Main Building)
• Given that the main building will contain an array of interior footings, coupled with the expected
wet conditions and possible sloughing/caving of any loose sands (overlying the sandy gravel), we
recommend that the entire foundation footprint area of the building be mass over-excavated
down to “target” bearing, sandy gravel. This approach may require building pad structural fill to
build back up to perimeter footing grade (depending on the footing grade relative to “target”
gravel); and it will require interior structural fill within the foundation walls to build back up to
interior footing and slab grades.
• If the south end of the main building will contain an enclosed garage, only the perimeter footing
and foundation wall alignments need to be over-excavated down to “target” gravel. The core
soils under the garage slab can remain in place, provided the slab is supported on the minimum
18-inch gravel section.
• If the south end of the main building will contain a covered canopy area, only the pad footing
and concrete column locations need to be over-excavated down to “target” gravel. The core
soils under the garage slab can remain in place, provided the slab is supported on the minimum
18-inch gravel section.
Foundation Excavation (ADU Building)
• Assuming the ADU building will be an elevated structure, only the pad footing and concrete
column locations need to be over-excavated down to “target” gravel.
• If the ADU building will be a slab-on-grade with perimeter footings and foundation walls, either
the building can be mass excavated (same as the main building); or assuming it will have no or
very limited interior footings, only the footing locations can be over-excavated. At a minimum,
all footings need to bear on “target” gravel and the slab needs to be supported on a minimum
18-inch gravel section.
Foundation Excavation (Pedestrian Bridge)
• The pad footings and abutment walls need to be over-excavated down to “target” gravel in two,
short trench excavations.
Foundation Excavation (Tall Retaining Wall)
• The retaining wall footing needs to be over-excavated down to “target” gravel. We expect this
will be completed in a wide trench excavation.
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Footings
• The “target” bearing material for all building, retaining wall, and bridge footings (incl. perimeter,
interior, and exterior footings) is the native sandy gravel that underlies the site at depths of 5.0
to 6.5 feet.
• TP-5, located near the southeast corner of the property, was dug in an elevated section of the
driveway that is about 2.0 feet above existing ground grades. At this location, the gravel depth
was measured at 8.0 feet. However, since the driveway grade is elevated by about 2.0 feet, the
actual gravel depth is really only about 6.0 feet below existing ground (to the north and south of
the driveway).
• The “target” gravel is identifiable based on its brown color, “clean” sandy composition, and
abundant rounded gravels and big cobbles.
• All footings must bear directly on the “target” gravel or on compacted granular structural that in
is supported on the native gravel.
• Depending on footing grades relative to the top of the native gravel, some footing areas may
bear directly on the “target” gravel, while others will need to be over-excavated down to gravel
and built back up to footing grade with compacted, granular structural fill.
• As stated above, we recommend mass excavation/over-excavation down to the “target” gravel
under the entire foundation footprint area of the main building, with the possibility of only over-
excavating under the garage/cover canopy footings. For the ADU building, pedestrian bridge,
and retaining wall, only the footing alignments/locations will excavated/over-excavated.
• To minimize disturbance to the native gravel subgrade surface, the excavation should be dug
with a smooth-edge foundation bucket.
• If the native gravel subgrade is wet or contains standing groundwater, it should either be static
rolled or track-packed with the excavator.
• If the native gravel subgrade is dry, it should be vibratory compacted to a dense and unyielding
condition.
• As discussed throughout, the site is impacted by shallow groundwater. During most of the year
(if not all of it), the groundwater level is at the top of the sandy gravel or above it. As a result,
we expect all foundation over-excavations down to “target” bearing, sandy gravel will encounter
groundwater. Depending on the height of the groundwater over the top of the gravel, some
level of groundwater dewatering will likely be required during foundation earthwork to lower
the water as much as possible.
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• For the main building and tall retaining wall areas, we recommend lowering the groundwater to
the extent possible and then placing an initial layer of 1”-minus clean crushed rock to get above
the wet subgrade or shallow standing water. The rock layer must be vibratory compacted and
covered with a medium weight, 8 oz. non-woven geotextile separator fabric before continuing
the fill placement with granular structural fill.
• For the ADU building and pedestrian bridge areas, we expect a lot of water due to the proximity
to the creek. Most likely, the excavations will contain much more water. Rather than using any
granular structural fill (which requires a dry excavation), if structural fill is needed to build up to
footing grade it should consist entirely of 1”-minus, clean crushed rock. The rock can be placed
and vibratory compacted in standing water conditions.
• See Figures 3 through 8 for minimum width requirements for clean crushed rock and granular
structural fill beyond the edge of footing. In general, the minimum width of the fill material is
dependent on the fill thickness.
• All granular structural fill that is placed under footings must consist of either 3”-minus, sandy
(pitrun) gravel or 1.5”-minus, crushed (roadmix) gravel. Specifications for these materials are
provided in a later section of the report. Most of the time, 3”-minus sandy gravel is used.
• The granular structural fill section should be placed in multiple lifts (depending on thickness of
fill required and the size of the roller used) with each lift being vibratory compacted to a dense
and unyielding condition. See a later report section for additional compaction specifications. A
large, smooth drum roller should be used wherever possible. Small, walk-behind sheepsfoot
rollers and hand-held, jumping jack compactors should be used in narrow/confined excavations
and along edges and in corners of the excavation.
• The minimum depth of cover for frost protection of perimeter and exterior footings is four feet
(unless they are frost protected according to IBC standards). This dimension is measured from
bottom of footing up to the final grade of the ground surface.
Foundation Backfill
• Proper interior foundation wall backfill under slabs is critical (to minimize settlement potential
under the slab) and should be limited to high quality, granular structural fill (such as 3”-minus
sandy (pitrun) gravel or 1.5”-minus crushed (roadmix) gravel) or 1”-minus clean crushed rock.
These materials are easy to compact in tight and confined areas and do not have the settlement
potential that a native silt/clay or clean sandy soil has. See a later report section for additional
material specifications.
• Normally, exterior backfill can consist of on-site soil, provided it is not organic or too rocky, and
has a moisture content that will allow for proper compaction. Do not use wet soils for backfill.
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• For exterior backfill under site concrete areas adjacent to the foundation walls (such as patios,
decks, sidewalks, and at door entries), consideration should be given to fully backfilling these
areas with 3”-minus sandy gravel or 1”-minus clean crushed rock. By doing so, they will be well
supported on gravel/rock materials, settlement potential will be minimized, and frost heaving
potential will be eliminated. At a minimum, all exterior slabs should be supported on at least 18
inches of compacted gravel (of the which the top 6 inches must be clean crushed rock). The full
18-inch section can consist of clean crushed rock as well.
• For the ADU building (assuming an elevated structure) and the pedestrian bridge, all foundation
backfill can consist of dry native soils.
• For the tall retaining wall, the lower portion of the footing/wall backfill can consist of dry, native
soil. The native soils can be used on both sides of wall up to the finished grade elevation on the
front side of the wall (see Figures 6 through 8). For the backfill behind the upper, unsupported
section of the wall, this must either consist of 1”-minus clean crushed rock or geofoam blocks.
• All wall backfill materials (including clean crushed rock) must be placed in thin, level lifts and be
vibratory compacted with either a small, self-propelled roller, a walk-behind, sheepsfoot roller,
or a jumping jack compactor. Pay particular attention to proper compaction under interior and
exterior slabs.
Interior Slabs
• All organic topsoil must be stripped from under slabs.
• At a minimum, interior building slabs shall be underlain by a 6-inch (min.) layer of compacted,
1”-minus clean crushed rock that is supported on a 12-inch (min.) layer of compacted granular
structural fill. The structural fill materials can consist of 3”-minus sandy (pitrun) gravel or 1.5”-
minus crushed (roadmix) gravel. The purpose of the 18-inch thick, crushed rock and gravel
section is to provide adequate support of the slab.
• Since we are recommending mass over-excavation and structural fill replacement under slab-on-
grade foundations, the slab will bear on several feet of compacted granular structural ill.
• Depending on how the main building is excavated, the garage and/or covered canopy area (on
the south end of the building) can either be incorporated into the mass over-excavation of the
entire foundation footprint area or only the perimeter footings/walls/columns can be over-
excavated (thereby leaving the native soils under the garage slab). If the native soils are left
under the garage slab area, the minimum gravel support section that is recommended under the
slab is 18 inches. This shall consist of a 6-inch layer of crushed rock overlying a 12-inch layer of
granular structural fill; or the entire 18-inch section can be crushed rock.
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• Prior to any fill placement, the subgrade soils should be re-compacted to a dense and unyielding
condition. A large roller should be used wherever possible.
Exterior Slabs
• All organic topsoil must be stripped from under slabs.
• For exterior slabs adjacent to foundation walls and doors (like porches, patios, and decks), these
slabs should be supported on a minimum 18-inch gravel section. The section can consist of 18-
inches of clean crushed rock or 6 inches of rock overlying 12 inches of granular structural fill. As
stated in the foundation backfill section of the report, consideration should be given to fully
backfilling exterior walls (from footing grade up to slab grade) with crushed rock or structural fill
under exterior slab areas. The thicker the gravel section under exterior slabs, the less likely they
will be negatively impacted by frost heaving movements. Given that the site lies in a shallow
groundwater environment, a little extra gravel under exterior slabs will be a benefit. At a bare
minimum, these exterior slabs next to foundations and doorways must be supported on at least
12 inches of gravel consisting of 12 inches of crushed rock or 6 inches of crushed rock overlying
6 inches of granular structural fill.
• For the exterior garage aprons and driveways, these slabs should be supported on a minimum
18-inch gravel section. The section can consist of 18-inches of clean crushed rock or 6 inches of
rock overlying 12 inches of granular structural fill.
• For sidewalks away from buildings (not next to foundations or doorways), the City of Bozeman
minimum gravel section is 3 inches of clean crushed rock. We recommend this section thickness
be increased to 6 inches under all sidewalks. A little more rock will improve the slab support
and remove frost susceptible soils to a greater depth.
• Prior to any fill placement, the subgrade soils should be re-compacted to a dense and unyielding
condition. A large roller should be used wherever possible.
Moisture Protection and Subsurface Drainage (Slab-On-Grade Foundations)
• All interior building slabs shall be underlain by a 15-mil Stego vapor barrier. The barrier shall be
placed directly under the slab and above the 6-inch thick, clean crushed rock layer. It shall be
sealed along all seams and around pipe penetrations. Typically, vapor barriers are not installed
under garage slabs.
• Typically, perimeter frost walls around at-grade slab areas are not damp-proofed (per the code).
• Perimeter footing drains are not necessary for slab-on-grade foundations.
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Moisture Protection and Subsurface Drainage (Crawl Space Foundations)
• Since the site is not suitable for crawl space foundations, no moisture protection or subsurface
drainage recommendations are provided for this type of foundation configuration.
SURFACE DRAINAGE RECOMMENDATIONS
Final site grading next to the building must establish and promote positive surface water drainage away
from the foundation footprint in all directions. Absolutely no water should be allowed to accumulate
against or flow along any exposed wall (and thereby soak into the foundation wall backfill). Concrete or
asphalt surfacing that abut the foundation should be designed with a minimum grade of two percent;
while adjacent landscaped areas should have a slope of at least five percent within ten feet of the wall.
Steeper side slopes than five percent (in landscape areas) are encouraged wherever possible. By doing
this, any minor settlements in the foundation backfill should not negatively affect the positive drainage
away from the building. To further reduce the potential for moisture infiltration along foundation walls,
backfill materials should be placed in thin lifts and be well compacted, and in landscaped areas, they
should be capped by four to six inches of low permeable topsoil. With the exception of the locations
that will be surfaced by concrete or asphalt, finished grades (next to foundation walls) should be set no
less than six inches below the top of the interior concrete slab or below the bottom of the sill plate for
framed floor applications.
FOUNDATION-RELATED FILL MATERIAL RECOMMENDATIONS
Provided below are specifications for the fill materials that are recommended for use during foundation
earthwork construction. These include on-site excavated soils, sandy (pitrun) gravel, crushed (road mix)
gravel, and clean crushed rock. Fill placement and compaction criteria follow the specifications.
Excavated Foundation Soils
For more information on this, please refer to an earlier section of the report that is entitled “Excavation
and Re-Use of On-Site Soils”.
Sandy (pitrun) Gravel
Sandy (pitrun) gravel is a granular structural fill alternative for placement under footings and slabs and
behind walls. This material shall be a non-plastic, well-graded, mixture of clean, sand and gravel with
100 percent of its gravels/cobbles passing a three-inch screen and between 2 and 10 percent of its
silt/clay particles (by weight) finer than the No. 200 sieve. It should meet all material and gradation
specifications as presented in Section 02234 of the Montana Public Works Standard Specifications
(MPWSS) for 3”-minus, uncrushed, sub-base course gravel.
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Crushed (road mix) Gravel
Crushed (road mix) gravel is a granular structural fill alternative for placement under footings and slabs
and behind walls. This material shall be a non-plastic, well-graded, mixture of clean, sand and gravel
that is processed (crushed) such that 100 percent of its rock fragments pass a 1-1/2-inch screen and
between 0 and 8 percent of its silt/clay particles (by weight) are finer than the No. 200 sieve. It should
meet all material and gradation specifications as presented in Section 02235 of the MPWSS for 1-1/2”-
minus, crushed, base course gravel.
Clean Crushed Rock
The primary uses for crushed rock include placement under concrete slabs and behind foundation and
retaining walls for drainage-related purposes. Crushed rock shall be a clean assortment of angular
fragments with 100 percent passing a one-inch screen and less than 1 percent (by weight) finer than the
No. 100 sieve. This aggregate product needs to be manufactured by a crushing process and over 50
percent of its particles must have fractured faces. It is not acceptable to use rock containing abundant
spherical particles for foundation-related applications.
Note: For retaining wall Options 1 and 3, the wall shall be fully backfilled with compacted, clean crushed
up to a depth of 2.0 feet below finished grade. For Option 2, which utilizes light-weight geofoam block
for backfill, some crushed rock is required under the block and behind (upslope) of the block for leveling
and subsurface drainage.
Fill Placement and Compaction
All fill materials should be placed in uniform, horizontal lifts and compacted to an unyielding condition.
This includes clean crushed rock, which can be readily compacted by vibratory means. In general, the
maximum “loose lift thickness” for all fill materials (prior to compaction) should be limited to 12 inches
for large, self-propelled rollers, 6 inches for remote-controlled, dual drum rollers and walk-behind,
jumping jack compactors, and 4 inches for walk-behind vibratory plate compactors. The moisture
content of any material to be compacted should be within approximately two percent (+/-) of its
optimum value for maximum compaction.
Provided in Table 3 (on the following page) are compaction recommendations for general foundation
applications. These are presented as a percentage of the maximum dry density of the fill material as
defined in ASTM D-698.
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Table 3. Compaction Recommendations (Application vs. Percent Compaction)
APPLICATION % COMPACTION
Granular Structural Fill Under Footings and Slabs: 97
Interior Backfill under Slabs (Granular Structural Fill): 97
Backfill Behind Foundation and Retaining Walls: 95
Clean Crushed Rock Under Footings/Slabs and Behind Walls: N/A (Vibration Required)
Site Fill Around Building and Under Concrete and Pavement Areas: 95
RETAINING WALL DESIGN AND BACKFILL RECOMMENDATIONS
As stated in two, earlier sections of this report (including the geotechnical issues and structural design
parameters section), probably the most critical design and construction element on the project is the tall
retaining wall on the east side of the main building. In order to prevent this wall from experiencing
inward rotation (which will displace the top of the wall and make it be out of plume), the wall must be
properly designed and backfilled. At a minimum, the wall and footing must bear on “target” gravel or on
structural fill that in turn is supported on the gravel; and the wall must be backfilled with a 4-inch PVC or
PE drain pipe and drainage system to prevent the build-up of hydrostatic pressures behind the wall. The
drain pipe shall be placed at an elevation that is equal to the finished grade on the front of the wall, be
bedded in clean crushed rock, wrapped in non-woven fabric, and daylighted to drain. The outlet of the
drain pipe should be covered with a perforated cap to prevent rodent entry. In lieu of a drain pipe, the
wall can be poured the 1.5 to 2-inch sleeves for weep holes. The weep holes should be about 6 inches
above the final grade on the front of the wall and have a spacing of about 10 feet on-center.
In addition to foundation support and subsurface drainage, other key aspects of the wall performance
will be the structural design and the backfill. We have provided three options for the wall design and
the backfill materials. These are presented as Options 1, 2, and 3 on Figures 6, 7, and 8. Provided below
is a summary of each of the options. For all three options, the upper 2.0 feet of backfill shall be native
soils that are well compacted (to seal off the crushed rock backfill and geofoam backfill) and to prevent
surface water infiltration behind the wall. In the sidewalk and parking lot areas, the east side extents of
the wall backfill (further away from the wall) will be capped by the design gravel/pavement sections.
• Option 1: This option involves designing the wall using higher lateral earth pressures (which will
result in a thicker and more heavily-reinforced wall cross section) and backfilling the wall with
well compacted, fabric-encased, clean crushed rock. The crushed rock is free-draining, lighter
weight than 3”-minus sandy gravel, and is considered a high strength wall backfill material. We
recommend a medium-weight, 8 oz. non-woven geotextile fabric.
• Option 2: This option involves backfilling the wall with light-weight, geofoam blocks that are
locally available from Big Sky Insulations in Belgrade. We recommend using an EPS-15 block.
The blocks shall be installed on a thin leveling lift of compacted, clean crushed rock and must be
8.0 feet wide from the back of the wall (to minimize the soil stresses on the wall). The blocks
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are typically manufactured in 4.0’ x 8.0’ sheets and can be made in 6-inch thickness increments
up to about 30 inches. We typically recommend using 1.0, 1.5, or 2.0-foot thick blocks for ease
of installation and stacking. When I last checked on a cost for the geofoam block about a year
ago, it was about $60/CY. In back of the report, I have provided the contact information for
Chad Schauers at Big Sky Insulations. He is the EPS products manager and estimator. On the up-
slope side of the blocks, fabric-encased, clean crushed rock backfill must be placed that connects
to the crushed rock layer under the blocks. The top of the blocks must be covered with fabric as
well to prevent soil migration into the vertical joints between the blocks. We recommend a
medium-weight, 8 oz. non-woven geotextile fabric. In addition to the geofoam block backfill,
consideration should be given to designing the wall with higher earth pressures for increased
conservancy. This is not required; but should be evaluated as part of a cost vs. risk assessment.
• Option 3: This option involves designing the wall with concrete deadman and tie-backs and also
backfilling the wall with well compacted, fabric-encased, clean crushed rock (same as Option 1).
Similar to Option 2, the use of higher earth pressures for design is not required, but it should be
evaluated and considered. A thicker and more heavily reinforced wall will be much less likely to
undergo any noticeable deflection. As discussed in the structural design parameters section of
the report, a complicating issue with the wall is the fact that it will retain an elevated parking
area. As long as the parking stalls are 8.0 to 10.0 feet away from the wall (and separated by a
landscape strip and sidewalk), there should be no vehicle induced, lateral force component on
the back of the wall. Even though there should be enough separation, the fact that vehicle loads
will be behind the wall may be a good reason to design all options (even Options 2 and 3) with
higher earth pressures.
ASPHALT PAVEMENT SECTION RECOMMENDATIONS
Pavement Section Design
The southside driveway (leading to the garage or covered canopy area) and the parking lot (on the east
side of the building) may be surfaced with gravel or asphalt. Provided in Table 4 is the design section for
these improvements. If these areas will not be paved, the asphalt thickness can simply be omitted and
the total section thickness will be 15 inches instead of 18 inches.
Table 4. Pavement Section Design – On-Site Parking Lots – Option 1 – Stable Subgrade
COMPONENT COMPACTED THICKNESS (IN)
Asphalt Concrete: 3
Base Course – 1.5”-Minus Crushed (Roadmix) Gravel: 3
Sub-Base Course – 6”-Minus Uncrushed Sandy (Pitrun) Gravel: 12
315 lb. Woven Geotextile Fabric (Mirafi 600X or Approved Equal): Not Necessary
Stable Subgrade Soils (Less Topsoil): Compacted to 95%
TOTAL SECTION THICKNESS: 18
Final Geotechnical Report
705 S. Church Ave. – Bozeman, MT
Project: 21-207
January 14, 2022
Allied Engineering Services, Inc. Page 24
Important Note: This design pavement section is suitable provided the subgrade soils are dry, stable,
and can be compacted to 95 percent of ASTM D-698 prior to the placement of the sub-base gravel. If
unstable subgrade conditions are an issue, either the overly moist soils will need to be scarified and
dried OR the sub-base component of the pavement section will have to be thickened in order to bridge
the inferior soils. Depending on the level of severity of the soft subgrade conditions, additional sub-base
gravel thickness could range from 6 to 12 inches. If subgrade soils are wet/soft and deflect/rut under
construction traffic, they are considered to be unstable.
Pavement Section Materials, Placement, and Compaction
The sub-base and base course materials that comprise the granular parts of the pavement section shall
consist of 6-inch minus uncrushed sandy (pitrun) gravel and 1-1/2-inch minus crushed (road mix) gravel,
respectively. Both gravel courses shall meet the material and gradation specifications as presented in
the MPWSS, Sections 02234 and 02235. All gravels shall be placed in loose lifts not exceeding 12 inches
in thickness and be compacted to at least 95 percent of the material’s maximum dry density as defined
in ASTM D-698. Asphalt pavement shall meet specifications in MPWSS Section 02510 and be compacted
to a minimum of 93 percent of the Rice mix density.
PRODUCTS
Provided in Table 5 is a reference guide for all products (other than foundation-related fill material) that
have been recommended within this report. Listed below is the name of the product, its intended use,
and where it can be obtained. The manufacturer specification sheet for each of these products is
attached at the end of the report.
Table 5. Product Reference Guide
PRODUCT USE SOURCE PHONE
Stego 15-mil Vapor Barrier Sub-Slab Moisture Protection MaCon Supply – Bozeman 551-4281
Mirafi 180N Non-Woven Fabric Crushed Rock Separation Multiple Sources – Bzn/Blgd N/A
EPS-15 Geofoam Block Retaining Wall Backfill Material Big Sky Insulations – Belgrade 388-4146
Notes: 1) Use Stego 15-mil vapor barrier only. There are no approved equals for this product. 2) Stego 15-mil vapor barrier has a water transmission rate that meets national standards for vapor barriers.
3) A Stego 15-mil vapor barrier shall be placed under house interior slabs and in crawl spaces for moisture protection.
4) Use Mirafi 180N non-woven fabric or an approved equal that meets or exceeds Mirafi 180N fabric specifications.
5) Mirafi 180N non-woven fabric is a medium-weight, 8 oz. separation fabric. 6) Approved equals for Mirafi 180N non-woven fabric are available from multiple sources in the Bzn/Blgd area.
7) Mirafi 180N non-woven fabric shall be placed above crushed rock that is placed in a wet foundation excavations.
8) Mirafi 180N non-woven fabric shall be placed around crushed rock backfill behind tall retaining wall.
9) Mirafi 180N non-woven fabric shall be placed over geofoam blocks that are used for backfill of tall retaining wall.
10) Use EPS-15 geofoam block only. There are no approved equals for this product.
11) EPS-15 geofoam block is a rigid product that is used for light-weight backfill material behind retaining walls.
12) We provide three options for design and backfill of the tall retaining wall. The use of EPS-15 block is Option 2.
% WATERCONTENTSAMPLESDEPTH (FT)HORIZONTAL DISTANCE (FT):
JOB NUMBER: 21-207
PROJECT: 705 S. Church Ave.
DATE: December 3, 2021
BACKHOE TYPE: Volvo Mini-Excavator
BACKHOE OPERATOR: Zach W. - Walker Exc.
LOGGED BY: Lee Evans - AESI
SURFACE ELEVATION: N/A
TOTAL DEPTH: 7.0’
GROUNDWATER: 4.0’ (on 12/3/21)
TEST PIT DESIGNATION: TP-1
2
4
6
8
10
12
2 4 8 106
Civil Engineering
Geotechnical Engineering
Land Surveying
32 Discovery Drive
Bozeman, MT 59718
Phone: (406) 582-0221
Fax: (406) 582-5770
DESCRIPTION OF MATERIALS
4
LOCATION: Between Creek and Old Shed
(See Figures 1 & 2 for Approximate Location) Reviewed By: __________________ No Samples Collected
1
2
3
{0.0’ - 2.0’}: Native Topsoil
Medium stiff; black; organic silty
CLAY w/ roots; slightly moist.
Notes:
- Rich topsoil; heavily organic.
- A lot of root growth.
{2.0’ - 4.0’}: Native Silt/Clay
Medium stiff; dark brown to brown;
sandy SILT to sandy lean CLAY;
moist to very moist to wet.
Notes:
- Becomes wetter/softer w/ depth.
- Unsuitable found. bearing material.
{4.0’ - 5.0’}: Native Sand
Loose/soft; dark brown to brown;
“clean” SAND; wet.
Notes:
- Caving test pit walls (loose soils).
- Unsuitable found. bearing material.
{5.0’ - 7.0’}: Native Sandy Gravel
Medium dense to dense; brown;
sandy GRAVEL w/ some big gravels
and cobbles; wet.
Notes:
- “Clean” sandy gravel
- “Target” found. bearing material.
1
“Target” foundation bearing in
sandy GRAVEL below 5.0’ depth.
2
3
4
LSE, 12/30/21Note: TP-1 was dug on the
east side of Bozeman Creek.
Note: All soils in TP-1 are native.
No random fill material was observed.
Heavily organic topsoil
w/ a lot of root growth.
Generally a smaller, 3” to 4”-
minus gravel w/ some bigger
6” to 8” gravels and cobbles.
Medium stiff silt/clay
throughout. Moist to
very moist.
Silt/clay soils become
wetter/softer w/ depth.
Loose sand.
Identifiable based on big cobbles.
Test pit walls cave/flow (into pit).
% WATERCONTENTSAMPLESDEPTH (FT)HORIZONTAL DISTANCE (FT):
JOB NUMBER: 21-207
PROJECT: 705 S. Church Ave.
DATE: December 3, 2021
BACKHOE TYPE: Volvo Mini-Excavator
BACKHOE OPERATOR: Zach W. - Walker Exc.
LOGGED BY: Lee Evans - AESI
SURFACE ELEVATION: N/A
TOTAL DEPTH: 8.0’
GROUNDWATER: 5.0’ (on 12/3/21)
TEST PIT DESIGNATION: TP-2
2
4
6
8
10
12
2 4 8 106
Civil Engineering
Geotechnical Engineering
Land Surveying
32 Discovery Drive
Bozeman, MT 59718
Phone: (406) 582-0221
Fax: (406) 582-5770
DESCRIPTION OF MATERIALS
4
5
LOCATION: West Side of Building Site
(See Figures 1 & 2 for Approximate Location) Reviewed By: __________________ No Samples Collected
1
2
3
{0.0’ - 0.5’}: Gravel Surfacing
Dense; brown; 1”-minus roadmix
GRAVEL; slightly moist.
{0.5’ - 1.5’}: Native Topsoil
Stiff; black; organic silty CLAY
w/ roots; slightly moist.
{1.5’ - 5.0’}: Native Silt/Clay
Stiff to medium stiff; dark brown
to brown; sandy SILT to sandy lean
CLAY; moist to very moist to wet.
Notes:
- Soils down to 3.0’ (+/-) are stiff.
- Qu @ 2.0’ = 1.5 to 2.0 tsf.
- Less stiff/more moist below 3.0’.
- Becomes wetter/softer w/ depth.
- Unsuitable found. bearing material.
{5.0’ - 6.0’}: Native Sand
Loose/soft; dark brown to brown;
“clean” SAND w/ small gravels; wet.
Notes:
- Caving test pit walls (loose soils).
- Unsuitable found. bearing material.
{6.0’ - 8.0’}: Native Sandy Gravel
Medium dense to dense; brown;
sandy GRAVEL w/ some big gravels
and cobbles; wet.
Notes:
- “Clean” sandy gravel
- “Target” found. bearing material.
“Target” foundation bearing in
sandy GRAVEL below 6.0’ depth.
2
3
4
5
LSE, 12/30/21Note: With the exception of the gravel
surfacing, all soils in TP-2 are native.
No random fill material was observed.
Generally a smaller, 3” to 4”-
minus gravel w/ some bigger
6” to 8” gravels and cobbles.
Silt/clay soils become
wetter/softer w/ depth.
Stiff silt/clay
above 3.0’ (+/-).
Moisture break at 3.0’ (+/-).
Increasing moisture w/ depth.
Med. stiff silt/clay
below 3.0’ (+/-).
Loose sand w/ small gravels.
Test pit walls cave/flow (into pit).
1
Note: TP-2 was dug in the
gravel driveway to the west
of the existing house.
Identifiable based on big cobbles.
% WATERCONTENTSAMPLESDEPTH (FT)HORIZONTAL DISTANCE (FT):
JOB NUMBER: 21-207
PROJECT: 705 S. Church Ave.
DATE: December 3, 2021
BACKHOE TYPE: Volvo Mini-Excavator
BACKHOE OPERATOR: Zach W. - Walker Exc.
LOGGED BY: Lee Evans - AESI
SURFACE ELEVATION: N/A
TOTAL DEPTH: 8.0’
GROUNDWATER: 5.5’ (on 12/3/21)
TEST PIT DESIGNATION: TP-3
2
4
6
8
10
12
2 4 8 106
Civil Engineering
Geotechnical Engineering
Land Surveying
32 Discovery Drive
Bozeman, MT 59718
Phone: (406) 582-0221
Fax: (406) 582-5770
DESCRIPTION OF MATERIALS
4
5
LOCATION: Southwest Side of Building Site
(See Figures 1 & 2 for Approximate Location) Reviewed By: __________________ No Samples Collected
1
2
3
{0.0’ - 0.5’}: Gravel Surfacing
Dense; brown; 1”-minus roadmix
GRAVEL; slightly moist.
{0.5’ - 1.0’}: Native Topsoil
Stiff; black; organic silty CLAY
w/ roots; slightly moist.
{1.0’ - 2.0’}: Native Silt/Clay
Very stiff; dark brown to brown;
sandy SILT to sandy lean CLAY;
slightly moist to moist.
Notes:
- Soils are very stiff throughout.
- Qu @ 1.5’ = 2.0 to 2.5 tsf.
- Unsuitable found. bearing material.
{2.0’ - 6.0’}: Native Sand
Loose/soft; dark brown to brown;
“clean” SAND w/ multiple inter-
bedded seams of small, 1” to 2”-
minus gravels; slightly moist to wet.
Notes:
- Caving test pit walls (loose soils).
- Unsuitable found. bearing material.
{6.0’ - 8.0’}: Native Sandy Gravel
Medium dense to dense; brown;
sandy GRAVEL w/ some big gravels
and cobbles; wet.
Notes:
- “Clean” sandy gravel
- “Target” found. bearing material.
“Target” foundation bearing in
sandy GRAVEL below 6.0’ depth.
2
3
4
5
LSE, 12/30/21Note: With the exception of the gravel
surfacing, all soils in TP-3 are native.
No random fill material was observed.
Generally a smaller, 3” to 4”-
minus gravel w/ some bigger
6” to 8” gravels and cobbles.
Very stiff silt/clay.
Loose sand w/ small gravels.
Test pit walls cave/flow (into pit).
1
Note: TP-3 was dug in the
gravel driveway to the SW
of the existing house.
Identifiable based on big cobbles.
% WATERCONTENTSAMPLESDEPTH (FT)HORIZONTAL DISTANCE (FT):
JOB NUMBER: 21-207
PROJECT: 705 S. Church Ave.
DATE: December 3, 2021
BACKHOE TYPE: Volvo Mini-Excavator
BACKHOE OPERATOR: Zach W. - Walker Exc.
LOGGED BY: Lee Evans - AESI
SURFACE ELEVATION: N/A
TOTAL DEPTH: 8.0’
GROUNDWATER: 6.5’ (on 12/3/21)
TEST PIT DESIGNATION: TP-4
2
4
6
8
10
12
2 4 8 106
Civil Engineering
Geotechnical Engineering
Land Surveying
32 Discovery Drive
Bozeman, MT 59718
Phone: (406) 582-0221
Fax: (406) 582-5770
DESCRIPTION OF MATERIALS
4
5
LOCATION: South Side of Building Site
(See Figures 1 & 2 for Approximate Location) Reviewed By: __________________ No Samples Collected
1
2
3
{0.0’ - 0.5’}: Gravel Surfacing
Dense; brown; 1”-minus roadmix
GRAVEL; slightly moist.
{0.5’ - 1.0’}: Native Topsoil
Stiff; black; organic silty CLAY
w/ roots; slightly moist.
{1.0’ - 5.0’}: Native Silt/Clay
Very stiff to stiff; dark brown to
brown; sandy SILT to sandy lean
CLAY; slightly moist to moist.
Notes:
- Soils down to 3.0’ (+/-) are very stiff.
- Qu @ 2.0’ = 2.5 to 3.0 tsf.
- Qu @ 3.0’ = 2.0 to 2.5 tsf.
- Less stiff/more moist below 3.0’.
- Unsuitable found. bearing material.
{5.0’ - 6.5’}: Native Sand
Loose; dark brown to brown; “clean”
SAND w/ small gravels; very moist.
Notes:
- Caving test pit walls (loose soils).
- Unsuitable found. bearing material.
{6.5’ - 8.0’}: Native Sandy Gravel
Medium dense to dense; brown;
sandy GRAVEL w/ some big gravels
and cobbles; wet.
Notes:
- “Clean” sandy gravel
- “Target” found. bearing material.
“Target” foundation bearing in
sandy GRAVEL below 6.5’ depth.
2
3
4
5
LSE, 12/30/21Note: With the exception of the gravel
surfacing, all soils in TP-4 are native.
No random fill material was observed.
Generally a smaller, 3” to 4”-
minus gravel w/ some bigger
6” to 8” gravels and cobbles.
Very stiff silt/clay
above 3.0’ (+/-).
Moisture break at 3.0’ (+/-).
Increasing moisture w/ depth.
Stiff silt/clay
below 3.0’ (+/-).
Loose sand w/ small gravels.
Test pit walls cave/flow (into pit).
1
Note: TP-4 was dug in the
gravel driveway to the south
of the existing house.
Identifiable based on big cobbles.
% WATERCONTENTSAMPLESDEPTH (FT)HORIZONTAL DISTANCE (FT):
JOB NUMBER: 21-207
PROJECT: 705 S. Church Ave.
DATE: December 3, 2021
BACKHOE TYPE: Volvo Mini-Excavator
BACKHOE OPERATOR: Zach W. - Walker Exc.
LOGGED BY: Lee Evans - AESI
SURFACE ELEVATION: N/A
TOTAL DEPTH: 9.0’
GROUNDWATER: 8.0’ (on 12/3/21)
TEST PIT DESIGNATION: TP-5
2
4
6
8
10
12
2 4 8 106
Civil Engineering
Geotechnical Engineering
Land Surveying
32 Discovery Drive
Bozeman, MT 59718
Phone: (406) 582-0221
Fax: (406) 582-5770
DESCRIPTION OF MATERIALS
4
6
5
LOCATION: Southeast Side of Building Site
(See Figures 1 & 2 for Approximate Location) Reviewed By: __________________ No Samples Collected
1
2
3
{0.0’ - 1.0’}: Gravel Surfacing
Dense; brown; 1”-minus roadmix
GRAVEL; slightly moist.
{1.0’ - 2.0’}: Driveway Fill Material
Dense; black; dirty GRAVEL w/
some silt/clay; slightly moist.
{2.0’ - 3.0’}: Native Topsoil
Stiff; black; organic silty CLAY
w/ roots; slightly moist.
{3.0’ - 6.5’}: Native Silt/Clay
Very stiff to stiff; dark brown to
brown; sandy SILT to sandy lean
CLAY; slightly moist to moist.
Notes:
- Soils down to 5.0’ (+/-) are very stiff.
- Unsuitable found. bearing material.
{6.5’ - 8.0’}: Native Sand
Loose; dark brown to brown; “clean”
SAND w/ small gravels; very moist.
Notes:
- Unsuitable found. bearing material.
{8.0’ - 9.0’}: Native Sandy Gravel
Medium dense to dense; brown;
sandy GRAVEL w/ some big gravels
and cobbles; wet.
Notes:
- “Clean” sandy gravel
- “Target” found. bearing material.
“Target” foundation bearing in
sandy GRAVEL below 8.0’ depth.
3
4
6
5
LSE, 12/30/21Note: With the exception of the gravel
surfacing and driveway fill material, all
soils in TP-5 are native. No random fill
material was observed.
Generally a smaller, 3” to 4”-
minus gravel w/ some bigger
6” to 8” gravels and cobbles.
Very stiff silt/clay
above 5.0’ (+/-).
Moisture break at 5.0’ (+/-).
Increasing moisture w/ depth.
Stiff silt/clay
below 5.0’ (+/-).
Loose sand w/ small gravels.
Test pit walls cave/flow (into pit).
1
2
Note: TP-5 was dug in the
gravel driveway to the SE
of the existing house.
Note: At the TP-5 location,
the driveway grade is about
2.0’ above existing ground.
Identifiable based on big cobbles.
LIMITATIONS OF YOUR GEOTECHNICAL REPORT
GEOTECHNICAL REPORTS ARE PROJECT AND CLIENT SPECIFIC
Geotechnical investigations, analyses, and recommendations are project and client specific. Each project
and each client have individual criterion for risk, purpose, and cost of evaluation that are considered in the
development of scope of geotechnical investigations, analyses and recommendations. For example, slight
changes to building types or use may alter the applicability of a particular foundation type, as can a
particular client’s aversion or acceptance of risk. Also, additional risk is often created by scope-of-
service limitations imposed by the client and a report prepared for a particular client (say a construction
contractor) may not be applicable or adequate for another client (say an architect, owner, or developer for
example), and vice-versa. No one should apply a geotechnical report for any purpose other than that
originally contemplated without first conferring with the consulting geotechnical engineer. Geotechnical
reports should be made available to contractors and professionals for information on factual data only and
not as a warranty of subsurface conditions, such as those interpreted in the exploration logs and discussed
in the report.
GEOTECHNICAL CONDITIONS CAN CHANGE
Geotechnical conditions may be affected as a result of natural processes or human activity. Geotechnical
reports are based on conditions that existed at the time of subsurface exploration. Construction operations
such as cuts, fills, or drains in the vicinity of the site and natural events such as floods, earthquakes, or
groundwater fluctuations may affect subsurface conditions and, thus, the continuing adequacy of a
geotechnical report.
GEOTECHNICAL ENGINEERING IS NOT AN EXACT SCIENCE
The site exploration and sampling process interprets subsurface conditions using drill action, soil
sampling, resistance to excavation, and other subjective observations at discrete points on the surface and
in the subsurface. The data is then interpreted by the engineer, who applies professional judgment to
render an opinion about over-all subsurface conditions. Actual conditions in areas not sampled or
observed may differ from those predicted in your report. Retaining your consultant to advise you during
the design process, review plans and specifications, and then to observe subsurface construction
operations can minimize the risks associated with the uncertainties associated with such interpretations.
The conclusions described in your geotechnical report are preliminary because they must be based on the
assumption that conditions revealed through selective exploration and sampling are indicative of actual
Allied Engineering Services, Inc. Page 2
conditions throughout a site. A more complete view of subsurface conditions is often revealed during
earthwork; therefore, you should retain your consultant to observe earthwork to confirm conditions and/or
to provide revised recommendations if necessary. Allied Engineering cannot assume responsibility or
liability for the adequacy of the report’s recommendations if another party is retained to observe
construction.
EXPLORATIONS LOGS SHOULD NOT BE SEPARATED FROM THE REPORT Final explorations logs developed by the consultant are based upon interpretation of field logs (assembled
by site personnel), field test results, and laboratory and/or office evaluation of field samples and data.
Only final exploration logs and data are customarily included in geotechnical reports. These final logs
should not be redrawn for inclusion in Architectural or other design drawings, because drafters may
commit errors or omissions in the transfer process.
To reduce the likelihood of exploration log misinterpretation, contractors should be given ready access to
the complete geotechnical report and should be advised of its limitations and purpose. While a contractor
may gain important knowledge from a report prepared for another party, the contractor should discuss the
report with Allied Engineering and perform the additional or alternative work believed necessary to
obtain the data specifically appropriate for construction cost estimating purposes.
OWNERSHIP OF RISK AND STANDARD OF CARE
Because geotechnical engineering is much less exact than other design disciplines, there is more risk
associated with geotechnical parameters than with most other design issues. Given the hidden and
variable character of natural soils and geologic hazards, this risk is impossible to eliminate with any
amount of study and exploration. Appropriate geotechnical exploration, analysis, and recommendations
can identify and lesson these risks. However, assuming an appropriate geotechnical evaluation, the
remaining risk of unknown soil conditions and other geo-hazards typically belongs to the owner of a
project unless specifically transferred to another party such as a contractor, insurance company, or
engineer. The geotechnical engineer’s duty is to provide professional services in accordance with their
stated scope and consistent with the standard of practice at the present time and in the subject geographic
area. It is not to provide insurance against geo-hazards or unanticipated soil conditions.
The conclusions and recommendations expressed in this report are opinions based our professional
judgment and the project parameters as relayed by the client. The conclusions and recommendations
assume that site conditions are not substantially different than those exposed by the explorations. If
during construction, subsurface conditions different from those encountered in the explorations are
observed or appear to be present, Allied Engineering should be advised at once such that we may review
those conditions and reconsider our recommendations where necessary.
RETENTION OF SOIL SAMPLES
Allied Engineering will typically retain soil samples for one month after issuing the geotechnical report.
If you would like to hold the samples for a longer period of time, you should make specific arrangements
to have the samples held longer or arrange to take charge of the samples yourself.