HomeMy WebLinkAboutGeotech Repot_05042021MONTANA | WASHINGTON | IDAHO | NORTH DAKOTA | PENNSYLVANIA
JOB NO. 21-006-001 FEBRUARY 2021
REPORT OF GEOTECHNICAL INVESTIGATION
CLIENT ENGINEER
L’Heureux, Page, Werner
Mr. Timothy Peterson, AIA
15 Fifth St S
Great Falls, MT 59401
Email: timothyp@lpwarchitecture.com
Ahren Hastings, PE
ahren.hastings@tdhengineering.com
REPORT OF GEOTECHNICAL INVESTIGATION
PROJECT NAME
PROJECT LOCATION 406.586.0277
tdhengineering.com
234 East Babcock, Suite 3
Bozeman, MT 59715
TRADITIONS TOWNHOMES
BOZEMAN, MONTANA
Geotechnical Report Traditions Townhomes Table of Contents
Bozeman, Montana i
Table of Contents
1.0 EXECUTIVE SUMMARY .................................................................................................... 1
2.0 INTRODUCTION .................................................................................................................. 3
2.1 Purpose and Scope....................................................................................................... 3
2.2 Project Description ........................................................................................................ 3
3.0 SITE CONDITIONS.............................................................................................................. 3
3.1 Geology and Physiography .......................................................................................... 3
3.2 Subsurface Conditions ................................................................................................. 3
3.2.1 Soils .......................................................................................................................... 3
3.2.2 Ground Water ......................................................................................................... 4
4.0 ENGINEERING ANALYSIS ................................................................................................. 5
4.1 Introduction ..................................................................................................................... 5
4.2 Site Grading and Excavations ..................................................................................... 5
4.3 Conventional Shallow Foundations on Structural Fill .............................................. 5
4.4 Building Slab-on-Grade and Exterior Flatwork ......................................................... 6
5.0 RECOMMENDATIONS ....................................................................................................... 7
5.1 Site Grading and Excavations ..................................................................................... 7
5.2 Conventional Shallow Foundations ............................................................................ 8
5.3 Foundation and Retaining Walls ................................................................................. 9
5.4 Exterior Concrete Flatwork ........................................................................................ 11
5.5 Interior Concrete Floor Slabs ...................................... Error! Bookmark not defined.
5.6 Pavements ..................................................................... Error! Bookmark not defined.
5.7 Continuing Services .................................................................................................... 11
6.0 SUMMARY OF FIELD AND LABORATORY STUDIES ............................................... 13
6.1 Preliminary Field Explorations ................................................................................... 13
6.2 Laboratory Testing ...................................................................................................... 13
7.0 LIMITATIONS ..................................................................................................................... 14
Geotechnical Report Traditions Townhomes Appendix
Bozeman, Montana ii
APPENDIX
Test Pit Location Map (Figure 1)
Logs of Exploratory Test Pits (Figures 2 through 5)
Laboratory Test Data (Figures 6 through 8)
Soil Classification and Sampling Terminology for Engineering Purposes
Classification of Soils for Engineering Purposes
Geotechnical Report Traditions Townhomes Executive Summary
Bozeman, Montana Page 1
GEOTECHNICAL REPORT
TRADITIONS TOWNHOMES
BOZEMAN, MONTANA
1.0 EXECUTIVE SUMMARY
A geotechnical investigation was performed for the proposed new Traditions Townhomes, located at
the intersection of Durston Road and Rosa Way in Bozeman, Montana, on January 15, 2021.
Subsurface soil conditions consist of lean clay overlying native poorly-graded gravel with sand and
clay. In all test pits a surficial layer of topsoil classifying as lean clay with elevated organics was
encountered. The topsoil and majority of the lean clay was frozen at the time of our investigation.
Ground water was observed in all test pits at depths ranging from 5.0 to 6.5 feet below existing site
grade. This area is known for high ground water which could rise or fluctuate seasonally; thus,
ground water should be expected in excavations for the building and associated utilities at certain
times of the year. Based on the findings of our preliminary field investigation and experience in this
area, the seismic site class for this site is D. The risk of seismically induced liquefaction is
considered low and does not warrant additional consideration.
The primary geotechnical concern regarding this project is the presence of weak surficial lean clay
and relatively high ground water. Based on the site conditions encountered, it is our opinion that the
clay soils are not suitable for the support of the planned structure due to the weak nature and
presence of organics. Objectionable settlement is expected for foundations and slabs supported on
the weak clay material. The recommended bearing stratum for this project is the native gravel which
was encountered in all four test pits at a depths between 2.1 to 3.5 feet. Our recommendation is to
completely remove the topsoil and clay to the underlying native gravel and replace this zone with
compacted structural fill up to the desired building foundation. This approach may require over-
excavation beneath conventional foundation and slab systems. The amount of over-excavation will
vary depend on site grading and the finished floor elevation.
Geotechnical Report Traditions Townhomes Introduction
Bozeman, Montana Page 3
2.0 INTRODUCTION
2.1 Purpose and Scope
This report presents the results of our geotechnical investigation for the proposed new Traditions
Townhomes, located at the intersection of Durston Road and Rosa Way in Bozeman, Montana. The
purpose of this preliminary study was to determine the general surface and subsurface conditions
across the existing site for use by the owner and design team in site planning and design for the
buildings. This report describes the field work and laboratory analyses conducted for the site
investigation, the surface and subsurface conditions encountered, and presents our engineering
assessment and design recommendations.
The site investigation was performed on January 15, 2021 and consisted of four test pits excavated
across the proposed project area. Excavation depths ranged from 7.4 to 8.2 feet below existing site
grades. Samples from the investigation were returned to our Great Falls laboratory for testing.
Laboratory testing was performed on selected soil samples to obtain engineering properties of the
subsurface materials.
2.2 Project Description
We understand the proposed project includes construction of a two-story, five-plex townhome
apartment building and associated site improvements on Lots 4A, 4B, 4C, 4D and 4E, Block 1,
Traditions Subdivision-Phase 1, Bozeman, Montana. Based on the “Preliminary Not For
Construction” Building Drawings dated November 3, 2020, we understand that both a conventional
shallow spread footing foundation with a crawlspace and thickened-edge slab-on-grade system are
being considered for this project. Due to known high ground water in the area and know n issues
with water accumulations in crawlspaces, we highly recommend that slab-on-grade construction be
utilized in this area.
Structural loads had not been provided for the structure at the time of this report. Our preliminary
analysis assumes the building will be loaded typical of residential construction. Our understanding
is that asphalt pavement will not be a part of this project and has been excluded from our analysis.
Geotechnical Report Traditions Townhomes Site Conditions
Bozeman, Montana Page 3
3.0 SITE CONDITIONS
3.1 Geology and Physiography
The site is geologically characterized as gravel (Qgr) and upper Tertiary Sediments (Tsu). The
gravel areas shown on the geologic map below are generally comprised of variable deposits ranging
from pebble to boulder size including sand, silt, and clay. They are dominantly associated with
alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.
The sections denoted Tsu are Upper Tertiary Sediments or sedimentary rock comprised of
conglomerate, tuffaceous sandstone and siltstone, marlstone, and equivalent sediment and ash
beds.
Geologic Map of Montana, Edition 1.0 (2007)
Montana Bureau of Mines & Geology
Based on the subsurface conditions encountered and our experience in the area, the site falls under
seismic Site Class D. The structural engineer should utilize the site classification above to
determine the appropriate seismic design data for use on this project in accordance with current
applicable building codes in use by the City of Bozeman at the time of design. The likelihood of
seismically-induced soil liquefaction is low and does not warrant additional evaluation.
3.2 Subsurface Conditions
3.2.1 Soils
The subsurface soil conditions appear to be relatively consistent based on our exploratory
excavations and soil sampling. In general, the subsurface soil conditions encountered within
the test pits consist of approximately 0.4 to 0.6 feet of surficial topsoil, visually classified as
lean clay which contains organic material. This is underlain by native lean clay ranging in
Geotechnical Report Traditions Townhomes Site Conditions
Bozeman, Montana Page 4
thickness of 1.6 to 3.1 feet. Native poorly-graded gravel with sand was encountered in all
test pits at depths of 5.0 to 6.5 feet below existing ground, and similar gravels extend to
depths of at least 8.2 feet, the maximum depth investigated.
The subsurface soils are described in detail on the enclosed test pit logs and are
summarized below. The stratification lines shown on the logs represent approximate
boundaries between soil types, and the actual in situ transition may be gradual vertically or
discontinuous laterally.
TOPSOIL
The topsoil was visually classified as lean clay and was observed to be frozen and
considered relatively stiff based on difficulty with excavating the material. Generally organics
were observed in varying amounts at our test pit locations.
LEAN CLAY
Lean Clay was encountered in all test pits between the surficial topsoil and the underlying
native gravel. The clay is considered stiff and was predominantly frozen during excavation.
A sample of the material contained 3 percent gravel, 7.8 percent sand, and 89.2 percent
fines (clay and silt). The lean clay was found to have a liquid limit of 43, a plastic limit of 23
percent, and a plasticity index of 20 percent based on Atterberg Limits result. The natural
moisture contents varied from 14.4 to 19.8 percent and averaged 18.0 percent.
POORLY-GRADED GRAVEL WITH SAND AND CLAY
Native poorly-graded gravel with sand and clay was encountered in all test pits beneath the
surficial clay and extends to depths of at least 8.2 feet. The native gravels are considered
relatively dense based on the effort required during excavation. A sample of the material
contained 59.3 percent gravel, 22.3 percent sand, and 5.8 percent fines (silt and clay). The
natural moisture contents varied from 2.6 to 5.8 percent and averaged 4.2 percent.
3.2.2 Ground Water
Ground water was encountered in all four test pits at depths ranging from 5.0 to 6.5 feet
below existing ground surface. Numerous factors contribute to seasonal ground water
occurrences and fluctuations, and the evaluation of such factors is beyond the scope of this
report. This area is known for high ground water and should be expected in excavations for
the building and associated utilities at certain times of the year. A monitoring well was
installed in TP-1 for future ground water monitoring.
Geotechnical Report Traditions Townhomes Engineering Analysis
Bozeman, Montana Page 5
4.0 ENGINEERING ANALYSIS
4.1 Introduction
The primary geotechnical concern regarding this project is the presence of weak surficial lean clay
and high ground water. The native clays were frozen and generally considered stiff based on the
difficulty of excavation. However, during spring conditions similar clays are generally very soft and
highly compressible leading to concerns with settlement.
Due to the shallow ground water and the potential for seasonal fluctuations the use of a crawlspace
is not advised for this project. This option carries an elevated risk of water accumulation within the
below grade space which can lead to mold and other health risks. Conventional foundation drain
systems utilizing mechanical pumping systems, such as would be required due to the flat terrain, are
not intended for permanent ground water control and will not resolve the ground water issues.
4.2 Site Grading and Excavations
The ground surface at the proposed site is consistent and can be described as relatively flat. The
majority of the site appears to be lower than the surrounding curb level indicating that fill may be
necessary to build up the site during construction. Based on our field work, variable clay
thicknesses overlying native poorly-graded gravel with sandy and clay will be encountered in
excavations across the site. Depending on the time of year, ground water should be anticipated in
all utility and foundation excavations.
4.3 Conventional Shallow Foundations on Structural Fill
The existing clay soils encountered across the site are not suitable to support foundation loads due
to their relatively weak strength and the resulting risk of settlement. It is recommended that the
native clays be completely removed down to the underlying native gravel and replaced with
compacted structural fill to the desired foundation elevation beneath all conventional footing and
stem wall or thickened-edge foundation systems. This depth of over-excavation and resulting
structural fill thickness will depend on the finished floor elevation and foundation system selected for
the proposed building. Transferring foundation loads directly to the native gravel through
compacted structural fill will minimize total and differential settlement risk. Based on this construction
methodology, our experience, and using an allowable bearing pressure of 3,000 pounds per square
foot, we estimate the total settlement for footings will be less than ¾-inch. Differential settlement
across the building should be on the order of one-half this magnitude.
4.4 Foundation Walls
Foundation walls associated with a crawlspace configuration, if used, will be subjected to horizontal
loading due to lateral earth pressures. The lateral earth pressures are a function of the natural and
backfill soil types and acceptable wall movements, which affect soil strain to mobilize the shear
Geotechnical Report Traditions Townhomes Engineering Analysis
Bozeman, Montana Page 6
strength of the soil. More soil movement is required to develop greater internal shear strength and
lower the lateral pressure on the wall. To fully mobilize strength and reduce lateral pressures, soil
strain and allowable wall rotation must be greater for clay soils than for cohesionless, granular soils.
The lowest lateral earth pressure against walls for a given soil type is the active condition and
develops when wall movements occur. Passive earth pressures are developed when the wall is
forced into the soil, such as at the base of a wall on the side opposite the retained earth side. When
no soil strain is allowed by the wall, this is the "at-rest" condition, which creates pressures having
magnitudes between the passive and active conditions.
The distribution of the lateral earth pressures on the structure depends on soil type and wall
movements or deflections. In most cases, a triangular pressure distribution is satisfactory for design
and is usually represented as an equivalent fluid unit weight. Design parameters are given in the
recommendations section of this report.
4.5 Building Slab-on-Grade
In our opinion, the use of slab-on-grade construction is best suited to this site and the shallow
ground water conditions; however, the clay soils present on site are not suitable to support these
systems directly. Therefore, it will be necessary to improve the underslab conditions by removing
the surficial clay soils down to native gravel and replacing this zone with compacted structural fill up
to the planned bottom of slab elevation. This can result in a significant amount of soil removal and
imported gravel depending on the final site grading and finished floor elevation; however, this
method of construction will help control the potential for slab displacements, optimize slab
performance, and control issues related to the shallow ground water condition in the area.
4.6 Exterior Concrete Flatwork
Exterior flatwork is generally more tolerable to vertical movement and can be repaired or replaced
more economically than interior slab systems. Thus, it is common for exterior flatwork applications
to utilize conventional construction consisting of a limited base course layer (four to six inches)
beneath the concrete surfacing. Similar construction is anticipated for this project assuming the
Owner understands and is willing to accept the risks and resulting need for future maintenance.
Exterior flatwork on this site will be exposed to greater risk as a result of the lean clay which is
compressible and highly frost susceptible.
Should the Owner desire a higher level of performance, improvements beneath exterior flatwork
systems can be considered. The cost of improving the subgrade below exterior flatwork can be
substantial and generally consist of thicker base course sections and possible geogrid
improvements to help reduce the overall risk to within the desired level. Optimal performance is
realized when the clay soils are completely removed and replaced with compacted structural fill as is
recommended beneath interior slab systems.
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 7
5.0 RECOMMENDATIONS
5.1 Site Grading and Excavations
1. All topsoil and organic material should be removed from the proposed building and
any areas to receive site grading fill. Additionally, the existing native lean clay should
be completely removed from beneath all planned building foundations and interior
slab-on-grade systems.
2. All fill and backfill should be non-expansive, free of organics and debris and should
be approved by the project geotechnical engineer. The on-site soils, exclusive of
topsoil, are suitable for use as exterior backfill and general site grading fill on this
project.
All fill should be placed in uniform lifts not exceeding 8 inches in thickness for fine-
grained soils and not exceeding 12 inches for granular soils. All materials compacted
using hand compaction methods or small walk-behind units should utilize a
maximum lift thickness of 6 inches to ensure adequate compaction throughout the
lift. All fill and backfill shall be moisture conditioned to near the optimum moisture
content and compacted to the following percentages of the maximum dry density
determined by a standard proctor test which is outlined by ASTM D698 or equivalent
(e.g. ASTM D4253-D4254). On-site moisture conditioning should be anticipated for
any fine-grained clay or silt soils to be utilized as fill or backfill as these materials are
likely to exhibit excessively high moisture contents to be properly compacted.
a) Below Foundations or Spread Footings ...................................... 98%
b) Below Interior Slab-on-Grade Construction ................................. 98%
c) Exterior Backfill, Crawlspace Infill, & Exterior Flatwork ............... 95%
d) General Landscaping or Nonstructural Areas ............................. 92%
e) Site Utility Trench Backfill, To Within 2 Feet of Surface ............. 95%
For your consideration, verification of compaction requires laboratory proctor tests to
be performed on a representative sample of the soil prior to construction. These
tests can require up to one week to complete (depending on laboratory backlog) and
this should be considered when coordinating the construction schedule to ensure
that delays in construction or additional testing expense is not required due to
laboratory processing times or rush processing fees.
3. Imported structural fill, when required, should be non-expansive, free of organics and
debris, and conform to the material requirements outlined in Section 02234 of the
Montana Public Works Standard Specifications (MPWSS). All gradations outlined in
this standard are acceptable for use on this project; however, conventional proctor
methods (outlined in ASTM D698) shall not be used for any materials containing less
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 8
than 70 percent passing the ¾-inch sieve. Conventional proctor methods are not
suitable for these types of materials, and the field compaction value must be
determined using a relative density test outlined in ASTM D4253-4254.
4. Develop and maintain site grades which will rapidly drain surface and roof runoff
away from foundation and subgrade soils; both during and after construction. The
final site grading shall conform to the grading plan, prepared by others to satisfy the
minimum requirements of the applicable building codes.
5. It is the responsibility of the Contractor to provide safe working conditions in
connection with underground excavations. Temporary construction excavations
greater than four feet in depth, which workers will enter, will be governed by OSHA
guidelines given in 29 CFR, Part 1926. The soil conditions on site can change due
to changes in soils moisture or disturbances to the site prior to construction. Thus,
the contractor is responsible to provide an OSHA knowledgeable individual during all
excavation activities to regularly assess the soil conditions and ensure that all
necessary safety precautions are implemented and followed.
5.2 Conventional Shallow Foundations
The design and construction criteria below should be observed for conventional shallow foundation
systems including thickened-edge slab options. The construction details should be considered
when preparing the project documents.
6. Both interior and exterior footings should bear on properly compacted native poorly-
graded gravel with sand and clay or structural fill extending to native gravels and
should be designed for a maximum allowable soil bearing pressure of 3,000 psf
provided settlements as outlined in the Engineering Analysis are acceptable. The
limits of over-excavation and replacement, when needed, should extend at least 18
inches beyond the outer faces of the footing in all directions.
7. Soils disturbed below the planned depths of footing excavations should either be re-
compacted or be replaced with suitable compacted backfill approved by the
geotechnical engineer.
8. Footings shall be sized to satisfy the minimum requirements of the applicable
building codes while not exceeding the maximum allowable bearing pressure
provided in Item 6 above.
9. Exterior footings and footings beneath unheated areas should be placed at least 48
inches below finished exterior grade for frost protection. For shallow thickened-edge
slabs, use of rigid board, extruded polystyrene insulation (XEPS) is recommended to
provide adequate frost protection. One inch of XEPS is approximately equivalent to 1
foot of soil cover.
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 9
10. The bottom of the footing excavations should be free of cobbles and boulders to
avoid stress concentrations acting on the base of the footings. If the native gravels
cannot be rolled smooth, the use of a thin cushion course between the native gravels
and the footings should be considered. Cushion course gravels should conform to
the material requirements of MPWSS Section 02235 and be compacted to
requirements of Item 2a above.
11. Lateral loads are resisted by sliding friction between the footing base and the
supporting soil and by lateral pressure against the footings opposing movement. For
design purposes, a friction coefficient of 0.45 and a lateral resistance pressure of
200 psf per foot of depth are appropriate for footing bearing on properly compacted
native gravel or structural fill and backfilled with compacted native soils.
12. A representative of the project geotechnical engineer should be retained to observe
all footing excavations prior to the placement of concrete formwork in order to
document that native gravels have been encountered beneath all foundation
elements and to conduct compaction testing to verify compliance with Item 2a during
subgrade preparation.
5.3 Foundation Walls
The design and construction criteria presented below should be observed for foundation walls if a
crawlspace configuration is selected for this project. The construction details should be considered
when preparing the project documents.
13. Foundation stem walls which are laterally supported and can be expected to undergo
only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of 60 pcf for exterior backfill
consisting of properly compacted native soils.
14. When a crawlspace configuration is utilized, fill should be placed and compacted
(Item 2c) on the interior of the crawlspace to an elevation equal to the top of the
exterior footings. This fill is intended to provide lateral support to the wall during
exterior backfill and help control the potential for water accumulation in the
crawlspace associated with exterior sources.
15. Backfill should be selected, placed, and compacted per Item 2c above. Care should
be taken not to over-compact the backfill since this could cause excessive lateral
pressure on the walls. Only hand-operated compaction equipment should be used
within 5 feet of foundation walls.
16. Exterior footing drains are required to remove ground water seepage and infiltrated
surface runoff away from foundation soils when a crawlspace configuration is
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 10
utilized. This will be a critical component to help control the potential for water
accumulation within the crawlspace but the inclusion of a foundation drain alone will
not prevent water accumulations from occurring. We highly advise against the use
of a crawlspace for this project due to the inherent ground water issues and
associated risks which can pose health concerns to future tenants should mold or
other moisture related issues develop.
If a crawlspace is used, the preferred system would be directly connected to the
storm water system to eliminate the need for mechanical pumps, which can fail and
operate with restrictions increasing the potential for water accumulations on site such
as this where the system may be operating to lower the ground water table on site.
5.4 Building Slab-on-Grade
17. As discussed in the engineering analysis section above, the native clay soils are not
considered suitable to support interior building slabs and should be completely
removed down to the surface of the native gravels and replaced with properly
compacted structural fill back to the design slab elevation. A thin cushion course of
finer graded gravel is permissible directly beneath the slab-on-grade construction as
needed for plumbing installation or as a leveling material.
18. Structural fill materials should conform to the gradation shown in Item 3 above.
Cushion course materials utilized beneath interior and exterior slab-on-grade
applications should conform to the requirements outlined in Section 02235 of the
Montana Public Works Standard Specifications (MPWSS). All gradation outlined in
this specification are acceptable for this application. Prior to placing the cushion
course, the upper six inches of subgrade (exterior) or underlying structural fill should
be compacted per Item 2 above.
As an alternative, a clean crushed chips material has been utilized on previous
projects in lieu of conventional base course materials. This product is acceptable for
use on this project but should be compacted using at least three passes of a smooth
bottom vibratory plate following installation and we advise thicknesses not exceed six
inches.
19. Concrete floor slabs should be designed using a modulus of vertical subgrade
reaction no greater than 300 pci when designed and constructed as recommended
in Item 17 above.
20. Geotechnically, an underslab vapor barrier is recommended for this project due to
the shallow ground water table and the potential for moisture transmission through
the building slab. We recommend a minimum 10-mil vapor barrier which is taped
and sealed at all joints be utilized unless otherwise specified by the architect or
structural engineer.
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 11
5.5 Exterior Concrete Flatwork
21. For normally loaded, exterior concrete flatwork, the native lean clay soils may pose
some risk to the performance of these features due to the potential compressive
properties. Conventional construction consisting of approximately four to six inches
of free-draining, crushed gravel placed beneath the concrete and compacted to the
requirements of Item 2c above is considered suitable provided the Owner is aware of
and willing to accept the risks associated with this construction method. The
magnitude of displacement will vary depending the depth of the clay soils or fill in
that location as well as drainage conditions, irrigation locations, slab loading
conditions, and other factors. Slab movements could result in the need for more
frequent repair or replacement of the exterior concrete if they become too great.
22. If the Owner desires to reduce the risk of movements beneath exterior slab systems
and improve anticipated performance a variety of measures are possible and these
can be discussed with the design team. Mitigative efforts can range from the use of
a greater base course thickness to the complete removal and replacement of the
problematic soil. The various options need to be considered by the Owner based on
the expected performance and related construction cost so they can select the most
appropriate system for their project.
5.6 Continuing Services
Three additional elements of geotechnical engineering service are important to the successful
completion of this project.
23. Consultation between the geotechnical engineer and the design professionals during
the design phases is highly recommended. This is important to ensure that the
intentions of our recommendations are incorporated into the design, and that any
changes in the design concept consider the geotechnical limitations dictated by the
on-site subsurface soil and ground water conditions.
24. Observation, monitoring, and testing during construction is required to document the
successful completion of all earthwork and foundation phases. A geotechnical
engineer from our firm should be retained to observe the excavation, earthwork, and
foundation phases of the work to determine that subsurface conditions are
compatible with those used in the analysis and design. If construction services are
performed by someone other than our firm, the entities performing these services
must be directed to contact us immediately upon changes in subsurface conditions
so we may re-evaluate our recommendations in a timely manner.
25. During site grading, placement of all fill and backfill should be observed and tested to
confirm that the specified density has been achieved. We recommend that the
Geotechnical Report Traditions Townhomes Recommendations
Bozeman, Montana Page 12
Owner maintain control of the construction quality control by retaining the services of
an experienced construction materials testing laboratory. We are available to
provide construction inspection services as well as materials testing of compacted
soils and the placement of Portland cement concrete and asphalt. In the absence of
project specific testing frequencies, TD&H recommends the following minimum
testing frequencies be used:
Compaction Testing
Beneath Column Footings 1 Test per Footing per Lift
Beneath Wall Footings 1 Test per 50 LF of Wall per Lift
Beneath Slabs 1 Test per 1,500 SF per Lift
Foundation Backfill 1 Test per 100 LF of Wall per Lift
LF = Lineal Feet SF = Square Feet
Geotechnical Report Traditions Townhomes Summary of Field and Laboratory Studies
Bozeman, Montana Page 13
6.0 SUMMARY OF FIELD AND LABORATORY STUDIES
6.1 Field Explorations
The geotechnical investigation was performed on January 15, 2021 and consisted of four test pits
excavated to depths ranging from 7.4 to 8.2 feet across the property. The approximate location of
each test pit is shown on Figure 1. The test pits were excavated by Earth Surgeons using a
Komatsu 88 excavator. The subsurface exploration and sampling methods used are indicated on
the attached test pit logs. The test pits were logged by Mr. Ahren Hastings, PE of TD&H
Engineering.
During excavation, composite samples of the subsurface materials were collected from the
excavation spoils at distinct changes in the subsurface stratigraphy. Logs of all test pits, which
include soil descriptions and sample depths, are presented on Figures 2 through 5. During
excavation of TP-1, a perforated pipe was installed vertically and backfilled with excavation spoils to
be utilized in any future ground water monitoring.
6.2 Laboratory Testing
Samples obtained during the field exploration were returned to our materials laboratory where they
were observed and visually classified in general accordance with ASTM D2487, which is based on
the Unified Soil Classification System. Representative samples were selected for testing to
determine the engineering and physical properties of the soils in general accordance with ASTM or
other approved procedures.
Tests Conducted: To determine:
Natural Moisture Content Representative moisture content of soil at the time of
sampling.
Grain-Size Distribution Particle size distribution of soil constituents describing the
percentages of clay/silt, sand and gravel.
Atterberg Limits A method of describing the effect of varying water content on
the consistency and behavior of fine-grained soils.
The laboratory testing program for this project consisted of eight moisture-visual analyses, two sieve
(grain-size distribution) analyses, and one Atterberg Limits analysis. The results of the water content
analyses are presented on the test pit logs, Figures 2 through 5. The grain-size distribution curve
and Atterberg Limits results are presented on Figures 6 through 8.
Geotechnical Report Traditions Townhomes Limitations
Bozeman, Montana Page 14
7.0 LIMITATIONS
This report has been prepared in accordance with generally accepted geotechnical engineering
practices in this area for use by the client for design purposes. The findings, analyses, and
recommendations contained in this report reflect our professional opinion regarding potential
impacts the subsurface conditions may have on the proposed project and are based on site
conditions encountered. Our analysis assumes that the results of the exploratory test pits are
representative of the subsurface conditions throughout the site, that is, that the subsurface
conditions everywhere are not significantly different from those disclosed by the subsurface study.
Unanticipated soil conditions are commonly encountered and cannot be fully determined by a limited
number of test pits and laboratory analyses. Such unexpected conditions frequently require that
some additional expenditures be made to obtain a properly constructed project. Therefore, some
contingency fund is recommended to accommodate such potential extra costs.
The recommendations contained within this report are based on the subsurface conditions observed
in the test pits and are subject to change pending observation of the actual subsurface conditions
encountered during construction. TD&H cannot assume responsibility or liability for the
recommendations provided if we are not provided the opportunity to perform limited construction
inspection and confirm the engineering assumptions made during our analysis. A representative of
TD&H should be retained to observe all construction activities associated with subgrade
preparation, foundations, and other geotechnical aspects of the project to ensure the conditions
encountered are consistent with our assumptions. Unforeseen conditions or undisclosed changes
to the project parameters or site conditions may warrant modification to the project
recommendations.
Long delays between the geotechnical investigation and the start of construction increase the
potential for changes to the site and subsurface conditions which could impact the applicability of
the recommendations provided. If site conditions have changed because of natural causes or
construction operations at or adjacent to the site, TD&H should be retained to review the contents of
this report to determine the applicability of the conclusions and recommendations provide
considering the time lapse or changed conditions.
Misinterpretation of the geotechnical information by other design team members is possible and can
result in costly issues during construction and with the final product. Our geotechnical engineers are
available upon request to review those portions of the plans and specifications which pertain to
earthwork and foundations to determine if they are consistent with our recommendations and to
suggest necessary modifications as warranted. This service was not included in the original
geotechnical scope of the project and will require additional fees for the time required for
specification and plan document review and comment. In addition, TD&H should be involved
throughout the construction process to observe construction, particularly the placement and
compaction of all fill, preparation of all foundations, and all other geotechnical aspects. Retaining
the geotechnical engineer who prepared your geotechnical report to provide construction
observation is the most effective method of managing the risks associated with unanticipated
conditions.
Geotechnical Report Traditions Townhomes Limitations
Bozeman, Montana Page 15
This report was prepared for the exclusive use of the owner and architect and/or engineer in the
design of the subject facility. It should be made available to prospective contractors and/or the
contractor for information on factual data only and not as a warranty of subsurface conditions such
as those interpreted from the test pit logs and presented in discussions of subsurface conditions
included in this report.
Prepared by: Reviewed by:
Ahren Hastings, PE Craig Nadeau PE
Geotechnical Engineer Geotechnical Manager
TD&H ENGINEERING TD&H ENGINEERING
J:\2021\21-006 Traditions Townhomes\GEOTECH\Traditions Townhomes Geotech FINAL.doc
QUALITY CHECK:DESIGNED BY:DRAWN BY:CAD NO.JOB NO.DATE:21-006 FIG-1MONTANAWASHINGTONIDAHOGREAT FALLS-BOZEMAN-KALISPELL-SHELBYLEWISTONSPOKANEEngineeringtdhengineering.comNORTH DAKOTAWATFORD CITY1FIGURE21-0061.31.2021ACHAPPROXIMATE TEST PIT LOCATIONSBOZEMAN, MONTANATRADITIONS TOWNHOMES GEOTECHNICAL INVESTIGATION
Log of Test Pit TP-1
Figure No.
Sheet of
2
1 1LOGGRAPHIC01020304050
0
0 10 30 40 5020SAMPLEDEPTHWATERGROUNDSOIL DESCRIPTION
LEGEND DEPTH(FEET)Traditions Townhomes
Geotechnical Investigation
THOMAS, DEAN & HOSKINS, INC.ENGINEERING CONSULTANTS
January 15, 2021 21-006(FEET)1
8
12
Ground Water EncounteredDuring ExcavationAPPROXIMATE SURFACE ELEVATION:
SURFACE:
Logged By: Ahren Hastings, P.E.Excavated By:Earth Surgeons
Komatsu 88
4
6
10
14
16
CLAY
2
3
5
7
9
11
13
15
Bottom of Test Pit
GRAVEL
CLAY
Log of Test Pit TP-2
Figure No.
Sheet of
3
1 1LOGGRAPHIC01020304050
0
0 10 30 40 5020SAMPLEDEPTHWATERGROUNDSOIL DESCRIPTION
LEGEND DEPTH(FEET)Traditions Townhomes
Geotechnical Investigation
THOMAS, DEAN & HOSKINS, INC.ENGINEERING CONSULTANTS
January 15, 2021 21-006(FEET)1
8
12Ground Water EncounteredDuring ExcavationAPPROXIMATE SURFACE ELEVATION:
SURFACE:
Logged By: Ahren Hastings, P.E.Excavated By:Earth Surgeons
Komatsu 88
4
6
10
14
16
CLAY
2
3
5
7
9
11
13
15
Bottom of Test Pit
GRAVEL
CLAY
Log of Test Pit TP-3
Figure No.
Sheet of
4
1 1LOGGRAPHIC01020304050
0
0 10 30 40 5020SAMPLEDEPTHWATERGROUNDSOIL DESCRIPTION
LEGEND DEPTH(FEET)Traditions Townhomes
Geotechnical Investigation
THOMAS, DEAN & HOSKINS, INC.ENGINEERING CONSULTANTS
January 15, 2021 21-006(FEET)1
8
12Ground Water EncounteredDuring ExcavationAPPROXIMATE SURFACE ELEVATION:
SURFACE:
Logged By: Ahren Hastings, P.E.Excavated By:Earth Surgeons
Komatsu 88
4
6
10
14
16
CLAY
2
3
5
7
9
11
13
15
Bottom of Test Pit
GRAVEL
CLAY
Log of Test Pit TP-4
Figure No.
Sheet of
5
1 1LOGGRAPHIC01020304050
0
0 10 30 40 5020SAMPLEDEPTHWATERGROUNDSOIL DESCRIPTION
LEGEND DEPTH(FEET)Traditions Townhomes
Geotechnical Investigation
THOMAS, DEAN & HOSKINS, INC.ENGINEERING CONSULTANTS
January 15, 2021 21-006(FEET)1
8
12Ground Water EncounteredDuring ExcavationAPPROXIMATE SURFACE ELEVATION:
SURFACE:
Logged By: Ahren Hastings, P.E.Excavated By:Earth Surgeons
Komatsu 88
4
6
10
14
16
CLAY
2
3
5
7
9
11
13
15
Bottom of Test Pit
GRAVEL
CLAY
Tested By: BC Checked By:
LIQUID AND PLASTIC LIMITS TEST REPORT
PLASTICITY INDEX0
10
20
30
40
50
60
LIQUID LIMIT
0 10 20 30 40 50 60 70 80 90 100 110
CL-ML
C L o r O L
C H o r O H
ML or OL MH or OH
Dashed line indicates the approximate
upper limit boundary for natural soils
47
WATER CONTENT42.4
42.8
43.2
43.6
44
44.4
44.8
45.2
45.6
46
46.4
NUMBER OF BLOWS
5 6 7 8 9 10 20 25 30 40
MATERIAL DESCRIPTION LL PL PI %<#40 %<#200 USCS
Project No. Client:Remarks:
Project:
Location: TP-3
Sample Number: A-22731 Depth: 2.0 ft
Figure
Lean CLAY 43 23 20 CL
21-006-001 LPW
Report No. A-22731-207
Date: 1-28-2021Traditions Subdivision 5-Plex
Bozeman, Montana
6
Tested By: WJC Checked By:
1-30-2021
(no specification provided)
PL= LL= PI=
D90= D85= D60=
D50= D30= D15=
D10= Cu= Cc=
USCS= AASHTO=
*
Poorly-Graded GRAVEL with Silt and Sand (Visual)
6"
4"
3"
2.5"
2"
1.5"
1"
3/4"
1/2"
3/8"
#4
#10
#20
#40
#60
#80
#100
#200
100.0
96.8
87.4
77.4
70.5
65.3
54.9
48.6
40.5
36.4
28.1
20.7
15.1
11.1
8.9
7.8
7.2
5.8
80.5163 72.9042 30.7716
20.3480 5.6570 0.8349
0.3284 93.70 3.17
GP-GM
Report No. A-22730-206X
LPW
Traditions Subdivision 5-Plex
Bozeman, Montana
21-006-001
Material Description
Atterberg Limits
Coefficients
Classification
Remarks
Location: TP-2
Sample Number: A-22730 Depth: 4.0 - 5.0 ft Date:
Client:
Project:
Project No:Figure
SIEVE PERCENT SPEC.
*PASS?
SIZE FINER PERCENT (X=NO)PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.00010.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
12.6 38.8 20.5 7.4 9.6 5.3 5.86 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report
7
Tested By: WJC Checked By:
1-30-2021
(no specification provided)
PL= LL= PI=
D90= D85= D60=
D50= D30= D15=
D10= Cu= Cc=
USCS= AASHTO=
*
Lean CLAY
1/2"
3/8"
#4
#10
#20
#40
#60
#80
#100
#200
100.0
98.8
97.0
95.9
95.0
94.0
93.2
92.6
92.0
89.2
0.0904
CL
Report No. A-22726-206
LPW
Traditions Subdivision 5-Plex
Bozeman, Montana
21-006-001
Material Description
Atterberg Limits
Coefficients
Classification
Remarks
Location: TP-1
Sample Number: A-22726 Depth: 2.0 ft Date:
Client:
Project:
Project No:Figure
SIEVE PERCENT SPEC.
*PASS?
SIZE FINER PERCENT (X=NO)PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.00010.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 0.0 3.0 1.1 1.9 4.8 89.26 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report
8
Great Falls, Kalispell, Bozeman, Montana
Spokane, Washington, Lewiston, Idaho
THOMAS, DEAN & HOSKINSEngineering Consultants SOIL CLASSIFICATION AND
SAMPLING TERMINOLOGY
FOR ENGINEERING PURPOSES
12" 3" 3/4" No.4 No.10 No.40 No.200 <No.200
SILTS & CLAYSBOULDERSCOBBLESGRAVELSSANDS
PARTICLE SIZE RANGE
(Distinguished By
Atterberg Limits)FineCoarse FineMediumCoarse
Sieve Openings (Inches)Standard Sieve Sizes
CL - Lean CLAY
ML - SILT
OL - Organic SILT/CLAY
CH - Fat CLAY
MH - Elastic SILT
OH - Organic SILT/CLAY
SW - Well-graded SAND
SP - Poorly-graded SAND
SM - Silty SAND
SC - Clayey SAND
GW - Well-graded GRAVEL
GP - Poorly-graded GRAVEL
GM - Silty GRAVEL
GC - Clayey GRAVEL
* Based on Sampler-Hammer Ratio of 8.929 E-06 ft/lbf and 4.185 E-05 ft^2/lbf for
granular and cohesive soils, respectively (Terzaghi)
STANDARD PENETRATION TEST (ASTM D1586)
RELATIVE DENSITY*RELATIVE CONSISTENCY*
Granular, Noncohesive
(Gravels, Sands, & Silts)Fine-Grained, Cohesive
(Clays)
Very Loose
Loose
Medium Dense
Dense
Very Dense
Very Soft
Soft
Firm
Stiff
Very Stiff
Hard
0-2
3-4
5-8
9-15
15-30
+30
0-4
5-10
11-30
31-50
+50
Standard
Penetration Test
(blows/foot)
Standard
Penetration Test
(blows/foot)
PLASTICITY CHART
0 10 16 20 30 40 50 60 70 80 90 100 110
60
50
40
30
20
107
4
C L or O LC H or O H
ML or OL
MH or OH
CL-ML "U - LIN E""A - LIN E"LIQUID LIMIT (LL)PLASTICITY INDEX (PI)For classification of fine-grained soils and thefine-grained fraction of coarse-grained soils.
Equation of "A"-line
Horizontal at PI = 4 to LL = 25.5,
then PI = 0.73 (LL-20)
Equation of "U"-line
Vertical at LL = 16 to PI = 7,
then PI = 0.9 (LL-8)
Great Falls, Kalispell, Bozeman, Montana
Spokane, Washington, Lewiston, Idaho
THOMAS, DEAN & HOSKINSEngineering Consultants ASTM D2487
CLASSIFICATION OF SOILS
FOR ENGINEERING PURPOSES
Flow Chart For Classifying Coarse-Grained Soils (More Than 50 % Retained On The No. 200 Sieve)
Flow Chart For Classifying Fine-Grained Soils ( 50 % Or More Passes The No. 200 Sieve)
<5% fines
5-12% fines
>12% fines
<5% fines
5-12% fines
>12% fines
Well-graded GRAVELWell-graded GRAVEL with sandPoorly-graded GRAVELPoorly-graded GRAVEL with sand
Well-graded GRAVEL with silt
Well-graded GRAVEL with silt and sandWell-graded GRAVEL with clay (or silty clay)Well-graded GRAVEL with clay and sand (or silty clay and sand)
Poorly-graded GRAVEL with silt
Poorly-graded GRAVEL with silt and sand
Poorly-graded GRAVEL with clay (or silty clay)Poorly-graded GRAVEL with clay and sand (or silty clay and sand)
Silty GRAVELSilty GRAVEL with sandClayey GRAVELClayey GRAVEL with sandSilty, clayey GRAVEL
Silty, clayey GRAVEL with sand
Well-graded SAND
Well-graded SAND with gravel
Poorly-graded SANDPoorly-graded SAND with gravel
Well-graded SAND with silt
Well-graded SAND with silt and gravel
Well-graded SAND with clay (or silty clay)Well-graded SAND with clay and gravel (or silty clay and gravel)
Poorly-graded SAND with siltPoorly-graded SAND with silt and gravelPoorly-graded SAND with clay (or silty clay)
Poorly-graded SAND with clay and gravel
(or silty clay and gravel)
Silty SANDSilty SAND with gravelClayey SAND
Clayey SAND with gravel
Silty, clayey SAND
Silty, clayey SAND with gravel
<15% sand>15% sand
<15% sand
>15% sand
<15% sand>15% sand
<15% sand
>15% sand
<15% sand>15% sand<15% sand>15% sand
<15% sand>15% sand<15% sand>15% sand<15% sand
>15% sand
<15% gravel
>15% gravel
<15% gravel>15% gravel
<15% gravel>15% gravel<15% gravel>15% gravel
<15% gravel
>15% gravel<15% gravel>15% gravel
<15% gravel
>15% gravel<15% gravel>15% gravel<15% gravel>15% gravel
Lean CLAYLean CLAY with sandLean CLAY with gravelSandy lean CLAY
Sandy lean CLAY with gravel
Gravelly lean CLAY
Gravelly lean CLAY with sand
Silty CLAY
Silty CLAY with sand
Silty CLAY with gravel
Sandy silty CLAYSandy silty CLAY with gravelGravelly silty CLAYGravelly silty CLAY with sand
SILT
SILT with sandSILT with gravelSandy SILTSandy SILT with gravel
Gravelly SILT
Gravelly SILT with sand
Fat CLAYFat CLAY with sand
Fat CLAY with gravel
Sandy fat CLAYSandy fat CLAY with gravelGravelly fat CLAYGravelly fat CLAY with sand
Elastic SILT
Elastic SILT with sand
Elastic SILT with gravelSandy elastic SILTSandy elastic SILT with gravelGravelly elastic SILT
Gravelly elastic SILT with sand
%sand > %gravel
%sand < %gravel
<15% gravel>15% gravel<15% sand>15% sand
%sand > %gravel
%sand < %gravel<15% gravel>15% gravel<15% sand
>15% sand
%sand > %gravel%sand < %gravel
<15% gravel>15% gravel<15% sand>15% sand
%sand > %gravel%sand < %gravel<15% gravel>15% gravel<15% sand
>15% sand
%sand > %gravel
%sand < %gravel
<15% gravel>15% gravel<15% sand>15% sand
fines=ML or MH
fines=CL or CH (or CL-ML)
fines=ML or MH
fines=CL or CH (or CL-ML)
fines=ML or MH
fines=CL or CH
fines=CL-ML
fines=ML or MH
fines=CL or CH
(or CL-ML)
fines=ML or MH
fines=CL or CH
(or CL-ML)
fines= ML or MH
fines=CL or CH
fines=CL-ML
<30% plus No. 200
>30% plus No. 200
<30% plus No. 200
>30% plus No. 200
<30% plus No. 200
>30% plus No. 200
<30% plus No. 200
>30% plus No.200
<30% plus No. 200
>30% plus No. 200
Cu>4 and 1<Cc<3
Cu<4 and/or 1>Cc>3
Cu>4 and 1<Cc<3
Cu<4 and/or 1>Cc>3
Cu>6 and 1<Cc<3
Cu<6 and/or 1>Cc>3
Cu>6 and 1<Cc<3
Cu<6 and/or 1>Cc>3
CL
CL-ML
ML
CH
MH
PI>7 and plotson or above"A" - line
4<PI<7 andplots on or above"A" - line
PI<4 or plotsbelow "A" - line
PI plots on orabove "A" - line
PI plots below"A" - line
GRAVEL%gravel >
%sand
SAND%sand >%gravel
LL>50(inorganic)
LL<50(inorganic)
GW
GP
GW-GM
GW-GC
GP-GM
GP-GC
GM
GC
GC-GM
SW
SP
SW-SM
SW-SC
SP-SM
SP-SC
SM
SC
SC-SM
<15% plus No. 20015-29% plus No. 200
%sand > %gravel
%sand < %gravel
<15% plus No. 200
15-29% plus No. 200
%sand > %gravel
%sand < %gravel
<15% plus No. 200
15-29% plus No. 200
%sand > %gravel
%sand < %gravel
<15% plus No. 20015-29% plus No. 200
%sand > %gravel
%sand < %gravel
<15% plus No. 20015-29% plus No. 200
%sand > %gravel
%sand < %gravel