HomeMy WebLinkAbout19 Courtyard_Geotechnical Report_102519MONTANA | WASHINGTON | IDAHO | NORTH DAKOTA | PENNSYLVANIA
JOB NO. B19-079-001 OCTOBER 2019
REPORT OF GEOTECHNICAL INVESTIGATION
CLIENT ENGINEER
Black Ridge Companies
PO Box 11890
Bozeman, MT 59719
Craig Nadeau, PE
Craig.nadeau@tdhengineering.com
REPORT OF GEOTECHNICAL INVESTIGATION
PROJECT NAME
PROJECT LOCATION 406.761.3010
tdhengineering.com
1800 River Drive North
Great Falls, MT 59401
LEWIS & CLARK COMMERCIAL DEVELOPMENT HOTEL
BOZEMAN, MONTANA
Lewis & Clark Commercial Development Hotel Table of Contents
Bozeman, Montana i
Table of Contents
1.0 EXECUTIVE SUMMARY ......................................................................................................... 1
2.0 INTRODUCTION...................................................................................................................... 2
2.1 Purpose and Scope .......................................................................................................... 2
2.2 Project Description ........................................................................................................... 2
3.0 SITE CONDITIONS ................................................................................................................. 3
3.1 Geology and Physiography .............................................................................................. 3
3.2 Surface Conditions ........................................................................................................... 4
3.3 Subsurface Conditions ..................................................................................................... 4
3.3.1 Soils ........................................................................................................................... 4
3.3.2 Ground Water ........................................................................................................... 5
4.0 ENGINEERING ANALYSIS ..................................................................................................... 6
4.1 Introduction ....................................................................................................................... 6
4.2 Site Grading and Excavations .......................................................................................... 6
4.3 Conventional Shallow Foundations .................................................................................. 6
4.4 Soil Retaining Walls ......................................................................................................... 7
4.5 Floor Slabs and Exterior Flatwork .................................................................................... 7
4.6 Pavements ........................................................................................................................ 8
5.0 RECOMMENDATIONS.......................................................................................................... 10
5.1 Site Grading and Excavations ........................................................................................ 10
5.2 Conventional Shallow Foundations ................................................................................ 11
5.3 Soil Retaining Walls ....................................................................................................... 12
5.4 Floor Slabs and Exterior Flatwork .................................................................................. 13
5.5 Pavements ...................................................................................................................... 14
5.6 Continuing Services ....................................................................................................... 16
6.0 SUMMARY OF FIELD AND LABORATORY STUDIES ........................................................ 17
6.1 Field Explorations ........................................................................................................... 17
6.2 Laboratory Testing ......................................................................................................... 17
7.0 LIMITATIONS ........................................................................................................................ 19
Lewis & Clark Commercial Development Hotel Appendix
Bozeman, Montana ii
APPENDIX
Test Pit Location Map (Figure 1)
Logs of Exploratory Test Pits (Figures 2 through 6)
Laboratory Test Data (Figures 7 through 10)
USGS Design Maps Summary Report
LTTPBind Online PG Asphalt Binder Analysis Summary
Soil Classification and Sampling Terminology for Engineering Purposes
Classification of Soils for Engineering Purposes
Lewis & Clark Commercial Development Hotel Executive Summary
Bozeman, Montana Page 1
GEOTECHNICAL REPORT
LEWIS & CLARK COMMERCIAL DEVELOPMENT HOTEL
BOZEMAN, MONTANA
1.0 EXECUTIVE SUMMARY
The geotechnical investigation for the proposed hotel site to be located within the Lewis and Clark
Commercial Subdivision at Section 36, Township 01 South, Range 05 East, Block 2, Lot 4,
encountered surficial lean clay soils overlying water-bearing, poorly-graded gravel with clay and
sand. Based on our experience in the vicinity of this project, the seismic site class is D and the risk
of seismically-induced liquefaction or soil settlement is considered low and does not warrant
additional evaluation. The primary geotechnical concern regarding this project is the presence of
relatively soft compressible soils at and below the planned depth of foundation and slab elements
for this project. Additionally, the shallow ground water table in the area warrants consideration and
will impact construction of footings and site utilities.
Based on the subsurface conditions encountered, the site is suitable for the use of conventional
shallow foundations bearing on properly compacted structural fill extending down to the native
gravels beneath all footings. Such construction should be designed using a maximum allowable
bearing pressure of 4,000 pounds per square foot (psf) provided the recommendations included in
this report are followed. Interior slab-on-grade construction will not require the complete removal of
the clay soils; however, an increased thickness of structural fill is warranted between the clay soils
and the interior slab due to soft compressible condition and elevated moisture which will preclude
proper compaction.
Lewis & Clark Commercial Development Hotel Introduction
Bozeman, Montana Page 2
2.0 INTRODUCTION
2.1 Purpose and Scope
This report presents the results of our geotechnical study for the proposed hotel site located within
the Lewis & Clark Commercial Subdivision at Section 36, Township 01 South, Range 05 East, Block
2, Lot 4. The purpose of the geotechnical study is to determine the general surface and subsurface
conditions at the proposed site and to develop geotechnical engineering recommendations for
support of the proposed structure and design of related facilities. This report describes the field work
and laboratory analyses conducted for this project, the surface and subsurface conditions
encountered, and presents our recommendations for the proposed foundations and related site
development.
Our field work included excavating five test pits across the proposed site. Samples were obtained
from the test pits and returned to our Great Falls laboratory for testing. Laboratory testing was
performed on selected soil samples to determine engineering properties of the subsurface materials.
The information obtained during our field investigations and laboratory analyses was used to
develop recommendations for the design of the proposed foundation systems.
This study is in accordance with the proposal submitted by Mr. Ahren Hastings, PE of our firm dated
September 6, 2019. Our work was authorized to proceed by Mr. Will Ralph, Director of Development
of Black Ridge Companies by his signed acceptance of our proposal.
2.2 Project Description
It is our understanding that the proposed project consists of, in part, a multi-story, wood-framed hotel
structure approximately 20,000 square feet in plan. The structure is proposed to be supported on
conventional shallow foundations incorporating interior slab-on-grade construction. An interior
swimming pool is also planned which we understand to have a maximum depth of approximately
five feet below finished floor elevation. Structural loads had not been developed at the time of this
report. However, based on our experience with similar structures and for the purpose of our
analysis, we have assumed that wall loads will be less than 6,000 pounds per lineal foot and column
loads will be less than 150 kips.
Site development will most likely include landscaping, grade separation retaining structures, exterior
concrete flatwork, and asphalt pavement for parking lots and access roads. If the assumed design
values presented above vary from the actual project parameters, the recommendations presented in
this report should be reevaluated.
Lewis & Clark Commercial Development Hotel Site Conditions
Bozeman, Montana Page 3
3.0 SITE CONDITIONS
3.1 Geology and Physiography
The site is geologically characterized as alluvium (Qal). These deposits are generally valley fill
comprised of mixtures of gravel, sand, and silt; however, some terrace deposits and glacial drift are
present in areas, and the glacial drift is generally of clay composition. The alluvial formations are
typical of areas proximate to stream and river channels and floodplains. Areas near the site also
encounter upper tertiary sediments or sedimentary rock (Tsu). This formation includes
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, the site falls under seismic Site Class D. The
appropriate International Building Code (IBC) seismic design parameters for the site include site
coefficients of 1.225 and 1.981 for Fa and Fv, respectively. The recommended design spectral
response accelerations at short periods (SDs) and at 1-second period (SD1) are 0.587g and 0.277g,
respectively. These values represent two-thirds of the mapped response accelerations following
correction for the appropriate site classification and assume the proposed construction to fall into
risk category I, II, or III. The likelihood of seismically-induced soil liquefaction or settlement for this
project is low and does not warrant additional evaluation.
Approximate
Location
Lewis & Clark Commercial Development Hotel Site Conditions
Bozeman, Montana Page 4
3.2 Surface Conditions
The proposed project site is located at within the Lewis and Clark Commercial Subdivision on the
east side of Boot Hill Court, west of Interstate 90, and north of East Baxter Lane. The site is
presently undeveloped and vegetated with native pasture grasses. Based on background
information and site observations, the topography of the site is considered nearly level.
3.3 Subsurface Conditions
3.3.1 Soils
The subsurface soil conditions appear to be relatively consistent based on our exploratory
excavating and soil sampling. In general, the subsurface soil conditions encountered within
the test pits consist of approximately 0.9 to 1.2 feet of surficial lean clay topsoil containing
relatively high levels of organics. The topsoil is underlain by native lean clay which extends
to depths of approximately 4.0 to 7.2 feet below existing site grade. The lean clay is
underlain in all test pits by native poorly-graded gravel with clay and sand extending to
depths of at least 8.0 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.
LEAN CLAY
Lean clay soils were encountered in all five borings from the ground surface to depths of 4.0
to 7.2 feet. The uppermost 0.9 to 1.2 feet contained abundant organics and was classified
as topsoil. The lean clay is generally considered soft to firm based on the ease of
excavation and is likely compressible. A single sample of the material contained no gravel,
6.6 percent sand, and 93.4 percent fines (clay and silt). A separate sample exhibited a liquid
limit of 34 percent and a plasticity index of 12 percent. The natural moisture contents varied
from 21 to 44 percent and averaged 32 percent.
A bulk sample of the native clay was tested in accordance with ASTM D698 to establish the
moisture-density relationship of the material. This test indicates that a maximum dry density
of 96.8 pounds per cubic foot (pcf) is achievable when compacted at the optimum moisture
content of 23.5 percent.
Lewis & Clark Commercial Development Hotel Site Conditions
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POORLY-GRADED GRAVEL WITH CLAY AND SAND
The native gravels were visually classified as poorly-graded gravel with clay and sand. They
are considered relatively dense based on the effort required during excavation using a mini-
excavator. A bulk sample of the native gravels were tested using the relative density method
outlined in ASTM D4253 and D5254 to establish the moisture-density relationship for the
material. This test indicates that a maximum dry density of 143.4 pcf is achievable when
compacted at the optimum moisture content of 6.2 percent.
3.3.2 Ground Water
Ground water was encountered within all five test pits at depths ranging from 4.0 to 7.5 feet
below the ground surface at the time of the investigation. Thus, ground water is anticipated
to be encountered in foundation and utility excavations for this project and should be
considered both during design and construction. Temporary dewatering systems should be
provided by the contractor to facilitate construction. The design of site dewatering systems
was not included as part of our scope of work and must be performed by others.
The presence or absence of observed ground water may be directly related to the time of the
subsurface investigation. Numerous factors contribute to seasonal ground water
occurrences and fluctuations, and the evaluation of such factors is beyond the scope of this
report.
Lewis & Clark Commercial Development Hotel Engineering Analysis
Bozeman, Montana Page 6
4.0 ENGINEERING ANALYSIS
4.1 Introduction
The primary geotechnical concern regarding this project is the presence of somewhat variable
thicknesses of relatively soft compressible clay beneath the proposed structure. Based on the
planned size of the building and the anticipated foundation loads associated with similar multi-story
construction, settlements are a concern for foundation elements supported on this material. The
settlement concern will be exacerbated by the anticipated inability to properly compacted the native
soils due to excessive moisture present at the time of our investigation and likely during
construction. Subgrade improvements to remove and replace the unsuitable fine-grained clay will
be required beneath all foundations and increased structural fill thicknesses beneath interior slab
systems to ensure the long-term performance of the building.
Additionally, shallow ground water was encountered on site and will impact construction and
potential below grade portions of the structure, such as the planned swimming pool.
4.2 Site Grading and Excavations
The ground surface at the proposed site is considered nearly level. Based on our field work,
relatively soft, moist lean clay soils overlying water-bearing poorly-graded gravel with clay and sand
will be encountered in foundation and utility excavations to the depths anticipated. Based on the
test pits, ground water should be expected in all utility or foundation excavations for this project.
Ground water was observed as depths of four to eight feet below current site grades at the time of
our investigation; however, seasonal fluctuations in this depth should be expected and water may be
much shallower depending on the time of construction. Evaluation of the magnitude of seasonal
fluctuation were not included as part of our scope of work and can vary from year to year depending
on several factors.
4.3 Conventional Shallow Foundations
Considering the subsurface conditions encountered and the nature of the proposed construction,
the native lean clay soils are not considered suitable for the support of foundation elements and are
likely to result in oversized footing elements and possible objectional settlements. Thus, we
recommend that all footings be supported on properly compacted structural fill extending to native
gravels. Excavation and placement of structural fill should anticipate the need for dewatering
systems to lower the ground water table to facilitate construction.
Based on our experience, the theory of elasticity, and using an allowable bearing pressure of 4,000
psf, we estimate the total settlement for footings will be less than ¾-inch when constructed as
described above. Differential settlements between bearing locations should be on the order of one-
half this magnitude and will be largely controlled by the placement and compaction of the structural
Lewis & Clark Commercial Development Hotel Engineering Analysis
Bozeman, Montana Page 7
fill during construction. A one-third increase in the design bearing pressure is permitted for the
consideration of short-term, dynamic loading conditions.
The lateral resistance of spread footings is controlled by a combination of sliding resistance between
the footing and the foundation material at the base of the footing and the passive earth pressure
against the side of the footing in the direction of movement. Design parameters are given in the
recommendations section of this report.
Alternative subgrade improvement methods, such as engineering aggregate piers (EAPs), can
achieve similar design bearing pressures and foundation performance without the need to remove
and replace the native clay soils completely. These systems are proprietary and need to be
designed by the licensed installer if they are considered for use. We can refer you to experienced
EAP designer / installers if you wish to discuss this alternative further.
4.4 Soil Retaining Walls
Soil retaining structures are only anticipated for the indoor swimming pool which will lie below
finished floor elevation. Differential soil heights are not anticipated at other locations within the
development due to the intended use of conventional slab-on-grade construction and the relatively
flat site grades. Wall systems for the indoor pool 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 strength of the
soil.
Swimming pool walls are generally stiff and designed such that inward rotation of the wall system is
restrained. Thus, these walls are anticipated to experience an "at-rest" condition which creates
lateral pressures having magnitudes between the passive and active conditions. 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.
Due to the elevated moistures within the clay, these materials will be difficult to properly compact
and will impart greater lateral forces on the pool walls. For these reasons, we advise that native clay
not be used as backfill adjacent to the swimming pool area. Rather, native gravels (if available) or
properly compacted structural fill should be utilized in this application.
4.5 Floor Slabs and Exterior Flatwork
The natural on-site soils, exclusive of topsoil, are suitable to support lightly to moderately loaded,
exterior slab-on-grade construction provided some risk of differential movements and settlement
area acceptable. At a minimum, a leveling course of granular fill should be placed beneath the
concrete flatwork to provide a structural cushion, a capillary-break from the subgrade, and a
drainage medium. Similar conventional construction typically utilizes six inches of compacted
granular fill beneath exterior concrete; however, the requirements may vary locally.
Lewis & Clark Commercial Development Hotel Engineering Analysis
Bozeman, Montana Page 8
The natural on-site soils are not considered suitable to support interior floor slabs due to the very
soft compressible condition of the clay and the potential for settlement. The incorporation of
structural fill beneath the slab is advised to improve slab support and control potential settlements.
This structural fill will also account for the anticipated inability to compact the clay due to excessive
moisture within the material. A structural fill depth of at least 24 inches of is recommended beneath
interior slab systems and should be separated from the underlying native clay soils by a woven
geotextile fabric. This layer will help to distributing the anticipated interior slab loads to the subgrade
and control differential displacements.
4.6 Pavements
A pavement section is a layered system designed to distribute concentrated traffic loads to the
subgrade. Performance of the pavement structure is directly related to the physical properties of the
subgrade soils and the magnitude and frequency of traffic loadings. Pavement design procedures
are based on strength properties of the subgrade and pavement materials, along with the design
traffic conditions. Traffic information was not available at the time of this report. We have assumed
that traffic for the access roads and parking lots will primarily consist of passenger-type cars with
occasional truck traffic associated with deliveries, trash collection, and commercial users of the hotel
facility. We have estimated a 20-year design equivalent single axle load (ESAL) of up to 150,000
based on the size of the facility and the anticipated traffic conditions. The recommended pavement
section provided within this report has not considered potential construction traffic associated with
this project. At this time, there are too many unknowns to reasonably estimate construction traffic
volumes and vehicles sizes which may utilize finished roadways areas; thus, additional evaluation of
section improvements in any areas to be utilized for construction traffic are warranted once a
contractor is selected based on their preferred access plan.
The worst-case subgrade material is the native lean clay which is classified as an A-6 soil in
accordance with the American Association of State Highway and Transportation Officials (AASHTO)
classification. AASHTO considers this soil type to be a relatively poor subgrade due to its reduced
permeability and reduced strength when wetted. Typical California Bearing Ratio (CBR) values for
this type of soil range from 4 to 8 percent when the subgrade can be properly moisture conditioned
and compacted as part of construction. However, the on-site soils exhibit an average moisture
content which is approximately ten percent higher than the optimum value for compaction. Thus,
moisture within the clay is anticipated to preclude proper compaction of the subgrade which will
result in a reduced CBR value.
A geotextile fabric is recommended between the pavement section gravels and the relatively soft
clay subgrade to provide separation and assist in providing some reinforcement. Due to the
relatively shallow ground water, this geotextile should also provide the ability for water migration
through the fabric in the event of ground water fluctuations. The geotextile will prevent the upward
migration of fines and the loss of aggregate into the subgrade, thereby prolonging the structural
integrity and performance of the pavement section.
Lewis & Clark Commercial Development Hotel Engineering Analysis
Bozeman, Montana Page 9
The pavement section presented in this report is based on an assumed CBR value of 1.5 percent to
account for the soft non-compactible subgrade, assumed traffic loadings, recommended pavement
section design information presented in the Asphalt Institute and AASHTO Design Manuals, and our
past pavement design experience in Bozeman, Montana.
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 10
5.0 RECOMMENDATIONS
5.1 Site Grading and Excavations
1. All topsoil and organic material should be removed from the proposed building and
pavement areas and any areas to receive site grading fill. For planning purposes, a
stripping thickness of 12 to 14 inches is anticipated; however, thicker stripping
depths may be warranted to remove all detrimental.
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 backfill and general site grading fill on this project but
should be expected to require moisture conditioning. The native soils exhibit in-situ
moisture contents which are approximately 10 percent higher than the optimum
compaction level; thus, this material will require significant moisture conditioning
prior to use to achieve the required compaction levels.
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).
a) Structural Fill Below Foundations ................................................ 98%
b) Structural Fill Below Interior Slabs & Interior Backfill .................. 98%
c) Exterior Foundation Wall Backfill ................................................. 95%
d) Below Paved Areas and Exterior Concrete Flatwork .................. 95%
e) General Landscaping or Nonstructural Areas ............................. 92%
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 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
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 11
methods (outlined in ASTM D698) shall not be used for any materials containing less
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 and assumed
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. For planning purposes, subsoils
encountered in the test pits are considered Type B for the native lean clay and Type
C for the native gravels. 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 a spread footing foundation
system. The construction details should be considered when preparing the project documents.
6. Both interior and exterior footings should bear on properly compacted structural fill
(Item 3) extending to native gravels and should be designed for a maximum
allowable soil bearing pressure of 4,000 psf with an allowable one-third increase for
the consideration of short-term dynamic loading conditions. Based on the
preliminary test pits and an assumed footing depth of approximately four feet,
structural fill thickness of up to three feet are anticipated for this project. The limits of
over-excavation and replacement with compacted structural fill should extend at
least 24 inches beyond the outer face of the footing elements in all directions.
Dewatering of the site should be anticipated to facilitate the placement and
compaction of the structural fill as well as foundation construction. Water levels
were measured at depths ranging from 4 to 8 feet below existing site grades and will
vary seasonally.
7. Soils disturbed below the planned depths of footing excavations should either
replaced with properly compacted structural fill (Item 3).
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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.
10. 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
150 psf per foot of depth are appropriate for footings bearing on properly compacted
structural fill and backfilled with moisture conditioned and recompacted native lean
clay. If native gravels or structural fill are to be utilized as backfill within five feet of
foundation elements, an increased lateral resistance pressure of 300 psf per foot of
depth may be utilized.
11. In accordance with Section 1704.7 of the International Building Code (IBC), a
representative of the project geotechnical engineer should be retained to provide soil
special inspections during construction. Special inspections should include
subgrade verification (including the removal of lean clay) and material placement /
compaction at a minimum.
5.3 Soil Retaining Walls
The design and construction criteria presented below should be observed for soil retaining walls
associated with the below grade pool and any other site grading features. The construction details
should be considered when preparing the project documents.
12. Below grade swimming pool walls and other retaining 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 as summarized below based on the intended backfill type.
Backfill Material At-Rest Equivalent
Fluid Pressure (pcf)
Native Lean Clay * 90
Structural Fill or
Native Gravel 60
* Native lean clay will require significant moisture conditioning prior to use
as backfill to achieve the recommended compaction level.
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Bozeman, Montana Page 13
13. Site graining retaining structures, if required, which can deflect sufficiently to mobilize
the full active earth pressure condition, approximately three percent of the exposed
wall height, may be designed for an active lateral earth pressure as summarized
below based on the intended backfill type.
Backfill Material Active Equivalent
Fluid Pressure (pcf)
Native Lean Clay * 70
Structural Fill or
Native Gravel 40
* Native lean clay will require significant moisture conditioning prior to use
as backfill to achieve the recommended compaction level.
14. Backfill should be selected, placed, and compacted per Items 2 and 3 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 and retaining walls.
15. Retaining walls over 4 feet in height should incorporate backfill drainage systems
and/or weep holes to prevent the accumulation of hydrostatic pore pressures.
5.4 Floor Slabs and Exterior Flatwork
16. For normally loaded, exterior concrete flatwork, a typical cushion course consisting of
free-draining, crushed gravel should be placed beneath the concrete and compacted
to the requirements of Item 2 above. A cushion course thickness of six inches is
typically utilized but requirements may vary locally. Conventional construction, as
has been described, is not intended to mitigate expansion or settlement concerns
associated with the subsurface conditions encountered. If no risk can be accepted
for exterior flatwork, additional improvement may be warranted as discussed in Item
17 below.
17. For normally loaded, interior slab-on-grade construction, a minimum thickness of 24-
inches of compacted structural fill (Item 3) should be placed beneath the slab and
compacted to the requirements of Item 2 above. The structural fill should be
separated from the native lean clay soils using a woven geotextile consisting of a
Mirafi RS380i. Prior to geotextile installation, the subgrade should be cleared of all
loose debris and smoothed to ensure a level and taught installation.
18. Cushion course materials utilized beneath slab-on-grade applications should
conform to the requirements outlined in Section 02235 of the Montana Public Works
Standard Specifications (MPWSS). All gradations outlined in this standard are
considered acceptable for this application based on local availability and contractor
preference.
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 14
19. Interior concrete floor slabs should be designed using a modulus of vertical
subgrade reaction no greater than 200 pci when designed and constructed as
recommended above.
20. Geotechnically, an underslab vapor barrier is recommended beneath all interior slab
systems which will receive moisture sensitive flooring systems. Relatively shallow
ground water was observed, and fluctuations of the ground water level are likely. A
minimum 15-mil vapor barrier is recommended unless otherwise specified by the
Architect or Structural Engineer for the project.
21. If no acceptable risk of slab movement can be assumed by the Owner, the only
positive method to control potential slab movements is to completely remove and
replace the native lean clay soils with compacted structural fill. This will require up to
seven feet of structural fill at some locations but would address potential
compressibility concerns associated with the native clay.
5.5 Pavements
22. The following pavement section or an approved equivalent section should be
selected in accordance with the discussions in the Engineering Analysis. The
subgrade is expected to exhibit elevated moisture contents making them weak, non-
compactible, and problematic for construction. The soft subgrade conditions have
been considered in the section below; however, this section has not considered any
construction traffic which may utilize final pavement areas.
Pavement Component Component Thickness
Asphaltic Concrete Pavement 3”
Crushed Base Course 6”
Crushed Subbase Course 12”
Total 21”
Due to the unknowns regarding the size, volume, and location of construction traffic
at this time, the use of construction traffic has not been considered in the
recommended section above. Construction traffic utilized with the provided section
may result in undesirable wear and distress to the pavement or base course
surfaces and is not recommended. If construction traffic will utilize any final
pavement areas, additional improvements are warranted to account for the additional
vehicle loads during the construction period and should be evaluated prior to
construction to avoid unnecessary delays and expense associated with
reconstruction of an inadequate section.
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Bozeman, Montana Page 15
23. Gradations for the crushed base courses shall conform to Section 02235 of the
Montana Public Works Standard Specifications (MPWSS). The gradation for the
subbase shall conform to Section 02234 of the MPWSS. All gradations outlined in
these specifications are acceptable for this application based on the local availability
and contractor preference.
24. Where the existing grades will be raised more than the thickness of the pavement
section, all fill should be placed, compacted and meet the general requirements
given in Item 2 above.
25. A geotextile is recommended between the pavement section and the prepared
subgrade to prevent the migration of fines upward into the gravel and the loss of
aggregate into the subgrade. A Mirafi RS380i is recommended with the section
provided above. This product will provide superior reinforcement to the soft, non-
compactible subgrade while maintaining high permittivity to accommodate ground
water fluctuations.
As an alternative, the combination of a non-woven geotextile and a high-strength
biaxial geogrid can be considered in lieu of the woven fabric outlined above. If this
option is considered, we recommend a Mirafi 140N be placed directly over the
subgrade with a BX1500 Biaxial geogrid over top.
26. The asphaltic cement should be a Performance Graded (PG) binder having the
following minimum high and low temperature values based on the desired pavement
reliability.
Reliability Min. High
Temp Rating
Min. Low
Temp Rating Ideal Oil Grade
50% 33.8 -30.6 PG 52-34
98% 37.5 -39.4 PG 52-40
In our experience, neither of the ideal oil grades based on the local climate
conditions are available in the Bozeman area. The use of these products is likely to
result in unnecessary additional project expense associated with the import and
design for a custom oil product. Thus, we recommend that asphalt mixes for this
project utilize a PG 58-28 grade oil. This is the most commonly available product
locally which provides the highest level of cold weather performance. It should be
sufficient to achieve reliability level of just below 50 percent which is typically
acceptable for most parking lot and low volume roadway applications.
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 16
5.6 Continuing Services
Three additional elements of geotechnical engineering service are important to the successful
completion of this project.
27. 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.
28. 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.
29. 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
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 Interior Slabs 1 Test per 1,000 SF per Lift
Foundation Backfill 1 Test per 100 LF of Wall per Lift
Parking Lots & Exterior Flatwork 1 Test per 2,500 SF per Lift
LF = Lineal Feet SF = Square Feet
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 17
6.0 SUMMARY OF FIELD AND LABORATORY STUDIES
6.1 Field Explorations
The field exploration program was conducted on October 1, 2019. A total of five test pits were
excavated to a depth of approximately eight feet each at the approximate locations shown on Figure
1 to observe subsurface soil and ground water conditions. The tests pits were excavated by Earth
Surgeons using a Komatsu PC88MR mini-extractor. The subsurface exploration and sampling
methods used are indicated on the attached test pit logs. The test pits were logged by Mr.
Christopher Shaw of TD&H Engineering. The location of the test pits as shown on Figure 1 are
approximate based on the relative distance from existing site features.
Composite grab samples of the subsurface materials were taken at various depths and at changes
in the subsurface stratigraphy. Logs of all test pits, which include soil descriptions and sample
depths, are presented on the Figures 2 through 6.
Measurements to determine the depth of ground water in the test pits were made using a steel tape
measure shortly after the completion of excavating. The depths or elevations of the water levels
measured, if encountered, and the date of measurement are shown on the test pit logs.
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.
Moisture-Density Relationship A relationship describing the effect of varying moisture
content and the resulting dry unit weight at a given
compactive effort. Provides the optimum moisture content
and the maximum dry unit weight. Also called a Proctor Curve
or relative density curve.
Lewis & Clark Commercial Development Hotel Summary of Field & Laboratory Studies
Bozeman, Montana Page 18
The laboratory testing program for this project consisted of ten moisture-visual analyses, one sieve
(grain-size distribution) analysis, and one Atterberg Limits analysis. The results of the water content
analyses are presented on the test pit logs, Figures 2 through 6. The grain-size distribution curve
and Atterberg limits are presented on Figures 7 and 8. In addition, one proctor (moisture-density)
test and one relative density test were performed. These results are shown on Figures 9 and 10.
Lewis & Clark Commercial Development Hotel Limitations
Bozeman, Montana Page 19
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. We strongly advise that TD&H
be retained 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. 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.
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
Lewis & Clark Commercial Development Hotel Limitations
Bozeman, Montana Page 20
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:
Craig Nadeau PE Ahren Hastings PE
Geotechnical Manager Prjoect Manager
TD&H ENGINEERING TD&H ENGINEERING
LEWIS AND CLARK COMMERCIAL DEVELOPMENT HOTEL BOZEMAN, MONTANA TEST PIT LOCATION MAP FIGURE 1
PR
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.
0
1.5
3
4.5
6
7.5
9
10.5
TOPSOIL: Lean CLAY, appears soft, dark brown, slightly
moist
Lean CLAY, appears soft to firm, light brown, moist
Poorly-Graded GRAVEL with Clay and Sand, relatively
dense, brown, moist to wet
Bottom of Test Pit
0.9
4.0
8.0
G
G
LEGEND LOG OF TEST PIT TP-1Atterberg Limits
Field Moisture content Lewis & Clark Commercial Subdivision Hotel
Bozeman, MontanaGroundwater Level
Grab/composite sample
Logged by:Christopher Shaw
Excavated by:Earth Surgeons
Komatsu PC 88MRGNP = Granular and Nonplastic
Note: The stratification lines represent approximate
boundaries between soil types. Actual boundaries
may be gradual or transitional.
October 1, 2019 B19-079-001
Figure No. 2
Sheet
GR
A
P
H
I
C
LO
G
SOIL DESCRIPTION
SURFACE:Native Pasture Grass
SURFACE ELEVATION:Not Measured
DE
P
T
H
(
F
T
)
GR
O
U
N
D
WA
T
E
R
SA
M
P
L
E
DE
P
T
H
(
F
T
)
MOISTURE CONTENT
0 10 20 30 40 50
= MOISTURE CONTENT
1 of 1
0
1.5
3
4.5
6
7.5
9
10.5
TOPSOIL: Lean CLAY, appears soft, dark brown, slightly
moist
Lean CLAY, appears soft to firm, light brown, moist
Poorly-Graded GRAVEL with Clay and Sand, relatively
dense, brown, moist to wet
Bottom of Test Pit
1.0
4.0
8.0
G
G
LEGEND LOG OF TEST PIT TP-2Atterberg Limits
Field Moisture content Lewis & Clark Commercial Subdivision Hotel
Bozeman, MontanaGroundwater Level
Grab/composite sample
Logged by:Christopher Shaw
Excavated by:Earth Surgeons
Komatsu PC 88MRGNP = Granular and Nonplastic
Note: The stratification lines represent approximate
boundaries between soil types. Actual boundaries
may be gradual or transitional.
October 1, 2019 B19-079-001
Figure No. 3
Sheet
GR
A
P
H
I
C
LO
G
SOIL DESCRIPTION
SURFACE:Native Pasture Grass
SURFACE ELEVATION:Not Measured
DE
P
T
H
(
F
T
)
GR
O
U
N
D
WA
T
E
R
SA
M
P
L
E
DE
P
T
H
(
F
T
)
MOISTURE CONTENT
0 10 20 30 40 50
= MOISTURE CONTENT
1 of 1
0
1.5
3
4.5
6
7.5
9
10.5
TOPSOIL: Lean CLAY, appears soft, dark brown, slightly
moist
Lean CLAY, appears soft to firm, light brown, moist
Poorly-Graded GRAVEL with Clay and Sand, relatively
dense, brown, moist to wet
Bottom of Test Pit
1.2
7.2
8.0
G
G
LEGEND LOG OF TEST PIT TP-3Atterberg Limits
Field Moisture content Lewis & Clark Commercial Subdivision Hotel
Bozeman, MontanaGroundwater Level
Grab/composite sample
Logged by:Christopher Shaw
Excavated by:Earth Surgeons
Komatsu PC 88MRGNP = Granular and Nonplastic
Note: The stratification lines represent approximate
boundaries between soil types. Actual boundaries
may be gradual or transitional.
October 1, 2019 B19-079-001
Figure No. 4
Sheet
GR
A
P
H
I
C
LO
G
SOIL DESCRIPTION
SURFACE:Native Pasture Grass
SURFACE ELEVATION:Not Measured
DE
P
T
H
(
F
T
)
GR
O
U
N
D
WA
T
E
R
SA
M
P
L
E
DE
P
T
H
(
F
T
)
MOISTURE CONTENT
0 10 20 30 40 50
= MOISTURE CONTENT
1 of 1
0
1.5
3
4.5
6
7.5
9
10.5
TOPSOIL: Lean CLAY, appears soft, dark brown, slightly
moist
Lean CLAY, appears soft to firm, light brown, moist
Poorly-Graded GRAVEL with Clay and Sand, relatively
dense, brown, moist to wet
Bottom of Test Pit
1.2
7.0
8.0
G
G
LEGEND LOG OF TEST PIT TP-4Atterberg Limits
Field Moisture content Lewis & Clark Commercial Subdivision Hotel
Bozeman, MontanaGroundwater Level
Grab/composite sample
Logged by:Christopher Shaw
Excavated by:Earth Surgeons
Komatsu PC 88MRGNP = Granular and Nonplastic
Note: The stratification lines represent approximate
boundaries between soil types. Actual boundaries
may be gradual or transitional.
October 1, 2019 B19-079-001
Figure No. 5
Sheet
GR
A
P
H
I
C
LO
G
SOIL DESCRIPTION
SURFACE:Native Pasture Grass
SURFACE ELEVATION:Not Measured
DE
P
T
H
(
F
T
)
GR
O
U
N
D
WA
T
E
R
SA
M
P
L
E
DE
P
T
H
(
F
T
)
MOISTURE CONTENT
0 10 20 30 40 50
= MOISTURE CONTENT
1 of 1
0
1.5
3
4.5
6
7.5
9
10.5
TOPSOIL: Lean CLAY, appears soft, dark brown, slightly
moist
Lean CLAY, appears soft to firm, light brown, moist
Poorly-Graded GRAVEL with Clay and Sand, relatively
dense, brown, moist to wet
Bottom of Test Pit
1.0
7.0
8.0
G
G
LEGEND LOG OF TEST PIT TP-5Atterberg Limits
Field Moisture content Lewis & Clark Commercial Subdivision Hotel
Bozeman, MontanaGroundwater Level
Grab/composite sample
Logged by:Christopher Shaw
Excavated by:Earth Surgeons
Komatsu PC 88MRGNP = Granular and Nonplastic
Note: The stratification lines represent approximate
boundaries between soil types. Actual boundaries
may be gradual or transitional.
October 1, 2019 B19-079-001
Figure No. 6
Sheet
GR
A
P
H
I
C
LO
G
SOIL DESCRIPTION
SURFACE:Native Pasture Grass
SURFACE ELEVATION:Not Measured
DE
P
T
H
(
F
T
)
GR
O
U
N
D
WA
T
E
R
SA
M
P
L
E
DE
P
T
H
(
F
T
)
MOISTURE CONTENT
0 10 20 30 40 50
= MOISTURE CONTENT
1 of 1
Tested By: TF Checked By:
10-9-2019
7
(no specification provided)
PL= LL= PI=
D90= D85= D60=
D50= D30= D15=
D10= Cu= Cc=
USCS= AASHTO=
*
Lean CLAY
#4
#10
#20
#40
#60
#80
#100
#200
100.0
99.3
98.5
97.8
97.2
96.8
96.4
93.4
CL
Report No. A-20343-206
Black Ridge Companies
Lewis & Clark Commercial Subdivision Hotel
Bozeman, Montana
B19-079-001
Material Description
Atterberg Limits
Coefficients
Classification
Remarks
Location: TP-1
Sample Number: A-20343 Depth: 2.0 ft Date:
Client:
Project:
Project No: Figure
SIEVE PERCENT SPEC. *PASS?
SIZE FINER PERCENT (X=NO)
PE
R
C
E
N
T
F
I
N
E
R
0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0001 0.001 0.01 0.1 110 100
% +3" Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 0.0 0.0 0.7 1.5 4.4 93.4
6
i
n
.
3
i
n
.
2
i
n
.
1½
i
n
.
1
i
n
.
¾ i
n
.
½ i
n
.
3/
8
i
n
.
#4 #1
0
#2
0
#3
0
#4
0
#6
0
#1
0
0
#1
4
0
#2
0
0
Particle Size Distribution Report
Tested By: MS Checked By:
LIQUID AND PLASTIC LIMITS TEST REPORT
PL
A
S
T
I
C
I
T
Y
I
N
D
E
X
0
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
WA
T
E
R
C
O
N
T
E
N
T
32.4
32.8
33.2
33.6
34
34.4
34.8
35.2
35.6
36
36.4
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-20348 Depth: 4.0 ft
Figure
Lean CLAY 34 22 12 CL
B19-079- Black Ridge Companies
8
Report No. A-20348-207
Date: 10-11-2019 Lewis & Clark Commercial Subdivision Hotel
Bozeman, Montana
Tested By: TF Checked By:
Moisture-Density Test Report
Dr
y
d
e
n
s
i
t
y
,
p
c
f
89
91
93
95
97
99
Water content, %
16 18.5 21 23.5 26 28.5 31
23.5%, 96.8 pcf
ZAV for
Sp.G. =
2.65
Test specification: ASTM D 698-12 Method A Standard
Clay
BULK CL 2.65 0.0
Lean CLAY
B19-079- Black Ridge Companies
Report No. A-20354-204
Date: 10-9-2019
Elev/ Classification Nat. Sp.G. LL PI % > % <
Depth USCS AASHTO Moist. #4 No.200
TEST RESULTS MATERIAL DESCRIPTION
Project No. Client: Remarks:
Project:
Location: TP-2 Sample Number: A-20354
Figure
Maximum dry density = 96.8 pcf
Optimum moisture = 23.5 %
Lewis & Clark Commercial Subdivision Hotel
Bozeman, Montana
Technician:
Test Procedure
2.70
0.0
-----
Figure 10 Peter Klevberg, P.E.
Laboratory Manager
Relative Density, (ASTM D-4253, ASTM D-4254)
% Retained on 3"
Project: Lewis & Clark Commercial Subdivision Hotel
Date Sample Received: 10/7/2019
Attn:
Address:
Sample Source: TP-2
REPORT OF RELATIVE DENSITY
1800 River Drive North
Great Falls, Montana 59401
Mr. Will Ralph
Report Date: 10/9/2019
Telephone: (406) 761-3010 Fax: (406) 727-2872
Bozeman, MT 59719 Sample Number: A-20353
PO Box 11890 Project Number: B19-079-001
Client: Black Ridge Companies Report Number: A-20353-209
Thomas, Dean & Hoskins, Inc.
MS / CRN
Pessimum Moisture = 2.9 % Passing No. 200
Poorly-Graded GRAVEL with Clay and Sand
6.2
Specific Gravity
Unified Classification
Optimum Moisture =
Minimum Dry Density =
143.4
114.2
Maximum Dry Density =
110.0
115.0
120.0
125.0
130.0
135.0
140.0
145.0
150.0
0.0 2.0 4.0 6.0 8.0 10.0
Dr
y
D
e
n
s
i
t
y
(
p
c
f
)
Water Content (%)
110
120
130
140
150
0 10 20 30 40 50 60 70 80 90 100
lb
s
.
/
c
u
.
f
t
.
Percent Relative Density
ZAV Curve