B08_140P1424R0004_Attachment_3_Fort_Smith_Admin_Building_Geotechnical_Report_Final.pdf

Report Cover Page

Rehabilitation of the Fort Smith

Administration Building (BICA)–

PMIS #312083

Geotechnical Engineering Report

November 30, 2022 | Terracon Project No. 26225043

Prepared for:

DJ&A, P.C.

2000 Maple Street

Missoula, Montana 59808

1392 13th Ave SW

Great Falls, MT 59404

P (406) 453-5400

Terracon.com

Facilities | Environmental | Geotechnical | Materials

Report Cover Letter to Sign

November 30, 2022

DJ&A, P.C.

2000 Maple Street

Missoula, Montana 59808

Attn: Mr. Perry Palmer, Senior Project Manager / Group Lead

P: (720) 768-6385

E: Perry@djanda.com

Re: Geotechnical Engineering Report

Rehabilitation of the Fort Smith Administration Building (BICA)–PMIS #312083

NPS – Bighorn Canyon National Recreation Area (BICA)

Contract No. 140P1220D0002 / Task Order No. 140P1420F0127

Fort Smith, MT

Terracon Project No. 26225043

Dear Mr. Palmer:

We have completed the scope of Geotechnical Engineering services for the above referenced project in general accordance with the Scope of Services (SOS) for Task

Order No. 140P1420F0127 and our signed Professional Services Agreement dated August

2, 2022. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of foundations and floor slabs for the proposed project.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report or if we may be of further service, please contact us.

Sincerely, Terracon

Matthew D. Hoffmann, P.E. Travis Goracke, P.E.

Senior Associate / Office Manger Senior Engineer

Rehabilitation of the Fort Smith Administration Building (BICA)–PMIS #312083 | Fort Smith, MT

Facilities | Environmental | Geotechnical | Materials i

Table of Contents Introduction

Project Description

Site Conditions

Geotechnical Characterization

Seismic Site Class

Geotechnical Overview

Earthwork

Demolition

Site Preparation

Subgrade Preparation

Existing Fill

Excavation

Fill Material Types

Fill Placement and Compaction Requirements

Utility Trench Backfill

Grading and Drainage

Earthwork Construction Considerations

Construction Observation and Testing

Shallow Foundations

Design Parameters – Compressive Loads

Construction Adjacent to Existing Building

Foundation Construction Considerations

Floor Slabs

Floor Slab Design Parameters

Floor Slab Construction Considerations

Lateral Earth Pressures

Design Parameters

Subsurface Drainage for Below-Grade Walls

Frost Considerations

General Comments

Figures GeoModel

Attachments

Exploration and Testing Procedures Photography Log

Site Location and Exploration Plans

Facilities | Environmental | Geotechnical | Materials ii

Exploration and Laboratory Results

Supporting Information

Note: This report was originally delivered in a web-based format. Blue Bold text in the report indicates a referenced section heading. The PDF version also includes hyperlinks which direct the reader to that section and clicking on the logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.

Refer to each individual Attachment for a listing of contents.

http://client.terracon.com/

Facilities | Environmental | Geotechnical | Materials 1

Introduction

This report presents the results of our subsurface exploration and Geotechnical

Engineering services performed for the proposed Rehabilitation of Fort Smith

Administration Building located at NPS – Bighorn Canyon National Recreation Area

(BICA) in Fort Smith, MT. The purpose of these services was to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil conditions

■ Groundwater conditions

■ Seismic site classification per IBC

■ Site preparation and earthwork

■ Foundation design and construction

■ Floor slab design and construction

■ Lateral earth pressure

■ Frost considerations

The geotechnical engineering Scope of Services for this project included the advancement of four test borings extending to depths of approximately 13.0 to 16.5 feet below existing grade, laboratory testing, engineering analysis, and preparation of this report.

Drawings showing the site and boring locations are shown on the Site Location and

Exploration Plan, respectively. The results of the laboratory testing performed on soil samples obtained from the site during our field exploration are included on the boring logs and as separate graphs in the Exploration Results section.

Project Description

■ Our initial understanding of the project was provided based on our review and assumption inclusions to the Scope of Services (SOS) document and was discussed during project planning

Facilities | Environmental | Geotechnical | Materials 2

Item Description

Information

Provided

We were provided several documents from Mr. Palmer via email on June 8, 2022 that were related to previous work through SD

Design phase at the facility. The documents included:

■ BICA Ft. Smith Draft Final Basis for Design Report dated

09.29.2021

■ BICA Ft. Smith Final SD Plans dated 09.30.2021

■ RFP Letter dated 05.24.2022

■ SOS BICA Ft. Smith Admin. Rehab DDCD Package dated

04.27.2022

Project

Description

This project renovates the NPS Administration Building in Fort

Smith, Montana to address component renewal and cyclic maintenance issues, bring the building into compliance with accessibility code and federal energy efficiency regulations, and create a modern workplace that can be shared by NPS and the

U.S. Bureau of Reclamation (BOR or USBR) staff. Built in 1961 to house support offices during construction of the Yellowtail

Dam, this 8,100 square foot two-story building currently serves as offices for both BOR administration of the Yellowtail Dam and

NPS administrative, maintenance, and law enforcement personnel assigned to the north district of Bighorn Canyon

National Recreation Area, an NPS unit covering over 120,000 acres in two states

Proposed

Structure(s)

Planned rehabilitation includes the construction of two arctic entryways located on the north and south sides of the building, along with a mechanical yard enclosure on the north side of the building.

Building

Construction

Wood-framed construction with frost-depth, reinforced concrete foundations for arctic entryways. It is anticipated that new foundation elements will be embedded to the same bearing elevation as existing elements from the 1961 construction of the split-level structure, at approximately 6 feet below existing grade.

Finished Floor

Elevation

To match existing finished floor elevation, with access to main floor at Elevation 3292.1 feet per provided drawings.

Maximum Loads

Not provided at the time of development of this report. We have assumed the following loading conditions for our analysis.

■ Columns: 25 kips maximum

■ Continuous: 4 to 6 kips per lineal foot (klf) maximum

■ Slabs: 150 pounds per square foot (psf)

Facilities | Environmental | Geotechnical | Materials 3

Grading/Slopes Site grading is anticipated to match existing grade; therefore, no major grading changes planned.

Below-Grade

Structures

Split-level construction includes 6 foot (daylight) basement within the existing structure. New construction will not include occupiable below-grade space.

Free-Standing

Retaining Walls None planned.

Pavements Not included in this SOS

Terracon should be notified if any of the above information is inconsistent with the planned construction, especially the grading limits, as modifications to our recommendations may be necessary.

Site Conditions

The following description of site conditions is derived from our site visit in association with the field exploration and our review of publicly available geologic and topographic maps.

Item Description

Parcel

Information

The project is located at 5 Avenue B within the NPS – Bighorn

Canyon National Recreation Area (BICA) in Fort Smith, MT.

See Site Location

Existing

Improvements

The site is occupied by the existing Fort Smith Administration

Building, which was constructed in 1961.

Current Ground

Cover

Asphalt pavement north and south of the building and grassed area cover the additions footprint.

Existing

Topography

Relatively flat, with a slight grade from east to west across the property, with elevations between 3291 to 3288 based on the

Site Plan from the Final Schematic Design set.

Geotechnical Characterization

We have developed a general characterization of the subsurface conditions based upon our review of the subsurface exploration, laboratory data, geologic setting and our understanding of the project. This characterization, termed GeoModel, forms the basis of our geotechnical calculations and evaluation of the site. Conditions observed at each exploration point are indicated on the individual logs. The individual logs can be found in

Facilities | Environmental | Geotechnical | Materials 4 the Exploration Results and the GeoModel can be found in the Figures attachment of this report.

As part of our analyses, we identified the following model layers within the subsurface profile. For a more detailed view of the model layer depths at each boring location, refer to the GeoModel.

Model

Layer Layer Name General Description

1 FILL

Lean Clay with Sand (CL) in landscaping areas

Clayey Gravel with Sand (GC) in south entry way

2 SAND

Poorly Graded Sand with Silt and Gravel (SP-SM), brown, moist, dense to very dense

3 GRAVEL

Well Graded Gravel with Silt and Sand (GW-GM), brown to light brown, moist, subangular to subround gravels, fine to coarse sand

The borings were advanced in the dry using hollow-stem auger technique that allow short term groundwater observations to be made while drilling. Groundwater seepage was not encountered within the maximum drilling depth at the time of our field exploration. Groundwater conditions may be different at the time of construction.

Groundwater conditions may change because of seasonal variations in rainfall, runoff, and other conditions not apparent at the time of drilling. Long-term groundwater monitoring was outside the scope of services for this project.

Seismic Site Class

The seismic design requirements for buildings and other structures are based on Seismic

Design Category. Site Classification is required to determine the Seismic Design

Category for a structure. The Site Classification is based on the upper 100 feet of the site profile defined by a weighted average value of either shear wave velocity, standard penetration resistance, or undrained shear strength in accordance with Section 20.4 of

ASCE 7 and the International Building Code (IBC). Based on the soil properties observed at the site and as described on the exploration logs and results, our professional opinion is that a Seismic Site Classification of C be considered for the project. Subsurface explorations at this site were extended to a maximum depth of 16.5 feet. The site properties below the boring depth to 100 feet were estimated based on our experience and knowledge of geologic conditions of the general area. Additional deeper borings or geophysical testing may be performed to confirm the conditions below the current boring depth.

Facilities | Environmental | Geotechnical | Materials 5

Geotechnical Overview

The site appears suitable for the proposed construction based upon geotechnical conditions encountered in the test borings, provided that the recommendations contained in this report are implemented in the design and construction phases of this project.

The subsurface materials generally consisted of dense to very dense sand and gravel with varying amounts of silt extending to the maximum depth of the borings.

Groundwater was not encountered within the maximum depths of exploration during or at the completion of drilling.

Based on the conditions encountered and estimated load-settlement relationships, the proposed additions to the Fort Smith Administration Building can be supported on conventional continuous or spread footings.

The near surface, very stiff lean clay could become unstable with typical earthwork and construction traffic, especially after precipitation events. The establishment of effective drainage should be completed early in the construction sequence and maintained after construction to avoid potential issues. If possible, the grading should be performed during the warmer and drier times of the year. If grading is performed during the winter months, an increased risk for possible undercutting and replacement of unstable subgrade will persist. Additional site preparation recommendations, including subgrade improvement and fill placement, are provided in the Earthwork section.

The Shallow Foundations section addresses support of the building additions directly bearing on native dense to very dense sand or gravel. The Floor Slabs section addresses slab-on-grade support of the building using overexcavation techniques.

The recommendations contained in this report are based upon the results of field and laboratory testing (presented in the Exploration Results), engineering analyses, and our current understanding of the proposed project. The General Comments section provides an understanding of the report limitations.

Earthwork

Earthwork is anticipated to include demolition of adjacent sidewalks, clearing and grubbing of isolated landscape areas, excavations, and engineered fill placement. The following sections provide recommendations for use in the preparation of specifications for the work. Recommendations include critical quality criteria, as necessary, to render the site in the state considered in our geotechnical engineering evaluation for foundations and floor slabs.

Facilities | Environmental | Geotechnical | Materials 6

Demolition

The proposed building additions will be constructed in areas of current sidewalk/flatwork on the north and south sides of the administrative building which will need to be demolished. We recommend existing exterior slabs and utilities be removed from within the proposed building additions footprints and at least 5 feet beyond the outer edge of new foundations.

Site Preparation

Prior to placing fill, existing vegetation, topsoil, and root mats should be removed.

Complete stripping of the topsoil should be performed in the proposed building additions.

Mature trees are located near the footprint of some of the proposed buildings south arctic entryway, which may require removal at the onset of construction. Tree root systems can remove substantial moisture from surrounding soils. Where trees are removed, the full root ball and all associated dry and desiccated soils should be removed. The soil materials which contain less than 5 percent organics can be reused as engineered fill provided the material is moisture conditioned and properly compacted.

Site grade raising fill (lean clay) was encountered on the north side of the building. At the planned south arctic entryway, a deeper zone of clayey gravel with sand fill was encountered, which is likely associated with the construction of the split-level basement backfill zone during the 1961 construction period. The removal of all fill, to an approximate depth of 6.5 feet below existing grade, at the south entryway is recommended such that new foundations are bearing on native gravels.

Subgrade Preparation

We recommend that the soils within the footprint of the proposed structures be removed to the bottom of footings, consistent with the bearing elevation of existing foundations.

Where isolated overexcavation of existing fill may be required on the south side of the building, Structural Fill should be placed beneath the entire footprint of the foundations and should extend horizontally a minimum distance of 2 feet beyond the outside edge of footings. On-site native sand and gravel soils are considered suitable to be used as general fill materials.

All exposed areas which will receive foundation concrete or fill, once properly cleared and benched where necessary, should be scarified to a minimum depth of 10 inches, moisture conditioned as necessary, and compacted per the compaction requirements in this report. Compacted Structural Fill soils should then be placed to the proposed design grade and the moisture content and compaction of subgrade soils should be maintained until foundation or flatwork construction.

Facilities | Environmental | Geotechnical | Materials 7

Based upon the subsurface conditions determined from the geotechnical exploration, subgrade soils exposed during construction are anticipated to be relatively workable;

however, the workability of the subgrade may be affected by precipitation, repetitive construction traffic or other factors. If unworkable conditions develop, workability may be improved by scarifying and drying.

Existing Fill

As noted in Geotechnical Characterization, all borings encountered previously placed fill to depths ranging from about 1.0 to 6.5 feet below existing grades. We have no records to indicate the degree of control, and consequently, the fill is considered unreliable for support of foundation loads. This risk of unforeseen conditions, and specifically for this project differential settlement between new and old foundations, cannot be eliminated without completely removing the existing fill.

Excavation

We anticipate that excavations for the proposed construction can be accomplished with conventional earthmoving equipment. The bottom of excavations should be thoroughly cleaned of loose soils and disturbed materials prior to backfill placement and/or construction.

Fill Material Types

Fill required to achieve design grade should be classified as Structural Fill and general fill. Structural fill is material used below, or within 5 feet horizontally of structures.

General fill is material used to achieve grade outside of these areas.

Reuse of On-Site Soil: Excavated on-site soil may be selectively reused as fill outside the footprint of planned additions, such as for exterior backfill, landscaped areas or general site grade raising.

Material property requirements for on-site soil for use as general fill and Structural Fill are noted in the table below:

Property General Fill Structural Fill

Composition Free of deleterious material Free of deleterious material

Maximum particle size

4 inches

(or 2/3 of the lift thickness)

2 inches

Facilities | Environmental | Geotechnical | Materials 8

Property General Fill Structural Fill

Fines content Not limited Less than 10% Passing No. 200 sieve

Plasticity Not limited Maximum plasticity index of 10

GeoModel Layer

Expected to be Suitable1 1, 2, 3 2, 3

1. Based on subsurface exploration. Actual material suitability should be determined in the field at time of construction.

Imported Fill Materials: Imported fill materials should meet the following material property requirements. Regardless of its source, compacted fill should consist of approved materials that are free of organic matter and debris. Frozen material should not be used, and fill should not be placed on a frozen subgrade.

Soil Type 1

USCS

Classification

Acceptable Parameters (for Structural

Fill)

Structural Fill

GW, GP, SW, SP

(and dual symbols)

100% passing 1 ½”

30-60% passing the No. 4 screen

Less than 12% passing No. 200 sieve

Maximum Liquid Limit of 25

Maximum Plasticity Index of 10

1. Structural and general fill should consist of approved materials free of organic matter and debris. Frozen material should not be used, and fill should not be placed on a frozen subgrade. A sample of each material type should be submitted to the Geotechnical Engineer for evaluation prior to use on this site.

Additional geotechnical consultation should be provided prior to use of uniformly graded gravel on the site.

Fill Placement and Compaction Requirements

Structural and general fill should meet the following compaction requirements.

Item Structural Fill General Fill

Maximum Lift

Thickness

9 inches or less in loose thickness when heavy, self-propelled compaction equipment is used

4 to 6 inches in loose thickness when hand-guided equipment (i.e. jumping jack or plate compactor) is used

Same as

Structural Fill

Facilities | Environmental | Geotechnical | Materials 9

Item Structural Fill General Fill

Minimum

Compaction

Requirements 1,2,3

98% of max. below foundations and below floor slabs

95% of max. above foundations elevation

(exterior backfill)

92% of max.

Water Content

Range

Low plasticity cohesive: -2% to +3% of optimum

High plasticity cohesive: 0 to +4% of optimum

Granular: -3% to +3% of optimum

As required to achieve min.

compaction requirements

1. Maximum density and optimum water content as determined by the standard

Proctor test (ASTM D 698).

2. High plasticity cohesive fill should not be compacted to more than 100% of standard Proctor maximum dry density.

3. If the granular material is a coarse sand or gravel, or of a uniform size, or has a low fines content, compaction comparison to relative density may be more appropriate. In this case, granular materials should be compacted to at least

70% relative density (ASTM D 4253 and D 4254). Materials not amenable to density testing should be placed and compacted to a stable condition observed by the Geotechnical Engineer or representative.

Utility Trench Backfill

Any soft or unsuitable materials encountered at the bottom of utility trench excavations should be removed and replaced with Structural Fill or bedding material in accordance with public works specifications for the utility to be supported. This recommendation is particularly applicable to utility work requiring grade control and/or in areas where subsequent grade raising could cause settlement in the subgrade supporting the utility.

Trench excavation should not be conducted below a downward 1:1 projection from existing foundations without engineering review of shoring requirements and geotechnical observation during construction.

On-site materials are considered suitable for backfill of utility and pipe trenches from 1 foot above the top of the pipe to the final ground surface, provided the material is free of organic matter and deleterious substances.

Trench backfill should be mechanically placed and compacted as discussed earlier in this report. Compaction of initial lifts should be accomplished with hand-operated tampers or other lightweight compactors. Where trenches are placed beneath slabs or footings, the backfill should satisfy the gradation and expansion index requirements of engineered fill discussed in this report. Flooding or jetting for placement and compaction of backfill is not recommended.

Facilities | Environmental | Geotechnical | Materials 10

Grading and Drainage

All grades must provide effective drainage away from the building during and after construction and should be maintained throughout the life of the structure. Water retained next to the building can result in soil movements greater than those discussed in this report. Greater movements can result in unacceptable differential floor slab and/or foundation movements, cracked slabs and walls, and roof leaks. The roof should have gutters/drains with downspouts that discharge onto splash blocks at a distance of at least 10 feet from the building.

Exposed ground should be sloped and maintained at a minimum 5 percent away from the building for at least 10 feet beyond the perimeter of the building. Locally, flatter grades may be necessary to transition ADA access requirements for flatwork. After building construction and landscaping have been completed, final grades should be verified to document effective drainage has been achieved. Grades around the structure should also be periodically inspected and adjusted, as necessary, as part of the structure’s maintenance program. Where paving or flatwork abuts the structure, a maintenance program should be established to effectively seal and maintain joints and prevent surface water infiltration.

Earthwork Construction Considerations

Shallow excavations for the proposed structure are anticipated to be accomplished with conventional construction equipment. Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of grade-supported improvements such as floor slabs. Construction traffic over the completed subgrades should be avoided. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over or adjacent to construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the affected material should be removed, or the materials should be scarified, moisture conditioned, and recompacted prior to floor slab construction.

As a minimum, excavations should be performed in accordance with OSHA 29 CFR, Part

1926, Subpart P, “Excavations” and its appendices, and in accordance with any applicable local and/or state regulations.

Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety or the contractor's activities; such responsibility shall neither be implied nor inferred.

Facilities | Environmental | Geotechnical | Materials 11

Excavations or other activities resulting in ground disturbance have the potential to affect adjoining properties and structures. Our scope of services does not include review of available final grading information or consider potential temporary grading performed by the contractor for potential effects such as ground movement beyond the project limits. A preconstruction/ precondition survey should be conducted to document nearby property/infrastructure prior to any site development activity. Excavation or ground disturbance activities adjacent or near property lines should be monitored or instrumented for potential ground movements that could negatively affect adjoining property and/or structures.

Construction Observation and Testing

The earthwork efforts should be observed by the Geotechnical Engineer (or others under their direction). Observation should include documentation of adequate removal of surficial materials (vegetation, topsoil, and sidewalks), evaluation and remediation of existing fill materials, as well as mitigation of unsuitable areas observed by the

Geotechnical Engineer during foundation observations.

Each lift of compacted fill should be tested, evaluated, and reworked, as necessary, as recommended by the Geotechnical Engineer prior to placement of additional lifts. Each lift of fill should be tested for density and water content at a frequency of at least two tests for every lift of compacted fill at each addition location. Where not specified by local ordinance, one density and water content test should be performed for every 100 linear feet of compacted utility trench backfill and a minimum of one test performed for every 12 vertical inches of compacted backfill.

In areas of foundation excavations, the bearing subgrade should be evaluated by the

Geotechnical Engineer. If unanticipated conditions are observed, the Geotechnical

Engineer should prescribe mitigation options.

In addition to the documentation of the essential parameters necessary for construction, the continuation of the Geotechnical Engineer into the construction phase of the project provides the continuity to maintain the Geotechnical Engineer’s evaluation of subsurface conditions, including assessing variations and associated design changes.

Shallow Foundations

The primary geotechnical considerations for the planned additions to the Administration

Building are providing uniform bearing support, consistent with existing foundations, while limiting potential for differential movement between the new and old foundation systems. The existing dense to very dense native sand and gravel deposits provide substantial bearing capacity with limited settlement potential under the light-loads anticipated for the additions. Therefore, quality earthwork construction and proper

Facilities | Environmental | Geotechnical | Materials 12 preparation of the native sand/gravel subgrade soils will have the greatest impact on addition performance. Based on discussions with the design team, it is our understanding that the new foundation systems for the north and south arctic entryways are to be constructed at the same bearing elevation as the existing footings, or approximately 6.0 feet below existing grade. Embedment of footings at this elevation will satisfy local frost-depth criteria and provide support within native very dense sand/gravel in general. Where isolated columns are to be placed for entryways or the signage that are not immediately adjacent the existing structure, ensuring bearing within the native sand/gravel soils will be critical.

If the site has been prepared in accordance with the requirements noted in Earthwork, the following design parameters are applicable for shallow foundations.

Design Parameters – Compressive Loads

Item Description

Maximum Net Allowable Bearing

Pressure 1, 2

3,000 psf - foundations bearing upon properly prepared native sand or gravel subgrade

Required Bearing Stratum

GeoModel Layer 2 or 3 or Structural Fill extending to prepared native subgrade

Minimum Foundation Dimensions 15 inches

Per IBC 1809.7

Ultimate Passive Resistance

(equivalent fluid pressures) 500 pcf (granular backfill)

Sliding Resistance

0.35 allowable coefficient of friction -granular material

Minimum Embedment below

Finished Grade

Exterior footings in unheated areas: 48 inches

Interior footings in heated areas: 12 inches

Estimated Total Settlement from

Structural Loads

Less than about ½ inch

Estimated Differential Settlement 2, 7

About ½ of total settlement

1. The maximum net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. Assumes proper preparation of bearing surface un accordance with Earthwork section. Based on a minimum factor of safety of 3.

2. Values provided are for maximum loads noted in Project Description. Additional geotechnical consultation will be necessary if higher loads are anticipated.

3. Unsuitable or soft soils should be overexcavated and replaced per the recommendations presented in Earthwork.

Facilities | Environmental | Geotechnical | Materials 13

4. Use of passive earth pressures require the sides of the excavation for the spread footing foundation to be nearly vertical and the concrete placed neat against these vertical faces or that the footing forms be removed and compacted structural fill be placed against the vertical footing face. Assumes no hydrostatic pressure. A minimum factor of safety of 2 should be applied to ultimate values.

5. Can be used to compute sliding resistance where foundations are placed on suitable soil/materials. Frictional resistance for granular materials is dependent on the bearing pressure which may vary due to load combinations. For fine-grained materials, lateral resistance using cohesion should not exceed ½ the dead load. Based on a minimum factor of safety of 2.

6. Embedment necessary to minimize the effects of frost and/or seasonal water content variations. For sloping ground, maintain depth below the lowest adjacent exterior grade within 5 horizontal feet of the structure.

7. Differential settlements are noted for equivalent-loaded foundations and bearing elevation as measured over a span of 50 feet.

Construction Adjacent to Existing Building

Differential settlement between the additions and the existing building is expected to approach the magnitude of the total settlement of the addition. Expansion joints should be provided between the existing building and the proposed addition to accommodate differential movements between the two structures. Underground piping between the two structures should be designed with flexible couplings and utility knockouts in foundation walls should be oversized so minor deflections in alignment do not result in breakage or distress. Care should be taken during excavation adjacent to existing foundations to avoid disturbing existing foundation bearing soils.

New footings should bear at or near the bearing elevation of immediately adjacent existing foundations. Depending upon their locations and current loads on the existing footings, footings for the new addition could cause settlement of adjacent walls. To reduce this concern and risk, clear distances at least equal to the new footing widths should be maintained between the addition’s footings and footings supporting the existing building.

Foundation Construction Considerations

As noted in Earthwork, the footing excavations should be evaluated under the observation of the Geotechnical Engineer. The base of all foundation excavations should be free of water and loose soil, prior to placing concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance. Care should be taken to prevent wetting or drying of the bearing materials during construction. Excessively wet or dry material or any loose/disturbed material in the bottom of the footing excavations should be removed/reconditioned before foundation concrete is placed.

Facilities | Environmental | Geotechnical | Materials 14

If unsuitable bearing soils are observed at the base of the planned footing excavation, the excavation should be extended deeper to suitable soils, and the footings could bear directly on these soils at the lower level or on lean concrete backfill placed in the excavations. This will be of particular importance to reducing the risk of undermining existing footings if overexcavation is required near existing foundations. The lean concrete replacement zone is illustrated on the sketch below.

Floor Slabs

Design parameters for floor slabs assume the requirements for Earthwork have been followed. Specific attention should be given to positive drainage away from the structure and positive drainage of the aggregate base beneath the floor slab.

Floor Slab Design Parameters

Item Description

Floor Slab

Support1

Use 4 inches base course meeting material specifications of ACI

Subgrade compacted to recommendations in Earthwork

Estimated Modulus of Subgrade

Reaction 2

200 pounds per square inch per inch (psi/in) for point loads

1. Floor slabs should be structurally independent of building footings or walls to reduce the possibility of floor slab cracking caused by differential movements between the slab and foundation.

2. Modulus of subgrade reaction is an estimated value based upon our experience with the subgrade condition, the requirements noted in Earthwork, and the

Facilities | Environmental | Geotechnical | Materials 15 floor slab support as noted in this table. It is provided for point loads. For large area loads the modulus of subgrade reaction would be lower.

The use of a vapor retarder should be considered beneath concrete slabs on grade covered with wood, tile, carpet, or other moisture sensitive or impervious coverings, when the project includes humidity-controlled areas, or when the slab will support equipment sensitive to moisture. When conditions warrant the use of a vapor retarder, the slab designer should refer to ACI 302 and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder.

Saw-cut contraction joints should be placed in the slab to help control the location and extent of cracking. For additional recommendations, refer to the ACI Design Manual.

Joints or cracks should be sealed with a waterproof, non-extruding compressible compound specifically recommended for heavy duty concrete pavement and wet environments.

Where floor slabs are tied to perimeter walls or turn-down slabs to meet structural or other construction objectives, our experience indicates differential movement between the walls and slabs will likely be observed in adjacent slab expansion joints or floor slab cracks beyond the length of the structural dowels. The Structural Engineer should account for potential differential settlement through use of sufficient control joints, appropriate reinforcing or other means.

Floor Slab Construction Considerations

Finished subgrade, within and for at least 10 feet beyond the floor slab, should be protected from traffic, rutting, or other disturbance and maintained in a relatively moist condition until floor slabs are constructed. If the subgrade should become damaged or desiccated prior to construction of floor slabs, the affected material should be removed, and structural fill should be added to replace the resulting excavation. Final conditioning of the finished subgrade should be performed immediately prior to placement of the floor slab support course.

The Geotechnical Engineer should observe the condition of the floor slab subgrades immediately prior to placement of the floor slab support course, reinforcing steel, and concrete. Attention should be paid to high traffic areas that were rutted and disturbed earlier, and to areas where backfilled trenches are located.

Facilities | Environmental | Geotechnical | Materials 16

Lateral Earth Pressures

Design Parameters

Structures with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to values indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction, and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown in the diagram below. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The “at-rest” condition assumes no wall movement and is commonly used for basement walls, loading dock walls, or other walls restrained at the top. The recommended design lateral earth pressures do not include a factor of safety and do not provide for possible hydrostatic pressure on the walls (unless stated).

Lateral Earth Pressure Design Parameters

Earth

Pressure

Condition 1

Coefficient for

Backfill Type 2

Surcharge

Pressure 3 p1 (psf)

Equivalent Fluid Pressures

(psf) 2,4

Unsaturated 5 Submerged 5

Active (Ka) Granular - 0.26 (0.26)S (35)H (80)H

At-Rest (Ko) Granular - 0.41 (0.41)S (55)H (90)H

1. For active earth pressure, wall must rotate about base, with top lateral movements 0.002 H to 0.004 H, where H is wall height. For passive earth pressure, wall must move horizontally to mobilize resistance. Fat clay or other expansive soils should not be used as backfill behind the wall.

2. Uniform, horizontal backfill, with a maximum unit weight 130 pcf for granular soils.

3. Uniform surcharge, where S is surcharge pressure.

4. Loading from heavy compaction equipment is not included.

Facilities | Environmental | Geotechnical | Materials 17

Lateral Earth Pressure Design Parameters

Earth

Pressure

Condition 1

Coefficient for

Backfill Type 2

Surcharge

Pressure 3 p1 (psf)

Equivalent Fluid Pressures

(psf) 2,4

Unsaturated 5 Submerged 5

5. To achieve “Unsaturated” conditions, follow guidelines in Subsurface Drainage for Below-Grade Walls below. “Submerged” conditions are recommended when drainage behind walls is not incorporated into the design.

Backfill placed against structures should consist of granular soils. For the granular values to be valid, the granular backfill must extend out and up from the base of the wall at an angle of at least 45 degrees from vertical for the active case.

Footings, floor slabs or other loads bearing on backfill behind walls may have a significant influence on the lateral earth pressure. Placing footings within wall backfill and in the zone of active soil influence on the wall should be avoided unless structural analyses indicate the wall can safely withstand the increased pressure.

The lateral earth pressure recommendations given in this section are applicable to the design of rigid retaining walls subject to slight rotation, such as cantilever, or gravity type concrete walls. These recommendations are not applicable to the design of modular block - geogrid reinforced backfill walls (also termed MSE walls). Recommendations covering these types of wall systems are beyond the scope of services for this assignment. However, we would be pleased to develop a proposal for evaluation and design of such wall systems upon request.

Subsurface Drainage for Below-Grade Walls

A perforated rigid plastic drain line installed behind the base of walls and extends below adjacent grade is recommended to prevent hydrostatic loading on the walls. The invert of a drain line around a below-grade building area or exterior retaining wall should be placed near foundation bearing level. The drain line should be sloped to provide positive gravity drainage to daylight or to a sump pit and pump. The drain line should be surrounded by clean, free-draining granular material having less than 5% passing the

No. 200 sieve, such as No. 57 aggregate. The free-draining aggregate should be encapsulated in a filter fabric. The granular fill should extend a minimum of 2 feet above top of footing, and the backfill zone should be capped with a minimum of 12 inches of compacted cohesive fill to reduce infiltration of surface water into the drain system.

Facilities | Environmental | Geotechnical | Materials 18

Frost Considerations

The soils on this site are frost susceptible, and small amounts of water can affect the performance of the slabs on-grade and sidewalks. Exterior slabs should be anticipated to heave during winter months if surface water is not properly addressed as discussed above and moisture conditions increase in the native gravel and sand materials. If frost action needs to be eliminated in critical areas, we recommend the use of non-frost susceptible (NFS) fill or structural slabs (for instance, structural stoops in front of building doors). Placement of NFS material in large areas may not be feasible; however, the following recommendations are provided to help reduce potential frost heave:

■ Provide surface drainage away from the building and slabs, and toward the site drainage system.

■ Install drains around the perimeter of the building, stoops, below exterior slabs and connect them to the site drainage system.

■ Grade clayey subgrades so groundwater potentially perched in overlying fill or aggregate base, slope toward a site drainage system.

■ Place NFS fill as backfill beneath slabs critical to the project.

■ Place a 3 horizontal to 1 vertical (3H:1V) transition zone between NFS fill and other soils.

■ Place NFS materials in critical sidewalk areas.

General Comments

Our analysis and opinions are based upon our understanding of the project, the geotechnical conditions in the area, and the data obtained from our site exploration.

Variations will occur between exploration point locations or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. Terracon should be retained as the

Geotechnical Engineer, where noted in this report, to provide observation and testing services during pertinent construction phases. If variations appear, we can provide further evaluation and supplemental recommendations. If variations are noted in the absence of our observation and testing services on-site, we should be immediately notified so that we can provide evaluation and supplemental recommendations.

Our Scope of Services does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken.

Facilities | Environmental | Geotechnical | Materials 19

Our services and any correspondence are intended for the sole benefit and exclusive use of our client for specific application to the project discussed and are accomplished in accordance with generally accepted geotechnical engineering practices with no third-party beneficiaries intended. Any third-party access to services or correspondence is solely for information purposes to support the services provided by Terracon to our client. Reliance upon the services and any work product is limited to our client and is not intended for third parties. Any use or reliance of the provided information by third parties is done solely at their own risk. No warranties, either express or implied, are intended or made.

Site characteristics as provided are for design purposes and not to estimate excavation cost. Any use of our report in that regard is done at the sole risk of the excavating cost estimator as there may be variations on the site that are not apparent in the data that could significantly effect excavation cost. Any parties charged with estimating excavation costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing. Site safety and cost estimating including excavation support and dewatering requirements/design are the responsibility of others.

Construction and site development have the potential to affect adjacent properties. Such impacts can include damages due to vibration, modification of groundwater/surface water flow during construction, foundation movement due to undermining or subsidence from excavation, as well as noise or air quality concerns. Evaluation of these items on nearby properties are commonly associated with contractor means and methods and are not addressed in this report. The owner and contractor should consider a preconstruction/precondition survey of surrounding development. If changes in the nature, design, or location of the project are planned, our conclusions and recommendations shall not be considered valid unless we review the changes and either verify or modify our conclusions in writing.

Figures

Contents:

GeoModel

3,274

3,276

3,278

3,280

3,282

3,284

3,286

3,288

3,290

3,292

E L

E V

A T

IO

N

M

S L

(f ee t)

Rehabilitate Fort Smith Administration Building (BICA) - PMIS #312083 Fort Smith, MT Terracon Project No. 26225043

Layering shown on this figure has been developed by the geotechnical engineer for purposes of modeling the subsurface conditions as required for the subsequent geotechnical engineering for this project.

Numbers adjacent to soil column indicate depth below ground surface.

NOTES:

B-1 B-2 B-3 B-4

GEOMODEL

This is not a cross section. This is intended to display the Geotechnical Model only. See individual logs for more detailed conditions.

Well Graded Gravel with Silt and Sand (GW-GM), brown to light brown, moist, subangular to subround gravels, fine to coarse sand

LEGEND

Lean Clay with Sand

Well-graded Gravel with silt and sand Poorly-graded Sand with Gravel

Clayey Gravel with Sand

Model Layer General DescriptionLayer Name

Lean Clay with Sand (CL) in landscaping areas Clayey Gravel with Sand (GC) in south entry way1

Poorly Graded Sand with Silt and Gravel (SP-SM), brown, moist, dense to very dense2

GRAVEL

FILL

SAND

1.5

16.5

5.5

10.5

13.5

13.5

6.5

Attachments

Exploration and Testing Procedures

Field Exploration

Number of Borings Approximate Boring

Depth (feet) Location

4 13.0 to 16.5 Improvement areas

Boring Layout and Elevations: Terracon personnel provided the boring layout using handheld GPS equipment (estimated horizontal accuracy of about ±10 feet) and referencing existing site features. Approximate ground surface elevations were obtained by interpolation from the site specific topographic survey data provided in the SD drawings.

Subsurface Exploration Procedures: Our exploration was completed on August 15, 2022, when we advanced the borings using our Terracon Bismarck truck-mounted CME

75 drill rig using continuous flight hollow-stem augers. In general, four samples were obtained in the upper 10 feet of each boring and at intervals of 5 feet thereafter. In the split-barrel sampling procedure, a standard 2-inch outer diameter split-barrel sampling spoon was driven into the ground by a 140-pound automatic hammer falling a distance of 30 inches. The number of blows required to advance the sampling spoon the last 12 inches of a normal 18-inch penetration is recorded as the Standard Penetration Test

(SPT) resistance value. The SPT resistance values, also referred to as N-values, are indicated on the boring logs at the test depths. We also observed the boreholes while drilling and at the completion of drilling for the presence of groundwater. Groundwater was not observed at these times in the boreholes. For safety purposes, all borings were backfilled with auger cuttings after their completion.

The sampling depths, penetration distances, and other sampling information was recorded on the field boring logs. The samples were placed in appropriate containers and taken to our soil laboratory for testing and classification by a Geotechnical Engineer. Our exploration team prepared field boring logs as part of the drilling operations. These field logs included visual classifications of the materials observed during drilling and our interpretation of the subsurface conditions between samples. Final boring logs were prepared from the field logs. The final boring logs represent the Geotechnical Engineer's interpretation of the field logs and include modifications based on observations and tests of the samples in our laboratory.

Laboratory Testing

The project engineer reviewed the field data and assigned laboratory tests. The laboratory testing program included the following types of tests:

■ Moisture Content

■ Grain Size Distribution

■ Atterberg Limits

The laboratory testing program often included examination of soil samples by an engineer. Based on the results of our field and laboratory programs, we described and classified the soil samples in accordance with the Unified Soil Classification System.

Site Location and Exploration Plans

Site Location Plan

Exploration Plan

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