B08_Attch_5_Geotech_Rpt_2-8-23.pdf

Report Cover Page

Visitor Center Replacement Geotechnical Engineering Report

February 8, 2023 | Terracon Project No. 26225036

Prepared for:

AJC Architects PC

703 East 1700 South

Salt Lake City, UT 84105

2110 Overland Avenue, Suite 124

Billings, MT 59102

P (406) 656-3072

Terracon.com

Facilities | Environmental | Geotechnical | Materials

Report Cover Letter to Sign

February 8, 2023

AJC Architects PC 703 East 1700 South Salt Lake City, UT 84105

Attn: Ms. Jill A. Jones, AIA, LEED AP BD+C P: (801) 466 8818 E: jjones@ajcarchitects.com

Re: Geotechnical Engineering Report Contract No. 140P2019D0015 / PMIS No. 241430 Visitor Center Replacement Little Bighorn National Monument Crow Agency, MT Terracon Project No. 26225036

Dear Ms. Jones:

We have completed the scope of Geotechnical Engineering services for the above referenced project in general accordance with Terracon Proposal No. P26225036 dated June 23, 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

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

Senior Engineer Senior Associate | Great Falls Office Manager

Geotechnical Engineering Report

Visitor Center Replacement | Crow Agency, MT

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Table of Contents Introduction Project Description Site Conditions Geotechnical Characterization Seismic Site Class Corrosivity Geotechnical Overview Earthwork

Demolition Site Preparation Subgrade Preparation Soil Stabilization 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 Foundation Construction Considerations

Floor Slabs Floor Slab Design Parameters Floor Slab Construction Considerations

Pavements General Pavement Comments

Frost Considerations General Comments

Figures GeoModel

Attachments

Exploration and Testing Procedures Photography Log Site Location and Exploration Plans Exploration and Laboratory Results Supporting Information

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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.

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Facilities | Environmental | Geotechnical | Materials 1

Introduction

This report presents the results of our subsurface exploration and Geotechnical Engineering services performed for the proposed Visitor Center Replacement project to be located at the Little Bighorn National Monument in Crow Agency, 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

Demolition considerations

Foundation design and construction

Floor slab design and construction

Lateral earth pressure

Pavement design and construction

Frost considerations

The geotechnical engineering Scope of Services for this project included the advancement of three test borings (only two were advanced due to site safety restrictions), 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 final understanding of the project conditions is as follows:

Item Description

Information Provided

Provided via email correspondence with AJC on June 17, 2022.

Supporting documents included:

A04_LIBI-242031 DD-CD-SOSv1.pdf

LIBI 241430 Project Schedule, 220615.pdf

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Item Description

Project Description

The project includes demolition and replacement of the existing

Monument.

Proposed Structure

Although preliminary drawings were not provided, we anticipate the new visitor center will include a single-story, wood or light gauge metal framed building with a footprint of about 10,000 square feet.

Finished Floor Elevation

Not provided; however, it is anticipated to be within 2 feet of existing grade.

Maximum Loads

Columns: 120 kips (assumed)

Walls: 6 kips per lineal foot (klf) (assumed)

Slabs: 150 pounds per square foot (psf) (assumed)

Grading/Slopes

Grading plans were not provided at the time of report preparation. We have anticipated relatively minor site grading modifications would be required during construction to develop final grade with final slope angles of as steep as 5H:1V (Horizontal: Vertical) expected.

Below-Grade Structures

None anticipated.

Free-Standing Retaining Walls

Retaining walls are not expected to be constructed as part of site development to achieve final grades.

Pavements

Paved driveway and parking will be constructed on the parcel.

We have considered both rigid (concrete) and flexible (asphalt) pavement sections. Please confirm this assumption.

Anticipated traffic is as follows:

Autos/light trucks: 1,000 vehicles per day Light delivery, RV, and trash collection vehicles: 10 vehicles per day Tractor-trailer trucks: 1 vehicle per day

The pavement design period is 20 years.

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.

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Site Conditions

The following description of site conditions is derived from our site visit in association with the field exploration.

Item Description

Parcel Information

The project is located at the Little Bighorn National Monument in Crow Agency, MT.

Latitude/Longitude (approximate) 45.56992° N, 107.43032° W See Site Location

Existing Improvements

The site includes an existing visitor center, landscaped grasses, paved parking areas and drive lanes.

Current Ground Cover

Earthen, lightly to moderately vegetated, existing visitor center, asphalt paved parking lot

Existing Topography

Based on our review of aerial imagery and our site visit findings, the site is relatively flat with site contours ranging from approximately 3,250 to 3,255 feet above mean sea level (MSL).

We also collected photographs at the time of our field exploration program.

Representative photos are provided in our Photography Log.

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 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.

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Model Layer

Layer Name General Description

1 Fat Clay Medium stiff to very stiff fat clay with variable amounts of sand and gravel encountered in both borings.

2 Sand Medium dense clayey sand with variable amounts of clay and silt encountered in Boring B-1.

3 Bedrock Sedimentary shale bedrock with interbedded sandstone encountered below the overburden soil in both borings.

The borings were advanced in the dry using hollow stem auger drilling techniques 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/bedrock properties observed at the site and as described on the exploration logs and results, our professional opinion is for that a Seismic Site Classification of C be considered for the project. Subsurface explorations at this site were extended to a maximum depth of 25.4 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.

Corrosivity

The table below lists the results of laboratory soluble sulfate, electrical resistivity, and pH testing. The values may be used to estimate potential corrosive characteristics of the on-site soils with respect to contact with the various underground materials which will be used for project construction.

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Corrosivity Test Results Summary

Boring Sample Depth (feet)

Soil Description

Soluble Sulfate (mg/L)

Electrical -cm) pH

B-3 0.5 5 CH 5,020 119 7.9

Results of soluble sulfate testing can be classified in accordance with ACI 318 Building Code Requirements for Structural Concrete. Numerous sources are available to characterize corrosion potential to buried metals using the parameters above.

ANSI/AWWA is commonly used for ductile iron, while threshold values for evaluating the effect on steel can be specific to the buried feature (e.g., piling, culverts, welded wire reinforcement, etc.) or agency for which the work is performed. Imported fill materials may have significantly different properties than the site materials noted above and should be evaluated if expected to be in contact with metals used for construction.

Consultation with a NACE certified corrosion professional is recommended for buried metals on the site.

Geotechnical Overview

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

The subsurface materials generally consisted of fat clay and clayey sand materials overlying weathered sedimentary shale bedrock which extended 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 structures can be supported on conventional continuous or spread footings.

The near surface, medium stiff to very stiff, high plasticity fat 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.

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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, clearing and grubbing, 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, floor slabs, and pavements.

Demolition

The proposed building will be constructed within the footprint of the existing Visitor Center which will need to be demolished, as well as exterior sidewalks, pavements, and utilities. We recommend existing foundations, slabs, and utilities be removed from within the proposed building footprint and at least 5 feet beyond the outer edge of foundations.

For areas outside the proposed building footprints and foundation bearing zones, existing foundations, floor slabs, and utilities should be removed where they conflict with proposed utilities and pavements. In such cases, existing foundations, floor slabs, and utilities should be removed to a depth of at least 2 feet below the affected utility or design pavement subgrade elevation.

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 and parking/driveway areas.

Mature trees are located within or near the footprint of the proposed new Visitor Center, 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 in accordance with the recommendations below.

Where fill is placed on existing slopes steeper than 5H:1V, benches should be cut into the existing slopes prior to fill placement. The benches should have a minimum vertical

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Facilities | Environmental | Geotechnical | Materials 7 face height of 1 foot and a maximum vertical face height of 3 feet and should be cut wide enough to accommodate the compaction equipment. This benching will help provide a positive bond between the fill and natural soils and reduce the possibility of failure along the fill/natural soil interface.

Although no evidence of fill or underground facilities was observed during the exploration and site reconnaissance, such features could be encountered during construction. If unexpected fills or underground facilities are encountered, such features should be removed, and the excavation thoroughly cleaned prior to backfill placement and/or construction.

Subgrade Preparation

We recommend that the soils within the footprint of the proposed structures be removed and replaced with Structural Fill to a minimum depth of 2 feet below the bottom of footings. Structural Fill placed beneath the entire footprint of the foundations should extend horizontally a minimum distance of 2 feet beyond the outside edge of footings.

Portions of the near-surface materials anticipated to be developed as excavation spoils are not considered suitable for use as Structural Fill.

Pavement subgrade should be proofrolled with an adequately loaded vehicle such as a fully loaded tandem-axle dump truck. The proofrolling should be performed under the observation of the Geotechnical Engineer or representative. Areas excessively deflecting under the proofroll should be delineated and subsequently addressed by the Geotechnical Engineer. Excessively wet or dry material should either be removed, or moisture conditioned and recompacted.

All exposed areas which will receive 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 pavement construction.

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.

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Soil Stabilization

Methods of subgrade improvement, as described below, could include scarification, moisture conditioning and recompaction, and removal of unstable materials and replacement with granular fill (with or without geosynthetics). The appropriate method of improvement, if required, would be dependent on factors such as schedule, weather, the size of area(s) to be stabilized, and the nature of the instability. More detailed recommendations can be provided during construction as the need for subgrade stabilization occurs. Performing site grading operations during warm seasons and dry periods would help reduce the amount of subgrade stabilization required.

If the exposed subgrade is unstable during proofrolling operations, it could be stabilized using one of the methods outlined below.

Scarification and Recompaction - It may be feasible to scarify, dry, and recompact the exposed soils. The success of this procedure would depend primarily upon favorable weather and sufficient time to dry the soils. Stable subgrades likely would not be achievable if the thickness of the unstable soil is greater than about 1 foot, if the unstable soil is at or near groundwater levels, or if construction is performed during a period of wet or cool weather when drying is difficult.

Removal and Replacement with Structural Fill - The use of Structural Fill is a common procedure to improve subgrade stability. Typical undercut depths would be expected to range from about 12 to 24 inches below finished subgrade elevation. The use of high modulus geotextiles (i.e., engineering fabric or geogrid) could also be considered after underground work such as utility construction is completed. Prior to placing the fabric or geogrid, we recommend that all below grade construction, such as utility line installation, be completed to avoid damaging the fabric or geogrid. Equipment should not be operated above the fabric or geogrid until one full lift of crushed stone fill is placed above it. The maximum particle size of granular material placed over geotextile fabric or geogrid should not exceed 1-1/2 inches.

Further evaluation of the need and recommendations for subgrade stabilization can be provided during construction as the geotechnical conditions are exposed.

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 10 feet horizontally of structures, pavements or constructed slopes. General fill is material used to achieve grade outside of these areas.

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Reuse of On-Site Soil: Excavated on-site soil may be selectively reused as exterior foundation wall backfill, pavement subgrade, and landscaping areas.

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

6 inches

(or 2/3 of the lift thickness)

1.5 inches

Fines content Not limited Less than 12% Passing No. 200 sieve

Plasticity Not limited Maximum plasticity index of 10

GeoModel Layer

Expected to be Suitable1 1, 2 ---

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)

Granular GW, GP, SW, SP 100% passing 1.5-inch sieve; 30 to 60% passing No. 4 sieve; less than 12% passing No. 200 sieve

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.

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Item Structural Fill General Fill

Maximum Lift Thickness

8 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

Minimum Compaction

Requirements 1,2

98% of max. below foundations

95% of max. above foundations, below floor slabs, utility trench backfill, and pavement subgrade

92% of max.

Water Content

Range 1

Low plasticity cohesive: -2% to +2% 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. 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 1H:1V 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 or pavement subgrade elevation, 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

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Facilities | Environmental | Geotechnical | Materials 11 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.

For low permeability subgrades, utility trenches are a common source of water infiltration and migration. Utility trenches penetrating beneath the building should be effectively sealed to restrict water intrusion and flow through the trenches, which could migrate below the building. The trench should provide an effective trench plug that extends at least 5 feet from the face of the building exterior. The plug material should consist of cementitious flowable fill or low permeability clay. The trench plug material should be placed to surround the utility line. If used, the clay trench plug material should be placed and compacted to comply with the water content and compaction recommendations for Structural Fill stated previously in this report.

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% 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 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 and pavements. 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, February 8, 2023 | Terracon Project No. 26225036

Facilities | Environmental | Geotechnical | Materials 12 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 or pavement construction.

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

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 pavements), evaluation and remediation of existing fill materials, as well as proofrolling and mitigation of unsuitable areas delineated by the proofroll.

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 one test for every 2,500 square feet of compacted fill in the building areas and 5,000 square feet in pavement areas. 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 conditions, including assessing variations and associated design changes.

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Shallow Foundations

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 1,500 psf - foundations bearing upon

Structural Fill

Required Bearing Stratum 3 A minimum 2-foot-thick layer of Structural Fill

Minimum Foundation Dimensions Column Footings: 30 inches Continuous Footings: 18 inches

Ultimate Passive Resistance4

(equivalent fluid pressures)

170 pcf (cohesive backfill) 480 pcf (granular backfill)

Sliding Resistance 5 0.45 allowable coefficient of friction granular Structural Fill material

Minimum Embedment below

Finished Grade 6

Exterior footings in unheated areas: 42 inches

Interior footings in heated areas: 24 inches

Estimated Total Settlement from

Structural Loads 2 Less than about 1 inch

Estimated Differential Settlement 2, 7 About ½ to ¾ 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. Values assume that exterior grades are no steeper than 20% within 10 feet of structure.

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.

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.

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.

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Item Description

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.

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.

Overexcavation for Structural Fill placement below footings should be conducted as shown below. The overexcavation should be backfilled up to the footing base elevation, with imported Structural Fill placed, as recommended in the Earthwork section.

To limit surface water runoff into the 2-foot thick layer of Structural Fill below the Visitor Center footings, it is imperative that positive grading away from the Visitor Center be maintained throughout the life of the structure. We also recommend that the exterior foundation wall backfill consist of low permeability material, such as the native clay soils, to further limit the potential for the native clay soils below the building footings to become saturated. Installation of a perimeter drainage detail incorporated into the construction drawing package should be reviewed by the Geotechnical Engineer to ensure the system is designed as intended to reduce potential for moisture migration to the base of the Structural Fill replacement zone.

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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.

The subgrade soils are comprised of high plasticity clays exhibiting the potential to swell with increased water content. Construction of the floor slab, combined with the removal of trees, and revising site drainage creates the potential for gradual increased water contents within the clays. Increases in water content will cause the clays to swell and damage the floor slab. To reduce the swell potential to less than about 1 inch, the subgrade soils should be replaced with Structural Fill so the floor slab is supported on at least 2 feet of compacted Structural Fill.

Floor Slab Design Parameters

Item Description

Floor Slab

Support1, 3

A minimum 2 feet thick zone of properly placed and compacted imported Structural Fill.

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 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.

3. Crushed aggregate base course in accordance with Montana Public Works Standard Specifications (MPWSS) 7th Edition, Section 02235 and any applicable City of Belgrade modifications to the MPWSS.

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.

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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.

Pavements

General Pavement Comments

Based on conversations with AJC Architects and the National Park Service, we understand a pavement thickness design has already been developed by the Federal Highway Administration (FHWA) for the Visitor Center parking area and drive lanes based on a geotechnical report prepared by SK Geotechnical and issued February 13, 2020, for the Little Bighorn Park Road.

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Frost Considerations

The soils on this site are frost susceptible, and small amounts of water can affect the performance of the slabs on-grade, sidewalks, and pavements. Exterior slabs should be anticipated to heave during winter months. 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 pavements, 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 and pavements 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.

As an alternative to extending NFS fill to the full frost depth, consideration can be made to placing extruded polystyrene or cellular concrete under a buffer of at least 2 feet of NFS material.

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

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Facilities | Environmental | Geotechnical | Materials 18 is concerned about the potential for such contamination or pollution, other studies should be undertaken.

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 affect 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.

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Figures

Contents:

GeoModel

February 8, 2023 | Terracon Project No. 26225036

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Attachments

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Facilities | Environmental | Geotechnical | Materials

Exploration and Testing Procedures

Field Exploration

Boring Approximate Boring

Depth (feet) Location

B-1 25.4 Planned building footprint

B-3 21.5 Planned building footprint

Boring Layout and Elevations: Based on conversations with AJC and NPS personnel, Terracon staked the boring layout provided by AJC using handheld GPS equipment (estimated horizontal accuracy of about ±10 feet) and referencing existing site features.

The borings were placed in areas previously discussed with AJC and NPS personnel to be accessible by truck-mounted drilling equipment. Approximate ground surface elevations were estimated using Google Earth. If elevations and a more precise boring layout are desired, we recommend borings be surveyed.

During the field exploration, the drill crew could not safely access the location of Boring B-2 as it sat atop a small slope. Based on the above and recent inclement weather, the drill crew did not think they could safely drill Boring B-2 at the original staked location.

The NPS architect on-site was notified. In order to keep the drill schedule and avoid another trip to the site from the drill crew and the public and private utility locators to clear another location, Boring B-2 was omitted from the field exploration.

Subsurface Exploration Procedures: We advanced the borings with a truck-mounted, rotary drill rig using continuous flight, hollow stem augers. In general, four samples were obtained in the upper 10 feet of each boring and at attempted intervals of 5 feet thereafter. Bulk samples were collected from auger cuttings in the upper 5 feet of subgrade, as needed. In the thin-walled tube sampling procedure, a thin-walled, seamless steel tube with a sharp cutting edge was pushed hydraulically into the soil to obtain a relatively undisturbed sample. 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 observed and recorded groundwater levels during drilling and sampling. 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

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Facilities | Environmental | Geotechnical | Materials 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

Dry Unit Weight

Atterberg Limits

Grain Size Analysis

Unconfined Compressive Strength

Moisture Density Characteristics (Proctor)

California Bearing Ratio

Corrosivity Suite Testing (pH, resistivity, soluble sulfates)

The laboratory testing program 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.

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Photography Log

Near Boring B-1, looking north Near Boring B-1, looking south

Near Boring B-3, looking south Near Boring B-3, looking east

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Site Location and Exploration Plans

Contents:

Site Location Plan Exploration Plan

Note: All attachments are one page unless noted above.

February 8, 2023 | Terracon Project No. 26225036

Facilities | Environmental | Geotechnical | Materials

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Site Location

DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES MAP PROVIDED BY MICROSOFT BING MAPS

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Exploration Plan

DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES MAP PROVIDED BY MICROSOFT BING MAPS

Exploration and Laboratory Results

Boring Logs (B-1 and B-3) Atterberg Limits Grain Size Distribution Unconfined Compressive Strength Consolidation/Swell Moisture Density Relationship

CBR

Corrosivity (7 pages)

PROJECT: Little Bighorn Visitor Center Replacement PROJECT NO: 26225036 LOCATION: Crow Agency,…