Amendmnet_3_140P2025B0001_Geotech_Gravel_Pit_Answers_7_28_25_0003.pdf

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1 copies of the amendment; (b) By acknowledging receipt of this amendment on each copy of the offer submitted ; or (c) By separate letter or electronic communication which includes a reference to the solicitation and amendment numbers. FAILURE OF YOUR ACKNOWLEDGEMENT TO BE

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OFFER. If by virtue of this amendment you desire to change an offer already submitted , such change may be made by letter or electronic communication, provided each letter or electronic communication makes reference to the solicitation and this amendment, and is received prior to the opening hour and date specified.

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PDS

Denver CO 80225-0287 P.O. Box 25287 1 Denver Federal Center, Building 50 NPS, DSC Contracting Services Div

07/18/20250003

13. THIS ITEM ONLY APPLIES TO MODIFICATION OF CONTRACTS/ORDERS. IT MODIFIES THE CONTRACT/ORDER NO. AS DESCRIBED IN ITEM 14.

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FACILITY CODE CODE

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4. REQUISITION/PURCHASE REQ. NO.3. EFFECTIVE DATE2. AMENDMENT/MODIFICATION NO. 5. PROJECT NO. (If applicable)

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AMENDMENT OF SOLICITATION/MODIFICATION OF CONTRACT

07/09/2025

CHECK ONE A. THIS CHANGE ORDER IS ISSUED PURSUANT TO: (Specify authority) THE CHANGES SET FORTH IN ITEM 14 ARE MADE IN THE CONTRACT

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14. DESCRIPTION OF AMENDMENT/MODIFICATION (Organized by UCF section headings, including solicitation/contract subject matter where feasible.)

Yellowstone 311631 - Replace Mammoth Wastewater Collection System

The purpose of this amendment is to:

a) Provide Geotechnical Investigation Report;

b) Provide Gravel Pit Locations;

c) Provide answers to questions received to date.

16A. NAME AND TITLE OF CONTRACTING OFFICER (Type or print)15A. NAME AND TITLE OF SIGNER (Type or print)

15C. DATE SIGNED 16B. UNITED STATES OF AMERICA 15B. CONTRACTOR/OFFEROR 16C. DATE SIGNED

(Signature of person authorized to sign) (Signature of Contracting Officer)

Jason Longshore

STANDARD FORM 30 (REV. 11/2016)

Prescribed by GSA FAR (48 CFR) 53.243

Previous edition unusable

Except as provided herein, all terms and conditions of the document referenced in Item 9 A or 10A, as heretofore changed, remains unchanged and in full force and effect .

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GEOTECHNICAL | ENVIRONMENTAL

MATERIALS TESTING | SPECIAL INSPECTION

AN EMPLOYEE-OWNED COMPANY

2720 Palmer St Unit A., Missoula, MT 59808 Phone: 406.206.5911 • Fax: 208.762.0942

Hayden, ID • Lewiston, ID • Meridian, ID • Spokane Valley, WA • Missoula, MT www.allwesttesting.com

September 8, 2022

Mr. Jarrett Moran, P.E.

HDR Engineering, Inc.

2150 Analysis Drive, Suite A Bozeman, Montana 59718

RE: Geotechnical Evaluation

Mammoth Wastewater Improvements Mammoth, Yellowstone National Park, Wyoming

ALLWEST Project No. 722-070G

Mr. Moran, ALLWEST has completed the authorized geotechnical evaluation for the proposed wastewater improvements project located in Mammoth, Yellowstone National Park, Wyoming. The purpose of this evaluation was to characterize the soil and geologic conditions on the property. The attached report presents the results of the field evaluation and our recommendations to assist with design and construction of the proposed project.

We appreciate the opportunity to provide these services to you on this project. If you have any questions or need additional information, please call us at (406) 206-5911.

Sincerely, ALLWEST

Prepared by: Reviewed by:

Andrew Warren, P.E Shawn Turpin, P.E.

Senior Geotechnical Engineer Senior Geotechnical Engineer

WWW.ALLWESTTESTING.COM

GEOTECHNICAL EVALUATION

MAMMOTH WASTEWATER IMPROVEMENTS

MAMMOTH, YELLOWSTONE NATIONAL PARK, WYOMING

ALLWEST PROJECT NO. 722-070G

September 8, 2022

Prepared for:

Mr. Jarrett Moran, P.E.

HDR Engineering, Inc.

2150 Analysis Drive, Suite A Bozeman, Montana 59718

Prepared by:

ALLWEST

2720 Palmer St Unit A Missoula, Montana 59808

09/08/2022

Geotechnical Evaluation ALLWEST Project No. 722-070G

Mammoth, Yellowstone National Park, Wyoming

TABLE OF CONTENTS

EXECUTIVE SUMMARY

1.0 SCOPE OF SERVICES

2.0 PROJECT DESCRIPTION

3.0 EVALUATION PROCEDURES

4.0 SITE CONDITIONS

4.1 General Geologic Conditions

4.2 General Soil Conditions

4.3 Seismicity

5.0 SUBSURFACE CONDITIONS

5.1 Silt

5.2 Clay

5.3 Gravel

5.4 Sand

5.5 Groundwater Conditions

6.0 INFILTRATION TESTING

7.0 LABORATORY TESTING

7.1 Water Content

7.2 Classification

7.3 Moisture-Density Relationship

8.0 CONCLUSIONS AND RECOMMENDATIONS

8.1 Site Grading

8.1.1 Clearing and Stripping

8.1.2 Excavation

8.1.3 Subgrade Preparation

8.1.4 Cut and Fill Slopes

8.1.5 Materials

8.1.6 Fill Placement and Compaction

8.1.7 Wet Weather Construction

8.1.8 Cold Weather Construction

8.2 Slope Stability Analysis

8.3 Stormwater and Drainage

9.0 ADDITIONAL RECOMMENDED SERVICES

10.0 EVALUATION LIMITATIONS

TABLE OF CONTENTS (continued)

Important Information About Your Geotechnical Engineering Report (Published by Geoprofessional Business Association)

APPENDICES

Appendix A –

Vicinity Map (Figure A-1) Boring and Infiltration Test Location Map (Figure A-2)

Appendix B –

Boring Logs Unified Soil Classification System

Appendix C –

Summary of Laboratory Results Laboratory Test Results (Figures C-1 through C-4)

Appendix D –

Summary of Double-Ring Infiltration Testing Results Photograph Log of Double-Ring Infiltration Testing

Appendix E –

Slope Stability Analyses Outputs (Figures E-1 through E-6)

Appendix F –

Typical Fill Benching Detail (Figure F-1)

EXECUTIVE SUMMARY

ALLWEST has completed the authorized geotechnical evaluation for the proposed improvements to the wastewater treatment system serving the community of Mammoth in Yellowstone National Park, Wyoming. The general location of the project is shown on the Vicinity Map, Figure A-1, in Appendix A of this report. The purpose of the evaluation was to assess the subsurface conditions throughout the site with respect to the proposed design and construction. This report details the results of the field evaluation and presents recommendations to assist in the design and construction of the proposed development. A summary of geotechnical considerations follows:

The natural soils removed from excavations are suitable for re-use as embankment fill, provided they are thoroughly mixed to achieve a uniform texture and composition prior to placement.

Slope stability analyses suggest that proposed embankment configurations using on-site soils consisting of 4H:1V slopes and heights less than 10 feet tall will be stable under static and pseudo-static (seismic) conditions.

Dewatering may be required for construction depending on the season and excavation depths required for construction of the proposed improvements. Dewatering means and methods will be the responsibility of the selected earthwork contractor. As a minimum, dewatering should extend at least 2 feet below the bottoms of excavations.

Material excavated from near and below the groundwater table will require drying prior to placement as fill.

This geotechnical evaluation was prepared based on preliminary plans that were made available at the time of exploration. The geotechnical engineer must be informed of future changes to the site layout, proposed structure locations/layout, and/or loading criteria that differ from the assumptions stated in this report.

Close monitoring of the construction operations discussed herein will be critical in achieving the design subgrade support. If we are not retained to provide required construction observation and materials testing services, we cannot be responsible for soil engineering related construction errors or omissions. This summary should be used in conjunction with the entire report for design purposes. It should be recognized that details were not included or fully developed in this section, and the report must be read in its entirety for a comprehensive understanding of the items contained herein. Report section titled 10.0 EVALUATION LIMITATIONS should be read for an understanding of the report limitations.

Mammoth Wastewater Improvements Page 1

1.0 SCOPE OF SERVICES

To complete this geotechnical evaluation, ALLWEST accomplished the following scope of services:

1) Performed a field evaluation by observing the drilling of nine hollow-stem auger borings near the proposed wastewater improvements. Subsurface conditions observed in the borings were described and visually classified and the subsurface profiles were logged.

2) Performed three infiltration tests in general accordance with the methods outlined in WYDEQ Chapter 25 and two double-ring infiltration tests in general accordance with the methods outlined in ASTM D3385.

3) Performed laboratory tests on select soil samples to assess the appropriate engineering soil properties and characteristics for the proposed development.

4) Performed engineering analyses and prepared recommendations to assist project planning, design, and construction.

Services were provided in general accordance with ALLWEST’s proposed scope of services submitted via email dated August 17, 2022 and subsequently approved by HDR, Inc.

2.0 PROJECT DESCRIPTION

The project area is located adjacent the west side of Lower Mammoth Service Road and the existing pump house. The project will consist of the addition of multiple percolation ponds and a membrane bioreactor (MBR) facility to assist in the treatment of wastewater from Mammoth.

Preliminary grading plans were not available to ALLWEST at the time of this report; however, site grading is anticipated to consist of cut and fill on the order of 5 to 8 feet or less to construct the various ponds and MBR facility. The scope of services for this evaluation does not include foundation recommendations for structures for the proposed project at the request of HDR, Inc. If the proposed design assumptions vary from those stated, ALLWEST should be notified to review the recommendations herein.

3.0 EVALUATION PROCEDURES

To complete this evaluation, ALLWEST reviewed soil and geologic literature for the project area. Subsurface conditions were evaluated by drilling nine hollow-stem auger borings on August 22 through 24, 2022. Drilling was completed using a Simco 2800 truck-mounted drill rig equipped with 6-inch diameter hollow-stem, continuous-flight augers. Approximate locations of the borings are shown on Figure A-2, Boring and Infiltration Test Location Map in Appendix A.

Prior to drill rig mobilization, One Call of Wyoming was contacted by Park personnel to request the location and clearance of public underground utilities. Review of the site was also

Mammoth Wastewater Improvements Page 2 performed to determine possible access limitations to proposed boring locations prior to drilling. Samples of the subsurface materials were collected as described below.

Split spoon samplers having an outside diameter of 2 inches were driven into the various subsurface materials from the bottom of a borehole using a 140-pound hammer falling 30 inches (ASTM International (ASTM) Method D1586). The number of blows required for the sampler to penetrate each 6-inch increment to a total depth of up to 18 inches is recorded. The sum of the number of blows required for the second and third 6-inch increments of penetration is termed the "standard penetration resistance" or "N-value". In cases where 50 blows are insufficient to advance through a 6-inch interval, the penetration after 50 blows is recorded and the test concluded.

The N-value provides an indication of the relative density or consistency of the subsurface materials.

Disturbed bulk samples representative of soil conditions from select locations were obtained from hollow-stem auger cuttings returned to the surface.

Subsurface conditions observed in the borings were visually described and classified in general accordance with ASTM D2488 and the subsurface profiles were logged by an ALLWEST geotechnical engineer. Detailed descriptions of the soil observed in the borings are presented on the boring logs found in Appendix B of this report. The descriptive soil terms used on the boring logs, and in this report, can be referenced by the Unified Soil Classification System (USCS). A summary of the USCS is included in Appendix B.

4.0 SITE CONDITIONS

The wastewater improvements project is located within an undeveloped area just south and east of the community of Mammoth. The project area is characterized by rolling topography which varies in steepness from relatively flat to moderately steep. Native grasses and small brush are present throughout. The project area is immediately bordered to the northeast by Lower Mammoth Service Road, to the north by a fenced grass covered helicopter pad, and to the west and south by open area. Two existing wastewater treatment ponds and connecting works also exist to the south and east of the project area.

4.1 GENERAL GEOLOGIC CONDITIONS

The project area is mapped geologically on the “Surficial Geologic Map of the Mammoth Quadrangle and part of the Gardiner Quadrangle, Yellowstone National Park, Wyoming and Montana”, published by the United States Geological Survey, 1973.

The mapping depicts the project area as being underlain by Pleistocene glacial till deposits from the Pinedale Glaciation period (pt - B, Gi) and by Holocene alluvial fan deposits (fa) alluvial deposits within the lower lying areas along the west and south edges of the site. The mapped glacial till unit (pt) is described as an unsorted, non-stratified, compact mixture of round-stones, sand, silt, and clay deposited by glaciers. The glacial till specific to the project area (B, Gi) is described as gray, calcareous, and clayey with a diversity of cobble-sized erratics. The alluvial deposits (fa) are described as fine-grained alluvium consisting of silt and subordinate sand, clay, and plant material that underlie wet ground along drainages, in upland

Mammoth Wastewater Improvements Page 3 depressions or in original depressions on top of rhyolite flows and supports lush stands of tall grasses and sedges. Conditions observed in the borings were generally consistent with the geologic mapping.

4.2 GENERAL SOIL CONDITIONS

The USDA Natural Resources Conservation Service (NRCS) has mapped the soil throughout the site as Pesowyo-Beartooth-Sawbuck families, complex. The complex is described as being well drained to somewhat excessively drained soil. The complex soils originate from a variety of mechanisms, including: alluvial fans, flood plains, moraines, and outwash terraces.

The typical profile in the upper five feet consists primarily of very gravelly and cobbly sandy loam with intermittent layers of sandy loam and loam. The most limiting layer within the complex is reported to have the capacity to transmit 0.71 to 2.13 inches per hour of water based on the mapping. In general, the subsurface conditions observed at the project site are not consistent with the NRCS mapping.

4.3 SEISMICITY

ALLWEST anticipates the 2018 International Building Code (IBC) will be used as the basis for design of the proposed improvements as part of this project. Based on laboratory testing results, boring information, and knowledge of the local geology, the natural soils at the site can be characterized as Site Class D for seismic design, in accordance with the previously referenced standard. Soils characterized as Site Class D have a generally stiff relative consistency, with average standard penetration resistance values ranging from 15 to 50 blows per foot in the upper 100 feet. These blow counts correlate to average undrained shear strengths of 1,000 to 2,000 pounds per square foot (psf).

The following seismic parameters may be used for design of the proposed structure:

Parameter Value Description Latitude 44.969270° Project site geographic position

Longitude -110.687102° Project site geographic position Seismic Site Class D Seismic Design Site Classification

Risk Category II Seismic design risk category SS 0.988 MCER ground motion (period = 0.2s) S1 0.282 MCER ground motion (period = 1.0s) SDS 0.728 Numeric seismic design value at 0.2s SA SD1 0.345 Numeric seismic design value at 1.0s SA Fa 1.105 Site amplification factor at 0.2s Fv 1.836 Site amplification factor at 1.0s

PGA 0.384 MCEG peak ground acceleration FPGA 1.116 Site amplification factor at PGA

PGAM 0.429 Site modified peak ground acceleration

5.0 SUBSURFACE CONDITIONS

General characterization of the subsurface profiles observed follows, grouping soils with similar physical and engineering properties. Boring logs should be referenced for more

Mammoth Wastewater Improvements Page 4 detailed descriptions of the soil types and their estimated depths. It should be noted that depths shown as boundaries between various strata on boring logs are approximate.

Transitions between soil types/layers may be gradual. In addition, subsurface conditions may vary between exploration locations from those observed at discrete boring locations. Such changes in conditions would not be apparent until construction. If subsurface conditions deviate from those observed in the borings, construction timing, plans, and costs may change.

The general subsurface soil profile observed in the borings conducted in the area of the proposed MBR facility consisted of a thin topsoil layer overlying silt containing variable sand and gravel content. Below the silt, a layer of gravel or sand was encountered. Below the granular layer, additional silt with varying sand and gravel content or silty sand with varying gravel content was observed to the maximum exploration depth (approximately 31 feet). Silty clay with varied sand and gravel content was also observed at various depths in boring BH- 09.

In the area of the proposed percolation ponds, subsurface conditions observed generally consisted of a thin topsoil layer overlying silt containing variable sand and gravel content to depths of approximately 4.5 to 7 feet. Below the silt, lean clay with varied silt, sand, and gravel content was observed to depths on the order of 10 to 18 feet. Sand with variable silt and gravel content was then observed to the maximum exploration depth (21.5 feet).

5.1 SILT

Silt with varied sand and gravel content was observed in each of the borings below topsoil to depths ranging from approximately 4 to 22 feet, occasionally interrupted by sand or clay layers. The silt was characterized as varying in color from light brown to dark brown and gray, generally slightly moist to moist above the measured water table, and low plasticity to non-plastic. Field blow counts (uncorrected N values) ranging from 4 to greater than 50 blows per foot were recorded in the silt strata, indicating a medium stiff to hard soil.

5.2 CLAY

Lean clay with sand varied amounts of silt, sand, and gravel was encountered in borings BH- 07 and BH-08 below layers of silt. The clay was observed to be gray to brown in color, very moist, and exhibited low to medium plasticity. The clay is generally characterized as soft to medium stiff (uncorrected N values generally ranging from 3 to 13), with one value recorded as 34 blows per foot (hard).

5.3 GRAVEL

Poorly graded gravel with sand or poorly graded gravel with silt and sand was observed in borings BH-01 and BH-02, respectively. The gravel was encountered below layers of silt at depths on the order of 10.5 and 15 feet, and was generally 0.5 to 1.5 feet in thickness. The gravel was noted as brown, moist to wet, fine to coarse grained, and subangular to subrounded. Uncorrected N values recorded in the gravel (greater than 50 blows per foot) indicate a very dense relative density.

5.4 SAND

Sand with varied amounts of silt, clay, and gravel was encountered beginning from depths of approximately 3.5 to 18 feet below grade in each of the borings except for BH-04 and BH-06.

Mammoth Wastewater Improvements Page 5

The sand was gray to dark brown in general, very moist to wet, and medium to coarse grained in general, and contained occasional cobbles. Uncorrected N values generally ranged from 26 to greater than 50 blows per foot recorded in the material indicate the sandy strata to be medium dense to very dense, however, blow counts in layers of clayey sand with gravel encountered in BH-05 and BH-09 generally ranged from 4 to 9 blows per foot, indicating a loose relative density.

5.5 GROUNDWATER CONDITIONS

Groundwater was observed in borings BH-01, BH-02, BH-03, BH-08, and BH-09 at depths ranging from approximately 9.5 to 15 feet during drilling (August 2022). Changes in precipitation, irrigation, construction, or other factors may impact depth to groundwater and surface water flow on the property and therefore, conditions may be different during construction.

6.0 INFILTRATION TESTING

In-situ infiltration testing was performed at five locations throughout the project site to assist in percolation pond design. Approximate locations of the infiltration tests are shown on Figure A-2, Boring and Infiltration Test Location Map in Appendix A. Three infiltration tests were performed in general accordance with the methods outlined in WYDEQ Chapter 25, Appendix A (two tests performed near the proposed percolation ponds, one test performed near the existing percolation pond), while two double-ring infiltration tests in general accordance with the methods outlined in ASTM D3385 near the proposed percolation ponds.

At two of the testing locations in the area of the proposed percolation ponds, National Park Service (NPS) personnel excavated overburden soils to depths of approximately 1 foot (east location) and 4.5 feet (west location – approximately 6 inches deeper than desired) below existing grades spanning a horizontal area of approximately 4 feet by 4 feet using a rubber-tired backhoe. Following excavation to the approximate desired depths as determined by HDR, Inc., or from the surface at the testing location near the existing percolation pond, ALLWEST excavated a smaller area sized approximately 12 inches in diameter by 18 inches in depth below the previously excavated region. The smaller excavations were hand-excavated to an approximate 18-inch depth to allow for the prescribed pre-soak depth described the testing methods outlined in WYDEQ Chapter 25. Approximately 2 to 3 inches of pea gravel was placed in the bottom of the 12-inch diameter excavations to act as a splash guard.

At the east infiltration testing location, natural groundwater was encountered at a depth on the order of 6 feet below surrounding grade. As such, ALLWEST re-excavated the smaller approximately 12-inch diameter by 18-inch-deep hole further up the ramp slope from a depth of approximately 3 feet below surrounding grade to avoid influence from groundwater.

Following excavation and placement of pea gravel, water was introduced into the approximately 12-inch diameter holes, commencing a four-hour saturation period using an approximate 18-inch head of water. Following the saturation period of four hours, an approximate 12-inch head of water was used to begin each trial, and the distance of the water column drop during a 10-minute trial was recorded. The final trial following three consecutive

Mammoth Wastewater Improvements Page 6 trials not varying by more than ⅛-inch from one another is reported as the infiltration rate.

These data are presented in the following table. It is recommended the civil engineer apply appropriate factors of safety to the measured values or select lower values for design based on previously observed and documented performance of percolation into these soils in the vicinity of the project.

Test Location

Latitude Longitude Depth of Test Below Ground Surface (in)

Infiltration Rate (in/hr)

Near BH-06 44.969317 -110.687855 48 12

Near BH-08 44.969018 -110.686243 30 9½ Near Existing Pond No. 2

44.966176 -110.681969 18 7¼

Two additional locations near the proposed percolation ponds and near borings BH-06 and BH-08 were tested using a double-ring infiltrometer. At each test location, a driving plate was placed on top of two concentric steel rings and the rings were driven into the ground using a sledgehammer to a depth of up to 6 inches if ground conditions were conducive to do so. The concentric steel rings have diameters of approximately 12 and 24 inches. Following driving, Mariotte tubes filled with water of known volume were connected to each of the steel rings.

Water was used to fill the inner and outer rings to an arbitrary height and equalized. Once the fluid levels were constant, the test began with the release of water from the Mariotte tubes to maintain a constant head of water within the rings. Readings of volume consumed by each ring were recorded at regular time intervals until a relatively constant rate was obtained. A summary of the double-ring infiltration testing results is provided in Appendix D.

7.0 LABORATORY TESTING

ALLWEST performed laboratory testing to supplement field classifications and to assess the appropriate soil engineering properties for use in design of the proposed improvements.

The laboratory testing program conducted for this evaluation included the following tests:

Test Performed: Information Acquired:

Natural Water Content

(ASTM D2216)

Water content representative of soil conditions at the time and location samples were collected

Particle-size Distribution

(ASTM D6913)

Size and distribution of soil particles (i.e., gravel, sand, and silt/clay) of a particular sample

Atterberg Limits

(ASTM D4318)

Effects of varying water content on the consistency of fine-grained soils present in a particular sample

Moisture-Density Relationship

(ASTM D698)

Relationship between the laboratory maximum dry density and corresponding water content of a soil for a particular compaction effort

Mammoth Wastewater Improvements Page 7

Laboratory test results are presented and summarized in Appendix C. The following sections discuss some of the laboratory testing results in detail.

7.1 WATER CONTENT

Natural water content of tested split spoon samples of the various materials observed in the borings above the observed groundwater table varied from 6 to 35 percent at the time of drilling (August 2022). Boring logs should be referenced for more detailed natural water content information of tested samples and their associated depth.

7.2 CLASSIFICATION

Gradation analysis in conjunction with Atterberg limits testing was performed on disturbed bulk samples of representative material collected from auger cuttings returned to the surface from borings BH-03 (1 to 5 feet), BH-05 (5 to 9 feet), and BH-08 (5 to 9 feet). Atterberg limit testing indicates the tested materials to have liquid limits ranging from 29 to 44 percent and plasticity indices ranging from of 11 to 19 percent. The testing determined material classifications of sandy silt, clayey sand with gravel, and lean clay with sand (Figures C-1 through C-3, Appendix C).

7.3 MOISTURE-DENSITY RELATIONSHIP

Moisture-density relationship testing was performed on disturbed bulk samples obtained from borings BH-03 (1 to 5 feet) and BH-08 (5 to 9 feet) from auger cuttings returned to the surface.

The testing performed indicates the material tested from BH-03 to have a rock corrected maximum dry density of approximately 102.9 pounds per cubic foot (pcf) at an optimum moisture content of approximately 19.6 percent (Figure C-4, Appendix C). At the time of this report, moisture-density relationship testing was not complete for the sample from BH-08.

Results will be provided upon completion.

8.0 CONCLUSIONS AND RECOMMENDATIONS

The following conclusions and recommendations are presented to assist in planning and design of the proposed improvements. Recommendations are based on ALLWEST’s understanding of the proposed construction, conditions observed in the borings, laboratory testing, and engineering analyses. If the construction scope changes, or if conditions are encountered during construction which are different than those described in this report, ALLWEST should be notified so the recommendations herein can be reviewed revisions can be provided, if necessary. Additionally, ALLWEST should be given the opportunity to review plans and specifications to determine whether the recommendations presented in this report were properly incorporated as intended.

8.1 SITE GRADING

The following recommendations are provided for site grading considerations.

8.1.1 Clearing and Stripping

Prior to placement of fill, the site should be stripped of topsoil and organics. Based on observations in the borings, the stripping depth for removal of topsoil or organics is estimated to be less than 6 inches.

Mammoth Wastewater Improvements Page 8

8.1.2 Excavation

Based on conditions observed in the borings, it is anticipated that excavation of the on-site soil can be achieved with typical heavy-duty excavation equipment. Cobbles and boulders were encountered during the subsurface exploration at various locations and depths and should be anticipated by the earthwork contractor. Difficult excavation conditions should be anticipated in sporadic locations due to the presence of cobbles, boulders, and potential bedrock. The use of specialty excavation equipment may be required to excavate to the required depths if such conditions are encountered during construction.

Unsupported vertical slopes or cuts deeper than 4 feet are not recommended if worker access is necessary. Cuts should be adequately sloped, shored, or supported to prevent injury to personnel from local sloughing and spalling. Excavations should conform to applicable federal, state, and local regulations. Regarding trench wall support, the site soil is considered Type C soil according to OSHA guidelines and therefore should not exceed a 1.5H:1V temporary slope.

Dewatering may be required for construction depending on the season and depths required for construction of the proposed improvements. Dewatering means and methods will be the responsibility of the selected earthwork contractor. As a minimum, dewatering should extend at least 2 feet below the bottoms of excavations.

8.1.3 Subgrade Preparation

ALLWEST defines the subgrade as the native soil exposed at the base of excavation prior to placement of fill. Natural soils at subgrade elevations for the various proposed improvements are anticipated to vary depending on the specific location but are generally anticipated to consist of silt with varying sand and gravel content. Boring logs should be referenced for more detailed descriptions of the soil types and estimated depths.

The subgrade requires an evaluation by the geotechnical engineer-of-record or staff under their supervision to confirm the site conditions are consistent with those observed during our geotechnical evaluation. Following clearing and stripping, the subgrade should be compacted to a firm and unyielding condition and proof rolled with heavy rubber-tired construction equipment such as a loader with a full bucket or a loaded dump truck.

The site soils are susceptible to pumping and rutting if subjected to significant and repeated traffic by rubber tire construction equipment. It is recommended tracked construction equipment be used to traffic the site and rubber tire equipment be limited to haul routes.

In the event the exposed subgrade becomes unstable, yielding, or unable to be compacted due to high moisture conditions or construction traffic, the materials should be removed to a sufficient depth to develop stable subgrade soils that can be compacted to the minimum recommended levels. The severity of construction problems will be dependent, in part, on the precautions that are taken by the contractor to protect the subgrade soils.

Prior to placement of construction of embankments, or placement of imported granular structural fill where necessary, the natural subgrade soils should be properly moisture conditioned and compacted to a firm and unyielding condition. Moisture conditioning of the subgrade surface may involve wetting or drying of the soil to help facilitate compaction. No

Mammoth Wastewater Improvements Page 9 moisture specification for subgrade soil preparation is provided herein but the earthwork contractor should adhere to typical good practice and not attempt to compact soils that are visually either too dry or too moist. Please refer to the in-situ moisture content laboratory test results for an estimation of existing soil-moisture conditions (at the time of exploration).

Weather conditions should be given careful attention during subgrade preparation to prevent excess moisture from collecting on or penetrating and possibly saturating the subgrade before and after compaction. It is recommended that the subgrade be temporarily sloped to provide drainage to a low area of the excavation and any excess water pumped from the excavation.

Such collection and discharge must be in compliance with the Contractor’s site-specific storm water pollution prevention plan (SWPPP). Should portions of the subgrade become excessively saturated, those areas should be sufficiently excavated, replaced with properly moisture conditioned soil, and properly compacted.

8.1.4 Cut and Fill Slopes

Based on information received by HDR, Inc., embankment fill slopes up to approximately 5 to 8 feet tall and cut slopes up to approximately 5 to 6 feet tall are anticipated. Fill slopes constructed with on-site soil should be no steeper than 3:1 (horizontal:vertical). Final grading requirements are not known to ALLWEST at the time of preparation of this report. If final grading requirements are substantially different than described herein, ALLWEST should be notified to review the final grading plan and revise the recommendations herein, if necessary.

New embankment fill material should be keyed into the existing soils using proper benching techniques, as shown on Figure F-1, Typical Fill Benching Detail, Appendix F. Fill slopes should be over-constructed and cut back to the final grade to allow for a properly compacted slope face. Cut slopes should be no steeper than 3:1. Exposed soil slopes should be protected from erosion as soon as practical after construction. Erosion protection could consist of vegetation, wattles, erosion control blankets, riprap, etc. Topsoil may be placed on exposed cut or fill slope faces to assist with growth of vegetation to reduce erosion potential.

8.1.5 Materials

8.1.5.1 On-site Soil

The on-site soils present in the project area to the anticipated excavation depths are considered suitable for re-use as engineered fill for construction of embankments for the percolation ponds. Depending upon the specific requirements of the embankments, clay or other low-permeability soil may need to supplement the natural soils to achieve desired construction and transmissivity/permeability requirements.

Upon excavation, the excavated on-site materials should be thoroughly mixed to achieve a uniform texture and composition prior to placement as embankment fill. The excavated soils should be periodically observed by the geotechnical engineer to evaluate excavated materials and to determine the appropriateness for use as embankment fill. If encountered, oversize cobbles and boulders larger than 6 inches should be removed from on-site materials and should not be used within the embankment.

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8.1.5.2 Import Soil

Import soil, where required, should approved by the Geotechnical Engineer prior to delivery to the site. Import material should be free of organics, debris, and other deleterious material.

In addition, import soil used for construction of embankments should be thoroughly mixed with the available on-site soils prior to placement as fill to achieve a uniform texture and composition.

Should structural fill be necessary as part of this project, it should consist of an imported well-graded granular soil and meet the requirements presented in the following table.

Fill Type Recommendations

Import Granular Structural Fill1,2

Sieve Percent Passing 3-inch 100 ¾-inch 70 – 100 No. 40 10 – 20

No. 200 0 – 15 1 Soils with more than 30% retained on the ¾-inch sieve are considered ‘oversized’ and may require method-based compaction methods.

2 Material should be non-plastic

8.1.6 Fill Placement and Compaction

Fill should be placed in lift thicknesses appropriate for the compaction equipment used.

Typically, six to eight-inch loose lifts are appropriate for typical rubber tire and steel drum compaction equipment. Lift thicknesses should be reduced to a maximum of four inches for hand operated compaction equipment.

The on-site silty and clayey soils will be very sensitive to changes in moisture content and great care should be taken to keep the material at or near optimum moisture content.

Fill should be moisture conditioned to within 3 percent of the optimum moisture content prior to placement to facilitate compaction. Material excavated from near and below the groundwater table will require drying prior to placement as fill.

Fill placed for on-site improvements should be compacted to the following percentages of the maximum dry density as determined by ASTM D698 (standard Proctor).

Fill Area Compaction (%) Site Grading (i.e., embankments) 95 Utility Trench Backfill 95

8.1.7 Wet Weather Construction

Due to the climatic effects in this region during late fall, winter, and spring (generally wet conditions), it is recommended that construction (especially site grading) take place during the summer and early fall season, if possible. If construction occurs during or immediately after excessive precipitation, it may be necessary to over-excavate and replace wet subgrade soil which might otherwise be suitable.

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If construction is undertaken in wet periods of the year, it will be important to slope the ground surface to provide drainage away from construction. In addition, groundwater levels will likely be higher during wet periods of the year.

8.1.8 Cold Weather Construction

If construction is anticipated during cold weather, proper winter construction practices should be observed. Snow and ice should be removed from excavated and fill areas (i.e., utility trenches, roadway subgrade) prior to additional earthwork or construction. Frozen soils should not be used as fill.

8.2 SLOPE STABILITY ANALYSIS

Models of the proposed embankment slope topography and assumed subsurface geometry were developed to perform stability analyses using the slope stability analysis software SLIDE developed by RocScience, Inc. SLIDE performs iterations of calculations utilizing various industry accepted analysis methods to determine possible failure surfaces and the associated factor(s) of safety. Calculations performed by the software are largely based on surface and subsurface geometry, soil strength characteristics, and groundwater considerations.

Slope conditions for this project were modeled based on the proposed construction, soils and groundwater information observed in the exploratory borings, and assumed soil strength properties based on boring information, available laboratory testing, previous experience similar soils, and published correlations.

The stability analyses included the following assumptions:

1. The topographical profile (existing and proposed) used for the analyses was based on the preliminary site plan (sheet C6), provided by HDR, Inc. via email September 7, 2022. The cross section used for stability modeling was approximately perpendicular to the proposed embankments, designated as Cut C1 on sheet C4, shown on Figure A-2 in Appendix A.

2. The subsurface materials consist of materials similar in both lateral and vertical extent across the construction area to those encountered and sampled during the subsurface exploration.

3. Embankment materials will consist of a relatively homogeneous blend of excavated on-site soils moisture conditioned and placed in accordance with the recommendations provided herein.

Soil strength parameters used in the slope stability analyses were developed based on engineering properties correlated from standard penetration testing (SPT) values obtained during the subsurface explorations, available laboratory testing, and ALLWEST’s previous experience with similar soil types in this area.

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Soil parameters used in the stability analyses are presented in the following table.

Material Unit Weight

(lb/ft3) Friction Angle

(degrees) Cohesion

(lb/ft2) Embankment 105 23 300

Gravelly/Sandy Silt 100 25 350 Silty Clay with Sand/Gravel 120 20 400

Silty Sand 115 28 200

The following table presents a summary of the global slope stability analysis for the cases modeled in SLIDE. Cases were modeled under static and pseudo-static (seismic) conditions.

Analysis Case

Factor of Safety

Static

Static (Saturated

Embankment) Seismic

Seismic (Saturated

Embankment)

Existing Topography >10.00 - 1.84 -

Proposed Topography 5.54 4.38 2.09 1.43

Factors of safety of 1.5 and 1.1 or greater are typically considered acceptable for permanent slopes under static and seismic conditions, respectively. The seismic coefficient is assumed as 40 percent of the SDS value (assumed to be 0.728g at this project location). A horizontal acceleration of 0.291g was therefore utilized for the slope stability analyses.

The slope stability analysis performed for the existing topography indicates the slope is stable in its current condition. Analyses performed for the proposed embankment topography indicate factors of safety in excess of 1.5 and 1.1 for static and seismic conditions, respectively, suggesting the embankments will remain stable or the desired lifetime of the structure. Graphical depiction of the results of each of the analysis cases modeled are presented in Appendix E (Figures E-1 through E-6).

8.3 STORMWATER AND DRAINAGE

The ground surface adjacent to foundations should be sloped a minimum of five percent within 10 feet of structures. If the adjoining ground surface consists of hardscapes it may be sloped a minimum of two percent in the first 10 feet. Water should not be allowed to infiltrate or pond adjacent to foundations.

Exterior slopes of pond embankments should be vegetated as soon as possible after grading to help reduce the potential for erosion caused by stormwater runoff. Placement of jute netting or similar should be considered in conjunction with hydroseeding. Over- and under-watering of slopes should be avoided during vegetation establishment.

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9.0 ADDITIONAL RECOMMENDED SERVICES

ALLWEST should be retained to provide construction materials testing and observation to verify the soil and geologic conditions and the report recommendations are incorporated into the actual construction. The design engineer-of-record should determine applicable testing and special inspection requirements in accordance with the governing code documents. If ALLWEST is not retained to provide required construction observation and materials testing services, ALLWEST cannot be responsible for soil engineering related construction errors or omissions.

10.0 EVALUATION LIMITATIONS

This report has been prepared to assist the planning and design for the proposed Mammoth Wastewater Improvements project located in Mammoth, Yellowstone National Park, Wyoming. The evaluation was provided based on preliminary plans that were made available at the time of exploration. The geotechnical engineer must be informed of significant changes that differ from the assumptions stated in this report. Reliance by any other party is prohibited without the written authorization of ALLWEST. Services consist of professional opinions and conclusions made in accordance with generally accepted geotechnical engineering principles and practices in the local area at the time this report was prepared. This acknowledgement is in lieu of all warranties, express or implied.

Geotechnical-Engineering Report Important Information about This

Subsurface problems are a principal cause of construction delays, cost overruns, claims, and disputes.

While you cannot eliminate all such risks, you can manage them. The following information is provided to help.

The Geoprofessional Business Association (GBA) has prepared this advisory to help you – assumedly a client representative – interpret and apply this geotechnical-engineering report as effectively as exposure to problems associated with subsurface conditions at project sites and development of them that, for decades, have been a principal cause of construction delays, cost overruns, claims, and disputes. If you have questions or want more information about any of the issues discussed herein, contact your GBA-member geotechnical engineer.

Active engagement in GBA exposes geotechnical engineers to a wide array of risk-confrontation everyone involved with a construction project.

Understand the Geotechnical-Engineering Services Provided for this Report Geotechnical-engineering services typically include the planning, collection, interpretation, and analysis of exploratory data from widely spaced borings and/or test pits. Field data are combined with results from laboratory tests of soil and rock samples obtained from field exploration (if applicable), observations made during site reconnaissance, and historical information to form one or more models of the expected subsurface conditions beneath the site. Local geology and alterations of the site surface and subsurface by previous and proposed construction are also important considerations. Geotechnical engineers apply their engineering training, experience, and judgment to adapt the requirements of the prospective project to the subsurface model(s). Estimates are made of the subsurface conditions that will likely be exposed during construction as well as the expected performance of foundations and other structures being planned and/or affected by construction activities.

The culmination of these geotechnical-engineering services is typically a geotechnical-engineering report providing the data obtained, a discussion of the subsurface model(s), the engineering and geologic engineering assessments and analyses made, and the recommendations developed to satisfy the given requirements of the project. These reports may be titled investigations, explorations, studies, assessments, or evaluations.

Regardless of the title used, the geotechnical-engineering report is an engineering interpretation of the subsurface conditions within the context of the project and does not represent a close examination, systematic inquiry, or thorough investigation of all site and subsurface conditions.

Geotechnical-Engineering Services are Performed

Geotechnical engineers structure their services to meet the specific needs, goals, and risk management preferences of their clients. A geotechnical-engineering study conducted for a given civil engineer will not likely meet the needs of a civil-works constructor or even a different civil engineer. Because each geotechnical-engineering study is unique, each geotechnical-engineering report is unique, prepared solely for the client.

Likewise, geotechnical-engineering services are performed for a specific project and purpose. For example, it is unlikely that a geotechnical-engineering study for a refrigerated warehouse will be the same as one prepared for a parking garage; and a few borings drilled during a preliminary study to evaluate site feasibility will not be adequate to develop geotechnical design recommendations for the project.

Do not rely on this report if your geotechnical engineer prepared it:

• for a different client;

• for a different project or purpose;

• for a different site (that may or may not include all or a portion of the original site); or

• before important events occurred at the site or adjacent to it;

e.g., man-made events like construction or environmental remediation, or natural events like floods, droughts, earthquakes, or groundwater fluctuations.

Note, too, the reliability of a geotechnical-engineering report can be affected by the passage of time, because of factors like changed subsurface conditions; new or modified codes, standards, or regulations; or new techniques or tools. If you are the least bit uncertain about the continued reliability of this report, contact your geotechnical engineer before applying the recommendations in it. A minor amount of additional testing or analysis after the passage of time – if any is required at all – could prevent major problems.

Read this Report in Full Costly problems have occurred because those relying on a geotechnical-engineering report did not read the report in its entirety. Do not rely on an executive summary. Do not read selective elements only. Read and refer to the report in full.

You Need to Inform Your Geotechnical Engineer About Change Your geotechnical engineer considered unique, project-specific factors when developing the scope of study behind this report and developing the confirmation-dependent recommendations the report conveys.

Typical changes that could erode the reliability of this report include those that affect:

• the site’s size or…