Att 8 Geotechnical Subsurface Investigation.pdf

Project 623-20-123 Campus Global Stability Assessment

Jack C. Montgomery

VAMC

Muskogee, Oklahoma

Specialized Engineering Solutions Omaha, Nebraska

February 17, 2023

TTL Project No. 2257001

TTL Associates, Inc.

1915 North 12th Street Toledo, OH 43604-5305 419-324-2222 www.ttlassoc.com

Final Report Geotechnical Subsurface Investigation

1915 North 12th Street

Toledo, OH 43604-5305

T 419-324-2222

F 419-241-1808 www.ttlassoc.com

Teamwork - Trust - Leadership Since 1927

February 17, 2023 TTL Project No. 2257001

Mr. Brad Carne Specialized Engineering Solutions 10360 Ellison Circle Omaha, Nebraska 68134

Final Report Geotechnical Subsurface Investigation

Project 623-20-123 Campus Global Stability Assessment Jack C. Montgomery VAMC

Muskogee, Oklahoma

Dear Mr. Carne:

Following is the report of the geotechnical subsurface investigation performed by TTL Associates, Inc. (TTL) under the direction of Toledo Testing Laboratories, Inc. for the referenced project. This investigation was performed in general accordance with TTL Proposal No. 2257001R, dated July 21, 2022, and was authorized with a subcontract agreement prepared by Specialized Engineering Solutions (SES), dated August 8, 2022.

Change Order No. 1 for additional drilling mobilization and reduction of the scope by one test boring was provided September 28, 2022.

This final report contains the results of our study, our engineering interpretation of the results with respect to the project characteristics, evaluations and conclusions regarding on-site slope stability, consideration of areas of concern, and potential options to provide mitigation of areas with slope stability concerns. A draft version of this report was provided December 21, 2022 for review by SES and VA. You provided via email on January 26, 2023 authorization to finalize the report, indicating there were no comments to the draft version of the report.

Soil samples collected during this investigation will be stored at our laboratory for 45 days from the date of this report. The samples will be discarded after this time unless you request that they be saved or delivered to you.

Should you have any questions regarding this report or require additional information, please contact our office.

Sincerely, TTL Associates, Inc.

Christopher P. Iott Curtis E. Roupe, P.E.

Geotechnical Project Manager Vice President

T:\Projects\2257001 \Reports and Other Deliverables\2257001 TTL FINAL Geotech Report VAMC Campus Global Stability Muskogee OK.doc

FINAL REPORT

GEOTECHNICAL SUBSURFACE INVESTIGATION

PROJECT 623-20-123 CAMPUS GLOBAL STABILITY ASSESSMENT

JACK C. MONTGOMERY VAMC

MUSKOGEE, OKLAHOMA

FOR

SPECIALIZED ENGINEERING SOLUTIONS

10360 ELLISON CIRCLE

OMAHA, NEBRASKA 68134

SUBMITTED

FEBRUARY 17, 2023

TTL PROJECT NO. 2257001

TTL ASSOCIATES, INC.

1915 NORTH 12TH STREET

TOLEDO, OHIO 43604

(419) 324-2222

(419) 321-6257 (FAX)

Specialized Engineering Solutions February 2023 TTL Project No. 2257001 Page i

TABLE OF CONTENTS

Page No.

1.0 Introduction

2.0 Investigative Procedures

3.0 Project Description

4.0 General Site and Subsurface Conditions

4.1 General Site Conditions

4.2 General Subsurface Conditions

4.3 Groundwater Conditions

5.0 Evaluations and Recommendations

5.1 Global Stability

5.2 Areas of Concern and Potential Mitigation

5.2.1 Site Preparation

5.2.2 Fill

6.0 Qualification of Recommendations

PLATES

Plate 1.0 Site Location Map Plate 2.0 Test Boring Location Plan

FIGURES

Logs of Test Borings Legend Key Grain Size Distribution Proctor Results Direct Shear Test Results Ground Surface Topography Survey Results Global Stability Output

TTL Project No. 2257001 Page 1

1.0 INTRODUCTION

This geotechnical subsurface investigation report has been prepared for the campus global stability assessment (Project 623-20-123) at the Jack C. Montgomery Veterans

Administration Medical Center (VAMC) in Muskogee, Oklahoma. Assessment was requested for the north, east, and south sides of the VAMC campus, and was limited to the on-site campus rather than extending downslopes beyond the campus limits. The VAMC campus is located at 1011 Honor Heights Drive, as shown on the attached Site Location Map

(Plate 1.0).

This report summarizes our understanding of the project characteristics, describes the investigative and testing procedures, presents the findings, as well as discusses our evaluations and conclusions evaluations and conclusions regarding on-site slope stability, consideration of areas of concern, and potential options to provide mitigation of areas with slope stability concerns.

This investigation was performed in general accordance with TTL Proposal No. 2257001R, dated July 21, 2022, and was authorized with a subcontract agreement prepared by

Specialized Engineering Solutions (SES), dated August 8, 2022. Change Order No. 1 for additional drilling mobilization and reduction of the scope by one test boring was provided

September 28, 2022.

The purpose of this investigation was to evaluate the subsurface conditions at this site to provide assessment regarding campus global stability for the referenced project. To accomplish this, TTL performed six test borings, field and laboratory soil testing, a geotechnical engineering evaluation of the test results, and review of available soils and bedrock information for the project vicinity.

This report includes:

A description of the subsurface soil, rock, and groundwater conditions encountered at the boring locations.

Evaluation of campus global stability.

This investigation did not include an environmental assessment of the subsurface materials at this site.

TTL Project No. 2257001 Page 2

2.0 INVESTIGATIVE PROCEDURES

This investigation included six test borings, designated as B-1 through B-6, which were drilled by Drilling Services of Oklahoma on October 20, 2022, under the direction of TTL.

The borings were performed at various elevations throughout the northern, eastern, and southern portions of the VAMC campus. The test borings were located in the field by a

Ground Penetrating Radar (GPR) subconsultant to TTL, based on a proposed boring location plan that was reviewed and approved by VA. The boring locations were then scanned using

GPR to clear existing underground structures and utilities.

As part of the project, TTL subcontracted with Honor Engineer Co. (Honor) to obtain ground surface topography for the site to support global stability evaluations. A plan showing the ground surface topography with contours at 1 foot intervals, as well as cut cross-sections from the north going southeast and from the west going east, which were prepared by Honor, are attached to this report. It should be noted that the boring locations shown on the survey were originally located by the surveyor but were subsequently relocated due to presence of existing underground utilities in those areas.

Ground surface elevations at the as-drilled boring locations were interpolated to the nearest foot based on the Honor plan containing ground surface topography. It should be noted that

Boring B-4 was located beyond the extents of the survey. However, LiDAR was performed as part of the survey, and Boring B-4 was within the LiDAR area. The ground surface elevation of Boring B-4 was provided to TTL by Honor based on the LiDAR results. The approximate locations of the as-drilled borings, as well as an aerial of the site, are shown on the Test Boring Location Plan (Plate 2.0).

The borings were drilled with a drill rig utilizing 6-inch diameter augers. The borings were extended to auger refusal at depths ranging from 14 to 28½ feet below existing grade. During auger advancement, soil samples were generally collected at 2½-foot intervals to a depth of

10 feet, and at 5-foot intervals thereafter to boring termination. Due to split-spoon refusal

(SSR, 50 or more blows for 6 inches or less penetration during a Standard Penetration Test

(SPT) as described below) over the first four sample intervals in Boring B-3, the boring was advanced without additional sampling to auger refusal at a depth of 14 feet. Split-spoon (SS) samples were obtained by the SPT Method (ASTM D 1586), which consists of driving a 2-inch outside diameter split-barrel sampler into the soil with a 140-pound weight falling freely through a distance of 30 inches. The sampler was driven in three successive 6-inch increments with the number of blows per increment being recorded. The sum of the number

TTL Project No. 2257001 Page 3 of blows required to advance the sampler the second and third 6-inch increments is termed the Standard Penetration Resistance (N-value) and is presented on the Logs of Test Borings attached to this report. The samples were collected in re-sealable bags and transported to our laboratory for further classification and testing.

Bulk samples from the auger cuttings were obtained from four selected borings. The samples were collected in 5-gallon buckets and returned to our laboratory for further classification and testing of selected samples.

All recovered samples were visually or manually classified in accordance with the Unified

Soil Classification System (USCS) (ASTM D 2487 and D 2488) and tested in our laboratory for moisture content (ASTM D 2216). Unconfined compressive strength estimates were obtained for the intact cohesive samples using a calibrated hand penetrometer. These test results are presented on the Logs of Test Borings attached to this report.

Standard Proctor moisture-density relationship tests (ASTM D 698) and direct shear tests

(ASTM D 3080) were performed on bulk samples from Borings B-1 and B-2. The direct shear tests were performed using normal pressures of 500; 1,000; and 2,000 pounds per square foot (psf). These test results are attached to this report.

Atterberg limits tests (ASTM D 4318) and particle size analyses (ASTM D 6913 and D

7928) were performed on selected samples to determine soil classification and index properties. These test results are presented on the Logs of Test Borings and Grain Size

Distribution sheets attached to this report.

Soil conditions encountered in the test borings are presented in the Logs of Test Borings, along with information related to sample data, SPT results, water conditions observed in the borings, and laboratory test data. It should be noted that these logs have been prepared on the basis of laboratory classification and testing as well as field logs of the encountered soils.

Experience indicates that the actual subsoil conditions at a site could vary from those generalized on the basis of test borings made at specific locations. Therefore, it is essential that a geotechnical engineer be retained to provide soil engineering services during any site improvement projects. This is to observe compliance with the design concepts, specifications, and recommendations, and to allow design changes in the event subsurface conditions differ from those anticipated prior to the start of construction.

TTL Project No. 2257001 Page 4

3.0 PROJECT DESCRIPTION

It is our understanding that the project consists of a global stability assessment of the Jack

C. Montgomery VA Medical Center (VAMC), located at 1011 Honor Heights Drive, in

Muskogee, Oklahoma. The VAMC campus includes 17 acres, but it was indicated that 14 acres of the campus is included in this study. It was indicated that the hillside parking lot currently under construction in the western portion of the site is not included in this study. As such, the study will consist of global stability assessment in the northern, eastern, and southern portions of campus.

The campus includes 24 buildings constructed from 1922 to 2014, as well as various parking lots and access roads. Grades on campus vary from approximately Elevs. 745± in the north, to 650± in the east, to 745± in the south, to 715± in the west (pre-construction of new parking lot).

Grades surrounding the campus slope down in all directions for approximately ½ mile to 1 mile. The toe grades of the slopes are on the order of Elevs. 570 in the north, 590 in the east, and 640 in the south. The slopes extending down from campus are estimated to be generally on the order of 5 horizontal to 1 vertical (5H:1V) to 15H:1V, although some zones of steeper and flatter grades are present at the site.

It was indicated that this global stability study was requested since fill materials were encountered in the western portion of the campus as part of the hillside parking lot project that is currently under construction. There is concern that similar conditions may be present in other portions of the site. Therefore, exploration was requested to evaluate soil conditions at the campus that may be suspect to potential future slope instability. This exploration was requested to be limited to the VAMC campus, without evaluation of soils and bedrock conditions downslope from campus. Although instability of the slopes beyond campus would affect the global stability of the campus, it was indicated that there are not currently concerns regarding those slopes extending down from campus since they are relatively flat and there are not apparent signs of slope instability.

TTL Project No. 2257001 Page 5

4.0 GENERAL SITE AND SUBSURFACE CONDITIONS

4.1 General Site Conditions

General site conditions were discussed in Section 3.0.

The surface materials encountered in the borings consisted of topsoil on the order of 2 inches in thickness. Distinct surface cover was not present at the locations of Borings B-4 and B-6.

Existing fill materials were encountered in the upper 3½ feet in Borings B-4, B-5, and B-6.

In Borings B-4 and B-6, the fill consisted of very stiff to hard sandy silt (Layer 1 later in the text) for which SPT N-values of 18 blows per foot (bpf) and 30 bpf, as well as moisture contents of 9 percent and16 percent, were determined for the recovered samples. In Boring

B-5, the fill consisted of very dense silty, clayey gravel with sand (Layer 2 later in the text) for which a moisture content of 2 percent was determined for the recovered sample. The SPT within the gravel fill zone resulted in split-spoon refusal (SSR, 50 or more blows for 6 inches or less penetration). Non-soil materials consisting of roots and tree limb fragments were present in the fill materials.

4.2 General Subsurface Conditions

Based on the USDA Web Soil Survey, the near surface soils mapped at the site consist of

Eram-Coweta-Rock outcrop association consisting of residual materials weathered from sandstone and shale.

The stratigraphy encountered underlying the surface and existing fill materials consisted of sand and gravel exhibiting varying degrees of sandstone weathering, as well as clays and silts exhibiting varying degrees of shale weathering. The materials can be generally described as three layers of soils, identified in this text as Layers 3 through 5, as well as weathered shale.

Layer 3 consisted of predominantly medium dense granular soils encountered in Boring B-1 underlying the topsoil to a depth of 3½ feet, as well as from 8½ to 13½ feet, and in Boring B-

2 underlying Layer 4 soils from 3½ to 6 feet. These granular soils consisted of silty sand

(SM), silty, clayey sand (SC-SM), as well as silty, clayey gravel (GC-GM). SPT N-values ranged from 14 to 28 blows per foot (bpf). Moisture contents ranged from 4 to 13 percent.

TTL Project No. 2257001 Page 6

Layer 4 consisted of dense to very dense granular soils encountered underlying the topsoil in

Boring B-2 to a depth of 3½ feet and from 6 to 19 feet, underlying to topsoil in Boring B-3 to a depth of 6 feet, the fill in Borings B-4 and B-6 to auger refusal at a depth of 18.7 feet and to a depth of 6 feet, as well as from 3½ to 8½ feet in Boring B-1. These granular soils consisted of silty sand (SM). SPT N-values generally ranged from 39 to 73 bpf. Moisture contents ranged from of 2 to 14 percent.

Layer 5 consisted of hard cohesive soils encountered underlying the granular fill materials in

Boring B-5 to auger refusal at a depth of 19.3 feet, as well as underlying Layers 3 and 4 in

Borings B-1, B-3, and B-6, to depths ranging from 13½ to 18½ feet. These cohesive soils consisted of lean clay (CL) with varying amounts of sand and gravel, as well as silt (ML) with varying amounts of sand. The SPT resulted in N-values ranging from 32 to 82 bpf, as well as SSR. Unconfined compressive strengths for intact specimens were greater than 9,000 pounds per square foot (psf). Moisture contents ranged from of 8 to 15 percent.

Underlying Layer 5 in Borings B-1 and B-6, weathered shale bedrock was encountered to auger refusal at depths of 28 ½ feet and 19 feet, respectively.

4.3 Groundwater Conditions

Groundwater was initially encountered during drilling in only Boring B-1 at a depth of 6½ feet below existing grade (Elev. 646.5±), and upon completion of drilling at a depth of 19.3 feet (Elev. 634±). Groundwater was not encountered during drilling or observed upon completion of drilling in the remaining borings. Each of the borings was drilled and backfilled within the same day. Stabilized water levels may not have occurred over this limited time period. Instrumentation was not installed to observe long-term groundwater levels.

Based on the soil characteristics and groundwater conditions encountered in the borings, it is roundwater levels at the site are generally expected below the extent of the borings performed for this exploration. However, it should be noted that groundwater elevations can fluctuate with seasonal and climatic influences. In particular, perched water may be encountered in granular soils underlain by relatively impermeable cohesive soils, or at the soil/bedrock interface. The groundwater conditions encountered in

Boring B-1 appear to reflect perched conditions. Therefore, the groundwater conditions may vary at different times of the year from those encountered during this investigation.

TTL Project No. 2257001 Page 7

5.0 EVALUATIONS AND RECOMMENDATIONS

The following evaluations and recommendations are based on our understanding of the project and on the data obtained during the field investigation. If the project information or location as outlined is incorrect or should change significantly, a review of these recommendations should be made by TTL. These recommendations are subject to the satisfactory completion of the recommended site preparation and fill placement operations described herein.

5.1 Global Stability

Based on the topographic survey performed by Honor Engineer Co. as part of this investigation, cross-sections were prepared from the northern portion of the site extending east, as well as from the southern portion of the site to the east for purposes of global stability evaluations. The cut sections are shown on the attached Topographic Survey results.

This exploration was requested to be limited to the VAMC campus, without evaluation of soils and bedrock conditions downslope from campus. Although instability of the slopes beyond campus would affect the global stability of the campus, it was indicated that there are not currently concerns regarding those slopes extending down from campus since they are relatively flat and there are not apparent signs of slope instability.

With the exception of retaining walls, the majority of the campus grades are relatively flat.

Global stability evaluations were performed for both cross-sections in the relatively flat portions of the site. Grades slope down steeper to the area of Boring B-1. As such, global stability evaluations were also performed for both cross-sections for the steeper portion of the slope.

Recommended factors of safety for slopes are summarized as follows:

Table 5.1.A. Recommended Factors of Safety

Potential Method of Failure Recommended

Factor of Safety Global Stability

(Wall/Slope Not Supporting Structure on Shallow Foundations)

1.3

Global Stability (Wall/Slope Supporting Structure on Shallow Foundations)

End-of-Construction: 1.3

Long-Term: 1.5

TTL Project No. 2257001 Page 8

Global stability evaluations were performed using SLIDE. Analyses were performed using circular and block potential failure surfaces to determine factors of safety. Traffic surcharge loads were modeled in pavement areas in the upper portions of the evaluated cross-sections.

The retaining wall areas were assumed to be constructed with suitable internal and external factor of safety, so only global stability around the retaining wall areas was considered.

Analyses for the slopes were performed to establish long-term (drained) safety factors using effective stress soil parameters (ESSP) for the existing slope conditions. If new construction is performed, short-term, end-of-construction (undrained) safety factors using total stress soil parameters (TSSP) would need to be considered. Assuming those modifications would be relatively minor with respect to overall site grades, we also performed global stability evaluations for the end-of-construction conditions using the existing site grades.

Soil strengths were evaluated based on direct shear test results, hand penetrometer readings, as well as SPT N-values, moisture content, and soil plasticity data of the encountered soils.

Correlations with published data were also utilized to estimate soil properties.

It should be noted that existing soils at the site may have been excavated and used for re-grading at the site. As such, there may be areas of fill that are difficult to differentiate from native soils, particularly if they are placed in an engineered manner. In any case, based on the conditions encountered in the borings, the site soils would not be modeled differently if they were fill materials or native soils for the global stability evaluations.

It should be noted that the properties of the soil strata vary somewhat with layer and depth;

the layers and assigned soil properties used in the analyses are detailed in the attached figures that illustrate the slope geometry and potential critical failure surfaces associated with the global stability of the slopes.

Global stability factors of safety determined from our analyses are summarized in the following table.

TTL Project No. 2257001 Page 9

Table 5.1.B. Global Stability Evaluation Results

Evaluation Location

Factor of Safety Short-Term, Undrained Case (End-of-Construction)

Long-Term, Drained Case

(Post-Construction) Existing North to East Slope (B-4 to B-1) Lower Steep Portion

1.405 Shallow Sloughing: 1.303

Deep Seated: >1.5 Existing North to East Slope (B-4 to B-1) Upper Flatter Portion

2.131 Shallow Sloughing: 0.926

Deep Seated: >1.5

Existing South to East Slope (B-6 to B-1) Lower Steep Portion

Shallow Sloughing: 0.716 Shallow Sloughing: 0.716

Deep Seated: >1.3 Deep Seated: >1.5 Existing South to East Slope (B-6 to B-1) Upper Flatter Portion

Shallow Sloughing: 0.508 Shallow Sloughing: 0.643 Deep Seated: >1.3 Deep Seated: >1.5

For the upper relatively flat portions of the site, evaluations for the end-of-construction condition generally indicate a suitable factor of safety of greater than the minimum required value of 1.3 for deep seated failure envelopes. Evaluations for the long-term, post-construction condition generally indicate a suitable factor of safety of greater than the minimum required value of 1.5 for slopes supporting structures bearing on shallow spread foundations for deep seated failure envelopes. Factors of safety less than 1.3 or 1.5 in the short-term and long-term cases, respectively, were determined in areas of existing retaining walls. However, the model only included soils data and did not include retaining wall systems/reinforcement, and therefore these factors of safety are unrealistic at the retaining wall locations. Local stability at the retaining wall(s) for the retained soil is the responsibility of the retaining wall designer.

For the lower steep portion of the site, an unsuitable factor of safety of less than 1.3 was determined for the end-of-construction and long-term conditions when evaluating relatively shallow sloughing surface failures along the steep slope based on the modeled design parameters. Regular monitoring should be performed in these areas to evaluate if surface sloughing is occurring. If surface sloughing occurs and is not remediated expeditiously, these surface sloughs may progress into deeper seated failures. If these areas are performing suitably, it may be a case where the existing site soils in the areas exhibit more favorable strength parameters than assigned based on the borings performed at the site. Additional geotechnical exploration could be performed in these areas to obtain samples for better delineation of the soil layers specifically in these areas, and additional strength testing.

For the lower steep portion of the site, deep seated failure envelopes for the end-of-construction condition indicate a suitable factor of safety of greater than the minimum required value of 1.3. Evaluations for the deep seated failure envelopes considering long-

TTL Project No. 2257001 Page 10 term, post-construction condition also indicate a suitable factor of safety of greater than the minimum required value of 1.5 for slopes supporting structures bearing on shallow spread foundations.

5.2 Areas of Concern and Potential Mitigation

It was indicated that this investigation was requested due to existing fill materials encountered in the western portion of the site and concern regarding encountering these materials in other portions of the site. The conditions encountered in the borings indicated fill materials in only Borings B-4 through B-6 extending to a relatively shallow depth of 3½ feet below existing grades. The global stability evaluations results in Section 5.1 do not indicate a deep seated global stability concern based on the conditions encountered in the borings.

However, as discussed in Section 5.1, for the lower steep portion of the site, unsuitable factor of safety was found for the end-of-construction and long-term conditions when evaluating relatively shallow sloughing surface failures along the steep slope based on the modeled design parameters. Regular monitoring should be performed in these areas to evaluate if surface sloughing is occurring. If surface sloughing occurs and is not remediated expeditiously, these surface sloughs may progress into deeper seated failures. If these areas are performing suitably, it may be a case where the existing site soils in the areas exhibit more favorable strength parameters than assigned based on the borings performed at the site.

Additional geotechnical exploration could be performed in these areas to obtain samples for better delineation of the soil layers specifically in these areas, and additional strength testing.

5.2.1 Site Preparation

If new construction is planned at the site, prior to proceeding with construction operations, all structures, topsoil, root mats, vegetation, and other deleterious non-soil materials should be removed from the proposed construction areas. The actual amount of required stripping should be determined in the field by a geotechnical engineer or qualified representative.

It should be noted that trace organics were observed in some upper profile soil and fill samples. If structures or pavements will be constructed, they should bear on soils containing trace or less organics.

Due to the existing development at the site, the preparation of this site will require careful inspection. Voids remaining after removal of existing structures, and other exposed

TTL Project No. 2257001 Page 11 excavations should be backfilled with engineered fill, placed in controlled lifts and tested for suitable compaction in accordance with the criteria in Section 5.2.2 of this report. If such excavations are randomly filled or graded without compaction control, excessive differential settlement could occur.

Upon completion of stripping and clearing, the areas intended to support new fill and pavements should be carefully inspected by a geotechnical engineer, including observation of proof rolling. Inspection should also include verification of suitable bearing in footing excavations. Additional geotechnical exploration should be performed in areas of new development to prepare specific design and construction recommendations.

5.2.2 Fill

Material for engineered fill or backfill required to achieve design grades may consist of any non-organic soils having a maximum dry density as determined by the Standard Proctor

(ASTM D 698) of 90 pounds per cubic foot (pcf) or greater. On-site soils may be used as engineered fill materials provided that they are free of organic matter, debris, excessive moisture, and rock or stone fragments larger than 3 inches in diameter.

Depending on seasonal conditions, the on-site soils may be wet of optimum and could require scarification and aeration to achieve satisfactory compaction. If the construction schedule does not allow for scarification and aeration activities, it may be more practical or economical to utilize imported granular fill.

All engineered fill in structure and pavement areas should be placed in layers no more than 8 inches thick (loose measure) and adequately keyed into stripped and scarified soils. Due to the existing slopes at the site, it will be particularly important to provide suitable stripping and scarification of the subgrade soils, along with stair-step benching where fill is placed on existing slopes. Fill should be compacted to not less than 100 percent of the maximum dry density as determined by ASTM D 698 (Standard Proctor). Compaction for granular soils should be performed using a smooth-drum roller. A sheepsfoot roller should be used for cohesive soils.

All fill material should be within 3 percent of the optimum moisture content to facilitate compaction. Furthermore, fill material should not be frozen or placed on a frozen base. It is recommended that all earthwork and site preparation activities be conducted under adequate specifications and properly monitored in the field by a geotechnical testing firm.

TTL Project No. 2257001 Page 12

6.0 QUALIFICATION OF RECOMMENDATIONS

Our evaluation of on-site campus global stability has been based on our understanding of the site and project information and the data obtained during our field investigation. The general subsurface conditions used were based on interpretation of the subsurface data in the borings.

Regardless of the thoroughness of a subsurface investigation, there is the possibility that conditions between borings will differ from those at the boring locations, that conditions are not as anticipated by the designers, or that the construction process has altered the soil conditions. This potential is increased for sites with previous development. Therefore, experienced geotechnical engineers should perform project specific explorations and observe earthwork, retaining wall, foundation, and pavement construction to confirm that the conditions anticipated in design are noted. Otherwise, we assume no responsibility for compliance with the design concepts, specifications, or recommendations.

The recommendations in this report have been developed on the basis of the previously described project characteristics and subsurface conditions. If project criteria or locations change, we should be permitted to determine whether the recommendations must be modified. The findings of such a review will be presented in a supplemental report.

Our professional services have been performed, our findings derived, and our recommendations prepared in accordance with generally accepted geotechnical engineering principles and practices. This warranty is in lieu of all other warranties, either expressed or implied. TTL is not responsible for the conclusions, opinions, or recommendations of others based on this data.

2257001 leg VAMC Campus Global Stability

Notes:

1. Exploratory borings were performed on October 20, 2022 by Drilling Services of Oklahoma under the direction of TTL Associates, Inc.

2. These logs are subject to the limitations, conclusions, and recommendations in the report and should not be interpreted separate from the report.

3. The test borings were located in the field by GPRS under the direction of TTL Associates, Inc.

Ground surface elevations at the boring locations were generally estimated to the nearest foot based on a plan with ground surface topography prepared by Honor Engineer Co. (Honor). It should be noted that Boring B-4 was located beyond the extents of the survey. However, LiDAR was performed as part of the survey, and Boring B-4 was within the LiDAR area. The ground surface elevation of Boring B-4 was provided to TTL by Honor based on the LiDAR results.

4. Unconfined Compressive Strength (tsf):

NP = Non-Plastic NI = Non-Intact NR = No Recovery