MS ERFO NPS VICK 2020 -1(2) - Final Geotechnical Report.pdf

SOILS AND FOUNDATIONS REPORT

REPORT NO. 01-24

PROJECT: LANDSLIDE REPAIRS AT GRANT CIRCLE AND UNION

AVENUE

MS ERFO NPS VICK 2020 -1(2)

WARREN COUNTY, MISSISSIPPI

US DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

22001 LOUDON COUNTY PARKWAY

BUILDING E-2, SUITE 200

ASHBURN, VA 20147

MAY 2024

TABLE OF CONTENTS

1. INTRODUCTION

1.1 General

1.2 Project Description

2. GEOLOGIC SETTING

3. SUBSURFACE EXPLORATION PROGRAM

4. SUBSURFACE CONDITIONS

5. ANALYSIS AND RECOMMENDATIONS

5.1 Geotechnical Evaluation

5.2 Design Alternatives

5.3 Recommendations

5.4 Analyses

6. DISCLAIMER/LIMITATIONS CLAUSE

REFERENCES

APPENDICES

Appendix A – Figures Appendix B – Geotechnical Engineering Report and Boring Logs Appendix C – Slope Stability Analysis Results Appendix D – Selected Photographs

Note: Design changes subsequent to publication of this report and prior to the project’s advertisement will be documented by a memo attached after the title page.

PROJECT: AR ERFO FS 2020 - 1(1)

LANDSLIDE REPAIRS AT GRANT CIRCLE AND UNION AVENUE

WARREN COUNTY, MISSISSIPPI

PAGE 3 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

SOILS AND FOUNDATIONS REPORT

REPORT NO. 01-24

PROJECT: LANDSLIDE REPAIRS AT GRANT CIRCLE AND UNION AVENUE

MS ERFO NPS VICK 2020 -1(2)

WARREN COUNTY, MISSISSIPPI

1. INTRODUCTION

1.1 General

This report presents the results of the subsurface exploration, laboratory testing, geotechnical analysis, and recommendations for the landslide and roadway repair of Grant Circle and Union Avenue in the Vicksburg National Military Park, Vicksburg, Warren County, Mississippi. The site location is shown on Figure 1, Appendix A.

1.2 Project Description

The embankment failure at Grant Circle (Site 5) is located at the northeastern end of the park, at the end of Grant Avenue. The failure consists of a large, eroded gully that has left nearly vertical head and side scarps over 40 ft high along the roadway. The failure has accelerated in the past couple years due to the exceptionally high rainfall.

Three areas of the roadway at Union Avenue showed pavement, subgrade, and embankment failures that appear to be related to subgrade and slope failures. In some areas the ground has moved laterally and/or vertically causing pavement and ground cracking and scarps of up to 5-ft deep.

2. GEOLOGIC SETTING

Available geologic data indicated that the site is underlain by the Miocene age Paynes Hammock sand, which overlays the Vicksburg group and Chickasawhay limestone of the Oligocene age (Belt et al. 1945).

Paynes Hammock sand is a sandy limestone with cross-bedded fine sand, and thin-bedded sand and clay.

Chickasawhay limestone is only present in eastern Mississippi and is a sandy limestone and sand. The Vicksburg group is predominantly limestone and marl. However, the dominant, governing geologic feature of all four sites are the thick loess deposits at the surface.

The general Geologic Map of the site is shown on Figure 2 in Appendix A.

Site Geology is addressed in more details in the Geotechnical Engineering Report (Appendix B).

3. SUBSURFACE EXPLORATION PROGRAM

A subsurface exploration program consisting of drilling one (1) soil boring at Grant Circle (Site 5) and seven (7) soil borings at Union Avenue (Sites 7, 8, and 9) was conducted under the supervision of Schnabel

WARREN COUNTY, MISSISSIPPI

PAGE 4 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

Engineering, between March 1 and March 2, 2020. The Standard Penetration Test (SPT) was performed at selected depths in the borings. The borings were drilled to their pre-determined termination depths and no auger refusal was encountered. Detailed information of the subsurface exploration, equipment used, boring locations plan, and the boring logs are included in Schnabel’s Geotechnical Engineering Report in Appendix B.

In addition, Burns Cooley and Dennis, Inc. performed a geotechnical study for Vicksburg Park-Wide Road System Study in April-August 2021. The relevant subsurface information from this study that was also used in our evaluation and is included in Appendix B.

4. SUBSURFACE CONDITIONS

A brief description of the soil conditions encountered during the subsurface exploration is presented below.

The boring logs, soil profiles, detailed subsurface conditions encountered in the borings, and results of laboratory testing are included in Appendix B.

The stratification lines designating the interfaces between soil types on the boring logs represent approximate boundaries. The transition between materials may be gradual. It should be noted that one or more of the units may be absent at specific locations. Subsurface conditions may vary between and beyond the boring locations. Also, cobbles and boulders may exist at different locations and depths.

STRATUM F (Fill)

Fill soils, probably associated with the existing embankment, were encountered along Union Avenue (Sites 7,8 and 9) immediately beneath the pavement and extending to depths of 3.5± ft below existing site grades.

The fill was classified as a Lean Clay (CL), Silt (ML), and Silty Sand (SM) with various amounts of rock fragments, and gravel. The fill possessed plasticity indices ranging from 10 to 11 with 19.1 to 96.7 percent passing the #200 sieve. The N-values of the fill ranged from 2 to 17 indicating soft to very stiff consistencies. The moisture contents of the samples varied from 7.2 to 24.4 percent.

STRATUM A (Loess)

Loess soils were encountered at the ground surface or underneath the fill soils and extended to the termination depths of the soil borings. The Loess was predominately classified as SILT (ML) and LEAN CLAY (CL), and Silty SAND (SM), though two samples were classified as a FAT CLAY (CH). The lean clay and silty sand variation of the loess were encountered at approximate depths of 53.5 and 72 feet, respectively. The loess possessed plasticity indices ranging from NP (Non-Plastic) to 11 with 85.5 to 99.5 percent passing the #200 sieve. The N-values of the Loess ranged from 2 to 46 indicating soft to hard consistencies. The moisture contents of the samples varied from 11.1 to 29.8 percent.

WARREN COUNTY, MISSISSIPPI

PAGE 5 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

Groundwater

Groundwater was encountered along Union Avenue in borings TA-01 through TA-04 at depths of 3.0 to

3.5 ft below existing site grades, but the groundwater dissipated at the completion of drilling. In borings TA-05 to TA-07 groundwater was encountered at depths of 2 to 6.5 ft below existing site grades at the time of drilling and 3 to 13 ft at the completion of drilling.

Groundwater level readings were made in the boreholes at the times and under the conditions stated on the boring logs. However, fluctuations in groundwater levels due to seasonal variations, rainfall, temperature, and other factors not evident at the time measurements were made should be expected.

The boring logs and soil profiles are included in Appendix B.

5. ANALYSIS AND RECOMMENDATIONS

The previous geotechnical analyses and recommendations by WSP are superseded by the analyses and recommendations presented below.

5.1 Geotechnical Evaluation

Landslides occur as a result of unbalanced pressures along the sloping ground. The presence of water acts as a trigger mechanism by increasing the driving forces and/or reducing the shear strength of the in-situ materials. To mitigate slope stability problems, various approaches can be used. One approach is to reduce the driving forces and increasing shear strength. The shear strength can be increased by lowering the groundwater table which can be attained by improving the surface and subsurface drainage systems.

Another approach is increasing the resisting forces. Increasing the resisting forces can be achieved by buttressing the slope, adding a retaining structure, or by reinforcing the slope.

5.2 Design Alternatives

Various alternatives were considered given their feasibility and effectiveness, site subsurface conditions, constructability, cost, and our experience with similar projects. A brief discussion of each alternative is presented below.

1) Grant Circle (Site 5) – Landslide Alternative No. 1 – Reinforced Soil Slope (RSS)

Reinforced soil slopes are constructed using common construction materials, and conventional equipment and techniques, providing an economical and durable structures. These types of walls are cost effective, self-draining, and can tolerate larger differential settlements compared to other repair alternatives and they can be more esthetically pleasant. The reinforcement can be achieved by placing horizontal geosynthetics (geogrids) at certain intervals.

WARREN COUNTY, MISSISSIPPI

PAGE 6 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

Alternative No. 2 – Gabion basket wall with soil nails

This alternative consists of re-establishing and stabilizing the slope by constructing a gabion basket wall.

These walls consist of wiremesh baskets that are filled with stone and placed on top of each other to form a self-draining retaining wall. Gabion basket walls are essentially gravity type walls and would require considerable thickness in order to provide adequate stability. They can sustain considerable deformation and differential settlements. The soil nails provide lateral resistance resulting in reduced volume of excavation and backfill behind the gabion wall.

Alternative No. 3 – Gabion-faced Reinforced Soil Slope (RSS) with soil nails

This alternative is a combination of the previous options.

Alternative No. 4 – Gabion-faced Reinforced Soil Slope (RSS) with Drilled Shafts (Shear Pins)

This alternative consists of re-establishing the roadway and stabilize the slope by constructing a reinforced soil slope (RSS) with gabion baskets facing. Large diameter drilled shafts (shear pins) are installed near the toe of the RSS at a determined interval to intercept the sliding plane and provide resistance. The drilled shafts include a wide flange steel column as a reinforcing core.

2) Union Avenue (Sites 7, 8, and 9) – Pavement and Embankment Failures

Alternative No. 1 – Eight (8) ft thick load transfer platform (LTP), consisting of layers of #57 stone separated by geogrids, supported on natural soils.

Alternative No. 2 – Four (4) ft thick load transfer platform (LTP), supported on a system of impact rammed aggregate piers (RAP)

A 60-mil vapor retarder membrane sandwiched by a geofabric shall be placed at the bottom of the LTP.

5.3 Recommendations

Based on our observations during the site reconnaissance, the results of our subsurface exploration program, geotechnical analyses, and a discussion with our partner, the following alternatives were selected.

Grant Circle (Site 5) – Landslide

A gabion-faced Reinforced Soil Slope (RSS) with Drilled Shafts (Shear Pins) at the toe is the preferred approach to repair the landslide at Grant Circle.

The minor pavement depression near the landslide shall be repaired with a 4.5 ft. deep patch system.

WARREN COUNTY, MISSISSIPPI

PAGE 7 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

Union Avenue (Sites 7, 8, and 9) – Pavement and Embankment Failures

A four (4) ft thick layer of geogrid reinforced, granular backfill (#57 stone) load transfer platform (LTP) supported on a 25 feet deep system of impact rammed aggregate piers (RAP) is the preferred approach to repair the roadway damage at Union Avenue. A 60-mil vapor retarder membrane sandwiched by a geofabric shall be placed at the bottom of the LTP. The rammed aggregate piers (RAP) should be designed and constructed by qualified specialty contractor with a proven record of success in such applications.

The repair alternatives recommended above should include a subsurface drainage system composed of granular backfill and a perforated 6” diameter pipe. The 6” diameter perforated pipe should be surrounded by a filter fabric to prevent the migration of fines and will drain subsurface water into a 6” diameter non-perforated outlet pipe located in the lowest point behind the wall. To keep wildlife out of the retaining wall drainage system a 1/4” mesh should be installed to cover the 6” diameter non-perforated pipe outlet.

5.4 Analyses

The slope stability analyses of the proposed earth retaining systems were conducted using limit equilibrium methodology, in general accordance with the design concepts and procedures presented in FHWA Publication FHWA-GEC 11 “Geotechnical Engineer Circular No. 11 – Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” (2009), and following “AASHTO LRFD Bridge Design Specifications, 9th Ed.” (2020) design requirements. A uniform distributed live surcharge load was applied along the road to account for traffic loading following AASHTO Section 3.11.6.4 requirements. A summary of the soil engineering parameters assumed in the external, internal, and global stability analyses are presented in Table 1.

Table 1 - Summary of assumed engineering properties for analysis.

Material Type Unit Weight (pcf) Cohesion (psf) Internal Friction Angle ϕ (°)

New Fill 120 0 34

Saturated Loess 90 0 20

Loess 90 300 20

Loess (deep) 95 500 20

Loess Deep (Silty Sand) 110 0 32

# 57 Stone 120 0 38

Geomembrane and geofabric 50 0 10

Gabion Face 110 0 40

WARREN COUNTY, MISSISSIPPI

PAGE 8 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

The selected AASHTO load factors and combinations are presented in Table 2. The AASHTO resistance factors and Factors of Safety requirements for the analysis of MSE walls are presented in Table 3.

Table 2 – Retaining wall load factors and combinations.

Load Combination EH EV ES LS EQ

Strength I (Max) 1.50 1.35 1.50 1.75 -

Strength I (Min) 0.90 1.00 0.75 1.75 -

Service I 1.00 1.00 1.00 1.00 -

Extreme Event I 1.00 1.00 1.00 1.00 or 0.50 1.00

Table 3 – Reinforced Soil Slope resistance factors.

Analysis Resistance Factor

Sliding Resistance Φ: 1.00 (FS:1.5)

Overall/Global Φ: 0.65 (FS:1.5)

Overall/Global Extreme Event I Φ: 0.9 (FS:1.1)

The global/overall stability analysis computations were performed using the computer program Slide 2 by Rocscience, Inc. Copies of the computer output are included in Appendix C.

According to AASHTO 11.5.4.2, a seismic design shall not be considered mandatory for walls located in Seismic Zones 1 through 3, or for walls at Sites where the site adjusted peak ground acceleration is less than or equal to 0.4g, unless one or more of the following is true:

• Liquefaction induced lateral spreading or slope failure, or seismically induced slope failure, due to the presence of sensitive clays that lose strength during the seismic shaking, may impact the stability of the wall for the design earthquake.

• The wall supports another structure that is required, based on the applicable design code or specification for the supporting structure, to be designed for seismic loading and poor seismic performance of the wall could impact the seismic performance of that structure.

In addition, according to C11.5.4.2, paragraph 7, a seismic analysis should be considered mandatory if the wall is located in seismic zone 2 or higher, and if either of the following is greater than 30 feet:

• The exposed wall height plus the average depth over the width of the wall of any soil surcharge present.

• For tiered walls, the sum of the exposed height of all the tiers plus the average soil surcharge depth.

WARREN COUNTY, MISSISSIPPI

PAGE 9 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

Based on the above listed criteria, we concluded that the retaining structure system selected does not require an external or internal seismic evaluation due to an adjusted PGA less than 0.4g, and the retaining structures are in Seismic Zone 1 to 3. However, an Extreme Event I global stability analysis is required for all sites.

For this analysis, an assumed allowable lateral deformation was established based on the flexible nature of the types of structure. See the evaluation calculations in Appendix C for details.

A summary of the retaining structure and load transfer platform parameters used in the analysis are presented in Tables 4 below.

Table 4 – Reinforced Soil Slope (RSS) and Load Transfer Platform (LTP) Parameters.

Site Repair Type

Approximate Reinforced Height (ft.)

Slope face inclination

Geogrid Spec.

Nominal Long- Term

Strength (lb/ft.)

Minimum geogrid length

(ft.)

Maximum geogrid vertical spacing

(ft.)

Grant Circle (Site 5) RSS 50 2V:1H Type V 3,000 35 1.5

Union Avenue (Sites 7, 8, and 9)

LTP 4 1V:3H Type V 3,000 30 1.0

Large diameter drilled shafts (shear pins) will be installed near the toe of the RSS at Grant Circle (Site 5).

The drilled shafts include a wide flange steel column as a reinforcing core. Geotechnical parameters of the drilled shafts (shear pins) are shown in Table 5.

Table 5 – Drilled Shafts (Shear Pins).

Site Depth (ft.)

Diameter (in.)

Center to Center

Spacing (ft.)

Assumed Reinforcement

Member

Assumed minimum factored shear resistance

(kips)

Min.

Concrete Strength

(psi)

Grant Circle (Site 5)

50 24 4 W16 x 100 854 4,000

The proposed repair technologies satisfied the minimum static and Extreme Event I (seismic) global stability requirements stated above. Details of the global stability analyses results are included in Appendix C.

WARREN COUNTY, MISSISSIPPI

PAGE 10 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

6. DISCLAIMER/LIMITATIONS CLAUSE

The subsurface explorations and tests described in this report have been conducted in accordance with standard practices and procedures (except as specifically noted). The results of these exploration and tests represent conditions at the specific locations, to the depths explored, and on the dates indicated. Subsurface conditions below the depths explored, between and beyond the boring locations may vary. The Geotechnical Evaluation and Recommendations Sections of this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Prepared by:

Girmay Weldegiorgis, P.E.

Geotechnical Engineer

Reviewed by:

Mounir Abouzakhm, P.E.

Division Geotechnical Engineer

P ROJECT: AR ERFO FS 2020 - 1(1)

LANDSLIDE REPAIR OF MORGAN MOUNTAIN ROAD (NFSR 1504)

WARREN COUNTY, MISSISSIPPI

PAGE 11 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VA

REFERENCES

AASHTO LRFD Bridge Design Specifications, 2020, 9th Ed., Washington, D.C. American Association of State Highway and Transportation officials.

FHWA. (2014). Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-14). US Department of Transportation. Washington D.C.: US Department of Transportation.

FHWA. (2015). Strength Characterization of Open-Graded Aggregates for Structural Backfills - FHWA-HRT-15-034. McLean: FHWA.

FHWA (2009) FHWA-NHI-10-024 & -025, “Geotechnical Engineer Circular No. 11 – Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes”. Washington DC

P ROJECT: AR ERFO FS 2020 - 1(1)

LANDSLIDE REPAIR OF MORGAN MOUNTAIN ROAD (NFSR 1504)

WARREN COUNTY, MISSISSIPPI

PAGE 12 DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

APPENDICES

DEPARTMENT OF TRANSPORTATION

APPENDIX A – FIGURES

SHEET

NO. PROJECT TOTAL

SHEETS

STATEREG

MS ERFO NPS VICK

2020 - 1(2)MS

FIGURE 1

SITE LOCATION MAP

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL LANDS HIGHWAY DIVISION

EASTERN FEDERAL LANDS HIGHWAY DIVISION

ASHBURN, VIRGINIA

Sites 7, 8, & 9

Sites 5

Source:

USGS National Goologic Map Database: https://ngmdb.usgs.gov/ngm-bin/pdp/zui_viewer.pl?id=7361

STATE TOTAL

SHEETS

EASTERN FEDERAL LANDS HIGHWAY DIVISION MS ERFO NPS VICK

2020 - 1(2)

SHEET

NO. PROJECT

ASHBURN, VIRGINIA MS

FIGURE 2

GEOLOGIC MAP

U.S. DEPARTMENT OF TRANSPORTATION REG

FEDERAL LANDS HIGHWAY DIVISION

SITE 5

SITES 7, 8 & 9

DEPARTMENT OF TRANSPORTATION

APPENDIX B – GEOTECHNICAL ENGINEERING REPORT AND BORING LOGS

WSP USA

Suite 1600 851 SW 6th Avenue Portland, OR 97204 503.478.2800 wsp.com

June 12, 2020

Mr. Mounir Abouzakhm Contracting Officer’s Representative Federal Highway Administration Eastern Federal Lands Highway Division Loudoun Tech Center, Room 105 21400 Ridgetop Circle Sterling, VA 20166-6511

Reference No. HFAC-15

Subject: Contract No. DTFH7015D00005/693C7320F00002 2015 Architect & Engineering Multi-Discipline IDIQ

Project: VICK 12(4), 20(1), 21(1) Design and Recommendations for Landslide, Embankments, and Pavement Repair – Vicksburg National Military Park

Dear Mr. Abouzakhm:

WSP USA Inc. (WSP), along with our subconsultant, Schnabel Engineering, LLC, is pleased to submit the final geotechnical engineering report for this project.

We appreciate this opportunity of serving Eastern Federal Lands Highway Division on this important project. It was a pleasure working with you and your staff and look forward to future opportunities of assisting you on other challenging project assignments.

I can be reached at 703.742.5827 (office) or via timothy.ramey@wsp.com if you have any questions or require any additional information.

Sincerely, WSP USA INC.

Timothy J. Ramey, PE Project Manager

Attachment: GEOTECHNICAL ENGINEERING REPORT; Concept Design and Recommendations for Landslides and Pavement; Vicksburg National Military Park; Vicksburg, Mississippi

GEOTECHNICAL ENGINEERING REPORT

Concept Design and Recommendations for Landslides and Pavement Vicksburg National Military Park Vicksburg, Mississippi

EFLHD Project VICK 12(4), 20(10), 21(1) Schnabel Reference 20C23001 June 12, 2020

June 12, 2020

Mr. Timothy J. Ramey, PE WSP USA, Inc.

13530 Dulles Technology Drive Suite 300 Herndon, VA 20171

Subject: Project 20C23001, Geotechnical Engineering Report, Concept Design and Recommendations for Landslides and Pavement, Vicksburg National Military Park, Mississippi (EFLHD Project VICK 12(4), 20(1), 21(1))

Dear Mr. Ramey:

SCHNABEL ENGINEERING, LLC (Schnabel) is pleased to submit our draft geotechnical engineering report for this project. This study was performed in accordance with our revised proposal dated September 17, 2019, our proposal modification dated January 21, 2020, and our amended Task Order No. 2, dated July 25, 2019, executed under the Eastern Federal Lands Highway Division (EFLHD) Prime Agreement No. DTFH7015D00005, 2005, Task Order No. 693C7320F000002.

We appreciate the opportunity to be of service for this project. Please call us if you have any questions regarding this report.

Sincerely, SCHNABEL ENGINEERING, LLC

G. Allen Bowers, Jr., PE, PhD Project Engineer

Ken Derrenbacher, PE Principal

GAB:KD:sam G:\2011-2020\2020\Sterling\Projects\20C23001 - EFL Vicksburg Military Park Repairs\03-SE Products\03-Reports\02- Final\20C23001 - EFL Vicksburg NMP Repairs- Master.Docx

WSP USA, Inc Vicksburg National Military Park Repairs, Mississippi

June 12, 2020 Page i Schnabel Engineering, LLC Project 20C23001 ©2020 All Rights Reserved

GEOTECHNICAL ENGINEERING REPORT

CONCEPT DESIGN AND RECOMMENDATIONS FOR LANDSLIDES AND PAVEMENT

EFLHD PROJECT VICK 12(4), 20(1), 21(1)

VICKSBURG, MISSISSIPPI

TABLE OF CONTENTS

1.0 EXECUTIVE SUMMARY

2.0 SCOPE OF SERVICES

3.0 INTRODUCTION

3.1 Background

3.2 Site Description and Review of Existing Data

4.0 SITE GEOLOGY

5.0 SUBSURFACE CONDITIONS

5.1 Exploration Methods

5.2 Laboratory Testing Methods

5.3 Generalized Subsurface Stratigraphy

5.4 Strength Parameters for Design

6.0 ASSESSMENT OF GEOTECHNICAL AND GEOLOGIC HAZARDS

6.1 Louisiana Circle

6.2 Ransom’s Gun Path

6.3 Thayer’s Approach

6.4 Grant Circle

7.0 RECOMMENDATIONS

7.1 Louisiana Circle

7.2 Ransom’s Gun Path

7.3 Thayer’s Approach

7.4 Grant Circle

7.5 New Pavements

7.6 General Recommendations

7.7 Conceptual Design Cost Estimates

8.0 LIMITATIONS

9.0 REFERENCES

June 12, 2020 Page ii Schnabel Engineering, LLC

FIGURES

Figure 1: Site Vicinity Map Figure 2: Geologic Map Figure 3: Louisiana Circle Topographic Site Plan and Mapping Figure 4: Louisiana Circle Recommended Options’ Extents Figures 5A to 5B: Louisiana Circle Section A-A’ Figure 6: Ransom’s Gun Path Topographic Site Plan and Mapping Figure 7: Ransom’s Gun Path Recommended Options’ Extents Figure 8: Ransom’s Gun Path Section A-A’ Figures 9A to 9G: Thayer’s Approach Topographic Site Plan and Mapping Figure 10: Grant’s Circle Topographic Site Plan and Mapping Figure 11: Grant’s Circle Recommended Options’ Extents Figures 12A to 12C: Grant’s Circle Section A-A’

APPENDICES

Appendix A: Subsurface Investigation Appendix B: Laboratory Investigation Appendix C: Slope Stability Analyses and Design Calculations Appendix D: Photo Log Appendix E: WSP Drainage Report

June 12, 2020 Page 1 Schnabel Engineering, LLC

1.0 EXECUTIVE SUMMARY

Based on the information provided to us, our field observations, and the results of our stability analyses, we have developed the following conclusions and recommendations for the areas of on-going slope instability within Vicksburg National Military Park.

We are providing this executive summary solely for purposes of overview. Any party that relies on this report must read the full report. This executive summary omits several details, any one of which could be very important to the complete understanding of our conclusions and recommendations.

The following applies to all four project sites:

1. All four project sites are underlain by loess soils. The loess was primarily a non-plastic silt with less than 10 percent clay and fines.

2. Undisturbed Vicksburg loess soils possess a unique internal structure that give them a high apparent cohesion. Upon saturation or disturbance, the structure collapses resulting in sudden loss of strength. This ultimately results in several failure modes that are unique to loess.

3. Because of the unique structure of undisturbed loess, it cannot be modeled using conventional methods. Furthermore, in order to develop its peak frictional angle, undisturbed loess must undergo large amounts of deformation.

4. Repairing and improving site drainage to prevent surface flow onto and over the slopes is required for all of the sites.

5. We recommend to develop and implement a regular slope monitoring program to asses, catalogue, and monitor the various slope assets at the Park.

6. We believe that regular inspections and maintenance of the Park slopes can significantly reduce the risk for future slope failures.

The following applies to the Louisiana Circle site:

1. The subject slope, though currently stable, exhibits factors of safety below the acceptable threshold. Specifically, the results of the stability analyses revealed the potential for deep seated failures.

2. We believe that past failures downslope were instigated as the result of water infiltration into the slope, resulting strength collapse and columnar shear failure. We believe that the pavement subsidence and guardrail movement at the top of the slope are a result of a shallower failure related to the previous fill placement.

June 12, 2020 Page 2 Schnabel Engineering, LLC

3. Among several slope stabilization options we considered, we believe the two that are most economical are a reinforced soil slope (RSS) and soil nail stabilization. Because of the low factors of safety with respect to deeper-seated failures, both options will require toe stabilizing elements consisting of piles.

4. Control of water at the top and along the slope is critical to maintaining stability. Water must not be allowed to infiltrate the slope through cracks or preferential drainage channels.

5. In addition to the control of water, we recommend the implementation of several risk reduction methods that are designed to control what we believe to be the primary instigators of failure.

These methods can effectively lessen the overall risk of failures by reducing the probability of the instigation of a failure and through lessening the consequences, should a failure occur.

The following applies to the Ransom’s Gun Path site:

1. We believe that the present condition was primarily caused by a water drainage issue that has now escalated to a slope stability issue. The present condition was likely created when water flowing down Union Avenue from the south crested the slope to the east of the road.

2. Though currently limited to the slope supporting the roadway, we believe that if nothing is done to mitigate the current situation in terms of stability and drainage, the failure will progress to impact more of the slope.

3. Among several slope stabilization options we considered, we believe the two that are most economical are a reinforced soil slope (RSS) and soil nail stabilization. Both options can likely be installed from the road. The RSS would allow reclamation of the shoulder, however the soil nail stabilization would not necessitate road closure for duration of construction.

4. The water flowing down Union Avenue must be redirected and prevented from flowing over the crest of the slope.

The following applies to the Thayer’s Approach site:

1. We identified three areas where we believe that lateral ground movements have caused pavement and slope failures. We also identified block and edge cracking in the pavements at other locations. Some localized cracking was observed in some wheel paths, suggesting some structural failures, but was not prevalent.

2. We suspect that the slope failures were gradual, with progressive ground movements, leading to cracks, which allowed more surface water to infiltrate, and eventually causing the observed failures.

3. Complete reconstruction of the pavements will be required in the three locations where slope failures have occurred. During restoration, the pavement subgrade and slope fills should be excavated, graded, and replaced with a well-draining structural fill.

June 12, 2020 Page 3 Schnabel Engineering, LLC

4. Complete reconstruction of the inclined pavements between the slope failures is also recommended. Replacing the slope fills and pavements for the entire inclined pavement area should reduce the risk of future slope failures and pavement subgrade problems in this study area. Complete replacement of the pavements will allow for a more robust curb, ditches, and well draining aggregate base below the asphalt, as well as pavement and slope subdrainage measures to account for the surface and ground water. The new pavement section should include a subdrainage layer to reduce water related future pavement subgrade and slope failures.

5. We considered repairs to the non-sloped portion of Thayer’s Approach by either grinding, patching, and overlaying or using Full Depth Reclamation (FDR) and a new asphalt pavement.

However, we recommend that complete replacement of the pavement section be considered, particularly if the project is combined with the complete replacement of the inclined portion of Thayer’s Approach. Complete replacement of the pavement section and subgrade will allow for installation of an aggregate drainage layer below the asphalt, subdrainage pipes along the roadway, changes to the surface grades to achieve adequate surface drainage, and other measures to control the runoff. The project should include the drainage ditch north of the roadway. Complete replacement of the pavements and the surrounding drainage system should significantly reduce future maintenance and repairs of the Thayer’s Approach pavements in the study area.

The following applies to the Grant Circle site:

1. We believe that the past failures are a combination of some of the failure modes that loess is susceptible to including gullying and shear – both of which were caused by the introduction of water into the slope.

2. Among several slope stabilization options we considered, we believe there are three that are most economical. They include a reinforced soil slope (RSS), soil nail stabilization, and mass fill.

Depending on construction method, all three options would likely require access to the base of the failed area during construction.

3. The mass fill and RSS options allow for reclamation of Union Avenue. The soil nail option would require relocation of Union Avenue further west unless the failed area is filled after the soil nails are installed. The soil nail option would also require toe stabilizing elements.

4. It is imperative that a robust drainage system be constructed that conveys surface water away from the slope and prevents it from infiltrating into the ground on or behind the slope.

5. We recommend that the selected stabilization option be extended to address the vertical sidewalls to prevent failures from propagating and impacting a non-stabilized portion of slope.

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2.0 SCOPE OF SERVICES

Our revised proposal dated September 17, 2019, and proposal modification date January 21, 2020 define the scope of services for this project. In accordance with the scope of services, we have performed the following tasks:

• Performed an initial site reconnaissance to observe the distressed slopes, embankments, pavements, and drainage conditions.

• Performed subsurface exploration field services including soil borings, hand augers, and pavement cores.

• Performed laboratory testing of selected samples collected during the subsurface exploration.

• Prepare a Geotechnical Engineering Report (this report) in accordance with Section 6.5 of the

PDDM, which includes:

o A summary of field exploration activities;

o A boring location plan and boring logs for each site;

o Estimated subsurface profiles within the areas explored based on data collected in the subsurface exploration;

o Laboratory testing results;

o Calculations;

o Conceptual recommendations for landslide, embankment, and pavement remediation at each site;

o A preliminary engineer’s cost estimate for the recommended remediation options.

Our scope of services for this project also included presenting the results of our study and recommended design concepts to EFLHD during a Draft Report Review Meeting, which occurred on June 4, 2020. This report is the final version, incorporating the comments we received from EFLHD.

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3.0 INTRODUCTION

3.1 Background

Four locations within Vicksburg National Military Park in Vicksburg, Mississippi have either experienced a failure or show potential for failure. A site vicinity map is included as Figure 1. The four sites are summarized below.

1. Louisiana Circle: This site is an overlook adjacent to the Mississippi River and located along US-61, approximately 0.5 miles north of Interstate 20, Mississippi River Bridge. In conversations with National Park Service (NPS) personnel during our site visits, we understand that this overlook has experienced two prior failures, the latest of which occurred in 2017. The failure in 2017 was a slide that occurred on the northwest side of the overlook and resulted in soil being deposited into the county park below. There is evidence of past subsidence in the pavement along the northeast portion of the circle, however we understand that this condition has not changed for a long period of time. There are currently jersey barriers and cones set up to prevent vehicles from driving too close to the edge of the pavement and the overlook is closed to buses.

2. Ransom’s Gun Path: This site is located along Union Avenue. Embankment distress has been caused by shallow sloughing of the embankment slope materials adjacent to the road. At the time of our site visit, an approximately 10-15 ft high scarp was located about 2 ft from the edge of the road. During rain events, water flows across the road and directly over the scarp. To prevent this, NPS personnel have placed sandbags to direct water down to an inlet and prevent it from flowing directly over the edge of the embankment. They have noted that no further failure has been observed since implementing the sandbags in early 2020.

3. Thayer’s Approach: This site is an approximately 1,500 ft long section of road located along Union Avenue. Numerous areas of pavement along this stretch have experienced settlement and cracking, some of which have resulted in scarps of up to 5 ft high. New asphalt was placed in two locations in December 2019, however both locations experienced failures resulting in cracking of the asphalt after periods of heavy rain in mid-January 2020. NPS personnel observed that hairline cracks would form first, and the cracks would progress to larger failures following periods of rain. NPS personnel now cover these areas with plastic to prevent as much water from infiltrating into the subsurface. Due to the magnitude of the failures, the NPS has been forced to close the road.

4. Grant Circle: This site is located at the northern end of the park at the end of Grant Avenue.

During our site visits, the failure consisted of a large gully that has eroded into the hollow below.

The erosion has resulted in nearly a vertical head of 40 ft high and side scarps of 5 to 20 ft high.

In conversations with NPS personnel, we understand that the failure has progressively occurred, but has accelerated recently due to the exceptionally high rainfall. As of our site visit, the failure has removed about half of the roadway. Due to the failure, the NPS has been forced to close the road and move some of the monuments. Sand bags have been placed to direct surface water away from the scarp.

We obtained the site information from the topographic site plans and documents provided by EFLHD on February 10, 2020, through our site visits, and in conversations with NPS personnel. We believe the topography shown on the maps was collected in late 2019. The progressive nature of the failures and our observations in the field indicate that the topography provided in the previously referenced files may no longer completely match field conditions. However, in the absence of updated topographic data, we have

June 12, 2020 Page 6 Schnabel Engineering, LLC based our analyses on the data from the topographic files. Thus, in this report, analyses and figures that reference “existing conditions” are referencing the conditions as portrayed by the topographic files provided to us. We recommend that prior to final design, new topographic surveys be conducted so that the final designs will be more reflective of field conditions.

3.2 Site Description and Review of Existing Data

The descriptions below are based on site topographic data we believe was collected in late 2019 and our site observations during our visits on February 19-20, 2020 and on March 1-12, 2020.

3.2.1 Louisiana Circle

Louisiana Circle is an overlook at the top of the eastern bank of the Mississippi River and located on the western side of US-61, approximately 0.5 miles north of Interstate 20, Mississippi River Bridge. We were provided a topographic file of Louisiana Circle by EFLHD on February 10, 2020. The topography, along with our field observations, are included in Figure 3 at the end of this report. The paved portion of the overlook is a semi-circle with elevations between EL 224 and EL 218 ft. A hill with a top elevation of EL 253 ft is between the paved circle and US-61 as shown in the topographic survey (Photo 1 of Appendix D).

The embankment supporting the paved area descends to an elevation of approximately EL 130 ft, directly to the west. The slope of the hill below the overlook is irregular, ranging from an average of 2H:1V along the northwest face of the slope, to localized values of 1H:1V or steeper along portions of the western and southwestern faces. An access road to a city park at the base approximately delineates the extent of the slope.

We observed several signs of past and possibly ongoing failures during our site visit. Most pronounced was an area of pavement subsidence and cracking along the southwestern portion of the circle (Photos 2 to 3). In some locations, the vertical offset was 2-3 inches. In addition to the pavement settlement, the wooden guardrail had rotated approximately 20-30 degrees from the vertical (Photo 4). There is also evidence of erosion approximately 50 to 70 ft downslope from the pavement as shown in Photos 6 and 7 and marked on Figure 3. The topography also shows signs of smaller, localized failures that extend about half the height of the slope. These failures are located along the western and southwestern faces.

In conversations with NPS personnel during our site visits, we understand that this overlook has experienced two prior failures, the latest of which occurred in 2017. These failures are likely the ones observed in the topography. The failure in 2017 was a slide that occurred on the northwest side of the overlook and resulted in soil being deposited into the county park below.

3.2.2 Ransom’s Gun Path

The failure at Ransom’s Gun Path is on the eastern side of a north-south stretch of Union Avenue as Union Avenue snakes around a hollow. We were provided a topographic file of Ransom’s Gun Path by EFLHD on February 10, 2020. The topography, along with our field observations, are included in Figure 6 at the end of this report. The elevation of the road at the area of the failure gently rises from an elevation of EL 229 ft at the north end to EL 304 ft at the southern end. The base of the hollow is located about 80 ft east from the edge of the road and has an elevation between EL 255 and EL 260 ft. The slope continues to rise to the west of the road to a maximum surveyed elevation of EL 325 ft.

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At the time of our site visits in February and March 2020, the failure consisted of an approximately 10-15 ft high scarp that is located about 2 ft from the edge of the road (Photo 11 of Appendix D). The scarp extends about 20 ft along the road and the southern end, makes a 90 degree turn down the slope with 3 to 5 ft high scarp faces (Photo 14). The scarp perpendicular to the road is approximately 30 ft long.

A smaller 3 to 4 ft high scarp face was observed approximately 30 ft to the north of the main scarp face.

This smaller scarp is in the shape of a “U”, with the vertex of the “U” about 2 ft from the road.

On the western side of the road, opposite the scarp, an approximately 2 ft diameter by 2 ft deep hole was observed adjacent to the road (Photo 12). In conversation with NPS personnel, we learned that this hole has formed several times and has subsequently been filled with soil. There is no known drainage structure at that location. The hole itself, however, is located at the end of a gully that drains the slope to the west (Photo 12). The hole fills with water during rain events and once filled, the water flows over the road (Photo 13).

During rain events, water also flows across the road and directly over the scarp (Photo 15). To prevent this, NPS personnel have deployed sandbags (observed in Photo 15) to direct water further down the road and into an inlet, thus preventing it from flowing directly over the edge of the embankment. They have noted that no worsening of the failures have been observed since implementing the sandbags in early 2020.

Based on cross sections developed from the topography file, we believe that Union Avenue along this section was created through cut and fill. The slope to the west of Union Avenue was partially cut, and the slope to the east of Union Avenue was built up to create the road. The interpolated grade of the slope prior to construction of Union Avenue appears to have been approximately 2H:1V. After construction, the fill slope supporting the road to the east of Union Avenue falls at approximately 1.25H:1V until it meets the natural slope. Portions of the slope that have not failed, which are to the north and south of the current failure, still display this grading.

3.2.3 Thayer’s Approach

The Thayer’s Approach study area is about a 1,500 ft long section of Union Avenue between Grant Avenue and the USS Cairo Museum. We were provided a topographic file of Thayer’s Approach by EFLHD on February 10, 2020. The topography, along with our field observations, are included in Figures 9A to 9F at the end of this report. It should be noted that topography data was not included for a recently failed area on the west side of the study area (west of the 106th Illinois Infantry monument).

The elevation for the eastern portion of the study area is generally flat with road elevations between EL 217 and EL 220 ft for the eastern 825 ft of the study area, between the bridges, shown on Figures 9B, 9C, and 9D. West of the bridge shown on Figure 9D, the road elevations climb when moving about another 700 ft further west to where the road approaches a gentle “S” curve, west of the 106th Illinois Infantry monument. The highest elevation in the topography file is located in the middle of the “S” curve at EL 246

ft. The elevation continues to rise as the road reaches the western-most failure area. However, we do not have topography information for this area.

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From the bridge shown on Figure 9D and to the east, the pavements are open sections. The road grades in this area are generally slightly higher than the surrounding ground. Thus, a swale is present on the north side of the road. The pavements in this section appear to be slightly crowned. Thus, surface water appears to flow to the pavement edges and into the swale on the north side and downslope on the south side.

A curb is present on the downslope (southwest) side of the road from the bridge on Figure 9D and west to the gentle curve on Figure 9F. We suspect the curb is to collect surface water and convey it to inlets, rather than allowing it to flow onto the downslopes west of the road.

The pavement distresses we observed consist of pavement cracking, slight rutting, edge cracks, and some depressions where ponded water was observed (Photos 19, 22, 26, 30 in Appendix D). Standing water was also observed on either side of the road and in some instances, on the road itself (Photos 20, 21, 23).

We also observed three areas where it appears that slope failures have occurred. The ground has moved laterally and/or vertically causing pavement and ground cracking and scarps of up to 5-ft deep (Photos 17, 18, 24, 25, 27-29). Descriptions of these three areas are provided in Section 6.3.1.

3.2.4 Grant Circle

The failure at Grant Circle is located at the northern end of the park at the end of Grant Avenue. The failure consists of a large, eroded gully that has left nearly vertical head and side scarps over 40 ft high.

We were provided a topographic file of Grant Circle by EFLHD on February 10, 2020. The topography, along with our field observations, are included in Figure 10 at the end of this report. A large gully has formed on the eastern side of Grant Avenue. The topography in this area indicates that Grant Circle is a local high point with an elevation of EL 378 ft in the grassy area in the middle of the circle. The terrain then slopes down on all sides with the exception of the Grant Avenue, which is at the top of a ridge. On the eastern side, where the failure is located, the ground slopes from an elevation of EL 375 ft at the road to an elevation of EL 310 to EL 315 ft at the base of a hollow, before rising on the other side of the hollow towards Sherman Avenue.

During our site visits, the failure consisted of an erosional head scarp approximately 35-45 ft wide along Union Avenue, with an approximately 40 ft high vertical face along the roadway (Photos 31-34 of Appendix D). The head scarp is part of a gully with near vertical side scarps along the edges with vertical, or near vertical faces, 10 to 40 ft high (Photos 35-36). The width of the gully varies from 40 to 50 ft and extends approximately 120 ft downslope into the hollow. In conversations with NPS personnel, we understand that the failure has progressively occurred, working from the base of the hollow and gradually propagating westward towards the road. During rain events, ‘slices’ of the scarp would shear off, expanding the failure area as the scarp edge moved up the slope. The failure has accelerated in the past couple years, especially in the past year due to the exceptionally high rainfall. As of March 12, 2020, the failure has removed about half of the roadway. It is our understanding that the NPS desires to reclaim the lost roadway if possible.

During our field visits, we observed a tension crack to the north of the failed area (Photo 38 and the location is shown on Figure 10). The tension crack was approximately 10 ft behind the side scarp, was up to 1 ft wide and up to 5 ft deep.

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The Grant Avenue roadway is a low elevation at the top of the slope in this area and drains much of Grant Circle. We observed the remnants of a storm drain inlet within the failed area. The park has installed a sandbag barrier to restrict surface water flow into the failure. We observed ponded water outside of the sandbags (Photos 39-40).

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4.0 SITE GEOLOGY

A geologic map is presented in Figure 2. The site is underlain by the Miocene age Paynes Hammock sand, which overlays the Vicksburg group and Chickasawhay limestone of the Oligocene age (Belt et al.

1945). Paynes Hammock sand is a sandy limestone with cross-bedded fine sand, and thin-bedded sand and clay. Chickasawhay limestone is only present in eastern Mississippi and is a sandy limestone and sand. The Vicksburg group is predominantly limestone and marl. However, the dominant, governing geologic feature of all four sites are the thick loess deposits at the surface.

Loess is defined as wind-blown (eolian) sediment of primarily silt-size particles with potentially small amounts of sand and clay. Because loess is a depositional soil, the underlying geology does not impact the characteristics of the soil, except when mixed by colluviation.