Geotech Report.pdf

REPORT OF GEOTECHNICAL EXPLORATION

TN ERFO FS CHRKE804 2020-2(5)

Introduction July 24, 2025 i

Table of Contents

1.0 INTRODUCTION

2.0 GEOLOGIC SETTING

3.0 SUBSURFACE EXPLORATION PROGRAM

3.1 SUBSURFACE EXPLORATION

3.2 FIELD TESTING AND SAMPLING

3.3 LABORATORY TESTING

4.0 SUBSURFACE CONDITIONS

4.1 GENERAL

4.2 SPRING CREEK

4.2.1 MP/Boring 406

4.2.2 MP/Boring 411

4.2.3 MP 428 (Borings 428-A, 428-B)

4.2.4 MP/Boring 431

4.2.5 MP/Boring 433

4.2.6 MP/Boring 452

4.3 CITICO CREEK

4.3.1 MP 466 (Borings 466-A, 466-B)

4.3.2 MP/Boring 469

4.3.3 MP 481 (Borings 481-A, 481-B)

4.3.4 MP/Boring 485

4.3.5 MP/Boring 486

4.3.6 MP/Boring 487

4.3.7 MP/Boring 508

4.3.8 MP/Boring 515

4.3.9 MP 518 (Boring 518-A)

4.3.10 MP 518-B (Borings 518-B, 518-C)

5.0 ANALYSIS AND RECOMMENDATIONS

5.1 GENERAL

5.2 SEISMIC HAZARD ANALYSIS

5.3 CULVERT AND AOP CROSSINGS

5.4 SLOPE STABILIZATION RECOMMENDATIONS

5.4.1 MP 466

5.4.2 MP 481

5.4.3 MP 485

5.4.4 MP 486

5.4.5 MP 487

5.4.6 MP 508

5.4.7 MP 515

5.4.8 MP 518-C

5.4.9 MP 518-B & C

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5.4.10 Slope Stabilization Results

5.5 CONSTRUCTION CONSIDERATIONS

5.5.1 General Considerations

5.5.2 Culvert Construction

5.5.3 Embankment Construction

6.0 REFERENCES

7.0 CLOSING

LIST OF TABLES

Table 1. Summary of Soil Laboratory Test Results Table 2. Summary of Rock UCS Tests Table 3. Summary of Boring Locations Table 4. Summary of Culvert Structures Table 5. Summary of Culvert Bearing Resistances Table 6. Summary of AOP Culvert Bearing Resistances Table 7. Summary of Proposed Slope Remediation Table 8. Summary of Slope Stability Analyses Table 9. Summary of Biaxial Geogrid Placement

LIST OF FIGURES

Figure 1. Aerial View Figure 2. Shearing Strength of Rockfill from Large-Scale Triaxial Tests……………..…Appendix F

LIST OF APPENDICES

Appendix A. Alignment and Exploration Location Plan Appendix B. Boring Logs Appendix C. Laboratory Results Appendix D. Seismic and Culvert Analyses Appendix E. Subsurface Profiles Appendix F. Slope Stability Analyses Appendix G. Geocell Information Appendix H. Site and Rock Core Photographs

1.0 INTRODUCTION

The United States Department of Transportation, specifically the Federal Highway Administration (FHWA)

– Eastern Federal Lands Highway Division, is planning to repair storm and flood damage at two locations in the eastern part of the Cherokee National Forest in Tennessee. The two locations are designated as

Spring Creek Road (FS 27) in Polk Co. and Citico Creek Road (FS 35-1) in Monroe Co. The damage along the two locations generally includes downslope landslides and erosion and culvert upgrade/replacement.

Figure 1 shows the approximate locations of the damaged areas.

Figure 1. Aerial View (Google Earth Pro, 2024)

Tennessee

Georgia

North Carolina Cherokee National Forest

Geologic Setting

2.0 GEOLOGIC SETTING

The two sites along Citico Creek Road and Spring Creek Road are located in the Unaka Mountains

Physiographic Region. The Unaka Mountains are segments of the Blue Ridge and Appalachian Mountain systems. The Unakas have been severely dissected by stream erosion and in general are characterized by steep slopes and deep, narrow valleys that are clothed with hardwood forest. The overburden soils generally consist of relatively thin deposits of alluvial, colluvial, and material weathered from the parent bedrock.

The Geologic Map of Tennessee, East-Central Sheet (Tennessee Department of Conservation, 1966) indicates that the Citico project site is underlain by Precambrian age Wilhite, Shields, and Licklog formations described as follows: Wilhite formation as gray to green siltstone and slate with interbeds of pebble conglomerate, sandstone, and quartzite; Shield Formation as massive conglomerate, sandstone, argillaceous slate; and Licklog Formation as feldspathic sandstone, greenish phyllite, and bluish gray slate.

The Spring Creek area is underlain by Precambrian age Sandsuck and Cochran Conglomerate formations described as follows: Sandsuck Formation as olive green and gray, argillaceous, micaceous shale with coarse feldspathic sandstone and quartz-pebble conglomerate; Cochran Conglomerate as Quartz-pebble conglomerate, gray pebbly arkose, siltstone and shale.

According to the Tennessee Landforms website, more than 54,000 sinkholes have been observed in the state, of which over 40 percent are located in the Ridge and Valley Physiographic Region. This region’s eastern boundary is located about 1 mile west Spring Creek and 5 miles west Citico project sites.

3.0 SUBSURFACE EXPLORATION PROGRAM

3.1 SUBSURFACE EXPLORATION

Stantec created an exploratory plan that included 19 test borings to depths of about 20 to 30 feet below the existing roadway surface at the specific slide areas. The Tennessee 811 One Call System was contacted by Stantec to locate and clear the public utilities. A Stantec engineer provided oversight during the field exploration. Upon completion of the field work, Stantec surveyed the as drilled locations of the test borings and provided ground surface elevations. From April 16 through 19, 2024, the borings were advanced in general accordance with AASHTO guidelines. Borings were performed with a CME 45 truck-mounted drill rig (811) using 3¼-inch inside diameter (ID) hollow stem augers to advance the borings through the overburden materials. The tested efficiency of the automatic hammer is 87 percent (Appendix B).

3.2 FIELD TESTING AND SAMPLING

Each boring was logged and sampled in the field by a geotechnical engineer. Soils were logged paying particular attention to the soil type, consistency, moisture content, and color. Standard penetration test

(SPT) samples were obtained at selected 2½ foot intervals within the upper 20 feet until auger refusal was

Subsurface Exploration Program reached. The SPT is performed by advancing a 2-inch O.D. split-spoon sampler, in three 6-inch increments, with a 140-pound automatic hammer dropping 30 inches at select depth intervals in the boring. The number of hammer blows needed to advance the sampler each 6-inch increment is recorded. The sum of the blow counts from the last two 6-inch increments is called the field N-value. Where auger refusal was encountered, rock coring was performed over a depth of about 5 to 15 feet.

Where bedrock coring was performed in borings NQ-size equipment was used. Recovery, core loss, and rock quality designation (RQD) values were recorded as percentages for each coring run. The recovery is a measurement of the core sample obtained from a core run. The loss is the difference between the core run and the recovery. The RQD is measured by dividing the sum of all pieces of intact rock core longer than four inches in a run by the total length of the core run. These values are shown on the boring logs contained in Appendix B.

The borings were checked for the presence of groundwater during drilling. Groundwater levels were not recorded at completion due to the addition of water during rock coring. The borings were backfilled with soil cuttings upon completion and capped with loose gravel.

3.3 LABORATORY TESTING

The soil and rock samples obtained from the borings were returned to Stantec’s geotechnical laboratory for additional laboratory testing. Engineering classification testing was performed on samples reflecting each of the main soil horizons. The engineering classification tests conducted on the samples were sieve and hydrometer analysis (AASHTO T 88) and Atterberg limits (AASHTO T 89 & T 90). The samples were classified according to the Unified Soil Classification System (USCS) and the American Association of State

Highway Transportation Officials (AASHTO) classification method. Moisture Content tests followed ASTM

D 2216 and AASHTO T 265 procedures.

Unconfined compression testing was performed samples of rock core strength testing (ASTM D 7012, Method C). Laboratory test results are presented in Appendix C. Tables 1 and 2 represent a summary of soil and rock laboratory test results, respectively.

Table 1. Summary of Soil Laboratory Test Results

Boring No.

Sample Depth (ft)

AASHTO

Classification

USCS

Classification

MC

Gravel

Sand

Silt/Clay

406 1.0-10.0 A-2-4 (0) GC 8.7 48.7 37.8 13.5

406 13.5-20 A-4 (0) SC-SM 29.6 4.9 54.5 40.6

406 21.0-25.0 A-4 (1) CL-ML 34.3 0.0 42.3 57.7

411 1.0-7.5 A-2-4 (0) SC 7.9 36.1 42.6 21.3

411 8.5-12.5 A-1-b (0) SM 13.3 31.1 56.2 12.7

428-A 1.0-7.5 A-1-a (0) GP-GM 4.1 47.5 43.2 9.3

433 3.5-10.0 A-2-4 (0) SC-SM 9.7 30.8 51.1 18.1

452 3.5-7.5 A-2-4 (0) SC-SM 6.8 28.4 53.7 17.9

466-A 1.0-10.0 A-2-4 (0) SM 13.1 24.0 40.6 35.4

466-A 11.0-15.0 A-5 (1) SM 29.1 13.7 46.0 40.3

Subsurface Exploration Program

Boring No.

Sample Depth (ft)

AASHTO

Classification

USCS

Classification

MC

Gravel

Sand

Silt/Clay

466-A 16.0-20.0 A-4 (0) SM 15.3 7.4 52.0 40.6

466-B 1.0-7.5 A-6 (1) SM 14.2 27.8 35.1 37.1

466-B 8.5-10.0 A-4 (1) SM 14.2 3.3 52.0 44.7

466-B 13.5-20.0 A-4 (0) SM 18.1 0.5 61.1 38.4

469 3.5-7.5 A-1-b (0) GM 14.7 39.7 35.3 25.0

481-A 1.0-7.5 A-1-a (0) SC-SM 3.1 35.4 49.6 15.0 481-B 1.0-2.5 A-1-a (0) SC-SM 3.8 33.1 52.8 14.1

485 0.0-1.5 A-2-4 (0) GP-GC 7.9 66.9 21.9 11.2

486 1.0-5.0 A-1-a (0) GP-GC 7.9 58.5 29.9 11.6

487 1.0-5.0 A-2-4 (0) GP-GC 5.3 57.2 31.9 10.9

487 8.5-12.5 A-1-a (0) GW-GM 12.9 47.7 43.1 9.2 508 1.0-3.5 A-1-b (0) SC-SM 3.5 39.8 40.7 19.5 515 1.0-5.0 A-1-a (0) SW-SM 2.4 32.8 58.6 8.6

518-A 1.0-5.0 A-1-a (0) GP-GC 3.6 54.4 34.5 11.1

518-B 1.0-5.0 A-2-4 (0) SC-SM 13.8 37.0 42.1 20.9

518-B 6.0-9.0 A-1-b (0) GC-GM 12.7 38.4 37.8 23.8 518-B 11.0-17.5 A-1-b (0) SC-SM 5.9 30.0 53.3 16.7 518-C 1.0-7.5 A-1-b (0) SC-SM 7.3 31.0 45.5 23.5 518-C 1.0-7.5 A-1-b (0) SC-SM 7.3 31.0 45.5 23.5

Table 2. Summary of Rock UCS Tests

Boring No. Sample Depth (ft)

Compressive Strength psi ksf

411 17.8-18.2 8200 1181 428-B 7.7-8.1 19,480 2805 431 8.1-8.5 19,000 2736 433 13.7-14.1 13,640 1964 452 18.7-19.1 9340 1345

466-A 22.0-22.4 13,230 1905 469 13.3-13.7 10,060 1449

481-A 10.1-10.5 14,560 2097 481-B 9.4-9.8 6080 876 485 8.0-8.4 14,090 2029 486 9.7-4.1 18,130 2611 487 13.1-13.5 16,070 2314 508 5.8-6.2 5460 786

518-A 8.1-8.5 19,600 2822 518-B 20.1-20.5 17,550 2527 518-C 10.9-11.3 18,510 2665 518-C 18.2-18.6 10,820 1558

Subsurface Conditions

4.0 SUBSURFACE CONDITIONS

4.1 GENERAL

Due to the steep terrain, which would require clearing and benching and affect the sensitive nature of the surrounding ecosystem, Stantec performed the exploratory activities on the existing roadways. A summary of the borings is presented in Table 3. Exploration locations are shown on the site plans provided in

Appendix A and the boring logs are provided in Appendix B.

Table 3. Summary of Boring Locations

General Location

Boring No.

Latitude Longitude

Ground Surface

Elevation (feet)

Explored Depth (feet)

Spring Creek

406 35.2271935 -84.5367807 737.6 25.0 411 35.2249292 -84.5306409 734.3 20.4

428-A 35.2231921 -84.5130957 758.9 8.0 428-B 35.2232098 -84.5131006 759.1 14.2 431 35.2245684 -84.5069973 765.1 13.2 433 35.2244383 -84.5039145 777.1 16.3 452 35.2360103 -84.4920105 821.8 30.4

Citico Creek

466-A 35.3974928 -84.0986871 2053.8 29.8 466-B 35.3974979 -84.0986199 2055.6 32.0 469 35.4042798 -84.0910297 1912.2 15.5

481-A 35.4078652 -84.0826902 1617.9 15.0 481-B 35.4079827 -84.0826011 1616.3 15.3 485 35.4106247 -84.0874076 1566.3 14.5 486 35.4125589 -84.0873614 1519.3 14.6 487 35.4159338 -84.0891832 1452.8 21.0 508 35.431403 -84.1044271 1244.7 10.7 515 35.4474285 -84.1093968 1047.3 29.7

518-A 35.4506197 -84.1183505 1009.5 19.5 518-B 35.4505427 -84.1181346 1010.1 21.2 518-C 35.4504526 -84.1178033 1010.2 20.3

The surface conditions along the roads for Citico and Spring Creeks typically consisted of a crushed aggregate road base.

The subsurface conditions at the sixteen slope/wall and culvert locations generally consisted of varying thicknesses of granular soils overlying relatively shallow rock. Most of the samples tested were of relatively low plasticity (i.e., predominately granular). and classified as SM, SC, SC-SM, SW-SM, GC, GM, GP-GM, GP-GC, GW-GM, and CL-ML under the Unified Soil Classification System (USCS) and as A-2-4(0), A-1-a(0), A-1-b(0), A-4 (0), A-4 (1), A-5(1) and A-6(1) under the AASHTO method. SPT N-values ranged from

0 to over 50 blows per foot. Some of the blow counts and subsequent N-values may be misleading due to the larger particle sizes encountered (i.e., gravel, cobbles, and boulders). Moisture contents of the samples ranged from 0.7 to 36.6 percent with the majority of values between 5 and 15 percent. Corrosivity tests were performed on select soil samples with pH values ranging from 4.0 to 9.0 and soil resistivity values ranging from about 6700 to 23,100 ohms-cm. Typically, a pH value of 4.5 or less or a resistivity value less than 2000 ohms-cm is indicative of an aggressively corrosive environment.

The borings encountered auger refusal at depths of about 2 to 20 feet below the ground surface. Auger refusal is a point in the drilling process where if the driller were to continue drilling then there is a greater chance that significant damage to the augers or the drill rig is likely to occur. Rock coring was generally performed where auger refusal was encountered. Bedrock encountered during rock coring consisted of sandstone, siltstone, limestone, shale, and conglomerate. Rock Quality Designation (RQD) values ranged from 0 to 100 percent with the majority of values between 0 and 50 percent. Unconfined compressive strength tests on select rock core samples ranged from 5460 to 19,600 psi (786 to 2822 ksf).

During the performance of the explorations, groundwater was encountered at depths ranging from 7 to 19.6 feet in nine (9) of the borings performed for this project. Groundwater levels were not taken upon completion of drilling due to the addition of drilling fluids during the coring process. Boreholes were not left open for extended periods of time and were backfilled upon completion of drilling due to their location on the existing roadway and for safety concerns.

4.2 SPRING CREEK

4.2.1 MP/Boring 406

Soil Overburden

The overburden soils at the boring location consisted of loose to medium dense clayey gravel with sand

(GC) and silty, clayey sand (SC-SM) to a depth of about 20.5 feet. Underlying the granular soils was very soft to soft sandy, silty clay (CL-ML) extending to a depth of 25 feet below the existing ground surface.

Bedrock was not encountered at this boring location.

Groundwater

Groundwater was encountered at 12 feet during exploration activities.

4.2.2 MP/Boring 411

Soil Overburden

The overburden soils at the boring locations consisted of clayey sand with gravel (SC) and silty sand with gravel (SM) extending to a depth of 12.7 feet below the existing ground surface. The relative density of these materials ranged from medium dense to dense. Significant sandstone boulders were encountered below the granular soils to a depth of 20.4 feet.

Auger Refusal

Auger refusal was encountered on an apparent boulder at a depth of about 12.7 feet below the existing ground surface.

Rock Coring

Rock coring was performed from 12.7 to 20.4 feet and encountered sandstone boulders. Unconfined compressive strength of an intact piece of the sandstone was 8200 psi.

Groundwater

Groundwater was encountered at about 8.5 feet during exploration activities.

4.2.3 MP 428 (Borings 428-A, 428-B)

Soil Overburden

The overburden soils at the boring locations consisted of gravel with silt and sand (GP-GM) and silty sand with gravel (SM) extending to depths of 2.5 to 7.5 feet below the existing ground surface. The relative density of these materials ranged from medium dense to dense. Significant boulders were encountered below these materials.

Auger Refusal

Auger refusal was encountered on an apparent boulder at depths of 3.5 and 8 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 14.2 feet at boring 428-B and encountered shale and sandstone boulders/cobbles. Unconfined compressive strength test of a sample of the sandstone was 19,480 psi.

Groundwater

Groundwater was not encountered during exploration activities.

4.2.4 MP/Boring 431

Soil Overburden

The overburden soils at the boring location consisted of clayey sand with gravel (SC) extending to a depth of 4 feet below the existing ground surface. The relative density of these materials was medium dense.

Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at a depth of 4.2 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 13.2 feet below the existing ground surface and encountered limestone with an unconfined compressive strength of 19,000 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.2.5 MP/Boring 433

Soil Overburden

The overburden soils at the boring location consisted of silty, clayey sand with gravel (SC-SM) extending to a depth of 11 feet below the existing ground surface. The relative density of these materials was loose to medium dense. Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at a depth of 11 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 16.3 feet below the existing ground surface and encountered shale with an unconfined compressive strength of 13,640 psi.

Groundwater

Groundwater was encountered at 7.5 feet during exploration activities.

4.2.6 MP/Boring 452

Soil Overburden

The overburden soils at the boring location consisted of silty gravel with sand (GM) and silty, clayey sand with gravel (SC-SM) extending to a depth of 8.5 feet below the existing ground surface. The relative density of these materials was medium dense. Significant boulders were encountered below the overburden soils to a depth of 27.4 feet below the existing ground surface. Bedrock was encountered below these materials.

Auger refusal was encountered on boulders at a depth of 9 feet below the existing ground surface.

Rock Coring

Rock coring was performed through the boulder field at depths of 9 feet to 27.4 feet. Bedrock (limestone) coring was performed from 27.4 to 30.4 feet below the existing ground surface. The unconfined compressive strength of the sandstone boulder was 9340 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.3 CITICO CREEK

4.3.1 MP 466 (Borings 466-A, 466-B)

Soil Overburden

The overburden soils at the boring locations consisted of silty sand with varying amounts of gravel (SM) extending to depths of 21 to 23 feet below the existing ground surface. The relative density of these materials was very loose to very dense. Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at depths of 21 to 23 feet below the existing ground surface.

Rock Coring

Rock coring was performed to depths of 29.8 to 30.5 feet below the existing ground surface and encountered sandstone and conglomerate. An unconfined compressive strength test of the sandstone was

13,230 psi.

Groundwater

At the two borings, groundwater was encountered at 19 and 19.6 feet during exploration activities.

4.3.2 MP/Boring 469

Soil Overburden

The overburden soils at the boring location consisted of silty sand with gravel (SM) and silty gravel with sand (GM) extending to a depth of 7.5 feet below the existing ground surface. The relative density of these materials was loose to medium dense. Bedrock was encountered below these materials.

Auger refusal was encountered in bedrock at a depth of 8 feet below the existing ground surface.

Rock coring was performed to a depth of 15.5 feet below the existing ground surface and encountered shale. Unconfined compressive strength test of the shale was 10,060 psi.

Groundwater

Groundwater was encountered at 7 feet during exploration activities.

4.3.3 MP 481 (Borings 481-A, 481-B)

Soil Overburden

The overburden soils at the boring locations consisted of silty, clayey sand with gravel (SC-SM) extending to depths of 2.8 to 6 feet below the existing ground surface. The relative density of these materials was medium dense to dense. Bedrock was encountered below these materials.

Auger Refusal

Auger refusal was encountered in bedrock at depths of 3.3 to 6 feet below the existing ground surface.

Rock Coring

Rock coring was performed to depths of 15 to 15.3 feet below the existing ground surface and encountered sandstone. Unconfined compressive strength tests of the sandstone ranged from 6080 to 14,560 psi. Voids were encountered from 6.3 to 7.3 and 8.1 to 9.4 feet at 481-A and 481-B encountered a void from 6 to 8.5 feet.

Groundwater

Groundwater was encountered at 7 feet during exploration activities. Water was observed flowing through voids to the adjacent creek before the exploration/drilling.

4.3.4 MP/Boring 485

Soil Overburden

The overburden soils at the boring location consisted of poorly graded gravel with clay (GP-GC) extending to a depth of 1.5 feet below the existing ground surface. Bedrock was encountered below these materials.

Auger refusal was encountered in bedrock at a depth of 2 feet below the existing ground surface.

Rock coring was performed to a depth of 14.5 feet below the existing ground surface and encountered sandstone. The unconfined compressive strength test of the sandstone was 14,090 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.3.5 MP/Boring 486

Soil Overburden

The overburden soils at the boring location consisted of poorly graded gravel with clay (GP-GC) extending to depth of a 6.5 feet below the existing ground surface. The relative density of these materials was medium dense. Bedrock was encountered below these materials.

Auger Refusal

Auger refusal was encountered in bedrock at a depth of 7.3 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 14.6 feet below the existing ground surface and encountered limestone. The unconfined compressive strength test of the limestone was 18,130 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.3.6 MP/Boring 487

Soil Overburden

The overburden soils at the boring location consisted of poorly graded gravel with clay and sand (GP-GC) and well graded gravel with silt and sand (GW-GM) extending to a depth of 11.5 feet below the existing ground surface. The relative density of these materials was loose to medium dense. Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at a depth of 11.7 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 21 feet below the existing ground surface and encountered limestone. Unconfined compressive strength test of the limestone was 16,070 psi.

Groundwater

Groundwater was encountered at 11 feet during exploration activities.

4.3.7 MP/Boring 508

Soil Overburden

The overburden soils at the boring location consisted of silty, clayey sand with gravel (SC-SM) extending to a depth of 2.5 feet below the existing ground surface. The relative density of these materials was very dense. Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at depths of 2 and 3.5 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 10.7 feet below the existing ground surface and encountered shale. Unconfined compressive strength test of the shale was 5460 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.3.8 MP/Boring 515

Soil Overburden

The overburden soils at the boring location consisted of well graded sand with silt and gravel (SW-SM) extending to a depth of 4 feet below the existing ground surface. The relative density of these materials was medium dense. Bedrock was encountered below these materials.

Split Spoon Refusal

Split spoon refusal was encountered in bedrock at depths of 4 and 6.5 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 29.7 feet below the existing ground surface and encountered coal and calcareous and carbonaceous shale.

No groundwater was encountered during exploration activities.

4.3.9 MP 518 (Boring 518-A)

Soil Overburden

The overburden soils at the boring location consisted of poorly graded gravel with clay and sand (GP-GC) extending to depth of 5 feet below the existing ground surface. The relative density of these materials was loose to medium dense. Bedrock was encountered below these materials.

Auger Refusal

Auger refusal was encountered in bedrock at a depth of 5.5 feet below the existing ground surface.

Rock Coring

Rock coring was performed to a depth of 19.5 feet below the existing ground surface and encountered sandstone. The unconfined compressive strength test of the sandstone was 19,600 psi.

Groundwater

No groundwater was encountered during exploration activities.

4.3.10 MP 518-B (Borings 518-B, 518-C)

Soil Overburden

The overburden soils at the boring locations consisted of silty, clayey sand with gravel (SC-SM), clayey sand (SC), and silty, clayey gravel with sand (GC-GM) extending to depths of 6.3 to 9 feet below the existing ground surface. The relative density of these materials was very loose to medium dense.

Significant boulders with soil layers were encountered to depths of 11.7 and 18.2 feet below the existing ground surface. The relative density of these materials was very dense. Bedrock was encountered below these materials.

Auger Refusal

Auger refusal was encountered on boulders and in bedrock at depths of 6.3 to 18.2 feet below the existing ground surface.

Rock Coring

Rock coring was performed to depths of 20.3 to 21.2 feet below the existing ground surface and encountered sandstone, sandy siltstone, and sandy shale. Unconfined compressive strength tests of the sandstone ranged from 17,550 to 18,510 psi and 10,820 psi for the sandy shale.

No groundwater was encountered during exploration activities.

Analysis and Recommendations

5.0 ANALYSIS AND RECOMMENDATIONS

5.1 GENERAL

In February and April of 2020, a series of heavy rain events caused significant roadway damage within the Cherokee National Forest. The damaged roadways reduce/prevent roadway public access into various points of the Cherokee National Forest. Damages include landslides, roadway and shoulder washouts, clogged or damaged culverts, and other miscellaneous conditions. The FHWA has contracted with Stantec to perform geotechnical explorations laboratory testing and provide design recommendations for remediation of the affected areas. Two areas designated as Spring Creek and Citico Creek contain a total of sixteen specific slide and culvert locations.

The geotechnical explorations were performed on the existing roadways above the landslide or culvert.

The lack of access to the hillside and downslope areas of the slides, due to safety and environmental concerns, generally limited exploration activities in these areas. Observation of the slopes along the respective alignments and the overall condition of the roadways indicates the adjacent hillsides to be generally stable. From our observations the downhill slope failures do not appear to be significantly impacting the existing roadways, i.e., limited or no distress or apparent cracking in the roadway surface or sinking of the shoulder. From the photographs provided and those taken during exploratory activities it appears that most of the slide activity is erosional or surficial in nature and not a deep-seated failure. The recommendations that follow are based on the information discussed in this report, and the interpretation of the subsurface conditions encountered at the site during our fieldwork.

5.2 SEISMIC HAZARD ANALYSIS

The two project areas are mapped generally within or in the vicinity of the East Tennessee Seismic Zone

(ETSZ). Most earthquakes in the ETSZ generally occur in the highly tilted and folded rock layers and are thought to originate in the Precambrian basement structures not related to Appalachian orogeny. The

ETSZ is located mainly in the Valley and Ridge province of the southern Appalachians and is the second-most seismically active area in the eastern United States, after the New Madrid seismic zone.

The seismic hazard of the ETSZ is difficult to assess, as there is no obvious surface faults found previously except a few recent paleo seismic field surveys, and no recorded historical earthquakes of magnitude greater than 5 (C. Daniels, et al).

In accordance with AASHTO Section 3.10, the overburden soils at the two areas, Spring Creek and Citico

Creek, were classified by their stiffness for Site Class. Stantec conducted a review of the soil profile data for the borings completed for this exploration. Using the boring from each area with the deepest overburden profile, Stantec analyzed the soil and bedrock profile using the average Standard Penetration

Test (Method B) over the upper 100 feet of profile in accordance with AASHTO Section 3.10.3.1. In general, the soil profile over a depth of about 100 feet consists of layers of granular soils (2 to 23 feet) overlying bedrock. The granular soils were typically loose to very dense. Based on the analyses each of the two areas can be classified as Site Class D with the exception of MP 406 which was classified as

Site Class E. Stantec used these site class designations along with the values from the national ground motion maps (PGA, Ss, S1) to obtain the applicable site factors (Fpga, Fa, Fv). Applying the site factors and resulting acceleration coefficients indicates that these sites may be designed within Seismic Zone 2.

In accordance with AASHTO 11.5.4.2, a seismic design shall not be considered mandatory for wall located in Seismic Zones 1 through 3 or for walls at sites where the adjusted peak ground acceleration, As, 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 supported structure, to be designed for seismic loading and poor seismic performance of the wall could impact the seismic performance of that structure.

In addition, a seismic design should be considered for Seismic Zone 2 or higher if the following is greater than 30 ft:

the exposed wall height plus the average depth over the width of the wall of any soil surcharge present, or for tiered walls, the sum of the exposed height of all the tiers plus the average soil surcharge depth.

Based on the criteria listed above, the eight proposed wall/slope sites should not require a seismic design due to an adjusted PGA of 0.23 (< 0.4g), the sites are located in Seismic Zone 2, and the exposed wall heights plus surcharge is less than 30 feet high. Seismic analyses are included in Appendix D.

In accordance with AASHTO 11.6.5.2.1 the seismic horizontal acceleration coefficient (kh) for computation of seismic lateral earth pressures shall be determined on the basis of the PGA at the ground surface (i.e., kh0 = Fpga∙PGA = As), assuming zero wall displacement occurs. The vertical acceleration coefficient, kv, is assumed to be zero for determining lateral earth pressures. From AASHTO 11.6.5.2.2, a wall that is capable of displacement of 1 to 2 inches or more during the design seismic event, kh may be reduced to

0.5kh0 without conducting a deformation analysis. The reduction in kh can also be used to reduce the horizontal seismic forces applied to slope stability. Using a conservative Site Class analysis for Citico

Creek the resulting As is equal to 0.23. Therefore, the reduced kh = 0.5 As = 0.12 for the slope stability analyses.

5.3 CULVERT AND AOP CROSSINGS

A total of seven major culverts, six along Spring Creek Road and one along Citico Creek Road are planned for replacement. Concrete box culverts (4-sided) are planned for the stream/creek crossings and corrugated aluminum arch culverts are planned for the Aquatic Organism Passage (AOP) crossings. The proposed locations and size of culverts are summarized in the table below.

Table 4. Summary of Culvert Structures

MP

Location

Road

Crossing Size & Type

Length

(ft)

Slope

Proposed

Headwall

Height (ft)

Anticipated Subgrade

Material

Spring

Cr.

8’x5’ Concrete

Box

32.0 6.6 --

Clayey gravel w/sand

Spring

Cr.

8’x5’ Concrete

Box

32.0 27.2 5.0

Clayey sand w/gravel, Sandstone boulders

Spring

Cr.

8’x4’ Concrete

Box

30.0 11.1 2.0

Gravel w/silt and sand, Sandstone boulders

Spring

Cr.

4’x4’ Concrete

Box

22.0 7.1 1.0

Clayey sand w/gravel

Spring

Cr.

147”x53” Alum.

Arch Culvert

24.0 2.8 --

Clayey sand w/gravel

Spring

Cr.

218”x55” Alum.

Arch Culvert

26.0 7.6 --

Clayey sand w/gravel, Sandstone boulders

Citico Cr. 8’x4’ Concrete

Box

24.0 7.5 2.0

Silty gravel w/sand

Based on the conditions encountered, a yielding foundation system is recommended for the culverts.

Culverts are assumed to not require separate spread footings, except for the arch AOP structures.

Foundation widths for culvert structures are assumed to equal the base width of the structure.

Foundations for headwalls should be sized appropriately to resist the sliding, bearing, and lateral loads as necessary. The service limit state should use a resistance factor of 0.33 while the strength and extreme resistance factors used should be 0.45 and 1.0 for the strength and extreme limit state analyses. The table below summarizes the results of the foundation bearing resistance analyses for the proposed culverts.

Table 5. Summary of Culvert Bearing Resistances

MP

Location

Road

Crossing Size & Type

Bearing

Elevation

Bearing Resistance

(ksf) Coefficient of Friction, (tanδ) Inlet Outlet Nominal Factored

406 Spring Cr.

8’x5’

Concrete Box

732.4 730.3 11.7 5.3 0.53

MP

Location

Road

Crossing Size & Type

Bearing

Elevation

Bearing Resistance

(ksf) Coefficient of Friction, (tanδ) Inlet Outlet Nominal Factored

411 Spring Cr.

8’x5’

Concrete Box

729.4 720.7 11.7 5.3 0.53

428 Spring Cr.

8’x4’

Concrete Box

755.5 752.2 12.1 5.5 0.55

431 Spring Cr.

4’x4’

Concrete Box

763.5 761.9 7.2 3.3 0.55

469 Citico Cr.

8’x4’

Concrete Box

1908.0 1906.2 9.6 4.3 0.55

For the two AOP structures (MP 433 and 452), concrete spread footings are anticipated to range from 3 to 5 feet wide. Bearing resistance for a 3-foot-wide footing is provided in the table below. Additional bearing resistances can be provided upon request. Guidance for AOP structures recommends the footings extend down below the lower vertical adjustment potential for scour protection (LVAP). This places the footing base at about 4 to 5 feet below the bottom of the metal arch structure.

Table 6. Summary of AOP Culvert Bearing Resistances

MP

Location

Road

Crossing Size & Type

Bearing

Elevation

Bearing Resistance

3 ft wide footing (ksf) Coefficient of Friction, (tanδ) Inlet Outlet Nominal Factored

433 Spring Cr.

147”x60” Alum

Arch Culvert

773.0 772.3 9.2 4.1 0.50

452 Spring Cr.

218”x60” Alum

Arch Culvert

817.5 815.5 11.1 5.0 0.50

Because the culvert structures will be constructed on a slope the resistance to sliding should also be checked in general accordance with AASHTO Section 10.6.3.4. The coefficient of friction with respect to sliding is provided in the tables above. Bearing resistance analyses are included in Appendix D.

5.4 SLOPE STABILIZATION RECOMMENDATIONS

The issues associated with slope stability occur primarily in the Citico Creek area. Several slope remediation methods were suggested in the provided information for the various sites. These consisted of “hard faced” options such as gabion walls, a rip rap slope, and a rockery wall. Stantec reviewed provided photographs and photos taken during exploratory activities. Several of the proposed slope failures appear to be erosion problems associated with Citico Creek adjacent to the existing roadway.

Table 7. Summary of Proposed Slope Remediation

Embankment Location

Assumed Repair

Dimensions (HxL)

Chosen Remediation

Anticipated Bearing Material

Estimated Overburden Thickness

(ft)

Elev. Diff

– Road to Creek (ft)

MP 466 15’x80’ Reinforced Soil Slope

Silty Sand 21 N/A

MP 481 8’x70’ Special

Embankment Bedrock 4 - 6 4 - 6

MP 485 12’x52’ Geocell Sand & Gravel

2 - 3 N/A

MP 486 12’x30’ Special

Embankment Bedrock 6.5 7

MP 487 12’x25’ Special

Embankment

Gravel w/silt & sand

11.5 6

MP 508 6’x40’ Replace

Upper Wire Baskets

Existing gabion- Bedrock

2.5 8+

MP 515 12’x45’ Special

Embankment Coal Layer 4 5-6

MP 518-C 9’x20’ Special

Embankment Boulders 6 11-12

MP 518-B

15’x70

Special Embankment

Boulders 9 11-12 MP 518-C Boulders 6 11-12

Where possible, mitigation methods were selected/designed so as not to trigger potential 401/404 environmental permit requirements. However, in order to not sacrifice roadway width, the toe of the

Special Rock Embankment may encroach upon the edge of the creek at certain locations.

Stantec created several cross sections along the various alignments to evaluate the stability of existing slopes. Based on our visual observations along the roadway the existing soil slopes appear to be relatively stable. Most soil slopes do not appear to be excessively steep (steeper than 1H:1V) and given the relatively shallow depth to bedrock the soil cover is expected to be relatively thin. At several locations bedrock/boulders are exposed at the slope/stream bank surface. Based on the numerous rock outcroppings and boulders visible both up and down slopes and the size and density of foliage, we estimate the overburden soils on most slopes to be approximately 2 to 3 feet thick or less where bedrock is shallow. The overburden soils on slopes are likely residual in nature i.e., weathered from the parent bedrock and similar to what was encountered in the borings.

The Special Rock Embankment is controlled by FP-24, Section 252, Rockery, Special Rock

Embankment, and Rock Buttresses. EFLHD Detail E252-01 was used as a guide for determining the preliminary dimensions of the rockfill zone. Certain dimensions such as the offset at the toe and the grade of the rock fill slope were considered impractical due to the confined space between the apparent edge of creek and edge of shoulder of the road. In most cases the maximum toe offset is about 1 foot. At

MP481, MP487, MP515, and MP518 the toe of the special rock embankment may encroach on portions of the creek in order to “reclaim” lost embankment and to maintain the roadway width. Due to the potential for heave at the base of the special rock embankments and the difficulties with dewatering next to the flowing creek, the rock fill embankments were designed to be constructed at the waterline of the creek.

Soil and rock parameters used in the analyses were derived from the laboratory test results and from correlations from FHWA manuals. In addition, Stantec estimated the parameters of the Special Rock

Embankment. Coarse rockfill is difficult to characterize by testing. When a material contains cobbles or boulders, it is difficult to obtain representative samples and conduct meaningful tests with conventional laboratory equipment. Large-scale tests reported in the literature provide valuable data on the engineering properties of rockfill (See Figure 2 in Appendix F), but relatively few such tests have been published. However, appropriate properties can be usually selected on the basis of engineering judgment, where that judgment is supported by the performance of similar materials in other projects.

Fortunately, rockfill materials exhibit favorable engineering behavior. If the rockfill is relatively clean (voids between the larger rock particles are open), potential excess pore pressures will quickly dissipate.

Interlocking between large, angular pieces of durable rock will result in relatively high shear strength.

Satisfactory performance in these materials can usually be demonstrated, even when conservative estimates of strength are used. Based on our review of the available literature and provided the contractor constructs a dense well graded rockfill with strong particles in accordance with FP-24 Section

252, an internal friction angle of 48 degrees was used in our analyses. References are listed in Section 6.

5.4.1 MP 466

The existing slope at MP 466 is at approximately 1.5H to 2H:1V. From provided photographs and photos taken during field exploration minor surface sloughing appears to have occurred. Stantec performed a slope stability analysis of the existing conditions which resulted in a factor of safety of 1.28. With regrading and addition of rip rap to the slope, the factor of safety was 1.3. For the slope at 466-B, the application of seismic horizontal acceleration reduces the factor of safety to below 1.0. Removal and replacement of the existing soils with better quality aggregate does not sufficiently improve the factor of safety. Therefore, ground improvement consisting of a reinforced soil slope is recommended.

The suggested improvement plan consists of excavating the upper loose soils to a depth of about 8 to 9 feet below existing grade (about Elev. 2045 at 466-A and 2046 at 466-B). The base of the excavation should extend approximately 10 feet horizontally into the slope with a 1H:1V backslope. The Contractors responsible person shall make the final determination for the backslope in the field based on the exposed conditions. Using the existing granular soils and a biaxial geogrid with an ultimate tensile capacity of

2000 lbf/ft, reconstruct the 2H:1V slope with alternating layers of geogrid and 18-inch layers of compacted soil. The geogrid should wrap the face of each layer and extend a minimum of 3 feet beneath the next layer. A minimum of 4 inches of compacted soil should separate the 3-foot extension and the next overlying full-length reinforcement layer. The exposed geogrid face on the slope will likely require a filter fabric liner to prevent the loss of soil through the grid. The face of the reinforced slope as well as 5 feet below the reinforced zone should be covered with 12 inches of rip rap. The geogrid reinforcement should not be left exposed to sunlight. Because of the roadway slope and curve adjacent to the top of the slope, a catchment ditch or some other way to direct surface runoff should be considered. The preliminary reinforced soil slope analysis is provided in Appendix F along with the MSE wall analyses. The final design of the reinforced soil slope (by the contractor’s engineer) should also check to make sure that the selected geogrid meets pullout and tensile capacity requirements as outlined in Geotechnical Engineering

Circular 11, Vol 2.

5.4.2 MP 481

From provided photographs and photos taken during field exploration minor erosion appears to have occurred at spots along the creek bank. The elevation change from the road to the edge of water is estimated at about 4 to 5 feet and portions of the bank are already armored with boulders and existing rock. To create a rockery wall here would require excavation of the existing boulders and bedrock and would likely infringe upon the existing waterway, thus requiring environmental permitting. We recommend the use of a special rock embankment placed just at/above the water’s edge. This is similar to a rockery wall where gravity and friction keep the stacked stones in place. We recommend the slope face roughly match the slopes adjacent to the area being repaired but not be steeper than 1H:1V.

5.4.3 MP 485

At MP 485 the test boring indicates that bedrock is very shallow (at about 1.5 ft) and the provided photographs indicate possible minor surface sloughing of the slope. Creating a retaining wall or rock fill at the proposed 12-foot height would likely require significant rock excavation. We recommend using a geocell product to rebuild and protect the slope after grading has removed the scarp. A geocell solution provides a way to grow and maintain a vegetated slope as steep as 1H:1V. A geocell solution should also reduce the risk of erosion and future slides at this location. Stantec contacted a geocell supplier to determine the feasibility of this technique and provide a preliminary estimate. As geocell products tend to be supplier specific and supplier designed (propriety design), we have included some preliminary information in Appendix G to assist in the decision-making process. We would anticipate that regrading and the application of a “green type” of slope remediation would be more desirable and cost effective than extensive rock excavation. Near the bottom of the geocell system a two-foot-wide rip rap zone should be placed to reduce the risk of erosion at the toe.

5.4.4 MP 486

MP 486 is located along the inside curve of the creek where the erosion forces are the greatest. Based on provided photos, the height of the bank is approximately 7 feet to the water’s edge. From the test boring performed in the road, the subsurface consists of about 6.5 feet of granular soils overlying bedrock. From the provided photographs the creek bed is lined with boulders and apparent bedrock. For remediation of the approximately 30 feet stream bank, we recommend a special rock embankment placed just at/above the water’s edge. The rock fill is anticipated to bear on bedrock. We recommend the slope face roughly match the slopes adjacent to the area being repaired but not be steeper than 1H:1V.

5.4.5 MP 487

At the location of MP 487 the bank of the creek is eroding, likely due to an eddies pool that forms during high water flow just downstream of a large boulder. Based on provided photos, the height of the bank is approximately 5 to 6 feet to the water’s edge. From the test boring performed in the road, the subsurface consists of about 11 feet of granular soils overlying bedrock. From the provided photographs the creek bed is lined with boulders and apparent bedrock. For remediation of the approximately 25 feet stream bank, we recommend a special rock embankment placed just at/above the water’s edge. The rock fill is anticipated to bear on granular soils. We recommend the slope face roughly match the slopes adjacent to the area being repaired but not be steeper than 1H:1V. To stabilize the rock fill slope, biaxial geogrid is required within the rock fill due to the weaker anticipated soil subgrade.

5.4.6 MP 508

MP 508 is the approach to a bridge over Spring Creek. The existing approach on the north side of the creek is constructed of a short MSE wire basket framed wall over a gabion wall foundation. Both the

MSE wire basket wall and gabion wall appear to be bearing on the existing bedrock. From the provided photographs the maximum wall height is estimated at about 8 feet tall. The photographs also show the existing wire baskets and gabion wall bulging.

We understand that the existing bridge is planned to be replaced in the near future. The existing gabion portion of the wall appears to be functioning adequately. Therefore, we recommend that the upper two layers of MSE wire baskets be removed and replaced with new baskets and fill. This will allow any voids or low spots between the MSE baskets and the gabion foundation to be filled and leveled. No additional analysis was performed for this area. Once the new bridge is designed, we recommend that a new concrete wing wall be included to replace the MSE wire baskets and gabion walls bearing on bedrock.

5.4.7 MP 515

MP 515 is another location where erosion of the creek bank is occurring. Based on provided photos, the height of the bank is approximately 6 to 7 feet to the water’s edge. From the test boring performed in the road, the subsurface consists of about 4 feet of granular soils overlying bedrock (coal). From the provided photographs the creek bed is lined with cobbles and boulders and apparent bedrock. For remediation of the approximately 45 feet long stream bank, we recommend a special rock embankment placed just at/above the water’s edge. We recommend the slope face roughly match the slopes adjacent to the area being repaired but not be steeper than 1H:1V. To stabilize the rock fill slope, biaxial geogrid is required within the rock fill due to the weaker anticipated coal subgrade.

5.4.8 MP 518-C

MP 518C is a location where erosion of the creek bank has occurred and is threatening the roadway.

Based on the provided photos, the height of the bank is approximately 11 to 12 feet to the water’s edge.

From the test boring performed in the road, the subsurface conditions consist of about 7 feet of granular soil over interbedded boulders and soil overlying bedrock. From the provided photographs the creek bed is lined with cobbles and boulders and apparent bedrock. For remediation of the approximately 20 feet long stream bank, we recommend a special rock embankment placed just above the water’s edge. We recommend the slope face roughly match the slopes adjacent to the area being repaired but not be steeper than 1H:1.1V. To stabilize the rock fill slope, biaxial geogrid is required within the rock fill due to the anticipated weaker boulder and soil subgrade.

5.4.9 MP 518-B & C

MP 518B&C is another location where erosion of the creek bank has occurred and is threatening the roadway. Based on the provided photos, the height of the bank is approximately 11 to 12 feet to the water’s edge. From the test borings performed in the road, the subsurface conditions consist of about 7 to 9 feet of granular soils over interbedded soil and boulders overlying bedrock. From the provided photographs the creek bed is lined with cobbles and boulders and apparent bedrock. For remediation of the approximately 70 feet long stream bank, we recommend a special rock embankment placed above the water’s edge. We recommend the slope face roughly match the slopes adjacent to area being repaired but not be steeper than 1H:1.1V. To stabilize the rock fill slope, biaxial geogrid is required within the rock fill due to the anticipated weaker boulder and soil subgrade.

5.4.10 Slope Stabilization Results

Global stability analyses were performed for each of the special rock embankment locations and the reinforced soil slope. Typical highway minimum factors of safety of 1.3 (1.1 for seismic) are considered acceptable for slope stability.