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Final Geotechnical Report

Norris Peak Road Realignment Black Hills National Forest

Pennington County, South Dakota

SD FLAP CR223801 (1)

Yeh Project No. 215-096

November 2, 2015

Prepared for:

Atkins North America, Inc.

4601 DTC Boulevard, Suite 700 Denver, Colorado 80237

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433

Phone: 805-481-9590 i

Final Geotechnical Report

Norris Peak Road Realignment Black Hills National Forest

Pennington County, South Dakota

SD FLAP CR223801 (1)

Yeh Project No. 215-096

November 2, 2015

Prepared by:

Jonathan D. Blanchard, GE (Ca) Geotechnical Engineer

Simon P. Boone John D. Duffy, CEG (Ca) Staff Geologist Senior Engineering Geologist

Reviewed by:

Robert F. LaForce, P.E.

Senior Materials Engineer

Geotechnical Investigation Report Project No. 215-096 Norris Peak Road Realignment November 2, 2015 ii

Table of Contents

1. PURPOSE AND SCOPE OF STUDY

2. PROPOSED PROJECT

3. GEOLOGIC SETTING AND SITE CONDITIONS

3.1 GEOLOGIC SETTING

3.2 SITE CONDITIONS

3.3 CLIMATE

4. FIELD EXPLORATION AND TESTING

4.1 DRILLING

4.2 LABORATORY TESTING

4.3 SEISMIC REFRACTION

4.4 GEOLOGIC MAPPING

5. SLOPE STABILITY

5.1 ROCK SLOPE STABILITY

5.2 FILL SLOPE STABILITY

6. SUBSURFACE CONDITIONS

7. GEOTECHNICAL RECOMMENDATIONS

8. EARTHWORK DESIGN

8.1 GENERAL GRADING

8.1.1 EROSION AND SLOPE DRAINAGE

8.1.2 EARTHWORK FACTORS

8.1.3 EXCAVATION

8.2 SLOPE DESIGN

8.2.1 SUBEXCAVATION

8.2.2 EMBANKMENT ON SLOPING GROUND

8.2.3 ROCKFALL MITIGATION

8.3 CULVERT DESIGN

iii

9. PAVEMENT DESIGN RECOMMENDATIONS

9.1 DESIGN TRAFFIC VOLUME AND SUBGRADE

9.2 STRUCTURAL SECTION

9.3 BINDER AND MIX RECOMMENDATIONS

9.4 SUBGRADE STABILIZATION

9.5 PAVEMENT MAINTENANCE

10. NOTES TO DESIGNER

10.1 CORROSION CONSIDERATIONS

11. NOTES TO CONSTRUCTION

11.1 GROUNDWATER

11.2 COMPACTION

11.3 HARD ROCK

11.4 CONSTRUCTION OBSERVATION

11.4.1 SUBGRADE EVALUATION

11.4.2 GRADING OBSERVATION

11.4.3 ROCK SLOPES

12. LIMITATIONS

13. REFERENCES

List of Figures

FIGURE 1 - SITE LOCATION MAP

FIGURE 2: TYPICAL SIDEHILL GRADING

FIGURE 3: SUMMARY OF STRUCTURAL SECTION RECOMMENDATIONS

List of Tables

TABLE 1: ESTIMATED MATERIAL PROPERTIES

TABLE 2: RECOMMENDED CUT-FILL SLOPE INCLINATIONS

Plates

Regional Geologic Map iv

List of Appendices

Page

Appendix A - Boring Logs Field Exploration Plan ..................................................................................................................... A-1 to 3 Legend for Symbols used on Boring Logs ................................................................................................ A-4 Boring Logs 15E-01 to 15E-04 .......................................................................................................... A-5 to 8 Borings Logs 15P-01 to 15P-03 ...................................................................................................... A-9 to 11 Drilling Photos ............................................................................................................................ A-12 to 13 Appendix B - Laboratory Test Results Summary of Laboratory Test Results ...................................................................................................... B-1 R-value .........................................................................................................................................B-2 and 3 Consolidation Test, Constant Strain ........................................................................................................ B-4 Direct Shear Test ................................................................................................................................... B-5 Unconsolidated, Undrained Triaxial Test ................................................................................................ B-6 Unconfined Compressive Strength .................................................................................................. B-7 to 9 Appendix C - Seismic Refraction Results Olson Engineering (2015) Report .................................................................................................... C-1 to 5 Appendix D - Stereographic Analyses Results Discontinuity Data Collected .................................................................................................................. D-1 Stereographic Analyses ............................................................................................................... D-2 to D-6 Appendix E – Slope Stability Results Existing .................................................................................................................................................. E-1 Proposed (buried alluvium) .................................................................................................................... E-2 Proposed (subex alluvium) ..................................................................................................................... E-3 Appendix F – Pavement Design Calculations Required Structural Number .................................................................................................................. D-1 Structural Thickness ............................................................................................................................... D-2

Geotechnical Report Project No. 215-096 Norris Peak Road Realigment November 2, 2015

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates, Inc., as a sub-consultant to Atkins North America, Inc., Denver, Colorado was retained by the Federal

Highway Administration (FHWA), Central Federal Lands Highway

Division (CFL) to provide geotechnical recommendations for the design of the realignment of Norris Peak Road in the Black

Hills National Forest in

Pennington County, South

Dakota. Norris Peak Road experiences heavy motorcycle use during the Sturgis Rally held during the first week of every

August. There are two sharp reverse curves at 9 percent grade along Norris Peak Road with 15 miles per hour (mph) advisory speeds. Since 2004, there have been 16 reported accidents within the project limits. The improved horizontal geometry of the roadway will provide safer access to and through the Black Hills National Forest. The project location is shown on

Figure 1.

The geotechnical investigation included: research of the regional and local geology, seismic refraction surveys, and subsurface exploration by auger and air rotary borings. Laboratory testing was performed on selected soil samples obtained from the borings and rock samples obtained from the discontinuity mapping areas. Results of the laboratory testing were considered in the analyses to provide recommendations. Logs of the exploratory borings and results of the laboratory testing are included in the report. This report presents geotechnical recommendations for the design of roadway cuts, embankments and pavements.

Figure 1 - Site Location Map

2. PROPOSED PROJECT

The proposed project is shown on the design plans being prepared by Atkins (2015). The purpose of the project will be to remove two sharp reverse curves by straightening the Norris

Peak Road alignment and moving it toward the east. The proposed realignment will be approximately 0.75 miles in length extending along Norris Peak Road from about 0.3 miles north of the intersection with South Dakota Highway 44 (SD 44; Station 10+00) to 1.4 miles north of the intersection with SD 44 (Sta. 49+00). The re-alignment will improve safety and drivability in a section that includes steep grades and sharp curves.

The design for the new roadway is for a two-lane road with 12-foot travel lanes and 4-foot shoulders. The realignment is mostly through virgin terrain, and crosses the existing alignment once near Sta. 42+50. The new roadway transitions between cut and fill, with maximum centerline cuts of up to about 12 feet, and maximum centerline fills of up to about 30 feet.

Culverts will be provided at three drainage crossings: with dual 60-inch culverts to be provided at Sta. 15+30 and Sta. 28+48, and a single 48-inch culvert to be provided at Sta. 41+50.

3. GEOLOGIC SETTING AND SITE CONDITIONS

3.1 GEOLOGIC SETTING

The site is located in the Black Hills of South Dakota. The Black Hills physiographic province is an elongate dome-shaped uplift located in the Upper Great Plains of South Dakota and

Wyoming. The uplift occurred during the Laramide Orogeny, a period of mountain building that occurred in the western United States beginning during the late Cretaceous period and ending in the early Tertiary period, approximately 60-65 million years ago. The site is located within the core of the uplifted region, in an area that is characterized by metamorphic rock, mainly phyllite and quartzite.

The regional geology in the site vicinity, as mapped by Norton et al (2008), is shown on Plate 1.

The bedrock geology of the region consists of the core of Precambrian granite, phyllite and schist dome that is the Black Hills, flanked by younger sedimentary formations. Surficial deposits consisting of Tertiary terrace deposits, and recent colluvium, alluvium, and artificial fill locally conceal those units within the project limits.

3.2 SITE CONDITIONS

Terrain along the proposed Norris Peak Road re-alignment is hilly and forested with rolling, open meadows crossed by occasional drainages generally flowing west toward the existing alignment of Norris Peak Road. The ground elevation generally rises from approximately elevation (el.) 4,328 feet at Sta. 10+00 to about el. 4,656 feet at Sta. 48+35. The topography undulates along the proposed alignment and crosses various drainage and knolls. Existing site grades average about 12 percent along the alignment, with cross slopes that commonly range from approximately 30 to 45 percent, and local rock outcrops that stand near vertical.

Vegetation consists of mostly Ponderosa Pine forested areas with grasses and shrubs comprising the open meadows. The area shows evidence of prior logging operations.

3.3 CLIMATE

Climate data is based on available records for the Johnson Siding from the Western Regional

Climate Center from 1994 to 2011. Johnson Siding is approximately 0.3 miles from the jobsite.

The area has a typical midcontinent climate. The mean annual precipitation for the historic period available is 20.63 inches, most of which occurred in the spring and summer months

(March through August). The average snowfall during the historic period is 53.9 inches. The average annual high temperature is 58.6o F and average annual low temperature is 31.6o F.

The annual high temperatures occur in July and August and the annual low temperatures occur in December, January, and February. By contrast, the Rapid City Regional Airport normally receives 16 inches of precipitation annually, including an average of 40.9 inches of snowfall, and has normal annual maximum temperatures about 2o F higher that at Johnson Siding.

Freeze-thaw conditions are a consideration for the site. Frost depths are reported to exceed 3 feet in southwestern South Dakota.

4. FIELD EXPLORATION AND TESTING

The field exploration for this project consisted of exploratory drilling, seismic refraction studies, and geologic and discontinuity mapping of the project site. A total of 7 borings, 3 seismic refraction lines, and supporting laboratory testing were performed for this evaluation. The surficial geology was mapped in the field and discontinuity data was recorded where bedrock outcroppings existed within the project limits.

4.1 DRILLING

Inberg-Miller Engineers of Casper, Wyoming was the drilling subcontractor for the field exploration program. Inberg-Miller used a truck-mounted Mobile B-57 drilling rig outfitted to perform hollow-stem auger, tri-cone rotary and diamond tipped core barrel operations to perform the drilling. Seven (7) exploratory borings were drilled for the geotechnical investigation during the period of July 14 and 15, 2015. The borings were drilled to depths of up to approximately

16.5 feet below the ground surface. Upon completion, the borings were backfilled with cuttings and approved local soil. The borings were logged by a representative of Yeh and Associates.

The boring locations, logs, and photos of the boring locations are presented in Appendix A.

The holes were sampled by collecting bulk samples from the auger flights, and by driving 3-inch and 2-inch outside diameter split spoon samplers into the materials encountered, typically at 1.5 to 5-foot intervals within each boring. The 3-inch samplers contained brass liners. The 2-inch sampler was used without liners. The samplers were driven into the ground at the bottom of the hole using a 140-pound automatic trip hammer in general accordance with ASTM 1586 (the

Standard Penetration Test (SPT)). The blow count, N-value, reported on the logs is the number of blows needed to drive the sampler 12 inches after the sampler was seated 6 inches into the material at the bottom of the hole. The blow count can be used as a relative measure of the material stiffness or relative density.

4.2 LABORATORY TESTING

Laboratory tests for classification and selected geotechnical properties were performed on selected soil samples from the bulk samples, California, and split spoon samplers that were recovered from the field exploration program. The testing was performed in general accordance with applicable procedures of either AASHTO or ASTM, as noted on the test results.

Classification and R-value testing was performed in our laboratory facilities located in Grover

Beach, California and Denver, Colorado. Laboratory tests performed at our labs included moisture content and unit weight, Atterberg limits, pH, resistivity, soluble sulfate content, and chlorine ion content. Selected bedrock samples collected from rock outcrops within the discontinuity mapping areas along the project alignment were sent to Advanced Terra Testing

Labs in Lakewood, Colorado for unconfined compressive strength tests. Tests for direct shear strength, unconsolidated undrained (UU) triaxial compressive strength, and consolidation were performed at the California Polytechnic lab in San Luis Obispo, California. Laboratory test results are presented in Appendix B. Following the completion of the laboratory testing, the field descriptions were confirmed or modified as necessary and boring logs were prepared.

4.3 SEISMIC REFRACTION

Olson Engineering of Wheatridge, Colorado was the geophysical subcontractor for the field exploration program. A total of three (3) seismic refraction surveys were completed for the geotechnical investigation. The seismic refraction surveys were performed between July 14 and

July 15, 2015 using 24 and 48 channel geophone spreads. The locations of the seismic refraction surveys and the results are summarized on the plan and profile figures in Appendix A.

A copy of the Olson report, with their results and the interpreted seismic refraction surveys and profiles, is presented in Appendix C.

4.4 GEOLOGIC MAPPING

Yeh geologists mapped the surficial geology and outcrops within the project limits during the period of July 13 through 16, 2015 by physically traversing the proposed alignment within the project limits and recording the data on the plan sheets developed by Atkins. Where bedrock was exposed, discontinuity data was recorded. The data was used to assess the rock characteristics for the purposes of rock slope stability and rockfall analyses. The results of the mapping are summarized on the field exploration plan in Appendix A.

5. SUBSURFACE CONDITIONS

The subsurface conditions encountered in the exploratory borings drilled at the site along the proposed re-alignment are discussed in the sections below. Areas where bedrock outcroppings were observed are also described. The units encountered at the site generally consisted of artificial fill, colluvium or alluvium overlying predominantly decomposed phyllite or quartzite bedrock. The approximate locations of the borings, mapping of the surface geologic units, seismic lines, and a summary of the refraction data are shown on the project plan and profile are presented with the boring logs and photographs in Appendix A. A description of the units encountered is summarized below.

Artificial Fill (af). Artificial fill associated with the existing Norris Peak Road alignment was observed from approximately Station 10+00 to Station 13+50 and Station 41+50 to Station

42+75. The artificial fill was observed to form the base of the existing alignment of Norris Peak

Road. The artificial fill from Station 41+50 to Station 42+75 was observed to form an embankment approximately 15 feet high. The artificial fill appears to consist of a mixture of imported sand and gravel.

Alluvium (Qa). Alluvium was encountered near the surface in boring 15E-01, at approximately

Station 15+20. The alluvium generally consisted of dark brown silt and trace organics. Boring

15E-01 was located nearby a flowing creek, and the grass at the surface was laid down toward the direction of flow, providing evidence of recent flooding at the site. The alluvium was encountered to approximately 4 feet below the ground surface. Groundwater was encountered in boring 15E-01 at a depth of approximately 6 feet at the time of drilling. Alluvium was also observed to exist in a number of small drainages within the project limits, located at approximately Station 28+50, Station 41+50, and Station 42+75.

Colluvium (Qcol). Colluvium is exposed along the road side and adjacent to a ridge of resistant quartzite rock beyond Sta. 43+00. The colluvium observed in those cuts consisted of clayey gravel to cobble size material. As exposed in the cut, the colluvium was overlying local outcrops of phyllite and quartzite near the northern terminus of the project.

Phyllite (Xbp). The phyllite member (Xbp) of the Precambrian Buck Mountain Formation was observed in bedrock outcroppings near Station 17+00. As shown on Plate 1, Norton et al

(2008) describe the unit as dominantly gray to tan phyllite containing minor, thin quartzite beds, and is exposed in road cuts near the northern terminus of the project.

Decomposed phyllite of the Buck Mountain Formation was encountered in each of the exploratory borings drilled within the project limits. The decomposed phyllite has been mapped between Stations 18+20 and 29+00. The decomposed phyllite generally consisted predominantly of stiff reddish brown sandy clay. Fragments of moderately weathered phyllite, smaller than 1 inch, were observed within the decomposed unit. The decomposed phyllite was encountered to approximately 16.5 feet below the ground surface in the borings, the maximum depth explored.

Quartzite (Xnq). North from Station 32+00 to Station 41+50, the bedrock consisted of the quartzite member (Xnq) of the Norris Peak Rocks. The outcrops were consistent with those described by Norton et al, (2008) as thickly bedded tan to gray quartzite containing sparse, intra-formational conglomerate. The quartzite outcrops along a ridge near Station 44+00. The

Norris Peak Phyllite (Xnp) is then exposed intermittently in cuts along the roadside beyond the outcrop (at Station 44+00) to the terminus of the project.

6. SLOPE STABILITY

6.1 ROCK SLOPE STABILITY

Field mapped discontinuity data were organized and entered into the computer program DIPS

6.0, developed by Rocscience. Dips is a stereographic analysis software program, which allows three dimensional discontinuity data to be displayed in two dimensions, in order to visualize how they interact.

The software was used to evaluate the stability of proposed rock cuts along the project alignment by performing kinematic analyses of discontinuity data to evaluate the nature of potential planar sliding, wedge and toppling failures. Notations were made as to whether potential failures were global (rockslides) or local (rockfall) in nature. A friction angle of 40 degrees was used for the quartzite bedrock in the stereographic analyses (Rock Slope

Engineering). The results of selected plots from Dips 6.0 are presented in Appendix D.

Representative samples of bedrock outcrops were obtained from the discontinuity mapping areas for the purpose of laboratory testing to determine the unconfined compressive strength of the rock. The unconfined compressive strength of the rock is a measure of the hardness of the rock. Results of the laboratory testing are showing in Appendix B and the results of the stereographic analyses are shown in Appendix D.

6.2 FILL SLOPE STABILITY

The computer program SLIDE 6.0, developed by Rocscience, was used to evaluate the stability of the existing and proposed fill slopes near Station 42+50 on Norris Peak Road. The existing roadway lies atop an embankment fill that varies in height from about 5 to 15 feet in thickness.

Based upon observations and topographic data, slope inclinations range from 25 to nearly 30 degrees. The toe of the slope adjoins a natural drainage which is bordered by the subject fill slope and a natural slope immediately east of the channel. This channel will be redirected through a culvert as a part of the project.

During our site visit, longitudinal cracks were observed in the pavement along the east side

(northbound lane) of the existing road in this area. Slope stability analyses were performed to evaluate whether or not the longitudinal cracking was indicative of slope instability relative to both existing and proposed slope conditions. As specific material strength data was not available for this area, a back-calculation process was used to estimate strength parameters for the existing slope conditions. The estimated strength parameters from the back analysis were then used to evaluate the stability of the proposed roadway fill configuration. The new alignment will result in an additional 10 to 20 feet of material to be placed in the vicinity of Station 42+50 at slope inclinations of around 18 degrees (1V:3H) and oblique to the existing road alignment.

Surface and subsurface boundaries, groundwater levels, unit weight and strength parameters were assigned to specific soil or rock units to prepare the model for the SLIDE 6.0 analysis. An iterative back calculation method was used to estimate strength parameters for the existing slope. Strength parameters for the artificial fill (AF) and colluvium were adjusted to achieve a factor of safety near 1.0 corresponding to an incipient failure condition associated with the observed cracking in the roadway. The soil and rock strength parameters developed under existing conditions were then used to analyze the proposed fill at this location using SLIDE 6.0.

Table 1 presents the estimated parameters from the slope stability analyses.

Table 1: Estimated Material Properties

Material Unit Weight

(pcf) Phi

(degrees) Cohesion, psf Artificial Fill - Proposed (AF) 140 30 400

Artificial Fill - Existing (AF) 120 22 200

Colluvium 120 0 200

Alluvium 120 0 300

Quartzite 160 45 0

The results of selected plots from SLIDE 6.0 are presented in Appendix E. The plots include the analyses for existing roadway at this location and the proposed configuration considering two conditions. The alluvial channel along the east side of the roadway is characterized by alluvial soils. Analyses were performed considering removal of the alluvium and leaving the alluvium in place. If the alluvium is left in place, the calculated factor of safety for the new slope is approximately 1.4. If the alluvium is removed and artificial fill is placed, the factor of safety increases to approximately 2.3. Removal of the alluvium prior to placement of the fill and subsequent drainage culvert will result in an increase of overall stability.

7. GEOTECHNICAL RECOMMENDATIONS

The recommendations of this report assume that pavement and fill materials will be placed in general accordance with Standard Specification FP-14. Compliance with those specifications is needed for the recommendations in this report to remain applicable.

8. EARTHWORK DESIGN

8.1 GENERAL GRADING

All site grading should conform to the applicable sections of FP-14. Fill materials should not contain organic matter or other deleterious material. On-site soil and rock removed from excavations and cleansed of topsoil, organics and oversized rock can likely be used as borrow for general embankment fill and non-critical areas of construction. Excess fill or cut material should be hauled offsite for proper disposal, unless an onsite location has been specifically identified for placement of excess fill on the project plans. Temporary excavations should conform to OSHA requirements.

8.1.1 EROSION AND SLOPE DRAINAGE

Graded slopes will be vulnerable to erosion. Surface drainage should be maintained similar to existing conditions and without concentrated flows or runoff. Topsoil, erosion control matting (if needed) and appropriate vegetation should be placed on graded slopes to reduce the potential for erosion. Maintenance and repairs to gullied or eroded areas should be performed promptly while vegetation is being established. Concentrated flows of runoff should not be allowed to run over slopes. Overside drains, curbing, and divert drainage water and carry the water the base of slopes should be provided where needed. Energy dissipation devices should be provided at the outlet of pipes and along ditches where concentrated flows of runoff may cause erosion.

8.1.2 EARTHWORK FACTORS

Earthwork factors1 were estimated for the subsurface conditions encountered along the realignment. The subgrade encountered along the proposed realignment of Norris Peak Road

1 The earthwork factor represents the ratio of the existing in-situ unit weight of the soil before construction to the unit weight of the same soil after compaction. For example, an earthwork factor of 0.95 represents a 5 percent loss in the volume of soil following compaction. An earthwork factor of 1.05 represents a 5 percent gain in volume of the soil or pavement following compaction due to bulking of a relatively dense material.

generally consisted of either clayey residual soil and decomposed phyllite that has the consistency of stiff clay, and areas of hard quartzite bedrock. It is recognized that disturbance to drive samples occurs due to compaction of samples during driving, and loosening of soils when recovering the sampler from the drill hole and during trimming in the laboratory. Differences in the soil density occur in areas that have been previously graded (the existing road subgrade), rodent borrows (particularly within the upper 2 feet of a site), and inherent differences in soil type and consistency.

A 40 percent loss in volume should be used for the upper 1 foot of material in areas of undisturbed ground to account for losses due to grubbing, rodent burrows, and compaction.

Where on-site soil and decomposed phyllite will be excavated and used as compacted fill

(generally south of Sta. 30+00), an average earthwork factor of 0.95 should be used for areas where fill or subgrade will be placed to at least 95 percent relative compaction. Where on site quartzite will be excavated and used as embankment fill (generally north of Sta. 30+00), an average earthwork factor of 1.12 should be used to account for bulking of the rock (Caltrans

1978). The earthwork factors are approximate, and should be checked and adjusted as-needed as the grading work progresses to properly balance earthwork operations.

8.1.3 EXCAVATION

Hard quartzite bedrock was encountered in the proposed cut areas from Sta. 34+75 to 40+25 and from Sta. 43+75 to 45+00. Samples of quartzite rock taken from outcrops had an unconfined compressive strength of 10,260 psi to 13,290 psi. The average seismic velocity of rock in this areas was 3,000 feet per second within the planned depths of excavation (Olson

2015). Results of the lab testing are presented in Appendix B. The Olson (2015) report is presented in Appendix C, and summarized in Appendix A.

Excavation of quartzite is considered rippable with a D8R caterpillar due to the discontinuity spacing. Discontinuities observed in outcrops of the quartzite had three predominant discontinuities that resulted in a block-like spacing of about 1 to 6 feet. The term rippable does not imply that the rock will disintegrate when excavated. However, due to the hardness of the rock, localized use of mechanical hammers and blasting to assist with excavation of large blocks should be anticipated and provided for within the project specifications. Decomposed and soft phyllite rock was encountered in the cut areas proposed between Sta. 18+75 to 26+50.

Excavation of the overburden soil and phyllite is expected to be possible with conventional earthmoving equipment. Soil and bedrock materials derived from the cuts can be use as embankment fill in accordance with FP-14.

8.2 SLOPE DESIGN

The proposed realignment will involve various cuts and fill through undulating terrain, with intervening drainage, and sidehill construction. Cuts and embankment slopes are generally anticipated to have maximum heights less than 15 feet, except at the crossing of the existing alignment where the embankment slopes will be up to about 30 feet high. A summary of our recommended inclinations is presented below:

Table 2: Recommended Cut-Fill Slope Inclinations

Station Maximum

Cut/Fill Height

Recommended Slope

Inclination (or flatter) Material Type Comments from to

11+00 (begin) 12+75 <1 ft. fill

4 ft. cut 1V:2.5H Compacted Fill Decomposed

Phyllite and clay

Minor fill, at-grade with cut for ditch widening.

12+75 18+50 20 ft. fill 1V:2.5H Compacted Fill Minor centerline cuts in Phyllite near Sta. 17+00

18+50 27+00 15 ft. cut < 4 ft. fill 1V:2.5H

Decomposed Phyllite and clay

Compact fill Minor fills beyond Sta. 26+00 Phyllite contains blocks of hard rock

27+00 30+00 8 ft. fill < 4 ft. cut 1V:2.5H

Compacted fill Decomposed

Phyllite and clay

30+00 34+75 25 ft. fill 20 ft. cut

1V:2.5H fill 4V:1H cut

Compacted Fill Quartzite

Generally left side fill with right side cuts.

Contact with quartzite occurs near Sta. 29+00.

Transition to adjacent flatter cut slopes within these station limits.

34+75 40+00 25 ft. cut minor fill

4V:1H cut 1V:2.5H fill

Quartzite Compacted Fill Through cut.

40+00 40+70 15 ft. fill 20 ft. cut

1V:2.5H fill 4V:1H cut

Compacted Fill Quartzite

Embankment Fill. Covers the existing roadway embankment.

40+70 43+50 30 ft. fill 1V:3H fill 4V:1H cut Compacted Fill Fill over intersection with existing

Norris Peak Road alignment

43+50 45+25 30 ft. cut < 4 ft. fill

4V:1H cut 1V:2.5H fill Quartzite

Transition back to existing alignment with predominantly left widening of existing cut slope.

Station Maximum

Cut/Fill Height

Recommended Slope

Inclination (or flatter) Material Type Comments from to

45+25 48+35 (end)

15 ft. cut 10 ft. fill

1V:1.5H cut 1V:3H

Colluvium, Phyllite

Compacted Fill

Transition onto existing alignment with left and right widening of existing cut and embankment slopes.

8.2.1 SUBEXCAVATION

Topsoil should be removed from areas to receive fill in accordance with FP-14. Subexcavation is recommended along the entire realignment to provide better subgrade support for the pavement structural section, to help stabilize wet clay subgrade conditions that could be encountered in cut areas between Stations 11+00 and 30+00, and to provide frost protection in the cut areas and reduce frost heave potential. We recommend that the existing soil and rock be excavated to at least 2 feet below the proposed structural section, at least 3 feet below finished grade, and be replaced with compacted fill. The refill material for the subexcavation should consist of relatively free-draining, granular soil that has no more than 50 percent material retained on the No. 4 sieve, a sand equivalent value of at least 30, and an R-value of at least

25. If excavated on-site material is to be used as refill below pavement areas, the excavated material will like need to be processed and screened to remove oversized materials and fines before the material is considered suitable for placement.

Where the existing alignment crosses alluvium deposits near Stations 15+50, 28+75, and

41+60, along the existing drainages, the subgrade should be reviewed following the initial subexcavation to evaluate whether or not additional soft or compressible soil is in place. If needed, additional subexcavation should be provided to remove soft or compressible soil prior to placing the embankment fill.

8.2.2 EMBANKMENT ON SLOPING GROUND

New embankments will be constructed on ground steeper than 1v:3h. Keying and benching of the fill will be needed to provide suitable foundation support for the new embankment per FP-14.

Existing topsoil should be removed from fill placement areas. The fill should then be keyed and benched into the existing hillside such that at least the upper 2 feet of the existing soil is removed and the fill is keyed into stable material. The keying can be performed in association with placement of the fill, and such that the soil excavated from the existing embankment slope is simultaneously incorporated into the layer of the compacted fill that is being placed. The excavated material should be scarified and blended with the new fill to provide uniformity of material and moisture that is suitable for compaction. The benches do not need to be precut, and height of the benches can be reduced compared to CFL Standard Plan C204-50 to avoid additional rock excavation or blasting to form the benches. Figure 2 shows a typical detail with the recommended grading and slope recommendations for side hill areas.

Figure 2: Typical Sidehill Grading

8.2.3 ROCKFALL MITIGATION

Table 2 presents the recommended cut slope inclinations that are considered stable based upon the surface mapped discontinuities and the results of the kinematic analyses. Rock slopes will experience rockfall, and localized wedge and toppling failures within the proposed rock slopes should be anticipated. As designed (Atkins 2015), 8 feet of catchment with a 1V:4H ditch slope adjacent to the roadway will be provided at the base of the cut slopes. The 8-foot catchment width is considered adequate mitigation for rockfall originating from the proposed cut slopes in rock, and no additional special considerations to address rockfall are needed for design. Rockfall catchment width is the area between the base of the cut slope and the edge of traveled way

(ETW). Catchment ditch effectiveness was evaluated using the Rockfall Catchment Area Design

Guideline (Pierson et al. 2001).

8.3 CULVERT DESIGN

Culverts are proposed at Stations 15+50, 28+75, and 41+60. As shown on the Atkins (2015) plans the culverts will be corrugated metal pipe ranging from 36 to 60 inches in diameter. The creeks at Sta. 15+50 and 28+75 were flowing during our July 2015 field exploration program, and the invert of the drainage was wet at Sta. 41+60. Pipe bedding and backfill materials can be provided in accordance with the details shown on the Atkins plans. Control of surface water, groundwater and stabilization of the subgrade will be needed to allow for construction of the culverts. Culverts should be placed as ditch conduits. Where the culvert will be in fill (Sta.

41+60), the embankment should be constructed to at least 2 feet above the top of the proposed pipe, and a trench should be excavated in the compacted fill to allow for construction of the pipe and to reduce overburden loading on the pipe. Dewatering and diversion of stream flow should be provided before beginning the excavation for the culverts.

Foundation Preparation. The foundation soils in the culvert areas are anticipated to be wet and yielding if exposed to construction traffic. The bottom of the trench should be subexcavated to at least 1 foot below the bottom of the pipe and replaced with gravel bedding to provide support for the pipe and a working mat for construction. The gravel should consist of 3/8 to 1-inch open graded aggregate fully encased in a geotextile filter fabric. The geotextile should be placed without gaps or wrinkles and conform to Section 714.01 of FP-14 for Class 2 Geotextile, Type E. Pipe bedding can then be placed over the stabilized subgrade. The final excavation should be performed using an excavator bucket or similar equipment to reduce the potential for excavation equipment to disturb the subgrade below the proposed culverts.

Seepage. There is a potential for stream and groundwater to flow along the pipe bedding and backfill and result in erosion or piping of the backfill material. Trench plugs (as shown on

Standard Drawing 602-3 in Atkins (2015)) should be provided behind the headwalls and outlets for the culverts to reduce the potential for seepage to occur along the pipe. The plug should extend to at least 6 inches below the bottom of the bedding and gravel below the pipe and at least 6 inches into undisturbed material at the sidewalls of the trench.

9. PAVEMENT DESIGN RECOMMENDATIONS

The minimum design thickness for the Norris Peak Road Realignment is recommended based on pavement design standards provided in Chapter 11 of the CFLHD Project Development and

Design Manual (PDDM), and the AASHTO (1993) Guide for the Design of Pavement Structures.

The spreadsheet calculations for our recommended traffic loading and thickness recommendations are presented in Appendix F.

9.1 DESIGN TRAFFIC VOLUME AND SUBGRADE

The design traffic volume was estimated using the average daily traffic (ADT) volume and vehicle distributions presented in the Central Federal Lands (2015) Highway Design Standards for SD FLAP CR2238091 (1) and procedures in the Federal Lands Highway Project

Development and Design Manual (FHWA 2008). The current average daily traffic (ADT) for

Norris Peak Road is estimated to be approximately 525 vehicles per day with 7.4 percent trucks.

These data were used to estimate the number of 18-kip equivalent single axle loads (ESALs) considering 5-axle trucks and a design life of 20 years. This design ESAL value was 374,368.

The subgrade soil encountered along the alignment included clay soil derived from decomposed and weathered phyllite rock encountered at the site, classified as A-7-6 (20) and is considered poor subgrade support for the roadway. The R-value for a sample of the clay was 10. The recommended structural section thicknesses were estimated considering a resilient modulus

(Mr) of 3,562 psi for the subgrade soils encountered.

9.2 STRUCTURAL SECTION

The roadway structural section for the Norris Peak Road network should consist of at least 4 inches of hotmix asphalt concrete pavement (HACP) and 8 inches of aggregate base course

(ABC). The structural section should be placed on at least 2 feet of compacted granular fill placed according to the recommendations of this report and having an R-value of at least 25.

The 2 feet of compacted fill should have an R-value of 25, and consist of non-plastic, free draining granular material that is not frost susceptible. The base and fill material placed below the structural section should be moisture conditioned and compacted in-place to at least 95 percent relative compaction in accordance with FP-14 Standard Specifications. A summary of the structural section recommendations is shown in Figure 3.

Figure 3: Summary of Structural Section Recommendations

Shoulder backing materials should be placed along the new edge of pavement (NEP) to provide and reduce the potential for erosion at the pavement edge. Shoulder backing should consist of

Class D base conforming to 703.05 of FP-14 with a thickness of at least 3 inches at the NEP.

9.3 BINDER AND MIX RECOMMENDATIONS

The site is within the northern central section of the United States, with cold winter and moderately hot summers. We contacted Pennington County regarding the types of asphalt they typically allow on their projects. They recommended an asphalt binder of either PG 58-28 or 64-

28, which is further supported based on the LTPPBind Binder selection program based on the weather information from the Rapid City weather stations. The LTPPBind program output recommending PG 58-28 is presented in Appendix F following the pavement design information.

The site is relatively remote. Loss of temperature and segregation of the hot asphalt mix can occur during long distance transport. We recommend the project specifications require the use of a material transfer device at the point of placement to insure uniform temperatures and prevent segregation of the mix during placement.

We recommend a nominal ¾ inch or ½ inch aggregate mix in compliance with the gradations presented in Table 703-4 of FP-14. The mix should be designed using the Superpave compactor at 75 gyrations and the mix should contain the performance graded binder conforming to ASTM M320 for grade PG 58-28. If a locally available mix is desired, we recommend that a mix meeting the requirements a South Dakota DOT Class E, Type 1 mix with

PG 58-28.

9.4 SUBGRADE STABILIZATION

Subgrade stabilization should be provided in areas where unsuitable materials or soft subgrade conditions are encountered that will not allow for proper compaction of the subgrade or FDR materials, or the subgrade is composed of organic or other deleterious materials that will not provide suitable foundation support for the new roadway. Subgrade stabilization can consist of removing the existing soil to a depth at least 1 foot below the bottom of the structural section or to the bottom of the unsuitable material, whichever is deeper. If the subgrade is wet or yielding, subexcavation should be performed using backhoe type equipment such that construction equipment will not operate on the exposed subgrade during excavation.

A geotextile for stabilization should be placed over the undisturbed subgrade in accordance with Section 207 of P-14. The geotextile should be placed without gaps or wrinkles and conform to

Section 714.01 of FP-14 for Class 1 Geotextile, Type E. Gravel for stabilization should consist of uniformly graded 3/8 to 1.5-inch gravel conforming to Section 703.6, Crushed Aggregate, of FP-14. The aggregate should be fully encased in the geotextile to reduce the potential for the overlying base course or adjacent sand dune soils to erode into the gravel.

The project specifications should include a quantity for stabilizing 500 feet of the subgrade for the roadway or culverts, if needed, and for increasing, decreasing or deleting the quantity based on the conditions encountered during construction. The geotechnical professional should review the subgrade conditions encountered at the time of construction to evaluate whether or not stabilization of the subgrade is needed, and to recommend the depth and limits of the subexcavation and stabilization.

9.5 PAVEMENT MAINTENANCE

The pavement condition should be periodically evaluated to help plan and scope the need for maintenance and rehabilitation following the initial construction of the pavement. Maintenance of asphalt concrete pavements should typically include periodic fog, chip or slurry seals to reduce the potential for weathering, or overlaying with additional HACP when needed to strengthen and further the life of the pavement. Deflection surveys can be performed to evaluate the structural capacity of pavements and the need for future overlays.

10. NOTES TO DESIGNER

Groundwater was encountered in several of the borings drilled where deep cuts are proposed.

Seepage from the faces of new cut slopes will reduce slope stability and could require re-grading to a more stable configuration or permanent support with retaining structures.

10.1 CORROSION CONSIDERATIONS

FHWA foundation design manuals define aggressive conditions of fill, natural soil, and groundwater as the following:

• The soil pH is less than 4.5.

• The soil or water resistivity is less than 2000-ohm-cm.

• The sulfate ion content exceeds 200 parts-per-million (ppm) or the chloride content exceeds 100 ppm.

• Soils with resistivity greater than 5000 ohm-cm are considered non-aggressive.

• A corrosion rate of 0.003 inch/year is used for steel piles buried in fill or disturbed natural soil.

Tests for pH, electrical resistivity, water soluble chlorides and sulfates were performed on selected samples. The test results are presented in Appendix B. The pH for the samples tested ranged from 6 to 7.9 and the soil resistivity ranged from 1,984 to 12,994 ohm-centimeters.

Water soluble chlorides samples measured values of 0.03 to 0.07 ppm. The concentration of water-soluble sulfates measured in the laboratory on representative samples of soils measured at 0.01 ppm or less. The soil tested are not considered aggressive towards concrete.

Corrosion and corrosion test data should be considered in the design of buried concrete or steel structures.

11. NOTES TO CONSTRUCTION

11.1 GROUNDWATER

Creeks near the culvert locations at Sta. 15+50 and 28+75 were flowing during our July 2015 field exploration program. Groundwater was encountered in boring 15E-01 drilled near the creek at about 6 feet below the ground surface. Control of surface water and groundwater will be necessary to construct the culverts and place backfill at these locations. Groundwater control is likely to consist of flumes and ditches to bypass the construction area, and pumping from gravel-filled sumps placed in the bottom of the trench or excavation. Sumps, wells, and gravel used for dewatering should be equipped with proper geotextile filters or screens to reduce the potential for the soil to be eroded or removed from the adjacent soil or rock. A dewatering plan should be prepared and submitted for review by the geotechnical professional prior to beginning the excavation. Areas with wet or soft ground should be stabilized with gravel in accordance with Section 7.4 of this report prior to placing fill.

11.2 COMPACTION

The subgrade soil encountered in borings between about Sta. 10+00 and 30+00 consisted of clay soil that was typically at a moisture content higher than that needed for compaction.

Recommendations are provided for subexcavation, and placing a layer of compacted fill over the undisturbed subgrade in these areas. Although moisture conditions vary seasonally, the clayey material will likely need to be dried to a moisture content suitable for compaction.

Control of compaction layer thickness, moisture content, and providing an adequate amount of compactive effort will be necessary to achieve the minimum specified compaction.

Materials encountered beyond Sta. 30+00 were generally hard quartzite rock and more granular overburden soil. The compaction characteristics of clayey material from the south end of the project and the rocky material from the north end of the project will have different compaction characteristics, and likely require different types of processing and equipment to achieve compaction. Compaction of the subgrade and fill materials should be performed using suitable equipment for the type of material being placed, such as smooth drummed rollers and/or vibratory compaction in non-plastic soils and kneading or sheepsfoot compactors for clayey soils.

11.3 HARD ROCK

Outcrops and areas of hard rock are common between stations 34+00 and 40+50 and 43+50 and 45+00. The excavation characteristics of the rock are discussed in this report. It is anticipated that ripping and localized blasting will be needed to perform rock excavations. The term “rippable” does not imply that the rock will disintegrate when excavated, and processing and reducing particle sizes will be necessary to allow for the material to be properly placed and compacted per FP-14. Rock excavation should include removing the rock to at least 2 feet below the bottom of the structure section, or about 3 feet below finished grade, as recommended in this report.

11.4 CONSTRUCTION OBSERVATION

A geotechnical professional should continue to evaluate the subsurface conditions at site through construction, and observe that the work being performed and subsurface conditions encountered are consistent with the recommendations of this report. The following provides key items that should be evaluated or observed during construction.

11.4.1 SUBGRADE EVALUATION

The geotechnical professional should observe the base of excavations, the bottoms of trenches and roadway subgrade to evaluate whether or not the exposed subgrade is stable and suitable for fill placement. The project specifications should provide for review of the subgrade by the geotechnical professional, and for variations in the depth of excavation, if needed, to remove additional loose soil, undocumented fill, or unsuitable material.

11.4.2 GRADING OBSERVATION

The geotechnical professional should observe grading operations during construction on behalf of the owner to have reasonable certainty that fill placement and compaction is being performed according to the recommendations of this report and FP-14. Field density testing should be performed to help evaluate the compaction and moisture content of the materials being placed.

Fill and aggregates delivered to the site, and excavated onsite soil that will be reused as fill or backfill, should be sampled and tested for conformance with gradation and quality requirements for the project. The frequency and locations of the tests should be at the discretion of the geotechnical professional. The project specifications should include provisions for the contractor to allow for testing and to provide any shoring, ingress-egress, or traffic control needed to safely perform the testing at the locations and depths needed.

11.4.3 ROCK SLOPES

The geotechnical professional should review the excavation and blasting plans prior to the contractor beginning work.