Geotech Report.pdf

Geotechnical Report

Hyampom, Mad River, Van Duzen, Ruth-Zenia, and Alderpoint Bluff Road

FHWA Project No. CA ER 511(1)

Trinity County, California

Yeh Project No.: 218-524

August 26, 2020

Prepared f or:

Jacobs 2485 Natomas Park Dr #600

Sacramento, CA 95833 Attn: Mr. Ed Henderson, P.E.

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433 Phone: 805-481-9590

391 Front Street, Suite D Grover Beach, CA 93433

(805) 481-9590 www.yeh-eng.com

Colorado California Denver | Colorado Springs | Durango | Glenwood Springs | Grand Junction | Greeley Grover Beach | Ventura

Geotechnical Report

Hyampom, Mad River, Van Duzen, Ruth-Zenia, and Alderpoint Bluff Road

FHWA Project No. CA ER511(1)

Trinity County, California

Yeh Project No.: 218-524

August 26, 2020

Prepared by:

Jonathan D. Blanchard, GE2312 Reed Hooke, EIT Geotechnical Engineer Staff Engineer

Reviewer:

Gresham Eckrich, P.E., C.E.G.

Engineering Geologist http://www.yeh-eng.com/

Geotechnical Report Project No. 218-524 Trinity County CA ER Project August 26, 2020 iii

Table of Contents

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

2.1 SITE 001, VAN DUZEN ROAD MP4.1

2.2 SITE 006, HYAMPOM ROAD MP11.4

2.3 SITE 007, MAD RIVER ROAD MP7.5

2.4 SITE 008, MAD RIVER ROAD MP8.0

2.5 16SITE 009, MAD RIVER ROAD MP0.15

2.6 SITE 010, RUTH ZENIA ROAD MP15.4

2.7 SITE 011, RUTH ZENIA ROAD MP16.3

2.8 SITE 012, RUTH ZENIA ROAD MP17.7

2.9 SITE 013, RUTH ZENIA ROAD MP18.1

2.10 SITE 014, RUTH ZENIA ROAD MP18.7

2.11 SITE 015, ALDERPOINT BLUFF ROAD MP6.5

2.12 SITE 016, HYAMPOM ROAD MP20.2

3. GEOTECHNICAL INVESTIGATION

3.1 DRILLING

3.2 LABORATORY TESTING

3.3 GEOPHYSICAL SURVEY

3.4 GEOLOGIC RECONNAISSANCE

3.5 PREVIOUS STUDIES

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

4.2 TERRAIN AND VEGETATION

4.3 CLIMATE

4.4 SUBSURFACE CONDITIONS

5. SLOPE STABILITY EVALUATION

6. CONCLUSIONS AND RECOMMENDATIONS

6.1 EARTHWORK - GENERAL

6.1.1 GENERAL GRADING

6.1.2 EARTHWORK FACTORS

6.1.3 EROSION AND SLOPE DRAINAGE

iv

6.1.4 REUSE OF ONSITE SOIL

6.2 EMBANKMENT DESIGN

6.3 GEOGRID REINFORCED SOIL SLOPES (RSS)

6.4 CUT SLOPES

6.4.1 SITE 006, HYAMPOM ROAD MP11.4

6.4.2 SITE 015, ALDERPOINT BLUFF ROAD MP6.5

6.5 MICROPILE DESIGN

6.5.1 APPLIED LATERAL EARTH PRESSURE

6.5.2 MICROPILE ARRAY LAYOUT PARAMETERS

6.5.3 PILE LENGTHS, EMBEDMENT AND DIAMETER

6.5.4 INSTALLATION AND TESTING

6.6 PAVEMENT DESIGN

6.6.1 PAVEMENT REPLACEMENTS IN-KIND

6.6.2 VAN DUZEN ROAD REALIGNMENT

6.6.3 BINDER

6.6.4 SUBEXCAVATION

7. NOTES TO CONSTRUCTION

8. REFERENCES

List of Figures

FIGURE 1: VICINITY MAP

FIGURE 2: SITE 001, VAN DUZEN ROAD MP4.1

FIGURE 3: HYAMPOM ROAD MP11.4

FIGURE 4: MAD RIVER ROAD MP7.5

FIGURE 5: MAD RIVER ROAD MP8.0

FIGURE 6: MAD RIVER ROAD MP0.15

FIGURE 7: RUTH ZENIA ROAD MP15.4

FIGURE 8: RUTH ZENIA ROAD MP16.3

FIGURE 9: RUTH ZENIA ROAD MP17.7

FIGURE 10: RUTH ZENIA ROAD MP18.1

FIGURE 11: RUTH ZENIA ROAD MP18.7

FIGURE 12: ALDER BLUFF ROAD MP6.5

FIGURE 13: HYAMPOM ROAD MP20.2

FIGURE 14: DEVELOPMENT OF GEOGRID STRENGTH IN SLIDE

FIGURE 15: TYPICAL SLOPE WIDENING

FIGURE 16: LOADING ON MICROPILES

file://10.0.4.10/GB_Server/Projects%20-%20Grover/218-524%20ES%20CH2M%20-%20CFL%20Trinity%20County%20Ruth-Zenia%20Road/1.0%20Geotechnical%20Report/218-524%20GR%20for%20Trinity%20Co.%20final.docx#_Toc49337404 v

List of Tables

TABLE 1: GENERAL SUMMARY OF SITE CONDITIONS

TABLE 2: FIELD EXPLORATION SUMMARY

TABLE 3: SUMMARY OF PAVEMENT CONDITIONS ENCOUNTERED

TABLE 4: SUMMARY OF SUBSURFACE CONDITIONS BY SITE

TABLE 5: SUMMARY OF EMBANKMENT DESIGN

TABLE 6: SUMMARY OF TRAFFIC DISTRIBUTION

TABLE 7: SUMMARY OF RECOMMENDED SUBEXCAVATION

List of Plate Plate 1.1: Field Exploration Plan, Van Duzen Road MP4.1 Plate 1.2: Field Exploration Plan, Mad River Road MP7.5 Plate 1.3: Field Exploration Plan, Mad River Road MP0.15 Plate 1.4: Field Exploration Plan, Ruth Zenia Road MP16.3 Plate 1.5: Field Exploration Plan, Ruth Zenia Road MP17.7 Plate 1.6: Field Exploration Plan, Ruth Zenia Road MP18.1 Plate 1.7: Field Exploration Plan, Ruth Zenia Road MP18.7 Plate 1.8: Field Exploration Plan, Hyampom Road MP20.2 Plate 2.1 to 2.2 Regional Geologic Map

List of Appendices Page

Appendix A – Boring Logs Legend for Soil Classification ........................................................................................................ A-1 Legend for Rock Classification ...................................................................................................... A-2 Borehole Logs for 19S-01 to 11 ........................................................................................... A-3 to 20 Boring Location Photos ..................................................................................................... A-21 to 24 AGI Seismic Refraction Survey (Sites 016, 009, and 007) ................................................. A-25 to 34

Appendix B – Results of Laboratory Testing Moisture and Density Measurements ................................................................................. B-1 to 3 Mechanical Sieve Analysis ............................................................................................................. B-4 Proctor Compaction ...................................................................................................................... B-5 Atterberg Limits ............................................................................................................................ B-6 Direct Shear Test ................................................................................................................. B-7 to 10 Consolidated Undrained Triaxial Test ............................................................................... B-11 to 30 R-value Test Results .......................................................................................................... B-31 to 32

Appendix C – SLIDE Plots Site 007, Mad River Road MP7.5........................................................................................... C-1 to 3 Site 009, Mad River Road MP0.15......................................................................................... C-4 to 5 Site 011, Ruth Zenia Road MP16.3 ........................................................................................ C-6 to 8 Site 012, Ruth Zenia Road MP17.7 ...................................................................................... C-9 to 10 Site 014, Ruth Zenia MP18.75 ........................................................................................... C-11 to 12 Site 016, Hyampom Road MP20.2 .................................................................................... C-13 to 15 Micropile Design Calculations ........................................................................................... C-16 to 17

Appendix D – Pavement Design Calculations AASHTO (1993) Spreadsheet calculations............................................................................. D-1 to 3 s

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates was retained by Jacobs to provide geotechnical recommendations for the design of various road repairs as part of the Federal Highway Administration, Central Emergency Relief projects for Trinity County, California.

Roads were impacted by landslides, settlement, and erosion during 2017 and 2019 storms. The locations of the twelve sites that are a part of this project are shown on Figure 1 and summarized in Table 1.

The geotechnical evaluation consisted of a program of project coordination, reviewing data, site reconnaissance, field exploration, laboratory testing, and engineering analyses as a basis for providing the recommendations in this report. This report provides recommendations for the design of earthwork, geosynthetic reinforced slopes, drainage improvements, micropiles, and pavements.

2. PROJECT UNDERSTANDING

The project generally consists of the design of emergency repairs to County roads at the twelve (12) locations that were impacted by storm damage. The repairs will generally include debris removal, excavation, embankment slopes, reinforced soil slopes, drainage improvements, pavement patching, a road realignment, and other miscellaneous items. The routes that were impacted are Ruth-Zenia Road, Mad River Road, Hyampom Road, and Alderpoint Bluff Road. The overall purpose of this project is to restore two-way traffic at or near the condition prior to the storms. Table 1 provides a summary of the project locations and brief description and extent of the storm damage at each site that was observed during the scoping trip in May 2019.

Figure 1: Vicinity Map

Table 1: General Summary of Site Conditions

DAF Site No. Location 2017 Damage

Width along road

001 Van Duzen Road MP 4.1 Scour along Van Duzen River undercut southbound lane of traveled way. The road was reduced to one lane with two-way stop following the storms.

300 feet

006 Hyampom Road MP 11.4 Cut slope failure above the road. Slide debris and 15-foot scarp above the road. Both lanes were open to traffic. 160 feet

007 Mad River Road MP 7.5 Slipout below the road and 4- to 5-foot scarp along existing edge of pavement. The southbound lane was closed, and a two-way stop was installed following the storms.

215 feet

008 Mad River Road MP 8.0 Road settlement due to deep-seated landslide. The road had been patched with asphalt with both lanes open. No visual evidence of additional movement below the road was observed in May 2019.

160 feet

009 Mad River Road MP 0.15 Cracking in paved shoulder and active downslope movement below road to toe of slope. Both lanes have been detoured around slide area on the paved road.

130 feet

010 Ruth Zenia MP15.4 About 3 inches of subsidence in northbound lane of traveled way.

County patched with gravel and attributed to subsurface water. No evidence of continued movement in May 2019.

20 feet

011 Ruth Zenia Road MP 16.3

Slipout below road embankment failure with active movement of fill.

2 to 3-foot scarp at existing edge of pavement. The scarp was filled in with gravel. There were cracks in the gravel. Fill has cracks and evidence of continued downslope movement during in May 2019.

100 feet

012 Ruth Zenia Road MP 17.7

Slipout below road with downslope movement and settlement of the embankment to center of southbound lane. The road was patched with gravel following the storms and both lanes are open to traffic.

100 feet

013 Ruth Zenia Road MP 18.1

A deep-seated landslide has resulted in settlement and cracking of both lanes. Evidence of continued movement above and below road was observed in May 2019. The road has been patched with gravel patch with both lanes open to traffic.

130 feet (patch)

1,000 feet (slide)

014 Ruth Zenia Road MP 18.7

Slipout below road and embankment failure with active movement through northbound lane. An active debris flow chute was observed downslope of slide area. A gravel patch had been placed in northbound lane and both lanes were open to traffic.

120 feet

015 Alderpoint Bluff Road MP 6.5

Failure of 130-foot high slope above the road. Debris has been cleared from the road and both lanes were open to traffic. 100 feet

016 Hyampom Road MP 20.2

2019 storms caused additional damage to this site. A slipout below the road had taken out the westbound lane and left a 15-foot head scarp near centerline of road. A single lane detour with a two-way stop was cut into hillside on the upslope side of the road. The detour had been paved in October 2019.

100 feet

2.1 SITE 001, VAN DUZEN ROAD MP4.1

Location (lat/long): 40.4137, -123.5193.

Site 001 is located on the alluvial plain and east bank of the Van Duzen River, about 4.1 miles south of the intersection of Van Duzen Road with State Highway 35. Van Duzen Road is a 24-foot wide two-lane road with 1-foot paved shoulders, and an outside gravel shoulder that is typically about 3 feet wide. The road is within the Six Rivers National Forest and intersects with the Ruth Zenia Road about 10 miles south of Site 001. There is an unimproved campground on the west side of the road. The valley floor of the of the Van Duzen River is approximately 700 to 900 feet wide in the site vicinity and is mostly occupied by the braided channel of the river. The campground and river are furthered by forested hill and mountain sides typical of the region. A former alignment of Van Duzen Road appears to pass through the campground. The existing grade of Van Duzen Road generally appears to follow the grade of the river and descends to the north at a grade of approximately 0.2 percent.

Lateral scour and erosion along the riverbank undercut the southbound lane of Van Duzen Road during rainfall and high flows that occurred during the period of November 2016 through February 2017. The existing streambank was approximately 11 feet high below the road at the time of our May 8, 2019 scoping visit. The County had placed riprap along approximately 300 feet of the riverbank (top photo). The southern approximately 140 feet of the riprap had been undercut by erosion exposing the toe of the riprap several feet above the thalweg of the river. The thalweg of the river was composed of mostly cobbles (3- to 6-inches) in a matrix of medium- to coarse-grained sand. The riverbank downstream of the riprap (bottom photo) was near vertical to about 5 feet below the top of bank, with a gravel talus slope below sloping into the river at approximately 25 degrees.

The southbound lane was closed, and traffic was limited to one lane with a two-way stop through the site at the time of our fall 2019 field exploration program. The lateral migration of the river appeared to be associated with an accumulation of sediment that episodically filled the braided channel of the river and had driven the active channel toward the road. The sediment accumulation appeared to be associated with the river opening onto a wider floodplain just downstream of narrows that have been formed where the resistant Crooks Ridge crosses the river. This accumulation of sediment appeared to have drove the river easterly toward the road.

Design Concept. The concept for design is to relocate Van Duzen Road on a new alignment.

Relocation of the road would provide a buffer against potential further lateral migration of the river, and avoid environmental impacts associated with working in or encroaching on the river. The route would be near a previous alignment of Van Duzen Road and approximately 60 feet west of the

Figure 2: Site 001, Van Duzen Road MP4.1 Top: Looking south through lane closure

Bottom: Looking south through campground/proposed realignment current roadway. The realignment will be constructed near the existing grade of Van Duzen Road and will likely involve earthwork consisting of minor cuts and fills of no more than 2 feet.

2.2 SITE 006, HYAMPOM ROAD MP11.4

Location (lat/long): 40.6195, -123.3093.

Site 006 is located on a north facing cut slope within mountainous terrain traversed by Hyampom Road, 11.4 miles west of the intersection with State Highway 3 in Hayfork.

The road runs for approximately 22 miles within the Shasta-Trinity National Forest between Hayfork and Hyampom. Hyampom Road is generally a 22-foot wide two-lane road with 1-foot paved shoulders near the site. There is an approximately 4-foot wide paved shoulder with an asphalt curb at the base of the cut along the eastbound side of the road, with additional 3 to 4 feet of catchment behind the curb. There is a variable width unpaved outside shoulder along the westbound side of the road that is at least 3 feet wide. The cut slope is made on a north-facing mountain side. The mountain side is approximately 2,100 feet high with an average grade of about 26 percent that runs between the mountain top located about 1 mile southwest of Site 006, and Hayfork Creek located about 1,800 feet downslope of Site 006.

The surrounding hillsides are densely forested woodlands.

A rockslide occurred on the cut slope above the eastbound side of the road during rainfall that occurred in February 2017 filling the shoulder and entering the traveled way. The head of the slide was located approximately 85 feet above the road, with an approximately 20-foot high head scarp that was inclined at approximately 85 degrees. The body of the slide below the scarp was approximately 100 feet wide and inclined to about 41 degrees. The slope adjacent to the slide appears to have been graded cut sloped to approximately 1.2v:1h with slope rounding at the top of

Figure 3: Hyampom Road MP11.4 Photo taken May 6, 2019

Top: View of cut slope looking southeast Bottom: Base of cut slope below slide looking west cut as part of previous improvements to Hyampom Road that were constructed in 2008. The slope grading appeared more extensive and steeper than recommended in the Yeh (2006) report. Slide debris had buried the curb and filled the shoulder of the road at the time of our visit. The previous improvements involved construction of an 80-foot high sidehill embankment across the drainage on the west approach to the subject cut slopes, a new culvert, a rock buttress across from the cut, and a sliver type cut along the subject slope.

The 2017 slide occurred within decomposed granite that was estimated to mantle the outer approximately 15 feet of the slope. Intensely to moderately weathered rock was exposed within the head scarp and base of the adjacent cut outside the limits of the slide. A shear zone was observed along the south side of the slide. The rock within the shear zone was intensely fractured and relatively wet.

Movement and erosion of the landslide will likely continue to impact the road with the potential for large slope movements to impact or block the traveled way, particularly during periods of heavy or intense rainfall. The road was open to two lanes of traffic with a paved width of approximately 25 feet including a 4-foot paved shoulder at the base of the cut.

Design Concept. The concept for design is to recut the slope at a flatter inclination to maintain the current road width, improve slope stability and restore the catchment.

2.3 SITE 007, MAD RIVER ROAD MP7.5

Location (lat/long): 40.3626,-123.4263

Site 007 is located on a southwest facing hillside traversed by Mad River Road, about 175 feet above the edge of Ruth Lake and 7.5 miles south of the intersection of Mad River Road with Highway 36.

The road generally runs north-south along the Mad River drainage and within the Six Rivers National Forest. Mad River Road is generally a 22-foot wide two-lane road with at least a 3-foot wide outside shoulder. The road through the site was constructed mostly on cut, with a relatively shallow built-out shoulder beyond the edge of pavement.

A slipout occurred below the road that extends to the edge of pavement along the southbound side of Mad River Road in 2017. Arcuate shaped cracks above the scarp extend into the traveled way of the southbound lane. The head scarp is approximately 5 feet vertical below the road (see top inset photo, view north). The toe of the slide was observed at approximately 55 feet below the elevation of the road. The toe is furthered by a debris flow chute that extends downslope at approximately 32 degrees into Ruth Lake. Springs and wet ground were common within the lower half of the slide area. Springs and evidence of slope instability (hummocky ground and scarp features) were also observed on the slope above the road. The adjacent slopes north and south of Site 007 have evidence of slope instability, creep and landsliding below and above the road.

A subsequent review of aerial photography in the site vicinity (Google Earth) showed the subject slide is likely one of a series of a coalesced landslides within an overall larger landslide mass that extends about 100 feet above the road, 400 feet either side of Site 007, and descends into Ruth Lake below the road. The County has placed K-rail to direct traffic back from the edge of the road adjacent to the slide. Claystone and springs were observed on the cut slope. The head scarp of the slide is not considered stable, and there is a potential for the slide to move upslope into the road. There is an existing underdrain system along edge of the northbound lane below the cut.

Design Concept. The concept for design was to consider a multi-tiered tieback retaining wall.

However, if there is a deeper, larger, active landslide that is contributing to the slope movement then a “hard fix” solution may not be the preferred solution for this location. The design is now considering an earthwork solution. The earthwork solution is likely to consist of a buttress fill, shear key and geosynthetic reinforcement. Shoring or stabilization along the head scarp may be needed to allow for earthwork to be performed within the limits of the landslide.

Figure 4: Mad River Road MP7.5 Top: Looking north along edge of southbound lane and scarp

Middle: Looking south along slope from north approach Bottom: Slide debris at edge of lake below the road

2.4 SITE 008, MAD RIVER ROAD MP8.0

Location (lat/long): 40.3609, -123.4220

Site 008 is located on a west facing hillside traversed by Mad River Road about 500 feet from the edge of Ruth Lake, above a cabin type residence that is located on the shoreline below the site, and 8 miles south of the intersection of Mad River Road with Highway 36. The road generally runs north-south along the Mad River drainage and within the Six Rivers National Forest. Mad River Road is generally a 22-foot wide two-lane road with at least a 3-foot wide outside shoulder. The road through the site was constructed mostly on cut, with a relatively shallow built-out shoulder beyond the edge of pavement. Fill had been placed along the southbound shoulder of road that created an unpaved pullout that extended approximately 75 feet beyond the edge of pavement. The edge of the fill descends for about 40 feet below the road at a slope of approximately 28 degrees. The fill slope had evidence of past slope instability and small debris flows but they did not appear to be currently active.

The County reported arcuate shaped cracks and subsidence that extended across the entire paved width of the roadway during rainfall in February of 2017. The County subsequently placed a temporary asphalt patch over the area of cracked pavement that was approximately 185 feet long.

We looked for and did not observe evidence of cracking or reflecting cracks in the patch during the scoping trip in May 2019. The team concluded that the improvements at this location would be limited to repairing the roadway.

Design Concept. Remove and replace the temporary patch with new hot mix asphalt pavement.

2.5 16SITE 009, MAD RIVER ROAD MP0.15

Location (lat/long): 40.4434,-123.4946

Site 009 is located on a southwest facing hillside traversed by Mad River Road and about 300 feet south of its intersection with State Highway 36. The road through the site was constructed as a

Figure 5: Mad River Road MP8.0 View through site looking north on May 8, 2019 sidehill fill with a wide northbound shoulder. Instability of the slope below the road led to cracking and settlement of the paved shoulder and traveled way of the southbound lane that developed during rains in February 2017 (top of inset photos, view northwest). The slope below the road had evidence of past slope instability and repairs. The cracking in the road appeared more severe than reported in photos from 2017 but were generally constrained to the within the previous limits of the cracks. It appeared that slope movement was ongoing.

The slope below the road descended about 45 feet at approximately 22 degrees (middle inset photo, view looking north).

The terrain at the toe of the slope is relatively flat and is furthered by the Mad River located approximately 350 feet beyond the toe of slope. The base of the slope contained a toe bulge (bright green grass in middle photo) related to landsliding. The bulge extended horizontally out about 30 feet beyond the adjacent areas of the slope. Springs, wet ground, and surficial instability were observed along the toe bulge.

Highway 36 runs on a parallel alignment uphill of the site, with a distance of approximately 120 feet between centerlines, and approximately 15 to 20 feet higher in elevation than Mad River Road. We looked for and did not observe cracking in the relatively fresh-looking pavement that had been placed on Highway 36 during the May 2019 scoping trip. Subsequent review of aerial

Figure 6: Mad River Road MP0.15 Top: View looking west along edge of road

Middle: View looking northwest from base of slope Bottom: Aerial view of site and Highway 36 form Google Earth photography (Good Earth image from May 2016, bottom inset photo) shows evidence of subsidence and patching directly above the slope instability that impacted Mad River Road. The subsidence and cracking in Mad River Road and Highway 36 appeared to be associated with a larger landslide. Based on review of the photo, a larger landslide may extend approximately 160 feet in elevation above Mad River Road and appeared to be about 370 feet wide based on arcuate cracking of the pavement observed in the photo. The distress associated with this project and the 2017 storm damage appears to be constrained below the shoulder of the road.

Movement and widening of the cracks in the shoulder appeared to have continuing movement at the time of our October 2019 field exploration program. The slope movement is likely associated with movement of a slide within the larger landslide that has impacted both Highway 36 and Mad River Road at the site. Although the distress between the roads appears to be related, the cracking in Mad River Road appears to be constrained to the southbound shoulder and traveled way.

Design Concept. The design will consider the potential for further movement as a more comprehensive mitigation of the overall larger landslide extending above Mad River Road is beyond the scope of the current project. The design concept was initially to restore and rebuild the shoulder using an earthwork solution and geogrid reinforcement. However, 95% plans prepared by CFL from February 2013 show that riprap and a deep patch were previously constructed at this location. We understood from CFL that these improvements were constructed. The deep patch was shown on those plans 4 feet deep and about 17 feet wide extending from the centerline of the road to the outside shoulder of the northbound lane.

Our initial slope stability analyses suggested that more extensive grading of the slope would be needed to provide slope stability within the 2017 slide area. An earthwork solution could involve reconstruction of the slope from the toe of slope to the road elevation. Another alternative was therefore proposed by Yeh (2020) in January 2020 to consider a micropile wall to support the shoulder of the road and limit the amount of excavation that would be needed to reconstruct the slope. It was decided based on the memo and subsequent consultation between CFL, Jacobs, and Yeh to proceed with the micropile concept. Yeh is showing the micropile alternative as the concept for the slope improvements on the 70-percent plans; however, further development of the design, construction cost, and graded solution are still being considered.

2.6 SITE 010, RUTH ZENIA ROAD MP15.4

Location 40.2599, -123.4444

The site was located on northbound shoulder of Ruth Zenia Road where cracking and subsidence of the northbound lane of the traveled way was reported by the County during rains in February 2017. This site was initially not approved for FHWA funds and was not evaluated during the visit or subsequent field exploration program.

The distress appears to be associated with subsidence of the road that the County attributed to subsurface water in their damage assessment.

The subsidence was approximately 20 feet wide. The slope below the road is an embankment fill inclined at approximately 20 degrees.

Design Concept. The designed concept is to provide subexcavation of the northbound lane through the subsided area.

2.7 SITE 011, RUTH ZENIA ROAD MP16.3

Location (lat/long): 40.2490,-123.4457

Site 011 is located on the east-facing slope of a mountainside traversed by Ruth Zenia Road. The road through the site was constructed as a sidehill cut and fill road. A slipout below the road lead to cracking and settlement of the outside unpaved shoulder of the road during rains of February 2017. The County reported that the slipout resulted in a steep scarp that was located just off the edge of pavement. Figure 8: Ruth Zenia Road MP16.3

View looking north May 2019

Figure 7: Ruth Zenia Road MP15.4 Top: County photo taken January 2017 looking north

Bottom: View looking north in May 2019

The County subsequently placed a rock fill over the slipout to fill the steep drop off at the edge of pavement. We observed continued cracking in the fill during the scoping trip and continuing at the time of the October 2019 field exploration program (see inset photo).

The slide was about 100 feet wide along the edge of northbound shoulder of the road. The slope below the road descended at a slope of about 34 degrees with slope instability evidenced by pistol-grip trees and hummocky ground. The toe-bulge of the slip out was observed about 30 feet from the existing edge of pavement, and approximately 16 feet below the elevation of the road. The ground sloped away from the toe at a slope of about 20 degrees and had evidence of prevalent slope instability such as hummocky ground and more pistol grip trees.

Design Concept. The design concept is to restore and rebuild the shoulder using an earthwork solution consisting of a shear key excavated below slide at the toe of slope and rebuilding the slope above as a buttress fill with subsurface drainage.

2.8 SITE 012, RUTH ZENIA ROAD

MP17.7

Location (lat/long): 40.2371, - 123.4434

Site 012 is located on the east facing slope of a mountainside traversed by Ruth Zenia Road.

The road through the site was constructed as a sidehill cut and fill road. A slipout below the road lead to cracking and several

Figure 9: Ruth Zenia Road MP17.7 Top: View looking south in May 2019

Bottom Left: Shallow slipout in cut above the road.

Lower Right: Hummocky ground and leaning trees downslope of road.

inches of settlement of the northbound lane and part of the southbound lane of traveled way during rains of February 2017. The County subsequently placed a gravel patch over the slipout to fill the subsided area and improve surface drainage. There is also a shallow slip out above the road along a drainage from hillside above the road at the north end of the site (inset photo at bottom left). The slip out above the road is about 3 feet deep, 8 feet wide, and extends approximately 24 feet along the slope above the road.

The embankment below the road descended at a slope of about 34 degrees. There is 6-inch diameter subdrain outlet at the toe of the embankment located near the south end of the slipout. The slope below the toe of the embankment had the features of slope instability evidenced by pistol-grip trees, chaotic leans to trees, and hummocky ground. There was a relatively large toe-bulge observed at about 260 feet downslope of the existing edge of pavement, and approximately 150 feet below the elevation of the road. The slide area was evidenced by low and lush vegetation, side drainages, and springs over a total width of about 100 feet. The ground sloped away from the embankment toe at a slope of about 15 degrees (last inset photo with view to south across the slide). The appearance of the observed landslide features and boring suggest the slope instability could be associated with a coalesced series of relatively shallow slides that have occurred within a zone of weathered rock and overburden moving above the underlying bedrock surface. The roadway embankment may have been founded on the slide debris.

Design Concept. The design concept is to restore and rebuild the downhill side embankment using an earthwork solution consisting a shear key excavated below slide at the toe of slope and rebuilding the slope above as a buttress fill with subsurface drainage. The shallow slipout above the road would be cleaned out, lined with geotextile, and filled with angular riprap.

2.9 SITE 013, RUTH ZENIA ROAD MP18.1

Location (lat/long): 40.2292, -123.4368

Site 013 is located on the east facing slope of a mountainside traversed by Ruth Zenia Road (inset photo at top with view to south). The road through the site was constructed as a sidehill cut and fill road. Subsidence and cracking of the pavement across the traveled way was observed by the County during rains of February 2017. Poor surface drainage resulted in water flowing onto and ponding on the road. The County subsequently placed a gravel patch over the southbound lane to fill the subsided area and improve surface drainage.

The site appears to be underlain by a large deep-seated landslide. The larger landslide is apparent by a shear crack in the pavement observed about 650 feet north of the patched area (see inset photo below with view looking south toward site), a second shear crack in the pavement observed south of the patched area, and wide tension cracks (about 3 feet across) and a sunken graben feature along the southbound side of road. The County reported that there are additional features (ponds and grabens) upslope of the road. The landslide maybe 1,000 feet wide at the road level and could extend 10s of feet below the elevation of the road.

Landslide movement appeared to have resulted in poor surface drainage, wet subgrade conditions, and a separation in a 24-inch culvert that crossed the road at the site. A void that could be probed to a depth of 18 to 30 inches was found around the culvert below the centerline of the road. Intense fatigue cracking and sediment deposited on the pavement was observed is likely associated with wet and pumping subgrade conditions. The cracking was observed in the northbound lane adjacent to the gravel patch (see top inset photo with view looking south).

Design Concept. The design concept is to improve surface drainage, restore the pavement and do not try to mitigate the larger landslide. Evaluate whether an underdrain is needed to improve subgrade drainage.

Figure 10: Ruth Zenia Road MP18.1 Top: View looking south in May 2019

Bottom: Shear crack in road 650’ north of patched area

2.10 SITE 014, RUTH ZENIA ROAD MP18.7

Location (lat/long): 40.2371, -123.4434

Site 014 is located on the east facing slope of a mountainside traversed by Ruth Zenia Road.

The road through the site was constructed as a sidehill cut and fill road. A slipout occurred below the road during rains of February 2017 that extended into the unpaved shoulder and southbound lane of the traveled way. The County subsequently placed a gravel patch over the southbound lane to fill the subsided area and improve surface drainage. The head scarp of the slipout extended south beyond the patched area and existing edge of pavement. The estimated total width of the slide at the edge of road was approximately 120 feet. There was an existing culvert where the slipout entered the road. The culvert was separated and displaced below the road (see inset photo looking into the culvert from below the road).

The embankment from the edge of the road was approximately 20 feet high and sloped to approximately 1v:1.5h. The toe of the embankment was furthered by a descending slope with a hummocky ground surface and chaotically leaning trees. The slope descended about 50 feet beyond the toe of the embankment at a slope of 26 degrees to the head of a debris flow chute. The scarp of the debris flow was approximately 6 feet high vertically. The chute of the debris flow continued downslope to a point at least 110 feet below the road at a slope of approximately 34 degrees (see bottom inset photo with view looking downslope from top of scarp).

Design Concept. The design concept is to restore and rebuild the shoulder and southbound lane using an earthwork solution, geogrid reinforcement, and drainage. A geogrid reinforced shear key will be

Figure 11: Ruth Zenia Road MP18.7 Top: View looking south in May 2019

Bottom Left: Separated culvert. Bottom right: Debris flow chute downslope of slipout.

provided at the toe of the slope. The shear key will be founded in stable material with a vertical geosynthetic reinforcement to help maintain slope if the debris flow chute moves further upslope.

2.11 SITE 015, ALDERPOINT BLUFF ROAD MP6.5

Location (lat/long): 40.1926, -123.5431

Site 015 is located on a south facing hillside traversed by Alder Bluff Road. A rockslide occurred on the cut slope above the westbound side of the road during rainfall that occurred in February 2017. Debris from the rockslide blocked both lanes and crossed the road. The County subsequently removed the slide debris from the road.

The head of the slide was located approximately 100 feet above the road (see inset photo above).

Alder Bluff Road is a single lane roadway with a 16-foot paved travel way, a 4-foot wide outside unpaved shoulder on the eastbound side of the road, and a 6-foot wide inside shoulder that was observed either side of the rockslide. The main body of the rockslide remains on the slope with a slope of approximately 55 degrees (see inset photo). There is an approximately 15-foot high head scarp above the slide that is sloped to approximately 70 degrees. Displaced rock and slide debris have filled the catchment along the base of the cut and is present over the head of the rockslide on the slope above the road. The downslope side of the road drops off steeply, and there is small slipout near the edge of pavement across from the rockslide that is not part of the project. The slope is composed of moderately to intensely fractured graywacke sandstone, possibly with adversely sloping joints and fractures sloping toward the road.

Design Concept. The concept for design is to perform heavy scaling of the slope to remove loose rock and debris, remove potentially unstable trees along the head of the slide area, clear the catchment area of rockfall and debris, and to perform limited rock excavation at the toe of the slope to restore the previous 6-foot catchment width within the westbound shoulder of the road.

Figure 12: Alder Bluff Road MP6.5 Top: View from top of slope toward in November 2019

Bottom: View along edge of looking west.

2.12 SITE 016, HYAMPOM ROAD MP20.2

Location (lat/long): 40.6219, -123.4307

Site 016 is located on a north facing slope within mountainous terrain traversed by Hyampom Road, about 2 miles east of Hyampom. The road runs for approximately 22 miles within the Shasta-Trinity National Forest between Hayfork and Hyampom. The road through the site was constructed as a sidehill cut and fill road with westbound fill and eastbound cut. Hyampom Road is generally a 22-foot wide two-lane road with 1-foot paved shoulders near the site. Site 016 is located on a northwest facing mountain side traversed by Hyampom Road and about 1,750 feet above Hayfork Creek that runs along the base of the slope. The mountain side as viewed from Google Earth is composed of heavy timber and ponds pooled within hummocky terrain that may be associated with large-scale landslides extending at least 2.5 miles above Hyampom Road. The existing grade along the mountainside averages about 30 percent.

A slipout occurred below the road in 2017, and in early 2019 took out the westbound lane of the road. The head scarp of the slide was located at approximately the centerline of the previous road.

The head scarp is approximately 15 feet high below the road (see top photo in Figure 13). Springs and wet soil were present along the base of the scarp. The head of the slide below the scarp was wet, soft, and contained wide tension cracks. There was a break in the slope at about 50 feet below the head scarp. Debris from slide had flowed down the slope beyond that point. The slope below the main slide descended at approximately 41 degrees for a distance of at least 150 feet. The base of the slide maybe near the top of this steep slope, about 30 feet below the elevation of Hyampom Road.

The County has rerouted the road by cutting into the hillside opposite of the slide. The cut was approximately 25 feet high and graded to approximately 1v:1.5h. Claystone and springs were observed on the cut slope. The head scarp of the slide is not considered stable, and there is a potential for the slide to move upslope into the detour.

Figure 13: Hyampom Road MP20.2 Top: May 6, 2019 looking southwest from edge of slide Bottom: October 14, 2019: Looking west along detour

Design Concept. The concept for design is an earthwork solution, which will be a buttress fill, shear key, and geosynthetic reinforcement to allow for restoration of the road to two lanes through the site. The buttress will be founded in relatively stable bedrock below the slide debris.

3. GEOTECHNICAL INVESTIGATION

A field exploration program that included drilling, a seismic refraction survey and geologic reconnaissance of the cuts slopes was performed in October 2019. Field exploration plans, logs of the borings, and plotted results of the seismic refraction data are presented in Appendix A. Laboratory test data is presented in Appendix B. Table 2 summarizes the field exploration program for each site.

Table 2: Field Exploration Summary

DAF

Site No. Location Boring-Seismic

Line Nos. Completed Field Exploration 001-0 Van Duzen Road MP4.1 road realignment 19P-06, 07 Two borings to depths of 4.5 and 6.5 feet for the road realignment

006-0 Hyampom Road MP11.4 slide in cut above the road -- Geologic mapping and cross section using rope access.

007-1 Mad River Road MP7.5 slide below the road

19S-03, 04

SL-3

Two borings to depths of 32 and 40 feet. Refusal to sampling encountered at total depths of borings. Attempt to core rock unsuccessful due to intense fractures and shears in soft Franciscan mélange. Geophysical line through head of landslide.

008-0 Mad River Road MP8.0 subsidence-slide below the road

-- None planned. Road to be patched and maintained in kind.

009-1 Mad River Road MP0.15 slide below the road

19S-05

SL-2

One boring to depth of 40 feet. Geophysical line through head of landslide.

011-0 Ruth-Zenia Road MP16.3 slide below the road 19S-09 One boring to depth of 40 feet.

012-0 Ruth-Zenia Road MP 17.7 slide below the road 19S-10 One boring to depth of 40 feet.

013-0 Ruth-Zenia Road MP 18.1 large landslide 19S-08 One boring to depth of 11 feet for grading and drainage improvements

014-1 Ruth-Zenia Road MP 18.7 slide below the road 19S-11 One boring to depth of 25 feet. Refusal to sampling at total depth.

015-0 Alderpoint Bluff Road MP 6.5 slide in cut above the road -- Geologic mapping and cross section using rope access.

016-0 Hyampom Road MP 20.2 slide below the road

19S-01, 02

SL-1

Two borings to depths of 40 and 50 feet.

Geophysical lines through the head of landslide.

Refusal to sampling at total depths.

3.1 DRILLING

The drilling subcontractor for this project was Taber Drilling of West Sacramento, California. Taber used a CME55 truck-mounted rig equipped for auger, mud rotary, and rock core drilling. Eleven borings were drilled to depths ranging from approximately 5 to 50 feet during the period of October 14 and 18, 2019. The Yeh geologist scraped the sides of the borehole and recorded the thickness of the existing asphalt pavement and base course materials where borings were drilled through the existing pavement. The geologist also logged the subsurface conditions encountered during the drilling, secured soil and rock samples for subsequent laboratory testing, and observed the grouting and abandonment of the borings. The samples intervals, a description of the subsurface conditions encountered, field tests, and blow counts (N-Values) recorded during the drilling are presented on the logs.

Driven Samples. Sampling within the borings was performed by driving either a modified California or standard penetration test (SPT) split spoon sampler at typical 5-foot intervals. The SPT sampler has a 2-inch outside diameter, 1-3/8-inch inside diameter, and is equipped for but was used without liners.

The modified California sampler has a 3-inch outside diameter, 2-3/8-inch inside diameter, and was used with 1-inch high brass liners. Drive samples were collected using a 140-pound automatic trip hammer in accordance with ASTM D-1586 (the Standard Penetration Test) procedures. The hammer had an average rated efficiency of 82 percent (Taber 2016). Bulk samples of the subgrade soil were collected from the augers at the depth intervals noted on the logs.

Pocket Penetrometer. Pocket penetrometer tests were performed in the field on the trimmed end of selected samples to help estimate the undrained shear strength of cohesive materials. The penetrometer was pushed to the designated penetration and the shear strength was read from the spring scale on the device. The undrained shear strength results from the pocket penetrometer tests (noted by pp) are shown on the logs.

Abandonment. The borings were backfilled with cement slurry to within 5 feet of the surface and then topped with soil cuttings after drilling. Backfill was performed in compliance with the permit issued by Trinity County Environmental Health Services.

Upon completion, the borings were backfilled with approved native fill material collected from the auger cuttings, bentonite-cement grout in accordance with well permit requirements, and patches with hand-mixed rapid set concrete.

3.2 LABORATORY TESTING

Laboratory testing was performed on selected samples recovered from the field exploration program.

Tests for moisture content, unit weight, percent passing the 200 sieve, particle size distribution by mechanical sieve analysis, unit weight versus moisture content relation by the modified Proctor test, and pH and resistivity were performed at our laboratory in Grover Beach, California. R-value tests were performed by NV5 at their laboratory in Ventura, California. Strength tests for triaxial and direct shear strength were performed by the GEO-E lab at the Cal Poly Civil Engineering Department in San Luis Obispo, California. The results of laboratory tests are presented in Appendix B. Testing was performed accordance with ASTM and American Association of State Highway and Transportation Officials (AASHTO) methods as noted on the lab reports. After the completion of the laboratory testing, the field descriptions were confirmed or modified as necessary and boring logs were prepared.

3.3 GEOPHYSICAL SURVEY

Geophysical survey services consisting of seismic reflection and refraction were performed by Advanced Geosciences (AGI) of Torrance, California. AGI and Yeh performed seismic refraction surveys at the site between the period of October 19 through 21, 2019. Seismic refraction data was recorded along three lines designated at Mad River Road MP0.15, Mad River Road MP7.5 and Hyampom Road MP20.2. AGI processed the seismic refraction and created velocity plots from the recorded data. These data were then used to supplement the borings information and estimate the approximate depth of landslide deposits at the three sites. The AGI summary report and plots of the seismic refraction data are presented in Appendix A.

3.4 GEOLOGIC RECONNAISSANCE

A geologic reconnaissance of the cut slopes at Alder Point Bluff Road MP6.5 and Hyampom Road MP11.4 was performed on October 30, 2019. The reconnaissance consisted of using rope access to observe the geologic conditions on the slope relative to recutting the slope, assessing the need for scaling, characterizing rock conditions, and observing the performance and inclination of other slopes in the area.

3.5 PREVIOUS STUDIES

Yeh reviewed previous reports and plans available for the site. A summary of the documents we reviewed follows:

• Yeh (2006, 2008) prepared a geotechnical evaluation for the design of improvements and widening of Hyampom Road. The limits for that evaluation included the cut slope at Site 006, Hyampom Road MP11.4.

• Jacobs (2012) prepared plans for the rehabilitation of Ruth Zenia Road from MP14.8 to MP23.0, and a portion of Mad River Road that is outside the limits of this project. The plans show the structural section for Ruth Zenia Road was 3 inches of hot mix asphalt concrete over 6 inches of roadway aggregate.

• FHWA CFL (2008) prepared plans for improvements to Hyampom Road that included the cut slope Site 006, Hyampom Road MP11.4. The plans show that the slope cut with slope rounding details.

Cross sections that would show the design inclination of the proposed cut slope were not available.

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

The regional geology in the site vicinity as mapped by Strand (1962), Irwin et al (1974), and Irwin (2010) is shown on Plates 2.1 and 2.2 - Regional Geologic Map. Trinity County is located within the North Coast Ranges and Klamath Mountains geologic and geomorphic province. The North Coast Ranges are dominated by irregular, knobby, landslide-prone topography of the Franciscan Complex.

The Klamath Mountains are considered a northern extension of the Sierra Nevada, include the high mountain peaks of the Trinity Alps, and are composed of massive granitic rocks. The project sites along Mad River, Van Duzen, Ruth Zenia, and Alder Point Bluff are generally within the North Coast Ranges underlain by landslide-prone Franciscan Complex. The Hyampom Road sites and Mad River sites are at the margin between the provinces with diorite and granitic rocks exposed along Hyampom Road being representative of the Klamath Mountains. The project area is within the Six Rivers National Forest that includes major drainages along the Mad and Van Duzen Rivers. Alluvial sediments and landslides are mapped extensively throughout the region, many of which define entire mountain sides.

Regional geologic structure is characterized by a series of northwest-trending faults and mountain ranges. The Grogan fault is considered active and a seismic source within California. The Grogan fault is mapped along Mad River and is near the sites on Mad River Road.