Santa Anitia Canyon Site 1-5 Final Geotech.pdf

Geotechnical Design Report

CA FLAP LA CR2N40(1)

Santa Anita Canyon Road Site Improvements Los Angeles County, California

December 29, 2016

Submitted To:

Mr. Berwyn Wilbrink

Jacobs Engineering Group Inc.

707 17th Street, Suite 2400

Denver, Colorado 80202

By:

Shannon & Wilson, Inc.

1321 Bannock Street, Suite 200 Denver, CO 80204

23-1-01458-011

23-1-01458-011_R1/wp/ady 23-1-01458-011 i

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

1.1 General

1.2 Project Description

1.3 Scope of Work

1.4 Design Guidelines

2.0 SITE DESCRIPTION

2.1 General

2.2 Site 1

2.3 Site 2

2.4 Site 3

2.5 Site 4

2.6 Site 5

3.0 FIELD EXPLORATIONS AND LABORATORY TEST RESULTS

3.1 Subsurface Explorations

3.2 Laboratory Test Results

3.3 Geologic Mapping

4.0 GEOLOGY AND SUBSURFACE CONDITIONS

4.1 Regional Geology

4.2 Local Geologic Conditions

4.2.1 Site 1

4.2.2 Site 2

4.2.3 Site 3

4.2.4 Site 4

4.2.5 Site 5

4.2.6 Groundwater

5.0 GEOLOGIC HAZARDS

5.1 General

5.2 Seismic Hazards

5.2.1 Fault Surface Rupture

5.2.2 Ground Shaking

5.3 Rock Fall

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 Seismic Ground Motion

TABLE OF CONTENTS (cont.)

ii

6.1.1 Site Class

6.1.2 Design Parameters

6.2 Site 1 – Rock fall Hazard

6.2.1 Rock fall Hazard Mitigation Considerations

6.2.2 Site History and Observations

6.2.3 Kinematic Analysis

6.2.4 Rock Fall Hazard Mitigation

6.2.5 Additional Considerations

6.3 Site 2 – Basin

6.4 Retaining Wall Design

6.4.1 Wall Type

6.4.2 Cast-In-Place Concrete Retaining Wall

6.4.3 MSE Wall

6.4.4 Soil Nail Wall

6.4.5 Surcharge Loads

6.4.6 Drainage

7.0 CONSTRUCTION CONSIDERATIONS

7.1 General

7.2 Earthwork

7.2.1 Clearing and Grubbing

7.2.2 Temporary Excavations

7.2.3 Excavation Potential

7.2.4 Proof Roll and Subgrade Preparation

7.2.5 Backfill

7.2.6 Backfill Placement

7.3 Anchors

8.0 CONSTRUCTION OBSERVATION

9.0 LIMITATIONS

10.0 REFERENCES

TABLES

1 Existing Pavement Sections 2 Recommended Ground Motion Parameters 3 Recommended Parameters for Cast-In-Place Retaining Wall Design 4 Recommended Parameters for MSE Wall Design 5 Recommended Parameters for Soil Nail Wall Design

TABLE OF CONTENTS (cont.)

iii

PHOTOGRAPHS

1 Photograph 1 (Site 1) 2 Photograph 2 (Site 1) 3 Photograph 3 (Site 3) 4 Photograph 4 (Site 4) 5 Photograph 5 (Site 1)

FIGURES

1 Vicinity Map 2 Site and Exploration Plan (11 sheets) 3 Regional Geologic Map 4 Landslide Inventory Map 5 Geologic Cross Section at Station 28+65

APPENDICES

A Subsurface Explorations B Exploration Photographs C Laboratory Test Results D Geological Structure Mapping E Important Information About Your Geotechnical/Environmental Report

GEOTECHNICAL DESIGN REPORT

CA FLAP LA CR2N40(1)

SANTA ANITA CANYON ROAD SITE IMPROVEMENTS

LOS ANGELES COUNTY, CALIFORNIA

1.0 INTRODUCTION

1.1 General

This geotechnical design report provides our recommendations for the Santa Anita Canyon Road (SACR) Project (the Project) in southern California (Figure 1). The Project is a part of the Federal Lands Access Program (FLAP) which funds projects to rehabilitate, restore, and resurface roadways that provide access to federal lands. This report summarizes our subsurface explorations and laboratory testing, describes the geologic setting and associated hazards, and presents our design recommendations.

1.2 Project Description

The Project consists of spot improvements within a 3.3 mile stretch of SACR from Stations 10+00 to 184+52 (the Site). These improvements are planned at five sites along the roadway and include:

Site 1 (Stations 177+75 to 179+55) - Rock fall hazard mitigation by scaling, installation of a rock slope drape system, creating a catchment area by placing K-rails along the edge of the roadway, and replacement of adjacent roadway surfacing;

Site 2 (Stations 67+10 to 68+50) – Development of an attenuation basin and riser inlet on the upstream side of the road, installation of a pipe to transport water beneath the road, and installation of a flume drain outlet into the existing drainage channel;

Site 3 (Stations 28+00 to 29+40) – Replacement of the existing stone and mortar retaining wall with a new retaining wall (type to be determined);

Site 4 (Stations 21+25 to 24+70) – Stabilization of the existing subgrade and constructing a new pavement section, and construction of a cast-in-place barrier along approximately 500 linear feet of roadway (design by others); and

Site 5 (Stations 59+00 to 91+50) - Resurfacing of approximately 3,500 linear feet of roadway (design by others).

The location of each site is shown on the Vicinity Map (Figure 1), Site and Exploration Plan (Figure 2), Regional Geologic Map (Figure 3), and Landslide Inventory Map (Figure 4).

1.3 Scope of Work

The purpose of our geotechnical design study was to observe the surface conditions at the Site, explore the subsurface conditions, perform geotechnical engineering analyses, and provide geotechnical conclusions and recommendations for design and construction of the Project. Our services were completed in general accordance with our Subcontract Agreement No.

WVXY4050-S16-0001 with Jacobs Engineering Group, Inc. (Jacobs) dated October 14, 2015 and the Scope of Work prepared by Central Federal Lands Highway Division (CFLHD).

We completed the following tasks for this study:

Meetings – We attended the Project Scoping Meeting and site visit on May 20, 2015 (Jacobs, 2015) and the Design Kick-off Meeting and Project Review on October 14, 2015.

Document Review – We reviewed readily available documents regarding the topographic, geologic, and geotechnical conditions at the Site.

Field Preparation – We generated a Geotechnical Investigation Plan for project team and agency review, subcontracted traffic control and drilling services, generated a site specific health and safety plan, marked the proposed boring locations, and contacted Underground Service Alert of Southern California (i.e. DigAlert) for utility clearance.

Field Reconnaissance and Mapping – We completed a geologic reconnaissance at each of the five sites and mapped rock outcrops and discontinuities at Site 1.

Exploration – We advanced 10 borings through the SACR pavement and 2 borings adjacent to the roadway to explore the subsurface conditions. We performed geological structure mapping at Site 1 to characterize the existing rock slope.

Laboratory Testing – We conducted laboratory tests on selected soil samples obtained during our exploration to characterize pertinent physical and engineering characteristics of the geologic materials.

Geotechnical Engineering Analyses – We performed geotechnical engineering analyses for rock fall hazard mitigation (Site 1) and retaining wall design (Site 3).

Draft Report – We prepared this geotechnical report summarizing our findings, analyses, and recommendations.

We understand Los Angeles County will design the pavement sections and provide the corresponding material specifications and construction considerations.

1.4 Design Guidelines

Although SACR is managed by Los Angeles County, the Project will be developed using CFLHD standards and the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (2014; FP-14). As indicated at the kick-off meeting, the design manual for the Project is the 2008 Federal Lands Highway Project Development and Design Manual (PDDM). We followed AASHTO LRFD Bridge Design Specifications (2014) for seismic ground motion and the seismic increment lateral earth pressure. At the request of CFLHD, we provide foundation design criteria in accordance with the load and resistance factor design approach (LRFD).

2.0 SITE DESCRIPTION

2.1 General

The subject part of SACR that composes the Project is located within the southern part of the San Gabriel Mountains in Los Angeles County, California (see Figure 1). The two-lane, asphalt paved roadway was constructed within the hillside terrain apparently by excavating cut slopes on the upslope side and placing fills and/or earth retaining structures on the downslope side. Within the limits of the Project, there are three paved side roads that branch off SACR: the access road for the existing Los Angeles County Flood Control basin at Station 30+15, the access road to Santa Anita Dam at Station 94+10, and a paved access road at Station 100+55. Chantry Flat Parking Lot and Trail Head is located at the northern terminus of the roadway.

2.2 Site 1

Site 1 is the northern-most improvement location and extends from approximately Station 177+75 to 179+55. There is an unpaved shoulder along the west side of the roadway, with a variable width from approximately two to six feet. An existing bedrock slope ascends approximately 500 feet at an overall gradient of about 1 vertical (V): 1 horizontal (H) from the west shoulder to the ridgeline.

For the purposes of this report, we have divided the slope into three parts: lower, middle, and upper, as shown on Figure 2 (sheet 10 of 11). The lower portion of the slope ascends approximately 40 feet (vertically) from the SACR shoulder. This part of the slope has irregular topography with an overall gradient of about 2V:1H with some near vertical sections, as shown in Photographs 1 and 2 below. It contains a sparse growth of vegetation. This lower section may be a cut slope, excavated during SACR construction.

The middle section of the slope ascends approximately 75 to100 feet (vertically) at a gradient of about 1V:1H. The break in slope between the lower and middle sections is highlighted in Photographs 1 and 2. This break is well defined near the central area of the slope (measured parallel to the road) and becomes poorly defined toward the edges of Site 1. As with the lower section, this part of the slope is sparsely vegetated. Both the lower and middle sections were moderately well vegetated until at least December of 2003, as seen in historical Google Earth images. In the image from March 2005, the lower and middle parts of the slope are devoid of vegetation. From the image, it is possible that the lower and middle portions of the slope experienced a slope failure. Although the image provides little detail, the failure may be a debris flow that overran the roadway and continued downslope below SACR.

The upper part of the slope extends from the top of the middle part to the ridgeline. This section ascends at a gradient of approximately 1V:1H and is heavily vegetated. San Olene Road begins at the Chantry Flat Parking Lot and crosses the upper part of the slope twice. This road can provide access to Site 1 from above the lower and middle sections of the slope.

An H-beam and timber lagging impact wall is present just southeast of Site 1. The wall is approximately 40 feet long, has an exposed height of 15 feet, and currently retains three feet of accumulated debris. One of the H-beams is deflected toward SACR. This wall appears to be performing as intended and will remain in-place.

Photograph 1 (Site 1). Slope ascending from the west shoulder of SACR.

Break in slope

Photograph 2 (Site 1). Profile of slope ascending from west shoulder of SACR.

On the east side of SACR, the bedrock slope descends approximately 650 feet at an overall gradient of about 1V:1H to the Santa Anita Canyon drainage. The slope is covered with a modest growth of vegetation. The outbound shoulder of SACR is approximately two to four feet wide with a guard railing system consisting of an approximately 2-foot tall rock and mortar wall (in part) and row of boulders (in part).

2.3 Site 2

Site 2 is located west of SACR from Stations 67+10 to 68+50. The location of the planned basin is relatively flat and near the same elevation as SACR. It appears that this are of the road was consisted of filling a natural drainage area; the fill daylights against the hillside terrain. There is a guard rail and end-dump stockpiles of soil between SACR and the flat area. The flat area contains sparse grasses, while the natural slopes contain a modest to thick growth of vegetation.

The natural drainage buried beneath Site 2 extends below SACR and continues downstream toward the east. A gabion wall was constructed across part of this drainage, extending south from Stations 67+20 to 66+40.

Break in slope

2.4 Site 3

Site 3 is located from approximately Stations 28+00 to 29+40. The outside part of the southbound lane is supported by an existing near-vertical rock and mortar retaining wall, as shown in Photograph 3. The wall is sited within a topographically low area between two ridges and varies in exposed height from 2 to 12 feet. The existing grade in front of the wall descends 80 feet at a variable gradient between about 1V:1H and 1V:1-½H to SACR. The hillside below the wall is covered with a sparse to modest growth of vegetation.

A cut slope ascends up to 40 feet at a gradient of approximately 2V:1H from the outside edge of the northbound lane to the ridge top. The slope is irregular due to surficial deterioration and contains a sparse growth of vegetation.

The outer two feet of the southbound lane shows signs of distress, including edge deterioration longitudinal cracks and patching. There is a wood post and metal guard railing system between the top of wall and edge of pavement.

Photograph 3 (Site 3). Existing rock and mortar retaining wall.

2.5 Site 4

Site 4 is an arc-shaped stretch of roadway extending from Stations 21+25 to 24+70. There is no shoulder on either side of the roadway in this area. An existing cut slope ascends up approximately 30 feet from the western side of the roadway at a gradient of between 2V:1H and 1V:1H. A fill slope descends approximately 50 feet from the eastern side of the roadway at a gradient of about 1V:1-½H to 1V:2H to the existing residential development below.

The existing pavement contains longitudinal and transverse cracks, alligator cracking, and rutting. We observed longitudinal cracking in about 80 percent of the outside lane and about 10 percent of the inside lane. Between Stations 21+25 and 23+00, the longitudinal cracks are within the wheel path. The longitudinal cracks cross the lane between Stations 23+00 and 24+00. We observed alligator cracking and rutting in the outside lane around boring B-12 (see Photograph 4) and the inside lane approaching boring B-10. Approximately 75 percent of the cracks were sealed.

Photograph 4 (Site 4). Existing pavement condition at boring location B-12.

2.6 Site 5

Site 5 extends from Stations 59+00 to 91+50. In general, a cut or natural slope ascends from the western side of the roadway, while a descending fill slope, natural slope, or earth retaining structure supports the eastern side of the roadway.

The pavement condition varies within Site 5. The most common distress observed consists of longitudinal cracks, transverse cracks, and alligator cracking. We observed areas that were patched, including a utility trench, and sealed cracks. We observed longitudinal cracks in approximately 25 to 50 percent of the length of Site 5 and transverse cracks at a 5- to 20-foot interval. We noted alligator cracking in approximately 5 percent of the roadway. Approximately 25 to 50 percent of the cracks observed were sealed. We observed manhole covers between Stations 84+50 and 87+20.

3.0 FIELD EXPLORATIONS AND LABORATORY TEST RESULTS

3.1 Subsurface Explorations

Shannon & Wilson conducted a field exploration program on November 20, 2015 to evaluate subsurface conditions for the Project. The program consisted of drilling 12 geotechnical borings (designated as B-01 through B-12). Borings B-05 and B-09 were drilled outside the roadway with a hand auger. The remaining borings were drilled through the existing pavement, near the center of the lane on the downslope side of the roadway. These borings were drilled by 2R Drilling, Inc. using a truck-mounted CME 75 drill rig. The approximate boring locations are shown on Figure 2. Appendix A describes the procedures used to complete the drilling and sampling of the borings and presents individual boring logs. Appendix B contains photographs of each boring location.

3.2 Laboratory Test Results

We completed geotechnical laboratory testing to determine index and engineering properties of samples retrieved from the borings. Laboratory tests included natural water content, grain size distribution, Atterberg limits, R-value, and corrosion testing. Laboratory test results and a discussion of testing procedures are included in Appendix C. The natural water contents, Atterberg limits, and percent fines are also on the individual boring logs in Appendix A.

3.3 Geologic Mapping

In order to assess the rock fall hazard to SACR, we completed geologic mapping of bedrock exposures along the alignment of the road. An engineering geologist from Shannon & Wilson mapped bedrock exposures that were accessible standing on the shoulder of SACR (i.e. the bottom five feet of the lower section of slope). We did not map higher parts of the lower section of the slope or any of the middle section; our observations in those areas were limited to Google Earth images and observation from SACR.

Geologic mapping of known bedrock exposures was accomplished to identify and characterize lithologic units, boundaries and structures, including major geologic structures and minor geologic structure such as discontinuity sets, fillings, and coatings. Field data from this mapping were used to understand the possible failure modes at Site 1.

Information on the properties of rock discontinuities (i.e. fractures and joints) was collected from observations by cell mapping in three locations along the shoulder of SACR, as summarized in Table D-1. The properties mapped included orientation, persistence (length), spacing (distance between discontinuities), aperature (width), joint filling, and surface characteristics of the discontinuities (roughness). Orientations of discontinuities were measured in surface outcrop using a Brunton Compass. The data collected are summarized in Section 4.2.1 and presented in Appendix D. The purpose of the mapping was to estimate the Rock Mass Rating (RMR) of the slope and to provide recommendations for rock fall hazard mitigation. Our estimation of the RMR for the slope is presented in Appendix D.

4.0 GEOLOGY AND SUBSURFACE CONDITIONS

4.1 Regional Geology

The Site is located within the southern part of the San Gabriel Mountains, in the southern portion of the Transverse Ranges geomorphic province. The San Gabriel Mountains are a fault bound block with the San Andreas fault zone on the north, the Cucamonga-Sierra Madre fault complex on the south, and the San Jacinto zone on the east (United States Geological Survey [USGS], 2006a). These structural features separate the mountain range from Antelope Valley and San Gabriel Valley, respectively. The mountains consist of Upper Precambrian to Mesozoic granitic and metasedimentary rocks (USGS, 2006a).

The roadway is sited within the western wall of Santa Anita Canyon, increasing approximately 1,200 feet in elevation from the residential development at the southern end to Chantry Flat at the northern end. Santa Anita Canyon contains side slopes at gradients typically between 1V:2H and 1V:1H (although locally steeper). As shown in Figure 3, bedrock at the site consists of quartz diorite, often observed with pegmatite dikes and well-defined joints or discontinuities (Dibblee, 1998). Road cuts observed within the hillside terrain expose bedrock with varying degrees of weathering from fresh to highly weathered.

Landslide deposits were mapped by Treiman and Willis (2007) at the northern and southern ends of the Project, as shown on Figure 4. The northern landslide encompasses the entire Chantry Flat Parking Lot, toeing out at the bottom of the canyon and displacing the creek bottom. Treiman and Willis consider the northern landslide dormant (i.e. inactive) with no evidence of historic (last 200 years) movement. The southern landslide deposit encompasses SACR from its southern limit to approximately Station 76+00. They also consider this landslide dormant – with no evidence of historic movement, but noting it does containing evidence of significant erosional features. We did not observe evidence during the course of our study that would contradict the landslide characterization by Treiman and Willis that these features are inactive.

4.2 Local Geologic Conditions

Our characterization of the local geologic conditions is based on observations from the borings, exposures adjacent to the roadway, and topographic expression. In general, the pavement section is supported by fill consisting of loose to dense, silty sand (although boring B-03 dense silt with sand subgrade soils). We anticipate the fill thickness decreases, and may pinch out, toward the upslope side of the roadway. The existing pavement sections are summarized in Table 1 and the subsurface conditions are described for each site in the following sections.

TABLE 1

EXISTING PAVEMENT SECTIONS

Boring Asphalt

Concrete (in)

Untreated Aggregate Base

Course (in)

B-01 5 0 B-02 5.5 0 B-03 5 0 B-04 4 0 B-06 4 0 B-07 5 0 B-08 6 6 B-10 4 0 B-11 6 0 B-12 4 0

4.2.1 Site 1

We performed geologic mapping within the toe of slope along the SACR shoulder and cell mapping of Site 1, as described in Section 3.3 The bedrock slope ascending from the west side of SACR is composed of quartz diorite intersected by pegmatite dikes. The quartz diorite is fresh to slightly weathered, with an estimated unconfined compressive strength between 14,500 and 25,500 psi, and an estimated RMR ranging from good (74) to very good (84) (Bieniawski, 1989).

Stereographic projection of the geological structure data collected indicates at least three joint sets are present. In general, we observed the joints to have a persistence (i.e. length) of more than three feet and spacing of between two and six feet. The joint width varied from tight to open, was filled with non-cohesive soil, and had slightly rough surfaces. Considering the intersecting joint spacing observed, we estimate the typical rock block size to be approximately 12 to 18 inches. We also observed some larger blocks toward the north end of the site, with maximum dimensions of approximately six feet. Further discussion of the rock discontinuities is presented in Section 6.2 and Appendix D.

We did not drill borings in the vicinity of Site 1.

4.2.2 Site 2

We drilled boring B-05 northwest of SACR, within the proposed basin, and boring B-06 through the roadway in the vicinity of the culvert crossing. Boring B-05 was excavated with a hand auger and encountered refusal at 1.3 feet below the existing ground surface (bgs). We encountered fill to a depth of 40 feet bgs in boring B-06, consisting of very loose to medium dense sand with varying percentages of silt and gravel. We observed highly to completely weathered quartz diorite beneath the fill in boring B-06.

4.2.3 Site 3

Borings B-08 encountered fill consisting of loose to dense sand with varying percentages of silt and gravel in the first three feet bgs. Boring B-09 encountered fill and/or residual soil consisting of loose sand with varying percentages of silt and gravel, also in the upper three feet bgs. We drilled boring B-08 within the middle of the outside lane of SACR with a truck-mounted hollow-stem auger drill rig and boring B-09 approximately 10 feet (measured horizontally) in front of the existing wall with a hand auger. Beneath the fill and/or residual soil, we encountered moderately to highly weathered quartz diorite. We anticipate the fill thickness will decrease (and may be absent) toward the north side of the roadway, as shown in Figure 5.

The cut slope on the north side of SACR exposes highly weathered quartz diorite with fractures at an approximately 6- to 18-inch spacing. The highly weathered material is easy to excavate with a rock hammer. Although we did not observe signs of global slope instability, we did observe rilling and shallow scarps suggesting surficial instability.

4.2.4 Site 4

The borings at Site 4 were drilled to a depth of five feet bgs. The soils encountered typically consisted of loose to medium dense, silty sand. The borings were drilled in the outside lane. We anticipate the fill thickness decreases (and may be absent) toward the inside edge of the roadway.

4.2.5 Site 5

The borings at Site 5 were drilled to a depth of five feet bgs. The soils encountered typically consisted of loose to dense, silty sand. Layers of silt with sand and gravelly silt with sand were also encountered.

4.2.6 Groundwater

Groundwater was not encountered during drilling in any of the borings completed for this study.

5.0 GEOLOGIC HAZARDS

5.1 General

Geologic hazards that may impact the proposed improvements include fault surface rupture, seismic ground motion, and rock fall. Slope stability including landslides and debris flow were beyond the scope of this study (see Section 4.1).

5.2 Seismic Hazards

5.2.1 Fault Surface Rupture

The Project is located in seismically active southern California; therefore, the site has the potential to experience the effects of future earthquakes. Such earthquakes could occur on any of the active faults within the region. The USGS has mapped faults and identified fault activity in the region (USGS, 2006b). The numerous faults in southern California include fault displacement during the Quaternary period divided into four categories:

Historic displacement (last 200 years);

Holocene displacement (last 11,700 years);

Late Quaternary displacement (last 700,000 years), and;

Undifferentiated Quaternary displacement.

Classification for these major groups is based on criteria developed by the California Geological Survey (CGS) for the Alquist-Priolo (AP) Zone Act program. The AP Zone Act regulates development near active faults, which the CGS defines having ruptured within the historic and Holocene time (about the last 11,700 years). Known active faults are not mapped on the site. The closest established AP Zone to the site is the Raymond fault zone, which is located approximately one mile to the southeast of Site 3.

According to the USGS online mapping tool, faults associated with the potentially active Sierra Madre fault zone cross SACR in at least three locations, as shown in Figure 3. The Sierra Madre fault zone is a Late Quaternary fault with a most recent rupture of less than 130,000 years (Treiman, 2000). Considering the reported age of the fault zone’s most recent activity in this area, we consider the potential for surface rupture from displacement within this fault zone to be low.

5.2.2 Ground Shaking

Earthquakes along several active faults in the region could cause moderate to strong ground shaking at the Site. The intensity of earthquake motion will depend on the characteristics of the generating fault, distance to the earthquake fault, earthquake magnitude, earthquake duration, and site-specific geologic conditions. Recommendations for seismic ground motion are provided in Section 6.1.

To mitigate the potential for future damage from strong seismic events, we recommend that new structures be analyzed using seismic design criteria based on AASHTO. Section 6.1.2 presents our seismic design criteria recommendations.

5.3 Rock Fall

Our study included an assessment of the rock fall hazard at Site 1. The bedrock slope observed at Site 1 consisted of fresh to slightly weathered quartz diorite with pegmatite dikes and several joint sets (see Section 4.2.1 for a more detailed description of the rock mass). Evidence of global instability was not observed. The failure mode for previous slope instability appears to be related to either structurally controlled rock fall in the lower section of the slope and perhaps differential erosion and debris flow in both the lower and the middle sections (however, this was difficult to assess from the roadway). We observed evidence of previous rock falls at the site, including: topographic expression, open fractures and joints, scars in the SACR pavement surface, and distress of the existing H-beam and timber lagging wall. In the lower slope, where stability of the bedrock slope or blocks within the slope are potentially structurally controlled, a kinematic analysis was used to estimate the potential for rock blocks to dislodge. The results of our analysis are summarized in Section 6.2.

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 Seismic Ground Motion

6.1.1 Site Class

We determined the AASHTO site class at Site 3 using Standard Penetration Test (SPT) N-values and soil classifications derived from our explorations. Specifically, borings B-8 and B- 9 encountered 3 feet of fill and/or residual soil overlying quartz diorite rock. The SPT N-value for the fill was 39 blows per foot, while the blow counts in the quartz diorite ranged from 50 to

80. We anticipate the bedrock is less weathered at depth. As a result, we believe Site Class B – “Rock” is appropriate for characterizing the upper 100 feet of bedrock underlying the site.

6.1.2 Design Parameters

We estimated peak ground acceleration (PGA) at the site using the online probabilistic seismic hazard analysis (PSHA) tools available from the USGS. These PSHA tools are based on the 2008 updates to the national seismic hazard maps (Petersen and others, 2008). The PSHA is a method for estimating ground motions that takes into account uncertainties and randomness in potential earthquake source, size, location, recurrence, and source-to-site attenuation. Results of the PSHA for the site indicate that a PGA of 0.88 g (g = acceleration of gravity) has a 7-percent probability of being exceeded in 75 years (approximately a 1000-year return period) for Site Class B conditions.

Ground motion parameters were determined for the Project site using the U.S. Seismic Design Map Web Application (USGS, 2012) and the 2009 AASHTO LRFD Bridge Design Criteria (the design parameters have not changed in the current 2014 manual). Table 2 presents the recommended seismic design ground motion parameters for the site.

TABLE 2

RECOMMENDED GROUND MOTION PARAMETERS

Ground Motion Parameters Value

Peak Ground Acceleration (PGAB) 0.88 g Site Class B Short-period Spectral Acceleration, Ss 2.07 g Long-period Spectral Acceleration, S1 0.73 g Site Factor, Fpga 1.0 Site Factor, Fa 1.0 Site Factor, Fv 1.0 Peak Design Spectral Acceleration, AS 0.88 g Short-period Design Spectral Acceleration, SDS 2.07 g Long-period Design Spectral Acceleration, SD1 0.73 g T0 0.07 sec.

TS 0.35 sec.

Seismic Zone1 4

Notes:

1) Seismic zone from AASHTO (2014) Table 3.10.6-1.

6.2 Site 1 – Rock fall Hazard

6.2.1 Rock fall Hazard Mitigation Considerations

We understand that the Project was scoped for rock fall hazard mitigation at Site 1 assuming:

Approximately 10,000 square feet of drape mesh system;

A catchment area at the base of the drape mesh that consists of the existing shoulder of SACR and a K-rail along the edge of the pavement (i.e. a ditch will not be excavated within the SACR shoulder);

The catchment area will be maintained by removing rocks and debris, as needed; and

The existing alignment and elevation of SACR will not be changed as a part of the planned improvements.

We assessed the rock fall hazard potential at Site 1 using data from several sources including: conversations with Los Angeles County Department of Public Works (LAC) staff, observation of the existing topographic and pavement conditions, and geologic and cell structure mapping along the shoulder of SACR. The following sections present the data and analyses we performed to establish rock fall hazard mitigation recommendations.

In developing the rock fall hazard mitigation recommendations we considered two systems, an anchored mesh (i.e. a pretensioned system using anchors within the mesh) and a drape mesh (i.e. a passive system where the mesh rests on the slope face and is anchored at the top). An anchored system is typically used to provide near-surface support to the slope where there is insufficient room for a catchment area. The drape mesh system is typically used to control rock fall by dissipating the block’s energy and directing it’s trajectory into a catchment area. Based on the above Project scoping and subsequent discussions with the design team, the drape mesh system was selected for implementation. In our opinion, this system will reduce the risk from rock fall events originating from the lower part of the slope from traveling onto the roadway.

It should be noted that rock fall hazard mitigation can reduce the hazard but typically cannot eliminate it and some level of risk will exist to traveler safety, vehicles and traffic flow, and maintenance staff after mitigation. In particular, rock fall events could occur beyond the limits of any drape mesh system. It is also important to recognize that the effectiveness of any hazard mitigation will depend on the level of mitigation provided and the completion of required maintenance.

6.2.2 Site History and Observations

We discussed rock fall history at Site 1 with LAC staff during the project scoping meeting and project kickoff meeting. From those conversations, we understand that rock fall occurs infrequently, typically during significant rain events. The rocks are typically about 12 inches in maximum dimension, or smaller. Larger blocks (i.e. greater than two feet) have not fallen onto SACR in at least the two previous decades the LAC staff has been working in the area.

The existing slope face is irregular, as shown in Photograph 1 in Figure D-1. The topographic expression, as shown in Photograph 2 in Figure D-1, suggests the lower portion is a cut slope, presumably excavated during SACR construction. It is unclear if the irregular topography is the result of previous rock fall or due to the SACR construction.

The existing pavement contains a number of scars, as shown in Photograph 5, which may be the result of previous rock falls or maintenance equipment. The scars are located across the entire width of the road, although more concentrated on the inboard side. These scars are typically a couple a feet long and a couple inches wide, suggesting small blocks (i.e. ≤ two feet) as noted by the LAC staff.

Photograph 5 (Site 1). Existing pavement condition below the rock slope.

While we performed our geologic and cell mapping, a LAC maintenance crew was working along the shoulder of SACR. Their work included removing sediment and talus from the base of the ascending slopes with a grader and putting the material into dump trucks with a loader. We did not observe the equipment damaging the roadway during this time and believe it is unlikely these maintenance activities are the cause of the scars.

6.2.3 Kinematic Analysis

The discontinuity data collected at Site 1 was used to construct pole plots on equal area stereonets using the program Dips® Version 6.0 by Rocscience (2015). A pole represents an individual discontinuity and stereonets provide a two-dimensional representation of the three-dimensional discontinuity data. Next, we identified clusters or populations of discontinuities with similar orientations and plotted great circles for each major pole cluster. The pole clusters represent individual discontinuity sets. Photographs of the existing slope, a summary of rock mass properties, and stereonet plots are presented in Appendix D.

A kinematic analysis was used to estimate the kinematic potential for rock blocks to fail out of the slopes based on geometry. Three types of failure modes were analyzed: plane shear, wedge, and toppling. The information used to perform the analysis were the slope dip and dip direction, the orientation of the discontinuities within the rock mass, and the friction angle of the discontinuities.

A kinematically admissible plane shear failure mode occurs when:

the dip of the slope is greater than the dip of the potential sliding plane, the potential sliding plane daylights the slope, the dip of the sliding plane is greater than the friction angle of the sliding plane; and the dip direction of the sliding plane falls within ± 20° of the dip direction of the slope face (Goodman, 1980).

A wedge failure is similar to a planar failure except the sliding occurs on two or more intersecting planes. Potential failure surfaces can occur if the intersection of the planes daylight the slope face and the plunge of the line of intersection is greater than the friction angle of the sliding planes (Goodman, 1980).

Toppling is generally associated with blocks of rock formed by discontinuities with a joint that is steeply dipping into the slope. Toppling is considered admissible when planes can slide with respect to one another and the direction of the slip planes lies within ± 30° of the dip direction of the slope (Goodman, 1980).

The lower portion of the slope ascends approximately 40 feet (vertically) at an overall gradient of approximately 2V:1H (i.e. about 60 degrees). The slope gradient is variable with some sections near vertical. For our analysis, we assumed an average slope angle of 60 degrees (to model the lower portion of the slope) and a rock discontinuity friction angle based on the geomechanical information that we collected in the field, experience with similar rock types, and guidance from the Rock Slopes Reference Manual (Munfakh et al., 1998). For the purpose of preliminary design, we assumed a joint friction angle of 30 degrees for the discontinuities. A kinematic analysis is used to assess if a failure mode is admissible based on geometry alone, and limit equilibrium analyses are conducted to evaluate the stability of rock blocks and to estimate rock reinforcement requirements. Consequently, kinematic analyses are based on the assumption that the discontinuities are planar and laterally continuous.

Kinematic analyses indicate plane shear, wedge, and toppling failures are kinematically admissible. However, based on rock fall history these failure modes typically result in rock fall events composed of individual blocks with dimensions of less than two feet.

6.2.4 Rock Fall Hazard Mitigation

Potential rock fall mitigation options could include stabilization, protection, and warning systems. Because the anticipated failure mode typically consists of individual blocks ranging in size from 12 to 18 inches (and generally infrequent events), stabilization measures consisting of pattern rock bolting and rock bolt and shotcrete systems are not appropriate, in our opinion.

Protection measures could consist of either anchored mesh or drape mesh systems. Given the rock fall history, we concluded that scaling combined with a drape mesh and catchment area, to control the rock fall, would likely accomplish the project goals of reducing the rock fall hazard at a lower cost than an anchored mesh system.

We recommend mitigation begin with scaling in the lower and middle sections, followed by installing a draped mesh system covering at least the lower section. The purpose of the scaling is to remove loose rocks on the slope surface, while the drape system is intended to dissipate the energy of falling blocks and direct their trajectory into the allotted catchment area.

The mitigation area should begin approximately 20 feet northwest of the H-beam and timber lagging impact wall and extend approximately 120 feet toward the northwest (parallel to the road). We recommend extending the drape system at least 10 feet beyond the lateral limits of the mitigation area (i.e. total length of about 140 feet) and anchoring it at least 10 feet above it (i.e.

total height on the slope face of about 60 feet). The bottom of the drape mesh should stop about three to five feet above the catchment area. This mitigation area covers approximately 8,000 square feet of slope surface.

Considering the anticipated average block size is 12 to 18 inches, the mesh should consist of at least light weight double-twisted hexagonal mesh designed for blocks up to 2 feet. We recommend rock bolts be used to stabilize any potential blocks greater than 2 feet that are observed during construction. In this regard, we recommend including an allotment of 20-foot long rock bolts in the project. The anchors used to secure the drape system to the slope and construction details should be specified by the manufacturer.

Due to the limited space available for construction, we anticipate the contractor will install temporary barriers to collect the rocks and debris within the existing roadway. These activities will likely damage the existing pavement section. The plans and specifications should indicate responsibility for temporary repairs to the roadway during the scaling process, if the roadway is to remain operational during construction.

6.2.5 Additional Considerations

Depending on the project goals and budget, there are additional rock fall hazard mitigation measures that should be considered. We suggest considering installing the drape mesh on the middle section of slope or constructing an attenuation barrier at the break in slope between the lower and middle sections. Although we were unable to perform geologic mapping of this slope section, it is our opinion that there is a potential for rock fall hazard originating from this area, although to a lesser extent than the middle section because of the gentler slope gradient.

This area includes approximately 20,000 square feet of slope surface. An attenuator barrier, constructed in combination with the lower section drape mesh, would be an option to capture falling blocks and steer them under the drape mesh and ultimately to the catchment area

The recommendations in this report are intended to provide a basis for estimating the cost of a drape mesh system. The final drape mesh design will be completed by a specialty firm as part of construction.

6.3 Site 2 – Basin

Site 2 improvements include construction of an attenuation basin with a riser pipe in the basin and a culvert crossing under the roadway. We anticipate the basin side slopes will be on the order of five feet tall. Due to the very loose to loose, granular nature of the existing fill, there is a potential for settlement of the fill beneath the roadway and instability of the outer slope that could result from infiltration of water. The potential for settlement and slope instability would depend on the amount and frequency of water that infiltrates and is difficult to quantify. As such, consideration should be given to lining the basin to reduce the potential for surface water to infiltrate into the subsurface materials if the quantity of water is anticipated to be significant.

6.4 Retaining Wall Design

6.4.1 Wall Type

The Project includes replacing the existing rock and mortar wall at Site 3 because of distress observed along the wall face, specifically bulging of the central part of the wall and undermining of the wall at its ends. We understand that the roadway will maintain the current alignment, width, and elevation. Based on site conditions and constraints, we considered three types of retaining structures: a new cast-in-place concrete retaining wall, a new mechanically stabilized earth (MSE) wall, and a soil nail wall that incorporates the existing wall.

There are advantages and disadvantages to each of the three wall types. The cast-in place concrete wall is a common system that is easy to construct and relatively economical at heights of less than 15 feet. These walls, however, require creating a bench for construction by back-cutting to the heel of the footing and stockpiling the excavated material during construction. In addition, construction is typically more expensive than MSE walls at these heights. Similarly, MSE walls are a common system that are easy to construct and relatively economical. These walls also require a bench for construction by back-cutting to the bottom of the wall elevation at the back of the reinforced zone and stockpiling the excavated material. For each of these walls types, we anticipate there is enough room to allow one lane of traffic to remain open during construction after back-cutting.

A soil nail wall system that incorporates the existing wall face would not require excavation of a bench for construction. Considering the site conditions, we anticipate excavator-mounted drilling equipment can be staged on the outside traffic lane and reach over the slope.

The soil nails can be installed directly through the existing wall and a reinforced shotcrete facing constructed in front of the existing wall. In this case, the facing can be sculpted and/or colored to enhance the aesthetics of the wall. One disadvantage of this wall type is that it is less common, which requires a specialty contractor with experience on similar projects. We anticipate one lane of traffic can remain open during construction of a soil nail wall.

6.4.2 Cast-In-Place Concrete Retaining Wall

The existing wall can be replaced with a cast-in-place concrete wall. For construction, we anticipate the contractor will excavate a temporary slope from the existing roadway grade to a bench (at the base of the footing elevation) that daylights the existing slope face. The location of the top of the temporary slope will depend on the slope gradient, footing dimensions, and depth to bedrock. For planning purposes, we recommend assuming an overall slope of 1V:1H;

flatter slopes may be necessary in fill soils and steeper slopes may be possible in rock.

We recommend the retaining wall backfill (behind the wall and within the 1V:1H zone) consist of Select Granular Backfill material as described in Section 7.2.5 and placed in accordance with the recommendations provided in Section 7.2.6. Based on these assumptions, we recommend the parameters in Table 3, which were developed in accordance with AASHTO (2014).

TABLE 3

RECOMMENDED PARAMETERS FOR CAST-IN-PLACE RETAINING WALL DESIGN

Design Parameter Recommended Value Total Unit Weight of Backfill (pcf) 125 Friction Angle (degrees) 34 Cohesion (psf) 0

Lateral Earth Pressures

- Horizontal Backfill (pcf)1,2

Static Active Earth Pressure Coefficient, KA

0.25

Seismic Horizontal Acceleration Coefficient3, kh

0.44g

Seismic Active Earth Pressure Coefficient, KAE

0.72

Passive Earth Pressure Equivalent Fluid Density (pcf)4

Nominal Coefficient of Sliding Resistance5 0.78 Notes:

1) The active retaining condition assumes the wall is free to deflect 0.001 times the height of the wall.

2) The lateral earth pressures are based on the assumed friction angle of 34 degrees. Materials with a lower value will result in higher static and seismic active earth pressures.

3) The seismic horizontal acceleration coefficient assumes the wall is capable of displacing 1 to 2 inches during the design seismic event, in accordance with AASHTO (2014).

4) We recommend ignoring passive resistance due to the foreslope gradient.

5) The coefficient of sliding resistance provided is based on a friction angle of 38 degrees on rock and assumes a cast-in-place footing. If a pre-cast footing is used, the specified value should be multiplied by a reduction factor of 0.8 in accordance with AASHTO (2014).

6) All load and resistance factors for CIP wall design should be in accordance with AASHTO (2014).

The earth pressures assume the wall does not include any hydrostatic pressure related to accumulation of water in the backfill. As such, a drainage system will be required.

We recommend the wall be founded on bedrock. The existing fill and/or residual soil deposits should not be relied upon for vertical or lateral support. Based on borings B-08 and B- 09, we anticipate the existing wall will need to be deepened three feet to get through the existing fill and/or residual soil to expose bedrock. Additionally, due to bearing resistance and global stability considerations, the toe of the footing should be located a minimum horizontal distance of 1.5 feet from the face of the bedrock.

Assuming a horizontal distance of 1.5 feet between the toe of the footing and the face of the bedrock and a footing width of 6 feet, the footing may be designed for a nominal bearing resistance of 7 ksf. If necessary, distance between the toe of the footing and the face of the bedrock may be increased to increase the bearing resistance. Assuming a horizontal distance of

3.5 feet between the toe of the footing and the face of the bedrock and a footing width of 6 feet, the footing may be designed for a nominal bearing resistance of 13.5 ksf. In accordance with AASHTO (2014), a resistance factor of 0.55 should be applied to the bearing resistance at the strength limit state. We anticipate one inch or less of total settlement with a differential settlement of half of the total settlement over a distance of 30 feet for the above conditions.

6.4.3 MSE Wall

The existing retaining wall can be replaced with an MSE wall. For construction, we anticipate the contractor will excavate a temporary slope from the existing roadway grade to a bench (at the base of wall elevation) that daylights the existing slope face. The location of the top of the temporary slope will depend on the slope gradient, MSE wall and reinforced zone dimensions, and depth to bedrock. For planning purposes, we recommend assuming an overall slope of 1V:1H; flatter slopes may be necessary in fill soils and steeper slopes may be possible in rock.

We anticipate the MSE wall reinforced zone will be backfilled with select granular backfill material, as specified in FP-14 Section 704.08 and placed in accordance with the FP-14 Section 255.05. We further assume that the reinforcement will extend back to the weathered bedrock such that there will not be a retained backfilled material (i.e., soil placed between the reinforced zone and excavated rock face). Based on these assumptions, AASHTO (2014) and FHWA GEC 11 (2009) allow truncated reinforcement lengths (as short as 40 percent of the wall height) to account for the lack of loading on these wall systems (termed Stable Feature MSE walls or SFMSE walls). However, GEC 11 (2009) cautions about the use of truncated systems in seismically active areas.