FINAL_Geotechnical_Memorandum_CA_ERFO_3N17(2).pdf

To: Justin Henwood, Project Manager, CFLHD, Lakewood CO

From: Todd Hansen, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Marilyn Dodson, Lead Geotechnical Engineer CFLHD, Lakewood CO

Subject: Geotechnical Repair Recommendations Santa Clara Divide Road, Los Angeles County, California, CA ERFO 03N17(2)

I. PROJECT BACKGROUND INFORMATION

This geotechnical memorandum presents the field evaluation results, analysis, and recommendations for the reconstruction of Santa Clara Divide Road (FR 3N17.7 & FR 3N17.8).

The roadway was damaged during heavy precipitation events related to Spring storms in 2019.

Repairs are planned for the road as part of the Emergency Relief for Federally Owned Roads (ERFO) program.

The Santa Clara Divide Road Project is located in Los Angeles County, California (Figure 1).

Branching off Sand Canyon Road/Little Tujunga Canyon Road, the Santa Clara Divide Road provides access to the Magic Mountain Wilderness and the summit of Magic Mountain. The road provides recreational access to public lands in the Angeles National Forest. In general, the heavy precipitation events caused portions of the road section and embankment fill to fail and move downslope. The existing road is an asphalt or aggregate surfaced, two-way, one-lane road with widths between 15 and 18 feet. Traffic counts are not available, but this is assumed to be a low-volume road. At the time of the initial site visit, Santa Clara Divide Road was closed, and the public were directed to use alternate routes. The existing road is planned to be repaired back to its previous condition prior to damage caused by recent storm events.

Technical Memorandum

Date: July 31, 2019

Figure 1. Project location map (USFS Forest Road Map).

II. REGIONAL GEOLOGY

The site is mapped as underlain by Precambrian Mendenhall Gneiss bedrock (Dibblee and Ehrenspeck, 1991). The Mendenhall Gneiss displays severe metamorphism, is commonly undulated or contorted, may be severely fractured, and may contain complex intrusions by leucocratic granitic rocks. Younger Mesozoic aged leucocratic granite and quartz diorite rocks may be found nearby, south of the San Gabriel Fault Zone, along with Quaternary alluvial deposits.

Quaternary deposits consist of older, slightly consolidated, alluvial gravels and sands and are mapped to the south of the project vicinity. A map of the regional geology is presented in Figure 2.

Several known Quaternary faults are mapped within approximately 40 miles of the project area and are summarized on Table 1 below (CalTrans, 2017; U.S. Geological Survey, 2017).

FR 3N17.7

Storm Damage

FR 3N17.8

Storm Damage

Figure 2. Regional geologic map with damaged sites along Santa Clara Divide Road.

Table 1. Summary of Nearby Faults

FAULT OR

FAULT ZONE

DISTANCE

FROM

CENTER OF

PROJECT

FAULT

PARALLEL

SLIP RATE

FAULT

LENGTH AGE

(miles) (inch/year) (miles) (years)

San Gabriel 0.11 0.0236 Unspecified <15,000 Sierra Madre (San

Fernando) 3.11 0.0787 14.3 <15,000

Verdugo-Eagle Rock 5.91 0.0236 16.2 <15,000

Sierra Madre (Santa Susana) 3.69 0.236 21.7 <15,000

Northridge 5.41 0.0591 25.5 <15,000

PROJECT LOCATIONS

The American Association of State Highway and Transportation Officials (AASHTO) criteria for recommended seismic response parameters are based on the AASHTO LRFD Bridge Design Specifications, 7th Edition, 2014, and represents horizontal peak ground acceleration (PGA) with a 7 percent probability of exceedance in 75 years (approximate 1000-year return period). The 1000-year return period uniform hazard spectrum for Santa Clara Divide Road (34.360368° N latitude and 118.385512° W longitude) was obtained in accordance with the AASHTO ground motion maps. The site was classified as “Class C” according to site class definitions specified in Table 3.10.3.1-1 of AASHTO and soil profile assumptions based a think soil overlay on rock outcrops at the site and geologic mapping of the project vicinity. The recommended spectral acceleration coefficient values for probabilistic design are summarized on Table 2.

Table 2. Summary of Seismic Parameters Corrected for Site Class C

Peak Seismic Ground Acceleration Coefficient (As) 0.814

Horizontal Response Spectral Acceleration at Period of 0.2 sec (SDS) 1.934

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.920

Site Factor at Zero-Period of Acceleration Spectrum (FPGA) 1.0

Site Factor at Short-Period of Acceleration Spectrum (Fa) 1.0

Site Factor at Long-Period Range of Acceleration Spectrum (Fv) 1.3

Based on the long acceleration coefficient SD1 value of 0.920, the site is assigned to Seismic Hazard Zone 4, in accordance with Table 3.10.6-1 of AASHTO.

III. FIELD EVALUATION

To evaluate the field conditions, a Central Federal Lands (CFL) team visited the project site on April 16, 2019 and conducted limited field measurements and observations. This team included Justin Henwood (Project Manager), Lisa Hemesath (Environmental Protection Specialist), Sebastian Guzman (Highway Design Engineer), Brian Campbell, (Hydraulics Engineer) and Dominic Monarco (Geotechnical Engineer). The CFL team met with representatives of Angeles National Forest.

Field observations focused on evaluating the existing roadway conditions and soil properties.

Evaluations and recommendations presented in this memorandum are based on visual observations and the limited measurements taken during the site visit. As an ERFO project, time and budget are critical constraints to project completion. In the interest of faster and more economical project delivery, no subsurface soil sampling was conducted during this visit.

The Santa Clara Divide Road appears to have been constructed using cut and fill methods on native soil slopes that vary between about 1(V):1.6(H) and 1(V):1.3(H). Cut slopes for the roadway vary between 1(V):1(H) and 4(V):1(H). A nearby concrete crib wall appears to be constructed at a 6(V):1(H) batter. Native slopes in the area are typically vegetated with native brush and grasses.

Undisturbed areas were generally mantled in a layer of residual soil, with isolated alluvium deposits and bedrock outcrops. Bedrock was exposed in the cut slope and on the hillsides but is not readily apparent in the headscarp as shown in Figure 4. In this area, the bedrock varies from moderately fractured, hard gneiss to highly fractured, highly weathered, very weak rock. Evidence of instability was limited to the road section and embankment fill slopes as discussed below.

The existing roadway pavement is in fair condition, with some bumps, cracks, and debris. Overall, the existing cut and fill slopes are in good to fair condition. Areas with evidence of instability were noted during the site visit as summarized below.

A. Site #1 FR 3N17.8 Milepost 0.5

The existing roadway has partially collapsed due to undercutting as a result of a non-functioning drainage culvert allowing surface runoff from the road to cut into the hillside. The deteriorating pavement section is cracking in a crescent shape around and above the erosion gulley. No repairs have been made to the culvert or erosion feature except the addition of sandbag diversions to prevent runoff from further eroding the gulley. Road damage is limited to a 25-foot long section and impacts about 5-feet into the travel lane. Gulley erosion extents are estimated to be 28-feet vertically and 120-feet downslope. Figure 3 below shows the non-functioning drainage culvert and the gulley erosion. Figure 4 illustrates the headscarp impact to the travel lane.

Figure 3. Gulley Erosion at Site 1.

Figure 4. Headwall of Site 1 Gulley Erosion.

B. Site #2 FR 3N17.7 Milepost 0.4

A major slope failure has occurred in the cut bench native material at this location. The failure is approximately 82 feet wide along the road at its widest point and extends 25 feet, the full roadway width, to the inboard cut slope. At the edge of road, the failure is approximately 30 feet deep. The roadway subgrade and native soils consist of silty sand with gravel and cobbles. Intact bedrock may be present in the bottom of the failure.

Figure 5. Failure at Site 2. Initial slide appears to have occurred in outboard portion of the cut bench with subsequent headward erosion.

IV. ANALYSIS

A. Slope Stability

Slope stability analysis was completed for the failed embankment slopes at Sites 1 and 2. Analyses were conducted using the Slide 7.0 computer program developed by RocScience. Slide 7.0 uses limit equilibrium methods to determine the factors of safety for slope stability. Slope stability factors of safety were calculated for presumed pre-failure conditions and proposed repair methods as described below.

Conservative, presumptive soil properties at each location were estimated based on visual classification and experience with similar soil materials. Reports from the Angeles National Forest staff indicate that the failures occurred after a period of heavy precipitation. Therefore, a back analysis of the assumed pre-existing slope included a water table elevation at the ground surface.

A static factor of safety equal to approximately 1.0 was used to confirm the estimated soil parameters. Intact bedrock at Sites 1 and 2 was not observed, therefore bedrock was not included in the analysis at this site. This is a conservative assumption for slope stability analysis and does not necessarily reflect the actual subsurface profile. Difficult excavation conditions may be encountered at Sites 1 and 2 if bedrock does exist within 5 feet of the gulley bottom.

Slope geometry was measured at isolated locations using a handheld laser rangefinder. Actual slope geometry may be expected to vary from that measured in the field.

The back-analyzed soils parameters were used in further analyses to examine the effectiveness of repair options and are summarized in Table 3 below. Results of the slope stability analysis are attached to this memorandum.

Table 3. Material Properties Calculated from Back Analysis and Used in MSE Design.

SITE SOIL TYPE UNIT WEIGHT FRICTION

ANGLE COHESION

(pcf) (degrees) (psf) 2 MSE Fill 130 34 200 2 On-Site Soil 125 36 650

B. Mechanically Stabilized Earth Wall

Based on discussions with the project Cross Functional Team (CFT), a Mechanically Stabilized Earth (MSE) wall was analyzed as the preferred repair option for Sites 1 and 2. MSE walls are earth retaining structures constructed of soil reinforcing elements (either steel or geosynthetic) placed in a high quality backfill material. These structures rely on the soil reinforcement to create a coherent mass to resist the lateral pressures imposed by materials behind the reinforced fill. MSE walls have been utilized on many projects as context sensitive solutions preserving scenic, aesthetic, historic, and environmental resources.

A near vertical retaining wall system is expected to best meet the project requirements based on the existing site conditions, the proposed roadway widths, and potential impacts to the USFS identified assets. An MSE system is likely the most cost-effective retaining wall system for the proposed application.

Internal stability of the proposed MSE wall was analyzed using the MSEW computer program developed by ADAMA Engineering. The MSEW program uses limit equilibrium methods to determine factors of safety for circular, two-part wedge, and three-part wedge failure surfaces. The outboard slope batter of the MSE wall was assumed to be near vertical. The analysis assumed that the drainage ditch would intercept surface runoff and channel flow into the drop inlet on the inboard side of the roadway. Runoff channeled to the inlet will be channeled through a culvert pipe sized to fit in between the internal wall reinforcements and discharge out of the wall facing.

Reinforcement lengths were equalized at 83% and 88% of wall heights for Site 1 and Site 2, respectively. The top three reinforcements were lengthened an additional 3-feet to produce satisfactory factors of safety for the proposed systems.

External design including global stability, bearing resistance, sliding, and eccentricity was performed to evaluate the proposed wall geometries. Final MSE wall internal stability will be designed and submitted by the contractor in general accordance with the AASHTO LRFD Bridge Design Specifications 7th Edition including interim revisions, and FHWA Publication NHI-10-024 and NHI-10-025, Volumes 1 and 2, respectively, entitled “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” dated November 2009.

Acceptance criteria used for design is shown in Table 4.

The capacity to demand ratios (CDR) for bearing resistance and sliding are used to quantify the ratio of the factored resistance to the factored load. Resistance factors of 0.65 and 1.00 were used in preliminary design for bearing and sliding, respectively. Load factors for bearing, sliding, and eccentricity are shown in Tables 4-1 and 4-2 of FHWA (2009).

Seismic stability was evaluated in the preliminary analyses using 50% of the As for the horizontal acceleration, which corresponds to a peak ground acceleration of 0.407g. A satisfactory minimum global stability factor of safety will not be met when designed to this level of peak ground acceleration. However, in context of the emergency repairs being constructed to re-open the route and replace the roadway in kind to its original performance level, then a lower peak ground acceleration was used that corresponds to a minimum credible event on the close proximity, San Gabriel Fault, of PGA = 0.28g. The original route and retaining walls were likely never designed for seismic analysis, so this retaining wall will likely perform better than the rest of the route in an extreme event.

Table 4:- MSE Wall Design Minimum Acceptance Criteria

STABILITY MODE MINIMUM ACCEPTANCE

CRITERIA

Global Stability *FS = 1.25 (static) **1.0 (seismic) (PGA=0.28g)

Bearing Resistance CDR = 1.0 Sliding CDR = 1.0

Limiting Eccentricity emax = L/4 Notes: FS = Factor of Safety, CDR = Capacity to Demand Ratio, Eccentricity (emax) for soil foundations.

*Considered Acceptable for this Project **Peak ground acceleration meeting this criteria is noted in the report.

MSEW analysis was conducted for both geosynthetic and metallic reinforcements to verify constructability. For geosynthetic reinforcement, MSEW analysis assumed the use of Type II, Type IV and Type VI Reinforcement Geosynthetic (FP-14, 714.04) at 18-inch and 24-inch vertical spacings For metallic reinforcement, MSEW analysis assumed the use of W9.5 x W4.0 continuous metal mats with 8-in x 21-in spacing (longitudinal and transverse) at 24-inch vertical spacings with 12-inch intermediate layers. Metal mats are considered continuous with a 96-inch wide strip installed horizontally on 96-inch centers. Minimum yield strength of the steel (Fy) is 65 kips/in2.

Results of the analysis indicate either geosynthetic or metallic reinforcement can be used for the construction of these MSE walls.

C. Analysis Results

The MSE wall was designed in accordance with the guidelines provided in FHWA (2009) and AASHTO LRFD. The MSE wall was designed as a gravity retaining wall based on the proposed typical roadway section and cross sections prepared by the CFLHD Roadway Engineer.

Reinforcement design assumes 100 percent coverage of the placed fill on each lift. The MSEW software package published by ADAMA Engineering was used to check external (sliding, overturning, and bearing resistance) stability for the walls. Slide 7.0, published by RocScience, was used to check global stability. Slide 7.0 uses the conservative Bishop simplified method to calculate overall factors of safety for slope or retaining wall stability. Maximum wall heights of 28-feet and 35-feet were analyzed for Sites 1 and 2, respectively. Based on the design calculations, minimum reinforcement lengths of 83 and 88 percent of the design height are recommended for Sites 1 and 2, respectively. This is greater than the typical value of 70 percent of the design height due to the steepness of existing slopes below the walls and potential for elevated water levels resulting from intense precipitation. Additionally, due to the steep foreslope in front of the MSE walls (approximately 1V:1H), minimum embedment depths of 3 feet at Site 1 and 5 feet at Site 2 are required to meet global stability requirements.

Results of the global stability modeling are attached and summarized in Table 5. The static design values exceed the minimum values in Table 4. In the case of a large seismic event it is likely that multiple slope stability failures would occur along the forest road corridor. Therefore, based on the low probability of a large magnitude earthquake, the wall design will withstand seismic loading up to the previously stated accelerations.

Table 5:- Calculated Global Stability Factors of Safety

SCENARIO FACTOR OF SAFETY

Geotextile

18-inch Geotextile

24-inch

WWM

W9.5xW4.0

Static 1.5 1.5 1.5

Seismic (PGA = 0.28g) 1.0 1.0 1.0

V. RECOMMENDATIONS

A. Sites 1 and 2

For Sites 1 and 2, the roadway embankment should be reconstructed as an MSE wall. Excavate loose and disturbed material from the failure area prior to MSE construction, and key the MSE into the existing ground as shown on Standard Detail C204-50. Construct mechanically stabilized earth walls as described in Section 255 of FP-14 and the Project Special Contract Requirements (SCRs). Place rock fill to a minimum depth of 18-inches. Increase the thickness of the rock fill until a stable platform is formed. Maximum heights and minimum base reinforcement lengths for the MSE are summarized in the plan sheets. The top two layers of geosynthetic reinforcements should extend 3 additional feet in length than the minimum reinforcement length to prevent a tension crack at the top of the structure. Extend the MSE up and down-station past the failed area along the road alignment to key the MSE into stable ground. Provide drainage systems consisting of geocomposite sheet drains or benches at the back of the excavation, collector pipes, and discharge pipes per standard plan drawings. Discharge drainage systems away from the MSE.

Construction should be limited to the dry season to provide safe working conditions. At the end of each day, the top layer of backfill in the MSE should be sloped to drain water.

While not encountered in the erosion gullies, bedrock exposures in the vicinity are highly variable, from deeply weathered, weak rock, to fresh, hard rock. We anticipate that the proposed excavation can be completed using conventional heavy-duty construction equipment in good working order.

Hard bedrock is not apparent in the erosion channels at Sites 1 and 2, but may be present in the subsurface. Modifications to the MSE wall design may be required if bedrock encountered during

VII. REFERENCES

American Association of State Highway and Transportation Officials (AASHTO). “AASHTO LRFD Bridge Design Specifications.” Seventh Edition, 2014.

Berg, Ryan R., Christopher, Barry R., and Naresh C. Samtani (2009). “Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes – Volumes 1 & 2.” FHWA Publication NHI-10-024 and NHI-10-025.

California Department of Transportation (CalTrans). (2017). “ARS Online v.2.3.09.”

<http://dap3.dot.ca.gov/ARS_Online/index.php>

Dibblee, T.W. and Ehrenspeck, H.E., ed. (1991). “Geologic Map of the San Fernando and Van Nuys (North ½) quadrangles, Los Angeles County, California.” 1:24,000. Dibblee Geological Foundation Map DF-33.

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2014). “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP- 14.” Publication No. FHWA-FLH-14-001.

U.S. Geological Survey (2017). “Quaternary Fault and Fold Database of the United States.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/hazards/qfaults/>

U.S. Geologic Survey (2017). “U.S. Seismic Design Maps.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/designmaps/us/application.php>

1.5321.532

250.00 lbs/ft2

1.5321.532

Support Name Color Type Force Applica on Material Dependent Adhesion (psf) Fric on

Angle (deg)

Shear Strength Model

Force Orienta on Anchorage Strip Coverage

Tensile Strength

(lbs/ )

W9.5xW4.0, 8x21 GeoTex le Ac ve (Method A) No 100 34 Linear Bisector of Parallel and Tangent

Slope Face 100 49400

Material Name Color Unit Weight (lbs/ 3) Strength Type Cohesion

(psf) Phi (deg)

Water Surface Ru

Retained Soil 125 Mohr‐Coulomb 650 36 None 0

Backfill Soil 130 Mohr‐Coulomb 200 34 None 0

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-5

-1

-200 -150 -100 -50 0 50 100 150 200

Analysis Description Site 1 Static Analysis Company FHWA CFLHDScale 1:500Drawn By T. Hansen File Name ERFO_Angeles.slmdDate 7/2/2019, 1:42:45 PM

Project

CA ERFO 3N17(2)

SLIDEINTERPRET 7.018

0.9980.998

0.9980.998

Support Name Color Type Force Applica on Material Dependent Adhesion (psf) Fric on

Angle (deg)

Shear Strength Model

Force Orienta on Anchorage Strip Coverage

Tensile Strength

(lbs/ )

W9.5xW4.0, 8x21 GeoTex le Ac ve (Method A) No 100 34 Linear Bisector of Parallel and Tangent

Slope Face 100 49400

Material Name Color Unit Weight (lbs/ 3) Strength Type Cohesion

(psf) Phi (deg)

Water Surface Ru

Retained Soil 125 Mohr‐Coulomb 650 36 None 0

Backfill Soil 130 Mohr‐Coulomb 200 34 None 0

0.29

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-5

-1

-200 -150 -100 -50 0 50 100 150 200

Analysis Description Site 1 Seismic Analysis Company FHWA CFLHDScale 1:500Drawn By T. Hansen File Name ERFO_Angeles.slmdDate 7/2/2019, 1:42:45 PM

Project

1.0301.030

W

W

1.0301.030

Material Name Color Unit Weight (lbs/ 3) Strength Type Cohesion

(psf) Phi (deg) Water Surface Hu Type Hu

Retained Soil 125 Mohr‐Coulomb 650 36 Water Surface Custom 1

Method Name Min FS

Bishop simplified 1.030

Spencer 1.072

GLE / Morgenstern‐Price 1.071

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-5

-1

-150 -100 -50 0 50 100 150 200

Analysis Description Site 2 Backcalculated Failure Company FHWA CFLHDScale 1:500Drawn By T. Hansen File Name ERFO_Angeles.slmdDate 7/2/2019, 1:42:45 PM

Project

1.5191.519

1.5191.519

Support Name Color Type Force Applica on Material Dependent Adhesion (psf) Fric on

Angle (deg)

Shear Strength Model

Force Orienta on Anchorage Strip Coverage

Tensile Strength

(lbs/ )

W9.5xW4.0, 8x21 GeoTex le Ac ve (Method A) No 100 34 Linear Bisector of Parallel and Tangent

Slope Face 100 49400

Material Name Color Unit Weight (lbs/ 3) Strength Type Cohesion

(psf) Phi (deg)

Water Surface Ru

Retained Soil 125 Mohr‐Coulomb 650 36 None 0

Backfill Soil 130 Mohr‐Coulomb 200 34 None 0

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-5

-1

-250 -200 -150 -100 -50 0 50 100 150 200 25

Analysis Description Site 2 Static Analysis Company FHWA CFLHDScale 1:600Drawn By T. Hansen File Name ERFO_Angeles.slmdDate 7/2/2019, 1:42:45 PM

Project

0.9850.985

0.9850.985

Support Name Color Type Force Applica on Material Dependent Adhesion (psf) Fric on

Angle (deg)

Shear Strength Model

Force Orienta on Anchorage Strip Coverage

Tensile Strength

(lbs/ )

W9.5xW4.0, 8x21 GeoTex le Ac ve (Method A) No 100 34 Linear Bisector of Parallel and Tangent

Slope Face 100 49400

Material Name Color Unit Weight (lbs/ 3) Strength Type Cohesion

(psf) Phi (deg)

Water Surface Ru

Retained Soil 125 Mohr‐Coulomb 650 36 None 0

Backfill Soil 130 Mohr‐Coulomb 200 34 None 0

0.29

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-5

-1

-200 -150 -100 -50 0 50 100 150 200

Analysis Description Site 2 Seismic Analysis Company FHWA CFLHDScale 1:500Drawn By T. Hansen File Name ERFO_Angeles.slmdDate 7/2/2019, 1:42:45 PM

Project