12. NEW Via Amendment A00001 - Geotechnical Report.pdf

Trabuco Canyon Bridges

Trabuco Canyon Bridge Sites 1, 2, and 4 Geotechnical Assessment revision 1

Stephen Romero P.E.

Regional Geotechnical Engineer

Trabuco Canyon Bridges 2022

Contents

1.0 INTRODUCTION

2.0 PROJECT DESCRIPTION

3.0 GEOTECHNICAL INVESTIGATION

3.1 Field Exploration

3.2 Physical Testing

4.0 DISCUSSION OF FINDINGS

4.1 Site Conditions

4.2 Groundwater

4.3 Strength Characteristics

4.4 Equivalent fluid weight

4.5 Optimum moisture

4.6 Soil Gradation

4.7 Unstable Slopes

4.8 Scour Protection

5.0 ENGINEERING SEISMOLOGY

5.2 Faulting and Seismic Shaking

6.0 DESIGN RECOMMENDATIONS

6.1 Site Clearing and Earthwork

6.2 Foundation Recommendations

6.3 Unstable Slopes

6.4 Concrete Slabs

6.5 Lateral Load and Uplift Resistance

6.6 Subgrade Parameters

6.6.1 Portland Cement Concrete Pavement

6.7 Corrosion and Chemical Potential

6.7.1 Ferrous Metal Corrosion

6.7.2 Concrete Corrosion

7.0 POST INVESTIGATION/DESIGN REVIEW

8.0 CLOSURE

9.0 REFERENCES

FIGURES

APPENDIX A

A-1: Site 1 Refraction Result

A-2: Site 2 Refraction Result

A-3: Site 4 Refraction Result

A-4: Vs 100 Estimate

A-5: Allowable Bearing vs Depth

A-6: Liquefaction Evaluation

A-7: Deaggregation S1 Ss result

A-8: Deaggregation Sources

A-9: Site 1 Soil Classification

A-10: Site 2 Soil Classification

A-11: Site 4 Soil Classification

1.0 INTRODUCTION

This report presents the results of a geotechnical engineering study performed by the United

States Forest Service for the proposed work at the Trabuco Canyon on Trabuco Creek Road near in Orange County California. The project is shown on Figure 1.

The purpose of the investigation was to: 1) evaluate the nature and engineering properties of the soils encountered at the site ; 2) develop earthwork recommendations including compaction criteria for the site fill; 3) develop foundation recommendations for the proposed structures; and 4) develop design parameters for lateral earth pressures and frictional resistance.

The scope of the investigation included a field exploration, geophysics investigation, engineering analysis, and preparing this report.

This report has been prepared for Forest Service design team members for the sole purpose of developing the project described herein. This report may not contain sufficient information for other uses or the purposes of the other parties.

2.0 PROJECT DESCRIPTION

The site’s are located in the Trabuco Canyon approximately 3 miles to the East and upstream of

Rancho Santa Margarita, Ca. Site 1 is about 1355 feet above sea level. The exact locations of the sites are listed in Table 1.

Table 1

There are multiple routes into the sites starting from interstate 5. Some appear to be toll roads.

The gate on Trabuco Creek Road is locked. Coordination the Cleveland NF will be required for access.

Lat Long

Bridge Site 1 33.6745 -117.545

Bridge Site 2 33.67405 -117.532

Bridge Site 4 33.67522 -117.519

Potential Staging Area 33.6733 -117.549

Potential Staging Area 33.6746 -117.545

Potential Staging Area 33.6741 -117.531

Potential Staging Area 33.6760 -117.518

3.0 GEOTECHNICAL INVESTIGATION

The geotechnical investigation included field survey and geophysical study. Analysis of the data utilized explicit and empirical relationships in accordance with generally accepted procedures.

The field exploration and analysis are briefly described below. Results are presented in

Appendix A.

3.1 Field Exploration

A field survey was conducted in Trabuco canyon by Forest Service personnel in the Spring of

2022. The survey involved a visual inspection of soils and site conditions. Material was classified as alluvium - cohesionless, non-plastic, and variable in the stream channel. Evidence of past flooding and debris flows were observed throughout the canyon. Exposed soils in the stream channel and banks classified as SW/GW with entrained cobble and boulders. This was in general agreement with information contained in the web soil survey portal.

The web soil survey portal classify soils on the canyon slopes as SC/SM which was in general agreement with field observations. Bedrock outcrops were observed throughout the canyon.

Slope instability was noted at a number of locations including site 4 where buttressing may be required to minimize impacts to the proposed bridge.

Shear wave velocity testing was conducted on all three sites by GeoVision Inc.. Data from the testing was utilized to characterize the soils, bearing capacity, liquefaction potential, and estimate the response of the soil profile during the design earthquake. The location and alignment of the geophysics investigation is shown on Figure 3.

Interpreted soil profiles and measured shear wave velocities are contained in Appendix A.

3.2 Physical Testing

No laboratory testing was conducted.

4.0 DISCUSSION OF FINDINGS

The following discussion of findings for the site is based on the results of the field exploration and analysis of the data gathered.

4.1 Site Conditions

Alluvial sediments in the stream channel are generally dense well graded sands mixed with rock with cobble. Shale, sandstone, and meta-sedimentary rock underly the soils transitioning from weathered to fresh.

The soil profile along all alignments is stratified with shallow, lower velocity soil, overlying dense to very dense soil/weathered rock. The soil is highly variable but is

SW/GW on average with cobble and boulders.

Soil relative densities were determined using shear wave velocity measurements and soil type. They averaged about 85% along Line 1, 50% to 75% along Line 2, and 65% to

77% along Line 4.

Depth to hard bedrock varied by site. For site 1 the depth to hard rock is 10 feet on the west end deepening to 30 feet on the east end. For site 2 the depth to hard rock is 20 feet on the west end of the alignment decreasing to approximately 15 feet on the east end. For site 4 depth to hard rock is approximately 20 feet on the west end increasing to 30 feet approximately at the alignment midpoint transitioning to 20 feet on the east end.

4.2 Groundwater

Groundwater levels are assumed to be approximately equal to the stream free water surface at all sites. Groundwater is likely within a few feet of the surface under most conditions along all alignments.

4.3 Strength Characteristics

Effective in place friction angles were estimated at approximately 38 degrees on average for native materials.

4.4 Equivalent fluid weight

For cantilever assumptions the equivalent fluid weight is estimated at 30 pcf in place and as backfill if native materials are used. For constrained assumptions 46 pcf should be used.

4.5 Optimum moisture

The material is SW/GW. Optimum moisture of the native alluvial material should be approximately 7% but must be field verified.

4.6 Soil Gradation

The D60 of the alluvial material in the stream bed is approximately 1 mm. D10 for SW is about .2 mm. D10 for GW is about .8mm. D60/D10 ranges from 15 to 25. Significant stratification can be observed in stream banks with cobbly sand overlying well graded sand. Large boulders, likely transported under non-newtonian flow conditions (debris flows), were observed in the stream channel.

4.7 Unstable Slopes

There is evidence of historic slope instability in Trabuco Canyon. Slopes are comprised of low strength SC/SM soils. Consideration should be given to short and long term stability of any slopes adjacent to bridge alignments. Buttressing should be incorporated into the bridge approaches or as a stand alone solutions as warranted especially where disturbance and/or steepening of existing slopes is required.

4.8 Scour Protection

Due to unstable, erodible, slopes in the canyon and the propensity for intense rainfall events sediment laden flows and debris flows occur frequently. Thus, stabilized scour protection which accounts for significant bulking and increased density of sediment laden flow/debris should be planned.

5.0 ENGINEERING SEISMOLOGY

The site is situated in a seismically active area of Southern California. A seismic hazard assessment, which included a simple liquefaction evaluation, was conducted and is contained in

Appendix A. A discussion of the potential seismic shaking and mapped seismic hazards are discussed in the following sections.

5.1 Site Class

Based on field observations and Vs 30 measurements shear wave velocity measurements the Site Class, for the purpose of design, is chosen as B/C. Vs30 for site 1 was estimated at 3500 ft/s ( 1067 m/s), 3200 ft/s (951 m/s) for site 2, and 2515 ft/s (767 m/s) for site 4.

5.2 Faulting and Seismic Shaking

Multiple Holocene era faults exist near the sites and include the Elsinore Fault zone 7 miles to the Northeast of Site 1 and the Newport Englewood Fault zone 17.2 miles to the Southwest of site 1. The highest-magnitude earthquakes that would generate the strongest seismic shaking are expected to occur on the faults with highest slip rates and rupture lengths. Evidence indicates that the Elsinore Fault zone is the dominant seismic source for the sites.

Using the USGS unified hazard tool for site class B/C with a 1 percent probability of being exceeded in 50 years the peak ground acceleration (PGA) is .4863g. For the 1 second spectral period the peak acceleration is approximately .3394g. For .2 second spectral period the peak acceleration is approximately 1.1633g. These values yield Fv =

1.2302 and Fa = 1.0.

SMS = Fa x Ss = 1.0 x 1.1163g = 1.1163g SM1 = Fv x S1 = 1.2303 x .3394g = .41756g SDS = 2/3 x SMS = 2/3 x 1.1163g = .7442g SD1 = 2/3 x SM1 = 2/3 x .41756g = .2784g

Using the ASCE hazard tool for comparison assuming risk category 1, https://asce7hazardtool.online/

5.3 Liquefaction Potential

The soil profile along all alignments is stratified with lower velocity soil overlying dense to very dense soil/weathered rock. Alluvial soil is typically highly variable but is SW/GW on average in Trabuco Canyon.

Except for very near surface recently deposited materials soils are generally not liquefiable at any of the sites. Liquefication susceptibility was evaluated at the 1000 year recurrence interval using procedures outlined by Andrus, Stokoe, Juanga (2004).

Lower velocity soils exist in the upper 2 feet on the west end of each site deepening to 5 to 7 feet on the east end. The maximum depth of these lower velocity soils occurs on east end of Line 4 at approximately 7 feet.

Over excavation of at least 4 feet on the west end of each alignment and 6 feet on the east end prior to placement of the foundation is recommended to increase bearing and target soils that were not recently deposited.

6.0 DESIGN RECOMMENDATIONS

Based upon the results of the investigation and subsequent analysis, the proposed development is geotechnically feasible provided the recommendations present herein are incorporated into the design and construction. If changes in the design of the proposed structures occur or if https://asce7hazardtool.online/ variations or changed conditions are encountered during construction, an FS Geotechnical

Engineer shall be contacted to evaluate the effect of any changed field condition to these recommendations. The following geotechnical engineering recommendations for the proposed facilities are based on observation from the field investigation program and subsequent analysis.

6.1 Site Clearing and Earthwork

All earthwork, including excavation, backfill, and preparation of subgrade, will be performed in accordance with the geotechnical recommendations presented in this report and applicable portions of the grading code of local regulatory agencies.

Prior to the start of earthwork, the areas of the proposed improvements will be cleared of all vegetation and debris. Deleterious materials generated from the site clearing should be removed from the construction area and disposed off site.

The bottom of the excavation will be scarified to a depth of 6 inches, brought to at least

1 percent above optimum moisture, and mechanically compacted to at least 95 percent of the maximum dry density.

Cohesionless on-site soils may be used as compacted fill, provided that they are free of organic matter, demolition debris or other deleterious matter, material greater than 3 inches in size, and clayey soils. Fill soils will be moisture conditioned to at least 1 percent over optimum moisture, placed in lifts not exceeding 6 inches in thickness, and mechanically compacted to at least 95 percent of maximum dry density. The maximum dry density and optimum moisture content for each soil type used as engineered fill shall be determined in accordance AASHTO, T- 180.

6.2 Foundation Recommendations

Spread footings for the proposed bridges and other improvements may be designed using a maximum allowable bearing capacity of 6000 psf on Line 1, 3500 psf on Line 2, and 4500 psf on Line 4. This maximum allowable bearing pressure applies to combined dead and sustained live loads. The allowable bearing pressure may be increased by one third when including transient live loads, such as seismic and wind forces.

Minimum spread footer depth will be 4 feet below grade on the west end of each alignment. On the East/Northeast end of Lines 1 and 2 minimum spread footer depth will be set 6 feet below grade with Line 4 at 10 feet below grade. If rock is encountered at shallower depths remove weathered bedrock until the minimum depth is reached or fresh bedrock is encountered. Fresh bedrock shall have a minimum unconfined compressive strength of 2500 psi as measured using a Schmidt hammer or equivalent.

Footer dimensions may be adjusted to account for the increased bearing capacity provided by rock.

6.3 Unstable Slopes

There is evidence of historic slope instability in Trabuco Canyon. Rock outcrops were observed throughout the canyon. Soils on the slopes are comprised of low strength

SC/SM soils. Consideration should be given to short and long term stability of any slopes adjacent to bridge alignments. Buttressing should be incorporated into the bridge approaches or as a stand alone solutions as warranted especially where disturbance and/or steepening of existing slopes is required.

6.4 Concrete Slabs

Concrete slabs may be placed on compacted fill as outlined in Section 6.1 and meet the requirements of the AASHTO and IBC for the required loading. The moisture content of the subgrade soil will be maintained above optimum moisture until the slab is poured.

Any disturbed or yielding soils will be removed and recompacted to AASHTO, T- 180 prior to concrete placement.

6.5 Lateral Load and Uplift Resistance

For foundation walls and wingwalls the earth pressure - assuming drained conditions -are as follows,

• Cantilever configurations the equivalent fluid weight is estimated at 30 pcf for in place and backfill soils.

• Constrained configurations 46 pcf should be used.

• A coefficient of friction of .45 will be used for dead and sustained live load forces to compute the resistance of foundations constructed in contact with native soils.

• Under seismic and wind loading the pressure and frictional resistance coefficient values may be increased by one-third.

6.6 Subgrade Parameters

The near surface soils within the alluvial plane around the creek classify as SW/GW -

Soboba cobbly loamy sand/gravel. The slopes adjacent to the creek are classified as

SC/SM - Friant fine sandy loam. The road moves in and out of the two soil types along its alignment. For the purpose of design an R-value of 55 and CBR of 20 will be used for the subgrade soils. Pertinent design parameters are contained in Table 2

Table 2: Subgrade Design Parameters SM/SC

Resilient Modulus, MR 15,000 psi

CBR 20

R - Value 55

K value 250 pci

6.6.1 Portland Cement Concrete Pavement

Portland cement concrete pavement may be used for bridge approaches.

Assuming a modulus of subgrade reaction of 250 psi per inch for compacted subgrade soils, three-axle trucks with legal limit axle loads of 18,000 pounds trucks with tandem axle loads of no more than 36 kips, a minimum concrete section of 8 inches is recommended. The concrete will have a minimum compressive strength of 3500 psi at 28 days.

6.7 Corrosion and Chemical Potential

Information concerning the corrosion and chemical potential of native materials at the site was obtained from the NRCS Web Soil Server. The following information/recommendations will be observed and complied with.

6.7.1 Ferrous Metal Corrosion

There is not ferrous metal corrosion potential at the site.

6.7.2 Concrete Corrosion

There is not concrete corrosion potential at the site. No special concrete mix to account for such corrosion will be required.

7.0 POST INVESTIGATION/DESIGN REVIEW

Final plans and specifications will be reviewed prior to the construction to confirm that the full intent of the recommendations presented herein have been implemented in the design.

Should unexpected, or changed, conditions be observed during construction the geotechnical engineer will be immediately contacted to allow a sufficient and timely response to the changed condition.

Testing will be performed during construction as required to verify the foundation penetrates and is founded on anticipated soils and that fill is placed and compacted in an acceptable manner as prescribed in this report.

8.0 CLOSURE

The conclusions, recommendations, and opinions presented herein are: 1) based upon the evaluation and interpretation of available information, 2) subject to confirmation of the actual conditions encountered during construction, and 3) based upon the assumption that sufficient observation and testing will be provided during construction.

Test conducted by the Contractor shall be reviewed by a qualified Forest Service or Independent engineer.

9.0 REFERENCES

California Geological Survey Interactive Fault zonation map, http://www.conservation.ca.gov/cgs/shzp/Pages/Index.aspx

Revised Report Seismic Refraction Survey Trabuco Canyon Bridge Sites, Orange County, California GEOVision Report 22111-1 rev1 May 17, 2022

Ronald D. Andrus, Kenneth H. Stokoe II, C. Hsein Juanga ‘Guide for Shear-Wave-Based

Liquefaction Potential Evaluation’ Earthquake Spectra, Volume 20, No. 2, pages 285–308, May

2004, Earthquake Engineering Research Institute

Tezcan, S., Ozdemir, Z, Keceli, A., Erkal, A., ‘A Rapid Technique to Determine Allowable Bearing

Pressure’ International Earthquake Symposium Kocaeli 2007

Web Soil Survey, http://websoilsurvey.nrcs.usda.gov/app/

USGS 2014 Interactive Deaggregation Site, http://eqint.cr.usgs.gov/deaggint/2008/?PHPSESSID=gnqgj9nfp857bm1dpde2qfbr84 http://www.conservation.ca.gov/cgs/shzp/Pages/Index.aspx

FIGURES

Figure 1: Site map

Figure 2: Geologic Map

Figure 3: Geophysics Alignments

Location of three bridge sites

Figure 4: Large boulders observed in stream bed. Scoured bank exposed typical alluvial soil profile.

Figure 5: Large recent sediment deposition just downstream of the canyon outlet

APPENDIX A

A-1: Site 1 Refraction Result

A-2: Site 2 Refraction Result

A-3: Site 4 Refraction Result

A-4: Vs 100 Estimate

Soil Rock

Line 2 Depth Allowable Bearing Allowable Bearing feet KPA psf

0 123.22 2573.49

5 168.37 3516.41

10 199.16 4159.48

15 277.69 5799.75

20 277.69 5799.75

25 277.69 5799.75

30 477.73 9977.69

35 801.75 16744.93

Line 4 Depth Allowable Bearing Allowable Bearing feet KPA psf

0 123.22 2573.49

5 214.71 4484.27

10 222.52 4647.37

15 230.35 4810.89

20 277.69 5799.75

25 547.43 11433.24

30 801.75 16744.93

35 801.75 16744.93

A-5: Allowable Bearing vs Depth

Line 1 Depth Allowable Bearing Allowable Bearing feet KPA psf

0 277.69 5799.75

5 325.54 6799.12

10 341.58 7134.09

15 432.86 9040.54

20 571.31 11931.99

25 913.84 19086.03

30 942.88 19692.39

35 942.88 19692.39

Line 4

Factor of safety Depth against liquefaction feet

3.61 5

1.71 10

1.14 15

1.53 20

Rock 25

Rock 30

Line 2

Factor of safety Depth against liquefaction feet

1.31 5

1.17 10

2.21 15

1.53 20

1.14 25

Rock 30

A-6: Liquefaction Evaluation

Mw 6.47 1000 yr return period for bridges

R, km 10.21

Fault Type 1

(1 = strike slip, 2 = reverse slip, 3 = normal slip, 4 = all others)

Estimated Vs avg, m/s

(Vs30m)

50th percentile

PGA

0.4863 g

Approx 84th percentile PGA 0.73 g

Estimated fines content percent

Vs1* 215 m/s

Line 1

Factor of safety Depth against liquefaction feet

14.51 5

7.51 10

5.41 15

Rock 20

Rock 25

Rock 30

A-7: Deaggregation S1 Ss result

A-8: Deaggregation Sources

A-9: Site 1 Soil Classification

A-10: Site 2 Soil Classification

A-11: Site 4 Soil Classification