Expert Comments: Geotechnical and Engineering 1185 Sunset Aragon

8/10/2019 Expert Comments: Geotechnical and Engineering 1185 Sunset Aragon

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August 4, 2014

Mr. Hoover Tang

991 Everett Street

Apt. A (Owner's Unit)

Los Angeles, CA 90026

SUBJECT: Geotechnical and Engineering Geology Review of Geotechnical Engineering

Investigation, Proposed Apartment Complex, 1185 West Sunset Boulevard, LosAngeles, California, dated April 9, 2013, by Geotechnologies, Inc.

REFERENCES: See attached References Cited

Dear Mr. Tang:

At your request we have reviewed the subject reports with regard to the proposed grading, and the

geotechnical and engineering geology conditions at the project site. The subject report wasprovided as a 193 page Adobe Acrobat file with a Geologic Map that is the primary reference map

for the comments below. An 8 page scanned response report to address comments by the City ofLos Angeles Geologic and Soils Report Correction Letter was also reviewed.

Background

The proposed project property is an elongated north to south triangle, being approximately 200 feet

wide at the base (north end) and 820 feet long (Geotechnologies, Inc., 2013, Geologic Map). The

proposed project consists of two separate developments 1) a mixed use residential/retaildevelopment along Sunset Blvd. to the corner of Sunset and Everett St. and 2) a small lot

subdivision along Everett St. Project 1 above occupies 80 to 90 percent of the property from northto south and project 2 above occupies the far south end of the property.

Temporary and permanent cut slopes would border the north and east edges of project 1. The west-facing permanent maximum 51-feet high cut slope is proposed at approximately 1.3:1 to 1:1

(horizontal:vertical) and the temporary vertical cut slope is zero to 35-feet high.

Investigation methods employed at the site included surface geologic mapping, hand-dug test pitsand 8-inch diameter machine dug hollow-stem auger borings. Samples were collected for

laboratory testing. There is no indication that historic aerial photographs were interpreted to

determine pre-development geologic conditions at the site, for example evidence of landslides oroil/gas drilling/development.

General Geologic Conditions

Three geologic units are mapped within the project site: 1) Quaternary colluvium (map symbol

Qcol), 2) Quaternary alluvium (Qal), and 3) Tertiary (Miocene) Puente Formation bedrock (Tp).

Based on the grading plan it is understood that most areas of the Qcol and Qal would be removedfrom the site, so that in those areas only Tp would remain. A small area of man-made artificial fill

(af) is found at the extreme south end of the property and would be removed per the grading plan.


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Proposed Development 1185 Sunset Boulevard

August 4, 2014Page 2

Puente Formation is generally a well-bedded clayey siltstone with sandy siltstone (Geotechnologies,Inc., 2013) that is yellowish to grayish brown and brown in color. Geologic bedding (distinct

layering in the bedrock) in the Puente Formation is tilted (dips) to the south, with variations to the

southeast and southwest. Some less common easterly dips are noted. Bedding orientation (mainly

dip direction and amount in degrees) is critical to determining slope stability. Borings 1 and 2 byPetra indicate claystone and siltstone to depths of 30-feet along Sunset.

Additionally, fractures were mapped in test pits excavated and mapped in 2006 and 2013(Geotechnologies, Inc., 2013). These fractures are vertical and oriented (the strike) east-west,

northwest-southeast, and northeast-southwest. Fractures can be important to slope stability since

they are planes of weakness that can separate materials into smaller blocks and wedges on cutslopes where they intersect bedding planes.

Geologic faults, which can be classified as non-active, potentially active, or active, are not mappedat the subject site. Lamar (1970) maps a north-northwest to south-southeast trending fault through

the site area. It is not known if this fault is associated with the underlying active Elysian Park blind

thrust fault, and if any potential for surface rupture exists. The fault is mentioned byGeotechnologies, Inc. (2013) as bisecting the site and it is acknowledged that bedding seems to be

affected near boring B-5. No further definition of the fault location is made, and there is no

explanation of potential affects on site stability, groundwater flow or bedrock fracturing, or if the

fault may be active or potentially active. It is not known if the fault is related to potentially activefaults identified at Belmont High School that project generally toward the subject project site (Earth

Consultants International, 2003).

Slope Stability Considerations

Geotechnologies, Inc. (2013) prepared five geologic cross-sections A-A through E-E to analyzethe slope stability for temporary and permanent slopes planned for the proposed mixed use/retail

development area. Section D-D is oriented generally north-south through the approximately 35-

feet high vertical temporary cut slope at the north edge of the property. Sections E-E, B-B, and C-C are perpendicular to section D-D passing through the approximately 1.3:1, 51-feet high west

and west-southwest facing slopes. Section A-A is along the west edge of the property and not

discussed further in this report.

Section D-D shows geologic apparent bedding angles out of the temporary vertical slope at angles

up to approximately 25 degrees. No indications of cross-cutting vertical joints are shown. Sections

E-E, B-B, and C-C are perpendicular to section D-D, and section E-E is also perpendicular tothe west-facing slope. Sections B-B and C-C are not perpendicular to the west-southwest facing

slope which changes orientation by about 10-degrees south of section B-B. Depending on the

surface-mapped, test pit-mapped, or boring-mapped attitudes selected, this could change the neutralor into-slope apparent bedding shown in section B-B to an out-of-slope configuration with apparent

dips of 5, 10, or 15 degrees for data from individual borings or surface mapping locations examined

for this review (e.g., N78W 36S in B-5, 2006; 20-degree dip surface mapping east of B-6, 2006).No indications of cross-cutting vertical joints are shown in the cross-sections.

Bedrock Characteristics: The residual strength used for along bedding stability analyses appear to

be high for some of the claystone/clayey siltstone encountered. High or un-conservative residual


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shear test results may be obtained from the direct shear test (Watry and Lade, 2000). Published dataindicate that the correlated residual shear strength based on Atterberg Limits and clay content of

along-bedding materials should be significantly lower (Stark and McCone, 2005; Bowles, 1982)

than residual strength obtained from direct shear test. This problem arises principally from

difficulty in sampling and properly orienting test specimens in direct shear test devices. TheSouthern California Earthquake Center (SCEC) Committee for landslide hazards strongly

recommends that the direct shear test results be checked with correlated data (SCEC 2002, pages

35-45).

Geotechnologies, Inc. (2013) performed rotational failure slope stability analyses for cross-sections

B-B and E-E. However, a geologic cross-section perpendicular to slope in the area south ofsection B-B could be more critical from a slope stability standpoint than the analyzed section B-B.

Considering the geologic structures (bedding and joints) as discussed above, some mapped bedding

attitudes (e.g., see the attitude of 20 degrees above boring B-6 slightly east of section B-B) wouldexhibit an out-of-slope component. Translational failure slope stability analyses along such a

component could be more critical than the analyzed planes in section B-B, which could impact the

feasibility of the current slope design. In addition, some of the vertical joints might also presentareas of weaker strength affecting the slope stability calculations. Therefore, these factors should

be accounted for as necessary in translational failure slope stability analyses in the area south of

section B-B.

Surficial Stability: The development plan includes steep (steeper than 2:1) manufactured cut slopes.

Slopes steeper than 2:1 are susceptible to long term surficial instability due to weathering anderosion, and require more stringent maintenance. However, the consultant evaluated the surficial

stability of the cut slopes using unweathered strength of bedrock. This may not represent or reflect

the future surficial performance of the steep cut slopes.

The consultant performed surficial slope stability of cut slopes at 27 degrees gradient. Some of the

proposed cut slopes in bedrock (e.g., section C-C) are at 45 degrees (1:1 gradient). When slopeangels steeper than 27 degrees are considered, lower factors of safety are obtained.

Groundwater

Groundwater or seepage was noted in borings excavated at the site (Geotechnologies, Inc., 2013).

Structures upslope from the project site appear to be in areas where rainfall, irrigation, and run off

could infiltrate the ground surface and flow down gradient toward the project site. It is assumed

that the pathways would be predominantly along fractures and bedding planes in the PuenteFormation bedrock. This condition should be fully considered in slope stability analyses.

Other Geotechnical Considerations

The consultant indicates that on-site materials may be used for backfilling behind retaining walls

and provides recommended values for earth pressure for the design of retaining walls. Clayey

materials with medium and high expansion potential were encountered at the site. Considering this,the recommended earth pressure values appear to be significantly lower than published

recommended values for the type of on-site materials (e.g., NAVFAC, 1997; California Building

Standards Commission, 2014).


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No sand subbase layer or presaturation were recommended to mitigate the potential for expansivesoils action within slabs-on-grade areas.

Summary

Our review indicates technical issues that we feel should be considered and addressed prior toapproval of the project as it is currently defined. In particular, the surficial and bedrock slope

stability due to the slope angles, assumed material strengths, presence of fractures, up slope

groundwater/seepage, and orientation of the slope between sections B-B and C-C relative to thecurrent cross-section directions. Also, the choice of shear strength used in the retaining wall

analyses (e.g., using high cohesion intercept) appear to be inappropriate considering the clayey and

highly expansive nature of backfill materials. Addressing the above issues may result in a need foradditional design measures to demonstrate feasibility of the current design and to mitigate the

potential for adverse impacts on the proposed development and adjacent existing structures.

Closure

This report has been prepared for the sole use and benefit of our client. The analysis, results, and

conclusions were prepared in general compliance with normal industry practice in Los AngelesCounty. Other consultants may arrive at different results and conclusions with the same

information. The intent of the report is to advise our client of geotechnical and engineering

geologic conditions at the subject site, and the possible effects of these conditions on the proposed

development and surrounding properties. It should be understood that the geotechnical engineeringand engineering geologic consulting provided represents professional opinions and the contents of

this report are not perfect. Any errors or omissions noted by any party reviewing this report shouldbe reported to Wilson Geosciences Inc. and Geo-Dynamics, Inc. in a timely fashion. Only the client

can authorize subsequent use of this report. No warranty is either expressed or implied.

Please contact the undersigned if you have any questions.

Sincerely,

WILSON GEOSCIENCES INC. GEO-DYNAMICS, INC

_____________________________ ______________________________

Kenneth Wilson Ali Abdel-HaqPrincipal Geologist Principal Engineer

P.G. #3175, C.E.G. #928 P.E. 46989, G.E. 2308

(626) 791-1589 (805) 496-1222


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REFERENCES CITED

Bowles, J. E., 1982, Foundation Analyses and Design, Third Edition.

California Building Standards Commission, 2014, 2013 California Building Standards Code,http://www.bsc.ca.gov/codes.aspx.

Department of the Navy, Naval Facilities Engineering Command (NAVFAC), 1997,GeotechnicalEngineering Procedures for Foundation Design of Buildings and Structures (UFC 3-220-

01N, previously NAVFAC Design Manuals DM 7.1 and DM 7.2).

Dibblee, T. W. Jr., 1991, Geologic Map of the Hollywood and Burbank (South ) 7.5-minute

Quadrangles, Map No. DF-30, scale 1:24,000.

Earth Consultants International, 2003, Instigation of Faulting Potential on the Belmont Learning

Center Site.

Geotechnologies, Inc., 2013, Geotechnical Engineering Investigation, Proposed Apartment

Complex, 1185 West Sunset Boulevard, Los Angeles, California, dated April 9, 2013.

Geotechnologies, Inc., 2014, Response to Soils Report Correction Letter, Proposed ApartmentComplex, 1185 West Sunset Boulevard, Los Angeles, California, dated March 4, 2014.

Southern California Earthquake Center (SCEC), 2002, Recommended Procedure for

Implementations of DMG Special Publication 117, Guidelines for Analyzing and Mitigating

Landslide Hazards in California, In California Implementation Committee: T. F. BlakeChair, T. F. Blake, R. A. Hollingsworth, and J. P. Stewart Editors.

Stark, T.D., H. Choi, and S. McCone, 2005, "Drained Shear Strength Parameters for Analysis ofLandslides," Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 131,

No. 5, May, 2005, pp. 575-588.

Watry, S.M. and Lade, P.V., 2000, Residual Shear Strength of Bentonites on Palos Verdes

Peninsula, California, GeoDenver 2000, Proceedings, Geo-Institute of the American Society

of Civil Engineers.

Documents

Expert Comments: Geotechnical and Engineering 1185 Sunset Aragon