20844.000.001
September 13, 2024
APPENDIX E
Updated Geological and Soil Analysis
GEOTECHNICAL
ENVIRONMENTAL
WATER RESOURCES
CONSTRUCTION SERVICES
COASTAL/MARINE GEOTECHNICS
2010 Crow Canyon Place, Suite 250 San Ramon, CA 94583 (925) 866-9000 [email protected] www.engeo.com
Project No.
20844.000.001
October 3, 2022
Latest Revision September 13, 2024
Mr. David Smith
c/o Manatt, Phelps & Phillips, LLP
One Embarcadero Center, 30th Floor
San Francisco, CA 94111
Subject: Iglesia ni Cristo (INC)
650 Cape Breton Drive
Pacifica, California
LIMITED GEOTECHNICAL REPORT
Dear Mr. Smith:
We are providing this limited geotechnical reconnaissance and soil analysis as required in Section 2.B.7 of the agreement between the City of Pacifica and Iglesia ni Cristo (INC), property owner of 650 Cape Breton Drive (Property) (“Agreement”). Section 2.B.7 of the Agreement states “A geological and soils analysis of the site and conclusions and recommendations regarding the effect of the soil and geological conditions on potential grading, excavations, street and utility improvements, and structures.” We performed a site visit at the Property on Friday, September 16, 2022, and outline our observations and arena soil relocation recommendations below.
PROJECT BACKGROUND
The Property is located at 650 Cape Breton Drive in Pacifica, California (Figure 1). Based on discussions with you, we understand that 650 Cape Breton Drive contains a large 30,000-square-foot arena that was used by previous owners for equestrian purposes. Under previous ownership, the arena floor was covered with 1 to 3 inches of loose sandy material (arena soil) amounting to approximately 100 to 300 cubic yards of material. INC removed the arena soil from the floor and stockpiled it around the perimeter of the arena. Some arena soil was used to fill sandbags for erosion control measures around the site. The extent of displaced arena soil is depicted in the Site Plan, Figure 2.
As outlined in the Agreement between INC and the City of Pacifica, INC was cited for “Illegal Grading” for the actions described above. INC has agreed to obtain a retroactive grading permit for the displaced arena soil from the site. No further earthwork is proposed as part of this project.
Remaining work entails installation of best management practices (BMPs) to stabilize the displaced soil in place.
Additionally, a carport is proposed to bring the existing single-family residence into conformance with requirements of Hillside Preservation District. The proposed carport is to be prefabricated construction.
Mr. David Smith 20844.000.001
Iglesia ni Cristo (INC), 650 Cape Breton Drive October 3, 2022 LIMITED GEOTECHNICAL REPORT Latest Revision September 13, 2024
GEOLOGIC BACKGROUND
We reviewed regional geologic maps and aerial photographs within the vicinity of the Property.
Based on review of a geologic map prepared by Pampeyan (1994), the Property is mapped within Holocene-aged artificial fill (Qf1), Holocene-age slope wash, ravine fill, and colluvium (Qsr) underlain by Cretaceous and Jurassic-age greenstone of the Franciscan Complex (fg). Unit 1 of the artificial fill is described as poorly consolidated to well-consolidated gravel, sand, silt, and rock fragments with thickness varying by type of application and site. The Qsr is described to be unconsolidated to moderately consolidated deposits of sand, silt, clay, and rock fragments accumulated by slow downslope movement of weathered rock debris and soil. Pampeyan also notes that these deposits range between 5 to 20 feet thick depending on location along a slope.
The fg is described as dark-green to red, altered basaltic volcanic rocks, including flows, pillow lava, breccia, tuff, and minor related intrusive rock with strength varying between friable to hard depending on rock type and degree of weathering.
There are no known active faults crossing the Property; however, Pampeyan maps an unnamed fault trending northeast-southwest just north of the Property. Also, the San Andreas fault is mapped approximately 1.5 miles east of the Property (Reference 3). Pampeyan also mapped approximate locations of shallow landslides based on review of aerial photos from 1968 and shows one landslide within the vicinity of the Property (Figure 3). The site is not located within an Alquist-Priolo Earthquake Fault Zone.
Based on review of publicly available aerial photographs, a landslide occurred on the southern side of the Property. The landslide appears to be surficial in nature. Addressing this landslide is beyond the scope of this retroactive grading permit project.
According to the readily available aerial photographs, the area surrounding the Property appeared to be steeply sloping terrain with dense vegetation. Prior to development of the existing Property around 1968 or soon thereafter, the area was vacant, densely vegetated sloping terrain (Reference 5). According to topography mapped on topographic and geologic maps produced before and after development of the Property, some of the development appears to have been cut to a lower elevation during development; however, some placement of fill was likely used during development of the Property to create relatively level grades.
SITE VISIT AND SOIL ANALYSIS
On September 16, 2022, we visited the Property to inspect current site conditions and discuss proposed arena soil relocation options. We observed several locations where arena soil was relocated around the perimeter of the arena, as well as sandbags for erosion control measures throughout the site (Figure 2). The sandbags appeared to be in serviceable condition; however, some sandbags were damaged causing soil to disperse.
At the time of our site visit, we collected a bulk sample of the existing arena soil for laboratory testing. We classify the soil as gray silty sand, and the laboratory test performed produced a moderate dry density value of 119.4 per cubic foot and a relatively low moisture content value of
5.7 percent. The complete laboratory results can be found in Appendix D of the Environmental Sampling Results – Soil Stockpiles report dated June 11, 2024 (Reference 1).
Mr. David Smith 20844.000.001
Iglesia ni Cristo (INC), 650 Cape Breton Drive October 3, 2022 LIMITED GEOTECHNICAL REPORT Latest Revision September 13, 2024
A subsequent site visit was performed on May 16, 2024, to confirm present day conditions match those outlined in the September 16, 2022, site visit. The only notable change in condition is a growth of vegetation around some stockpiled arena soil and existing sandbags. Pictures taken during the recent site visit depicting present day conditions are provided on the site plan, Figure 2.
RECOMMENDATIONS
The scope of this project is limited to installation of BMPs to stabilize the displaced soil in its existing location. No placement of fill, further stockpiling, or other earthwork requiring compaction is included in the scope of this project.
Erosion Control
BMPs should be selected for long-term suitability. Arena soil should have debris removed. Care should be taken to protect existing established vegetation. Denuded soil should then be hydroseeded to establish vegetation for long-term erosion protection. The limits of arena soil requiring hydroseed application are depicted in the Site Plan, Figure 2.
Foundation Recommendations
The proposed carport is to be prefabricated construction. The carport should be anchored into the ground following the manufacturers’ requirements. Foundations may be required for carport anchors. Foundations should be embedded a minimum of 24 inches below the adjacent grade, a minimum of 18 inches in diameter, and support a maximum allowable load of 1,000 pounds.
If you have any questions, please do not hesitate to contact us.
Sincerely,
ENGEO Incorporated
Emily Mahon Kevin J. McFadden, PE
Robert H. Boeche, CEG
hjr/rhb/cb
Attachments: Selected References
Figures 1 through 3
20844.000.001
October 3, 2022
Latest Revision September 13, 2024
SELECTED REFERENCES
1. ENGEO. 2024. Environmental Sampling Results – Soil Stockpiles, Iglesia ni Cristo.
June 11, 2024. Project No. 20844.000.001.
2. Pampeyan, Earl H. 1994, Geologic Map of the Montara Mountain and San Mateo 7-1/2’ Quadrangles, San Mateo County, California. Miscellaneous Investigations Series Map I-2390.
3. UC Santa Barbara. 2001. Aerial Photography database; Flight CAS-65-130, Frame 10-27 and 10-28, Scale 1:12,000, January 1, 1965; Flight HM_2001_USA, Frame 3042-74 and 3042-73, Scale 1:10,800, January 1, 2001.
4. U.S. Geological Survey and California Geological Survey. Quaternary fault and fold database for the United States. accessed September 30, 2022, at: https://www.usgs.gov/naturalhazards/earthquake-hazards/faults.
5. Historic Aerials, Aerial Photography database. various aerial photographs dated between 1946 and 2020. www.historicaerials.com/viewer.
20844.000.001
October 3, 2022
Latest Revision September 13, 2024
FIGURES
Figure 1 – Vicinity Map
Figure 2 – Site Plan
Figure 3 – Geologic Map
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Expect Excellence
0 FEET 2000
VICINITY MAP
650 CAPE BRETON DRIVE
PACIFICA, CALIFORNIA
20844.000.001
SITE
1 KJM
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Expect Excellence
0 FEET 2000
REGIONAL GEOLOGIC MAP
650 CAPE BRETON DRIVE
PACIFICA, CALIFORNIA
20844.000.001
AS SHOWN 3
ANTICLINE SYNCLINE
AXIS OF FOLD
EXPLANATION
GEOLOGIC CONTACT-DASHED WHERE
GRADATIONAL OR APPROXIMATELY LOCATED
FAULT-DASHED WHERE INFERRED,
DOTTED WHERE CONCEALED, QUERIED
WHERE EXISTENCE IS DOUBTFUL.
SITE
Qya YOUNGER ALLUVIUM
Qac COARSE-GRAINED ALLUVIUM
Qsr SLOPE WASH, RAVINE FILL, AND COLLUVIUM
Qyl YOUNGER LANDSLIDE DEPOSITS
Qf1 ARTIFICIAL FILL
Qol OLDER LANDSLIDE DEPOSITS
Tsl SANDSTONE, SHALE, AND CONGLOMERATE
fg GREENSTONE
fs SANDSTONE
fc CHERT
fl LIMESTONE
fsr SHEARED ROCK
sp SERPENTINITE
KJM