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GROUND MOVEMENT POTENTIAL MAPTown of Portola Valley, California

EXPLANATION

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Relatively Stable GroundLevel ground to moderately steep slopes underlain by bedrockwithin approximately three feet of the ground surface or less;relatively thin soil mantle may be subject to shallow landsliding,settlement, and soil creep.Unconsolidated granular material (alluvium, slope wash, andthick soil) on level ground and gentle slopes; subject tosettlement and soil creep; liquefaction possible at valley floorsites during strong earthquakes.Naturally stabilized ancient landslide debris on gentle tomoderate slopes; subject to settlement and soil creep.Generally highly expansive, clay-rich soils and bedrock.Subject to seasonal shrink-swell, rapid soil creep, andsettlement. May include areas of non-expansive material.Expansive soils may also occur within other map untis.

Areas with Significant Potential for Downslope MovementSteep to very steep slopes generally underlain by weatheredand fractured bedrock subject to mass-wasting by rockfall,slumping, and raveling.Unstable, unconsolidated material, commonly less than 10 feetin thickness, on gentle to moderately steep slopes subject toshallow landsliding, slumping, settlement, and soil creep.Unstable, unconsolidated material, commonly more than10 feet in thickness, on moderate to steep slopes;subject to deep landsliding.Debris flows, (shallow, rapidly moving landlsides) includingrecognized source areas, flow paths and depostional runoutareas.

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Unstable Ground Characterized by Seasonally Active Downslope MovementMoving shallow landslides, commonly less than 10 feet inthickness.

Moving deep landslides, commonly more than 10 feet inthickness.

FEET

SCALE

0 500 1000 1500 2000250

COTTON, SHIRES AND ASSOCIATES, INC.CONSULTING ENGINEERS AND GEOLOGISTS

G0088JWTS 1"= 500'

SHEET NO.

PROJECT NO.

DATE JUNE 2017

SCALE

APPROVED BY

GEO/ENG BY

GROUND MOVEMENT POTENTIAL MAP

1 OF 1

Town of Portola ValleySan Mateo County, Calfornia

MAP SYMBOLSTraces of the San Andreas fault, dashedwhere approximate. En-echelon rupturebehavior depicted by series of diagonalslashes.Fault (other than San Andreas) ofundetermined activity, dashed whereapproximate, dotted where concealed, barbsare located on upthrownside of fault.Inactive Fault1906 Ground Cracks

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NOTE TO USERS:This is an interpretive map derived both from the GeologicMap of the Town of Portola Valley and from additional fieldobservations and geologic experience in the Portola Valleyregion. All boundaries between zones are locatedapproximately. Information on this map is NOT sufficient toserve as a substitute for detailed, site-specific geologicand geotechnical investigations necessary forconstruction. It illustrates the relative stability ormovement potential, in the Portola Valley area, of ground inits natural undisturbed state. Works of man may seriouslya;ter the natural stability of ground. Potential impactsof graded cut and fill slope are not addressed movementpotential interpretations.This map is an update of the initial Movement Potential of Undisturbed Ground Map prepared by J.D. Rodine (1975)revised by William Cotton and Associates, Inc. (1984), and revised by Cotton, Shires and Associates, Inc. (2010).For additional information about this map and the GeologicMap of the Town of Portola Valley, see: Geology andand Movement Potential within the Town of Portola Valley,California, February 1975, by J.D. RodinePlease see Town website at www.portolavalley.net forlatest map revisions.