112-11. Memo from Cotton, Shires, and Associates regarding geotechnical constraints of sloped single-family residential lots.pdfCOTTON, S1-11R1,.,S ANT) ASSOCIATES, INC.
CONSULTING ENGINEER'S AND GEOLOGISTS
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TO: George Schroeder, City of Cupertino
FROM: 'red Sayre, Cotton, Shires and Associates, Inc.
SUBJECT: City Questions Regarding Geotechnical Constraints
DATE: June 2,2011.
Q 1: Are (her(, different geologic/geotechnical concerns for Toe of Hill Lots versus
Enbankment Lots?
Reply: Generally Embankment Lots have relatively level front building areas with very
steep and high slopes toward the back portions of the property. Our geotechnical
concern is limited to proposed significant construction across the steep back
slopes and proposed construction within approximately 25 feet of the top of the
steep slopes,.
In contrast, the'foc of Hill Lots generally have a greater percentage of moderately
steep to steep slopes (greater than 15 percent). Cur geotecl-inical concern for these
lots includes proposed grading or significant construction on slopes greater than
20 percent. In addition, proposed construction should address any mapped
Geologic Hazard Zones.
Q2: From a geologic/geotechnical standpoint, should there be different FAR
restrictions for building, on the flat portion of the lot and off?
Reply: For flat portions of lots m,w,ay from steep slopes, we do not have a geotechnical
reason to restrict FAR as long as resulting increases in water runoff are property
controlled.
For very steep portions of lots (over 30 percent), we agree that geotechnical risks
are lowered by li.111iting the allowed square footage of development/grading
disturbance. For moderately steep slope areas in the range of 10 to 30 percent,
there are relatively standard geotechnical design measures to achic\,c project and
slope stability. As long as design measures are based on all adequate gcotechnical
investigation and peer reviewed, then we do not have geotechnical reasons to
restrict FAR on slopes less than approximately 30 pet -cent. We note that there are
potential aesthetic reasons to limit grading on moderately steep slopes,
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Q3: Should there be setback standards for development near slopes?
Reply: We suggest consideration of an approximate 25-foot setback for development.
near 30% or greater slopes exceeding 10 feet in height. Development within the
setback (if allowed) Should be supported by a geotechnical investigation
addressing slope stability and be peer reviewed.
Q4: Should current grading allowvances of 2,500 cubic yards and 2,,000 square feet for
the building, pad area be reevaluated?
Reply: We do not have geotechnical objections to these existing grading limits for R1-20
lots. For smaller lots, potential reduction of grading allowances seems to be
largely an aesthetic issue (if alterations of slopes greater than 20% are limited to
small areas or closely evaluated by required gcotechnical investigation).
Q5: Should there be stricter grading restrictions on tactual slopes?
Reply: ConstrlaGt.ion oaa slopes greater than 30% is restricted to saaaall areas and this is
consistent with geotechnical concerns. For slope areas in the range of 10 to 30
percent, there are relatively standard geotechnical design measures toachieve
slope stability. However, proposed signi licant grading of slopes exceeding 20
percent should be based on adequate geotechnical investigation that is peer
reviewed.
QG: What are the critical components to address in the geologic and geotechnical
reports?
Reply: Section 19,40.100 of the City RHS Ordinance provides minimal guidance
regarding preparation of technical reports. One of the best set Of guidelines wve
have identified to date is posted on the City of Calabasas wvebsite entitled
"Manual for the Preparation of Geologic and Geotechnical Reports", March 2010
(53 pgs). The City of San Diego also has a good set of guidelines that is available
on their wvebsite.
Q7: Do existing R1-20 lots have different geotechnical characteristics than other RI
logs with 15 to 30 percent slopes?
Reply: Fron:i a gcotechnical perspective, delineated RI--20 lots have essentially similar
slope constraints to other RI lots with 1.5 to 30 percent slopes. While volumes of
allowved grading might be scaled down to better fit smaller R1 lots, we do not
have geotechnical reasons to treat these two lot types differently.
Q8: Are there other geologic/geotechnical concerns for lots with 15 to 30 percent
slopes that should be regulated?
CC)r°rCDN, SHIRES AND AsSOCIATES, INC.
Reply: Based on current City Geohazard Maps, we (to not have a geotechnical basis to
recommend additional hillside regulation, If the City had a maj) prepared
delineating the distribL16011 of existing landslides in the hill areas, then we would
recommend additional restrictions associated Nvith areas of mapped landslides.
GrOUnd that has failed in the past by landsliding has a greater potential to fail in
the fUtUre by laiidsliding.
COTTON, SHIRES AND AssOCIATES, INC.