REVIEW OF GEOTECHNICAL REPORTS AND COASTAL BLUFF EVALUATION

709 Lucerne Road in Cayucos, San Luis Obispo County

APN 064-281-09

Bluff Setbacks

References:

(1) GeoSolutions Inc..; Geologic Site Condtions, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California.

(2) GeoSolutions, Inc..; Geologic Coastal Bluff Evaluation, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California; August 14, 2009.

(3) GeoSolutions, Inc..; Response to Comments, Geologic Coastal Bluff Evaluation, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California, Project No SL07201-2; January 15, 2010.

(4) GeoSolutions Inc..; Geotechnical Engineering Report, Proposed Replacement Residence, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California.

(5) Landset Engineers, Inc..; Review of Response to Review Comments: Coastal Bluff Evaluation, File No.: LSG-0576-01, SLO County File No. DRC 2009-00020; January 19, 2010.

(6) Landset Engineers, Inc.; Review of Geologic Coastal Bluff Evaluation; December 3, 2009.

References (Cont’d)

(7) GeoSolutions Inc..; Discussion of Groundwater, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California; January 18, 2012

(8) GeoSolutions Inc..; Review of Bluff Stability and Seepage, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California, Project No. SL07201-2; January 26, 2011.

(8) MID-COAST GEOTECHNICAL, INC.; Geotechnical Engineering Report, Proposed Replacement Residence, 709 Lucerne Road, Cayucos vicinity of San Luis Obispo County, File N0 09-6350, Report No. 13121.; July 30, 2009.

(9) GeoSolutions, Inc..; Geologic Coastal Bluff Evaluation, 709 Lucerne Road, APN: 064-281-009, Cayucos Area, San Luis Obispo County, California, Project No. SLO7201-1 ; August 14, 2009;

GeoSolutions, Inc..;

(10) John Black – Appellant; Agenda Item Th14a; April 12, 2012.

Bluff Evaluation

Bluff Evaluation

Shoreline Retreat  Shoreline retreat has been understated.  The photographs labeled

2002 through 2010, ones that have been presented at previous meetings of this board, show the erosion on the west-side very well that clearly demonstrates that a Slope-Retreat-Rate 2 inches per year is significantly understated.

Studies performed by GeoSolutions make reference to some of the items in the aerial photographs for comparison purposes.  Earlier photos of the bluff, if available, would be even more compelling if they could be compared with the most recent ones and a more accurate shoreline-retreat values could be computed from photos taken up-close and site-specific.

Bluff Evaluation

Shoreline Retreat 

Bluff Evaluation

Shoreline Retreat 

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Shoreline Retreat 

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Shoreline Retreat 

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Shoreline Retreat 

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Shoreline Retreat 

Bluff Evaluation

  Soil Liquefaction During a Strong Seismic Event

The slope-stability buffer would need to be increased significantly if the layer feeding the spring were to be become saturated and liquefy during a major seismic event.  The report prepared by MID-COAST GEOTECHNICAL, INC. (2) shows a layer described as “…Brown shells with some sand (terrace deposits)…” is at the level where groundwater was encountered.  There were no samples taken at the level where groundwater would have been found, 24.5 feet.  No classification tests or strength-tests were performed on the material classified as Shells and labeled (S1).   A report prepared by GeoSolutions and dated January 18, 2012 states:

  “The CHG letter states ‘The spring cistern receives flow from a pipe that extends

into the bluff.  The origin of the spring water is most likely from the basal sands and shell hash in the terrace deposits that are at an approximate elevation of 20-25 feet above mean sea level.”

  

Bluff Evaluation

This water may occur intermittently or perennially and may not have been present for borings drilled in summer and fall months.  Moreover, landscape irrigation may seep vertically and saturate these soils.  In the vicinity of the basement, if pervious soils are used to backfill buried walls, water seepage from overlying landscape irrigation could provide a direct conduit for seepage to the sandy materials described above.

  Seepage vertically through materials classified as clay, occurring

outside the building footprint, would have very low hydraulic-conductivity.  Moisture from this source would be less likely than from naturally occurring seepage collected in the fractured Franciscan Complex.

Regardless of the cause or source of soil saturation, the layer of shells and sand overlying the Franciscan Complex may become saturated, remain saturated, and liquefy during a strong seismic-event.

Bluff Evaluation

Accordingly, the slope-stability analysis used to establish a safe set-back from the edge of the bluff should include a non-circular, block-failure, for a block with a base coincident with the top one to two feet overlying the Franciscan Complex.  Moreover, this layer should be assumed to have liquefied by ground shaking caused by an earthquake that has a 975 year recurrence interval. 

The seismic accelerations used in the analyses should include both horizontal and vertical accelerations consistent with the California Building Code (2010).  I have attached a seismic analyses performed by our Licensed Engineering Geologist, Robert Becker, that shows a more appropriate acceleration to use would be one that corresponds to a seismic event with a Mean Return Time of 975 years, one with a 5% Probability of exceedance in 50 years.  This event has a probability of exceedance of slightly greater than 0.10%.  (See attached Analyses Sheets).  The acceleration that should be used to evaluate slope stability is 0.372 gravity approximately and is approximately 2.5 times greater than the acceleration reportedly used by GeoSolutions, Inc.. ( 2).

Bluff Evaluation

Spring at NW Corner of 707 Lucerne Avenue

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Exhibit – 1 Exhibit - 2

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Exhibit – 3 Exhibit - 4

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Exhibit – 5 Exhibit 6

Exhibit 7 – PSF Deaggregation on NEHRP 975 Years

Exhibit 8 – PSF Deaggregation on NEHRP 2475 Years

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Exhibit 9 – Liquefaction Analyses Boring 2

Exhibit 10 – Liquefaction Analyses Boring 1

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Cyclic Mobility

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Turnagain HeightsAlaska Earthquake 1964