COUNTY OF SAN MATEO

Inter-Departmental Correspondence

Department of Public Works

DATE:

October 18, 2007

BOARD MEETING DATE:

October 30, 2007

SPECIAL NOTICE/HEARING:

None

VOTE REQUIRED:

4/5^ths Vote

TO:

Honorable Board of Supervisors

FROM:

James C. Porter, Director of Public Works

SUBJECT:

La Honda Slides

Recommendation

A) Adopt a resolution:

1)

Authorizing a County contribution equivalent to funds that are made available from non-County sources, up to a maximum of $3,000,000 for the implementation of slope stabilization activities in the La Honda area and directing the Department of Public Works to facilitate slope stabilization projects utilizing these funds, if equivalent funds are received;

2)

Appropriating funds not to exceed $400,000 from the Public Works Road Fund to construct a french drain system to help dewater the failed slope areas;

3)

Authorizing the Department of Public Works to continue to winterize previously failed slope areas by re-grading and reseeding them annually.

B) Adopt a resolution:

1)

Authorizing an Appropriation Transfer Request in the amount of $250,000 from the General Fund, Departmental Reserves to the Road Fund for the cost of the improvements to Escondido Drive as per recommendation 2).

Vision Alignment

Commitment: Ensure basic health and safety for all.

Goal 7: Maintain and enhance the public safety of all residents and visitors.

Improving the stability of the landslide area will reduce the likelihood of further earth movement and improve public access via Escondido Drive.

Background

Previous Board Action

In 1999 the Board approved the County’s acquisition of ten (10) parcels through FEMA’s Hazard Mitigation Grant Program. In 2005, when Scenic Drive was damaged and the State Office of Emergency Services (OES) declared the landslide area a disaster area, the Board authorized the County to apply to OES for funding to make permanent repairs to Scenic Drive.

History

Two separate landslides occurred in 1998, one (1) between the closed portion of Recreation Drive (lower Recreation) and Escondido Drive and the other on the hillside above Recreation Drive, encompassing a portion of lower Scenic Drive, including several properties immediately adjacent to lower Scenic Drive.

The upper landslide destroyed nine (9) homes. FEMA’s Hazard Mitigation Grant Program provided financial assistance to property owners by financing the County’s purchase of the impacted properties. The County hired UPP Geotechnology in 1998 to investigate the landslide and to generate alternatives in addressing the slope instability and in reestablishing access along the portion of Scenic Drive that was damaged by the upper of the two landslides.

The UPP report presented a number of options for dealing with the slope and identified risks associated with each of the options. Upon review of the various repair alternatives, the County established a dewatering system, consisting of a network of wells, re-graded the distorted slopes to improve surface runoff and reconstructed the damaged portion of lower Scenic Drive.

Lower Recreation Drive, which was previously damaged in Winter 1995/1996 and repaired by County Workers, was displaced again during the 1998 slide and has not been accessible since that time.

Heavy winter rains in 2005 reinitiated landslide movement in the immediate vicinity of lower Scenic Drive. Lower Scenic Drive was once again displaced by landslide movement and, over the course of the following two (2) years, significant movement resulted in a much larger landslide mass which currently encompasses all of the 1998 landslides and significantly larger areas both above and below these landslides. The segment of Scenic Drive located above the failure (upper Scenic) is now located very near the uppermost limits of the visible slide and tension cracks have developed in the road indicating that additional expansion of the slide could have a direct impact on this segment of road.

In 2005, the State Office of Emergency Services (OES) declared the area a disaster area and determined that the County qualifies for State public assistance under the California Disaster Assistance Act (CDAA), and that an application can be made to obtain State funding to make permanent repairs to Scenic Drive. The County has applied to State OES for funding assistance and has received a funding commitment of seventy-five percent (75%) of costs associated with studies related to the stabilization of Scenic Drive only. OES is currently reviewing the studies performed by the County in order to determine whether or not repairs required to reestablish the road will also be eligible. Repairs that are not for the sole purpose of reestablishing the damaged road are not eligible under this funding program. As noted below, the recommendation includes the reestablishment of Scenic Drive only if all of the slope stabilization projects are constructed.

In 2005, San Mateo County contracted with UPP Geotechnical to reevaluate the lower Scenic Drive segment of the landslide, and with Parikh Consultants to geotechnically evaluate the upper segments of the slide and to provide recommendations on possible remedial measures that could stabilize the slope failures. San Mateo County also contracted with Soil Engineering Construction, Inc. (SEC), an engineering contractor, in order to develop further strategies for addressing the large-scale slope failures and to protect and/or restore threatened or damaged segments of the road. In 1998, SEC completed a large retaining wall on Polhemus Road in unincorporated San Mateo County to stabilize a failed slope. UPP Geotechnical and Parikh Associates continue to work with the County to provide peer review of SEC’s additional mitigation strategies.

To date, we have spent over $950,000 in our efforts to study options for remediation and also improve the stability of the slope through dewatering and grading efforts.

The following is a timeline of activity and various Public Works actions in the last 30 months:

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March 2005 – Reactivation of the 1998 Slide is observed.

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April 2005 – Public Works contracts with UPP Geotechnology, Inc. to perform a supplemental geotechnical investigation of the slide and to reevaluate their 1998 geotechnical investigation report.

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May 2005 – Public Works contracts with Griffin Dewatering Corporation for the installation of three (3) additional dewatering wells.

Ø

June 2005 – UPP Geotechnology submits letter of findings based on their investigation. Report finds that the 1998 report is still valid but that additional studies are necessary.

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September 2005 – Public Works contracts with Aero-Geodetic Corporation for completing an aerial survey of the slide area.

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October 2005 – Public Works contracts with Griffin Dewatering Corporation for the installation of five (5) additional dewatering wells.

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November 2005 – Public Works Road crew regrades slide area to eliminate low spots in preparation for winter hydroseeding.

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December 2005 – Public Works contracts with A to Z Hydroseeding for winter hydroseeding.

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December 2005 – Public Works authorizes UPP Geotechnology to conduct geotechnical study for repairing Scenic Drive with the construction of a buttress fill as recommended in their June 2005 letter. As part of their study, UPP drills large diameter holes to collect data of the geological features.

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March 2006 – Public Works contracts with Parikh Consultants for the preparation of a geotechnical study to stabilize the slide’s headscarp near upper Scenic Drive.

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May 2006 - UPP report recommends a buttress fill to support and allow for the reconstruction of lower Scenic Drive only. The buttress fill is estimated to cost $6 Million to construct.

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July 2006 – Parikh report based on Cone Penetration Tests (CPT) received. Slide defined in greater detail. However, additional studies are recommended.

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September 2006 – Public Works contracts with Half Moon Bay Grading & Paving, Inc. for the modifications to existing drainage swales on upper Scenic Drive to direct storm water away from the headscarp of the slide mass.

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November 2006 - Public Works contracts with Half Moon Bay Grading & Paving, Inc. for modifications to existing drainage swales on lower Scenic Drive and Entrada Way.

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March 2007 -Parikh “single fix” opinion letter received. Cost for repair estimated at tens of millions of dollars and would require several construction seasons to complete.

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March 2007 – Public Works contracts with SEC to evaluate constructability of the repair recommendations submitted by UPP Geotechnology and Parikh Consultants and provide alternative options for consideration.

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June 2007 – Alternate design presented by SEC. It includes shotcrete and tieback walls for stabilizing upper Scenic Drive and two pier-buttress subsurface barriers in the mid and lower section of the slides to stabilize the land mass. The team of geotechnical engineers affiliated with the studies meet and agree on the feasibility of the more cost effective SEC proposal, which can be implemented in components or in total to provide additional stability within the slide zone. Total cost of the alternative design options is about $6,355,500, about $250,000 of which is designated to improve Escondido Drive.

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June 2007 – Parikh report finalized based on data collected from additional borings. Parikh proposes that two tie-back walls be constructed to retain the existing Upper Scenic Drive.

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August 2007 – County meets with property owners and provides a cost matrix that outlines various types of mitigation measures that could be constructed and estimated costs. County arranges for meeting of County consultants and La Honda community geotechnical engineers and requests community input by the end of September on what mitigation options the community would like the County to pursue, including options for funding the work.

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September 2007 – County crew re-graded the slide area in order to eliminate low points that could be expected to pond water.

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On-going activities – Monitoring and maintenance of the twelve (12) dewatering wells within the slide area.

In addition to the above, the Department of Public Works has routinely notified the La Honda Guild of its work efforts; coordinated work with the La Honda Guild, where necessary; met with representatives of the La Honda Guild upon request; and facilitated meetings with various geotechnical engineers who have studied the area.

The Department is currently maintaining 12 well points within the slide area. These wells have pumped out over 5,000,000 gallons of water since 2005. Public Works has also redirected surface drainage on Upper Scenic to a creek that runs on the downhill side of the slide and installed drainage system enhancements uphill of the slide, further isolating surface drainage from the slide area.

Fiscal Impact

The cost for improving Escondido Drive is $250,000 and we recommend that funding for this project come from the General Fund. Because Escondido Drive is a private road owned by the Guild, the Public Works Road Fund cannot pay for this project. The french drain system is estimated to cost $400,000, and annual winterization costs are estimated to be $50,000. There is sufficient funding in the Public Works Road Fund to pay for these two projects.

The funding for any additional work beyond the french drain system and annual winterization has not been budgeted. It is recommended that County General Fund reserves be used for the improvements to Escondido Drive and to match any non-County funding to pay for any additional construction activities. The Road Fund does not have an adequate balance to absorb implementation of all of the recommended mitigation measures without the elimination of a significant portion of the road construction projects which have previously been budgeted.