Pacific Crest Engineering performed a geotechnical study to address a section of the Pajaro River Levee adjacent to the Watsonville Wastewater Treatment Plant. This portion of the levee is approximately 2,350 feet in length and located on the outboard side of a major bend in the Pajaro River. This section of levee has been subject to seepage and flooding damage in the recent past and the City proposed to employ sheet piling to accomplish a reduction in seepage and improve the stability of the levee embankment. The purpose of our investigation was to evaluate the effectiveness of using sheet piles to reduce seepage through the levee and for improving the levee stability. Our study included the advancement of borings and CPT soundings, laboratory analysis of soil samples, quantitative seepage analysis, quantitative slope stability analysis, report preparation and technical response to USACE reviews.
Los Padres Dam is an earth and rockfill structure approximately 133 feet tall and 600 feet long, with a crest elevation of 1,058 feet. The dam and spillway are located in steep mountainous terrain that was deeply dissected by the Carmel River prior to grading for the construction of the dam. The spillway is reinforced structure concrete 546 feet in length with an ungated ogee crest at elevation 1,040 feet. The spillway tapers in width from 110 feet at the ogee crest to 49 feet at the downstream end. Due to major storm events in California within the last decade, the government issued a plan to bolster the states dam safety plan that included detailed evaluation of dam appurtenant structures, such as spillways. A reconnaissance level assessment by the Department of Water Resources (DWR) concluded that the spillway at Los Padres Dam may have potential geological, structural, and hydraulic weaknesses as interacting parameters that might affect its performance in a critical flood event. California American Water Company retained the services of Pacific Crest Engineering Inc. to perform a condition assessment of the spillway. Pacific Crest Engineering liaised with our subcontractors SAGE Engineers and Zinn Geology to develop the geotechnical, geologic, structural and hydraulic components of this spillway assessment study. Pacific Crest Engineering performed geotechnical field exploration and laboratory testing to support development of Zinn Geology’s geologic map and cross sections, identified contrasting areas of soil and bedrock to either side of the spillway alignment, and developed supporting geotechnical criteria to assist the team in their respective assessments. As requested by DSOD, only non‐destructive methods were utilized in the development of this spillway condition assessment.
In 2014 Pacific Crest Engineering was contracted by the County of Santa Cruz to provide geotechnical consultation, observation and testing services for restoration of the levee near the confluence of the Pajaro River and Salsipeudes Creek. The project consisted of raising and widening portions of the levee to improve flood protection in the area. Members of our staff performed a pre-construction investigation consisting of excavation of test pits in selected areas along the landside toe of the levee, engineering analysis and development of geotechnical recommendations for the proposed levee improvements. Pacific Crest Engineering developed earthwork recommendations for construction of the keyways and benches of the proposed landside berm for the levee, consulted during construction regarding suitability of native materials, provided supplemental recommendations for compaction specifications to incorporate existing cobbles and rip rap within the engineered fill, performed compaction testing and QSP monitoring services, and provided technical assistance regarding other geotechnical related construction issues
Pacific Crest Engineering performed quantitative seepage analysis along a section of the Pajaro River Levee adjacent to Sakata Lane in Watsonville. The analysis was performed to address concerns that potential piping-related levee distress could result from the construction of nearby excavations. To address the piping concern, we performed a seepage analysis using geologic cross sections developed from our prior studies at the site. The cross-section analyzed included the existing levee and a proposed infiltration basin to be set back about 100 feet from the toe of the levee. The purpose of our seepage analysis was to make a preliminary assessment as to whether or not the proposed basin would pose a piping hazard the levee. Our analysis evaluated the seepage conditions at the toe of the levee and the base of the proposed infiltration basin. Analysis was performed using the groundwater finite element program SLIDE by Rocscience (Version 7.017). Our analysis concluded that following the construction of the proposed infiltration basin the vertical exit gradients were estimated to be below the required maximum vertical exit gradients as prescribed the Urban Levee Design Criteria of the California Department of Water Resources (2012). Based on the estimated magnitude of exit gradients it did not appear that the proposed infiltration basin significantly increased the probability of soil failure due to increased seepage forces and/or “piping of soils” at the levee toe or at the infiltration basin site. Our analysis also indicated that the construction of the basin would increase slightly the vertical exit grading at the infiltration basin location while reducing the vertical exit gradient at the toe of the levee.
444 Airport Blvd, Suite 106, Watsonville, CA 95076 · 831-722-9446
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