Agmt03 Bay Area Geotechn... (2) - Laserfiche WebLink

Browse Search

City of Redwood City Proposal No. 03-187 September 18, 2003 Page No. 4 Thus, it appears the drainage gallery was installed and at least temporarily "stabilized" the slide, which had the net result that no further work was perforated after the emergency removal of the slide debris behind the houses on From Hill Blvd and general smoothing of the scarp and slide area. While the material removal from the toe would have provided a safety margin for the houses below the slide, at the same time it removed a small buttressing effect from the remaining slide mass, somewhat increasing its likelihood of future movement. There is no evidence that the necessity of the buttress fill was ever "reviewed" as recommended by Shannon and Wilson in their report. Because we did not enter the properties bordering Farm Hill Blvd during our site visits, we were not able to confirm that the remedial grading had been done on the lower lots in accordance with the referenced grading plan. In addition, we were not able to locate evidence to confirm the horizontal drains were installed per the referenced subdrainage plan. While Mr. Mahoney, of Soil Engineering Construction, indicates he remembers a slide plane from the boring logs, it is not clear if this was an actually identified "slide plane," or the "sheared shale and clay" logged by Shannon and Wilson in their logs, where "sheared" is a general characteristic of the deposit, rather than a specific plane within it. In addition, there is some question as to the nature of the bedrock materials at the site, as the Shannon and Wilson report identifies the materials as the Butano formation (now called the Whiskey Hill formation) on both sides of a minor fault crossing the site, while other engineers referl to a fault separating Franciscan serpentine from Butano sandstone and shale as mapped by Brabb and Pampeyan in 1972, or mention2 Franciscan sandstone and shale. ~Letter by Hawke Engineers, Aug., 1977. 2Report by Soil Engineering Construction, Sept., 1979.