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City of Redwood ('~itv l)roposal No. 03-187 September 18, 2003 Page No. 5 APPROACH The slide has apparently not moved since 1967; however, in our opinion there is insufficient information to conclude wether the slide has been stabilized to an extent that would relieve the City of future liability, and there certainly is not enough information to evaluate whether the two lots could be developed in their present condition. Soil Engineering Construction stated that they have used Shannon and Wilson's information to calculate a safety factor of 1.1 with water buildup in the slope. At present, there is conflicting information regarding the effectiveness of the drainage gallery and the possible buildup of water in the slope, and no information regarding groundwater conditions within the old slide area. We are therefore proposing an investigation that would evaluate the present-day stability of the old slide. This would be accomplished by drilling three additional borings, with two of the borings downgradient of the drainage gallery being converted into new piezometers. We would also attempt to gain additional groundwater information (and additional monitoring capability) by trying to re- develop the two existing piezometers (Nos. 4 and 5) installed in 1966. These tasks would be intended to 1) investigate the effectiveness of the existing drainage gallery and the resulting current soil and groundwater conditions within and uphill of the slide area, and 2) investigate the presence of a "slide plane" uphill of the slide area that could threaten Nob Hill Road and the adjacent residences. Based on presently available info,~,ation, it is our opinion that the collection of soil and groundwater information would require three borings to depths varying from 25 to 50 feet, the installation of two potentially permanent piezometers (with locking covers to prevent vandalism), and laboratory strength testing of collected soil samples. Installation of the two new piezometers on City-owned property can be performed under one permit; however, the application will require a written work plan to describe the work to be accomplished and methods proposed. Also, because the borings would be attempting to find potential slide planes, particularly on the west side of the street, the conventional sampling interval will be substantially deceased, to nearly continuous sampling, within the zone expected to contain a slide plane.