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MEMORANDUM May 18, 2018 Review of Geotechnical Study Project No. 1-1089 Proposed Main Street Apartments Page 5 of 7 353 Main Street, Redwood City, California • Low Elevation and Tide Effects and Flooding The site sits on old mudflat 1 slough 1 tidal estuary terrain that interconnects with the Bay. From Google Earth, the elevation is under 10 -ft, possibly as low as 5 -ft above MSL. The portion of Redwood Creek proximal to the Project appears to be well within a zone that may be tidally affected. Not only does this expose the Project to potential adverse groundwater and related effects as already discussed herein, it also creates issues with respect to flooding hazards that may not be reflected 1 represented in the FEMA Flood Risk Maps relied on by the Project consultants and the City. There may be a significant environmental impact from flooding due to the tidal influences that create Monte Carlo type effects that are understood not to be a consideration by FEMA. An example of this Monte Carlo effect is the coincidental timing of heavy 1 exceptional rainfall with astronomically high (i.e., "King Tides") tide levels. A study should be conducted to confirm that the combination of sea level rise, king tides, storm surges, and rainfall will not result in significant water level increases in redwood Creek that could then flood the Project site. Also, we saw no analysis in the referenced materials that discusses possible tsunami impacts at the edge of the bay and/or seiche conditions in the bay that could result from a significant earthquake. • Constructability — Underlying Bay Muds, DDP, Import? Reference Nos. 1 and 2 state that the site is underlain by undocumented fills and underlying bay muds. These materials, as identified by the consultant, are expected to have poor engineering performance with respect to foundation support. The current plans for development of the Project, as indicated in Reference No. 2, indicate that these fill materials are to essentially remain in-place. The consultant further stated that the Project will be constructed on a structural slab to be supported by Drilled Displacement Piles (DDP) method. This method is intended to both reinforce and densify the surrounding soils. It is our professional opinion that the onsite soils would not likely result in densification as a result of the DDP method recommended. One cannot come to a conclusion whether the soils at the Project site are appropriate for the DDP method, without meaningful analyses and discussion regarding the soil type as it relates to its ability to include densification as well as provide proper mitigation for liquefaction 1 lateral spread, and static settlement potential. The noted undocumented fills in Reference Nos. 1 and 2 have unknown origin and pedigree. The underlying bay muds are soft and have low to very low densities. Over - excavation may be influenced by soft ground and potentially high groundwater. This is an important consideration in terms of achieving a suitable bottom for fill placement that should be addressed. Please refer above to our discussion of the interconnectedness of on-site water conditions and the adjacent creek and other groundwater. The DDP method may intersect and incidentally interconnect multiple groundwater horizons that were previously isolated from 350 Fischer Ave. Front • Costa Mesa, CA 92626 • P: 714 668 5600 • www.G3SoilWorks.com