Agmt09 Moffatt & NIchol - Laserfiche WebLink

Browse Search

" ',f Moffatt & Nichol Redwood Shores, Redwood City LEVEE CERTIFICATION 8585.000.000 June 11, 2009 Page 2 testing of varying quality and limited consolidation infonnation. Based on our review, the historic site usage and numerous borings highlight a general homogeneity within the subsurface stratigraphy. It is our opinion that the previous borings include enough stratigraphic information to establish general analysis cross sections. However, it is important to note that few of the borings were drilled in areas where the levees appear to have been constructed over previous natural drainage channels. Both the shear strength and settlement potential of the types of foundation soil generally anticipated at the project site are dictated by the stress state of the soil. Since we had limited recent strength and consolidation data for the site soil (the majority of these data are several decades old), we used very general assumptions regarding these two critical parameters in order to perform our analyses. As discussed in Section 3.8 of Reference 1: "Our conclusions and recommendations were based on historical research on the site development history and previous explorations and lab testing performed by others. Based on these previous studies, we have made estimations of the strength profiles and stress histories of the underlying soil deposits. Shearing strengths of soft ground with fill placed over it depend highly on the stress history of the foundation. In order to certify these levees, it would be necessary to confinn these estimates of strength profiles are correct. Accordingly, additional subsurface investigation is recommended as part of the final levee certification." As you know, certification submittals are subject to review by FEMA or their designated technical experts. In our experience, in-depth clarifying questions by reviewers are typical for the certification process and will likely include such issues as the adequacy of prior exploration. We opine that from a geotechnical perspective it would be prudent to perform an additional 4 to 6 borings with sampling for laboratory consolidation and shear strength testing as well as associated in.situ vane shear testing along the previous study area. The additional information would be used to: a. Confirm the shear strength model and consolidation information used to perform analyses or allow for modification of the associated analyses, and b. Evaluate subsurface conditions in some of the areas where historic data indicate levees were constructed over previous channels. We believe that the approach to the project to this point has been practical and appropriate to the data available; however, we also believe that gathering limited additional data would be prudent in order for a Registered Engineer to take on the responsibility associated with certifying the geotechnical aspects of a flood protection levee. With this additional information, provided it confirms that our strength and settlement models are appropriate, we would have confidence in certifying the subject levees after observing and testing construction conformance of proposed levee raises.