3400 RIVERFRONT BLVD Geotech Report 2022-10-25

• • • • 6.11.3 Surface Drainage • The finished ground surface around the buildings should be sloped away from foundations at a minimum 2 percent gradient for a distance of at least 5 feet. Downspouts or roof scuppers • should discharge to a solid pipe that carries the collected water to an appropriate stormwater • system. • 6.11.4 Subsurface Drainage for Footings and Walls • In our opinion, perimeter footing drains are not required for this project because the proposed • foundations will be well above the groundwater level. If the design changes and perimeter • footings extend below the groundwater level, then it might be necessary to include perimeter • drains. The discharge for the retaining wall drains should be tied into the stormwater drainage system. • • 6.12 FOUNDATIONS • Based on the results of our explorations and analysis, the proposed structures can be supported • on mat foundations. There are varying amounts of wood debris in the undocumented fill layer, which could result in excessive settlement if not mitigated through the use of mat foundations. • Explorations indicate that the wood deposits are up to 4.5 feet thick and consist of wood • shavings, logs, and dimensional lumber. Based on the explorations that have been performed at • the site, we have assumed that the pockets of wood debris will be less than 15 feet wide. This assumption should be verified before construction by completing additional explorations at each • building location. • • Our analysis indicates that lateral spreading will likely occur during a design-level earthquake • that will impact the row of townhomes along the eastern edge of the site that are closest to the riverbank. Although not adopted by the 2012 IBC, the recently released NEHRP 2015 (FEMA, • 2015) seismic provisions for new buildings indicate that spread footings can be tied together or • mat foundations can be used to mitigate up to 18 inches of lateral spreading for Risk Category I • and II structures. Supporting the proposed structures on mat foundations will mitigate lateral • spreading in addition to mitigating settlement caused by organic zones. All buildings and structures located with a horizontal distance of approximately 150 feet of the top of the slope • should be supported on mat foundations to mitigate lateral spreading. • • Mat foundations should be designed using a soil subgrade modulus of 100 psi per inch. The • mat foundation should also be designed to span zones of wood debris up to 15 feet wide. The bottom of the mat foundation should be at least 18 inches below the lowest adjacent final grade. IP • 6.12.1 Foundation Settlement For foundations designed in accordance with the recommendations provided above, the total post-construction settlement is anticipated to be less than 2 inches and the differential • settlement across the length of a typical residential townhome of approximately 100 feet is • anticipated to be 1 inch or less. Our settlement analysis for the site is discussed in further detail • in the "Long-Term Settlement Caused by Filling" section of this report. • • • • G EODESIGN? 19 Polygon-128-01:09161 5