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7.B. - Page 72 2 Environmental Analysis <br /> implement one of three options to stabilize the subgrade soils for the joint-use path profile that is <br /> above an elevation of 7 feet. The first option is lime treatment,which reduces the expansion of soils <br /> by chemically reacting with soils containing clay minerals (CE&G,2013). The second option the City <br /> could implement is encapsulation of the subgrade soils,which would reduce moisture-content <br /> changes within the soil (CE&G,2013). The third option the City could implement is to incorporate <br /> tensile elements into the structural section through use of a biaxial geogrid,which would reduce <br /> expansion and contraction of the subgrade soils. <br /> For the joint-use path profile that would be below seven feet elevation with wet and very saturated <br /> soils,the City would create a compacted crushed rock foundation and then drill the micropiles <br /> through the compacted rock to provide an adequate foundation for the joint-use path profile. This <br /> would reduce the uplifting buoyant forces and settlement of the soil. Therefore, the implementation <br /> these measures would reduce the project impacts from expansive soils to a less than significant level. <br /> Mitigation Measure GEO 1: Geotechnical Report Recommendations <br /> Cal Engineering and Geology has prepared a geotechnical report containing site-specific <br /> geotechnical recommendations for the proposed US 101 undercrossing project. The design <br /> team shall implement the following key recommendations into the final design of the project: <br /> • Joint-use path profile above an elevation of 7 feet: Stabilize the expansive subgrade soils <br /> for the portions of the joint-use path profile above an elevation of 7 feet using one of <br /> three options:lime treatment of the subgrade, encapsulating the subgrade material with an <br /> impervious membrane or incorporating a geogrid reinforcement into the structural <br /> section and using a nominally reinforced aggregate base layer. <br /> • Joint-use path profile below an elevation of 7 feet: Create a working platform by over- <br /> excavating the subgrade soil a minimum of 12 inches and placing a layer of woven <br /> geotextile fabric on top of the over-excavated subgrade and a layer of biaxial geo-grid <br /> placed directly on top of the fabric. A minimum of 12 inches of 3/4 inch compacted <br /> crushed rock shall be placed on top of the geo-grid. <br /> • The structural concrete slab foundation shall be supported using micropiles. The <br /> micropiles shall be advanced to a tip elevation between-15 to -45 feet. The final pile <br /> diameters and tip elevations shall be determined by a structural engineer in conformance <br /> with the Federal Highway Administration's Micropile Design and Construction Guidelines <br /> Manual(August 2000). <br /> • The retaining wall adjacent to the north abutment will be supported by tieback anchors <br /> that should be installed between the existing abutment piles. Install tieback anchors at a <br /> 10-degree inclination below horizontal with a minimum unbounded length of 10 feet with <br /> permanent Class I corrosion protection. The tieback wall shall be in conformance with <br /> the FHWA's Geotechnical Engineering Circular No.4,titled Ground Anchors and Anchored <br /> Systems (June 1999). <br /> 2-32 US 101 Pedestrian Undercrossing and Bair <br /> Island Road Storm Drain Pump Station Project <br />