2621 ROCKEFELLER AVE Geotech Report 2022-10-25

BELOW-GRADE WALLS AND RETAINING WALLS The following recommendations should be used for the design of below-grade walls that are intended to act as retaining walls and for other retaining structures that are used to achieve grade changes. Design Parameters Lateral earth pressures for design of below-grade walls and retaining structures should be evaluated using an equivalent fluid density of 35 pcf provided that the walls will not be restrained against rotation when backfill is placed. If the walls will be restrained from rotation,we recommend using an equivalent fluid density of 55 pcf. Walls are assumed to be restrained if top movement during backfilling is less than H/1000, where H is the wall height. These lateral soil pressures assume that the ground surface behind the wall is horizontal. For unrestrained walls with backfill sloping up at 2H:1V, the design lateral earth pressure should be increased to 55 pcf,while restrained walls with a 2H:1 V sloping backfill should be designed using an equivalent fluid density of 75 pcf: These lateral soil pressures do not include the effects of surcharges such as floor loads, traffic loads or other surface loading. Surcharge effects should be included as appropriate. Figure 5 illustrates recommended surcharge pressure equations based on the loading configuration. Below-grade walls for buildings should also include seismic earth pressures. Seismic earth pressures should be determined using a rectangular distribution of 8H in psf,where H is the wall height. 11 If vehicles can approach the tops of exterior walls to within 1/2 the height of the wall,a traffic surcharge should be added to the wall pressure. For car parking areas, the traffic surcharge can be approximated by the equivalent weight of an additional 1 foot of soil backfill (about 125 psf)behind the wall. Truck 11 traffic can be approximated by the equivalent weight of an additional 2 feet of soil backfill (250 psf) behind the wall. These recommendations are based on the assumption that any retaining walls at this project will be provided with adequate drainage. Positive drainage should be provided behind below-grade walls and retaining structures as discussed below. The values for soil bearing, frictional resistance and passive resistance presented above for foundation design are applicable to retaining wall design. Walls located in level ground areas should be founded at a depth of 18 inches below the adjacent grade. 1 Wall Drainage To reduce the potential for hydrostatic water pressure buildup behind the retaining walls, we 11 recommend that the walls be backfilled with a zone of free draining material with perforated pipes to discharge the collected water. Figure 3 summarizes our recommendations for wall drainage and backfill. 11 The zone of wall drainage material should be 2 feet wide and should extend from the base of the wall to within 2 feet of the ground surface. The wall drainage material should be covered with 2 feet of less permeable material,such as the on-site glacial till that is properly moisture conditioned and compacted. A 4-inch-diameter perforated drainpipe should be installed within the free-draining material at the base of each wall. We recommend against using flexible tubing for the wall drainpipe. The footing drain recommended above can be incorporated into the bottom of the drainage zone and used for this purpose. File No.12547-001-00 Page 12 February 3,2006 GEOENGINEERS