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June 13,2023 <br />HWA Project No. 2022-188-21 <br />The largest amount of differential settlement between foundations is likely to occur within the <br />transfer station building at the location of the wall west of the tunnel drive. Design of footings in <br />this area should take into consideration the abrupt change in settlement that could occur between <br />the foundations supported near the existing ground surface (on the east side) and those supported <br />within the fill pad placed to raise site grades up to the base of the tipping floor slab. Methods to <br />consider would be to structurally isolate foundations between the west and east sides of the wall <br />such that they can settle independent of each other. Alternatively, foundations could be extended <br />down to the same elevation and stem walls extended to the ground surface to limit potential for <br />differential settlement between spread footings. <br />4.5 Fr,oon Sr,a.ns <br />4.5.1 General <br />Grade-supported slabs may be employed for the various facilities, including floor slabs within <br />the buildings and the transfer station tipping floor. For floor slabs, we recommend the <br />underlying fine-grain materials be over-excavated and replaced with structural fill, as <br />recommended for spread footings. Subgrade preparation and structural fill placement for floor <br />slabs should follow the recommendations in Sections 4.12.1 and 4.12.2' <br />Grade-supported floor slabs for the various facilities should be placed on at least 6 inches of <br />freely draining, uniformly graded, crushed rock or washed pea gravel, which will serve as a <br />capillary break. This capillary break material should have a maximum particle size of 5/8-inch, <br />and less than l0o/o (by weight) should pass the #20 Standard U.S. sieve. A 10-mi1 (minimum) <br />thick plastic waterproofing membrane should be placed immediately over the crushed rock as a <br />vapor barrier. Placement of the vapor barrier should be undertaken with care and construction <br />activity on the membrane should be limited after placement to limit perforations in the <br />membrane. Floor slabs supported on a properly prepared subgrade may be designed on the basis <br />of a modulus of vertical subgrade reaction value of 120 pounds per cubic inch. <br />4.5.2 Settlement Considerations for Tipping Floor Slab <br />The tipping floor slab within the transfer station building will be supported by a thick structural <br />fill pad; however, it will butt up against the tunnel drive wall at the east side of the building. The <br />wall along the east side of the tipping floor will be founded up to 20 feet lower than the floor slab <br />and differential settlement between the tipping floor slab and the top of the tunnel drive wall <br />could be up to about a 1 inch; however, we anticipate that much of this settlement will occur as <br />the fill is placed. Future settlement after the slab is cast is expected to be small such that large <br />cracks do not form in the slab, provided drainage is directed away from the foundation of the <br />slab, <br />2022-188-21 KCTS - Final Geotechnical Report 11 HWA GEOSCIENCES INC.