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To determine when suitable bearing has been achieved and to verify proper rock placement, <br />the geotechnical engineer/engineering geologist must be present on a full-time basis during <br />footing trench excavation and backfill. Any ground water seepage entering the excavation <br />must be removed prior to commencing placement of rock fill. <br />After the bearing stratum has been reached, the trench should be immediately backfilled. We <br />recommend the use of "railroad ballast" or 2- to 4-inch-size crushed rock for backfill. The <br />crushed rock must be tamped into place to achieve a tightly packed mass; this may be done <br />with either a IHoepac"-type compactor mounted on the backhoe, or more typically, with the <br />bucket of the backhoe itself. Staging areas should be maintained so that the rock is not <br />contaminated by mud prior to placement in the trench. Equipment access to trench locations <br />should also be maintained. <br />Anticipated settlement of footings founded on suitable native sediments or approved rock <br />trenches should be on the order of 1 inch or less. However, disturbed material not removed <br />from footing trenches prior to footing placement could result in increased settlements. All <br />footing areas should be inspected by AESI prior to placing concrete to verify that the rock <br />trenches are undisturbed and construction conforms with the recommendations contained in this <br />letter -report. Such inspections may be required by the governing municipality. <br />It should be noted that the area bounded by lines extending downward at 1HAV from any <br />footing must not intersect another footing or intersect a filled area that has not been compacted <br />to at least 90 percent of ASTM:D 1557. In addition, a 1.511:1V line extending down and away <br />from any footing must not daylight because sloughing or raveling may eventually undermine <br />the footing. Thus, footings should not be placed near the edge of steps or cuts in the bearing <br />soils. <br />Based on subsurface conditions observed in our explorations, the soils on -site are consistent <br />with Site Class "C" (Table 1615.1.1) of the International Building Code (IBC), 2003, Section <br />1615. information presented in iBC Figure 1615(1) indicates a napped spectral acceleration <br />for short periods of Ss = 1.19. Information presented in IBC Figure 1615(2) indicates a <br />mapped spectral acceleration for a 1-second period of Si = 0.42. <br />Backfilled Walls <br />All backfill behind building walls or around foundation units should be placed as per our <br />recommendations for structural fill and as described in this letter -report. Horizontally <br />backfilled walls, which are free to yield laterally at least 0.1 percent of their height, may be <br />designed using a lateral pressure represented by an equivalent fluid equal to 35 pounds per <br />cubic foot (pet). Fully restrained, horizontally backfilled, rigid walls that cannot yield should <br />be designed for an equivalent fluid of 55 pcf. Walls with sloping backfill are not expected, but <br />if they are needed, we should be allowed to offer situation -specific surcharge <br />recommendations. If parking areas are adjacent to walls, a surcharge equivalent to 2 feet of <br />soil should be added to the wall height in determining lateral design forces. <br />7 <br />