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8 I 777X Static Test Building <br /> Retaining Wall and Structure Drainage <br /> Lateral earth pressures recommended in this section do not consider hydrostatic conditions. <br /> Therefore,we recommend providing drainage behind walls. To prevent lateral water pressure buildup <br /> against the wall, either a free-draining granular material should be used within an 18-inch-wide zone <br /> immediately behind retaining walls, or a fabric drain panel should be provided against the back of the <br /> wall. The drainage system should include weep holes through the face of the wall or a drain pipe at <br /> the base of the wall. The drain pipe should be at least 4 inches in diameter. Any collected drainage <br /> should be directed at a minimum 1 percent grade to an appropriate outlet location. <br /> Micropile Design <br /> We understand that the anticipated 777X static testing equipment and portions of the testing facility <br /> building will require significant tension, or uplift, resistance that cannot be met with shallow <br /> foundations. Testing loads on the order of 300 to 780 kips in tension are expected. We understand <br /> that the preferred foundation option for these heavily loaded areas are individual or groups <br /> of micropiles. <br /> Micropiles consist of small-diameter(usually 6 to 12 inches)drilled and grouted replacement <br /> (non-displacement) piles that are typically reinforced. A micropile is installed by drilling a borehole, <br /> placing reinforcement, and grouting from the bottom up. Often the micropiles are partially cased <br /> during construction,with casing sometimes left in place to increase the micropile strength. A drilling <br /> method suited to local conditions may be selected from a number of options. For example, rotary <br /> percussive or rotary duplex techniques may be used to penetrate obstructions. <br /> Because of its small diameter,the end-bearing resistance of a micropile is minor compared with the <br /> grout-to-ground bond resistance along its shaft and is typically neglected. The soil conditions and <br /> installation procedure strongly influence the grout-to-ground bond strength. In general, micropiles are <br /> classified into four types (A to D)depending on the construction classification details (FHWA 2005). <br /> For this analysis,we assumed that the micropiles will be constructed using pressure grouting through <br /> the casing as the casing is withdrawn slowly and incrementally,which corresponds to FHWA Type-B. <br /> Alternatively,to achieve additional grout-to-ground bond strength secondary pressure grouting may <br /> be performed,which corresponds to FHWA Type-C. The method used on the pre-construction <br /> verification tests must also be used for the production piles. <br /> Vertical Capacity of Micropiles <br /> We understand that vertical 8-inch-diameter micropiles will be used to support axial tension loads. <br /> Micropile capacity is largely a function of the means and methods of installation selected by the <br /> contractor. The contractor must choose appropriate means and methods to achieve the design bond <br /> stress based on experience on similar sites. We recommend the following for micropile design. <br /> ■ Contract the micropiles as design-build to allow the contractor to optimize the installation <br /> method. Prospective contractors may be required to provide information about their proposed <br /> methods of drilling and grouting to evaluate their applicability and suitability to site conditions. <br /> ss <br /> 19106-01 ALAT <br /> May 29,2015 NOON/,WE <br />