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onto the end of the pile, Drop hammers, however, must be used with care because they can damage wood piles, A much less desirable but frequently used method of inserting piles into sandy soil is "jet- ting," Jetting involves passing a high pressure stream of water through a pipe advanced alongside the pile, The water blows a hole in the sand into which the pile is continuously pushed or dropped until the required depth is reached, Sand is then tamped into the cavity around the pile and the end of the pile pounded with a sledge hammer or other available weight. Unfortunately, jetting loosens the soil around the pile, Decreased friction due to loose soil results in a lower load capacity, To increase the load capacity, jetted piles must be inserted deeper into the ground than driven piles,_____ ____ _ ____ _ _ _ . _ If the soil content has sufficient clay or silt (has a higher shear strength), a hole can be excavated oy an augenlrdtl'i!!rlTleans anctwill slay openin-- order to drop in a pile, Sand or pea gravel is then poured and tamped into the cavity around the pile, This technique does not provide as much load resistance as driving the pile into the ground, and longer piles are necessary, Some final driving with a sledge hammer can be helpful. As with post' foundations, piles are often partly backfilled with concrete to improve thel~ r~sistance to iaterai ioads. As shown in Figuie 3.2~1...t~e aiea at grade around each pile can be replaced by a concrete collar extending several feet below grade. , , , , , , , , , , , . , , , , , -~~.._.._..:~., , , , , , , , . , , , , , : ____ Relnforced .:.---- Concrete : Collar , , , , .' . . , , , , , :.... '~. .0.,:.'. ~ ' -._.1 Figure 3.20 Pile with Concrete Collar Such collars provide protection from minor ero- sion, add more deadweight to the structure, and increase the puJI-out resistance of the pile, The necessary pile size and depth of pile embedment depends on the number of piles used, size and weight of the manufactured home, wind and flood loads, soil bearing capacity, and poten- tial for scour and erosion, Calculations and sug- gested construction criteria are presented for piles in Chapter IV, Horizontal Beams-In a post or pile elevated foundation system, horizontal beams must be pro' vided to support the manufactured home, As shown in Figure 3,21. these horizontal members tie opposing posts or piles together and distribute any flood, wind, or roof live loads to those vertical members and into the ground, Figure 3.21 Horizontal Members Although steel channel sections could be used for this foundation component, sawn timber beams are considered preferable based on their availabil- ity, Chapter IV presents details on the required number and size of these horizontal beams to carry various imposed loads, Bracing Elevated Foundations-The vertical and horizontal members of an elevated founda- tion must be braced, as shown in Figure 3,22, to resist flood and wind induced loads, Even in areas where low loading is anticipated, bracing will pro- vide added assurance that the elevated founda- tion will withstand the impact of floating debris or excessive flood or wind forces, Although bracing placed underneath a manufac- tured home may be struck by floating debris or subject to other flood forces, the negative effects of these on a structure's survivability are generally outweighed by the beneficial effects of the bracing, Knee bracing, shown in Figure 3,23, is effec- tive in increasing lateral strength of posts or piles to resist all loads, 26