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58 7.2.2 On Stilts Sometimes when a structure is to be elevated, th~re are reasons why it should not or cannot be raised on fill. In some places, adequate fill material may not be available near the site. Also, as was mentioned, fill cannot be placed where it would encroach onto the floodway, A third reason for not elevating on fill relates to function. The space below an elevated structure may be used for some purpose such as parking, Elevating on posts, piles, piers, or walls may be considered if one of the conditions above exists, The most important factors when designing these kinds of supports are superstructure loads, hyd- rodynamic loads, and impact loads as explained in Chapter VI. The structure should be raised high enough that water does not reach the lowest floor level. Buoyancy, then, becomes a less significant factor. 7.2.2.1 Posts and Piles Both posts and piles consist of long slender columns made of wood, steel or concrete. The round, square, or rectangular section is the most common of many available shapes. Posts are set in pred- rilled or predug holes, and then a backfill is placed. Using concrete for a portion or all of the backfill will increase stability. Pile supports differ in that th~y are mechanically driven into the ground. Ver- tical loads are supported by friction between the pile surface and the soil, and also by the end bear- ing capacity. The depth of embedment for posts and piles is determined by soil conditions, anticipated loading and scour, and th~ spacing and size of the memb~rs. Wood posts are generally embedded deep enough to establish required bearing forces. Bearing forces depend on several soil characteristics and determination should be done using standard soil engineering procedures. Jt- L ~ \GRADE-Z- - -/' .. I' _1.... \ JI'J ~ - %... , rA' I.";' ,\ 1\ :", "- ...L.. ; /,J, \f 'II\)' r~I1..:..:~X': _~/PILECAP 3~1-I-T-- _If..! f--); //'. i ~ ,I t I 1..3L- T fr-/ ~;r L-t~_L~~/ I!~ l;-:-...J b,;: I PILES I I .<A.J WOOD POST FOUNDATION BEARING AREA Figure 7,2,Concrete pad to increase bearing area, (Reference 7) Figure 7,3,Post set on piles for very poor soils. (Reference 7)