Lecture13-Semi-Deep-Foundations

Lecture13-Semi-Deep-Foundations - Foundation Engineering...

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Unformatted text preview: Foundation Engineering Foundation Engineering Lecture #13 Lecture #13 Semi-Deep Foundations- Pit footings- Pedestal footings- Pin piles footings- Grade beams L. Prieto-Portar 2009 At some project sites, the depth of a suitable bearing stratum is neither sufficiently shallow for conventional spread footings, nor deep enough to warrant the expense of deep foundations. This transition depth is around 3 to 6 m from the surface. In this range, the designer may want to consider a " semi-deep " foundation. Pit Foundations. Pit foundations (Figure-1) are economical when it is possible to excavate without using sheetpiling to depths D f of 3 - 4 meter. This limitation is due to the reach of a backhoe arm, and for the easy access to the bottom of the pit. The size of the pit, B x L is dimensioned to carry the ormal load plus the now largely increased footing weight and the backfill normal load N , plus the now largely increased footing weight W f , and the backfill W s . A reduction in the cost of structural concrete can be effected by placing the footing on top of a block of a low-strength concrete, as shown in Figure-2. This block acts as an improved foundation support. The footing dimension B x L is calculated based on a low-strength concrete allowable stress, rather than on the soil allowable bearing qall . The block dimensions B1 x L1 are determined from the allowable bearing capacity qall of the soil at Df (Figure-3). If the column load N is not centered, or if the column transfers a moment M , the resulting lateral earth pressures must be included in the analysis. Figure-1. A pit foundation placed at depth Df (3 to 6 meter). Figure-2. An alternative pit foundation has a perched footing, built upon a low-strength concrete pour of depth h 1 . Figure 3. When the column normal load N is not centered, or when it transfers a moment M to the pit foundation, the surrounding soil is subjected to lateral passive pressure. Pedestal Foundations.Pedestal Foundations....
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Lecture13-Semi-Deep-Foundations - Foundation Engineering...

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