The method of analysis discussed earlier gives reliable results provided the width of the braced cut is larger than the depth of the excavation and that the braced cut is very long. In the cases where the braced cuts are rectangular, square or circular in plan or the depth of excavation exceeds the width of the cut, the following analysis should be used.

In this analysis the braced cut is visualized as a deep footing whose depth and horizontal dimensions are identical to those at the bottom of the braced cut. This deep footing would fail in an identical manner to the bottom braced cut failed by heave. The theory of Skempton for computing Nc (bearing capacity factor) for different shapes of footing is made use of. Figure 20.32 gives values of Nc as a function of H/B for long, circular or square footings. For rectangular footings, the value of Nc may be computed by the expression

Figure 20.32 Stability of bottom excavation (after Bjerrum and Eide, 1956)

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