Figure 17.10(a) shows a single drilled pier of diameter d, and length L constructed in a homogeneous mass of soil of known physical properties. If this pier is loaded to failure under an ultimate load Qu, a part of this load is transmitted to the soil along the length of the pier and the balance is transmitted to the pier base. The load transmitted to the soil along the pier is called the ultimate friction load or skin load, Qf and that transmitted to the base is the ultimate base or point load Qb. The total ultimate load, Qu, is expressed as (neglecting the weight of the pier)

If the pier is instrumented, the load distribution along the pier can be determined at different stages of loading.

Typical load distribution curves plotted along a pier are shown in Fig 17.10(b) (O'Neill and Reese, 1999).

These load distribution curves are similar to the one shown in Fig. 15.5(b). Since the load transfer mechanism for a pier is the same as that for a pile, no further discussion on this is necessary here. However, it is necessary to study in this context the effect of settlement on the mobilization of side shear and base resistance of a pier. As may be seen from Fig. 17.11, the maximum values of base and side resistance are not mobilized at the same value of displacement. In some soils, and especially in some brittle rocks, the side shear may develop fully at a small value of displacement and then decrease with further displacement while the base resistance is still being mobilized (O'Neill and Reese, 1999). If the value of the side resistance at point A is added to the value of the base resistance at point B, the total resistance shown at level D is overpredicted. On the other hand, if the designer wants to take advantage primarily of the base resistance, the side resistance at point C should be added to the base resistance at point B to evaluate Qu. Otherwise, the designer may wish to design for the side resistance at point A and disregard the base resistance entirely.

Figure 17.10 Typical set of load distribution curves (O'Neill and Reese, 1999)

Figure 17.11 Condition in which (Qb  + Qf) is not equal to actual ultimate resistance

Figure 15.5 (b) general shear failure in the strong lower soil

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