Page 322 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 20 Allowable Bearing Pressure for Shallow Foundations 279

The allowable vertical end bearing capacity of a pile is as shown next (neglecting the
side shearing resisting and overburden pressure of soil cover).

Qa ¼ Ap qa

¼ pð0:6Þ2 Â 280 ¼ 79 tonnes
4

The initial test pile should be tested up to two-and-a-half times the estimated load or up
to failure load. The safe load on a pile will be the least of the following:
1. Fifty percent of the load at 12 mm vertical settlement or the load corresponding to

the 6 mm vertical settlement of the rock mass
2. One-third of the ultimate failure load

The two initial test piles should also be tested up to failure by the lateral load. The
lateral safe load should be the least of following:
1. Fifty percent of the final lateral load, which corresponds to 8 mm total lateral

displacement
2. Final load, which corresponds to 4 mm total lateral displacement
A precaution should be taken to grout the slush of broken rocks first at the bottom of the
borehole in the rock mass before concreting the piles.

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