Piles and Foundation: Vincent Chu Answers

What is the purpose of post-grouting for mini-piles?

Post-grouting is normally carried out some time when grout of the initial grouting work has set (e.g. within 24 hours of initial grouting).

It helps to increase the bearing capacity of mini-piles by enhancing larger effective pile diameter.

Moreover, it improves the behaviour of soils adjacent to grouted piles and minimizes the effect of disturbance caused during construction.

In essence, post-grouting helps to improve the bond between soils and grout, thereby enhancing better skin friction between them.

During the process of post-grouting, a tube with a hole at its bottom is lowered into the pile and grout is injected.

The mechanism of post-grouting is as follows:

The pressurized grout is initially confined by the hardened grout and can hardly get away.

Then, it ruptures the grout cover and makes its way to the surrounding soils and into soft regions to develop an interlock with harder soil zones.

In order to enhance the pressurized grout to rupture the initial grout depth, a maximum time limit is normally imposed between the time of initial grouting and time of post-grouting to avoid the development of high strength of initial grout.

Consequently, the effect of soil disturbance by installation of casings and subsequent lifting up of casings would be lessened significantly.

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