Piles and Foundation: Vincent Chu Answers

What are the considerations in determining whether casings should be left in for mini-piles?

Contrary to most of pile design, the design of min-piles is controlled by internal capacity instead of external carrying capacity due to their small cross-sectional area.

There are mainly two reasons for designing mini-piles as friction piles:

(i) Due to its high slenderness ratio, a pile of 200mm diameter with 5m long has a shaft area of 100 times greater than cross-sectional area.

Therefore, the shaft friction mobilized should be greater than end resistance.

(ii) Settlements of 10%-20% of pile diameter are necessary to mobilize full end bearing capacity, compared with 0.5%-1% of pile diameter to develop maximum shaft resistance.

Left-in casings for mini-piles have the following advantages:

    • Improve resistance to corrosion of main bars;
    • Provide additional restraint against lateral buckling;
    • Improve the grout quality by preventing intrusion of groundwater during concreting;
    • Prevent occurrence of necking during lifting up of casings during concreting.