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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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

There are mainly two reasons to account 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:
(i) Improve resistance to corrosion of main bars;
(ii) Provide additional restraint against lateral buckling;
(iii) Improve the grout quality by preventing intrusion of groundwater during concreting;
(iv) Prevent occurrence of necking during lifting up of casings during concreting.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In designing min-piles, there are two approaches available:

(i) In the first approach, the axial resistance provided by the grout is neglected and steel bars take up the design loads only. This approach is a conservative one which leads to the use of high strength bars e.g. Dywidag bar. One should note that bending moment is not designed to be taken up by min-piles because of its slender geometry.

(ii) In the second approach, it involves loads to be taken up by both grout and steel bars together. In this way, strain compatibility requirement of grout and steel has to be satisfied.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In designing the axial capacity of mini-piles, grout may be taken into account in the contribution of axial load capacity. However, the total load capacity of min-piles may not be equivalent to the sum of individual capacity derived from grout and from steel H-section. The reason behind this is that the vertical loads on mini-piles are shared among grout and steel sections based on their Young’s modulus and areas. Basically, in order to comply with strain compatibility criterion, the steel bars and grout will deform as a whole though they possess different stiffness. A case may occur in which the sharing of loads for grout may be too high which cracks the grout section and fails the mini-piles already before the whole pile section could attain the full design load which is assumed to be the sum of individual capacities. Hence, strain compatibility has to be checked in designing the vertical capacity of min-piles.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The design of mini-piles somehow differs from other traditional pile types. For instance, the design of most common pile types is controlled by the external carrying capacity. However, owing to the small cross sectional area, the design of mini-piles is limited by internal carrying capacity. Hence, the choice of suitable load carrying element is of paramount importance in the design of mini-piles.

For steel pipes used as load carrying element, it is of circular cross section with a high radius of gyration. Moreover, it possesses a constant section of modulus in all directions, it serves the properties of excellent column. Bars are suitable when pure axial loading is required in confined situation.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.