Piles and Foundation Informations

Piles and Foundation




1. It is not necessary to design nominal reinforcement to piles. Is it true?
In BS8110 and BS5400 Pt.4, they require the provision of nominal reinforcement for
columns. However, for pile design the requirement of nominal reinforcement may not be
necessary. Firstly, as piles are located underground, the occurrence of unexpected loads to
piles is seldom. Secondly, shear failure of piles is considered not critical to the structure
due to severe collision. Moreover, the failure of piles by buckling due to fire is unlikely
because fire is rarely ignited underground.
However, the suggestion of provision of nominal reinforcement to cater for seismic effect
may be justified. Reference is made to J P Tyson (1995).
2. How do rock sockets take up loads?
The load transfer mechanism is summarized as follows:
When a socketed foundation is loaded, the resistance is provided by both rock socket wall
and the socket base and the load distribution is a function of relative stiffness of foundation
concrete and rock mass, socket geometry, socket roughness and strength. At small
displacements the rock-socket system behaves in an elastic manner and the load
distribution between socket wall and socket end can be obtained from elastic analysis. At
displacements beyond 10-15mm, relative displacement occurs between rock and
foundation and the socket bond begins to fail. This results in reduction of loads in
rock-socket interface and more loads are transferred to the socket end. At further
displacements, the interface strength drops to a residual value with total rupture of bond
and more loads are then distributed to the socket end.
3. In designing mini-piles, should the strength of grout be neglected during assessment
of loading carrying capacity?
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.
4. 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 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:
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
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(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.
5. 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.
6. In designing the lateral resistance of piles, should engineers only use the earth
pressure against pile caps only?
In some design lateral loads are assumed to be resisted by earth pressure exerted against the
side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do
contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance
of piles, earth pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs are adopted with
modulus of reaction kept constant or varying with depth. The normal practice of using a
constant modulus of reaction for soils is incorrect because it overestimates the maximum
reaction force and underestimates the maximum bending moment. To obtain the profile of
modulus of subgrade reaction, pressuremeter tests shall be conducted in boreholes in site
investigation. Reference is made to Bryan Leach (1980).