Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
Table 9.1 Advantages and Disadvantages of Machine-dug Piles
Advantages
i. No risk of ground heave induced by pile
driving.
ii. Length can be readily varied.
ii. Spoil can be inspected and compared with
site investigation data.
v. Structural capacity is not dependent on
handling or driving conditions.
v. Can be installed with less noise and vibration
compared to displacement piles.
vi. Can be installed to great depths.
vii. Can readily overcome underground
obstructions at depths.
Disadvantages
a. Risk of loosening of sandy or gravelly soils
during pile excavation, reducing bearing
capacity and causing ground loss and hence
settlement.
b. Susceptible to bulging or necking during
concreting in unstable ground.
c. Quality of concrete cannot be inspected after
completion except by coring.
d. Unset concrete may be damaged by
significant water flow.
e. Excavated material requires disposal, the
cost of which will be high if it is
contaminated.
f. Base cleanliness may be difficult to achieve,
reducing end-bearing resistance of the piles.
b) Mini / Micro Bored Piles
Mini-piles generally have a diameter between 100 mm and 400 mm. One or more high yield steel bars
are provided in the piles. In Malaysia, used high yield steel pipes are commonly used as the
reinforcement for micro piles.
Construction can be carried out typically to about 10 m depth or more, although verticality control will
become more difficult at greater depths. Mini-piles are usually formed by drilling rigs with the use of
down-the-hole hammers or rotary percussive drills. They can be used for sites with difficult access or
limited headroom and for underpinning. In general, they can overcome large or numerous obstructions
in the ground.
Mini-piles are usually embedded in rock sockets. Given the small-diameter and high slenderness ratio
of mini-piles, the load is resisted largely by shaft resistance. The lengths of the rock sockets are
normally designed to match the pile capacity as limited by the permissible stress of steel bars. A minipile
usually has four 50 mm diameter high yield steel bars and has a load-carrying capacity of about
1375 kN. Where mini-piles are installed in soil, the working load is usually less than 700 kN but can be
in excess of 1 000 kN if post grouting is undertaken using tube-a-manchette.
Pile cap may be designed to resist horizontal loads. Alternatively, mini-piles can be installed at an
inclination to resist the horizontal loads.
c) Large Diameter Bored Piles
Large-diameter bored piles are used in Malaysia to support heavy column loads of tall buildings and
highways structures such as viaducts. Typical sizes of these piles range from 1 m to 3 m, with lengths
up to about 80 m and working loads up to about 45,000 kN. The working load can be increased by
socketing the piles into rock or providing a bell-out at pile base. The pile bore is supported by
temporary steel casings or drilling fluid, such as bentonite slurry.
9-4 March 2009