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when the lateral loads are cyclic in nature, 5. Uncertainties of the Analytical Methods
the skin friction as well as the subgrade
reaction values must be reduced to allow The analysis of a pile system with vertical
for the gap in between the soil/pile due to and raker piles, subjected to vertical and
continual cyclic movement, e.g. reduction horizontal loads, has not been perfected
by 25% for soft clay, and 50% for stiff clay due to many reasons:
or dense granular material.
E.g. the use of soil modulus concept is
4. Analysis of Global Pile Group based on the unrealistic assumption of
linear elastic load/deflection behaviour.
The force method - a system of axial pile
loads is calculated which are simply in The elastical theories of soil pressure, on
equilibrium with applied loads Fig. 1). the other hand is based on failure in the
ultimate states.
The rivet group method piles are
considered vertical and in, equilibrium In the truss or frame system, the soil is
with the vertical component of the applied assumed to have no lateral resistance,
load. Then some piles are raked so that the which may be too severe an assumption. -
horizontal components of their axial loads
balancethe horizontal component of the In the simpler force or rivet group or
applied load.(Fig. 2) In some cases, small centroid methods, no account is taken of
unbalanced lateral forces may be tolerated secondary moments due to flexure of the
since most piles can withstand certain piles.
amount of bending in service condition.
E.g. Teng W.C. (1976) quotedthat for The assumption of uniform distribution of
reinforced concrete piles to 16 inches in forces of piles in any row is too simplified.
diameter, the allowable lateral load may be More common is that when the bearing
up to 21 tons in average .good soil strata settle, piles at the end rows support is
conditions. In any case, an allowable greater than piles in. the middle of the row.
lateral load of 0.5 ton is permissible for lack of field data since there are little tests
such piles. done on pile groups subjected to a
The centroid method - in Fig. 3. combination of vertical and lateral loads.
comparisons of different methods may
The frame method - the piles and the pile indicate surprisingly large differences in
cap are considered to form a frame or trust. the calculated loads acting on the same
The pile cap is assumed to be rigid but the piles. Judgement and experience are
piles are elastic and assumed to be rigidly important.
supported at certain point of the length.
The cap/pile and pile/support connections 6. Design Good Practice
can be pinned or fixed (say at least 2 feet
into the pile cap for fixed cap/pile a fan of piles is better than a group with all
connection). Some engineers design for central piles. vertical and edge piles raked.
the worst conditions of pinned and fixed
connections. The point of f ixity at the sup
port may be estimated by earth pressure
theories.(Fig . 4) other methods using
computers such as space frame or more
sophisticated plane frame or elastic half-
space.
Table 1
Allowable bearing capacity of rock
DESIGN OF PILE FOUNDATION load per pile is 5-10 kN. Bakau piles must be
installed below ground water table to ensure its
INTRODUCTION durability. Pressure treated timber piles are usu-
ally made of Kempas and they are available in
Piles are structural members used to transmit 125mm or 150mm sections. Allowable work-
the surface loads to firmer soil below. The load ing load per pile varies from 30 kN to
transfer may be by friction, g!nd bearing or 100 kN. It was instructed by Ketua Pengarah
combination of both. If the load is resisted by Kerja Raya in 1975 that pressure treated timber
skin resistance along the surface of pile, it is piles shall not be used for building more than 2
called frictional pile. If the pile derive most of storeys high. Also only one joint is allowed for
its carrying capacity from the resistance of the pressure treated timber pile.
dense stratum of soil or rock, it is called the
end bearing pile. 2.2.1.2 PRECAST REINFORCED
CONCRETE PILES
2. TYPES OF PILES
This is the most commonly used pile type in
2.1 CLASSIFICATION OF PILES JKR projects. The pile is designed as compres-
sion members and longitudinal steel is provid-
The British Standard Code of Practice for ed to withstand bending and tensile stress dur-
Foundation (CP 2004) places piles in three cat- ing handling and driving. The common sizes of
egories. They are as follows: reinforced concrete /piles used in JKR Projects
(i) Displacement (or large displacement) varies from 250mm to 400mm square section.
piles - These include all solid piles, The usual design load for these type of pile are
including timber, precast concrete, steel in the region of 300 - 600 kN.
and concrete tubes closed at the lower
end by a shoe or plug. 2.2.1.3 PRECAST PRESTRESSED
CONRCETE PILES
(ii) Small displacement piles - These
include rolled steel sections, open ended Precast prestressed concretes piles are
tubes, and screw piles. regularly used in JKR projects. Then' principal
advantage over ordinary reinforced concrete
(iii) Non-displacement piles - These are pile is the higher strength to weight ratio ,
formed by boring or other methods of enabling long slender units to be lifted and
excavation; the borehole may be lined driven. The second main advantage is the effect
with a casing or tube that is either left in of the prestressing in closing up cracks during
place or extracted as the hole is filled. handling and driving. This effect, combined
with the high quality concrete necessary for
2.2 COMMON TYPES OF PILES USED economic employment of prestressing, gives
IN JKR PROJECTS the prestressed concrete pile increased
durability. The common sizes for the
2.2.1 DRIVEN DISPLACEMENT PILES prestressed concrete pile is similar to precast
2.2.1.1 TIMBER PILES reinforced concrete piles, i.e. 250 - 400mm
square section. The usual design load per pile is
The common timber piles used in JKR 300 - 650 kN.
projects are bakau pile and ' pressure treated
timber piles. The common type of bakau tim- 2.2.1.4 STEEL PILES
ber used. are Bakau Minyak and Bakau Kurap.
Bakau piles are generally about 100mm in Steel piles have the advantage of
diameter and 5-6m in length. The allowable being robust, light to handle and'capable of
being driven hard to deep penetration to reach - Micropiles are classified as small diameter (less
a bearing stratum. They can carry high than 200mm) bored cast-in-situ piles. The
compressive loads when they are seated in a special feature of the micropile is its strength,
hard stratum. They can be designed as small resulting from the placing of a reinforcement
displacement piles, which is advantageous in and its seating in a hole of sufficient diameter
situations when ground heave and lateral dis- which may be bored in whatever direction is
placement must be avoided. They can be best suited to the requirements of the projects.
readily cut down and extended where the level The equipments used'in-the installation of
of the bearing stratum varies. H-section and micropiles are much smaller than those used in
pipe pile are the common steel pile used in this bored piles and so it is very convenient to
country. The design load for steel H--pile is move and install these-equipments. In areas
about 300 - 1800 kN where as for steel pipe where the ground consist of hard weatherd
pile it is in the region 300 - 3000 kN. rock they require special diamond-tipped drills
and for large diameter boreholes the process of
2.2.2. DRIVEN CAST-IN-SITU drilling will be quite difficult. Moreover only
DISPLACEMENT PILES few contractors can supply these drills.
Whereas for micropiles the drilling process can
Driven cast-in-situ piles are installed by be, easily done due to the small diameter.
driving to the desired penetration a -steel tube The micropiles are good foundation in
with its end closed. A reinforcing cage is next fractured rocks where the cracks/fractures can
placed in the tube which is then filled with con- be grouted at the same time of grouting the
crete. The tube is withdrawn while placi g the micropiles. For existing structures where the
concrete./This system of piling is usually/ foundation is found to fail these micropiles
patented basing on using different types of Ican be used because they do not require large
shoes of driving technique or casing equipments or space to work on and so the
withdrawal procedure. The system used in this other parts of the structure will not be affected.
country is'the patented Franki Pile. The design The working loads of micropiles as specified
load per pile is in the range of 650 - 1500 kN. by the soils lab are between 400KN to 800KN
Driven and cast-in-situ piles have the principal but some contractors claim that they can reach
advantage of being readily adjustable in length up till 1500 KN for 250 mm diameter
to suit the desired depth of penetration. micropiles.
Qf = Fc As
Where
Fc = unit local friction".measured using the
friction sleeve
As = area of shaft of pile
into the bearing stratum. allowable load must be the allowable structural
Ab = Area of base of Pile, capacity of the piles as determined from struc-
tural calculations based on the strengths of the
6. Evaluate the factor of safety of the pile. It materials making up the pile.
is arguable as to what is a suitable factor of
safety, and how it should be evaluated. Guidelines for the design of:
Driven piles in cohesionless soils (Sands)
*Tomlinson has quoted that if a factor of safety (Cu=O) (Based on borehole results)
of 2.5 is used on the total ultimate pile
resistance (Q u + Qb), then the settlement at The bearing capacity of piles in sand is mainly
working load is unlikely to exceed 10mm contributed by end bearing,-frictional resistance
(0.4)in, i.e. giving only a relatively small contribution..
The classic formula is:
Allowable load = ultimate pile resistance, Q u Qu = Nq Pd Ab + 1/2 Ks Pd tan SAs
---------------------------------- ----------- ------------------------
2.5 Qb Qs
However, for large diameter piles exceeding
600 mm. (2ft.), the problem of settlement Where
become more severe. This is because the Qu = Ultimate total pile resistance.
settlement required to mobilize the end bearing P d = effective overburden pressure at pile
resistance is much more than that required to base level.
mobilize skin friction (Skin friction can be Nq = bearing capacity factor.
mobilised with a downward movement of only Ab = base area of pile.
about 0.3 to 1% of the pile diameter, while to Ks = coefficient of earth pressure which
mobilize the full base resistance requires a depends on relative density of soil,
downward movement of about 10 to 20% of volume displacement of pile, material of
the base diameter). pile, shape of pile.
S = angle of friction between pile and soil.
*Tomlinson, M.J. - Pile design and construc- As = surface area of pile in contact with soil.
tion practice, viewpoint 1977.
The steps in the design of driven piles in cohe-
Thus the concept of partial factors of safety is sionless soils are as follows:
more appliable in the case of large diameter
piles. The method suggested by Burland, Butlre 1. Examine the soil profile and decide of a
and Dunican is to use do allowable load which bearing stratum of cohesionless soil. This
is the lower of the two given below: bearing stratum must be at least 6 Diameters
thick and should not be underlain by a softer,
(a) Allowable load = z (Qs + Qb) compressible material. If there is a
(b) Allowable load = Q+ Qs b. compressible material below it, then it is
advisible to penetrate the soft material to a
7. Having determined the allowable load on lower bearing stratum, unless the sand
the pile, check this against the required work- overlying the soft materialis very dense and
ing load. If the allowable load is less than the very thick (unusual).
required working load, try a longer length of
pile and go back to step 3. If the allowable load 2. Decide on a trial depth of pile, i.e., decide
is more than the required working load where the base of the pile is to be founded. The
a check must be made against the structural pile must be driven at least 5 Diameters into
capacity of the pile. If, the allowable structural the bearing stratum, but there is not much
capacity of the pile is exceeded, then. the actual point in driving the pile more than 20
PRINCIPAL REFERENCES