DEEP FOUNDATIONS (PILE FOUNDATIONS)
In case of deep foundations, the piles are used to transmit the load of structure to the soil. The term pile foundations is used to describe a construction for the foundation of a well or a pier, which in turn is supported on the piles. The piles may be placed separately or they may be placed in the form of a cluster throughout the length of the wall. This construction is adopted when the loose soil extends to a great depth. The load of the structure is transmitted by the piles to hard stratum below or it is resisted by the friction developed on the sides of piles.
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| PILE FOUNDATION |
Piles are deep foundations . They are relatively long, slender member and are either driven into the ground or bored cast-in- situ . The pile foundations become one of the choices when shallow foundations are not able to meet the required objectives. Such situations are often faced by the construction engineer due to weak foundation soils, which are not in a position to withstand the load from the superstructure be meeting the desired criteria of satisfactory foundation, i.e., no shear failure of the foundation soil and not exceeding the allowable settlement.
The most important factor in a pile selection is the soil condition and the type of piles i.e., a driven pile or a cast -in -situ pile , selection will be governed by the soil type and its consistency if cohesive soil and its degree of compactness of cohesion less soil.
USES OF PILES
The situations which demand piles as foundations are as follows:
(i) The load coming form the structure is very heavy and the distribution of load on soil is uneven .
(ii) The subsoil water level is likely to rise of fall appreciably. This may be seasonal or occasional variation.
(iii) The pumping of subsoil water is too costly for keeping the foundation trench in dry condition .
(iv) The construction of raft or grillage foundations is likely to be very expensive or is practically impossible.
(v)The firm bearing stratum exists at a greater depth. The piles upto 20 metres depth are common and under exceptional circumstances , they may even be taken to 30 metres depth. The piles are considered to be long when their length exceeds 30 metres.
(vi) The timbering to excavations is too difficult to maintain the sides of the foundation trench.
(vii) The pile foundation is to be adopted for the structures in the area where canals, deep drainage lines , etc., are to be constructed in near future.
(viii) The structure is situated on sea - shore or river bed and the foundation is likely to be affected by the scouring action of water. thus , the piles are useful for the marine structure.
(ix) The piles are also used as anchors. They may be designed to give lateral support to to resist an upward pressure or uplift pressure.
(x) The piles are used as fender piles in the construction of docks, piers and other marine structures.A fender pile protects the berthing ships form damage.
TYPES OF PILES
The piles are broadly classified into the following two categories:
(1) Load bearing piles
(2) Non-load bearing piles
Each of the above categories of piles will now be described in details.
LOAD BEARING PILES
These piles bear the load coming from the structure. The piles are generally driven vertically or in near vertical position. When a horizontal force is to be resisted, the piles ,may be driven in an inclined position and such inclined piles are termed as the batter piles. The design of batter piles should be made by considering the fact that they will resist more, if not all, of the horizontal loading . If batter piles are used together with vertical piles, it may be assumed that part of the vertical load will be transferred to the batter piles also. The load bearing piles may be resist the load by directly resting on a firm stratum or by friction developed at their sides . The former piles are known as the bearing or sustaining piles and the latter piles are known as friction or floating piles.
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| PILE FOUNDATION |
(1) Bearing piles: These piles penetrate through the soft soil and soil and their bottoms rest on a hard bed. Thus, they are end - bearing piles and act as columns or piers. The soft ground through which the piles pass also gives some lateral support and this increases the load carrying capacity of the bearing piles.
(2) Friction piles :When loose soil extends to a great depth, the piles are driven upto such a depth that the fractional resistances developed at the sides of the piles equals the load coming on the piles.Great care should be taken to determine the frictional resistances offered by the soil and suitable factor of safety should be provided in the design. The total frictional resistance of piles is obtained by multiplying frictional resistance of soil with the area of pile in contact with the soil. The circumference in the following ways:
(i) By increasing the diameter of the pile.
(ii) By driving the pile to a greater depth.
(iii) By making the surface of the piles rough
(iv) By placing the piles closely.
(v) By grouping the piles.
MATERIAL USED IN CONSTRUCTION OF LOAD BEARING PILES
The material which are used in the construction of load bearing piles are as follows:
(1) Cast-iron piles (4) Steel piles
(2) Cement concrete piles (5) Timber plies
(3) Sand piles (6) Wrought-iron piles
CAST IRON PILES
The cast-iron piles are generally hollow The diameter of piles is about 300 mm and thickness is about 25 mm . The length of piles is about 3 metres to 4 metres and with the help of suitable devices, it can be extended to any desired length. As cast iron is brittle, it is not possible to drive the piles into the ground by means of a hammer.Hence special screws are provided at the bottom of piles and then , they are driven like a screw into the ground. These are known as the cast iron piles.
(1) Advantage of cast-iron piles: following are the advantages of cast iron piles:
(i) The cast-iron piles are useful for areas where the timber piles will be attacked and damages by the insects or worms.
(ii) The cast iron piles are suitable for heavy vertical pressure.
(iii) If shocks or vibrations would endanger the adjacent properties, the cast iron piles are to be preferred.
Disadvantages of cast-iron piles; Following are the disadvantages of cast-iron piles :
(i) The cast-iron piles cannot be subjected to shocks or vibrations.
(ii) The cast- iron piles are unsuitable for works under sea water.
CEMENT CONCRETE PILES
The cement concrete possesses excellent compressive strength . With the advent of reinforced cement concrete, the R.C.c. piles are becoming more popular and they are fast replacing piles of other materials. The R C C. piles are divided into two groups :
(1) Cast - in- situ concrete piles
(2) Pre - cast concrete piles.
CAST IN- SITU CONCRETE PILES
In this type of piles , a bore is dug into the ground by inserting a casing . This bore is then filled with cement concrete after placing reinforcement, if any. The casing may be kept in position or it may be withdraw . The former piles are known as the cased cast-in -situ concrete piles and the later piles are known as the uncased cast -in situ piles.
SAND PILES
These piles are formed by making holes in the ground and then filling the holes by sand. If sand is kept confined, it possesses great crushing strength and becomes in compressible.
A bore of required diameter, usually 300 mm, is formed either by driving a wooden pile or by and auger or by forcing a pipe with closed end. The hole is then filled with sand and it is well closed end. rammed until the sand in the hole does not escape. The sand to be used should be moist at the time of placing. The top of sand pile is filled with concrete to prevent the sand to come upwards due to lateral pressure.
Advantages of sand piles: Following are the advantage of the sand piles:
(i) These piles are economical for small building and for embankments of the roads.
(ii) The sand piles are easy to construct.
(iii) It is possible to use gravel in place of sand . Such piles are known as the gravel piles.
(iv) The sand piles can be used irrespective to any position of the water tables.
Disadvantages of sand piles: Following are the disadvantages of sand piles:
(i) The sand piles are not suitable for loose or wet soils or where there is danger
of sour.
(ii) The sand piles cannot be adopted in region subjected to frequent earthquake.
STEEL PILES
The steel piles are used as the load bearing piles in three different forms;
(1) H- beam piles
(2) Box piles
(3) Tube piles.
TIMBER PILES
These piles are prepared from the trunks of trees and the use of timber as pile has been recorded since the beginning of the science of civil engineering . The wood to be used for timber should be free form knots,flaws shakes and other defects The common Indian timbers which are used as piles are : Babul , Chir , Deodar , Jarul, Poon , Sal , Semul , Teak , and white siris.
WROUGHT-IRON PILES
These piles are generally made solid. The diameter varies from 80 mm to 200 mm and the length is usually about 4 metres to 6 metres. The wrought - iron piles have suitable devices for lengthening and sinking These piles are mostly used for shafts of screw piles. The wrought- iron piles are most suitable for use under sea water. But as these piles are expensive, they are now replaced by the steel piles.
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