Tag: Pile Foundations

Sheet Piles – Types of Sheet Piles

Sheet Piles – Types of Sheet Piles

Sheet piles may he made up of wood, concrete or steel. Steel piles are driven side by side into the ground to form a continuous vertical wall for retaining soil. The alignment and resistance or thrusts are normally provided by horizontal wallers, braces or tiebacks. Factors affecting the choice of a particular type of pile include nature of ground, cost, ease of installation, availability of material, ability to withstand driving, lateral strength and ease of making connections. Depending upon the material used in their manufacture, some of the types of sheet piles are,

  1. Wooden sheet piles
  2. Precast concrete Sheet piles
  3. Prestressed concrete sheet piles
  4. Steel sheet piles

1. Wooden sheet piles:

Wooden sheet piles are made in various sizes and forms. The nature of site conditions determine, the choice of a particular type, In places where excavation is small and the ground water problem is not serious, 5 cm x 30 cm to 10 cm x 30 cm wooden planks arranged in a simple row will serve the purpose. If the water-tightness is required to a great extent, lapped sheet piling is used. In this case, each pile is made up of two planks, either spiked or bolted to one another. Thus if only earthen banks of small height are to be supported, a single or double row of  planks properly erected will perform the function of sheet piling. If complete water tightness is desired or pressure of the retained material wakefield or tongue and grooved sheeting is generally used. To facilitate the driving of the piles, they are usually bevelled at foot. This not only assists in driving but also prevents bruising, if the piles encounter hard stratum.

Wakefield piles:

This type of pile is made with three planks, 5 cm, 8 cm or 10 cm in thickness. The planks are nailed together with the middle plank offset forming a tongue on one edge and a groove on the other. The planks are connected by using a pair of staggered bolts at 80 cm centre to centre at intermediate points. The triple lap piles prove stronger in driving. There is no wastage in forming the tongue and groove joints and the piles have less tendency to warp. Timber sheet piles have light weight and as such the equipment required for pile driving is also light. This is considered to be an important advantage timber piles have over piles of other materials.

sectional plan of a wakefield pile

Sectional Plan of a Wakefield Pile

2. Precast concrete sheet piles:

Precast concrete piles are made in square or rectangular cross-section and are driven similar to wooden piles to form a continuous wall. The interlock between two piles is normally provided with the help of tongue and groove joint. The tongue and groove extend to the full length of the piles in most of the cases.

An alternative method of providing joint between two piles is shown below. In this method, after the piles are driven to the required. depth, the joint is grouted with cement mortar 1: 2 (1 cement : 2 sand).

sectional plan of different types of precast concrete piles

Sectional Plan of Different Types of Precast Concrete Piles

The piles are reinforced to avoid formation of cracks due to rough handling or shrinkage stresses. In order to reduce the possibility of damage due to driving impact, the stirrups should be spaced closely near the top and bottom of the piles. The piles are normally bevelled at their feet to facilitate tightly close driving of a pile against the already driven one.  Reinforced concrete sheet piles are bulky and heavy and as such they are gradually being superseded by prestressed concrete piles.

3. Prestressed concrete sheet piles:

On account of the numerous advantages the prestressed concrete members have over the conventional type of reinforced concrete members, prestressed concrete sheet piles are commonly used for sheet piling jobs. Similar to concrete sheet piles, they are reinforced on both the faces so that they could be handled from either side. They are comparatively lighter in weight, more durable and economical in the long run. They are advantageously used in sea water, since the danger of cracking of concrete is negligible and also the corresponding danger of corrosion of pile reinforcement is reduced.

4. Steel sheet piles:

Steel sheet pile is a rolled steel section consisting of a plate called the web with integral interlocks on each edge. The interlocks consist of a groove, one of whose legs has been suitably flattened. This flattening forms the tongue which fits into the groove of the second sheet. Commonly used sheet piles can be broadly divided into the following three categories,

  • Straight-web type
  • Shallow or deep arched-web type
  • Z web type

Special shapes and sizes of steel sheet piles are manufactured for meeting the requirement of junctions and other similar situations. Each of the above mentioned type of piles is manufactured in varying widths and lengths. The selection of the type of pile and the section to be adopted depend upon the depths up to which the pile is to be driven, the nature of soil to be penetrated the elevation of the earthen embankment, ground water level etc.

In general, Straight web type of piles are used where the piles are liable to he subjected to tensile forces and interlocking strength is of prime importance (Cellular cofferdam etc); Arched-web type are used where the piles are required to resist bending stresses (in cantilever retaining walls etc,) and Z-web type of piles arc used where the piles are required to resist bending stresses of very large magnitude.

Steel Sheet Piles

Steel Sheet Piles

Steel sheet piles are driven with the help of pile drivers which may be of drop hammer type or single or double acting hammer driven by steam or compressed air. The outstanding feature of steel sheet piles is that they  can be used for greater depths. The continuous interlocking arrangement of the piles gives strength and rigidity to the supported structure. A wall made from properly driven sheet piles leaks very little, hence steel sheet piling is used with advantage in the construction of deep cofferdams. They are commonly used in coastal defence works which are likely to be subjected to tidal action.

Types of Steel Piles

The types of steel piles commonly used are:

  • H-Piles
  • Pipe-piles
  • Screw piles
  • Disc piles


(a) H-piles:

The use of rolled steel H-beams to function as bearing piles is a comparatively recent development in piling industry. H-piles can withstand large impact stresses developed during hard driving. This type of pile has proved to be especially useful when the pile is expected to penetrate a rock or through hard substratum. On account of the smaller cross-sectional area of the pile, it can be driven to the desired depth without jetting, coring or adopting other methods of soil displacement. They require less storage space and can be handled without much difficulty. H- piles can be advantageously driven close to an existing structure as they produce very small soil displacement. H-piles can be securely spliced. Spliced H-piles have been driven to a maximum depth of 100 m. In situations where the piles are liable to corrode, they are coated with coal tar or some other type of suitable coating or they are encased in concrete. The smaller cross-sectional area of H-pile makes them function less effectively as friction or compaction piles. After the piles have been driven to the required depth, the pile heads are cut square and a steel plate cap is welded to each pile and finally the pile heads are embedded in reinforced concrete pile cap. H-piles are commonly used in the construction of bulkheads, trestles, retaining walls, bridges and cofferdams.

(b) Pipe-piles:

Seamless or welded steel pipes are often driven to function as end bearing or friction piles. The pipe piles may be driven either open ended or close ended. When the driving end of the pipe is left open, (without any driving point) it is called open end pile. Open ended piles are usually driven to penetrate rock or hard pan. As the open ended piles are sunk in the ground, the soil inside the pipe is cleaned out by means of compressed air, water jet etc simultaneously. When the pipes have been driven to the required depth, they are cleaned from inside and filled with concrete.

Pipe Pile

Pipe Piles

In case of closed end piles, the driving end of each pipe is closed by welding a conical steel or cast iron shoe to the pipe end. In this case also, after driving, the hollow space inside the pipe is normally, filled with concrete. The diameter of the pipes used for piling varies from 25 cm to 120 cm and their shell thickness varies from 8 mm to 12 mm The use of this type of pile for depths of 30 in or more is quite common.

(c) Screw piles:

A screw pile consists of a cast iron or steel shaft of external diameter normally varying from 15 to 30 cm and terminating into a helix or screw base. The pile shaft maybe hollow or solid. The diameter of the screw at its base varies from 45 cm to 150 cm. The pile is sunk by screwing the pile down inside the ground by use of an electric motor. Screw piles function most efficiently in soft clay or loose sand. In such a ground it is easy to install the piles and also the large bearing area provided by the screws makes the best use of the low bearing capacity of the soil.

Different Forms of Screw Pile

Different Forms of Screw Piles

(d) Disc piles:

Similar to a screw pile, a disc pile consists of hollow metallic pipe attached with a cast iron disc to its foot so as to enlarge the bearing area of the pile. There is a hole at the bottom of the pile to permit the water jet pipe to pass through during the sinking of the pile by jetting. Disc pile can he used only in sandy or soft soils which may permit sinking of the pile by water jets. Disc piles are used mostly in marine installations, where the total penetration of the pile in the ground is required to be large.

 Disc Pile

Disc Piles

Composite Piles

Composite Piles

Composite Piles are those piles of two different materials are driven one over the other, so as to enable them to act together to perform the function of a single pile. In such a combination, advantage is taken of the good qualities of both the materials. These prove economical as they permit the utilization of the great corrosion resistance property of one material with the cheapness or strength of the other.

The different stages in the construction of a composite piles having a timber pile at its lower part and precast concrete pile above are shown below. This type of composite pile is used with the object of achieving economy in the cost of piling work.

Composite Piles

Composite Piles

Another type of composite piles commonly used consists of a steel pipe or H-pile at the bottom and cast-in-situ concrete pile at the top. This type of composite pile is recommended in cases where the designed length of the pile works out to be greater than that available for the cast-in-situ type of pile.

Types of Cast-In-Situ Piles

The names of the various types of commonly used cast-in situ piles are:

  • Simplex pile
  • Franki pile
  • Vibro pile
  • Vibro-expanded pile
  •  Raymond pile
  • Mac Arthur pile


(1) Simplex pile:

This type of pile may be driven through soft or hard soil. A steel tube having an internal diameter equal to the diameter of the pile and 20 mm in thickness, is driven into the ground. To facilitate driving of the pile, the steel tube is fitted with a detachable steel shoe that completely closes the bottom of the tube. When the tube has been driven to the required depth, a charge of concrete is poured into the tube and the tube is gradually withdrawn leaving the charge of concrete below. Thus by alternately pouring the concrete and withdrawing the tube, the pile is constructed to its full length. The metallic shoe remains in place and hence a new one is needed for each pile In case the pile is required to be reinforced, the reinforcement cage is lowered into the steel tube prior to the pouring of concrete.

Simplex Pile

Simplex Pile

(2) Franki pile:

This type of pile has an enlarged base and a corrugated stem. A steel tube, having its internal diameter equal to the diameter of the pile required, is held vertical at the ground level with the help of leads. A charge of concrete is poured at its base filling the bottom 60 cm to 90 cm of the tube. The charge of concrete is consolidated into a solid plug by the blows of the drop hammer working inside the tube. Further blows of the hammer on the plug pull the tube down on account of the friction developed between the concrete and the inside surface of the tube. When the tube has been driven to the required depth, it is slightly raised’ and the plug is forced out of the tube by hammering. The reinforcement cage (if needed) is then lowered inside the tube. A fresh charge of concrete is then poured in the tube and rammed well by the drop hammer while the tube is pulled up a short distance. The repeated process produces a series of corrugations on the stem of the pile and the pile is thus completed.

Stages in Franki Pile

Stages in Franki Pile

(3) Vibro pile:

This type of pile is best suited for places where the ground is soft and offers little frictional resistance to the flow of concrete. A steel tube fitted with a cast iron shoe is first driven to the required depth. There is a water-tight joint between the shoe and the casing so that even if the pile is to be driven in water-logged ground, the soil and the sub-soil water cannot find an access in the tube before the concreting is done. The reinforcement cage (if needed) is lowered in the tube at this stage. The charge of concrete is then poured in the tube. The extraction of the tube and the ramming of concrete is effected by the upward and the downward blows of the hammer. The tube is connected to the hammer by extracting links. During the upward blow of hammer the tube is raised up by a short distance and the concrete moves down to fill the space left by the tube. During the downward blow, the concrete is compacted and rammed outwards thereby forming corrugated surface for the pile. This results in increased friction between the pile surface and the surrounding ground.

(4) Vibro-expanded pile:

In situations where it is desired to have increased frictional resistance between the pile stem and the surrounding ground, the surface of a vibro-pile is expanded greatly to achieve the object. this increases the bearing resistance of a vibro pile. In this process, a steel tube- of the required diameter of the pile having a detachable cast iron conical shoe at its base, is driven to the required depth. A charge of concrete (tilling a good length of tube) is poured and the tube is completely withdrawn leaving the cast iron shoe and the charge of concrete down in the pile hole. The withdrawn tube is fitted with a special flat iron shoe and once again driven in the same hole. The charge of concrete down below gets expanded to nearly double its area by this process. If required, another charge of concrete is poured and the process repeated. The reinforcement cage is there after lowered in the tube (if needed) and the pile is completed as usual.

Stages in Vibro-Expanded Pile

Stages in Vibro-Expanded Pile

(5) Raymond pile:

This type of pile is constructed in lengths varying from 6 to 12 m. The diameter of the pile varies from 40 to 60 cm at lop and the diameter at its base is slightly smaller, varying from 20 to 28 cm so as to give a uniform taper to the pile. The thickness of the outer shell depends upon the pile diameter and site conditions. The thin steel shell is reinforced with hard drawn wire spiral spaced at 8 cm centre to centre. The shell is closed at the bottom with a steel boot. The shell is placed over ii collapsible mandrel having the same taper as the pile and 1)0th arc driven to the desired depth. The mandrel is then withdrawn leaving the shell in the ground. The shell is gradually filled with concrete up to the lop. This forms a Raymond pile. The function of shell outside the concrete core is to prevent the adjoining soil and the sub-soil water coming iii contact with fresh concrete.

Stages in Raymond Pile

Stages in Raymond Pile

(6) Mac Arthur pedestal pile:

In this type of pile the apparatus consists of an outer casting (a hollow steel pipe ) and an inner core. The bottom of the core is of such a size that it completely closes the open base thickness with two point pick-up. Piles 500 mm square and smaller are usually cast solid, whereas pile above 500 mm square may be cast with 200 mm to 300 mm diameter cored hole (void).

Cast-in-situ Piles

Cast-in-situ piles

Cast-in-situ piles are those piles which are cast in position inside the ground. Since the cast-in-situ piles is not subjected  to handling or driving stresses, it is not necessary to reinforce the pile in ordinary cases or in places where the pile is completely submerged in the soil. Reinforcements are necessary to be provided in a cast-in- situ piles, when the pile acts as a column and is subjected to a lateral forces. Cast- in-situ piles can be divided into two types. In one the metallic shell of the pile is permanently left in place inside the ground along with the core while in the other type the outer shell is withdrawn.

Cast in situ piles for column footing

Cast in situ piles for column footing

Precast Concrete Piles

Precast Concrete Piles

Precast Concrete Piles may be defined as a reinforced concrete pile which is moulded in circular, square, rectangular or octagonal form. The precast concrete piles are cast and cured in a casting yard and then transported to the site for driving. In case space is available, pile can also be cast and cured near the site of works. They are driven in a similar manner as timber piles with the help of pile drivers. The diameter of the pile normally varies 1mm 35 cm to 65 cm and their length varies from 45 in to 30 m.

Precast Concrete Piles

Precast Concrete Piles

The function of reinforcement in a precast concrete piles are to resist the stresses produced on account of its handling, driving and the loading which the pile is finally expected to receive. Longitudinal reinforcement usually consists of one bar 20 mm to 50 mm in diameter at each angle of the section of the pile. The vertical rods are tied horizontally by bars 6 mm to 10mm in diameter. The horizontal bars may be provided in the form of stirrups wound around the verticals. For lengths of approximately 90 cm at head and toes, the spacing of the stirrups should be 8 cm c/c. Circular piles are seldom tapered but when tapering of the piles becomes necessary due to site conditions, their length is restricted to 12 m.

Precast Piles supporting Column footing

Precast Piles supporting Column footing

Timber Piles

Timber Piles

Transmission of load through timber piles takes place by the frictional resistance of the ground and the pile surface. Timber piles prove economical hit supporting light structures to be located in compressive soils constantly saturated with water. The timber piles are made from timber obtained from trees like sal, teak, deodar, babul, Khair etc. It has been found that piles made from Khair wood can stand action of sea water better and are thus commonly used for marine works. Timber piles may be circular or square in cross-section. Piles are driven with the help of pile-driving machine in which a drop hammer delivers blows on the pile head. To prevent the pile head from brooming, an iron ring about 25 mm less in diameter than the pile head is provided at the pile top. To facilitate driving, the lower end of the pile is pointed and is provided with a cast iron conical shoe. Piles should not be spaced less than 60 cm centre to centre. By driving piles much closer, the frictional resistance is destroyed. The best spacing for timber piles is 90 cm c/c. Maximum load on a wooden pile should normally not exceed 20 tonnes. Piles made from sound timber free from any defect or disease and driven in the soils which are either permanently wet or permanently dry, will remain in good condition for centuries. However, when subjected to alternate dry and wet conditions (on account of variations in ground water level) they get decayed. It is on this account that timber piles are cut a little below the lowest water-mark and capped with concrete, steel grillage, stone or timber. If timber capping is used, the cap should be permanently under water.

Timber Piles

Timber Piles

Advantages of timber piles:

  1. They are economical.
  2. They can be driven rapidly and as such there is great saving in time in execution of piling work.
  3. On account of their elasticity timber piles can be recommended for sites where piles are likely to the subjected to unusual lateral forces.
  4. They do not need heavy machinery and elaborate technical supervision

Disadvantages of timber piles:

  1. Timber piles must be cut off below the permanent ground water level to prevent them from decay and thus if the water table at a site is at a greater depth, extra cost of excavation needed to provide the pile cap may render the choice uneconomical.
  2. Timber piles can not be driven in filled up ground without injury and as such they can not be recommended for such sites.
  3. They are liable to decay or deteriorate by sail water or insects.
  4.  On account of their restricted length, they cannot be used for jobs  where long piles are needed-
  5. They have low bearing capacity.

 

Types of Piles – Deep Foundations

Depending upon their function, the different types of piles are,

  • Bearing piles
  • Friction piles
  • Sheet piles
  • Anchor piles
  • Batter piles
  • Fender piles
  • Compaction piles

 

Combination of Types of Piles

Combination of Types of Piles

(i) Bearing piles: Bearing piles are those which are driven into the ground until a hard stratum is reached. Such piles act as pillars supporting the super-structure and transmitting the load down to the level at which it can be safely borne by the ground. Thus bearing piles, by themselves do not support the load, rather they act as a medium to transmit the load from the foundation to the resisting sub-stratum.

Bearing Piles

Bearing Piles

(ii) Friction piles:  When piles are required to b driven at a site where the soil is weak or soft to a considerable depth, the load carried by a pile is borne by the friction developed between the sides of the pile and the surrounding ground (skin friction). In such cases the pile is named as friction or floating pile. Thus friction piles are driven in the type of soil whose strength does not increase with depth or, where rate of increase in strength with depth is very slow.

Friction Piles

Friction Piles

(iii) Sheet piles: Sheet piles differ from bearing or friction piles in that they are rarely used to furnish vertical support but are used to function as retaining wall. Sheet piles are used for retaining soil that is liable to escape laterally when subjected to pressure or to enclose the area required for some foundation and protect it from the action of running water or leakage.

(iv) Anchor piles: When piles are used to provide anchorage against horizontal pull from sheet piling walls or other pulling forces, they are termed as anchor piles.

(v) Batter piles: When piles are driven at inclination to resist large horizontal or inclined forces,, the piles are termed as batter piles.

(vi) Fender piles: 
When the piles are used to protect concrete deck or other water front structures from the abrasion or impact that may be caused from the ships or barges (when they are tied up at the deck) they are called fender piles. The fender piles are ordinarily made up of timber.

(vii) Compaction piles:
When piles are driven in granular soil with the aim of increasing the bearing capacity of the soil, the piles are termed as compaction piles.

Terms Used in Pile Foundations

Terms Used in Pile Foundations

Some of the terms used in pile foundations are,

Anvil: The part of a power operated hammer which receives the blow of the ram and transmits it to the pile.

Composite pile: A pile whose length is made up of more than one material, e.g., timber at bottom and concrete at top.

Dally: A cushion of hardwood or other material placed on top of the helmet to receive the blows of the hammer.

Driving Cap: A temporary cap placed on top of a pile to distribute the blow over the cross-section and to prevent the head being damaged during driving.

Drop or stroke: The distance which the weight is allowed to fall on to the head of the pile.

Drop hammer: A hammer, Ram or monkey raised by a winch and allowed to fall by gravity. A Single acting hammer is raised by steam, compressed air, or internal combustion, and allowed to fall by gravity. A double acting hammer is operated by steam, compressed air or internal combustion for lifting the ram and for accelerating the downward stroke.

Helmet: A temporary steel cap placed on top of a reinforced concrete pile to retain the packing in position and to prevent the head from being damaged during driving.

Pile bent: A number of piles projecting above the ground up to the bottom of bridge girders. The piles are connected by capping beams on which the bridge decking rests.

Ram: The rising and falling part of the hammer which delivers the blow.

Set: Is the penetration of the pile per blow during the final stages of driving.