Dolly Pile System

Easily moulded in fingers

Very soft

<20

Exudes between fingers if squeezed

8.10 Pile foundations

The use of sheet piling, which can be of timber, concrete or steel, for earth retaining structures has been described in Chapter 7. Piled foundations form a separate category and are generally used:

(i) to transmit a foundation load to a solid soil stratum;

(ii) to support a foundation by friction of the piles against the soil;

(iii) to resist a horizontal or uplift load;

(iv) to compact a loose layer of granular soil.

Classification of piles

There are two main classes of piles.

End bearing (Fig. 8.15a)

Derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile. The pile behaves as an ordinary column and should be

Soft

Soft

Hard

(a) End bearing Fig. 8.15 Classification of piles.

(b) Friction

Soft Firm

(c) Combination designed as such except that, even in weak soil, a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. it is in either air or water.

Friction (Fig. 8.15b)

Carrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile.

Combination of the two (Fig. 8.15c)

Really an extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay. The pile is driven far enough into the lower material to develop adequate frictional resistance. A further variation of the end bearing pile is piles with enlarged bearing areas. This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm layer to give an enlarged base. A similar effect is produced with bored piles by forming a large cone or bell at the bottom with a special reaming tool.

8.10.1 Driven piles Timber

Timber piles have been used from earliest recorded times and are still used for permanent work where timber is plentiful. In the UK, timber piles are used mainly in temporary works, due to their lightness and shock resistance, but they are also used for piers and fenders and can have a useful life of some 25 years or more if kept completely below the water table. However they can deteriorate rapidly if used in ground in which the water level varies and allows the upper part to come above the water surface. Pressure creosoting is the usual method of protection. In tropical climes timber piles above ground water level are liable to be destroyed by wood eating insects, sometimes in a matter of weeks.

Precast concrete

These are usually of square or octagonal section. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre-stressed concrete piles are also used and are becoming more popular than ordinary precast as less reinforcement is required.

Steel piles: tubular, box or H-section

These are suitable for handling and driving in long lengths. They have a relatively small cross-sectional area and penetration is easier than with other types. The risk from corrosion is not as great as one might think although tar coating or cathodic protection can be employed in permanent work.

Pile driving

The essentials of a standard pile driving rig are shown in Fig. 8.16. The frames are of steel construction and vary in height from 10 to 25 m. Obviously, much more complicated, and larger, arrangements than that illustrated are available.

To guide the hammer, and the pile as it is driven into the ground, a pair of steel members, called leaders, extend the full height of the piling frame. Although steam-powered winches are still available, most motors are now powered by diesel, petrol or electricity.

In the case of a simple drop hammer, as illustrated in Fig. 8.16 a hammer, of a mass between 2000 and 10000 kg, is raised by the winch and then released by some form of clutch arrangement and allowed to fall on to the pile. With this relatively simple equipment the rate of driving is very slow. A more satisfactory technique is to mount, directly above the pile, a single acting hammer consisting of a cylinder in which a heavy steel block, referred to as the ram or hammer, can slide up and down. Steam or compressed air is driven into the cylinder so that the ram is lifted some 1.5 m. When the hammer is in its raised position a valve may be opened and the hammer allowed to fall. Alternatively, the cylinder itself is made to lift up and fall down around a fixed ram. The order of blows is some 50 to 60 per minute.

An even more efficient system is that employing a double acting hammer in which the steam or air is used to both lift and then drive the ram downwards. The frequency of blows may be as high as 500 per minute.

A diesel hammer is similar to a single acting hammer. As the heavy piston falls it passes side ports and compresses the air below it. A lever at the side,

Piling Rig Dolly

struck by the upper part of the piston, activates a small pump that injects a small amount of diesel oil into a cup into which the shaped end of the piston fits. The resulting explosion both raises the piston and drives down the pile. The stroke, and hence the force of each blow, can be varied by the amount of oil injected. The hammer has to be started by raising the piston with a crane.

Protection of top of pile during driving

In order that the top of the pile is not damaged during driving it is protected by packing material, generally layers of hard wood, over which is placed a steel cap or dolly. The determination of the correct form of packing and dolly for the driving conditions is a highly specialised task.

Measurement of set

Piles are either driven to a specified depth or to a specified set. The set of a pile is the amount of downward movement of the pile per blow and need only be measured during the final stages of driving.

To measure the set an arrangement similar to that shown in Fig. 8.17a can be used. A paper sheet is attached to the pile and a straight edge, supported by a beam, is arranged to lie over the paper. As driving proceeds a pencil is drawn along the straight edge and a graph, similar to that shown in Fig. 8.17b is obtained.

The total downward movement of the pile following a blow consists of the set plus the temporary elastic compression of the pile and its protecting cap.

Jetted pile

When driving piles in non-cohesive soils the penetration resistance can often be considerably reduced by jetting a stream of high pressured water into the soil just below the pile. There have been cases where piles have been installed by jetting alone. The method requires an adequate supply of water and considerable experience, particularly when near to existing foundations.

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Responses

  • olli-pekka
    What is the dolly and packing of pile foundation?
    2 years ago
  • liisa
    What is dolly in pile equipment?
    1 year ago

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