Ground rupture hazard

The designer must check that a cellular foundation

.constructed in water-bearing ground will not float due to the hydrostatic head exceeding the imposed loading at any stage of construction or usage. This danger may prevail in the case of over-cornpensated foundations, such as empty submerged storage tanks, where loadings vary due to operational demands. In a similar manner the effect of ground heave when sucfxfoundations are supported on cohesive soils is an equally vital design condition requiring careful study.

4.5 Piled foundations

When a stratum sufficiently strong to support the imposed loading does not exist fairly near the surface, piled foundations may be used. A pile is a ‘column’ in the ground which is used to transfer the load to a stronger and deeper stratum.

Piles may be of timber, hollow steel section, prestressed or reinforced concrete; the last named being the most used for foundations of buildings. The load carried by a pile can vary from 300 to 10 000 kN depending on the size, type and ground conditions. The load is trans­ mitted from the pile to the ground either by end bear­ ing of the pile toe on a hard strata underlaying soft ground, or by ‘skin friction’ between the surface area of the pile and the surrounding soil, or by a combination of both.

♦ There is no one pile type which is best in all condi­ tions and all types have their advantages according to circumstances. Piles can be sub-divided into two main types — displacement piles and replacement piles.

4.5.1Displacement piles

Displacement piles are sub-divided into preformed (precast, in the case of concrete) piles and in-situ piles.

The precast piles in general use on power station sites are made either as reinforced or prestressed concrete usually of square or near-square section. Reinforce­ ment consists of heavy longitudinal bars with square links or helical binders. Prestressed piles are most usually pretensioned. In both types, the spacing of links­ and binders is decreased at the toe and the head to resist driving stresses; the longitudinal icinl'otcciisenI or prestress provided is greatly in excess of that required to take the structural load, but is needed to resist the stresses of lifting, handling and driving. The toe may be strengthened with a pointed cast-iron shoe, although this may not be necessary when driving in clay.

The casting, storing, transporting and driving of the precast piles for a major contract, which may call for some 20 (MX) piles, is a considerable undertaking. A large area of the site has to be set aside and levelled and accurately paved with concrete to provide a casting bed, and further areas are required for storage. Travel­ ling derricks are required for casting the piles and lifting and handling them, a batching plant is needed to produce the large quantity of concrete required, and a light railway system to transport the piles to the driving frames.

Piles precast in this manner have the advantage that they can be inspected before driving, and good control of the quality of concrete can be maintained. They are driven only after sufficient time has been allowed for the concrete to mature, and are thus resistant to attack from harmful substances in the soil.

The designer must check that a cellular foundation constructed in water-bearing ground will not float due to the hydrostatic head exceeding the imposed loading at any stage of construction or usage. This danger may prevail in the case of over-compensated foundations, such as empty submerged storage tanks, where loadings vary due to operational demands. In a similar manner *thefoundationseffect of areground heave when such

supported on cohesive soils is an equally vita) design condition requiring careful stifdy.

4.5 Piled foundations

When a stratum sufficiently strong to support the imposed loading does not exist fairly near the surface, piled foundations may be used. A pile is a ‘column’ in the ground which is used to transfer the load to a stronger and deeper stratum.

Piles may be of timber, hollow steel section, prestressed or reinforced concrete; the last named being the most used for foundations of buildings. The load carried by a pile can vary from 300 to 10 000 kN depending on the size, type and ground conditions. The load is trans­ mitted from the pile to the ground either by end bear­ ing of the pile toe on a hard strata underlaying soft ground, or by ‘skin friction’ between the surface area of

. the pile and the surrounding soil, or by a combination of both.

« There is no one pile type which is best in all condi­ tions and all types have their advantages according to circumstances. Piles can be sub-divided into two main types — displacement piles and replacement piles.

4.5.1Displacement piles

Displacement piles are sub-divided into preformed (precast, in the case of concrete) piles and in-situ piles.

The precast piles in general use on power station sites are made either as reinforced or prestressed concrete usually of square or near-square section. Reinforce­ ment consists of heavy longitudinal bars with square links or helical binders. Prestressed piles are most usually pretensioned. In both types, the spacing of links, and binders is decreased at the toe and the head to resist driving stresses; Ihe longitudinal reinforcemcnl or prestress provided is greatly in excess of that required to take the structural load, but is needed to resist the stresses of lifting, handling and driving. The toe may be strengthened with a pointed cast-iron shoe, although this may not be necessary when driving in clay.

The casting, storing, transporting and driving of the precast piles for a major contract, which may call for some 20 (XX) piles, is a considerable undertaking. A large area of the site has to be set aside and levelled and accurately paved with concrete to provide a casting bed, and further areas are required for storage. Travel­ ling derricks are required for casting the piles and lifting and handling them, a batching plant is needed to produce the large quantity of concrete required, and a light railway system to transport the piles to the driving frames.

Piles precast in this manner have the advantage that they can be inspected before driving, and good control of the quality of concrete can be maintained. They are driven only after sufficient time has been allowed for the concrete to mature, and are thus resistant to attack from harmful substances in the soil.

Some wastage of concrete is necessary as the tops of the piles have to be cut away to expose the pile rein­ forcement for inclusion in the foundations. An accurate

cut-off. It is possible to drive piles below ground level using a wooden ‘dolly’ as a temporary extension to the pile. Permanent extension of the pile and subsequent redriving is a tedious business as the head has to be stripped away, a new length of pile cast on in-xitu and left to mature before redriving can be undertaken.

A mobile pile frame — a tall steel structure on rails fitted with diesel-driven lifting gear — is employed to drive the piles. This hoists the pile into position, holds it and supports the pile hammer during driving, which continues until sufficient resistance is encountered, A variety of pile hammers are in use. Drop hammers varying in weight up to some 4 tonnes, dependent on the weight of pile, are still used, but diesel-driven hammers, which are much quicker in operation, are nowadays more common. The pile head is protected during driving with a steel helmet lined with a wooden or hard plastic packer.

Some of the problems related to uncertainty concern­ ing pile length can be addressed by jointed reinforced concrete piles which are offered as proprietary systems by some specialist contractors. Such piles can be pre­ cast in appropriate lengths (up to 10 in) and combina-’ tions. It is not desirable to have a joint just below

and with pointed toes, are frequently used to support jetties, especially where long pile lengths are required. Although more expensive than concrete piles, they are much lighter and do not require such careful handling. Lengthening, when required, is a simple matter of trimming the head and welding on a further section. A protective external coating of bituminous compound, or protective paint is required before driving. Once driven, the piles are sometimes filled with concrete or with inert water.

A number of systems for forming in-situ displace­ ment piles are used, each specialist firm having its own method. A heavy steel tube fitted with a detachable cast iron or concrete shoe, or a concrete tube formed of hollow precast sections and fitted with a concrete shoe, is driven using a special piling frame usually mounted on a crawler-tracked excavator. A cage of reinforce­ ment is lowered into the tube and the void concreted. The precast concrete shells are left in position and have the advantage that the outer skin of mature concrete is resistant to chemical attack. Where steel tubes are employed, these are withdrawn as concreting proceeds (great care must be taken to ensure that voids are not created by pulling sections of concrete with the tube). Piles of this type need be only lightly reinforced to take

i ypes or rounoanons

They are less certain in quality than the precast pile, particularly where a steel outer casing is used, as the ground pressure may cause the pile to close up after the casing is removed. A few systems employ a lighter steel permanent casing which is internally supported during driving by a mandrel in a similar way to the precast concrete shell type.

The greatest part of the loading on power station foundations is vertical, but some horizontal forces due to wind and operation loads have to be resisted. These forces are resisted by driving some piles at a rake of up to about 1 in 3. Raking piles are commonly used on such structures as jetties, cooling towers, chimneys, coal-handling structures and transmission .towers.

The very high column loadings now experienced in power station work cause the spacing of piles to be reduced to a point where the increase in ground pressure and consequential ‘heave’ caused by dis­ placement piles can become a serious problem. Great care has to be taken if ground movement is not to occur, and piles be displaced. Pre-boring at the location of each pile is sometimes undertaken to solve this problem. This procedure is also adopted to enable driven piles to penetrate thin hard layers of soil overlaying softer material.

4.5.2Replacement piles

Replacement piles arc constructed by specialist con­ tractors, with various methods of boring. It is rare for a single pile to be used on its own; piles are normally driven in groups at a spacing of 2.5 to 3 times their diameter and their heads incorporated into a thick capping of reinforced concrete designed to transmit the column load evenly to the piles. An arrangement of precast piles to support a column load of about 2000 kN is shown in Fig 3.10 and one to support about 30 000 kN from a major column using large diameter bored piles is shown in Fig 3.11.

If the piles are too close together, there will be considerable overlap of stress from adjacent piles in the soil or rock on which they rely for end bearing. Interaction is a particularly important consideration between piles which rely on skin friction. The bearing capacity of a group of piles is therefore, often less than the capacity of one pile multiplied by the number of piles in the group. This reduction in capacity is greatest for piles in cohesive soils, and in large groups can be as much as one third.

4.6Caisson foundations

For larger concentrated loads it may be necessary to use large diameter caissons either singly or in groups. The concrete shell of a cylinder is built from a steel cut­ ting edge at ground level, and supported vertically by temporary guides. The walls are in the order of 150 mm thick. Precast sections are used for cylinders between