Wednesday, February 13, 2013

Methods of Piling.

There is a wide variety of methods used for piling and every piling contractor has a number of variations for  their system – improvements in method and equipment continues. The main classes only are discussed below.

1 Driven piles
This method is used for piles of timber, precast concrete, prestressed concrete and the various types of steel piles.

The pile is hammered into the ground by pile-driving plant shown in outline in Fig. 14.10 (a). Methods of protecting the head of the pile from shattering are shown in Fig. 14.10 (b).

Driven piling.
Fig. 14.10 Driven piling.

Driven piles are classified as displacement piles and, where the soil can enter during driving, as small displacement piles (e.g. open ended tubular or other hollow sections often in steel).

2 Driven cast-in-place piles
A closed ended hollow steel or concrete casing is driven into the ground and then filled with fresh concrete. The casing may be left in position to form part of the whole  pile or withdrawn for reuse as the cast concrete is placed.

The cast concrete is rammed into position by a hammer as the casing is withdrawn ensuring firm contact with the soil and compaction of the concrete. Care must be taken to see that the cast concrete is not over-rammed or the casing withdrawn too quickly. There is a danger that as the liner
tube is withdrawn it can lift up the upper portion of in situ concrete leaving a void or necking in the upper portions of the pile. This can be avoided by good quality control of the concrete and slow withdrawal of the casing.

Driven cast in situ piles can prove to be economic for sands, loose gravels, soft silts and clays particularly when large numbers of piles are required. For small numbers of piles the on-site costs can however prove to be expensive.

3 Bored cast-in-place piles
The hole for the pile shaft is formed by drilling or augering and the toe of the hole can he enlarged by under-reaming in stiff clays to provide greater end-bearing capacity for the pile. The method tends to be restricted to clayey soils and, as with the driven cast-in-place pile, care must be exercised to prevent necking of the cast concrete. If they are used in loose sand or silt the inflow of soil into the bore must be  prevented. They can be installed in very long lengths and be of large diameter.

The relatively small on-site cost of bored piles means that smaller sites can be more economically piled than they can using a driven piling system. The bored pile is not usually economic in granular soils where loosening and disturbance of surrounding ground can cause excessive removal of soil and induce settlement in the surrounding area. During piling operations the hole can be lined with a casing which can be driven ahead of the bore to overcome difficulties caused by groundwater and soft sub-soil but sometimes difficulties of withdrawing the casing after casting can prove expensive.

4 Screw piles
Screw piles of steel or concrete cylinders with helical blades attached are screwed into the ground by rotating the blades. Their best application is in deep stratum of soft  alluvial soils underlain by firm strata. Due to the large diameter of the blades the piles have increased resistance  to uplift forces. Screw piles can be removed after use in  temporary works.

5 Jacked piles
Jacked piles are used where headroom for the pile and pile driver are limited as in underpinning within an existing building. The pile is jacked in short sections using the existing superstructure as a reaction frame.

6 Continuous flight auger piles
The flight auger pile system uses a hollow stem auger mounted on a mobile rig. The auger is drilled into the
ground with very little vibration and spoil removal. When the required depth has been reached (see Fig. 14.11), concrete or grout is injected through the auger shaft. Usually the concrete or grout mixing plant and the pumping equipment are located nearby but can, if such areas are sensitive, be located well away from such positions. Pile lengths of up to 25 m can generally be achieved with pile diameters from 300 mm to 600 mm. Piles can be raked up to an angle of 1 in 6 from vertical. The system is suitable for use in most virgin soils and fine granular fills and rigs can operate in areas with restricted headroom.

Flight auger pile.
Fig. 14.11 Flight auger pile.

7 Mini or pin piles
There are a number of mini or pin piles on the market. The systems range from water- or air-flushed rotary percussion augers to small-diameter driven steel cased piles which are driven to a set.

The pile diameters generally vary between 90 mm and  220 mm and can be used in most soils and with restricted access/limited headroom. Where necessary, noise and vibration can be kept to a minimum and piles can be driven within a few hundred millimetres of adjacent properties.

In underpinning they can be used to penetrate existing  concrete or masonry foundations, and can be bonded into the existing elements or form part of a new support system in conjunction with cast-in-situ ‘needle’ beams.

Slenderness of such small-diameter piles must however be taken into account and the need for good quality control particularly with regard to filling such small bores with concrete.

The piles are not generally suitable in mining areas where surface movements and lateral strains may be expected to distort or shear the piles.

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