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Isolators dramatically reduce the peak horizontal accelerations suffered by the structure, particularly the amplification caused by building resonance in the damaging 1 to 10 Hz frequency range, as illustrated in Fig 3.13.
Because of the complexities involved in designing connections between isolated and non-isolated plant.
Civil engineering and building works
lit. VIO Piled lotHukilions
2 m and 3 in diameter, but above that size, in-situ concrete is used. The soil from inside the cylinder is removed with an ‘orange peel' circular grab, and the cylinder is forced to sink by loading with kentledge. Grabbing can continue underwater if water-bearing strata are encountered, or an air lock chamber fitted and excavation continued by hand in compressed air. Further precast sections are added at intervals, or the in-situ walls extended until foundation level is reached. The bottom of the cylinder is plugged with mass con­ crete either placed under water or in compressed air, depending on the method of construction, and the remainder of the core concreted. Reinforcement is added if required. Cylinders are frequently used as the supports for jetties.
Construction through unstable ground is facilitated either by the use of temporary (or sometimes per­ manent) steei casing or by filling the boring with a thixotropic bentonite suspension.
Small diameter piles (up to 6(X) mm diameter) are traditionally constructed by a tripod rig which cuts the boring by cable-mounted cutting tools. In recent years

Chapter 3

pivvasl nmciclc piles

the steel can be inscribed either before or alter con­ creting. Considerable care has to be exercised during the concreting operation to avoid forming objects in the pipe, particularly with respect to the withdrawal of any temporary casing.

A recent development in the field of replacement piles is the continuous flight auger system where the soil is not removed until the flight auger has penetrated to the required depth. The auger is then withdrawn as the space beneath is continuously filled with pumped sand cement grout which passes down the centre of the auger.

4.7 Anti-seismic foundations

Anti-seismic foundations incorporate engineered de­ vices to isolate the main building structures and plant from the damaging effects of earthquake ground motions. /

The commonest form of isolator is similar to a bridge bearing pad made up of alternate bonded horizontal

layers of neoprene rubber and steel plates. Bridges and very small piles (micropiles) bored by small rotary drills other structures may also be isolated with devices con­

have appeared. Both systems have the advantage of

taining a solid lead core which, in plastically deforming,

being operable in conditions where the working space isabsorbs

vibrational energy. The isolators arc placed

restricted. Larger diameter piles up to about 2 m are

between

the structure to be isolated and the ground

bored by heavy lorry or crane-mounted rotary augers. foundations (see Fig 3.12).

In suitable conditions such piles are sometimes pivoted with an enlarged base using under-reamers.

On completion and cleaning up, of the boring, con­ creting of dry holes is usually by direct discharge. Where the boring is full of water or bentonite an adjusted concrete mix is placed using a trevice pipe. Depending on the quantity of reinforcement required.

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