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The origin of Daido piles comes from Japan where these prestressed concrete piles are used as replacement plies. Holes are pre-formed in the ground and Daido piles are placed inside these pre-formed holes with subsequent grouting of void space between the piles and adjacent ground. However, in Hong Kong Daido piles are constructed by driving into ground by hammers instead of the originally designed replacement method.
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In normal practice, reliance is placed on the self-compaction of specially designed concrete mixes to achieve adequate compaction. The use of vibrating devices like poker vibrators is seldom adopted for the compaction of concrete piles.
Based on the research by STS Consultants Ltd., it was found that concrete placed by free falling below 120 feet would not suffer from the problem of segregation and the strength of concrete would not be detrimentally impaired provided that the piles’ bore and base are dry and free of debris. Moreover, it is presumed in the past that during free falling of fresh concrete into the pile bores the hitting of falling concrete in the reinforcement cage causes segregation. However, in accordance with the experimental results of STS Consultants Ltd., the striking of reinforcement cage by fresh concrete does not have significant effect on the strength of concrete
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In current practice concrete cover is usually provided at the pile tips of pre-bored H-piles socketed in rock. The purpose of such arrangement is to avoid the potential occurrence of corrosion to H-piles in case concrete cover is not designed at pile tips. However, recent field and laboratory observations had reservation of this viewpoint.
For high-rise buildings constructed on steep cut slopes, these buildings are usually supported by large diameter piles. Though the piles are founded at some depth below the slopes, lateral load arising from wind on buildings may induce loads on the slop and causes slope failures. For shallow depths of slope which is marginally stable, it is more vulnerable to slope failure.
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In shaft grouting operation, tube-a-manchette pipes are fixed at regular spacing to the reinforcement cage. After concreting barrettes/bored piles, a small volume of water is injected under high pressure into these pipes to crack the concrete.
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Down the hole hammer has been used extensively to form pre-bored holes as rock sockets for mini piles and pre-bored H piles. The hammer functions by driving repeatedly a drill bit using compressed air on the rock. However, the use of down the hole hammer is normally limited to hole diameter of 600mm.
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Reversed Circulation Drill (RCD) is normally used for forming large diameter rock socket. The method involves the exertion of a download force of roller cutter bits on rock, together with the action of rotation and grinding of bits on rock. The cuttings are then removed by reverse circulation. The water and cuttings are airlifted through a central drill pipe, which is also used for rotating the drill bits.
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For unstable grout, particles will come out of the grout suspension leading to incomplete grouting and clogging of pipes. The stability of grout can be improved by adding additives such as bentonite. However, bentonite should not be used with very fine cements because its grain size is bigger than that of fine cements. Tests conducted previously confirm that a grout with bentonite is less stable under pressure.
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Compression of rockfill is normally caused by a reduction in dimension of fill and by rearrangement of particles into closer packing.
When the rockfill are saturated, the strength of rock would be reduced accordingly. In fact, wetting of rock surfaces does not reduce the coefficient of sliding friction between rockfills. Considerable settlement may result not from the lubricating effect of water but from a reduction of rock strength at its point of contact. The contact points would then be crushed under intergranular force and the contact area increases until contact pressure is less than the strength of rockfill.
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