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Is critical depth of piles a fallacy?

The critical depth of piles are normally assumed as 10-20 pile diameter deep and is the depth beyond which the resistance is constant and is equal to respective value at critical depth.
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How do fixed and pinned connections between piles and pile caps affect the load carrying capacity of piles?

The type of connection between piles and pile caps affects the load carrying capacity of pile groups. The fixity of pile head into pile cap, instead of pinning into pile cap, enhances higher lateral stiffness of the pile groups. For instance, for the same deflections, a cap with fixed connected piles can sustain far more loads than that of pinned connected piles. To satisfy the criterion of fixed connection, the minimum embedded length of piles into pile caps should be at least two times the diameter of piles.
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What are the methods to tackle negative skin friction?

(i) Use slender pile sections (e.g. H-pile or precast pile) because smaller pile area when subject to the same working load would produce higher deformation, thus increasing the relative downward movement of piles.

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What are the advantages of using top-down approach in basement construction?

The advantages of top-down approach are listed below:

(i) The structures above ground can be carried out simultaneously with the structures below ground. This greatly reduces the time for construction.
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How to measure rock during piling operation?

There is no strict rule in governing the measure of rock encountered in piling operation. There are two common practices in measuring rock during piling:

(i) Measure the quantity of obstruction taken out from the drilled out;
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