Full scale load test data give very variable and sometimes very low shaft resistance for large diameter bored piles founded in Hong Kong decomposed granite. These observations were discussed and explained in this paper.


Large diameter bored piles in Hong Kong are routinely used to support high column loads. Bored piles with an allowable pile loads in excess of 10MN are commonplace. Local design guidance, PNAP No. 141 (1994) published by Building Department, Hong Kong, gives an allowable bearing pressure of 5000kPa if the founding material is moderately decomposed rock or better, with a total (rock) core recovery of no less than 85%, and an unconfined compressive strength of at least 25 MPa. For more superior rock, an allowable bearing pressure of 7500 kPa may be used. For long piles, the required capacity may be obtained purely from shaft resistance in the decomposed granite. The decision to found large diameter bored piles in decomposed granite (as floating piles) is made when the bedrock is excessively deep (over 40m), or when the rock conditions are such that the acceptance criteria stipulated in PNAP141 are difficult to be fully met. The use of large diameter bored pile founded in decomposed granite began in the early eighties, notably in the projects of Telford Gardens (Holt et al, 1982) and the Island Eastern Corridor (Fraser, 1985). Since then, this type of foundation has been used both in public infrastructure and private development projects. A number of full-scale pile tests have been conducted in attempts to justify the design.

Sub-surface Conditions

Moderately weathered granite (MDG) manifested rock behaviour, but highly decomposed granite (HDG) and completely decomposed granite (CDG), often referred to as saprolites, is a soil like material in which some of the parent rock fabric is retained.

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