Performance of Large-Diameter Bored Pile with Short Socketed Length in Igneous Rock in Thailand-Paper

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The study evaluates the load-transfer and settlement behavior of more than 90 large-diameter bored piles socketed into granite or andesite in Thailand. The piles, constructed using rotary drilling, had rock socket lengths ranging from 0.3 m to more than three times their diameter (S/D). The analysis draws on static load tests and high-strain dynamic tests.<br /><br />Socket length strongly influenced performance. Piles with S/D ≥ 1 generally settled less than 0.5% of their diameter at the safe working load and less than 2% at test loads up to three times that load. Their resistance came from substantial shaft friction and end bearing, supported by a firm, stiff pile base. By contrast, piles with S/D around 0.5 showed greater and more variable settlement, sometimes approaching 5% of pile diameter at twice the working load. Their mobilized shaft friction and end bearing were only slightly greater than those typical of piles in soil. Short sockets may therefore create a risk of differential settlement, particularly if the pile base is poorly cleaned or the rock is weak.<br /><br />Settlement was mainly attributed to elastic shortening of the pile. An analytical method by Tomlinson provided reasonable settlement estimates; the authors estimated rock’s Young’s modulus at the pile base as 50 times its unconfined compressive strength. The data indicate that a socket length of at least one pile diameter is generally sufficient to develop significant end bearing, with no clear evidence that longer sockets consistently increase it.<br /><br />In a project where short-socketed piles caused concern, engineers assessed pile-group stiffness and re-struck selected piles with a drop hammer. Subsequent building monitoring recorded settlements of 2–4 mm and differential settlement within the project’s acceptance limit. The authors recommend evaluating stiffness by area and using pile re-striking where necessary to mitigate settlement differences.

Keywords

bored piles

rock socket

load transfer

settlement behavior

granite

andesite

shaft friction

end bearing

pile stiffness

Tomlinson method