Implication of Site Investigation on Piles Socketed in Weathered Rock-Paper

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The paper examines how geotechnical investigation quality affects the design and construction of piles socketed into weathered rock, using a railway bridge over the Balram River in Gujarat as a case study.<br /><br />Initial boreholes used rotary drilling with single-tube core barrels and produced poor core recovery and limited rock information. The rock was consequently assessed as very poor to poor and highly weathered over substantial depths, leading to conservative, potentially uneconomical foundation recommendations. During pile load-test construction, however, the contractor found the rock harder to drill than expected, and conventional chiselling was also difficult.<br /><br />A confirmatory investigation was therefore conducted at all supports using a hydraulic rig, NX diamond bits, and double-tube core barrels. Controlled drilling and geological supervision improved core recovery at several locations and clarified variations in rock weathering and strength. The revised data informed scour levels and pile-toe depths, while bored cast-in-situ piles socketed into rock were retained to meet capacity requirements.<br /><br />The case demonstrates that unsuitable drilling methods and incomplete reporting can cause design uncertainty, construction delays, and unnecessary cost. Investigations should follow relevant Indian Standards and use appropriate equipment, experienced operators, careful drilling control, and thorough documentation. The authors also caution that core recovery alone may be an unreliable basis for classifying weathered rock as scourable or non-scourable, since it depends on drilling methods. They recommend considering in-situ strength testing, such as pressuremeter tests, and combining geophysical surveys with boreholes to better assess rock variability across a site.

Keywords

geotechnical investigation

pile foundation

weathered rock

core recovery

rotary drilling

diamond core drilling

bridge foundation

rock socketed piles

pressuremeter test

Indian Standards