A Case History - Termination of Pile in Rock Based on Penetration Resistance for a Bridge across River Mandovi in Goa-Paper

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The Mandovi River bridge in Goa is supported on large-diameter piles socketed into rock. During construction at Pier 32, the BG-28 rotary rig encountered substantially stronger rock than indicated by the original geotechnical investigation. Drilling slowed to about 5 cm per hour, with equipment jamming and damaged rock-cutting tools, making the designed 5.4 m socket difficult to achieve and threatening project time and cost.<br /><br />Two confirmatory boreholes were drilled near the pier. Their improved core recovery and rock quality designation (RQD) showed that the rock was generally better quality than initially reported, although rock type and condition varied. The findings explained the unexpectedly low drilling productivity and provided additional information for reassessing the foundation.<br /><br />Rather than continue chiseling or deploy additional rigs, the project team assessed termination using pile penetration resistance (PPR), an estimate of the energy required to advance the bore into rock. On-site chiseling records and confirmatory borehole data indicated PPR values above 225 tonne-metres, signifying hard rock. The consultant consequently revised the socket depth for the 1.5 m piles from 5.4 m (3.6 times the diameter) to 3.75 m (2.5 times the diameter). This met the client’s minimum socket requirement and the design capacity requirement of 500 t/m².<br /><br />The revised termination criterion was accepted by the client and implemented. The case demonstrates that pile socket depth should be determined using site-specific rock conditions and penetration resistance, rather than relying solely on initial investigation data or fixed rules of thumb. It also highlights the value of confirmatory boreholes and controlled chiseling: excessive chiseling can damage rock around and below the pile tip, potentially increasing settlement.

Keywords

Mandovi River bridge

Goa

rock socket piles

rotary drilling

confirmatory boreholes

RQD

pile penetration resistance

socket depth revision

hard rock

foundation design