Unconventional Geotechnical Investigation for Construction of New General Cargo Terminal at Port Owendo, Gabon-Paper

Paper

Pdf Summary

The paper describes an unconventional geotechnical investigation used during construction of a new cargo terminal at Port Owendo, Gabon. The project required 675 bored, cast-in-place piles, 1.4 m in diameter, founded in weak to very weak sandstone and siltstone. Initial investigations indicated variable rock strength, while pile boring pulverized the rock into material resembling sand or paste, making it difficult to identify rock and determine pile termination depths. This uncertainty led to excessive socket lengths.<br /><br />Conventional approaches—including pile penetration ratio calculations, pressure-meter results, reverse SPT testing, and a marine-simulated pile load test—helped assess conditions but did not fully resolve concerns about rock strength. The team therefore extracted rock cores directly from working piles using a 600 mm core barrel attached to the piling rig. Samples were collected at successive depths and tested for unconfined compressive strength (UCS). Results ranged from 0.91 to 14.60 MPa, averaging approximately 6.18 MPa, confirming that the pulverized material was rock rather than sand.<br /><br />The investigation also established a practical method for identifying the top of rock: treat the point approximately 1 m below the refusal level of a vibro-driven liner as the start of the rock socket. Based on the strength findings, the adopted socket lengths were 6 pile diameters for land piles, where average UCS was about 6 MPa, and 9 diameters for marine piles, where rock was weaker.<br /><br />The revised approach reduced socket length from an initial 17.3 m to about 10.9 m in early piles. Across the project, it saved approximately 4,258 m of rock drilling, reducing cost, construction time, fuel use, and equipment wear. The authors conclude that direct sampling from working piles can provide useful strength data where conventional investigation is inconclusive and support safer, more economical pile design.

Keywords

geotechnical investigation

cargo terminal

Port Owendo

bored piles

rock socket

unconfined compressive strength

pile boring

working pile coring

weak sandstone

pile design optimization