An Approach for Optimization of Drilled Shaft Design in Dubai-Paper

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The paper examines why drilled-shaft designs in Dubai are often conservative. Static and dynamic load tests commonly show capacities greater than theoretical estimates, particularly because shafts in Dubai’s weak sedimentary rock (intermediate geomaterials, or IGMs) resist axial loads mainly through side friction.<br /><br />The commonly used Williams and Pells method estimates maximum unit side friction from rock compressive strength and a reduction factor, β, representing rock-mass discontinuities and shaft–rock interface conditions. Design estimates may be too low because they typically rely on unconfined compressive strength (UCS) and conservative assumptions about rock-mass quality. Installation method also affects interface roughness: bentonite slurry can reduce friction compared with cleaner drilling methods.<br /><br />The authors recommend improving estimates by selecting representative rock-mass parameters using high-quality drilling and by assessing stiffness through in-situ pressuremeter and down-hole seismic tests. They also propose laboratory consolidated-undrained triaxial (CIU) tests, conducted at pressures representative of in-situ conditions, with local-strain measurements. For Dubai IGMs, CIU compressive strengths were found to be about 1.5 times UCS values. Using these strengths, together with more appropriate rock-mass factors, raises the estimated unit side friction by approximately 30–40% relative to typical nominal estimates—consistent with observed loading-test results.<br /><br />Preliminary instrumented pile tests, including O-cell tests, are encouraged to calibrate design assumptions and improve predictions of capacity and load transfer. The recommended optimization depends on adequate investigation data and experienced engineering judgment when defining representative parameters for each geological layer. Overall, the proposed approach aims to reduce unnecessary conservatism while maintaining a design process grounded in site-specific testing and measured pile performance.

Keywords

drilled-shaft design

Dubai

side friction

intermediate geomaterials

Williams and Pells method

unconfined compressive strength

bentonite slurry

pressuremeter tests

CIU triaxial tests

O-cell pile tests