Instrumented static load test on rock-socketed micropile

Rock-socketed piles are often used to transfer heavy loads from a superstructure to competent underlying rock layers. The loads are transferred by the pile to the surrounding rock mass through shaft and base resistance. Several researchers have investigated the behavior of rock-socketed drilled shafts and related the uniaxial compressive strength of intact rock to pile-shaft resistance. However, the loadtransfer behavior and load-settlement response of micropiles are different from those of drilled shafts because of the large slenderness ratio (pile length=pile diameter) of micropiles. This study presents results from a fully instrumented field-scale load test on a 0.2-m-diameter micropile socketed 4.2 m into limestone layers (2.7 m into weathered limestone and 1.5 m into hard limestone). The results show that practically no base resistance is mobilized until the pile-head settlement reaches approximately 7% of the diameter of the test micropile. The measured limit shaft resistance values are compared with values predicted using methods available in the literature. The comparison indicates that use of the intact strength of the rock for estimation of limit shaft resistance may lead to an unconservative design when the rock mass quality is very poor, as indicated by, for example, low rock-quality designation values.