Instrumented pile load tests in limestones and granite formations - case studies with optical fibre strain sensors

Abstract

In instrumented pile load tests, obtaining a reliable strain profile is crucial to interpret the load-transfer behaviour of the test pile. For conventional instrumented pile load tests, an extensive quantity of point-based sensors such as vibrating wire strain gauges and tell-tale extensometers were installed to measure strains and pile deformation especially when the test pile is long and with large variation in soil stratum. The advancement of distributed fibre optic sensor (DFOS) technology, has significantly improved the conventional instrumented pile load test method. DOFS based on Brillion Optical Time Domain Analysis (BOTDA) is a novel technique for measuring strains in a continuous manner that outweighs the conventional point-based sensors. A full continuous pile length strain profile minimised misinterpretation of the load-transfer and mobilised shaft friction of the test pile, especially the cast-in-situ pile. This paper discusses two case studies of instrumented pile load tests performed using BOTDA technology for the Klang Valley Mass Rapid Transport (KVMRT) project found in limestones and granite formations. The interpreted results in terms of load transfer response, average unit shaft resistance, and elastic shortening of rock socketing piles are in exceptional agreement with conventional instrumentations such as Vibrating Wire Strain Gauges (VWSG) and Linear Variable Displacement Transducers (LVDT) at 6% difference. The maximum unit shaft friction acquired in the granitic layer was about 2,200 to 2,500 kN/m2 whereas the pile founded in limestones achieved maximum unit shaft friction of 1,800 to 1,900 kN/m2.