Field Studies of Well-Instrumented Barrette in Hong Kong

A large excavated rectangular pile (barrette) with lateral earth pressure and pore-water pressure cells was successfully constructed and tested in a sequence of marine, alluvial, and weathered granite soils. A "soft" base formed beneath the bottom of the barrette permitted over 100 mm of vertical settlement, completely mobilizing the shaft friction at the barrette-soil interface. During the vertical load tests, an unusual and complex response of pore-water pressures and earth pressures at the barrette-soil interface was measured. During each vertical loading cycle (except the last one) and before interface slippage of the barrette occurred, excess positive pore-water pressures were recorded in all soil layers. Upon the initiation of slip at the barrette-soil interface, a sudden drop in the measured pore pressures as well as a substantial drop in lateral earth pressures generally resulted. Subsequent loading or unloading slippage events did not show the same dramatic behavior unless a period of consolidation/recovery was allowed first. This implies that caution must be used in design of barrettes relying heavily on skin friction when shearing induces contractive soil behavior. The current test results indicated that the empirical uncorrected SPT-N value approach and the effective stress (beta)-method were inconsistent.