Field Studies of Well-Instrumented Barrette in Hong Kong

Abstract

Over the last 15 years, barette foundations have become increasingly popular in parts of Asia, such as Hong Kong and Malaysia, for many civil engineering structures and tall buildings. A large excavated rectangular pile (barette) 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 barette permitted over 100 mm of vertical settlement, completely mobilizing the shaft friction at the barette-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 and 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 N-value measured by Standard Penetration Tests (SPT-N approach) and the deduced shaft friction coefficient (beta) value based on the effective stress principle (effective stress beta-method) were inconsistent.