Axial Behaviour of Hollow Core Micropiles Under Monotonic and Cyclic Loadings

摘要

The use of hollow core bars in micropiles has greatly increased over the past ten years. Hollow core construction, also termed self drilled, is becoming a popular option because it allows a faster installation processes and ground improvement at the same time. This paper presents a field study on the behaviour of single hollow core micropiles in stiff silty clay deposit. Four hollow core micropiles were installed using an air flushing technique employing large drilling carbide bits. Ten axial tests were conducted on the four micropiles, including three compression tests, two tension monotonic axial tests, four compression cyclic tests, and one tension cyclic axial tests. The results of the full-scale loading tests are presented and analyzed in terms of load displacement curves. The results of the monotonic testing phase showed that the bond strength values (α_(bond)) suggested by the Federal Highway Administration in 2000 for the silty clay deposits may be underestimated when considering hollow core micropiles as type B micropile grouting. The response of the micropiles to cyclic loading is considered satisfactory. No sign of full debonding occurring at the pile-soil interface was observed after 15 load cycles with cyclic load amplitude of 33 % of the micropile design load. The stiffness of most micropiles remained almost constant after the cyclic loading. However, the pile head movement increased after the cyclic loading due to limited strain softening behaviour of the soil deposit occurs at the grout/ground interface. The results showed that the micopile's performance in stiff clay is not sensitive to minor changes in cyclic load amplitudes, but sensitive to the magnitude and rate of the total applied load.