Estimate of Ultimate Bearing Capacity of Timber Piles Driven in Medium to Very Soft Clay

Abstract

A method to estimate an ultimate bearing capacity, R_u, of timber piles driven in medium to very soft clay (whose N-value ranges between 0 and 8) is investigated. Firstly, as the skin friction and the end resistance composing R_u are uniquely dependent on the undrained shear strength of clay, c_u, a linear relationship between the N-value and c_u is proposed on the basis of statistical analysis of the unconfined compression test results. Then the values of R_u are calculated by the bearing capacity formula of pile using the N-c_u relationship proposed, and are compared with the second-limit-resistances measured in the static axial compressive load tests of a single pile, with the result that some 90% of estimation accuracy is found. As the timber pile tapers to the lower end, its ultimate bearing capacity becomes larger than that calculated using the sectional diameter of the lower end of pile. Taking this taper of timber pile into account the values of R_u are newly calculated and compared accurately with the second-limit-resistances. The above discussion leads to the conclusion that the N-c_u relationship proposed is effectively practical and accurate in estimating R_u of timber piles driven in medium to very soft clay. It's also particularly stated that a special effect of taper should be included into the calculation of R_u of timber piles.