On the Interpretation of the Bidirectional Static Load Test

Abstract

. The paper deals with the bidirectional test, aiming at interpreting its results, showing the factor that governs the upward movement and presenting approximate formulas to find the equivalent load-settlement curve of a conventional compressive test. The factor is the product c’.k = (1-c).k, where c’ and c are the Leonards-Lovell coefficients, related to the elastic shortening of the shaft under bidirectional and conventional static loading tests, respectively, and k is the relative stiffness of the pile-soil (shaft) system. The paper shows that when this product is constant the properly normalized upward curve is invariant, independently on shaft resistance distribution. Nomograms are presented to quickly determine c’ for two patterns of shaft friction associated to weak upper layers over deep more resistant soils. Under this condition, it is shown that c > c’, i.e., the load causing the downward movement in the conventional test induces greater elastic shortening than in the bidirectional test. Therefore, additional movement must be added to the measured displacement in the bidirectional test, which constitutes the basis of the proposed approximate formulas to find the equivalent curve of the conventional test. To validate these findings a mathematical model is used together with five case histories, comprising short to long piles. In one case a conventional compressive loading test was also available.