Static Load Tests by Bi-Directional Method in the Brazilian Context-Paper

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The paper reviews the use and interpretation of bi-directional static load tests for cast-in-place piles in Brazil, where the method was pioneered in the 1980s and has since been used extensively. Unlike a conventional test, which applies load at the pile head and requires a reaction frame, the bi-directional method uses one or more hydraulic Expendable Static Cells (ESCs), typically installed near the pile toe. Pressurizing a cell pushes the pile segments above and below it in opposite directions, producing separate load–displacement curves for each segment.<br /><br />To estimate how the pile would behave under a load applied at its head, the two measured curves must be combined. Silva’s method pairs equal displacements from the upper- and lower-segment curves and adds their corresponding loads. Other approaches, including Massad’s, account for pile compressibility and elastic shortening. Extrapolation may be needed when the two curves do not cover the same displacement range.<br /><br />The paper identifies elastic shortening as a key factor in interpreting results. For a rigid pile, Silva’s method can be applied directly. For a compressible pile, elastic shortening should be measured or calculated; otherwise, the equivalent pile-head load–settlement curve may be inaccurate. The authors describe the test’s installation, concreting, loading, and post-test grouting stages, and discuss analytical and mathematical methods for deriving an equivalent curve.<br /><br />Overall, bi-directional testing offers a practical alternative to conventional static tests by avoiding a separate reaction system. Its results can be correlated with conventional-test behavior, but reliable interpretation—especially for compressible piles—depends on accounting for elastic shortening.

Keywords

bi-directional static load test

cast-in-place piles

Brazil

Expendable Static Cells

pile toe

load-displacement curves

Silva method

elastic shortening

pile-head settlement

conventional static test