Bi-directional Load Test
The bi-directional load test is the only alternative static load testing method that uses a time and space saving approach of loading the foundation unit’s resistance from the mid-pile or near-toe position instead of top-loading.
The ability of this load test method to embed the sacrificial hydraulic jack near to the toe of the foundation unit enable the skin friction and end bearing of the foundation unit to be assessed separately. Unlike conventional maintained load test which load is apply from the top, the skin friction and end bearing is usually not distinguishable because the foundation unit is loaded as whole, and also due to the fact that large movement (usually 10% of the pile size) is required to mobilise the end bearing which may not practically achieved in conventional maintained load test.
Bi-directional load test perform similarly as to the conventional maintained load test with two important static features: 1.) ability to apply the test load in stages, and 2.) ability to maintain loading over specified duration. Due to these features, the static performance and soil behaviour of the foundation unit tested could be analyzed for comparison with the conventional maintained load test.
As in conventional maintained load test, bi-directional test pile instrumented with strain gauge provides essential information on the soil parameter, i.e. static load distribution and subsequent computation of mobilised unit skin friction and unit end bearing.
Bi-directional load testing is carried out via fully automated recording, by using displacement and pressure transducer which the readings are logged by data logger and display live on the PC at a close 1-minute interval. In the past years, the performance of bi-directional load test is often compared with conventional maintained load test, and with Finite Element Method (FEM). Many papers have cited good agreement between the comparison of performance of the two load test methods, and also comparison with FEM modelling.