Laboratory Study to Assess Load Capacity Increase of Drilled Shafts through Post Grouting

Abstract

While post grouting of the base of a drilled shaft is an acknowledged means of mobilizing shaft base resistance at low values of displacement, much uncertainty exists among engineering professionals as to the conditions under which this option may be entertained. The impact of diverse factors such as soil gradation and soil relative density, overburden pressures, and grouting methods utilized need to be properly understood. Furthermore, the complete benefit of post grouting, as a means of economizing construction expenditure, can only be realized when it is factored into the project at the design stage. For consideration of post grouting as a viable alternative, the designer must be able to effectively predict the load � displacement behavior of both the grouted and ungrouted shafts for direct comparison of one to another. Therefore, an effective numerical model capable of providing shaft base resistances at increasing levels of displacement is required. This report presents experimental and analytical research work carried out towards developing a numerical model capable of predicting shaft base load � displacement response with and without post grouting and a step-by-step procedure that is suitable for designing drilled shafts incorporating the post grouting option. Included in this report is the development and use of the large scale testing chamber, model shaft, grouting apparatus and instrumentation utilized to study the different factors affecting the usefulness of post grouting. The model shafts were tested in two types of soils each prepared at two different relative densities for both the ungrouted and grouted shaft tests. The developed soil deposition method is unique in enabling preparation of soil to uniform relative densities fairly accurately. Experimental results obtained include the variation in pressures induced during grouting as well as the load � displacement behavior of ungrouted and grouted shafts. These show definitive proof of the increase in resistance as a result of post grouting. The analytical model was developed based on refinement of the shallow foundation model and provides variation in loading resistance with base displacement. This model was calibrated using the experimental data obtained and accuracy demonstrated using case histories. The numerical model unload � reload loop was also obtained to enable prediction of the grouted shaft load � displacement behavior.