PREDICTION OF AXIAL GEOTECHNICAL RESISTANCE OF BASE GROUTED DRILLED SHAFTS-Paper

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The study evaluates base grouting (BG) as a way to increase the axial resistance and reduce settlement of drilled shafts. Grout is injected beneath a completed shaft to improve the soil at its base. Although BG is increasingly used, its performance—particularly in cohesive soils—has been difficult to predict.<br /><br />Researchers tested three pairs of shafts at two Wisconsin Zoo Interchange sites: one grouted and one ungrouted shaft in each pair. The shafts, 3.9–8.2 feet in diameter and about 52–71 feet long, were tested using Osterberg load cells. Ground conditions included clay, silt, and silty sand.<br /><br />The tests showed that BG increased base resistance by approximately 25% to 100% and total axial resistance by 27% to 43%. The smaller gains may reflect procedural problems: two of the three grouted shafts did not meet the intended grouting criteria. The results emphasize the importance of controlling and verifying grout pressure and shaft movement during installation.<br /><br />The study compared two prediction approaches: the Tip Capacity Multiplier (TCM) and Axial Capacity Multiplier (ACM) methods. Both generally underestimated resistance at a one-inch displacement and could overestimate nominal resistance, but TCM more closely matched measured performance and was recommended for estimating the contribution of BG. BG also appeared to enhance side resistance within roughly 1.5 shaft diameters above the base, though the authors advised against relying on this effect in design without further evidence.<br /><br />For ungrouted shafts, the FHWA GEC 10 alpha method predicted side resistance in cohesive soils reasonably well. Its beta method performed well for granular soils when the friction angle was capped at 40 degrees.<br /><br />Overall, the tests demonstrate that BG can improve shaft performance in both cohesive and granular soils, while highlighting uncertainty in prediction and the need for rigorous construction controls and further testing.

Keywords

base grouting

drilled shafts

axial resistance

settlement reduction

Osterberg load cell

cohesive soils

grout pressure

shaft base improvement

Tip Capacity Multiplier

FHWA GEC 10