Development and Assessment of Post-Grouting Methods to Increase the Load Carrying Capacity of Deep Foundations

摘要

The utilization of tip capacity in drilled shaft foundation applications remains unexploited in most design problems other than when the tip is within basement rock with adequate socketing. Commonly, tip capacity is neglected due to slough at the bottom of the borehole and the general necessity of excessive downward displacement of the shaft, at the tip, to develop adequate load carrying capacity therein. Inelastic materials, such as cohesionless soils, generally show increasing stiffness under repeated loading. Similar to precompression, if a load could be applied to the pile tip and then removed (compared to strain-hardening of steel), then the effect of the ensuing load from the superstructure could be considered a case of reloading of the tip. The reaction of the tip to reloading would be much stiffer than a virgin load-displacement response. This would then allow incorporation of the tip capacity into the design leading to shorter shafts. Initial loading of the tip can be carried out through the process of injecting low mobility compaction grout at the tip after the shaft concrete has hardened. This process of grouting of the tip, at high pressures, would be a case of loading the tip against the skin friction capacity of the shaft (in essence a cost effective load test).rnTwo field load tests were carried out in Washington on 6-ft. diameter shafts. In addition, a large scale lab testing program is in progress at the University of Nevada, Reno to yield a viable construction procedure and a recognized analysis/design approach to the effects of post-grouting of drilled shafts.