Post Stressing Drilled Shaft Tips

This paper presents a discussion of geotechnical design considerations when utilizing the post grouting construction technique. Post grouting of drilled shaft tips using pressure grouting techniques can significantly increase the ultimate capacity of the shafts. The paper also describes two case histories. The first case history is a project where exceptionally high end bearing capacities were developed. On one shaft, an ultimate end bearing capacity of over 925 kg was developed on a 1.2 m diameter shaft founded in a sand stratum. The second case history is a project consisting of multiple post-grouted shafts installed on a bridge overpass project where the density of the bearing sand stratum varied across the site. The project specified load test showed lower than expected achievable end bearing capacities, and therefore a revised design was developed to extend the depths of shafts to develop additional skin friction capacity. However, an alternate post grouted shaft design was proposed and approved. The alternate design included post grouting all the production shafts, and thus allowed the successful completion of the project without the need to increase the planned shaft depths.