Improvement of chalk solution feature infill by rapid impact compaction

This paper provides upper- and lower-bound limits of stiffness improvement observed from the treatment of infilled solution features in chalk as part of the Central 1 contract being delivered by the Align JV, which is part of the UK's High Speed 2 Phase 1 rail link. Infilled solution features were treated using rapid impact compaction (RIC) to achieve a sufficiently stiff subgrade beneath a wide range of temporary foundations and reduce the risk of collapse settlement. Improvement by RIC treatment was sufficient to ensure subsequent foundation performance or reduce the extent of compaction grouting subsequently required beneath the most heavily loaded foundations. The depth of improvement observed was up to 10 m and the improvement in elastic modulus observed was up to five times the pre-treatment value. Over 40 cone penetration tests were conducted before and after RIC. Typical lower- and upper-bound improvement curves are presented based on the observed minimum and maximum post-treatment stiffness. The degree of stiffness improvement was observed to generally reduce at greater than 5 m depth.