Compaction Grouting As Ground Modification in Karst Geology

Often times when a project site exhibits poor subsurface conditions not suitable for foundation support the most logical solution is a deep foundation system supported on/in an underlying dense/hard stratum. However, in some instances ground modification associated with shallow spread footings can be a more cost effective solution. Compaction grouting is an extremely flexible and customizable approach that can be applied to numerous poor subsoil conditions to stiffen and densify the subsoils to a desired degree applicable to the proposed construction. Compaction grouting is not a new technique; its application in areas of Karst geology where acute subsurface stabilization is necessary has been effectively used in many instances since the early 1950's (Warner 2004). Subsurface erosion in Karst areas is prevalent in an area of eastern Pennsylvania that spans from as far west as Shippensburg through Lancaster, Reading, and Allentown to the eastern border in Easton. Two case histories are presented — one involves "stiffening" the foundation subsoils encountered beneath additions to an existing 2-story structure in Millersville, PA. The second case history is the improvement of soils impacted by a water main break in Bethlehem, PA. These case histories describe subsurface conditions that compaction grouting was applied to and the methods used to ensure a successful grouting program. Also discussed are real time field results used to modify the compaction grouting program and monitor its effectiveness. Full-time monitoring was provided throughout the field work on each project.