ABSTRACT
Preconstruction soil improvement by compaction grouting in karst areas can be a cost effective alternate to conventional deep foundations or minipile foundation systems. This paper presents a case history of compaction grouting for site improvement on portions of a new corporate campus in Chester County, Pennsylvania. The new campus includes six-two and three story office buildings, and two-two story parking structures situated on a site of approximately 809,371 square meters (200 acres). The majority of the site is underlain by bedrock of the Conestoga Formation, which typically includes micaceous limestone, phyllite, and alternating beds of limestone and dolomite. Carbonite members of the Conestoga Formation are soluble and, therefore, susceptible to sinkhole development, and are present under significant portions of the site.
Site investigations included geologic site mapping and fracture trace analyses, standard penetration tests (SPT’s), static and dynamic cone penetrometer tests, and seismic refraction surveys. All subsurface information was managed through the use of computer databases and digital terrain models detailing the existing ground surface, estimated top of bedrock and estimated soft “ravelled” soil zone thickness throughout the building areas. This information was used by the geotechnical engineer and construction manager to compare foundation systems and develop estimated costs for foundation alternatives. Based on cost analyses and an evaluation of acceptable risk, the alternative of ground improvement (including compaction grouting and soil exchange) and shallow foundations was selected for the office buildings. A grade beam foundation, designed to span an unsupported zone of approximately 12.2 meters (40 feet), was selected for one of the two garages. The other garage was underlain by non-soluble phyllite and did not require special foundation considerations.
Based on an evaluation of the subsurface data and proposed building stresses, select areas requiring soil exchange and compaction grouting ground improvement were established. In addition, ground improvement performance criteria were established based on settlement and bearing capacity analyses.
Compaction grouting was performed in the selected areas and included “sealing” and grouting improvement in the upper 5 feet of bedrock and throughout the soil zone up to the proposed bottom of footing elevation. The effectiveness of the compaction grouting program was verified by review of grouting records and by use of post grouting SPT soil borings. In all, approximately 11,534 square meters (2.85 acres) of the total 48,562 sqaure meter (12 acre) structure footprint area was successfully treated with compaction grouting.
