The purpose of this project was to significantly reduce or eliminate the mine subsidence potential beneath a number of Phase II structures at the Wabash Valley Correctional Institution (WVCI) in Carlisle, Indiana. From the study conducted by Gennaro G. Marino Engineering Consultants, the Indiana Department of Administration Public Works (DAPW) determined that mine backfilling was the most costeffective solution for mine subsidence remediation of certain proposed structures.
As shown in Figure 1, existing Phase I structures were just north of the Phase II construction. Contrary to Phase I, many of the Phase II structures were undermined. Therefore, because of the approximately 300 ft. deep abandoned mine void, there was a significant potential for mine subsidence that could result at a number of the proposed Phase II structures. History shows that a structure ill prepared to handle mine subsidence performs very poorly with the associated damage resulting in abandonment of the structure. In order to limit the possible subsidence damage and to ensure safe operation of the structures, mine backfilling was performed in mine areas labeled Mine Areas 1, 2, and 3 which were subjacent to critical Phase II structures (see Figure 1).
Based on drilling information collected, Mine Area 1 contained more roof cave than Mine Area 2. Mine Area 3 appeared to have the most fall. This area, however, was small in comparison with the other two areas. The effect of the additional amount of rubble in the mine seemed nominal, as there was no substantial change in the grouting operation from Mine Areas 1, 2, and 3.
During the grouting operation, each mine area was first contained by strategically placing containment grout in certain entries and crosscuts. Once the mine areas were significantly contained, they were pumped with infill grout (which was lower in cost and more flowable than containment grout). Infill grouting generally proceeded across the mine with each injection hole grouted to refusal. In some highly rubblized areas, some retarder/plasticizer (calcium lignosulfonate) was used in the grout to increase penetration.
Once the grout operation began, the grout movement was monitored in adjacent holes by using electric resistivity probes (see Figure 2). Most of the holes were pumped to refusal. Refusal could not be achieved by only working the day shift despite pumping grout at typically 30-40 cy/hr., and so multiple shifts were necessary. Grout line refusal was taken at 600 psi or greater.
After the grouting operation was finished, core samples of the grouted zones were obtained for the verification. In-hole water pressure tests were conducted in grouted mine areas with low recovery. All tests were at refusal. The results obtained from the verification process showed the success of the operation.
At the completion, 9,559 cy of cement-flyash-sand grout was injected and 41,349 LF of drilling was performed. By implementing this backfill procedure, the potential subsidence at certain Phase II structures was suppressed at the prison site at Carlisle, Indiana. The backfilling was found to be significantly more cost-effective than designing each structure to be subsidence resistant.