Mann road sinkhole stabilization: A case history

On September 21,1988, a sinkhole suddenly developed beneath a 54 inch diameter effluent transfer line. The sinkhole occurred along Mann Road in Orange County, Florida, approximately 0.8 kilometer (0.5 mile) south of the intersection of S.R. 545 and Tilden Road. The sinkhole was initially about 9 meters (30 feet) in diameter and 2.4 meters (8 feet) deep at the center, and was beginning to undermine the pipe. During corrective deep grouting, the sinkhole suddenly opened to a diameter of 15 meters (50 feet). While emergency measures were being considered, the prestressed concrete pipeline spanned 15 meters (50 feet) of unsupported length. The vertical extent of the ravelled zone was approximately 38 meters (125 feet) below ground surface. Given the implications of a possible structural failure of the pipe at the unrestrained joints, it was necessary to react immediately and provide judgement based on field observations. With this in mind, the sinkhole was filled with sandy soil immediately. This filling operation maintained support for the pipeline until remedial measures could be taken. After partial stabilization of the pipeline was obtained through filling it with sand, a deep cement-grout stabilization program was implemented to densify ravelled soil conditions and retard future internal soil erosion. The stabilization program consisted of pumping sand-cement grout at depths 38 to 6 meters (125 to 20 feet) below ground surface. Pre- and post-grouting Standard Penetration Test (SPT) and auger borings, as well as Cone Penetration Test (CPT) soundings were made to assess the merit of the grouting program. Post-grouting borings showed that the deep conditions at the site greatly improved due to the cement grouting program. This was evidenced by the increase in the SPT blow count. Because upper sands could not be pressure grouted without heaving the pipeline, additional measures were also proposed to preserve the long-term integrity of the pipeline. The cement grouting program proved to be effective in stabilizing the sinkhole. However, additional remedial measures were also proposed to safeguard the integrity of the pipeline on a long-term basis. Alternatives included chemical grouting, shallow support beam, rerouting of the pipeline, and do-nothing. From these options, the chemical grouting of the near surface soil was adopted as the long-term remedial measure.