Repair of Voids Above Jack-and-Bore Pipeline Installations Under a Divided Highway

摘要

Two pipelines were installed approximately 10 years apart as bored crossings under a divided highway in an arid part of the southwestern United States. Sinkholes discovered locally above both lines alerted the pipeline company to a problem with subsurface voids and loose soils around the bored crossings. The sinkholes were initially filled with surface soils or flowable fill. A ground-penetrating radar survey was attempted in hopes of detecting locations and relative sizes of voids, but interference from nearby overhead 345 kV electrical transmission lines limited the technology's success. A vacuum truck with a plastic vacuum hose extension tube was used to create small-diameter potholes at 31 locations. The pothole procedure consisted of jetting with a water wand to loosen the silty and sometimes gravelly sandy soil so that it could be sucked out of the hole with the vacuum pressure. Loose soil was detected in places above the pipe, but abrupt loss of jetting water was the best indicator of voids. Limited-mobility displacement grout (compaction grout) was needed to fill voids and compact loose soil above the buried pipelines. Ranging data of the road surfaces were obtained with a three-dimensional laser scanner before and after grouting in northbound lanes to provide a basis for documenting elevation change that might have been caused by the grouting. Arbitrary coordinates were used for the scanner surveys, the locations of which were related to distinctive features on nearby steel lattice electrical transmission towers. Grouting was accomplished in vertical pipes in the northbound lanes and parts of the median and shoulder. Limited-mobility displacement grout was injected into 129 grout points spaced nominally at 5-ft (1.5-m) centers in two lines over the edges of each pipe. Grout-pressure criteria and local manometer readings on the ground surface were used to minimize expected ground heave. Both sets of scanner survey data showed that the grouting caused no detectible change in the road surface elevation. Error in the scanner data, which varied with distance from the scanner setup location, was attributed to windy conditions, including air blasts generated by passing vehicles, and an increasingly acute angle between the scanner laser and the road surface. The 3D laser scanner survey revealed a slight depression over one of the pipelines in the southbound left lane, which was subsequently addressed by a supplemental grouting program. The largest amount of grout (143 ft~3 [4.05 m~3]) injected at a single point on the entire void-repair project was in the shallow depression revealed by the scanner survey.