Methods for Post-Construction Evaluation of Horizontal Directional Drilled Utilities

Trenchless Technology has become an increasingly popular underground utility construction method, beginning in the early 1900s with pipe jacking beneath railroad lines. One method, horizontal directional drilling (HDD), became more common in the 1990s and into the current day. Since trenchless construction does not require an open cut or excavation, cave-in(s), that are potentially dangerous for workers, do not occur. In addition, conduits and utilities can be installed beneath existing structures and roadways without having to demolish current infrastructure. However, the utilization of HDD is associated with the potential risk for ground subsidence and soil heaving. Sinkholes and settlement can form after construction, damaging existing infrastructure and causing safety hazards. Trenchless methods, requiring an annual overcut, disturb soil around the outer diameter of the utility being installed. While the annular overcut is necessary for feeding pipes through the borehole, when used in conjunction with a liquid lubricant, the likelihood for developing voids increases. The annular overcut is also a cause for concern because the consistency and void ratio of the overburden soil change after boring. Inconsistent and void-ridden soil can cause void propagation through the overlying soil until it reaches the surface, where it will become a sinkhole or crack.
This paper addresses post-construction evaluation methods, especially pertaining to the annular space and void propagation region above and around a freshly installed utility in Southern Virginia. Two non-destructive evaluation methods are used to scan the surrounding soil: Ground penetrating-radar (GPR) and FutureScan. FutureScan, a radar device developed by Louisiana Tech University, is capable of being attached to pipe inspection robots and uses a means of penetrating radar to detect voids and inconsistencies in three dimensions. This study examines the difference between GPR and FutureScan, regarding the imaging techniques used and the measured void ratios. All soil conditions for this study pertain to The Virginia Piedmont region.
Relative elevations were recorded before, during, and after drilling, to measure surface movement caused by drilling efforts. The relative elevation was also recorded several months after utility installation, so it could be compared on a long-term scale. Long-term results indicate whether or not the imaging techniques imply subsidence in the long-term. After the analysis was conducted using both FutureScan and GPR, representative soil samples of the test site were retrieved and transported to a geotechnical laboratory for further testing. Based on the GPR and FutureScan findings, the utility of the two post-construction evaluation methods was determined. The results were compared to the relative elevations recorded in the field. Void ratio field data and equipment reliability are limited to Virginia’s piedmont region.