Review on sinkhole monitoring and performance of remediation measures by high-precision leveling and terrestrial laser scanner in the salt karst of the Ebro Valley, Spain

Abstract The need to apply sinkhole remediation and monitoring programs is increasing, concurrently with the rising engineering impacts of sinkholes in many regions. However, there is a significant gap in the scientific literature regarding the assessment of the performance of remedial works through subsidence monitoring. Monitoring techniques have experienced significant advances in the last few years thanks to the advent of new technologies. This work presents a review of sinkhole monitoring methods and discusses some practical considerations about their capabilities and limitations. It also documents a monitoring program (high-precision leveling and terrestrial laser scanner) and the concurrent remedial works (cement-based and polyurethane grouting) carried out in a highly active sinkhole that affects a flood-control dike and the adjacent village of Alcala (NE Spain). The subsidence activity in the sinkhole is characterised by progressive sagging and the development of catastrophic nested collapses. Monitoring data reveal subsidence expansion and acceleration in the sector of the dike treated by polyurethane grouting above the main cavities. In contrast, subsequent cement-based grouting of the large cavities significantly reduced subsidence rates. Displacement data also indicate substantial subsidence enhancement following flood recession and the associated water table drop (buoyancy loss). This case study illustrates that high-precision leveling and terrestrial laser scanning are adequate complementary methods for monitoring specific sinkholes, characterised by very high accuracy and detailed spatial resolution, respectively.