Abstract

In order to solve the problem of subsurface cavities in urban areas, industry, government and academia carried out collaborative research over 2018-2020. In this joint research, analysis of existing cavity data, laboratory model experiments, numerical analysis, and full-scale field test, monitoring of the cavity in the road, etc. were conducted. In this study, a full-scale test road was constructed with artificial cavities installed to evaluate the risk of collapse in consideration of the road structure. A series of plate loading tests and falling weight deflectometer tests was conducted on the road surface above a cavity to understand the cavity behaviour. A surface wave survey was also conducted to evaluate the wave propagation on and around cavities. It was found to be reasonable to evaluate the collapse risk of subsurface cavities by the ratio of depth and width of cavities, in which the thickness of the pavement should be excluded from the depth.

Abstract

This study examines the detection of subsurface cavities using seismic wave propagation. Both vertical and horizontal excitation methods were employed to measure the reflection characteristics of artificially constructed cavities in a test field. The study was conducted at the Le Petau golf course in the town of Abira, Hokkaido, Japan. In this study, a new horizontal excitation technique was introduced along with conventional vertical excitation to compare its effectiveness in identifying wave reflections caused by cavities. An artificial cavity was created using rubber balloons reinforced with cardboard to simulate subsurface anomalies. The measurement used 24 geophones connected to an automatic data logger, and the acquired data underwent extensive analysis with bandpass filtering, amplitude normalization, and deconvolution to elucidate the interaction between the elastic waves and the subsurface structures. The results showed that horizontal excitation provided a clearer view of the cavity due to reflected waves. This is because only SH waves are excited and observed in the horizontal excitation mode. However, this study also recognized that it is difficult to distinguish the boundary between direct and reflected waves at shallow cavity depths, suggesting the need for excitation methods capable of generating shorter wavelengths to improve resolution. This study highlights the potential of integrating vertical and horizontal excitation methods in seismic surveys to enhance subsurface cavity exploration, and highlights future research issues, such as improving waveform analysis techniques and searching for alternative excitation sources, to advance the field of subsurface exploration.

Abstract

The presence of karstification processes in the terrain poses a threat to the safety of the foundations of structures, which in the case of gypsum materials becomes more dangerous due to the rate at which they can progress. This work describes the case of a Spanish high-speed railway bridge whose route runs over a Tertiary gypsum formation belonging to the lower Miocene unit of the Madrid Basin. These are very firm materials in a healthy state, that allow the direct support of shallow foundations, which stand out for their high heterogeneity, with the presence of massive gypsum banks as well as alternating levels of gypsum and clay. However, these materials can be affected by karst processes of dissolution, weathering, and replacement by clays, in which case there is a marked degradation of their mechanical properties, and the eventual local appearance of cavities. In these circumstances, it is necessary to carry out soil improvement treatments, such as compaction grouting, or resort to deep foundations with piles that transmit the loads of the structure to a lower substrate in a non-disturbed state. By its nature, the detection of this type of processes by drilling rotary boreholes is not trivial, and there is often uncertainty associated with its spatial variability. This communication presents the results obtained after the execution of a geotechnical research campaign, consisting of the realization of seismic tomography borehole-profiles under each support of the viaduct, supported by the execution of conventional rotary boreholes. The results of this exploration have made it possible to understand and define the spatial extent of the materials degraded by karstification and the selection of robust foundation typologies that favour the sustainable and resilient nature of the infrastructure.