Integrated reflection seismics, 3D-GPR, leveling, DInSAR to detect and characterize high-risk sinkholes in urban cover evaporite karst. A NE Italian case study.

<p>Sinkholes linked to cover evaporite karst in urban environments still represent a challenge in terms of clear identification and mapping considering the anthropic rehash and the presence of man-made structures.</p><p>We propose and tested a methodology to identify the subsiding features in an urban area within a cover evaporite karst environment, through an integrated and non-invasive multi-scale approach combining seismic reflection, DInSAR, leveling and full 3D GPR.</p><p>The analysis was conducted in a small village in the Tagliamento valley (Friuli Venezia Giulia region, NE Italy) named Quinis, where sinkholes are reported since a long time as well as the hazard linked to their presence: within the years, several houses have been demolished and at present many of them are damaged.</p><p>First we applied each methodology independently and after we compared, combined and integrated them to obtain more coherent and cross-validates results. Seismic reflection imagined the covered karst bedrock identifying three depocenters; DInSAR investigation allowed to identify an area with higher vertical velocities; leveling data presented a downward displacement comparable with DInSAR results; 3D GPR, applied here for the first time in the study and characterization of sinkholes, clearly defined shallow sinking features imaging also under a shallow dense pipe network. Combining all the obtained results with accurate field observations we identified and map the highest vulnerable zones.</p><p>The final result is the combining of the geophysical, DInSAR and leveling information, while also locating the damaged buildings, the local asphalt pavement breaks or renovation and the buildings which are nowadays demolished, by using vintage photographs and historical maps. The data are consistent, being the most relevant present damages and the demolished building within the zones with higher sinking velocity on the base of both leveling and DInSAR. Geophysically imaged depocenters lie within the most critical area and perfectly correlate with the local pavement damages.</p><p>In a complex geological and hydrological framework, as in the study area, a multidisciplinary and multi-scale approach is mandatory to identify and map the zone most affected by sinking phenomena. While punctual data such as borehole stratigraphy, local groundwater level variations with time, extensometers measurements and geotechnical parameters are useful to highlight local hazard due to occurring deformation, the proposed integrated methodology addresses a complete and quantitative assessment of the vulnerability of the area. It’s fundamental, especially in anthropized environments, using different integrated techniques, without forgetting the role of the fieldwork of the geologists who can detect the precursors or already occurred, even elusive, signs of the ongoing or incipient sinking.</p>

Sinkhole collapse and hydrogeological hazard assessment in covered karst terrains: Case study of a bridge project across the Loire River (Orleans, France)

<p>The floodplain of the Loire River around Orleans is a sinkhole-prone area due to the highly karstified calcareous Beauce Formation overlying by few meters of weathered materials and alluvial deposits. Such layout makes it necessary to carry out detailed geotechnical and hydrogeological investigations for any important building project.</p><p>This paper presents the first results of a study carried out by the French Geological Survey for a bridge project across the Loire River. The geological setting indicates that the study area is near the front door of a supposed-major cave system in which water, coming from the Loire River, not only provides drinking water for an important part of the region, but also supplies the main spring of the Loiret River, located a few kilometers further. The overview of past sinkhole collapses confirms that the study area is regularly concerned by ground collapses of several meters of diameter. Field investigations include microgravimetry (26 hectares), two electrical resistivity profiles (720 m and 470 m long), 149 cone penetration tests (around 15 m deep), a first sequence of 11 drillings (40 m deep) and videos, gamma-ray and sonar logs. A first sinkhole hazard assessment is now quite complete: very weak layers (possibly caves) of several decimeters to a few meters thick need specific mitigation measures to secure the construction project. Other investigations are still being analysed (a second sequence of drillings, dye tracing, injection of salt brine with resistivity profiling) and should help specify the area hydrogeological hazard.</p>

Methodological Considerations in Cover-Collapse Sinkhole Analyses: A Case Study of Southeastern China’s Guangzhou City

Cover-collapse sinkholes can present significant hazards to human habitation and communal facilities in soil-covered karst regions. Therefore, for human security and land-use planning in sinkhole-prone areas, appropriate approaches are required prior to construction in order to understand the cover-collapse sinkhole genesis and its likely evolution. The study seeks to contribute to performing an integrated analysis of karst hazards in mantle karst regions where karst evidence can be masked, with the ultimate goal of developing a methodological framework utilizing different techniques and approaches. A small area located in Guangzhou City of southeastern China’s Guangdong Province was analyzed. The detailed typology, morphometry, and chronology inventory of 49 cover-collapse sinkholes in the study area were analyzed using various surface investigation methods, such as field surveys, aerial photography, and photogrammetry. The Quaternary deposits and indicators of the active underground karst features in the aforementioned mantle karst region were geotechnically characterized using drilling and geophysical techniques. These techniques included ground penetrating radar (GPR), electrical resistivity imaging (ERI), natural source audio frequency magnetotellurics (NSAMT), and micro-tremors. During this study’s investigations, three karst fissure zones covered by Quaternary soil were observed using multiple techniques. In addition, it was found that the groundwater dynamic monitoring data confirmed that the sinkholes in the study area were closely related to changes in groundwater levels. Therefore, the efforts which have been made to investigate and monitor the sinkhole development will be required to continue into the immediate future.