Popping of Cavities in the Near-Surface Coal-Bearing Strata of the Donets Basin

Long-term research has proved the connection between old mine workings preserved at the outcrops of coal seams and land subsidence, and sinkhole collapse. Now the causes and types of such disturbances are determined, the most dangerous of which are caves, over various types of the abandoned mine workings. In addition to it, a method has been developed and improved for assessing the hazard rate of any abandoned working (cavity) with relation to sinkholes for mining and geological conditions of the Donets Basin. At the same time, a number of cases are known when cavities wiped out due to the failure of the overlying rocks reappeared on the upper levels, threatening to generate surface depression. Thus, the former, but already liquidated cavity seemed to pop in the overlying rocks. The understudied process of cavities moving in the subsurface coal-bearing strata from the places of their original location towards the surface in the Donets Basin is described. The computation of stage-by-stage popping of relatively undestroyed mine opening (cavity) in coalbearing strata of different strength is made. The conditions and factors contributing to the popping of the remaining cavities towards the overlying levels are determined. An assumption (hypothesis) on the inevitability of popping of any abandoned working or cavity preserved on the upper levels, with a favorable combination of the affecting factors is made. The necessity for this process to be taken into account in assessing the hazard rate of the surface areas undermined by the abandoned mine workings at shallow depths due to their potential capability of making sinkholes is demonstrated. The effective reference documents regulating the procedure for assessing the hazard rate of abandoned mine workings (cavities) at shallow depths do not take into account popping of cavities that hurts correctness of the results obtained for them.

Sinkhole collapse propagation studies through instrumented small-scale physical models

Sinkholes are common geohazards, frequently responsible for sudden catastrophic ground collapse. Thus, effective monitoring would allow for further understanding of the mechanism of occurrence of sinkholes and lead to the development of a potential early warning system to provide an alarm or a warning of incipient col-lapse. In the current study, fiber Bragg gratings (FBGs) were used to instrument reduced scale models, simulating a sinkhole event. The tests were conducted by embedding optic fiber sensors in the soil and inducing failure until critical conditions were reached. FBG sensors were manufactured in a single optic fiber cable. The measurements of small horizontal strains were recorded simultaneously and in various positions. Failure mechanism was found to relate to the backfill density, and compaction.

Detection of sinkhole occurrence, experiences from South Africa

Sinkholes are alarming and dangerous events, they have a worldwide occurrence, and are imposing a potential risk to urban communities and the widely developed built environment. Losses due to catastrophic sinkhole collapse, foundation, pavement and structural repairs, occur more often, due to the increased pressure to develop even on sinkhole prone land, and the aging of existing water supply infrastructure in the majority of cities. Remote sensing earth observation methods have proved to be valuable tools during the last two decades in long-term sinkhole hazard assessment. Satellite air borne and ground earth observation methods have primarily facilitated the wide detection of continuous displacement on the earth's crust. National sinkholes catalogues are necessary for town planers decision makers, and government authorities. In many instances the ground collapse is the result of water ingress from old poorly maintained leaking pipelines, or extensive dewatering activities. In the current study a comprehensive review of the current literature is presented in order to show experiences from South Africa and present recent mapping using PSInSAR methodology in Centurion South Africa.

InSAR Detection of Localized Subsidence Induced by Sinkhole Activity in Suburban West-Central Florida

Sinkhole activity in west-central Florida is a major hazard for people and property. Increasing frequency of sinkhole collapse is often related to an accelerated use of groundwater and land resources. In this work, we use radar interferometry acquired over a selected region in Hernando County in west-central Florida to observe small localized deformation possibly caused by sinkhole activity. The data used for the study consist of acquisitions from one TerraSAR-X frame covering a time span of approximately 1.7 years with spatial resolution of 0.25 by 0.60 m. We applied the Persistent Scatterer Interferometry (PSI) technique using the Stanford Method for Persistent Scatterers (StaMPS). Results reveal several areas of localized subsidence at rates ranging from −3.7 to −4.9 mm yr−1. Ground truthing and background verification of the subsiding locations confirmed the relationship of the subsidence with sinkhole presence.

Geomechanical modelling of sinkhole development using distinct elements: model verification for a single void space and application to the Dead Sea area

Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth ∕ diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth ∕ diameter values in each material type may partly reflect sinkhole growth trends.

Growth of a sinkhole in a seismic zone of the northern Apennines (Italy)

Sinkhole collapse is a major hazard causing substantial social and economic losses. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkhole-related hazard challenging. Furthermore, more than 40 % of the sinkholes of Italy are in seismically hazardous zones; it remains unclear whether seismicity may trigger sinkhole collapse. Here we use a multidisciplinary data set of InSAR, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed in an active fault zone and grew through several events of unrest characterized by episodic subsidence and lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence at rates of up to 7.1 mm yr−1 occurred during 2003–2008, between events of unrest. Earthquakes on the major faults near the sinkhole do not trigger sinkhole activity but low-magnitude earthquakes at 4–12 km depth occurred during sinkhole unrest in 1996 and 2016. We interpret our observations as evidence of seismic creep at depth causing fracturing and ultimately leading to the formation and growth of the Prà di Lama sinkhole.