Sinkhole subsidence due to mining

This paper reviews the modes of formation of sinkhole subsidence associated with mining activities, drawing on examples in India. Sinkhole (pot-hole) subsidence is an abrupt local depression at the surface which can be hazardous to life and property due to its tendency to occur without warning. Shallow extraction, weak overburden and geological discontinuities are the main factors which cause them. Sinkholes occur due to the failure of a mine roof which migrates through the overlying strata until the failure zone intercepts the unconsolidated overburden. Alternatively they may occur by the creation of cavities in the overburden following the inflow of sand and soil from the overlying weathered and friable strata through faults. Overburden cavities eventually cave in and sinkholes appear at the surface. Sinkholing phenomena can be controlled to some extent by proper design of mining supports and construction of walls to create a barrier around an area prone to sinkholes in bord and pillar workings. Backfilling and grouting can be used to stabilize abandoned underground workings.     

Issues of environmental impacts of karst in standards on construction in Russia

There are three important ecological aspects to be considered regarding economic development in covered karst terrain: (1) natural karst processes, and karst-suffosion processes in particular, are very responsive to technogenic effects, (2) possibility of intensive and deep pollution of the geological environment and (3) risk of ecological disasters caused by karst collapses on the sites of ecologically dangerous industrial and transport objects. In view of these aspects, a number of Russian standards and normative documents on construction are analysed. Some methodological approaches to practical application of the guidelines stated in the documents are offered.     

Earth fissures triggered by groundwater withdrawal and coupled by geological structures in Jiangsu Province,China

Earth fissures in Jiangsu Province, China have caused serious damages to properties, farmlands, and infrastructures and adversely affected the local or regional economic development. Under the geological and environmental background in Jiangsu Province, this paper presents the earth fissures caused by excessive groundwater withdrawal and coupled by distinctive geological structures such as Ancient Yellow River Fault in Xuzhou karst area, and Ancient Yangtze River Course and bedrock hills in Suzhou, Wuxi, and Changzhou area. Although all the earth fissures are triggered by groundwater exploitation, the characteristics are strongly affected by the specific geological and hydrogeological settings. In particular, in the water-thirsty Xuzhou city, the cone of depression caused by groundwater extraction enlarged nearly 20 times and the piezometric head of groundwater declined 17 m over a decade. As groundwater is extracted from the shallowly buried karst strata in the Ancient Yellow River Fault zone, the development of earth fissures is highly associated with the development of karstic cavities and sinkholes and their distribution is controlled by the Ancient Yellow River Fault with all the 17 sinkholes on the fault. On the other hand, in the rapidly developing Southern Jiangsu Province, groundwater is mainly pumped from the second confined aquifer in the Quaternary, which is distributed neither homogeneously nor isotropically. The second confined aquifer comprises more than 50 m thick sand over the Ancient Yangtze River Course, but this layer may completely miss on the riverbank and bedrock hills. With a typical drawdown rate of 4–6 m per annum, the piezometric head of groundwater in the second confined aquifer has declined 76 m at Maocunyuan since 1970s and 40 m at Changjing since mid-1980s, and a large land subsidence, e.g., 1,100 mm at Maocunyuan, is triggered. Coupled with the dramatic change of the bedrock topography that was revealed through traditional geological drilling and modern seismic reflection methods, the geological-structure-controlled differential settlement and earth fissures are phenomenal in this area.     

A hydrochemical and dye- tracing investigation in the Posht-e-Naz Karstic Aquifer, Alburz Mountain, Northern Iran

A hydrochemical and tracer study (uranine injection) was conducted in Jurassic limestone of the Posht-e-Naz area in the Alborz belt to evaluate hydraulic relations between a large diameter (about 100 m) sinkhole and springs and aquifer parameters. A main goal of the project was to find out the source of turbidity of the Emarate drinking water supply spring (SP4) in rainy season. Springs discharge were measured and hydrochemical investigation were carried out. The uranine tracer was injected and eight springs, three wells and the Neka River were selected and totally 989 samples in 107 days were collected. Hydrochemical data demonstrated a relative connection between sinkhole (Sh1) and spring (SP4). The results of the tracing by sampling water indicated only a hydraulic connection between Sange- Nou spring (SP8) and injection point, while the charcoal packets analysis revealed tracer exits from spring numbers SP1, SP3, SP4, SP5, SP8, in wells W1 and W2, and in the Neka River. The concentration — time curves of charcoal packets for qualitative analysis and exit tracer for quantitative analysis is also assessed.     

5 12汶川地震震后龙门山山前地表塌陷成因探讨

5 12汶川地震20余天后, 在四川省安县的安昌镇东南的李家院子村附近陆续出现了几十个地表塌陷坑并持续扩大。本文以李家院子地表塌陷坑群为研究对象, 根据其所在的地层、岩性、地下水、溶蚀作用和上覆土体的工程地质特性定性的探讨了该塌陷坑群的成因。初步认定钙质胶结砂砾基岩的存在是地表塌陷的先决条件; 强烈的地震动导致基岩岩体碎裂、解体、塌陷、并压密, 同时出现超孔隙水压力, 这使上层土体失去支持并开始启动塌陷; 余震的地震振动荷载作用导致地下土洞的形成和发展, 地下水的变化起到加速作用; 上覆土体的工程地质特性决定了塌陷坑在地表的浑圆形状。同时本文也注意到机械溶蚀和隐伏断层活动两种诱导因素的可能, 并简要阐述了相关背景依据。     

Evaporite karst and sinkholes: a synthesis on the case of Camaiore (Italy)

In 1995 a sinkhole suddenly formed at Camaiore (Tuscany, Italy), causing destruction or heavy damages to several houses and resulting in the evacuation of many people. To understand the causes, for the formation and evolution of the collapse, surface and underground geologic features were investigated and reconstructed on the basis of geologic and geognostic surveys. The sinkhole area is underlain by thick alluvial deposits that cover a bedrock consisting of the Calcare cavernoso formation. This formation results from hydration and dissolution of Triassic evaporites and has a characteristic spongy and vacuolate texture. The bedrock contains karst cavities, generally filled by breccia and/or alluvial materials. Thus, the sinkhole disaster could be ascribed to deep collapse of a cave in the bedrock, and might be considered a distant effect of ancient karst phenomena in evaporites.     

Drainage and flooding in karst terranes

Flooding in karst terranes is a commonly occurring geo-hazard. It causes damage to property, businesses, and roadways. It can lead to the formation of cover-collapse sinkholes and groundwater contamination. Generally, three types of flooding or their combinations are related to karst: recharge-related sinkhole flooding, flow-related flooding, and discharge-related flooding. Understanding of the type of flooding is essential for solving the flooding problem. Areas prone to karst flooding should be recognized, and restrictions and laws on land use should be implemented. Runoff and erosion control plans should address the unique characteristics of karst features. Digging out clogged sinkholes, creating retention basins, or installing Class V Injection Wells are possible solutions to improve drainage of storm water. Solutions to flooding problems in karst areas should also be coordinated with the water quality control to prevent groundwater contamination.     

Groundwater withdrawal impacts in a karst area

During a 3000-gpm pump test on a groundwater supply well in Augusta County, Virginia, residential properties were impacted. The impacts included lowered farm pond water levels, development of a sinkhole, and water level decrease in residential wells. A study was performed to assess whether a lower design yield was possible with minimal impacts on adjacent property. This study included a 48-h 1500-gpm pump test that evaluated impacts due to: (1) sinkhole development and potential damage to homes, (2) loss of water in residential wells, and (3) water-quality degradation. Spring flows, residential well levels, survey monuments, and water quality were monitored. Groundwater and surface water testing included inorganic water-quality parameters and microbiological parameters. The latter included particulate analyses,Giardia cysts, and coliforms, which were used to evaluate the connection between groundwater and local surface waterbodies. Although results of the study indicated a low potential for structural damage due to future sinkhole activity, it showed that the water quality of some residential wells might be degraded. Because particulate analyses confirmed that groundwater into the supply well is under the direct influence of surface water, it was recommended that certain residents be placed on an alternate water supply prior to production pumping and that filtration be provided for the well in accordance with the Surface Water Treatment Rule. A mitigation plan was implemented. This plan included crack surveys, a long-term settlement station monitoring program, and limitation of the groundwater withdrawal rate to 1.0 million gallons per day (mgd) and maximum production rate to 1500 gpm.     

Geophysical investigation of a sinkhole in the northern Harz foreland (North Germany)

The creek Jerstedter Bach is part of the Ringelheimer Mulde along the northern rim of the Harz Mountains in Northern Germany, a trough structure comprising steeply dipping limestones. The limestones are well karstified and drain parts of the region efficiently via sub-surface flow. As the entire region is intensively used by agriculture, contamination of the fast-flowing karst aquifer is a potential problem. During 2006, a small sinkhole (Windmühlenweg) developed alongside the Jerstedter Bach close to the city of Goslar. The sinkhole enlarged rapidly, with a small cave within the gravel cover diverting part of the creek to the subsurface karst catchment. Geophysical methods (gravimetry, geoelectrics) were used around the sinkhole to determine its extent and possible evolution. A negative Bouguer anomaly was found over the sinkhole, indicating more voids further downstream. The geoelectric survey helped to identify the old meandering course of the small river, which was straightened during the cultural land reform in the twentieth century.