Subsidence hazards due to evaporite dissolution in the United States

Evaporites, including gypsum (or anhydrite) and salt, are the most soluble of common rocks; they are dissolved readily to form the same type of karst features that typically are found in limestones and dolomites, and their dissolution can locally result in major subsidence structures. The four basic requirements for evaporite dissolution to occur are: (1) a deposit of gypsum or salt; (2) water, unsaturated with CaSO₄ or NaCl; (3) an outlet for escape of dissolving water; and (4) energy to cause water to flow through the system. Evaporites are present in 32 of the 48 contiguous states of the United States, and they underlie about 35–40% of the land area. Karst is known at least locally (and sometimes quite extensively) in almost all areas underlain by evaporites, and some of these karst features involve significant subsidence. The most widespread and pronounced examples of both gypsum and salt karst and subsidence are in the Permian basin of the southwestern United States, but many other areas also are significant. Human activities have caused some evaporite–subsidence development, primarily in salt deposits. Boreholes may enable (either intentionally or inadvertently) unsaturated water to flow through or against salt deposits, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures above the cavity can cause land subsidence or catastrophic collapse.     

Evaporite karst in Sicily

Karst areas are distributed over most of Sicily. The most widespread karst rocks are carbonates, particularly limestones, but karst phenomena can also be seen in evaporites and particularly in salt mines. This report provides an overview of evaporite karst in Sicily, along with a “case history” that shows some of the evaporite karst risks to the environment. In the centre and south of Sicily, a thick sequence of Messinian evaporite rocks are subject to dissolution from meteoric and formation waters. In areas where potassium salts and rock salts are being mined, some geomorphologic changes result from the drilling of boreholes and the collapse of underground mines, thus lowering or collapsing the land surface. An example is the old salt mine “Muti-Coffari”, situated in the commune of Cammarata, where there is a modification of the surface flow of the River Platani. Meteoric waters and runoff flow down through a borehole, enter the underground mine cavity and dissolve the salts, and then the resulting brine flows into a branch of the river, making it salty. Field investigations showed the presence of salt along the edges and on the bed of the stream where it comes out of the cave. Therefore, interventions for risk mitigation are necessary since the old mine constitutes a serious danger for damage or collapse of nearby infrastructures, and can lead to degradation of the river ecosystem and the natural environment.     

Identification, prediction, and mitigation of sinkhole hazards in evaporite karst areas

Sinkholes usually have a higher probability of occurrence and a greater genetic diversity in evaporite terrains than in carbonate karst areas. This is because evaporites have a higher solubility and, commonly, a lower mechanical strength. Subsidence damage resulting from evaporite dissolution generates substantial losses throughout the world, but the causes are only well understood in a few areas. To deal with these hazards, a phased approach is needed for sinkhole identification, investigation, prediction, and mitigation. Identification techniques include field surveys and geomorphological mapping combined with accounts from local people and historical sources. Detailed sinkhole maps can be constructed from sequential historical maps, recent topographical maps, and digital elevation models (DEMs) complemented with building-damage surveying, remote sensing, and high-resolution geodetic surveys. On a more detailed level, information from exposed paleosubsidence features (paleokarst), speleological explorations, geophysical investigations, trenching, dating techniques, and boreholes may help in investigating dissolution and subsidence features. Information on the hydrogeological pathways including caves, springs, and swallow holes are particularly important especially when corroborated by tracer tests. These diverse data sources make a valuable database—the karst inventory. From this dataset, sinkhole susceptibility zonations (relative probability) may be produced based on the spatial distribution of the features and good knowledge of the local geology. Sinkhole distribution can be investigated by spatial distribution analysis techniques including studies of preferential elongation, alignment, and nearest neighbor analysis. More objective susceptibility models may be obtained by analyzing the statistical relationships between the known sinkholes and the conditioning factors. Chronological information on sinkhole formation is required to estimate the probability of occurrence of sinkholes (number of sinkholes/km² year). Such spatial and temporal predictions, frequently derived from limited records and based on the assumption that past sinkhole activity may be extrapolated to the future, are non-corroborated hypotheses. Validation methods allow us to assess the predictive capability of the susceptibility maps and to transform them into probability maps. Avoiding the most hazardous areas by preventive planning is the safest strategy for development in sinkhole-prone areas. Corrective measures could be applied to reduce the dissolution activity and subsidence processes. A more practical solution for safe development is to reduce the vulnerability of the structures by using subsidence-proof designs.     

A genetic classification of sinkholes illustrated from evaporite paleokarst exposures in Spain

This contribution analyses the processes involved in the generation of sinkholes from the study of paleokarst features exposed in four Spanish Tertiary basins. Bedrock strata are subhorizontal evaporites, and in three of the basins they include halite and glauberite in the subsurface. Our studies suggest that formation of dolines in these areas results from a wider range of subsidence processes than those included in the most recently published sinkhole classifications; a new genetic classification of sinkholes applicable to both carbonate and evaporite karst areas is thus proposed. With the exception of solution dolines, it defines the main sinkhole types by use of two terms that refer to the material affected by downward gravitational movements (cover, bedrock or caprock) and the main type of process involved (collapse, suffosion or sagging). Sinkholes that result from the combination of several subsidence processes and affect more than one type of material are described by combinations of the different terms with the dominant material or process followed by the secondary one (e.g. bedrock sagging and collapse sinkhole). The mechanism of collapse includes any brittle gravitational deformation of cover and bedrock material, such as upward stoping of cavities by roof failure, development of well-defined failure planes and rock brecciation. Suffosion is the downward migration of cover deposits through dissolutional conduits accompanied with ductile settling. Sagging is the ductile flexure of sediments caused by differential corrosional lowering of the rockhead or interstratal karstification of the soluble bedrock. The paleokarsts we analysed suggest that the sagging mechanism (not included in previous genetic classifications) plays an important role in the generation of sinkholes in evaporites. Moreover, collapse processes are more significant in extent and rate in areas underlain by evaporites than in carbonate karst, primarily due to the greater solubility of the evaporites and the lower mechanical strength and ductile rheology of gypsum and salt rocks.     

Gypsum-karst problems in constructing dams in the USA

Gypsum is a highly soluble rock and is dissolved readily to form caves, sinkholes, disappearing streams, and other karst features that typically are also present in limestones and dolomites. Gypsum karst is widespread in the USA and has caused problems at several sites where dams were built, or where dam construction was considered. Gypsum karst is present (at least locally) in most areas where gypsum crops out, or is less than 30–60 m below the land surface. These karst features can compromise on the ability of a dam to hold water in a reservoir, and can even cause collapse of a dam. Gypsum karst in the abutments or foundation of a dam can allow water to pass through, around, or under a dam, and solution channels can enlarge quickly, once water starts flowing through such a karst system. The common procedure for controlling gypsum karst beneath the dam is a deep cut-off trench, backfilled with impermeable material, or a close-spaced grout curtain that hopefully will fill all cavities. In Oklahoma, the proposed Upper Mangum Dam was abandoned before construction, because of extensive gypsum karst in the abutments and impoundment area. Catastrophic failure of the Quail Creek Dike in southwest Utah in 1989 was due to flow of water through an undetected karstified gypsum unit beneath the earth-fill embankment. The dike was rebuilt, at a cost of US $12 million, with construction of a cut-off trench 600 m long and 25 m deep. Other dams in the USA with severe gypsum-karst leakage problems in recent years are Horsetooth and Carter Lake Dams, in Colorado, and Anchor Dam, in Wyoming.     

Quantitative sinkhole hazard assessment. A case study from the Ebro Valley evaporite alluvial karst (NE Spain)

Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km² year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology (precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from an alluvial evaporite karst area (0.81 km²) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability of occurrence of 0.14 sinkholes/km² year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km² year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence. Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes.     

乐业-凤山世界地质公园岩溶地貌景观特征与价值分析

乐业-凤山世界地质公园发育在“孤岛”状的碳酸盐岩区上,以高峰丛深洼地、大洞穴、大峡谷、天坑群、天窗群、天生桥、地下河为特色,景观类型丰富、系统完整、珍稀典型,观赏价值与科研价值极高。公园内不同地貌景观由于成因差异而各具特色,经定性与定量评价:天坑群、天窗群是世界级品牌景观,为公园五级地貌景观;天生桥、峡谷是公园重要景观,为公园四级地貌景观;峰丛洼地、峰丛谷地、坡立谷、洞穴景观是公园地貌景观的重要组成部分,为公园三级地貌景观;穿洞、岩溶泉、地下河景观是公园地貌景观的重要补充,为二级地貌景观;落水洞、竖井是公园点缀地貌景观,为公园一级地貌景观。      

Models and mechanisms of drilling-induced sinkhole in China

The present paper distinguishes the four general types of collapsed sinkholes induced by drilling in regions where non-indurated sediments (clay, silt, and sand) overlie unknown caves or open voids in underlying lithified karst rocks. These sinkhole models are classified into balance arch, hourglass, vibration, and subjacent drilling-induced erosion forms. A mechanical model was built for each type to assess the collapse probability. Drilling operations using boreholes facilitate rapid, turbulent, and erosive flow. These activities resulted in the 26 sinkholes that have been attributed to constructions in China in the last several years. Awareness of potential collapse conditions should allow construction projects to avoid high-risk settings that result in economic losses, environmental concerns, and life-threatening accidents caused by such rapid collapse of the land surface. Awareness and close monitoring of high-risk conditions during borehole monitoring should minimize the hazard.     

Geological and environmental implications of the evaporite karst in Spain

In Spain, evaporite outcrops cover approximately 7% of the total area of the country. Most of the evaporitic formations are made up of Ca-sulfates (gypsum/anhydrite) or Ca-sulfates and halite. Certain Paleogene marine evaporites also contain K-Mg-chlorides, and some Tertiary continental formations bear substantial amounts of Na-sulfates in the subsurface (glauberite and thenardite). Mesozoic evaporitic formations commonly wedge out towards the ground surface, passing into condensed sequences and dissolution-collapse breccias. Some of these highly porous breccias constitute major regional aquifers. In several areas, interstratal karstification of the evaporites has given rise to gravitational deformations such as basin structures, monoclines, and collapse structures covering several square kilometers that record a cumulative subsidence in excess of 200 m (Teruel and Calatayud Grabens). A widespread consequence of evaporite dissolution processes in Spain is the hydrochemical degradation of surface waters. Some of the largest and most outstanding lake systems, from an environmental perspective, occur in karstic depressions developed in evaporitic formations (Fuente de Piedra, Gallocanta, Bujaraloz, and Bañolas lakes). Sinkhole activity is a major geohazard in several evaporite karst areas. The sinkhole risk has a particularly high impact in sectors where Tertiary evaporites are overlain by Quaternary alluvial aquifers (Calatayud, Zaragoza, and Madrid areas). Some of the detrimental effects of subsidence include severe damage to historical monuments (Calatayud), the demolition of a whole village (Puilatos), or the derailment of a freight train (Zaragoza area). The deepest gypsum caves are found in Triassic diapiric structures (El Sumidor Cave, 210 m deep), and the longest ones are developed in horizontally lying Neogene sequences (Sorbas caves, and Estremera maze cave). The Cardona diapir hosts salt caves up to 4,300 m long whose genesis is related to flooding of mine galleries caused by the interception of a phreatic conduit. The main anthropogenic impacts on the endokarstic systems are related to the disposal of wastewaters and the destruction of caves by quarrying. The fluvial valleys that cross Tertiary evaporitic outcrops commonly show peculiar geological characteristics related to dissolution-induced synsedimentary subsidence phenomena: (1) Thickened alluvium filling dissolution basins up to several tens of kilometers long and more than 100 m deep. The largest thickenings are found in areas where the bedrock contains halite and glauberite. (2) Superimposed alluvial units locally bounded by angular unconformities. (3) Abundant deformational structures and paleosinkholes related to the rockhead and/or interstratal karstification of the substratum. These fluvial valleys typically are flanked by a prominent gypsum escarpment. Rock-falls favored by the dissolutional enlargement of joints derived from these scarps are the type of mass movement which has caused the highest number of casualties in Spain.     

Karst unit mapping using geographic information system technology,Mower County,Minnesota, USA

Mower County is in southeastern Minnesota in an area underlain by sedimentary bedrock. These rocks are karsted Middle Devonian and Middle Ordovician limestone and dolomite. The karst features of the county were inventoried. These features include sinkholes, disappearing streams, caves, dry valleys, and springs. Previous karst mapping efforts for other counties in Minnesota have produced sinkhole probability maps and a springshed map. In Mower County, we have developed a new type of karst map using a geographic information system (GIS) to produce a karst unit map. Karst units are discrete three-dimensional bodies in which solution of the bedrock has resulted in the integration of surface water and groundwater. The field mapping and hydrologic investigations were done with conventional methods. The karst unit delineation was done using GIS technology, which allowed us to examine the county's karst using two-dimensional and three-dimensional views. Many different overlays of the karst elements were combined to better understand the landscape dynamics. Ultimately, it was the overlay of the karst features, hydrologic information, and depth-to-bedrock mapping on a shaded relief landscape morphology map which allowed us to best delineate the individual karst units.     

The GIS approach to evaporite-karst geohazards in Great Britain

Evaporite karst in Great Britain has formed in Permian and Triassic gypsum, and in Triassic salt. Active dissolution of these deposits can occur on a human rather than a geological timescale causing subsidence and building damage. The British Geological Survey has taken two approaches towards understanding and advising on hazards caused by dissolution of these soluble rocks. At a detailed level, a national database and GIS of karstic features is being populated. Information gathered includes dolines, springs, stream sinks, caves and building damage. At a national level, the soluble rocks in Great Britain have been identified and digital-map polygon information relating to them was extracted from the British 1:50,000-scale digital geological map. These areas have been assessed, and in places their margins extended to include some overlying rocks where subsidence features are known to penetrate upwards through the overlying sequence. The national areas have then been assessed using detailed local information to assign a susceptibility rating from A (extremely low) to E (high), depending on the nature and regularity of the subsidence events that occur. This national zonation of the soluble rocks can be used for planning, construction and in the insurance businesses. This has proved useful for assessing the potential stability of linear routes, such as roads and pipelines or for other important structures such as bridges and buildings. The information can also be used to delineate zones of karstic groundwater flow.     

The World Karst Aquifer Mapping project: concept,mapping procedure and map of Europe

Karst aquifers contribute substantially to freshwater supplies in many regions of the world, but are vulnerable to contamination and difficult to manage because of their unique hydrogeological characteristics. Many karst systems are hydraulically connected over wide areas and require transboundary exploration, protection and management. In order to obtain a better global overview of karst aquifers, to create a basis for sustainable international water-resources management, and to increase the awareness in the public and among decision makers, the World Karst Aquifer Mapping (WOKAM) project was established. The goal is to create a world map and database of karst aquifers, as a further development of earlier maps. This paper presents the basic concepts and the detailed mapping procedure, using France as an example to illustrate the step-by-step workflow, which includes generalization, differentiation of continuous and discontinuous carbonate and evaporite rock areas, and the identification of non-exposed karst aquifers. The map also shows selected caves and karst springs, which are collected in an associated global database. The draft karst aquifer map of Europe shows that 21.6% of the European land surface is characterized by the presence of (continuous or discontinuous) carbonate rocks; about 13.8% of the land surface is carbonate rock outcrop.

     

Evaluation of Karst Hazards for Civil and Industrial Buildings

The European part of Russia exhibits highly developed sulphate and carbonate karst. It mostly occurs within river valleys with relatively thin covering deposits. These conditions may induce karst collapses, which appear to be the main danger for civil and industrial buildings. Evolution of karst rocks includes several epochs of karst development, which causes complicated distribution of karst caves in karst rocks and, as the result, irregular distribution of karst caves on the surface. Karst hazards prediction is mostly reliable within the geotechnical system "Karst-Construction", using probability methods. This approach allows creating 3 types of antikarst protection (alternative design of construction arrangement on a plan, structural protection of a construction and plugging of karst caves beneath construction foundation) and selecting the optimum or the most effective version or their rational combination.     

Geomorphological, geochemical and geo-environmental aspects of karstification in the urban areas of Kerman city, southeastern, Iran

Kerman city has a semiarid-arid climate with an average annual precipitation of about 158 mm. The area is underlain by soluble subsoil and alluvial deposits, overlying highly fractured Cretaceous limestones. Geo-environmental studies indicate that both paleokarst and active karst features are developed in the area. The paleokarsts were developed in the Upper Cretaceous limestone during the cold, humid periods of Post Cretaceous and probably Early Quaternary time and include honeycombs, solution flutes, rillenkarren, caverns, and solution collapse dolines. Active karst landforms occur by combined piping-induced and limestone solution at depth in subsoils, and alluvial deposits and bajada that overly potent karstic limestones and cover subsidence sinkholes and subjacent alluvial karst collapse dolines. Many factors, such as soluble compounds (salt and gypsum), desiccation cracks, and Qanat (dug water wells), could contribute to the development of karstic landforms. The most immediate cause for active karst landforms is considered to be the drawdown of the water table in the area. There is an increasing demand for groundwater consumption to irrigate pistachio fields. Excessive pumping of the groundwater lowers the water table about 80 cm per year. This rate of drawdown accelerates land subsidence (about 6 cm per year), creates circular patterns of fractures in the ground and in buildings, disrupts agricultural work and urbanization projects, and tilts foundations. These geohazards indicate that ground sinking and karstification are in progress in the alluvial deposits and underlying limestones. The disturbance and expense caused by the geohazards could be mitigated by the application of overhead sprinkler irrigation for pistachio fields or by planting less thirsty plants.     

Halite karst geohazards (natural and man-made) in the United Kingdom

In the United Kingdom, Permian and Triassic halite (rock salt) deposits have been affected by natural and artificial dissolution producing karstic landforms and subsidence. Brine springs from the Triassic salt have been exploited since Roman times or possibly earlier, indicating prolonged natural dissolution. Medieval salt extraction in England is indicated by the names of places ending in "wich", indicating brine spring exploitation at Northwich, Middlewich, Nantwich and Droitwich. Later, Victorian brine extraction in these areas accentuated salt karst development, causing severe subsidence problems which remain a legacy. The salt was also mined, but the mines flooded and consequent brine extraction caused the workings to collapse, resulting in catastrophic surface subsidence. Legislation was enacted to pay for the damage and a levy is still charged for salt extraction. Some salt mines are still collapsing, and the re-establishment of the post-brine extraction hydrogeological regimes means that salt springs may again flow, causing further dissolution and potential collapse.     

Karst Hazard Assessment of Eastern Saudi Arabia

Karst phenomena exist in areas in the eastern part of Saudi Arabia, forming solution features such as sinkholes, collapsed dolines and solution caverns, as a result of the chemical leaching of the carbonate and evaporite formations by percolating water. The instability of these karst phenomena could produce land subsidence problems. This paper reviews the geology of documented karstic rock units in Saudi Arabia and proposes a simple engineering classification of the solution features characteristic of limestone. Two case histories in the Dhahran area, eastern Saudi Arabia, will be used as examples for the application of a modified engineering classification.     

桂南红层岩溶及其发育控制因素探讨

桂南红层岩溶形态主要表现为岩溶缓丘、地下溶洞、岩溶泉以及地下河、天窗等。在地下50m深度范围,溶洞高0.5～3m,多为无充填溶洞,发育数量随深度增加而减少。岩溶泉类型有上升泉和下降泉,泉水流量变化较大。目前尚未发现仅由红层作为含水层的地下河。桂南红层岩溶的发育程度及形态特征与岩性、构造及水文地质条件有关。红层岩溶主要发育于红层盆地边缘的钙质砾岩中。红层的层组结构可分为单一型和互层型,单一型岩溶发育相对强烈,互层型以岩溶顺层发育为特征。通常,在可溶岩与非可溶岩接触带附近红层溶洞发育;在断裂破碎带以及河谷地带红层岩溶发育强烈,溶洞规模较大。      

不同岩溶储水结构分析与地球物理勘察

岩溶区储水结构是寻找地下水最为有利的靶区。在可溶岩与非可溶边界呈串珠状发育的岩溶泉、漏斗等,常指示地下岩溶强烈发育,接触界面的可溶岩为找水的有利部位。在背斜轴部,由于来自侧翼的挤压作用,灰岩、白云岩等硬脆性岩石易形成张性裂隙密集带,成为地下水的有利赋存部位。断裂、构造裂隙密集带使地下水互为沟通,成为良好的储水载体。在这...     

地面岩溶塌陷的高分辨地震勘查

地面岩溶塌陷是地质灾害主要灾种,由于其突发性,对人类的生产和生活造成威胁。浅层高分辨地震技术可以查明形成塌陷的基岩地质条件,特别是横波反射法可以查明覆盖层中地下水作用形成的潜蚀带或土洞等覆盖层地质条件。勘查洞穴是岩溶塌陷地震勘查方法面临的特殊问题,为了解土洞、溶洞形成的地震反射信号特征,专门进行二维固体超声波模型试验。唐山市岩溶塌陷勘查实例表明,高分辨地震技术可探明基岩埋深、断层、破碎带及覆盖层潜蚀扰动区,为防冶工程设计提供地质背景资料。     

A review of natural sinkhole phenomena in Italian plain areas

Italian sinkholes, which are mainly related to karst phenomena (i.e., solution sinkholes, collapse sinkholes, etc.), are widespread along the Apennine ridge and in pedemontane areas where there are carbonatic bedrock outcrops. However, other collapses, which seem unrelated to karst dissolution, have been identified in plain areas with a thick sedimentary cover over buried bedrock. The main goal of this work is to study the geological, geomorphological, and structural setting of these areas to identify the possible mechanism of the generation and evolution of these collapses. About 750 cases were identified by research based on historical archives, specific geological literature, and information from local administrations. Geological, geomorphological, and hydro-geochemical surveys were conducted in 300 cases, supported by literature, borehole, and seismic data. A few examples were discarded because they could be ascribed to karst dissolution, volcanic origin (i.e., maar), or anthropogenic causes. Field studies regarding the other 450 cases are in progress. These cases occur along the Tyrrhenian margin (Latium, Abruzzo, Campania, Tuscany) in tectonic, coastal, and alluvial plains close to carbonate ridges. These plains are characterized by the presence of pressurized aquifers in the buried bedrock, overlaid by unconsolidated sediments (i.e., clay, sands, pyroclastic deposits, etc.). The majority of these collapses are aligned along regional master and seismogenetic faults. About 50% of the studied cases host small lakes or ponds, often characterized by highly mineralized springs enriched with CO₂ and H₂S. The Periadriatic margin does not seem to be affected by these phenomena, and only a few cases have been found in Sicily, Sardinia, and Liguria. The obtained scenarios suggests that this type of collapse could be related to upward erosion through vertical conduits (i.e., deep faults) caused by deep piping processes whose erosive strength is increased by the presence of acidic fluids. In order to distinguish these collapses from typical karst dissolution phenomena, they are defined as deep piping sinkholes (DPS).