Subsidence hazard avoidance based on geomorphological mapping in the Ebro River valley mantled evaporite karst terrain (NE Spain)

In the valley of the Ebro River to the southeast of the city of Zaragoza (NE Spain), the dissolution of evaporite sediments (gypsum, halite and Na-sulphates) which underlie alluvial deposits gives rise to numerous sinkholes. These sinkholes are a potential hazard to human safety, particularly where they develop in a catastrophic way. Even slow-developing sinkholes are problematic, as they damage urban and agricultural infrastructure, necessitating costly repairs and vigilant maintenance. To assist in developing avoidance strategies for these hazards, the factors controlling sinkhole occurrence have been assessed using geomorphological maps produced from aerial photographs for 1956 and 1981. Important controls on sinkhole development are found to include underlying geological structure (manifest in preferred orientations of sinkholes on the azimuths N130-150E and N30-40E), and the presence of glauberite in the groundwater flow path, which apparently promotes accelerated gypsum dissolution. Perhaps surprisingly, alluvium thickness does not appear to significantly correlate with the density of sinkholes on the floodplain in this area. The maps for 1956 and 1981 reveal that both human activity and natural processes can serve to obscure the true density of sinkhole development. For instance, a large number of sinkholes which were conspicuous in 1956 have since been back-filled by farmers. In the most fluvially active zone of the Ebro valley (the meander belt), the relatively low density of sinkholes compared with adjoining zones suggests that subsidence is being masked by morpho-sedimentary dynamic processes (aggradation and erosion). Careful geomorphological mapping for different time periods yields a much more accurate impression of the frequency of sinkhole development than would be gained from surveying currently visible sinkholes in the area of interest.     

The formation of sinkholes in karst mining areas in China and some methods of prevention

 Mining of coal, lead and zinc, gold, and iron ore deposits in karst areas has been closely associated with sinkholes in China. Surface collapse causes an increase in mine water drainage and the possibility of major water inflow from karst aquifers, which threatens the environment in mining areas and endangers mine safety. The origin of such sinkholes is analyzed quantitatively in this paper and a combination of factors including soil weight, buoyancy, suffosion process and vacuum suction can contribute to surface subsidence. The key measures to prevent sinkholes in mining areas are to control the amount of mine drainage, reduce water-level fluctuation, seal off karst conduits and subsurface cavities in the overlying soil, prevent water inflow, and to increase gas pressure in the karst conduits. Received: 2 May 1996 · Accepted: 29 July 1996     

Dead Sea sinkholes – an ever-developing hazard

 Sinkhole development along the western shore of the Dead Sea became a major concern in 1990 with the appearance of a series of holes 2–15 m diameter and up to 7 m deep in the Newe Zohar area. One of these sinkholes, below the asphalt surface of the main road along the western shore of the Dead Sea, was opened by a passing bus. Repeated infilling and collapse of these holes indicated the extent of this ongoing process and the significance of this developing hazard. Since then sinkholes have developed in other areas including Qalia, Ein Samar, Ein Gedi and Mineral Beach. Three main types of sinkholes have been recognized. Gravel holes occurring in alluvial fans, mud holes occurring in the intervening bays of clay deposits between fans and a combination of both types at the front of young alluvial fans where they overlap mud flats. Fossil, relict sinkholes have been observed in the channels of some old alluvial fans. Sinkhole development is directly related to the regression of the Dead Sea and the corresponding lowering of the regional water table. Continuation of this process widens the neritic zone enveloping the sea and increases the sinkhole hazard of the region. Received: 4 February 1999 · Accepted: 8 April 1999     

Subsidence-sinkhole development in light of mud infiltrate structures within interstratal karst of the Coastal Plain,southeast United States

Mud infiltrate structures (MIS) are a class of clastic sedimentary structures that have formed by high-angle gravitational sedimentation in vadose cavernous limestones beneath the Miocene Hawthorn Group. Observations of MIS in caves beneath the Pelham Escarpment, southwest Georgia, suggest three main types occurring either beneath, within, or contiguous to subvertical feeder pipes in the ceiling: (1) rubbly, conical masses; (2) polished, striated, cylindrical masses; and, (3) mud-flow, -drape, and-splash forms. MIS abundance suggests gravitative deposits are a major component to the total volume of cave mud. The muds apparently do not include soil-mantle materials; rather, their sources are poorly cemented clay-rich layers within the overlying Hawthorn Group. In cases where an individual MIS volume is large in proportion to the clastic-cover thickness, but no surface depression is found, the MIS location hypothetically predicts a directly overlying site of potential cover-collapse sinkhole development. Smaller-volume and/or shallower MIS predict the location of potential cover-subsidence sinkholes. The MIS forms suggest plastic and fluid behavior of the sediments occurs during displacements. The capacity for subsurface voids to accept mobile sediments depends on several factors that influence sinkhole development: (1) void size and interconnectedness; (2) spacing and size of subvertical solution pipes which can act as MIS feeders; (3) depth beneath the sediment source of any main horizontal cave development; (4) thickness of the cover sediments; and (5) the presence and depth of any breakout domes. The Hawthorn is present over more than 50 percent of the area underlain by the Floridan Aquifer system and is the system's major upper confining unit. MIS-forming processes are likely to be widely distributed within this stratigraphic setting, promoting ground subsidence and subsurface porosity obliteration. Given the existence of feeder pipes, MIS also are expected to form within the phreatic zone.     

GIS and ANN coupling model: an innovative approach to evaluate vulnerability of karst water inrush in coalmines of north China

Evaporites, including rock salt (halite) and gypsum (or anhydrite), are the most soluble among common rocks; they dissolve readily to form the same types of karst features that commonly are found in limestones and dolomites. Evaporites are present in 32 of the 48 contiguous states in USA, and they underlie about 40% of the land area. Typical evaporite-karst features observed in outcrops include sinkholes, caves, disappearing streams, and springs, whereas other evidence of active evaporite karst includes surface-collapse structures and saline springs or saline plumes that result from salt dissolution. Many evaporites also contain evidence of paleokarst, such as dissolution breccias, breccia pipes, slumped beds, and collapse structures. All these natural karst phenomena can be sources of engineering or environmental problems. Dangerous sinkholes and caves can form rapidly in evaporite rocks, or pre-existing karst features can be reactivated and open up (collapse) under certain hydrologic conditions or when the land is put to new uses. Many karst features also propagate upward through overlying surficial deposits. Human activities also have caused development of evaporite karst, primarily in salt deposits. Boreholes (petroleum tests or solution-mining operations) or underground mines may enable unsaturated water to flow through or against salt deposits, either intentionally or accidentally, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence and/or catastrophic collapse. Evaporite karst, natural and human-induced, is far more prevalent than is commonly believed.     

Design and routing of storm flows in an urbanized watershed without surface streams

In the karst geologic setting of Greenbrier County, West Virginia, USA, the drainage network in the watersheds do not support surface streams, but depend entirely on sinkholes, solution cavities, or injection wells as discharge points for accumulated storm water. By providing a systematic framework for designing and routing storms in this geologic setting, functioning retention and attenuation structures have been developed which are protective of water quality while still safely discharging storm water in a controlled manner to the subsurface. This article provides a rationale for the design methodology and then examines the successful implementation of an attenuation and storm water retention design to manage the surface discharges for an entire watershed. By examining the pre-development flows and evaluating future land use patterns (i.e., installation of impermeable surfaces over large areas), as well as sinkhole conveyance capabilities, it was necessary to examine alternative disposal options for collected storm water as well as devise a basin-wide management strategy to coordinate future development of the watershed. Additionally, innovative water quality measures were implemented to help prevent contamination from preferentially infiltrating into the subsurface as a result of these land development activities.     

Pseudosinkhole occurrences in Brasilia,Brazil

Brasilia, the capital city of Brazil, is located in the central region of the country. Climate in the area is semitropical with an annual rainfall of 1500 mm. The geological environment in the area consists of low-grade metamorphic rocks. Slates of varying colors, metasiltstone, and quartzite beds are present. Over the Precambrian rocks is a lateritic layer varying in thickness from centimeters up to 30 m. Latosol dominates the existing plateaus, while laterite crusts and immature soils are dominant in the transition zones between plateaus and river valleys. Erosional problems related to the lateritic terrains were known prior to the settlement of the city in 1961. During 1986, erosion became a serious threat when several pseudosinkholes occurred in the urban area. Occurrence of pseudosinkholes resulted in condemnation of an area of 300,000 m² and the demolition of several buildings. Preliminary studies indicated at the time that underground erosion and pseudosinkholes were generated by shortening of the percolation path of groundwater due to the progress, toward the residential area, of large gullies. This produced an increase in the hydraulic gradient, resulting in the removal of latosol particles. Occurrence of pseudosinkholes in other areas of the city has led the Institute of Geosciences to investigate the problem in more detail. Studies have been conducted looking for correlation between pseudosinkhole occurrence and geologic, geomorphic, geotechnical, and urban development features. Recent results show a much more complex process then previously thought. Field data suggests that termite activity and recharge of the water table by inadequate disposal of residential sewer systems are directly related to the problem.     

Sinkhole distribution in a rapidly developing urban environment: Hillsborough County, Tampa Bay area, Florida

Sinkhole formation in Florida is a common event. The Florida karst plain is significantly altered by human development and sinkholes cause considerable property damage throughout much of the state. We present in this paper a morphometric analysis of karst depressions in the Tampa Bay area, and the relation with the known distribution of sinkholes. We selected the Tampa Bay area because it is particularly susceptible to the evolution of karst depressions in relation with development of the built-up environment. Karst depressions were mapped from the 1:24,000 USGS topographic maps and a morphometric analysis was performed by using parameters such as shape, circularity index, perimeter, area, length, width, and orientation. Maps showing the distribution of depression density, and the sectors with greatest areas of karst depression were produced using a GIS. These results were compared with data compiled from the database of sinkhole occurrences in Florida maintained by the Florida Geological Survey. Our analysis demonstrates that the distribution of new sinkhole occurrences differs from the distribution of existing sinkholes, indicating that there are processes acting today that are influencing karst landscape formation that are different from those acting in the past.     

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.     

Dye trace and bacteriological testing of sinkholes: Sulphur Springs,Tampa, Florida

This paper summarizes over four years of studies and testing of a sinkhole/spring system in north Tampa. Sulphur Springs Pool delivers an average of 95 million l/d to the Hillsborough River, which is tributary to Tampa Bay. In 1986, owing to increasingly erratic bacterial levels at the natural bathing area adjacent to Sulphur Springs, the Hillsborough County Health Department closed the pool for swimming. The City of Tampa, Southwest Florida Water Management District (SWFWMD), Hillsborough County Environmental Protection Commission, and the United States Geological Survey have gathered data in an attempt to better understand the system and possible sources of contamination. The Sulphur Springs Action League is a civic group in the area, which has an objective of reopening the pool for recreational purposes. Environmental Engineering Consultants, Inc. provided pro bono technical assistance and expertise in assisting the Action League with its goal. The Action League obtained a grant from SWFWMD to outfit underwater divers for sinkhole exploration as well as water quality and dye trace analysis. The main suspects for bacterial contamination of the pool were two significant sinkholes located 1950 and 2300 m north of the spring. A series of dye tests and water-quality tests were performed. It was estimated that the underground velocity of water was between 90–100 m/h. Using a dye trace, bacteria testing, and travel time estimating, a new source of contamination was found in a Department of Transportation (DOT) stormwater retention basin in which a sinkhole had opened up and was receiving stormwater. The two significant sinkholes received stormwater from commercial and residential areas, and this stormwater brings a large amount of bacteria into the sinkhole, which funnels into the underground system and induces a bacteria spike at Sulphur Springs pool that exceeds the bathing water standards. The City of Tampa has constructed an experimental initial flush capture basin that will sand-filter stormwater to see if this will favorably affect bacteria levels. A mayor's task force in Tampa has recommended ultraviolet disinfection as an interim solution to the contamination problem.