Engineering geology

Sinkholes in Karst

With few exceptions, the ground collapses that constitute the karst geohazard in engineering activity in limestone terrains are induced by human activity. Subsidence sinkholes, formed entirely within the soil profile, constitute the most widespread karst geohazard but are largely induced by engineered works, either directly or accidentally.

Water table decline (as a result of pumped abstraction or quarry de-watering) and uncontrolled surface drainage input are the two key factors that induce subsidence sinkholes, especially where both are involved.

Collapse sinkholes, formed by a failure of bedrock over a cavity, are rare in natural karst landscapes, but may be induced by excessive loading imposed on limestone that lies above an open cave; the risks associated with this geohazard should be eliminated by the implementation of an appropriate site investigation that includes proof drilling.

Sinkholes are bowl-shaped, funnel-shaped, or vertical-sided depressions in the land surface that form over underground voids. These depressions can range from a few feet to several hundred feet in diameter, and usually, result from the natural collapse of soluble bedrock and overlying soil.

Sinkholes can also result from mining, groundwater pumping, and leaking water, sewer, and stormwater pipes. Subsidence of the ground is usually gradual, but on occasions, it can be sudden and dramatic.

When it rains, a portion of the slightly acidic water percolates through the soil, comes into contact with the bedrock, and slowly dissolves mineral grains. In regions of carbonate bedrock, this process can create underground fissures and caves.

The surface of such a region is often pocked with depressions called sinkholes. Sinkholes are a characteristic feature of karst terrain. In well-developed karst terrain, chains of sinkholes form what is known as solution valleys and streams frequently disappear underground.

One of the fantastic examples of karst sinkholes was filmed in Bosnia and Herzegovina and belongs to one of the most famous karst areas on Earth – the Dinaric highlands in Southeast Europe:

Sinkhole collapse, either slow or dramatic, can cause considerable damage to buildings, highways, rails, bridges, pipelines, storm drains, and sewers.  In addition, sinkholes provide a pathway for surface water to directly enter groundwater aquifers. The increasing potential for pollution is particularly high due to the minimal filtering of surface water.

Sinkholes

Most sinkholes form by the process of ‘suffosion’. This is where loose, unconsolidated material including soil, ‘head’, loess and clay overlies fissures and joints in the underlying limestone, and material is washed into these fissures and into the caves beneath.

Suffosion sinkholes tend to develop gradually (over months or years) as the covering sediment slumps into open fissures in the underlying limestone, creating a void which migrates towards the surface eventually creating a sinkhole.

Although a natural process, the formation of sinkholes is often accelerated or triggered by human actions. Broken land drains, water mains and sewerage pipes, increased rainfall, storm events, modified drainage and diverted surface water can all help wash sediment into the underlying limestone, causing subsidence.

There have been many well-documented occurrences of sinkholes forming beneath broken water mains, unlined storm-water culverts and leaking swimming pools.

A poor understanding of karst terrain has led to land-use practices that pose significant economic and environmental impacts on households and communities. Sinkhole formation is closely related to local hydrological conditions, and human-induced changes to the local hydrology can accelerate the process.  Diverting surface water, pumping groundwater, and constructing reservoirs can all contribute to sinkhole formation.

An extreme example occurred in Florida on February 25, 1998, when, during the flushing of a newly drilled irrigation well, hundreds of sinkholes up to a hundred and fifty feet across formed over a twenty-acre area within a few hours.  Runaway urbanization and development dramatically increases water usage, alters drainage pathways, and overloads the ground surface. 

According to the Federal Emergency Management Agency, the number of human-induced sinkholes has doubled since 1930, while insurance claims for related damages have increased 1,200 % from 1987 to 1991, costing nearly $100 million.  Subsidence is generally not covered by standard homeowners insurance.

Signs of sinkhole formation


Although a sinkhole can form without warning, specific signs can signal potential development:

-Slumping or falling fence posts
-Wilting vegetation
-Discoloured well water
-Structural cracks in walls, floors, or foundations.
-Cracks in soil/subsidence.

Silent danger

Infrastructure — buildings and transportation, communication and utility networks — is vulnerable to damage from a variety of geologic hazards, such as volcanoes and earthquakes. But karst geohazards are stealthy. They come silently from below, then unexpectedly make themselves known. And because they usually affect a segment of a utility line, or one home, or one short length of the highway, their cost is also stealthy. But the toll adds up.

Karst landscapes and aquifers form when water dissolves limestone, gypsum and other rocks. The surface expression of karst includes sinkholes, sinking streams and springs.

Sinkholes

The economic losses of karst hazards are largely hidden because they are scattered across an area the size of a state, and individually they affect small areas when compared to tornado damage, for example. Most people don’t realize how much they are affected because the costs appear in the form of higher taxes and an increased cost of living.

The cost of repairing roads, preparing special foundations for large buildings (schools, for example) and extending public water lines to replace polluted groundwater all add to the costs of public projects.

Researchers foresee an increasing need for research on karst geologic hazards because of the accelerating pace of suburban development. Delineating karst groundwater basins is vital to protecting the quality of water discharged from springs and wells, and is an important tool for understanding the hydrology of sinkhole flooding, one of the most common karst hazards.

Karst

Damaging homes

Karst hazards include sinkhole flooding, sudden cover collapse, leakage around dams, the collapse of lagoons resulting in waste spills and radon infiltration into homes. Most noticeable are sinkhole flooding and cover collapse. Seldom are collapses reported to any central agency.

Sinkhole flooding is one of the more tragic hazards because it affects private residences the most.

Sinkhole flooding usually occurs during the same storms that flood rivers, so it is sometimes not recognized as karst related. Unlike a normal stream channel, the karst conduit has a fixed area that cannot increase in cross-section in response to floods.

Because of the loss of energy to friction and the finite cross-sectional area of the karst conduits, large increases in water pressure are needed to increase, even by a small amount, the flow in cave passages.

Sinkholes can also flood when their outlets are clogged, preventing water from being carried away as fast as it flows in. Trash thrown into a sinkhole can clog its throat, as can soil eroded from fields and construction sites or a natural rockfall near the sinkhole’s opening. Sometimes the conduit itself is too narrow because it has recently (in the geologic sense) captured a larger drainage basin.

The reach of a conduit downstream from constriction could carry a higher flow than it is receiving were it not for this restriction. Sinkholes flood more easily around development — roofs, parking lots, highways — which increases both the total runoff and the rapidity of runoff from a storm.

A second reason that sinkholes flood is because of backflooding, the outcome when the discharge capacity of the entire karst conduit network is exceeded. Some upgradient sinkholes that drain normally during the short, modest accumulation of storms, may actually become springs that discharge water during prolonged rainfall.

Sudden collapse

Cover collapse occurs when the soil collapses into an underlying grike, a fissure made larger as water dissolves limestone. Cover collapse is similar to subsidence, except that it happens suddenly in a small, focused location.

Both heavy rains or extended droughts can bring on cover collapse. Each weakens the soil over a grike, either by saturating the soil with water or robbing it of cohesion. Near buildings, downspouts and leaking utility pipes can accelerate the process. Eroded soil falls into the grike and water moves the soil to an underlying cave, forming a cavity in the mantling soil.

In high- flow events, water in the cave may backflood into the overlying soil. As the water recedes, the cave and grike drain faster than the soil, which means that saturated soil spans the void in the grike. The overloaded soil arch falls into the soil cavity and the cover collapses.

The erosion of soil into the underlying conduit does not automatically stop when an impermeable surface, such as a highway, parking lot or building, is constructed over the sinkhole. Lateral flow can easily continue to erode the soil.

The typical scenario is when no local ordinance prevents a developer of a rural subdivision from filling sinkholes. The developer builds a house on a filled sinkhole from which the fill continues to be undermined. Subsidence results, sometimes decades after the developer is gone.

Sinkholes defined

Geologists classify sinkholes based on their geometry and how they developed. Understanding sinkhole dynamics is critical to detecting and mitigating damages these karst features can cause.

Collapse sinkholes occur when the bridging material over a subsurface cavern cannot support the overlying material. The cover collapses into the cavern and a large, funnel-shaped depression forms.

Solution sinkholes result from increased groundwater flow into higher porosity zones within the rock, typically through fractures or joints within the rock. An increase of slightly acidic surface water into the subsurface continues the slow dissolution of the rock matrix, resulting in slow subsidence as surface materials fill the voids.

Alluvial sinkholes are older sinkholes that have been partially filled with marine, wetland or soil sediments. These features are common in Florida, where the water table is shallow, and typically appear as shallow lakes, cypress “domes” and wetlands.

Raveling sinkholes form when a thick overburden of sediment over deep cavern calves into the void and pipes upward toward the surface. As the overlying material or “plug” erodes into the cavern, the void migrates upward until the cover can no longer be supported and then subsidence begins.

Sinkholes in Karst

Different size of sinkholes


Sinkholes can range in size from a few feet or meters to over 100 meters (300 feet) deep. They’ve been known to “swallow” cars, homes, businesses, and other structures. Sinkholes often caused by the loss of groundwater from pumping.

A sinkhole can even collapse through the roof of an underground cavern and form what’s known as a collapse sinkhole, which can become a portal into a deep underground cavern.

While there are caverns located around the world, not all have been explored. Many still elude spelunkers as there is no opening to the cave from the earth’s surface.

Difference between a sinkhole and a pothole


A sinkhole is a closed natural depression in the ground surface caused by the removal of material below the ground and either collapse or gradual subsidence of the surface into the resulting void.

A pothole is usually a fairly small feature caused by a failure of paving materials, usually associated with roads, parking lots, and airports. In the colder parts of the country, potholes become more abundant in late winter and spring because of freeze-thaw damage to pavements. But beware of international terminology: British cavers refer to caves as potholes and call cave exploring “potholing”.

There’s also another kind of pothole. Parts of Canada and the central United States are covered by a region of wetlands called prairie potholes that were formed as Pleistocene Epoch glaciers receded around 12,000 years ago. The wetlands formed where water accumulated in small depressions in a landscape that is underlain by low-permeability glacial till. Prairie potholes are NOT collapse features.

Karst Caves

Inside karst caves, one might find a wide range of speleothems – structures created by the deposition of slowly dripping calcium carbonate solutions. Dripstones provide the point where slowly dripping water turns into stalactites (those structures which hang from the ceilings of caverns), over thousands of years which drip onto the ground, slowly forming stalagmites.

When stalactites and stalagmites meet, they forum cohesive columns of rock. Tourists flock to caverns where beautiful displays of stalactites, stalagmites, columns, and other stunning images of karst topography can be seen.

Karst topography forms the world’s longest cave system – the Mammoth Cave system of Kentucky is over 350 miles (560 km) long. Karst topography can also be found extensively in the Shan Plateau of China, Nullarbor Region of Australia, the Atlas Mountains of northern Africa, the Appalachian Mountains of the U.S., Belo Horizonte of Brazil, and the Carpathian Basin of Southern Europe.

Adapted from DMME, Geotimes, USGS, British Geological Survey Thought.Co,by W.Schmidt,M.Rosenberg,J. Cobb and J. Currens,T.Waltham and dron.ba

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