Download 1 Sinkholes, Land Subsidence, and Swelling Soils

Lecture 13: Sinkholes, Land Subsidence, and Swelling Soils Ch. 9: p.236-­‐251; 255-­‐256 Slow Disasters: Ground movements other than those caused by rapid-­‐onset hazards such as earthquakes, volcanic eruptions, and landslides, can nonetheless cause great amounts of damage over long periods of time due to the gradual destruction of buildings, roads, bridges, etc. Examples include sinkhole development (natural) and land subsidence (commonly due to human interference). Sinkholes: Although we don’t hear of common instances of sinkhole development in the United States, it is a fairly common phenomenon, although usually not on the scale of widescale destruction. e.g., Winter Park, Florida (1981): a sinkhole formed with a diameter of 100 m / 324 ft. Damage: $2 million. What lies hidden beneath the surface?: Why do sinkholes form in the first place? It has everything to do with where we find limestone and what happens to limestone when it reacts with a weak acid like H2CO3 (carbonic acid). See By-­‐the-­‐numbers 9-­‐1 (p.237). We call this weak acid ACID RAIN, and its ability to dissolve is intensified by anthropogenic (human) influences (i.e., pollution). There’s a reason why so many caves are in limestone: Dissolution of limestone can occur as the rainwater seeps through cracks in the rock on its way down to the water table, but it can also occur below the water table. Large cavities are produced that may become open caves if the water table drops. Karst Topography: Any area that has a lot of limestone (such as is common in the eastern U.S.A.) is prone to sinkhole development. Limestone is often indicated by a rugged landscape called karst topography that marks the remnants of ancient sinkholes that have eroded down. e.g., Florida: having formed under the ocean as an ancient coral reef environment, much of Florida is composed of limestone, hence the large number of sinkholes. Sinkholes in the United States: Sinkholes are common throughout the United States, wherever limestones and evaporite rocks (e.g., salt) occur. Why are these types of rocks found on land? Sinkhole Development: Sinkhole development can be gradual or rapid depending on the local ground conditions and water table level. • Dissolution: if there is a thin layer of soil above the limestone, percolating rain water slowly seeps down into cracks and widens them. The soil sinks down, causing depressions on the surface that may fill with water to become ponds. • Cover Subsidence: if there is a thick cover of soil or sediment above the limestone, surface depressions can form very gradually, slowly damaging buildings and other infrastructure. • Cover Collapse: if the surface sediment is cohesive, it doesn’t sink into the underlying cavity immediately, which may result in sudden, catastrophic collapse. Cover Collapse: Collapse is least likely when the cavities are filled with water, which gives buoyancy to the overlying rocks. If the water table drops due to times of drought or due to pumping for human consumption, sinkhole collapse is more likely to occur. Although limestone is the most common source of sinkholes, other dissolvable rock types such as rock salt and gypsum-­‐rich rocks also cause problems. 1 Disaster in Guatemala: A 100 m deep hole opened up beneath Guatemala City in Feb. 2007, killing 3 people. It followed heavy rains but may have been caused by a ruptured sewage main. Guatemala City suffered another sinkhole in May 2010 when an 18 m wide and 30-­‐story (100 m) deep cavity opened up, swallowing a 3-­‐story building. A burst sewer pipe or storm drain was responsible for destabilizing the surface materials (pumice). This occurred after heavy rainfall (3 ft / 1 m) from tropical storm Agatha. No deaths were recorded. Land Subsidence: Sinkholes are localized effects caused by dissolution, but there are also ways in which the entire ground level can drop across broad region, referred to as land subsidence. This effect is usually not completely natural but is triggered by human activities. What human activities could cause land subsidence?: • groundwater removal – pumping of groundwater lowers the water table. • oil wells – oilfields pump large volumes of petroleum from the subsurface. • mining – any sort of mining produces subsurface cavities that can collapse. Natural phenomena can also cause land subsidence: • earthquakes – shaking causes water to be forcibly ejected from sediments. In each case, the removal of fluid from the subsurface somehow resulted in the land surface subsiding. Why? Groundwater Removal: The removal of too much water from an aquifer is called overpumping. The resultant compaction (reduction of porosity) of the aquifer is permanent so it can never recover. Thus, any land subsidence that occurs is also permanent. E.g., Land subsidence due to groundwater pumping in the Santa Clara Valley, CA (south of San Francisco). The ground dropped a total of 4 m. One of the more infamous examples of land subsidence in the United States is in the San Joaquin Valley in California, an agricultural area. Excessive removal of groundwater for crop irrigation caused compaction of the aquifer and subsidence of the land elevation by up to 9 m in 50 years. Overpumping is particularly problematic in low rainfall and desert environments due to the long period of time necessary to naturally replenish an aquifer. Overpumping in the Phoenix/Tucson region of Arizona prior to 1985 caused the water table to drop by 30 – 150 m (100 – 500 ft), causing land subsidence. Because some areas subside more than others, called differential subsidence, the boundary of the subsiding area was marked by earth fissures up to 6 m deep and 9 m wide. Why do we care? What’s the hazard?: Land subsidence is usually very slow (occurring over years or decades), and may not necessarily damage buildings unless there is differential subsidence. So what is the nature of the hazard posed by land subsidence? e.g., Venice, Italy: Venice was built on the delta of the Brenta River, approximately at sea level. Over hundreds of years, extraction of groundwater from the delta sediments for human consumption resulted in Venice subsiding beneath the waves. Many roads turned into canals. People abandoned the first floors of their homes. Venice, as with other subsided areas, is thus particularly prone to flooding at unusually high tides (e.g. storm surges). 2 Oil Wells: As with groundwater removal, pumping oil and gas out of the ground causes a permanent reduction in porosity in the oil reservoir. Even though these reservoirs may be many kilometers underground, the compaction can cause land subsidence. This phenomenon has occurred near Long Beach, CA and Houston, TX. Houston, TX is particularly vulnerable to storm surges associated with hurricanes due to coastal areas now being a lot lower than they used to be due to oil pumping. Permafrost Thaw: Melting of permafrost during seasonal cycles, and in response to global warming, causes differential subsidence that can damage structures in arctic regions. Swelling Soils: Swelling soils are clays that can absorb large amounts of water, expanding greatly in the process. When these soils dry out, they shrink and cause subsidence. Repeating cycles of activity can gradually tear structures apart. Clay minerals that do this include smectite and montmorillonite. The clay layers they produce are commonly referred to as bentonite. Popcorn clay provides evidence of swelling soils. Many parts of the U.S.A. are prone to the effects of swelling soils, which cause about $4.4 billion in damage annually. Case in point: some suburbs of Denver, CO are built on inclined beds of bentonite within the Pierre Shale. Differential expansion has badly damaged homes. 3