Subsidence
Introduction
Subsidence is the gradual sinking or settling of the Earth's surface with little or no horizontal motion. This phenomenon can occur due to natural processes such as the dissolution of underlying carbonate rock, the compaction of sediments, or the withdrawal of fluids like water, oil, or gas from the subsurface. Human activities, including mining, groundwater extraction, and construction, can also induce subsidence. Understanding subsidence is crucial for geotechnical engineering, urban planning, and environmental management.
Causes of Subsidence
Natural Causes
Karst Processes
Karst topography is formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. This dissolution creates underground voids and caverns, which can collapse, leading to subsidence. The process is often accelerated by the presence of acidic water, which enhances the dissolution rate of the carbonate rocks.
Sediment Compaction
Sediment compaction occurs when the weight of overlying materials compresses the underlying sediments, reducing their volume and causing the ground surface to sink. This is common in deltaic regions where thick deposits of unconsolidated sediments are present. Over time, the weight of these sediments compacts the underlying layers, leading to subsidence.
Tectonic Activity
Tectonic subsidence is associated with the movement of the Earth's lithospheric plates. This can occur at divergent boundaries where plates are moving apart, causing the crust to thin and sink. It can also happen in areas of crustal extension, where the stretching of the crust leads to subsidence.
Anthropogenic Causes
Groundwater Extraction
The extraction of groundwater for agricultural, industrial, and domestic use can lead to subsidence. When water is pumped from underground aquifers, the pressure that supports the overlying sediments is reduced, causing them to compact and the ground surface to sink. This is a significant issue in many urban areas where groundwater is a primary water source.
Mining Activities
Mining activities, particularly underground mining, can cause subsidence. The removal of material from beneath the Earth's surface creates voids that can collapse, leading to ground subsidence. This is especially prevalent in coal mining regions where extensive underground networks of tunnels and shafts are present.
Oil and Gas Extraction
Similar to groundwater extraction, the removal of oil and gas from subsurface reservoirs can lead to subsidence. As the fluids are extracted, the pressure within the reservoir decreases, causing the surrounding rock to compact and the ground surface to sink.
Effects of Subsidence
Structural Damage
Subsidence can cause significant structural damage to buildings, roads, bridges, and other infrastructure. The differential settling of the ground can lead to cracks in foundations, walls, and pavements. In severe cases, it can result in the collapse of structures.
Flooding
In coastal and low-lying areas, subsidence can exacerbate flooding by lowering the ground level relative to sea level. This increases the risk of inundation during high tides and storm surges. Subsidence can also impair drainage systems, leading to waterlogging and increased flood risk.
Environmental Impact
Subsidence can have various environmental impacts, including the loss of wetlands, changes in groundwater flow patterns, and the destruction of habitats. In karst regions, subsidence can lead to the formation of sinkholes, which can disrupt ecosystems and pose hazards to human activities.
Monitoring and Mitigation
Monitoring Techniques
Remote Sensing
Remote sensing techniques such as InSAR and LiDAR are commonly used to monitor subsidence. These methods provide high-resolution data on ground deformation over large areas, allowing for the detection and measurement of subsidence.
Ground-Based Methods
Ground-based monitoring techniques include the use of GPS stations, leveling surveys, and extensometers. These methods provide precise measurements of ground movement and are often used in conjunction with remote sensing data to validate and refine subsidence models.
Mitigation Strategies
Groundwater Management
Effective groundwater management is essential for mitigating subsidence. This includes regulating the extraction of groundwater, implementing artificial recharge programs, and promoting water conservation practices. In some cases, the injection of water into depleted aquifers can help to stabilize the ground and reduce subsidence.
Engineering Solutions
Engineering solutions to mitigate subsidence include the use of deep foundations, ground improvement techniques, and the construction of flexible infrastructure that can accommodate ground movement. In mining areas, backfilling of voids and the use of controlled collapse methods can help to minimize subsidence.
Policy and Regulation
The development and enforcement of policies and regulations are crucial for managing subsidence. This includes land-use planning, zoning regulations, and the establishment of monitoring and reporting requirements for activities that can induce subsidence. Public awareness and education programs can also play a role in promoting responsible practices and reducing the risk of subsidence.
Case Studies
San Joaquin Valley, California
The San Joaquin Valley in California is one of the most well-documented cases of subsidence due to groundwater extraction. Extensive pumping of groundwater for agricultural irrigation has led to significant subsidence, with some areas experiencing more than 30 feet of sinking over the past century. This has resulted in damage to infrastructure, reduced water storage capacity, and increased flood risk.
Mexico City, Mexico
Mexico City is built on a former lakebed, and extensive groundwater extraction has led to severe subsidence. The city has sunk by more than 30 feet in some areas, causing widespread damage to buildings, roads, and utilities. Efforts to mitigate subsidence in Mexico City include the reduction of groundwater extraction and the implementation of artificial recharge programs.
Venice, Italy
Venice is a well-known example of subsidence exacerbated by human activities. The city is built on a series of islands in a lagoon, and the extraction of groundwater for industrial use in the 20th century led to significant subsidence. Combined with rising sea levels, this has increased the frequency and severity of flooding in the city. Efforts to address subsidence in Venice include the cessation of groundwater extraction and the construction of barriers to protect against high tides.