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Primary reference(s)

Rogers, C.D.F., 1995. . Accessed 14 October 2020.

USDA, 1990. . Accessed 26 October 2020.

Additional scientific description

Volume change when a soil is subject to load, results from changes in pore volume, initially as a result of the loss of air and water from the voids and then as a consequence of more ordered grain packing. Organic matter in the soil may be highly compressible; peat soils being particularly susceptible to consolidation. Consolidation is the gradual reduction in soil volume resulting from an increase in compressive stress. The resulting increase in density is compaction (USDA, 1990).

Consolidation is the gradual reduction in soil volume resulting from an increase in compressive stress. It consists of initial consolidation, which is a comparatively sudden reduction in volume resulting from the expulsion and compression of gas; primary consolidation, which results principally from a squeezing out of water and is accompanied by a transfer of load from the soil water to the soil solids; and secondary consolidation, resulting principally from the adjustment of the internal structure of the soil mass after initial consolidation. Settlement is the displacement of a structure due to the compression and deformation of the underlying soil. Compaction is the densification of a soil by means of mechanical manipulation (USDA, 1990).

Collapsible soils differ from compressible soils in that they are low density soils with a structure that collapses upon wetting. They may have considerable strength when dry or moist. They lose strength and undergo sudden compression when they are saturated. Some will collapse under their own weight when saturated; others, only when loaded (USDA, 1990).

Compaction reduces the volume of void available for water, potentially affecting water infiltration into soil, crop root moisture uptake and penetration, and consequential crop yield. Soil compaction can lead to surface ponding of water and water logging, leading to chemical deterioration in soil quality. The potential for increased surface water run-off can lead to soil erosion (USDA, 1990).

The Food and Agriculture Organization of the United Nations (FAO) and Intergovernmental Technical Panel on Soils estimate that 33% of land is moderately to highly degraded due to the erosion, salinisation, compaction, acidification and chemical pollution of soils (FAO and ITPS, 2015). They noted that compaction-related soil degradation is increasing in Asia, Latin America and the Near East and North Africa; it is variable in Europe and Eurasia, the SW Pacific and North America (FAO and ITPS, 2015).

Metrics and numeric limits

The FAO and its partners in the Global Soil Partnership (GSP) are designing SoilSTAT, a system for monitoring, forecasting and reporting periodically on the status of global soil resources. The name of the system mirrors the FAOSTAT family of global status databases and monitoring. SoilSTAT is part of the Global Soil Information System. It will be built under GSP Pillar 4 as part of spatial data infrastructure for the exchange of web-based soil data services. The basic data elements of Pillar 4 include soil profiles, soil polygon maps and soil grids; SoilSTAT will be based on indicators describing the current condition of, and trends in, soil quality. According to the Status of the World’s Soil Resources report, indicators will address soil threats such as erosion, compaction, salinisation and the loss of soil organic matter. SoilSTAT will therefore be an important mediator in the harmonised reporting of national soil information for the Sustainable Development Goals (FAO and ITPS, 2015).

Examples of drivers, outcomes and risk management

Causes of compaction include agricultural vehicle tracks and poaching with farm mechanisation considered the primary cause on both agricultural and forestry land (Forest Research, 2020). Extensive poaching is also caused by livestock and herds of wild animals. Soil compaction on construction sites occurs either deliberately when foundations and sub-grades are prepared for construction or as an unintended result of vehicular traffic and excavation (cut and fill) works (DEFRA, 2006).

Compressible soils commonly occur in river estuaries where they may be associated with flooding. Building in these areas may necessitate the use of building platforms to avoid flooding. However loads with shallow foundations may be subject to large settlement over long durations. Methods such as the installation of sand drains can be used to manage the settlement. Where the compressible soils are relatively thin, foundations may be taken down to bear into more competent strata at depth. This scenario can lead to differential settlement. These soils are also sensitive to ground lowering as a consequence of dewatering (USDA, 1990).

Compressible soils can be characterised using consolidation testing in the laboratory and static cone testing in the field (USDA, 1990).

Collapsible soil foundations cause problems when they are saturated after being loaded by a structure. Examples are settlement of a dam foundation resulting in cracking of the dam when stored water saturates the foundation; settlement and cracking of an irrigation ditch, dike or lining when seepage from the ditch saturates the foundation; or settlement of a building when the foundation soil is saturated with water from a downspout or leaks in water or sewer pipes (USDA, 1990).

Many loessic soils are collapsible. Low density soils above the water table in this situation should be suspected of being collapsible. They can be identified by modified consolidation testing of undisturbed samples. In testing, samples are loaded in a moist condition, then saturated collapsible soils will show sudden settlement upon wetting (USDA, 1990).

For example, it is important to check the foundations of earth dams for strength, permeability, compressibility, dispersive clays (piping), water table elevation, and depth to bedrock (USDA, 1990).

Collapsible soils commonly occur as alluvial fans in sub-humid to arid areas. Consequential threats include salinisation, nutrient imbalance and loss of biodiversity that may result from soil compaction. Compacted soil may contribute to atmospheric warming through increased emissions of carbon dioxide, methane and nitrogen dioxide from such soils (USDA, 1990).