Searches User Guide: Natural instability
Natural ground subsidence
The natural ground subsidence section is based on the BGS GeoSure dataset. It is produced for use at 1:50,000 scale providing a 50m ground resolution.
Each category is rated on a 6 tiered scale with the following ratings: No data, Negligible, Very Low, Low, Moderate and High.
In Avista, Screening & Review the 6 categories are displayed individually. In all other residential searches, the are grouped together to provide a 3 tier rating (based on the highest risk found): Negligible - Very Low, Low, Moderate - High.
Shrink-swell clays
The potential hazard presented by soils that absorb water when wet (making them swell), and lose water as they dry (making them shrink). This shrink-swell behaviour is controlled by the type and amount of clay in the soil, and by seasonal changes in the soil moisture content (related to rainfall and local drainage).
| A - Negligible | Ground conditions predominantly non-plastic. |
| B - Very low | Ground conditions predominantly low plasticity. |
| C - Low | Ground conditions predominantly medium plasticity. |
| D - Moderate | Ground conditions predominantly high plasticity. |
| E - High | Ground conditions predominantly very high plasticity. |
Data source: British Geological Survey
Data update schedule: Quarterly
Running sands
The potential hazard presented by rocks that can contain loosely-packed sandy layers that can become fluidised by water flowing through them. Such sands can ‘run’, removing support from overlying buildings and causing potential damage.
| A - Negligible | Running sand conditions are not thought to occur whatever the position of the water table. No identified constraints on land use due to running conditions |
| B - Very low | Running sand conditions are unlikely. No identified constraints on land use due to running conditions unless water table rises rapidly |
| C - Low | Running sand conditions may be present. Constraints may apply to land uses involving excavation or the addition or removal of water. |
| D - Moderate | Running sand conditions are probably present. Constraints may apply to land uses involving excavation or the addition or removal of water. |
| E - High | Running sand conditions are almost certainly present. Constraints will apply to land uses involving excavation or the addition or removal of water. |
Data source: British Geological Survey
Data update schedule: Quarterly
Compressible deposits
The potential hazards presented by natural deposits that could compress when a load (such as a building) is placed on them or they become saturated with water.
| A - Negligible | Compressible strata are not thought to occur. |
| B - Very low | Compressibility and uneven settlement problems are not likely to be significant on the site for most land uses. |
| C - Low | Compressibility and uneven settlement potential may be present. Land use should consider specifically the compressibility and variability of the site. |
| D - Moderate | Compressibility and uneven settlement hazards are probably present. Land use should consider specifically the compressibility and variability of the site. |
| E - High | Highly compressible strata present. Significant constraint on land use depending on thickness. |
Data source: British Geological Survey
Data update schedule: Quarterly
Collapsible deposits
The potential hazards presented by natural deposits that could collapse when a load (such as a building) is placed on them or they become saturated with water.
| A - Negligible | Deposits with potential to collapse when loaded and saturated are believed not to be present. |
| B - Very low | Deposits with potential to collapse when loaded and saturated are unlikely to be present. |
| C - Low | Deposits with potential to collapse when loaded and saturated are possibly present in places. |
| D - Moderate | Deposits with potential to collapse when loaded and saturated are probably present in places. |
| E - High | Deposits with potential to collapse when loaded and saturated are present. |
Data source: British Geological Survey
Data update schedule: Quarterly
Landslides
The potential for landsliding (slope instability) to be a hazard assessed using 1:50,000 scale digital maps of superficial and bedrock deposits, combined with information from the BGS National Landslide Database and scientific and engineering reports.
| A - Negligible | Slope instability problems are not thought to occur but consideration to potential problems of adjacent areas impacting on the site should always be considered. |
| B - Very low | Slope instability problems are not likely to occur but consideration to potential problems of adjacent areas impacting on the site should always be considered. |
| C - Low | Slope instability problems may be present or anticipated. Site investigation should consider specifically the slope stability of the site. |
| D - Moderate | Slope instability problems are probably present or have occurred in the past. Land use should consider specifically the stability of the site. |
| E - High | Slope instability problems almost certainly present and may be active. Significant constraint on land use. |
Data source: British Geological Survey
Data update schedule: Quarterly
Ground dissolution of soluble rocks
The potential hazard presented by ground dissolution, which occurs when water passing through soluble rocks produces underground cavities and cave systems. These cavities reduce support to the ground above and can cause localised collapse of the overlying rocks and deposits.
| A - Negligible | Soluble rocks are present within the ground. Very few dissolution features are likely to be present. Potential for difficult ground conditions or localised subsidence are at a level where they need not be considered. |
| B - Very low | Soluble rocks are present within the ground. Few dissolution features are likely to be present. Potential for difficult ground conditions or localised subsidence are at a level where they need not be considered except in exceptional circumstances. |
| C - Low | Soluble rocks are present within the ground. Some dissolution features may be present. Potential for difficult ground conditions are at a level where they may be considered, localised subsidence need not be considered except in exceptional circumstances. |
| D - Moderate | Soluble rocks are present within the ground. Some dissolution features may be present. Potential for difficult ground conditions are at a level where they may be considered, localised subsidence need not be considered except in exceptional circumstances. |
| E - High | Soluble rocks are present within the ground. Numerous dissolution features may be present. Potential for difficult ground conditions should be investigated. Potential for localised subsidence is at a level where it should be considered. |
Data source: British Geological Survey
Data update schedule: Quarterly
Shrink-swell hazard
The Property Shrink-swell Hazard Score gives a property-specific indication of susceptibility to shrink-swell subsidence given underlying geological properties, the proximity of trees (using Bluesky National Tree Map), and the characteristics of local buildings. Clay-rich soil and rocks have the ability to shrink and swell with changes in moisture content. Ground moisture variations may be related to a number of factors, including weather variations, vegetation effects (particularly growth or removal of trees), and the activities of people. Such changes can affect building foundations, pipes or services. The dataset has 6 input factors which are given a rating (1-10) which contributes to the overall assessment.
Building characteristics are taken from Office for National Statistics Lower Super Output Area data and are generalised to give the most likely characteristics for the property. These characteristics cannot be relied upon if the property is a new build.
Data update schedule: Quarterly
Landslides
The BGS National Landslide Database holds nearly 18,000 records of landslides and is the definitive source of landslide information for Great Britain. It is drawn from BGS paper and digital maps, memoirs and sheet explanations, reports and articles as well as non-BGS reports, council records, media reports and inherited databases. 83% of the records in this database have been validated by the BGS.
It is a point dataset and does not reflect the total extent of the landslide. Where possible, each point is located at the highest point of the landslide backscarp feature. Where this is not known, the highest point of the mapped landslide is used if available, otherwise the point is assigned an approximate location.
Each landslide record has an accuracy level assigned. Over 86% of the records in the database have an accuracy of +/- 100m, and Groundsure will not display any records that do not meet this minimum level. The absence of data in this section does not confirm that a landslide has not occurred at this location.
Data update schedule: Quarterly
Natural cavities
Industry recognised national database of natural cavities. Sinkholes and caves are formed by the dissolution of soluble rock, such as chalk and limestone, gulls and fissures by cambering. Ground instability can result from the movement of loose material contained within these cavities, often triggered by water. This data is provided in point and polygon format. Where the ‘Data Type’ in the data table is classified as ‘polygon’, this indicates an area-based record, and that natural cavities are found somewhere within this area, but not necessarily beneath the property itself. The polygon areas can be large, so caution is advised in the interpretation of this data.
Data update schedule: Annually
Coastal erosion
Projections with intervention measures in place
The scenario with the highest likelihood of impacting the property, as projected within the National Coastal Erosion Risk Mapping (2018-2021) (NCERM) when modelled with active intervention measures in place (as planned within the current Shoreline Management Plan). Examples of active intervention measures could be holding the existing defence line such as maintaining current defences, managed realignment, allowing the shoreline to move to an agreed position - advancing the line, and new defences planned extending the land area out to sea.
NCERM shows potential erosion extents from the coastal baseline for three time periods (0 - 20 years, 20 - 50 years and 50 - 100 years), and to three degrees of likelihood (95%, 50% and 5%).
Data update schedule: Quarterly
Projections with no active intervention
This is the scenario with the highest likelihood of impacting the property, as projected within the National Coastal Erosion Risk Mapping (2018-2021) (NCERM) when modelled with no active intervention in place. No active intervention means that there are no current plans to build any defences.
NCERM shows potential erosion extents from the coastal baseline for three time periods (0 - 20 years, 20 - 50 years and 50 - 100 years), and to three degrees of likelihood (95%, 50% and 5%).
Data update schedule: Quarterly