CORE Subsidence · Module 1 of 6
Content
Catalyst Subsidence Services
- Catalyst offer the following services;
- Expert Triage - Policy coverage review, Initial damage analysis, Dedicated claims handler
- Investigations – Full Site Investigations, Trial/Borehole, Foundation drawings, Soil testing, Root testing, Drainage investigation, Pressure testing, Watermain testing
- Mitigation Works & Monitoring – Drainage repairs, Tree management works, crack & level monitoring
What is Subsidence
- Subsidence
- Vertical downward movement due to loss of support of the subsoil below the foundations
- Possible Causes:-
- Desiccation of clay soils by trees
- Leaking drains
- Natural or mining cavities
- Heave
- The expansion of the ground beneath the building caused by swelling of the subsoil
- Possible Causes:-
- Removal of trees
- Leaking drains
- Landslip
- The sudden movement of soil on a slope or gradual creep of a slope over a period of time
- Possible Causes:-
- Destabilisation by adjacent works
- Alteration of water courses
- Dumping of fill upslope
- Failure of retaining walls
Subsidence
The type of cracking gives the structural engineer information of what is happening and which way the building is moving.
What is not Subsidence?
Eaves Spread – a roof with untied rafters tends to spread outwards and push out the tops of the walls
Thermal Movement – mainly vertical cracks appear in long walls built without movement joints
Wall Tie Failure – corrosion of metal ties embedded in mortar bed joints causing cracking
Defective Design, Workmanship or Construction – inadequate strength of floors, roofs, masonry walls or foundations
Lack of maintenance – e.g. corroded metal beams, rotted timber beams
Instability of unrestrained walls – occurs when the connection between floor and walls is poor
Lintel Failure – cracking appears over window openings
Redistribution of loads through structural alteration
Direct Root Action – tree roots growing under the wall physically lift the wall
Heave
Is the expansion of the ground beneath part or all of the building. This is normally taken to mean an upward movement of the site caused by expansion or swelling of the subsoil, (or the opposite of subsidence).
The most common cause of heave is removal of trees and other vegetation, causing clay soil to re-wet and swell.
Possible Cause of Heave
Removal of vegetation
When a tree is removed in clay soils the soil will re-wet (rehydrate) and expand in volume. If the foundations were laid upon soil already desiccated and shrunken due to the influence of the tree, then the tree is felled, then the consequential expansion may lift the foundations and cause cracking to the building.
Drains
Occasionally a leaking drain will cause previously desiccated soil to swell and cause heave damage.
Land Slip
• Slope destabilisation caused by new adjacent works - undercutting.
• Alteration of water courses whether intentionally or not.
• Dumping of fill upslope causing a surcharge of the slope.
• Failure of a retaining wall allowing movement of retained earth.
Cross-references
- Public website: Cracks topic video.
- Reference memory: Subsidence Handbook 4th Ed digest — chapters 1 & 2.
Knowledge check
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