Subsidence
INTRODUCTION
Traditionally, Insurers would normally only consider providing subsidence cover (including heave and landslip) as the result of a special request. Nowadays such cover is normally given as part of the standard perils “package” or, more commonly for business within the SME sector, is covered within All Risks or Accidental Damage covers and, as such, forms part of standard survey activity.
In cases where subsidence is suspected, observations and enquiries on site are to be made discreetly to avoid initiating a potential claim. Where subsidence is a concern, this should be brought to the Underwriters attention in the body of the report leaving them to decide whether a Structural Engineers report is to be required or other course of action taken. For this reason, there are no risk improvements relating to subsidence in the standard wordings.
In circumstances where subsidence is excluded from the survey instructions, but an unfavourable risk is deemed to exist and/or where evidence of possible subsidence from the general condition of the premises is evident, mention of this should still be made in the report as an alert to Underwriters in the event that cover is subsequently requested. A comment along the lines of: “Underwriters should be aware that the building exhibits signs of possible subsidence. No investigation of this defect was undertaken, and no comment is made, however, should subsidence cover be requested at a future date, Underwriters should seek documentary evidence confirming that the issue has been fully and properly addressed.”
SUBSIDENCE
Subsidence can be defined as the vertical downward movement of a building foundation caused by the loss of support of the site beneath the foundations. Particular problems arise when the movement varies from one part of the building to another. This is usually associated with a change in the volume the subsoil (e.g. the shrinkage of clay) which occurs as a result of external factors. Possible causes of subsidence include:
The shrinkage of clay subsoil during long periods of dry weather which can be significantly aggravated by the presence of trees and shrubs.
Leaking or defective drains which soften the supporting subsoil or wash soil away from the foundations. Soils with high sand or gravel content are particularly vulnerable.
Mining activity – previous or current.
Construction on back-filled, made-up or unstable virgin ground which has been inadequately compacted and/or where foundations have been incorrectly designed (subsidence arising from the settlement or movement of made-up ground is often excluded from cover).
Foundation damage by tree roots or the rotting and subsequent cavity formation of roots below the foundations.
HEAVE
Heave is defined as the expansion of the ground beneath part of all of the building resulting in an upward movement of the foundations. The most common cause of heave is the removal of trees and other vegetation causing the clay subsoil to re-wet and swell. Leaking or defective drains in clay subsoils can have a similar effect.
LANDSLIP
Landslip is defined as the sudden movement of soil on a slope or the gradual creep of a slope over a period of time. Possible causes of landslip include:
The instability of sloping ground due to heavy rain, leaking services and poor subsoil drainage.
Slope destabilisation arising from new adjacent works (undercutting).
Alteration of water courses.
Failure of retaining walls.
IDENTIFYING SUBSIDENCE DAMAGE
The identification as to whether a building is suffering, or is likely to suffer, from subsidence requires the skills of a Chartered Surveyor or Structural Engineer and is likely to be beyond the expertise of most Consultants. The objective of this Technical Bulletin is to provide some general guidance, which may be of assistance to Consultants.
It is important to recognise that all buildings are subject to slight structural movement which can give rise to cracking in brickwork, block work, plaster and other rigid components. The majority of incidents are usually a consequence of moisture and temperature effects to the building fabric, or minor settlement, particularly to floor slabs, following initial construction.
Most subsidence damage will tend to be found in low rise (up to 4 storeys) buildings of traditional construction. Buildings constructed prior to 1950 are most at risk, as they frequently have comparatively shallow foundations. Subsidence rarely affects larger commercial and industrial buildings with correctly engineered structures and foundations, although it can be found in portal steel framed buildings, particularly those over 40 years old.
Key pointers to the recognition of potential subsidence damage include:
Vertical and diagonal cracking concentrated in specific areas and tapering in width between the top and bottom of the property, sometimes accompanied by the presence of glass tell tales embedded across cracks to monitor movement.
Cracks extending through the DPC down into the foundations.
External cracking reflected internally in the same area of the wall.
Rucking of wallpaper at the corner between wall and ceiling junctions.
Re-pointing of mortar joints diagonally or vertically up walls indicating past problems.
Distortion of openings, which are weak points in the structure, causing doors and windows to stick.
Rotation of buildings towards trees on shrinkable clay subsoils.
Cracks appearing after a prolonged period of dry weather.
Seasonal opening and closing of cracks.
Generally, it can be considered that hairline cracks which appear on an annual basis are not structurally significant, but cracks which increase in width gradually over a period of time should be investigated. When they become between 16-25mm, they are classified as severe under the BRE Digest 251 (classification of visible damage to walls).
Common Forms of Cracking in Walls
1. Expansion Cracks – walls are affected by temperature and moisture change. Materials can suffer from initial shrinkage and/or subsequent expansion and contraction, resulting in cracks. Often vertical but can sometimes follow the line of least resistance and can end up stepped. Expansion cracks are often visible above window and door openings where the opening itself relieves the crack. This type of cracking, unlike more serious cracks, is normally consistent in width.
2. Cracks above Openings – commonly as a result of the removal of windows or doors with inadequate propping (such as when installing replacement PVCU windows), inadequate overhang of bearings or insufficient or non-existent lintels.
3. Wall Tie Failure – cracks arising from the corrosion and subsequent expansion of metal wall ties. Commonly visible every sixth course horizontally in the mortar joints.
4. Subsidence – commonly raking cracks, tapering in width between the top and bottom of the wall. Can occur in the corners of a building or from the top to the bottom of the walls. May also be visible around door and window openings.
5. Heave – similar to subsidence cracks, but with the taper in reverse.
The above information is intended to put wall cracks into context. It has been included for general guidance and illustrative purposes only and is not to be read as a definitive statement on what is often an involved and variable subject.
TREES AND CLAY SUBSOILS
In the UK, it is uncommon for tree roots to cause direct physical damage to sound foundations and in rare circumstances is likely to only occur where a tree is planted extremely close to a building, or where dead tree roots decay, causing the formation of cavities below the foundations. Most reported damage is secondary in nature and occurs when trees are planted in soil types which shrink considerably on drying, mainly consisting of clay and some peaty, fen soils.
During prolonged periods of drought, trees in close proximity to buildings can dry out the soil beneath the foundations causing the soil to shrink, potentially leading to subsidence of the foundations and cracking in the building’s superstructure. As previously mentioned, buildings up to four storeys in height, constructed before the 1950’s are considered to be most at risk, as they frequently have comparatively shallow foundations. Whilst Building Regulations over the years have evolved requiring more substantial foundations, it should not be assumed that modern buildings erected on shrinkable clay are immune from subsidence damage.
Most trees in the UK have a significant radial root system, extending one to one-and-a-half times the height of the tree. With some species, this can be even greater.
There are no hard and fast rules appertaining to minimum safe distances between trees and buildings. This is a complex issue in which due regard should be taken of several factors, including the species and cultivar of the tree, its vitality in relation to moisture abstraction, the precise nature of the soil, foundation design and climate.
Reliable studies (ref 1) have been conducted in the UK of insurance subsidence claims data where tree roots were implicated, collated from information obtained in the field during arboricultural surveys and site investigations. The results of the maximum straight line root spread recorded for various species of trees in his research paper are summarised in the following Table:
Expert opinion emphasises that these dimensions are not to be interpreted as absolute safe planting distances, but should be viewed as general guidance to distances above which, damage is regarded as not reasonably foreseeable. However, tree biology dictates that the rooting patterns and strategies of trees are inherently variable and can depend of several factors which may not be taken into account in a simplistic ‘straight line’ approach. As a result, a more balanced view should be taken having regard to the individual circumstances, which in some cases can lead to recommended safe planting distances being reduced from the figures shown, sometimes by as much as 50%. Equally, distances in excess of those of the Table may on rare occasions have to be taken into consideration.
Where mature trees are encountered below the “safe” planting distances and are deemed to present a subsidence risk, it is essential that specialist advice is sought from an Arboricultural Association Registered Consultant regarding pollarding, as the sudden removal of trees in these circumstances may result in foundation heave.
MINING ACTIVITY
Where there is an absence of local knowledge, it is possible to obtain an indication as to whether a particular city, town or village requires a mining search (and therefore potentially at risk) by visiting the GOV.UK online gazetteer at http://coal.decc.gov.uk/en/coal/cms/services/reports/gazetteer/gazetteer.aspx.
In the event that a mining report is required, this can be obtained for a specific property at cost, but would not be part of RSS standard survey activity.
FURTHER REFERENCE
1. The relationship between trees, distance to buildings and subsidence events on shrinkable clay soil (Messrs. Mercer, Reeves and O’Callaghan), published in the Arboricultural Journal 2011, Vol. 33, Issue 4.
