Since the early 1970's, building insurance policies have included 'damage caused by subsidence, heave or landslip' or similar wording.
Subsidence, heave and landslip all involve foundation movement, but so does settlement. Typical insurance policy definitions are :-
The downward movement of the ground (upon which the building is founded) for reasons unconnected to the loading of the ground from the building
The downward movement of the ground upon which the building is constructed due to the inability of the ground to satisfactorily support the load of the building
The expansion of the ground (beneath a building) for reasons unconnected to the existence of the building
Downward movement of sloping ground due to its self weight or imposed loading exceeding the shearing capacity of the ground
So subsidence is caused by some factor or other affecting the soil under the foundations.
- Drying out and shrinkage of cohesive (clay) soils
- Softening of cohesive soils by leaking drains
- Erosion of sandy soils by leaking drains
- Collapse of mine workings or natural cavities
If a building is subsiding uniformly, and by moderate amounts, there may not be much of a concern. In fact it probably will not be noticed. The problem is that subsidence usually occurs to only one part of the building, or more severely to one part than the rest. It is this differential subsidence which damages buildings, mainly by cracking and pulling apart the structure.
Clay shrinks as is dries out, and the shrinkage of clay soils is the most common factor causing subsidence in the UK. It first hit the headlines after the 1976 drought. Claims increased during the dry summers of the mid-1980's, rocketed in the early 1990's, and continued to be a major problem during the early 2000's. This was certainly influenced by the dry summers (and dry winters for some years) but was also very much boosted by public awareness of subsidence and the consequent difficulties in selling and affected property.
Why does clay shrink?
Clay is made up of very small particles, shaped like plates and less than 0.002 mm in diameter. They are so small that the molecular forces and properties affect the way the clay feels and behaves. In particular water (moisture) is held within the molecular structure causing the plates, and the soil as a whole, to swell or shrink with increasing or decreasing moisture content. The amount of swelling or shrinkage depends on the particular minerals of which the clay is composed, some are more reactive than others.
To determine how much a clay may swell or shrink requires soil samples to be collected – usually by hand dug trial pits, hand augers, or drive-in sampler rigs. The samples are then tested in a soils laboratory to determine two empirical factors known as the Atterberg Limits - or more commonly known as the Plastic Limit and the Liquid Limit.
The Plastic Limit is the moisture content required to bind the particles together in a plastic or mouldable manner. It is determined by rolling a sample into 3 mm diameter threads, on a glass plate. The sample is continually moulded and rolled which causes it to dry out. When the threads begin to break up, the moisture content is determined and this is defined as the 'Plastic Limit'.
The Liquid Limit is the moisture content of a sample which is on the point of changing from a solid to a liquid state. It is determined by the Cone Penetrometer Method which involves measuring the depth of penetration of an 80 gram, 60o cone into a number of samples of varying moisture content. A graph of penetration against moisture content is drawn from the results, and the moisture content for 20 mm penetration is calculated. This is defined (by the British Standard) as the 'Liquid Limit'.
The Plastic Limit and Liquid the Limit values are percentages. The difference between the values is called the 'Plasticity Index'. It is a measure of how much water the clay can hold whilst in a 'plastic' state. It follows from this, that the bigger the Plasticity Index, the more water a clay can hold in this state, and the more it will have to swell in order to do so. Consequently, the amount by which a clay can swell (or shrink) is proportional to the plasticity index. This is borne out by tests and experience in the field. For everyday use, the concept of 'Shrinkage Potential' has been developed, with categories related to the plasticity index thus:-
Plasticity Index % Shrinkage Potential
0 to 20 low
20 to 40 medium
40 to 60 high
over 60 very high
During a normal summer, on an open site, clay may dry out to depths of about 1 – 1.5 m. Modern properties would have foundations to about this depth and problems to do not normally occur. Older properties may have shallower foundations and slight (seasonal) movements may occur, but not usually enough to cause serious damage.
Serious problems begin to occur if the foundations are not deep enough, if the summer is long and dry and/or if there are significant trees nearby. The roots of such trees may extend into the soils beneath the foundations and suck the moisture out, causing subsidence. With clay soils of high shrinkage potential and large trees nearby, the ground can be affected up to 6m or more deep.
In modern construction the foundation depths in clays are determined from tables in the NHBC standards. The factors affecting the depth are the shrinkage potential of the soil, the type and mature height of any nearby trees, their distance from the property, and the geographical location of the property - generally as you move west and north from the south east of England, the climate is wetter and the required foundation depths are less.
How does one go about inspecting a building for subsidence, or other structural movement, and assessing the cause? Start by asking the homeowner about the property :-
- How long have they lived there?
- When was it built? Has it been altered?
- Have there been any previous structural problems?
- Have neighbouring properties had any structural problems?
- When did the cracking (or whatever) first appear?
- Has it got any worse?
Sometimes it may be fairly obvious what's happening – eg. an extension has pulled away from the main building, it's the end of a dry summer, there is a big tree nearby, and you know there are clay soils. In such a case you can be 99% certain about what is happening.
However, it is not always so straight forward so you need to be methodical. Start by inspecting all the external elevations in turn and the surrounding areas :-
- Is there roof spread or other roof problems?
- Are horizontal lines level are true level?
- Are vertical lines are true and plumb?
- Do the faces of the elevations bow out or show any distortion?
- Are door and window openings distorted?
- Are there any cracks to the walls, or gaps between window frames and reveals?
- Is the ground level or sloping?
- In what condition is any paving or garden walls?
- Are there any trees or large bushes nearby?
- Are there any drains nearby?
- Are there any recent building works nearby?
- What type of soil is it likely to be?
- What is the condition of neighbouring properties?
Use a spirit level to check that bed joints, window cills, etc. are level, and that corners, door and window reveals, and the faces of walls are plumb. Construction tolerances may well be present in the structure, sometimes these can be quite severe so you need to be alert to this. A slight slope or lean on its own, in one part of the building may not be significant, but if there is an overall pattern, or if there are other indications of movement they may add up to a problem. The spirit level may show up historic movement which may be identified as such by the lack of cracking which would otherwise be visible if it was recent movement.
Inspect the elevations for cracking to the brickwork and gapping to door and window openings between the frames and the masonry. Note which way any cracks or gaps have opened. Note the width, uniformity, or taper of any cracks, this often indicates the direction of the overall movement.
Briefly look at adjacent properties and see whether there are any similar cracks or other structural movement to these. subsidence and other cracks
Then move to the internal inspection and check the following :-
- Are there any defects with the roof structure?
- Do the ceilings sag or are they cracked?
- Is there any cracking to the insides faces of the external walls?
- Is there any distortion to the door and window openings through the external walls?
- Are the inside faces of the external walls true and plumb?
- Is there any cracking to the internal partition walls?
- Is there any distortion to the internal door openings?
- Are chimney breasts true and plumb?
- Are the floors flat and level?
All the above information should be recorded as survey notes and kept for future reference. Consider scanning the notes and filing away on your computer.
Take photographs - these are excellent for referring to along with your survey notes when preparing the report. They may also be useful to refer to on a future visit if necessary to compare extent and widths of cracks etc.
Before leaving the property you should have a good idea as to the pattern of the movement and the cause. But it always pays to sit down in the office and review the information - sometimes other possible causes become apparent.
ASSESSING THE SEVERITY
How much movement is acceptable? When should one be worried? When is action required?
Insurers give no guidance on how much movement is required to be considered in the case of an insurance claim. The Building Research Establishment has published their views on how wide cracks should be before action is taken, although these are not widely accepted.
It really depends on the property, its size and age in particular. Significant sloping floors and leaning walls are generally acceptable in older properties, so long as the problems are not getting noticeably worse. In old, traditional timber-framed properties severe distortions are the norm. For the modern house however, a hairline crack is likely to be concern to the homeowner and anything more than a millimetre in width may well be of concern to a Chartered Surveyor (or Chartered Engineer) if caused by foundation movement.
Movement is best assessed according to the severity in relation to the use and appearance of the property thus:-
Structural stability - Has the movement affected the structural stability of the property?
Is it going to fall down? Generally it is unlikely that a wall leans so much that it will fall over, but it is important to check (eg.) that any floor joists built into the wall have not been pulled from their bearings which could lead to the floor collapsing.
Serviceability - Has the movement affected the use of the property?
Is the floor slope or distortion so bad that you might trip or loose your balance? Is the structural movement damaging services such as pipework? Are door and windows impossible to open and close?
Appearance - Is the movement noticeable to the untrained eye?
Can you see the cracks or do you have to go looking for them? Are floor slopes noticeable? Are distortions to door and window openings noticeable?
Progression - Is the movement going to get worse? You may need to carry out investigations and/or monitoring to answer this question.
INVESTIGATIONS and MONITORING
Subsidence can normally be diagnosed, and the likely cause can usually be determined, from a visual inspection. However, it is usually necessary to carry out investigations to prove the cause to insurers, to determine if the subsidence is going to progress, and to gather necessary information for the design of remedial works.
Initial investigations would usually comprise hand-dug trial pits to determine the depths of the foundations and the recovery of soil samples from the underlying ground to check the shrinkage potential. Soil samples can be recovered from shallow depths using hand augers, or deeper using drive-in sampler rigs.
If deep piled underpinning is anticipated it may be necessary to use a larger borehole rig capable of boring to depths of 5 – 15 metres or more.
If there are any drains in the area of subsidence it is usual to test these for watertightness and often to carry out a CCTV survey in them to identify any damage or root infiltration.
Monitoring is often required by insurers because they want to be certain that movement is progressive before they spend large sums on underpinning, or because they want to be certain that movement has ceased before they spend money on crack repairs and the like. Monitoring normally lasts for about 3 months in the case of drainage repairs having been carried out, up to a year or more when clay shrinkage is involved.
REPAIRS and REMEDIAL WORKS
With subsidence, the cause must be removed or the foundations must be underpinned.
Trees can often be pruned or removed, but a risk assessment of heave should be carried out first. Leaking drains must obviously be identified and repaired. If the cause is removed then movement should cease, and after a period of few months for the structure to settle down, repairs can be carried out to the damage.
If the cause cannot be removed, the foundations must be underpinned. This means taking the foundations down to a deeper depth, either below the zone of influence of the tree roots or down to better and firmer ground in the case of soft soils.
Traditional underpinning involves digging (usually by hand) under the existing foundations, in small sections at a time, and backfilling with concrete. Gradually the building load is transferred from the old, shallow foundations, to the new, deeper underpinning.
As underpinning depth increases it can be more economical to dig deep pads at the corners of the building and to construct reinforced concrete ground beams spanning between these to support the structural walls.
Health and safety issues are an increasing concern with regard to the deep excavations required of traditional underpinning and it is also becoming very difficult to get labour prepared to endure the cramped conditions required. Once traditional underpinning depths reach 2½ - 3 m then a piled underpinning solution is often more economic and is much safer.
There is also less waste soil to dispose of, which has environmental benefits.
Once the structure has been stabilised, cracks need to be structurally repaired, usually using resin bonding or bed joint reinforcement to restore the original structural strength. Simply filling and decorating over cracks is likely to result in re-cracking in the future due to seasonal thermal/moisture changes in the wall.
Distortions, slopes and leans may also require correction depending upon the severity. This may mean jacking the structure back to line and level, partial re-building, or in severe cases demolition and total re-build may be required.
Cracking to buildings may be caused by many factors. Without knowing the cause, there is always a fear that cracking may be caused by subsidence and will require a huge expense to put it right. This is why mortgagees and insurers are always wary about taking on a new property when there is cracking present. However, most cases turn out not to be subsidence and rarely is the cracking so serious as to require major repairs or remedial.
Other causes of cracking are :-
typically the pattern of cracking and distortions will be the same as for subsidence, but the parts which have moved down are likely to be more heavily loaded (such as the party walls in a terrace), or may have shallower foundations (such as the front bays and rear extensions to Victorian terraces). Occasionally settlement may occur in an apparently random location due to a localised area of poor ground.
typically vertical or stepped diagonal cracks which have opened in the horizontal direction and are of uniform width or are not tapering. Look for long runs of brickwork on external elevations. Vertical cracks often occur 100 mm from corners. Common internally in lightweight aerated blockwork, usually vertical as the blocks are weak and easily cracked.
Beam or lintel failure:
Typically stepped diagonal cracking in the wall above, extending from one or both ends of the beam or lintel in towards the middle. Sometimes bed joints above the beam or lintel open up.
Concrete floor slab attacked by sulphates expand and rise up in the middle of rooms. In extreme cases the floor slabs can push the external walls out causing a step at dpc level. Sulphates attack in mortar will expand the bed joints and can lift the eaves by up to 50 mm. Has been known to lift the roof off the inner leaf wall plate in severe cases.
Occurs when the feet of the rafters and/or the ceiling joists are not adequately tied to the wall plates. Identified by gapping between eaves soffit and wall if the rafters are slipping or pushing the wall plate, or by the wall leaning out at the top if the wall itself moves.
Wall tie failure or other embedded corroding steel :
Wall tie corrosion in the outer leaf causes every third bed joint to crack and open up. If the ties corrode in the cavity and break the outer leaf can bow out - inner leaf usually stays plumb. Occasionally other steel embedded in the brickwork can corrode and cause cracking.
Poor detailing / construction can lead to cracking and distortions simply because the building elements are not well tied together. Typical examples are the vertical cracks which occur where the first floor, side walls of bay windows join to the main front wall.
Usually a vehicle and usually a culprit is identified.
Long term deterioration:
Typically occurs to finishes rather than brickwork. Old plaster, including lathe and plaster ceilings deteriorate and crack with age. Old render cracks - damp enters the cracks and it gets worse. Timber beams and lintels sag with age causing cracking to walls above.
Mike Royall can be contacted at email@example.com
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