What Is Subsidence?
Movement and resultant cracking in any structure during its lifetime is inevitable, however most of the movement is not foundation related. The term ‘subsidence’ is regularly interchanged with the term ‘settlement’ which are two very different phenomenon. This is addressed in ‘Subsidence v Settlement’.
Foundations in buildings can move up (heave) or down (subsidence). This is closely related to the reaction of the underlying soil to water ingress or its position in the ‘Modified Plasticity Index’ (I’p).
The infamous ‘London Clays’ have a very high volume change potential and its water content may fluctuate from between 15% in dry summers to up to 40% during wet winters.
However, most of Ireland is overlain with well graded glacial tills and ‘loamy’ soils, which are regarded as having a very low volume change potential and are not vulnerable to such changes.
So, if you hear or see the word ‘heave’ in relation to foundation movement, it’s probably time to run a mile as someone has been reading the wrong website or doing too much ‘cut and paste’.
Most subsidence literature is published in the UK. It focuses on shrinkable clay soils and trees which represent about 70% of subsidence claims. The Guardian and BBC News reported that the dry spell in June 2018 (the lowest on record since 1925) has resulted in a 20% increase in insurance claims in the UK, the highest since the last heatwaves in 2006 and 2003.
It is difficult to quantify the cost of subsidence claims in Ireland as the majority were covered under the insured peril of ‘escape of water’ which also represents damage to the building and contents from burst pipes. This is addressed in ‘What causes Subsidence’.
The approach to subsidence in shrinkable clay soils, like those in the UK, is totally different to the that taken to subsidence in non-shrinkable soils, like those in Ireland. One of the main reasons behind this is how the soil can recover from an event – volumetric change due to moisture variations in shrinkable clay soils is a physical change and can be carefully addressed by management of trees or repair of leaking drains before monitoring.
However, the loss of fines in granular soils is a chemical change in the soil and such loss of bearing capacity is non-reversible without the intervention of foundation remedial works such as micro-piling or ground improvement. BRE (Building Research Establishment) briefly states how it is not possible to adequately predict how granular soils will perform over a long period after such an event.
Subsidence v Settlement
A foundation is the part of a structure that connects it to the ground. Its purpose is to transfer the loads from the structure into the ground safely whilst also limiting transfer of subsoil movement back to the building. This principle is essential in understanding the difference between ‘subsidence’ and ‘settlement’.
‘Subsidence’ and ‘settlement’ are terms used interchangeably when discussing superstructure cracking. However, by definition, they are very different, particularly from an insurance aspect.
Settlement is defined as ‘the downward movement of the site on which the building stands due to the application of superimposed loadings from the buildings’ whereas ‘subsidence’ is defined as ‘the downward movement of a site which the building stands from causes unconnected with loading from the buildings’.
Settlement occurs for a period after the building is constructed and will inevitably result in cracks on walls. This settlement period is a factor of the soil type where granular soils (such as sands and gravels) tend to consolidate relatively quickly under load by rearrangement of the particles and expulsion of water whereby cohesive soils (silts and clays) tend to compress over many years due to their low permeability. It occurs due to the weight of the building acting on the soils and is normally nothing to worry about. However, if a building is inadequately designed or constructed, settlement cracks may be excessive and require further investigation or intervention works. Appropriate ground investigation and competent supervision during pouring is essential to reducing the risk of excessive settlement in any new building. See ‘Ground Investigation’ for further details.
Subsidence, on the other hand, does not occur purely due to the weight of the building, it is more related to the supporting soils. For subsidence to develop, particularly from an insurance point of view, an ‘event’ or ‘change in conditions’ must occur to affect the supporting soils ability to sustain the loads from the building. This will normally occur in older buildings due to many different factors. See ‘What causes subsidence?’ for further details.
What Are The Main Signs Of Subsidence?
Diagnosing subsidence through a simple present condition survey may sometimes prove difficult without further investigation. The following outlines the main signs that subsidence is occurring.
Cracking in walls is the most common sign of foundation movement and may form within or between components. However, most superstructure cracking does not relate to subsidence. See ‘What are not signs of subsidence?’ Subsidence cracking tend to be concentrated where the maximum structural distortion and structural weak points coincide. They occur in some of the following patterns and should be evident on either side of the wall:
- Diagonal cracks around door and windows
These cracks will be wider at the top.
- Horizontal cracking which taper toward corners
These cracks will increase in width, most likely towards a corner of the building
- Vertical cracks at the interface of a building and extension
These cracks will be wider at the top as the extension rotates away from the original structure. The corner of the extension will normally act as the pivot point.
Doors and Windows Sticking
As a foundation moves, the building will become distorted, particularly around openings in walls.
This can lead to doors and windows sticking as the frame will lose its squareness. In some instances, glass can crack!!!!
Floors off level
In most cases, the floor and the walls of older houses are constructed independently – the walls are supported on the foundation and the floor slab is supported by the ground. Many cases of floors off level is not related to foundation movement and is usually due to inappropriate or poorly compacted fill. The fill material will begin to ‘consolidate’ from the first day of placement and it may be quite some time before the floors begin to move. Homebond recommend that if the depth of fill beneath a ground supported floor slab exceeds 900 mm, the floor must be suspended on the external walls. However, if the upstairs floors are sloping considerably, then further investigation will be required.
In most cases, cracks in ceilings are not due to subsidence and are normally due to under sizing of joists.
When buildings constructed on traditional strip foundations move, the building can become distorted. However, in raft foundations, which are much stiffer, the movement may be very different. Raft foundation movement is often as a result of poor understanding of the principles of load transfer. Although a raft helps spread the load, the loadings are still concentrated on the external and internal load bearing walls. A raft, if designed and constructed properly, will bridge over soft spots.
Tilt occurs when the building moves as one unit, limiting obvious distortion. The Building Research Establishment provide indicative values for tilting of low-rise building ranging from 1:400 (maximum acceptable) to 1:50 (ultimate limit) where demolition may have to be considered.
Is movement progressive?
Despite the width of any crack, a key aspect of any subsidence investigation is to determine if movement is ongoing or not. Although a lot of information can be gained from a simple present condition survey, in some instances it may not be clear if the movement is historic (has finished) or progressive (has not finished).
Fresh cracks have a very clean appearance compared to older cracks which would have accumulated dirt over time. There may be evidence of previously repaired or filled cracks that have grown in width and length. Some of the following questions may help to determine if movement is ongoing:
When was the crack first noticed?
Have any cracks been repaired and subsequently opened?
When were repairs completed?
How were the repairs completed?
In areas of repair, has any cracks not been repaired?
When was the wall last painted?
Subsidence monitoring can help answer this question in a more quantitative manner but takes time. This may involve the installation of crack monitors where distortion has resulted in cracking or perhaps level monitoring in situations where tilt has occurred.
What are not signs of subsidence?
Cracking in walls is normally the first sign of foundation movement; however, most superstructure cracking does not relate to subsidence. Diagnosing subsidence through a simple present condition survey may sometimes prove difficult without further investigation. Other cracking can be caused by the following:
Thermal and Moisture variations
Thermal and moisture movement cracking will be found in almost all older buildings and generally form perpendicular to the stresses that cause them. These cracks form within individual buildings, particularly where two different materials meet or between individual buildings where no movement joints are installed. These do not tend to cause serious damage yet may be difficult to hide due to the normal cyclical opening and closing during variations in temperature. It is important to note that movement due to thermal expansion of blockwork will always be greater than that produced by the movement due to the subsequent contraction.
Roof spread normally occurs when alteration works are completed which result in a loss of ‘triangulation’. The outward force pushes the walls out just beneath the wall plate, resulting in a horizonal crack with lateral displacement or a ‘lip’. Cracking can also occur at the junction of the external walls and internal partitions. The normal approach to remediation is the installation of suitable collars.
Lintel Deflection / Failure
Lintels are structural members located over doors and window which transfer the load to either side of the opening. They are constructed of reinforced precast concrete or prefabricated steel in newer properties whilst in older properties, can be constructed of stone, timber or steel. More modern dwellings may have a combined lintel which performs the function of the cavity tray and lintel itself. Some older properties had a cast in situ ring beam at first and second floor level. Sometimes, these would sag over time, particularly in longer spans, but generally would not be a concern.
Older lintels are more likely to fail due to moisture. For example, unprotected mild steel will eventually corrode, and the resulting iron oxide will expand to four times the thickness of the parent material. This forces the walls upwards.
Vibration, due to heavy traffic or other human activity, may also result in cracking. Granular soils are more prone to consolidation than cohesive soils. Generally, crack repair and decoration is the normal approach to remediation.
What causes subsidence or cracking?
Subsidence or foundation movement, particularly in older buildings, is primarily caused by two types of event or change in circumstances – escape of water and variations in groundwater levels.
Escape of water, through leaking drains or leaking water supply pipes, is the most common cause of foundation movement. Services which are leaking over long periods gradually reduce the load bearing capacity of the soils by removal of the finer particles in granular or cohesionless soils or having a ‘softening’ effect in fine or cohesive soils.
Old vitreous clay drains, installed up to the 1970s, are most vulnerable to leaking. These were jointed with a dry sand and cement mortar between spigot and socket. This rigid construction did not allow the flexibility that modern day Upvc drains offer and so the clay drain deteriorated over time. This inevitably resulted in escape of water. Although the introduction of a polypropylene sleeve coupling with sealing ring was introduced to allow movement, the clay drain became obsolete and was replaced by the modern day Upvc. Although Upvc drains do not deteriorate like clay, they are regularly found to be leaking also as a result of the poor installation methods. See Drain Testing |& CCTV Drain Surveys for more information.
The second cause of subsidence relates to periods of intense rainfall. Ground water or water table levels vary naturally due to seasonal variations and do not affect the foundations of a building. However, in exceptional weather conditions, they may rise to abnormal levels close to the existing foundations before subsiding (pardon the pun!!!) taking with them the finer particles. This event has a much more sudden effect on a building where the loss of bearing capacity to the eroded soil occurs immediately relative to the gradual effect associates with leaking drains.
A good example of this event was the winter of 2015/2016 in Cork City which experienced rainfall levels of over 600 mm, nearly five times its average rainfall. 28% of the 2015 annual rainfall occurred during December while 21% of the 2016 annual rainfall occurred during January.
Substruck estimates that about 75 cases of subsidence around the Cork area was attributed to this event.
What are the effects of subsidence on a building?
Subsidence can affect a building in many ways and to varying degrees. The Building Research Establishment provide three broad categories of damage: ‘aesthetic’, ‘serviceability’ and ‘stability’.
The aesthetic performance of any building will naturally be compromised by cracking. In the case of tilt, noticeability will occur at about 1:250. It has been documented that the publics tolerance to the aesthetic consequences of foundation movement has reduced considerably over time.
Subsidence can have varying effects on the ability of a building to performs its primary functions in terms of stability and serviceability. The loss of stability must be considered in the most extreme of circumstances, although this is in fact quite rare. Subsidence will usually affect the serviceability of the building in many ways including the following:
- Doors and windows jamming,
- Sloping floors,
- Glass panels cracked,
- Rain penetration,
- Heat loss,
- Air infiltration
- Reduced sound insulation.
After careful examination of the evidence above ground, any investigation into alleged subsidence will require information from below ground.
Drain Testing & CCTV Surveying / Inspection
Substruck recommends that a drain test and CCTV survey should generally always be completed as part of any subsidence investigation for two reasons. Firstly, it may establish that leaking drains have caused the movement, or perhaps acted as a contributory factor. And secondly, it will also identify the drainage system layout which will still have to be considered during design and construction of the repair solution. See Services – Drain Testing and CCTV Surveying
Water supply Test
Substruck recommends that a water supply test should always be completed with the drainage investigation. This may establish that a leaking water supply has caused the movement, or perhaps acted as a contributory factor. Secondly, it may also provide the line of the water supply which may also need to be considered during design and construction of the repair solution. Furthermore, the water supply test may identify the construction of the supply such as lead. See Services – Water Supply Testing
Trial holes involve the opening up of the ground around the house at different locations to examine the foundation and underlying soils. Trial holes are opened at strategic locations to aid in determining the cause of the movement.
Ground investigation in subsidence issues normally involve dynamic probing and soil sampling. Dynamic probing and soil sampling will determine the resistance of underlying soils and soil type respectively at depth.
Although a lot of information can be gained from a simple present condition survey, in some instances it may not be clear if the movement is historic (has finished) or progressive (has not finished).
Subsidence monitoring can help answer this question but takes time. Unfortunately, homeowners who are trying to sell their property may not have this luxury. In some instances, as insurance claims take quite a period of time, it may be prudent to commence monitoring as soon as possible. Anyhow, the degree of movement that is related to subsidence can also be subjective.
Crack width monitors or tell tales
Crack width monitors or ‘tell tales’ are the most common form of subsidence monitoring in Ireland. The ‘Standard’ monitor consists of two plates which are independently fixed to either side of the crack whilst overlapping for part of their length. The bottom plate is calibrated in millimetres and the top plate is transparent and marked with a red hairline cursor; the plates are fixed so that the cursor is in line with the centre of the scale.
Readings can be taken over a defined period to determine if the crack is still opening (or closing!!!). Vertical movements can also be taken.
The ‘Plus’ series monitor has three extra features. The first is the addition of pre-set pegs which allows for quick and easy fixing and reading whilst spigots attached to each plate can be measured with a vernier callipers within 0.1 mm. It is also manufactured with a polycarbonate rather than the acrylic making it stronger and more impact resistant.
Crack monitors are not without their disadvantages – tiny vertical movements can be difficult to measure, particularly if time is a concern. Furthermore, they can be somewhat unsightly and may be prone to vandalism depending on their location.
Level Monitoring is rarely used in Ireland, particularly in domestic houses and is usually completed by specialist surveying companies.
Although it is generally more expensive, it has many advantages over tell tales as it can tell when, where and by how the building is moving. It can measure vertical movements to ± 0.5mm which may not have any actual effect on crack width.
Special steel discs can be fixed to walls and the distance measured by inserting a calipers into the small cavity on each disc. This system is less obtrusive to tell tales but will only tell the degree of movement as opposed to the direction from tell tales.
Subsidence Repair Approach
The optimum approach to remedying subsidence can only be determined through a systematic examination of many different factors. In the UK, the Building Research Establishment have completed significant research into mitigation of further movement in shrinkable clay soils through tree removal or pruning, root pruning and drain replacement. However, in Ireland, we must take a different approach due to the nature of our soils which are more granular in nature. See What is Subsidence?
In simple terms, the most appropriate solution will be determined by risk. This risk is subject to several different factors which need to be considered.
The basis of any subsidence investigation is to determine the cause of movement and the potential for reoccurrence. If the cause is an escape of water, then the drains (or water supply) will have to be replaced to prevent further erosion. Sometimes, monitoring is then proposed. However, the Building Research Establishment states that the loss of bearing capacity in granular soils due to escape of water from drains is impossible to predict and may take years until the effects are noticed!!!!!
On the other hand, if the cause is that of rising groundwater levels, then there is no control of reoccurrence and again, any loss of bearing capacity to granular soils may continue for years unless foundation repair works are completed. And of course, there is a risk of reoccurrence.
Scale of damage
The scale of the damage must also be assessed. This can be considered from several aspects such as the width of cracking or the area of which the building is affected. There may be a localised problem or the whole of the building may be affected. Partial ‘underpinning’ must be carefully considered and will mostly always require a transition zone to limit differential settlement between zones of which have been treated and zones which have not.
Nature of the building
The depth, profile and construction of the foundation has a significant influence over the remedial design. For example, if foundations are overly deep, then needle beams may have to project through the rising walls to provide a more economical solution. Also, if the foundations are overly shallow, the remedial solution must always pay respect to existing services such as drains.
The pile loadings and centres will be dependent on the weight of the building and therefore will need to be much closer for a bungalow compared to a two-storey dwelling. However, then, the ability of the existing foundation to span these loads must also be considered.
Ground conditions will always play a part in deciding the best approach. For example, grouting or ground improvement cannot be completed in clay soils due to the particle size.
Subsidence Repair Types
Pile and Beam
Pile and beam construction is generally applied in high risk buildings where external damage is both significant and progressive. Unlike ground improvement, the system can be easily proven by design and/or testing. Micropiles, unlike screw piles or displacement piles, can provide enough loadings in any ground conditions due to its ability to penetrate boulders and bedrock. In many instances, our design may be purely based on the length of rock socket.
This system uses micropiles to support short needle beams installed transversely through the rising walls or beneath the foundation. The two most common configurations are straddle beams and cantilever beams. Straddle beams are formed on two micropiles supporting a needle on either side of the wall or at adjacent corners with both micropiles under compression. Cantilever beams are formed by a pair of micropiles support a needle entering the wall from one side, usually externally; the micropile closest to the wall is under compression whilst that furthest is under tension.
As micropiles are passive installations, it will always take time for the micropile to begin to mobilise (or work). Therefore, a settlement period should be allowed before any superstructure repair works are completed. In very rare cases, the specification may require a more active solution due to time constraints or client requirements. In this instance, we can post tension cantilever beams to allow superstructure repair works to commence immediately.
A piled raft is generally applied in high risk buildings where internal and external damage is both significant and progressive. Unlike ground improvement or raking micropiles, the system can be easily proven by design and/or testing. The primary advantage of this system over the traditional pile and beam is that the piled raft will naturally cover the whole footprint of the building thereby providing support for the floor also. Therefore, where appropriate, a pile raft can be considered the best solution to foundation movement.
It is constructed of a reinforced concrete slab, cast in one pour, supported on micropiles, which carries the load from the external walls via needle beams. These needle beams are short reinforced concrete beams that project from the raft into the rising walls. The raft will also support the internal walls via smaller needles. The normal requirement for weldmesh is two layers of A393. The centres of the needles will depend on the pile loadings, raft design and arching ability of the walls.
This system can only be applied if enough rising walls are present to allow for the raft (c. 300 mm) and insulated concrete screed (150 mm) to be installed. Otherwise, traditional pile and beam may be the only viable option.
Raking micropiles through foundations can be applied in low to medium risk buildings where internal and external damage is significant and progressive. The difference between raking and traditional pile and beam is of course the piles are raking but secondly, the absence of ground beams allows for direct load transfer from the foundation to the pile.
This system may be more cost effective than traditional pile and beam but is not without its limitations. Unlike pile and beam construction, raking micropiles can only be proven by design as it is very difficult to test in situ micropiles for axial loadings. In some instances, such as raft foundation failure, micropiles can be installed vertically through the raft.
The micropiles should be installed through traditional strip foundations so may not be appropriate in old random rubble walls or foundations of little or no projection. It is also important to account for the shear forces during load transfer from the micropile to the foundation. Therefore, the surface area of the borehole though foundation must be as large as possible with borehole diameters of 90 mm as an absolute minimum. This may also limit the system to lightly loaded structures if increasing the centres will render the system non-viable.
Furthermore, as micropiles have a very low bending moment, such piles must be installed alternating on either side of the wall to negate such lateral and torsional forces. This system is most appropriate during a renovation of a single storey building where the internal floors are being replaced in any instance.
Ground improvement or pressure grouting is applied in low risk buildings in areas of loose permeable soils. Unlike pile and beam construction, this process is near impossible to prove by design and/or testing and should only be applied in low risk properties.
The process involves drilling boreholes through foundations and into the subsoil along the area in distress. This is usually completed by hand held rock drills or micro piling rigs to depths of about 2 metres, depending on the ground investigation. Then, a neat cementitious grout is pumped or filled into the subsoils to fill voids, increase strength and reduce compressibility of the underlying soils to stabilise the structure. The grout is very mobile and whilst following the path of least resistance, travels through the voids providing both compaction and densification, therefore increasing the bearing capacity of the soil. The borehole is finally reinforced with a reinforcement bar along its length.
Pressure grouting may in fact be described as an art based on natural and scientific laws but it requires experience and engineering judgement. Rigid rules for the exercise of this art cannot be established, and only general procedures and guidelines can be applied on the ground. Of course, nobody can tell where the grout is travelling beneath the ground but we address this by controlling grout pressures, grout volumes, flow rates and consistent site monitoring.
Unlike the UK, traditional underpinning is rarely used in Ireland to address subsidence problems. It is more often applied where a foundation needs extending downwards such as basement construction in older buildings. However, it can still be applied in buildings of any risk where suitable bearing soils can be found within a metre. Otherwise, the cost and risk of deep excavations may render the process non-viable, so ground improvement or pile and beam construction may be a more appropriate solution.
The Building Research Establishment defines underpinning as ‘the extension of the existing foundation to reach stiffer or more stable ground’. The process involves systematically excavating beneath the foundation at alternating locations until a good load bearing stratum is obtained, usually less than 2-3 metres. Concrete is then poured to 50 mm below the existing foundation which is dry-packed the following day to ensure a tight joint between the underpin and the original foundation. In some cases, it can be flooded up, i.e. poured above the level of the existing formation level.
The most recent traditional underpinning project that Substruck completed was in 2018 where a shallow random rubble foundation needed to be extended downwards both to mitigate the risk of further movement and accommodate a new insulated concrete floor. The key difference in this project to normal underpinning works was that a 300 mm projection on the new underpin would not accommodate the new insulated concrete floor and services, both internally and externally. To overcome this, rising walls were installed between the new underpin and formation level. The final pinning up was completed by dry packing with a semi-dry non shrink grout rammed into place from both sides.
Foundations are the most important structural element of any building. They transmit the loads from the walls, floors and roof into the ground. However, foundations can also transfer any ground movement back to the structure. Here are a few frequently asked questions.
Am I insured?
Most household policies will protect the homeowner against ‘subsidence’ in some manner that may not always be apparent. The claim may be considered under the perils of ‘escape of water’, ‘subsidence’ or even flooding, each of which involve different excesses. If you think you are not covered, it is always worth a review of your policy by an experienced professional. Contact Substruck
How long does an insurance claim take?
Now this is the million-dollar question and unfortunately, there is no set time frame. This claim is one of the most complicated involving a series of many different steps completed by many different professionals. But generally, from first discovery of damage, works should commence within one or two years.
How much does subsidence cost?
In order to provide a costing for subsidence works, we will require information such as a drain test report, trial hole and ground investigation report and specification of works. If you do not have this information, we will need to visit the site. You may not even have subsidence!
I hope to sell my home soon
If you are considering selling your property and think you have subsidence, we recommend that a brief survey is completed of your property by Substruck. If your property does have subsidence problems, you may find you’ll have great difficulty in selling it. Subsidence problems will devalue your home and many mortgage lenders won’t be happy to finance a property that has subsidence damage, until all subsidence issues are resolved.
But my house is only one year old?
Don’t panic. Cracks are not uncommon in newly built houses or extensions. They are likely to be the result of the building settling under its own weight or shrinkage within or between components, one of which are anything to worry about. If your building is several years old and you think that cracking is still active, please contact this office to arrange a site visit.
Is my home in danger?
Damage caused by subsidence will rarely cause a building to become unstable over a short period of time. Subsidence will normally affect the serviceability of a home such as doors and windows jamming, reduction of airtightness and insulation capability. See What are the effects of subsidence on a building??
Will I need to move out of my home?
This depends on the nature and extent of the works required to address the problem or indeed your own personal circumstances, but it is quite rare that you will need to move out.
What is a loss adjuster?
A loss adjuster is an independent professional appointed by the insurance firm to negotiate the settlement within the constraints of your policy schedule.
What is a loss assessor?
A loss assessor is an independent insurance professional appointed by the policy holder to represent the policy holder in the claim.