CRACKS TO WALLS NORMALLY CAUSED BY FOUNDATION DEFECTS
Buildings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause of movement in buildings can usually be related to one or more problems in the foundation soil. It is important for the homeowner or the homeowner’s builder to identify the oil type in order to ascertain the measures that should be put in place in order to ensure that the problem in the foundation soil can be prevented, thus protecting against building movement.
The types of soils usually present under the topsoil in land zoned for residential buildings can be split into two approximate groups – granular and clay. Quite often, foundation soil is a mixture of both types. The general problems associated with soils having granular content are usually caused by erosion. Clay soils are subject to saturation and swell/shrink problems.
Classifications for a given area can generally be obtained by application to the local authority. But these are sometimes unreliable and if there is doubt, a geotechnical engineers report should be commissioned. As most buildings suffering movement problems are found on clay soils, there is an emphasis on the classification of soils according to the amount of swell and shrinkage they experience with variations of water content.
CAUSES OF MOVEMENT
There are two types of settlement that occur as a result of construction:
· Immediate settlement occurs when a building is first placed on its foundation soil, as a result of compaction of the soil under the weight of the structure. The cohesive quality of clay soil mitigates against this, but granular (particularly sandy) soil is susceptible.
· Consolidation settlement is a feature of clay soil and may take place because of the expulsion of moisture from the soil or because of the soil’s lack of resistance to local compressive or shear stresses. This will usually take place during the first few months after construction but has been known to take many years in exceptional cases.
The problems are the province of the builder and should be taken into consideration as part of the preparation of the site for construction.
All soils are prone to erosion, but sandy soil in particularly susceptible to being washed away. Even clay with a sand component of say 10% or more can suffer from erosion.
This is particularly a problem in clay soils. Saturation creates a bog-like suspension of the soil that causes it to lose virtually all its bearing capacity. To a lesser degree, sand is affected by saturation because saturated sand may undergo a reduction in volume – particularly imported sand fill for bedding and blinding layers. However, this usually occurs as immediate settlement and should normally be the province of the builder.
SEASONAL SWELLING AND SHRINKAGE OF SOIL
All clays react to the presence of water by slowly absorbing it, making the soil increase in volume. The degree of increase varies considerably between different clays, as does the degree of decrease during the subsequent drying out caused by fair-weather periods. Because of the low absorption and expulsion rate, this phenomenon will not usually be noticeable unless there are prolonged rainy or dry periods, usually of weeks or months, depending on the land and soil characteristics.
The swelling of soil creates an upward force on the footings of the building and shrinkage creates subsidence that takes away the support needed by the footing to retain equilibrium.
This phenomenon occurs when the foundation soil does not have enough strength to support the weight of the footing. There are two major post-construction causes;
· Significant load increase
· Reduction of lateral support of the soil under the footing due to erosion or excavation.
· In clay soil, shear failure can be caused by saturation of the soil adjacent to or under the footing.
GENERAL CONDITIONS OF SITE CLASSES
A – Most sand and rock sites with little or no ground movement from moisture changes.
S – Slightly reactive clay silt sites with only slight ground movement from moisture changes.
M – Moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes.
H – Highly reactive clay sites, which can experience high ground movement from moisture changes.
E – Extremely reactive sites that can experience extreme ground movement from moisture changes.
A to P – Filled sites
P – Sites which include soft soils such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.
TREE ROOT GROWTH
Trees and shrubs that can grow in the vicinity of footings can cause foundation soil movement in two ways:
· Roots that grow under footings may increase in cross-sectional size, exerting upward pressure on footings.
· Roots in the vicinity of footings will absorb much of the moisture in the foundation soil, causing shrinkage or subsidence.
UNEVENESS OF MOVEMENT
The types of ground movement described above usually occur unevenly throughout the building’s foundation soil. Settlement due to construction tends to be uneven because of;
· Different compaction of foundation soil prior to construction.
· Different moisture content of foundation soil prior to construction.
Movement due to non-construction causes is usually more uneven still. Erosion can undermine a footing that traverses the flow or can create the conditions for shear failure by eroding soil adjacent to a footing that runs in the same direction as the flow.
Saturation of clay foundation soil may occur where subfloor walls create a dam that makes water pond. It can also occur wherever there is a source of water near footings in clay soils. This leads to a severe reduction in the strength of the soil which may create local shear failure.
Seasonal swelling and shrinkage of clay soil affect the perimeter of the building first, then gradually spreads to the interior. The swelling process will usually begin at the uphill extreme of the building, reaches the interior soil as absorption continues. Shrinkage usually begins where the sun’s heat is greatest.
EFFECTS OF UNEVEN SOIL MOVEMENT ON STRUCTURES
Erosion and saturation
Erosion removes the support from under footings, tending to create subsidence of the part of the structure under which it occurs. Brickwork walls will resist the stress created by this removal of support by bridging the gap or cantilevering until the bricks or the mortar bedding fail. Older masonry has little resistance. Evidence of failure varies according to circumstances and sy7mptoms may include:
· Step cracking in the mortar beds in the body of the wall or above/below openings such as doors or windows.
· Vertical cracking in the bricks (usually but not necessarily in line with the vertical beds or perpends).
· Isolated piers affected by erosion or saturation of foundations will eventually lose contact with the bearers they support and may tilt or fall over. The floors that have lost this support will become bouncy, sometimes rattling ornaments, etc.
Seasonal swelling/shrinkage in clay
Swelling foundation soil due to rainy periods first lifts the most exposed extremities of the footing system then the remainder of the perimeter footings while gradually permeating inside the building footprint to lift internal footings. This swelling first tends to create a dish effect, because the external footings are pushed higher than the internal ones.
The first noticeable symptom may be that the floor appears slightly dished. This is often accompanied by some doors binding on the floor or the door head, together with some cracking of cornice mitres. In buildings with timber flooring supported by bearers and joists, the floor can be bouncy. Externally there may be visible dishing of the hip or ridgelines.
As the moisture absorption process completes its journey to the innermost areas of the building, the internal footings will rise. If the spread of moisture is roughly even, it may be that the symptoms will temporarily disappear, but it is more likely that swelling will be uneven, creating a difference rather than a disappearance in symptoms. In buildings with timber flooring supported by bearers and joists, the isolated piers will rise more easily than the strip footings or piers under walls, creating noticeable doming of flooring.
As the weather pattern changes and the soil begins to dry out, the external footings will be first affected, beginning with the locations where the sun’s effect is strongest. This has the effect of lowering the external footings. The doming is accentuated, and cracking reduces or disappears where it occurred because of dishing but other cracks open. The rooflines may become convex.
Doming and dishing are also affected by the weather in other ways. In areas where warm, wet summers and cooler dry winters prevail, water migration tends to be towards the interior and doming will be accentuated, whereas where summers are dry and winters are cold and wet, migration tends to be towards the exterior and the underlying propensity is towards dishing.
Movement caused by tree roots
In general, growing roots will exert upward pressure on footings, whereas soil subject to drying because of tree or shrub roots will tend to remove support from under footings by inducing shrinkage.
Complications caused by the structure itself
Most forces that the soil causes to be exerted on structures are vertical – i.e. either up or down. However, because these forces are seldom spread evenly around the footings, and because the building resists uneven movement because of its rigidity, forces are exerted from one part of the building to another. The net result of all these forces is usually rotational. This resultant force often complicates the diagnosis because the visible symptoms do not simply reflect the original cause. A common symptom is binding of doors on the vertical member of the frame.
Effects on full masonry structures
Brickwork will resist cracking where it can. It will attempt to span areas that lose support because of subsided foundations or raised points. It is therefore usual to see cracking at weak points, such as openings for windows and doors.
In the event of construction settlement, cracking will usually remain unchanged after the process of settlement has eased.
With local or shear or erosion, cracking will usually continue to develop until the original cause has been remedied, or until the subsidence has completely neutralized the affected portion of footing and the structure has stabilized or other footings that remain affected.
In the case of shrink/swell effects, the brickwork in some cases return to its original position after completion of a cycle, however, it is more likely that the rotational effect will not be exactly reversed, and it is also usual that brickwork will settle in its new position and will resist the forces trying to return it to its original position. This means that in a case where swelling takes place after construction cracking occurs, the cracking is likely to at least partly to remain after the shrinking segment of the cycle is complete. Thus, each time the cycle is repeated, the likelihood is that the cracking will become wider until the sections of brickwork become virtually independent.
With repeated cycles, once the cracking is established if there is no other complication, it is normal for the incidence of cracking to stabilize, as the building has the articulation it needs to cope with the problem. This is by no means always the case, however and monitoring of cracks in walls and floors should always be treated seriously.
The upheaval caused by the growth of tree roots under footings is not simple vertical shear stress. There is a tendency for the root to also exert lateral forces that attempt to separate sections of brickwork after initial cracking has occurred.
The normal structural arrangement is that the inner leaf of brickwork in the external walls and at least some of the internal walls (depending on the roof type) comprise the load-bearing structure on which any upper floors, ceilings and the roof are supported. In these cases, it is internally visible cracking that should be the main focus of attention, however there are a few examples of dwellings whose external leaf of masonry plays some supporting role, so this should be checked if there is any doubt, In any case, externally visible cracking is important as a guide to stresses on the structure generally, and it should also be remembered that the external walls must be capable of supporting themselves.
Effects on framed structures
Timber or steel framed buildings are less likely to exhibit cracking due to swell/shrink than masonry buildings because of their flexibility. Also, the doming/dishing effects tend to be lower because of the lower weight of walls. The main risks of framed buildings are encountered because of the isolated pier footings used under walls. Where erosion or saturation causes a footing to fall away, this can double the span that a wall can bridge. The additional stress can create cracking in wall linings, particularly where there is a weak point in the
structure caused by a door or window openings. It is, however, unlikely that the framed structure will be so stressed as to suffer serious damage without first exhibiting some or all the above symptoms for a considerable period. The same warning period should apply in the case of upheaval. It should be noted, however, that where framed buildings are supported by strip footings there is only one leaf of brickwork and therefore the externally visible walls are the supporting structure for the building. In this case, the subfloor masonry walls can be expected to behave as full brickwork walls.
Effects on brick veneer structures
Because the load-bearing structure of brick veneer building s the frame that makes up the interior leaf of the external walls plus perhaps the internal walls, depending on the type of roof, the building can be expected to behave as a framed structure, except that the external masonry will behave in a similar way to the external leaf of a full masonry structure.
Water service and drainage
Where a water service pipe, a sewer or stormwater drainage pipe is in the vicinity of a building, a water leak can cause erosion, swelling or saturation of susceptible soil. Even a minuscule leak can be enough to saturate a clay foundation. A leaking tap near a building can have the same effect. In addition, trenches containing pipes can become watercourses even though backfilled, particularly where broken rubble is used as fill. Water that runs along these trenches can be responsible for serious erosion. Interstate seepage into subfloor rea and saturation.
Pipe leakage and trench water flow also encourage tree and shrub roots to the source of water, complicating and exacerbating the problem.
Poor roof plumbing can result in large volumes of rainwater being concentrated in a small area of soil:
· Incorrect falls in roof guttering may result in overflows, as may gutters blocked with leaves etc.
· Corroded gutters or downpipes can spill water to the ground.
· Downpipes not positively connected to a proper stormwater collection system will direct a concentration of water to the soil that is directly adjacent to footings, sometimes causing large-scale problems such as erosion, saturation, and migration of water under the building.
Seriousness of cracking
In general, most cracking found in masonry walls is a cosmetic nuisance only and can be kept in repair or even ignored.
AS 2870 also publishes figures relating to cracking in concrete floors, however, because wall cracking will usually reach the critical point significantly earlier than cracking in slabs – a reference to this should be made in this Australian Standard.
Where building movement is caused by water service, roof plumbing, sewer or stormwater failure, the remedy is to repair the problem. It is prudent, however, to consider also rerouting pipes away from the building where possible and relocating taps o positions where any leakage will not direct water to the building vicinity.
Even where gully traps are resent, there is sometimes enough spill to create erosion or saturation, particularly in modern installations using smaller diameter PVC fixtures. Indeed, some gully traps are not situated directly under the taps that are installed to charge them, with the result that water from the tap may enter the backfilled trench that houses the sewer piping.
If the trench has been poorly backfilled, the water will either pond or flow along the bottom of the trench. As these trenches usually run alongside the footings and can be at a similar depth, it is not hard to see how any water that is thus directed into a trench can easily affect the foundations’ ability to support footings or even gain entry to the subfloor area.
In all soils, there is a capacity for water to travel on the surface and below it. Surface water flows can be established by inspection during and after heavy or prolonged rain. If necessary, a grated drain system connected to the stormwater collection system is usually an easy solution.
It is, however, sometimes necessary when attempting to prevent water migration that testing is carried out to establish a water table height and subsoil water flows.
Protection of the building perimeter
It is essential to remember that the soil that effects footings extend well beyond the actual building line. Watering of gardens plants, shrubs and trees causes some of the most serious water problems.
For this reason, particularly where problems exist or are likely to occur, it is recommended that an apron of paving be installed around as much of the building perimeter as necessary. This paving should extend outwards a minimum of 900 mm (more in highly reactive soils) and should have a minimum fall away from the building of 1:60. The finished paving should be no less than 100 mm below the brick vent bases and 70-75 mm below the building weep holes.
It is prudent to relocate drainage pipes away from this paving, if possible, to avoid complications from future leakage. If this is not practical, earthenware pipes should be replaced with PVC and backfilling should be of the same soil type as the surrounding soil and compacted to the same density.
Except in areas where freezing of water is an issue, it is wise to remove taps in the building area and relocate them well away from the building.
It may be desirable to install a grated drain at the outside edge of the paving on the uphill side of the building. If the subsoil drainage is needed this can be installed under the surface drain.
In buildings with a subfloor void such as where bearers and joists support flooring, insufficient ventilation creates ideal conditions for condensation, particularly where there is little clearance between the floor and the ground. Condensation adds to the moisture already present in the subfloor and scientifically slows the process of drying out. Installation of an adequate subfloor ventilation system, either natural or mechanical is desirable.
WARNING Although the above deals with cracking in buildings, it should be said that subfloor moisture can result in the development of other problems notably:
· Water that is transmitted into masonry, metal or timber building elements causes damage and/or decay to those elements.
· High subfloor humidity and moisture content create an ideal environment for various pests, including termites and spiders.
· Where high moisture levels are transmitted to the flooring and walls, an increase in the dust mite count can ensue within the living areas. Dust mites, as well as dampness in general can be a health hazard to inhabitants, particularly those who are abnormally susceptible to respiratory ailments.
The ideal vegetation layout is to have lawn or plants that require only light watering immediately adjacent to the drainage or paving edge, then more demanding plants, shrubs and trees spread out in order.
Overwatering due to the misuse of automatic watering systems is a common cause of saturation and water migration under footings. If it is necessary to use these systems, it is important to remove garden beds to a completely safe distance from the building.
Where a tree is causing a problem of soil drying or there is the existence or threat of upheaval of footings, if the offending roots are subsidiary and their removal will not scientifically damage the tree, they should be severed and a concrete or metal barrier placed vertically in the soil to prevent future root growth in the direction of the building. If it is not possible to remove the relevant roots without damage to the tree, an application to remove the tree should be made to the local authority. A prudent plan is to transplant likely offenders before they become a problem.
Excavation around footings must be properly engineered soil supporting footings can only be safely excavated at an angle that allows the soil under the footings to remain stable., This angle is called the angle of repose (or friction)_ and varies significantly between soil types and conditions. Removal of soil within the angle of repose will cause subsidence.
Where erosion has occurred that has washed away soil adjacent to footings, the soil of the same classification should be introduced and compacted to the same density. Where footings have been undermined, augmentation or other specialist work may be required. Remediation of footings and foundations is generally the realm of a specialist consultant.,
Where isolated footings rise and fall of swell/shrink affect the homeowner may be tempted to alleviate floor bounce by filling the gap that has appeared between the bearer and the pier with blocking. The danger here is that when the next swell segment of the cycle occurs, the extra blocking will push the floor up into an accentuated dome and may also cause local shear failure in the soil.