Focus on Foundations
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Cracks and crevices in the walls of old houses are as natural as the patina on antique furniture, and usually signify nothing more sinister than age and experience.
They are normally caused by temperature changes, moisture differences, external vibrations, or the natural movement, decay or friability of traditional building materials. In certain cases a surveyor or structural engineer may diagnose foundation problems, assess their cause and suggest remedies. Practical measures such as the repair of a leaking drain or attending to nearby trees can often restore the status quo, because either of these can affect the subsoil’s moisture content, and thus its ability to support your home’s foundations. In extreme cases underpinning may be your only option, but, although an invasion of spade-wielding builders is inevitable, reputable companies plan such action carefully, and a supervising structural engineer or surveyor co-ordinates operations.
‘Subsidence’ is the downward movement of a building foundation caused by loss of support to the site beneath the foundation. This change is the result of outside influences or mechanisms which alter the ground’s ability to support the load of any building placed upon it.
‘Heave’ is just the opposite, occurring when the ground presses upwards against house foundations, which subsequently move.
‘Settlement’ is the normal downward compressive movement of a building into the ground as a result of its own weight. While this is more likely to be a problem with new buildings, it can occur if major structural alterations or additions are made to a property, when it tilts foundations sideways. This is termed ‘differential settlement’, because only part of the building is affected. Settlement can often be intermittent, cyclical or slow or have stabilised, so that it is only when some external changes occur that fresh instability occurs. The Leaning Tower of Pisa’s characteristic angle is attributable to the settlement of one side sinking into silty sand. Until Mussolini inadvisably tinkered with its foundations, the building was perfectly stable.
Before the invention of concrete, bricks were laid straight onto the soil and these courses of underground bricks were, and still are, referred to as ‘footings’. More recently, concrete footings became the norm, these load-bearing strips being wider than the walls.
Structural engineer Robert Afia explains, “Many old houses have foundations that are less than a metre deep, in contrast to modern building regulation requirements which insist on greater depth. Old London houses in particular have relatively shallow foundations. As trees in the vicinity grow taller, at some point their roots may draw water from the soil below the foundations, thereby causing shrinkage and collapse.”
A usual first investigative step is to excavate to determine the depth and condition of the foundations, as well as the condition and type of soil beneath. Very old buildings may have timber ground cills beneath their walls, and these may have decayed.
“Stand underneath a length of plastic guttering on a hot summer’s day, and you are likely to hear harsh cracking sounds,” says Roy Ilott, ??? of the Institute of Chartered Surveyors (RICS). “Whilst we acknowledge we are hearing the plastic shrink and move with heat, not many people realise that brickwork behaves in exactly the same way. Walls expand and contract because of thermal variations, resulting in cracks that are perfectly natural and of no consequence whatsoever. Some cracks might signify minor subsidence that has settled down. In many cases just because a wall has moved it doesn’t matter. But I would say that cracks of over 25mm are potentially serious.”
Subsidence cracks are usually diagonal and tapering, with the widest edge at the top - referred to as ‘sagging’ mode. They start, and are widest, at a free edge, such as corner of a house or top or end of a wall, or the edge of door or window, and they travel around openings. External cracks are likely to be mirrored internally, even though internal wall finishes are likely to have had cracks filled. The phenomenon can never crack a wall on only one side: even cavity walls are affected on both skins, even though there may be a difference of position. Cracks around doorways and windows also suggestive. Heave cracks are wider at the bottom than the top, (hogging mode) - opposite to subsidence cracks.
Once you have identified cracks, take note of any dip in the horizontal coursing of stone or brick, indicating a drop. Checking the level of floors with a spirit level can also give a good indication. Other dangerous signs are sloping lintels (the concealed supports above windows and doors), bulges in solid walls, the separation of double thickness walls, and walls which are out of true.
Monitoring of cracks, over periods of between six months to two years, is the first likely option whenever subsidence is suspected. There may have been subsidence in the past but its cause has since been rectified, leading to a new soil condition that has stabilised, meaning that here is no further risk to the property. Alternatively, monitoring is wise after remedial action has been taken to find out if it has worked. Roy Ilott: “You can draw a line, 30 - 40mm long across the split, marking off exact dimensions, and using calipers to measure any changes in its length. Or you can just fill the crack and wait and see if it appears again.” Other methods are the use of a special ‘crack measurement gauge’ - which is designed to measure the crack itself, or a ‘calibrated tell-tale’, comprising two pieces of clear plastic fixed either side of the crack, one of which has cross hairs through which you can view calibrations on the other. Seasonal cracking is an encouraging discovery, denoting marginal natural movements according to the wetness of the soil, and posing no structural risk.
WHAT YOU SHOULD DO
Immediately reporting any subsidence suspicions to your insurance company may not always be your wisest course of action, even though they will pay for any subsequent investigations. On the other hand if you are actually aware that your home may have subsidence it is a condition of many insurance policies that companies should be immediately informed. Roy Ilott: “It can be tricky, because your insurance company will keep records of any investigations of suspected problems, even if it’s subsequently proved groundless or a case of past subsidence that poses no future risk. These records can come to light if you sell the house, and the insurers may not want to give continuity of cover to the new owners - a very valuable asset for anyone selling their home. If you want a diagnosis for peace of mind, you can keep all options open by paying around £200 for an independent report from a surveyor or structural engineer, and he can either set your mind at rest and then monitor the crack over time, or present his report to your insurance company if action is required. In the former case, if a future purchaser’s surveyor sees cracks, you can afford to be totally upfront with him, and present documented records of the cracks’ satisfactory behaviour over time.”
For ancient buildings, the conservation officer of your local authority or the Society for the Protection of Ancient Buildings (SPAB) can give names of experts familiar with such property. Alternatively, the RICS have a helpline to match their members with clients, and the Institution of Structural Engineers (IStructE) offer their list of countrywide consultants.
Such is the specialised nature of this work, that properly qualified people who are authorised to design underpinning systems are principally engineers and scientists, not work-hungry builders. Professional ethics, plus their symbiotic relationship with major insurance companies, should ideally ensure that only necessary work is authorised. Underpinning older buildings can introduce unwelcome rigidity to an otherwise flexible structure: a fact that will be considered carefully by an engineer who is familiar with older types of construction.
Reputable companies carry their own professional indemnity insurance to guard against major disasters during a project’s duration. Larger companies, such as Roger Bullivant, may be authorised by the insurance companies to do their own surveys. John Patch (of Roger Bullivant), ex chairman of the Association of Specialist Underpinning Contractors, says reassuringly, “The bulk of our work is on domestic property, so all our operatives are used to dealing with clients in a diplomatic way - they understand how upsetting it can be. Speaking generally, I would say that it is vitally important that the householder checks that a professional person administers the actual project.”
Ivor Roberts, managing director of ‘Advanced Underpinning’, says, “There are many situations where underpinning can be done externally, meaning we don’t have to disturb the internal decorations, or cause major disruptance to the household. If internal work is required, homeowners usually move out for a period ranging from two weeks to 6 months. Unfortunately the nature of the process is such that it is necessary to temporarily remove some of the building’s remaining support in order to replace it with something more substantial. In the interim this can result in even more cracks and further damage to internal decorations. Most homes require complete redecoration afterwards. “
‘Mass concrete underpinning’, or ‘traditional underpinning’ involves digging by hand below the present foundations and pumping in concrete, as a way of adding effective depth to the entire length of the existing foundations. Holes are dug and filled in separate blocks or bays, thereby leaving areas of support throughout the operation. ‘Hit and miss’ underpinning, a cheaper option, is where only certain bays are excavated and concrete-filled.
‘Beam and Pier’ underpinning involves the incorporation of a horizontal concrete beam below the walls, either above, or in place of, the existing footings. An area of masonry is removed from the base of the wall, which is temporarily propped, then the ‘beam’ is cast into this void. Large areas beneath the beam are excavated vertically in order to cast large supportive piers at strategic intervals beneath them.
‘Piles’ - either standard size or the newer ‘mini piling’ systems, are basically circular holes drilled into the earth and then filled with a cement-based mortar. Sometimes steel or plastic cases are first inserted, to act as a sleeve for the mix. There are various different ways of installing piles, either straight through the existing foundations or floor, or on concrete supports set at right angles to the base of the wall.
‘Beam and pile’ underpinning works on a similar principle to beam and pier, but the beams are cast with extensions built out at right angles, so that piles can be drilled and filled with concrete beneath these outcrops either side of the wall, meaning that areas directly beneath the wall’s footings do not need to be excavated.
‘Pressure grouting’ is a method of stabilising the ground beneath a building. Liquid mortar is injected at pressure straight into the ground, so that any voids between rocks, hardcore or granular material are filled. This method is unsuitable for soils such as clays, where voids are unlikely. ‘Partial underpinning’ is a cost-saving measure for reinforcing only certain areas that are judged to need special support.
Graham Abrey, conservation expert and director of Ingram Consultancy, advises that you should always repair subsidence cracks in pre-Edwardian buildings with flexible materials that are sympathetic to the lime mortars that would have been used originally. Cement mortars are generally to be avoided, because they would create hard disruptive spots within the structure. Defects are, typically, cracking of mortar joints, displacement of window and door heads and arches, cracking of plaster finishes and possibly fracturing of brickwork, stone or terracotta.
“Resins can be used where additional strength is required for a repair,” Graham says, “These are available in varying thicknesses, so as to provide a way of controlling their degree of absorption into a fracture. Hydraulic lime grouts can be too liquid, running through the cracks and spreading to unintended locations, or else too thick or gritty, impeding their penetrability. Always find a craftsman who is familiar with the use of whatever materials your engineer or surveyor specifies.”