Repairs: walls

Most foundation failures in older buildings are probably caused by either drains failure or subsequent changes in soil volume water demand resulting from tree and vegetation planting since the building was constructed.

Underpinning is considered by some to be a black art requiring a very specialist design. It may not be necessary, providing you look carefully for the clues as to any likely cause.

However, if the building is in an area that is subject to issues such as mining subsidence, or if there are 'water solution features' – that is to say, big holes caused by washing out due to prevailing limestone and underground water courses or similar – then specialist solutions beyond the scope of this section may well apply.

If you decide to proceed, consider carefully whether there is any likelihood of the building being locked into place in one section with foundations free to float elsewhere. This can be a problem on shrinkable clay soils where further stress cracking can occur once part of the building is set on a rigid foundation, when other previously unnoticed movements suddenly become apparent as the building 'floats' up and down.

Insurance companies have historically been extremely keen to minimise cost exposure by only underpinning partial elements of the building, but the client needs to be aware that this can cause further distress to the structure.

There has been an increasing trend recently for engineers to recommend underpinning of the whole building to avoid this, but circumstances vary in individual cases: it will sometimes depend on whether a loss adjuster is involved with a particular insurance company policy. If there is no insurance company involved, you will have to form a judgement.

Solution

It is paramount to establish the cause of the problem. The first stage is to dig a trial pit and locate the extent of the problem. If drains are the cause, drain survey information will be required, and any leaking pipework will need to be repaired as part of the underpinning work.

The trial pit should establish the depth of the original foundation and should excavate down to sufficient depth to establish a reasonable load-bearing area in the soil. This is a relatively basic operation – when you can’t dig any further with ease and the spade is bouncing off the ground, you have probably reached a suitable depth (typically 1–2m below ground level).

The underpinning bays generally should not exceed 1m in width (beyond this the wall probably cannot support itself).

If the wall is particularly old with lime mortar in poor condition and the brick bond is not likely to adhere to itself, some additional support will be required. Use of screw jacks is the most straightforward option.

Note: It can be extremely difficult to dig sufficient trenching and remove the screw jacks from the holes. In practice it is easier to leave the screw jacks in situ and pour the concrete around them. Although a slight additional expense, the increased speed of working often offsets this.

The section of wall to be underpinned needs to be divided into equal bays so that each bay can be dug, filled and pinned up under the wall, leaving sufficient room for the concrete to cure and another bay to be constructed elsewhere. In this way, no section of the wall is unduly weakened at any one time.

It may be more economical to cast in 'temporary' screw jack supports when underpinning:

• it is quicker;
• it offers assured support to the soffit of the old footing (safety); and
• the soffit of any footing is forced up so any packing out between the top of the new concrete underpin bay and the underside of old footing can be securely rammed in.

Once the concrete has been poured, the remaining gap of about 50–75mm beneath the soffit of the original foundation should be pinned up by hand using a dry mix, or sometimes a self-expanding mix rammed in.

In areas where shrinkable clay soils are a problem, remember to use some form of slip plane such as a folded sheet of damp-proof membrane on the outside of the wall to reduce the friction caused by any soil heave that might subsequently occur once underpinning bays are back filled.

Also consider using stainless steel helical bars bedded into mortar joints to effectively create a reinforced beam along the base of the wall or footing, and assist with spreading load across any localised soft spots, coupled with an underpinning.

The same technique can be applied when carrying out extension works or modifying existing building layouts:

• It is generally appropriate to excavate and reinforce by underpinning the reveals of any wall openings constructed by breaking through a building – either removing a load-bearing internal wall or when creating openings into extended sections of the building.
• As the wall is broken open, increased loading or point loading will occur around the newly formed reveals. Even if the original foundations are broadly capable of supporting this, some redistribution of loads and distress cracking is likely to occur unless the footings are given some more substantial reinforcement.

The benefits of self-expanding cement In most cases, the author has used a dry pack self-expanding cement, which will fill any voids as it matures and absorbs any natural ground moisture to start the curing process. Although named 'dry pack' it is in fact rammed in by hand, slightly damp. The process is straightforward, providing you consider the steps carefully. More details for simple domestic low-rise structure underpinning are available in the BRE Guide, Foundation movement and remedial underpinning in low rise buildings. Any reasonably competent building surveyor should be capable of designing simple underpinning solutions to typical 2-storey domestic structures or their equivalent.