What's wrong with leaning walls?
Brian Morton considers appropriate remedial treatment for free-standing and retaining walls that have become causes for concern.
In our office the most frequent request for assistance probably relates to freestanding or retaining walls following either a collapse or a concern about the lean or out-of-plumbness of the wall.

It is normal to find a garden wall or a churchyard wall which has a degree of lean brought about by the raised soil level in a churchyard, or by trees, either affecting the wall physically by the force from roots, or indirectly by their roots draining water from the soil beneath the wall.

When considering the construction of these ancient walls generally, it is apparent that they have an enormous amount of flexibility and can lean-and become out of line to a point where, by modern standards, the construction not only does not comply with regulations but is considered to be potentially dangerous nevertheless, these walls remain standing.

I regularly pass a wall which 1 inspected some 25 years ago because cars regularly parked beneath it and it had a lean of about 250 mm in its head of 1.8 m. At the time I inspected it, there was nothing unusual about the situation, there were no trees causing a problem but clearly over the years the garden had been built up and was tending to push the wall over. 1 formed the conclusion that there were no additional forces acting on the wall that had not been acting for some considerable period and the wall probably leaned during the early part of its life, it was in fact reasonably stable. 1 decided to do nothing and 25 years later it is still standing.

A wall is said to be stable if the thickness at any point is at least a third of its height. Associated with this, if one takes a cross-section through the wall, the centre of load from the leaning wall should fall within the middle third. These rules apply where the wall is simply freestanding and has no other restraint mechanism, but it has to be realised that the wall spans horizontally as well as vertically, and thus a long wall that has retained its integrity is more stable than a short length of wall that does not have continuity in its length.

I was recently urgently called to look at a wall retaining a churchyard in Suffolk where there was a fear that the wall might collapse onto parked cars beneath it. The wall was quite long and there was only one significant problem associated with it and this was the lean and outof-line of the wall, associated with the very large tree directly behind it.

There were diagonal cracks in the wall on either side of the tree, undoubtedly caused by root pressure. My simple recommendation was that the brickwork either side of the crack should be taken down over a length of about 500 mm and rebuilt incorporating stainless steel ladder reinforcement within every fourth course of the brick joint.

In my report, 1 suggested that the wall would perhaps have a life of 10 years without the pressure from the roots causing a problem, but in the longer term it would be sensible to cut out a section at the base of the wall and build a lintel in over the area of the tree roots to stop the pressure.

A more significant problem was where a collapse had occurred to a rubble stone wall of some height. The district council had called an engineer in to investigate the bulging of the wall. The digger driver had parked his vehicle at the top of the slope and when he started to excavate the hole directly behind the wall, the digger and the wall had slid down the hillside.

The proposals for rebuilding the wall involved a thick reinforced concrete wall designed to restrain the full weight of the earth behind it. The council's conservation officer was concerned that this was over-engineering the situation and asked me to look.

An initial view during a casual visit was that we would need to thicken the wall significantly but it could be built of stonework; perhaps we would also need some restraining buttresses.

The ground strata in the area were quite rocky. Standing back and looking at the situation we realised that the only way the wall could have been built was to excavate out an enormous hole and then build the wall, retaining the footways, or terraces, starting from the bottom and working up. When we considered how this was actually done, we realised that the ground must have considerable stability in its own right and, thus, we asked if the face of the earth behind the collapsed area could be opened up for us.

In trying to cut into the slope with a pick axe the workman found a vertical face and also revealed that the front face of the wall was bound with mortar for perhaps 200 mm but that behind that the stone was loosely laid. The wall itself was restraining only the overburden placed between the natural face of the excavation and the back of the wall.

Our proposal is simply to rebuild the wall over the collapsed length, placing stainless steel ladder reinforcement into stone courses to restrain the wall in its length and link it to the existing part and to do very little else. It is important to drain the base of the remaining length of the retaining wall to stop the earth behind the existing wall consolidating due to penetration of water.

Every wall needs to be looked at separately and there should be a detailed inspection to see that the integrity of the wall remains: that it is still working horizontally without significant cracks and there are no signs of recent movement or causes of a problem. If there are none of these indications, it is likely that the wall can be left even in its leaning and bulging stage.

Brian Morton is a consultant to the Morton Partnership