They say that the east side of Britain is gradually sinking and the west side rising, but that is a very gradual process affecting a wide area that has been going on for thousands of years before anyone ever thought of building the city of Norwich. However, in recent times, hardly a month goes by without some report of local buildings having to be evacuated because of cracking, or a hole opening up in the road outside someone's front window.
So what's going on? Is Norwich particularly prone to subsidence problems? Should property owners be worried? What can we do about it?
In another article, we have looked at the various soil layers beneath Norwich. Unlike the craggier parts of the UK, we don't have much in the way of solid rock beneath us, or at least not within about 250 metres of the ground surface. What we do have is a considerable depth of firm-ish chalk covered by a highly variable thickness of sand, with some soft peat and clay down near the river and patches of firmer clay in some higher parts of the city.
Given this lack of rock to build on, there's actually nothing unique about the ground conditions in Norwich. Most cities where there is intensive building and other types of reliance upon the ground experience a certain amount of subsidence. The nature and frequency of the subsidence events in Norwich are readily explicable by the set of conditions we have. We just seem to make the most of them!
One thing about chalk is that it can sometimes get holes in it, either from natural processes of solution or from man-made tunnelling. And the natural sand we have in the area, usually in a layer above the chalk, is quite silty, which has always made it prone to being washed from A to B if subject to water flows, either suddenly during floods or gradually during years of leaking drains. So if we have holes or cracks at depth in the chalk, the sand can end up being washed down into these, leaving a space at a higher level. In fact, this was happening long before Man came on the scene, as we can sometimes observe natural 'pipes' of sand in otherwise solid chalk - for example, these 'solution features' were unexpectedly found to occupy more than 10 per cent of the bottom of the deep excavation for the Castle Mall development.
We noted that the ground can be quite soft (not unexpectedly) in the marshy areas near to the river. That's one reason why our forebears often tipped soil and other debris onto these marshes to reclaim the land, raise the level and allow useful development. The only trouble is, the tipped stuff often sunk gradually into the mire, and was itself difficult to compact, so subsequent buildings have sometimes ended up sinking, or starting to crack when extra floors were added or other alterations made. Even in the higher parts of the city, there can be a thick layer of infilled or raised ground that can squash down or decay under the foundations of buildings if they don't go deep enough. That probably happened to the Duke of Norfolk's palace at St Andrew's before it was demolished in 1711.
As with any busy and ancient city, there are all sorts of other possible localised causes of subsidence, from slopes that are too steep (like at Norwich City's former ground at The Nest, Rosary Road, where a big wall fell down in the 1990s), patches of shrinkable clay affected by tree growth (a bigger problem in London) and occasionally traffic vibration (rarely a primary factor).
Much as insurers and lawyers like to have a nice simple 'A caused B' mechanism (so they know who to blame), subsidence is rarely that simple. In the real world, it often results from a 'critical combination of circumstances' (CCC) in which it is difficult to isolate a single cause. Usually it's induced by a combination of natural and artificial factors.
For example, the famous bus-in-hole incident at Earlham Road could be regarded as a CCC of the underlying proximity of chalk mines, a natural solution pipe and a busy highway with water services running beneath it. No one factor was to blame.
Having said that, in the majority of cases of ground movement, it is water flow that is involved and implicated. Water has always fallen on the ground as rain and soaked in, so it is difficult to treat it as a fundamental cause of problems. However, concentrated flow of water do tend to be the trigger mechanism that disturbs the status quo in many cases, leading to the ground movement that is manifested as a cracked wall or a hole at the ground surface. In general, the more sudden the water flow, the greater the severity of subsidence, especially when added to a situation that is already unstable.
These days, nobody talks about altogether avoiding risk in any field. Instead it's 'risk management', recognising that there are rational things we can all do to greatly reduce (but not altogether eliminate) the possibility of a problem.
We saw above that water is a very common factor in the CCC underlying subsidence problems in Norwich. It therefore stands to reason that we can do a lot to reduce the risks by minimising the chances of water percolating into the ground, especially in the vicinity of foundations and roadways. That means regularly checking the condition of gutters, downpipes and drains, and not letting surface water accumulate at any one spot. Any structural cracks should be regularly monitored to see whether there is ongoing movement.
Obviously there are plenty of other precautionary things, to do with siting of new buildings, proper ground investigation and the design of foundations to suit the known ground conditions. But for existing property owners in Norwich, keeping water out of the equation is likely to be one of the most effective measures to avoid that sinking feeling.
© Matthew Williams
PICTURE 1: Subsidence problem in Norwich 2010 © Matthew Williams
PICTURE 2: Diagrammatic cross section through Norwich city centre © Matthew Williams
PICTURE 4: Bus in hole, Earlham Road 1988 © nosher.net
PICTURE 5: Cracked wall 2007 © Matthew Williams