From the previous pages the reader will have realised the extent of underground spaces in Kent and Sussex. Considering the number of such sites, the actual instances of ground subsidence are fortunately small, but this is little consolation to the person whose field or garden collapses. It is a traumatic experience and there has been a tendency in the past to adopt the principle of ‘out of sight, out of mind’ and to quickly fill in the hole with rubble. This, however, is a short-sighted view since the subsidence can happen again in the future! The correct treatment of subsidence depends on the cause and there are four main reasons why it happens.
Throwing loosely compacted rubble or other rubbish into a hole can fill it up quickly but this is only short term. After a few years, items like wood, etc. can rot and the weight of infill causes loose rubble to become compacted. This causes settling of the infill and it often happens that a large void is formed, not necessarily continued to the surface. If there are side chambers or passages, the weight of infill causes the bottom part to be pushed sideways into the spaces and this again leaves a large void. Over the years, the void gradually works its way upwards as more infill falls into the space until just a thin ‘crust’ is left at the surface. This can suddenly collapse under its own weight or, even worse, under the weight of a person or vehicle passing over it.
Collapse of Steining
Old wells (and sometimes deneholes) were walled around at the top by bricks or flints called ‘steining’. Until solid chalk was reached, this was necessary to stop surrounding soil, etc. from collapsing into the shaft. Many such features have been covered over in the past without filling the shaft itself but, over the years, the bricks or flints can fail away leaving the steining in a weakened condition. This can be aggravated if the feature has been adapted as a cesspit or land drain, since the water failing down the sides erodes the mortar and bricks. Once the steining falls away, the surrounding soil collapses into the shaft and a large crater is formed which can undermine any covering, causing it to fall into the shaft.
Slippage of Shaft Blockage
Many deneholes were not infilled upon abandonment but were blocked a few feet down by throwing in branches and tree trunks. Flints and soil were then thrown in to fill the shaft to the surface level. This may seem a risky method to us but we should remember that deneholes were sunk along field boundaries or in small copses and the landowners knew exactly where they were. Over the years, however, land changes hands and the locations are forgotten. Nowadays, many of these field boundaries have been removed to make larger fields or housing sites and this is one of the most common causes of subsidence in Kent. The original blockage has rotted away over the years and the soil layer now forms a compacted plug at the top. During alternating periods of dry and wet weather, the plug contracts and expands which causes it to slip slowly down the shaft (this explains many of the hollows found in woods). After a particularly heavy rain, the plug may suddenly disintegrate completely and fall down the shaft, especially if it is assisted by the vibration of agricultural machinery or the increased weight of high density housing.
The chambers excavated for deneholes or chalk mines are quite high and the roof level is usually just below the top of the chalk strata. Unless particular care was taken in shaping the roof the chalk has a tendency to crack and, being unsupported, will fall into the chamber below. This process can be accelerated if the chamber is at a shallow depth and extra weight is placed at the surface, e.g. a house, road, etc. Once the chalk layer has disappeared, the overlying strata is exposed and also begins to fall into the chamber below. This is especially so with Thanet Sand since wet weather will quickly cause draining water to erode the sides of a void. The void works its way up to the surface and failing material cannot block the void since it rolls sideways into the chamber below. As soon as the void reaches the surface, it appears as a spectacular crater. A similar effect can happen in natural cave chambers in chalk when the chamber becomes so big that it is no longer stable.
The best treatment is obviously preventative and more care should be taken in checking ground cleared for housing or larger fields. Members of the Kent Underground Research Group can advise on the likely existence of such problems and a computerised database of all known underground sites in the South East is maintained. if it is too late, then it is vital to find the cause of the subsidence before treating it and the Group can also assist with this. They can carry out an underground survey which will indicate if there are other likely danger points. As a rule of thumb, it can be stated that loose rubble infill alone must never be used since they will not be a permanent solution.
If the subsidence has caused a large crater, then it is likely that the only treatment is a complete infill. This must be done in such away that ALL voids are filled and it is necessary to use a material that can flow into every crevice and then set hard. The ideal material is Pulverised Fuel Ash (PFA) which is the waste product from power stations. This is mixed into a slurry and poured into the hole, where it spreads to fill every void and sets as hard as concrete. If the underground system is extensive, such as a chalk mine, then an alternative solution may be required since the quantities of PFA would be extremely expensive. In such cases, the best solution is to strengthen weak points of the roof with rock bolts, steel mesh and sprayed on concrete. The mine passages themselves can be left open with permanent access to check on the condition of the roof.
If a subsidence reveals that a shaft is still reasonably intact, there are two alternative solutions to a complete infill if the bottom chambers are stable. All that is required is to replace any missing steining and remortar it at the top. A concrete cap can then be installed over the shaft, below ground level if necessary, making sure that it extends at least 3ft beyond the shaft edges all round. If an access lid is incorporated, this will allow future checks to be carried out for stability. Instead of a concrete cap, a hinged metal grid can also be used, which will make the feature safe but will allow people to look down and perhaps allow access for bats. It could become a feature of the garden! With such methods, it is not advisable to continue using the shaft as a drain but, if this is necessary, any pipe should direct water down the centre of the shaft rather than the sides.
If access is not required, then a cheap alternative is a mixture of the two methods. The shaft can be filled with rubble to within 15ft of the top and the steining repointed. The last 15ft is then filled with PFA which is also carried on for 1ft above the shaft top and 3ft to the sides. The PFA sets to form a plug which will then remain, even if the the rubble settles below. The plug can be further secured by inserting a number of horizontal bars into the shaft sides before filling with PFA.