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The sandstone, taken ‘en masse’, is a competent rock. Despite the fact that the upper unweathered horizons can be readily excavated without the requirement for rock breakers, bearing pressures within the formation can range between 400kPa and 2000kPa¹. This competency can be attributed to the stratum’s apparent lack of fractures. While relatively closely-spaced joint sets can be viewed on the escarpment west of the Castle, fractures are generally widely spaced, and are not as common throughout the formation as might be anticipated.

Under the microscope, however, we see a completely different picture, the formation’s upper sections exhibiting a notable lack of cement. Under the electron microscope, the predominant mineral components (quartz grains) are not ‘bound’ together. Any cement within the formation consists mainly of calcite, which through the percolation of groundwater over the last 250 million years has been leached out. This is not surprising, given that the formation is a sub-division of the largest major aquifer in the UK (namely, the Sherwood Sandstone Group, Lower and Middle Triassic). Groundwater is therefore a common feature. The calcite that has been dissolved out tends to have been transported downwards and redeposited closer to the base of the formation.

Softness, combined with the lack of fractures has, particularly from the 16th through 19th centuries, provided ideal conditions for cave excavation, and numerous examples of troglodyte dwellings still exist in Nottingham City Centre – many still in day-to-day use (see map). These are often located at very shallow depths below rock-head. The only physical controls over the location of these caves are the presence or absence of groundwater, and roof stability. At shallow depths the maximum possible roof span is about five metres.

The dental practice is situated about one kilometre west of Nottingham City Centre, and 500m north of Nottingham Castle. The basement of the structure housing the practice comprises a cave excavated into the sandstone bedrock. Excavation marks on the faces surrounding the fissure show that the cave was hand-dug with metal tools. While many of Nottingham’s caves date back to pre-mediaeval times, the cave in question was excavated in the 19th Century, and probably over 150 years ago. It is located beneath a two-storey Georgian townhouse, approximately 3.5 - 4m below street level. It is mainly used for storage, in addition to housing two small compressors used by the dental practice.

During the summer of 2007, heavy rainfall inundated the grey-water storage system used at the practice, causing a leak that transported water outside a drainage pipe positioned vertically through the sandstone beneath the practice. Inspecting the cave after the heavy rainfall, the dentist discovered that a large fissure had developed.

Send for a geologist

Upon discovery of the problem, a consulting engineer was called out to make an inspection and identify the risks in, and possible solutions to, this unusual problem. In late June 2007 the fissure was at least 150mm wide and opened to a maximum height of 1.70m into the cave roof. Parts of the footings of the townhouse above were exposed in the top of the fissure.

The fissure trended approximately north-south. This tied in well with the overall joint/fracture system in the Nottingham Castle Sandstone, which runs roughly NNE-SSW in the cliffs on which Nottingham Castle is built². All the evidence indicated that we were dealing with a pre-existing fracture that had been subsequently infilled by soft sediment in more recent geological times.

So what to do? If the fissure were left untreated, clearly the townhouse would be living on borrowed time. Rapid cave collapse is not uncommon in Nottingham City Centre and when failures do occur they generally do so without warning. Ultimately the costs of resolving such an issue are far lower than those of catastrophic failure, and after much consultation the dental practice opted to remediate the fissure. Given the various spatial constraints on the job, we saw immediately that this would not be an easy task.

By late summer 2008 the consulting engineers had devised a remedial solution and a contractor had been engaged to install it. The design solution advocated would take five days to complete. The sectional diagram (Figure 5) presents a view looking downwards from the cave roof, showing how we intended to infill the fissure using a medium that could be then be injected with an expanding resin.

The black medium seen in the picture is a 2mm-thick rubber sock, specially fabricated for this job and fitted with two valves (top and bottom). The sock needed two valves for two reasons - to ensure that the resin could be injected into the sock, and to ensure adequate compression was achieved.

The first step was to ensure that the sock was firmly bound to the rock. This was achieved by buttering the faces with an epoxy resin, which was left to cure. After curing the sock was filled with a polyurethane resin, ensuring that maximum compression was achieved between the sock and the adjacent stratum. The elastic properties of the polyurethane would ensure that the remedial solution would be able to cope with any movements within the fissure up to 12.5mm in any direction.

The main design challenge was to maintain support to the foundation loading, while coping with the compressive and tensile forces generated by lateral movement of the fissure sides. Small vertical shear movements would also have to be accommodated by the remedial solution. Once or twice, the sock became over-filled with resin - which resulted bursts exposing the resin within. Luckily, these were confined to the southern part of the fissure and were remediated by filling behind the backing seal with mortar.

This backing seal was inserted across the whole of the fissure and ensured that the resin was in compression across all axes. The mouth of the fissure could then be filled primarily with a cementitious mortar, with a polystyrene board being used as formwork. The final step was to fill the gap left by the formwork with a polysulphide sealant, to ensure that the fissure was waterproof.


* Andrew J Brown is an engineering geologist with OPUS JOYNES PIKE Ltd.

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References

1. BGS Technical report WA/90/1
2. www.emgs.org.uk/files/publications/castlerock.htm