Up until recently, any local newspaper article reporting on a hole opening up somewhere in the city usually had 'cut-and-paste' paragraphs tagged on referring to how Norwich had a long history of chalk mining and was known to be 'riddled with tunnels'.
Ever since a large red bus fell into a hole in Earlham Road in 1988, and actually for years before that, subterranean Norwich has been associated in many people's minds with underground chalk workings. You only have to google to find accounts of ancient secret tunnels criss-crossing beneath the city linking various landmarks, including:
Although far from comprehensive, this is a varied list. These claimed links are usually involved in illicit activities or in escapes, and not all of them necessitate deep tunnels in the chalk. The alleged 15 km long tunnel leading all the way to Ludham is far-fetched, and as with other tunnels would involve crossing beneath the river where the ground is saturated, so the ancient users would have needed an aqualung!
While tunnels undoubtedly do exist in places, the proliferation of these tales has undoubtedly been encouraged by a combination of vivid teenage imaginations, wishful thinking, journalistic licence, natural reluctance on the part of the City Council to release information, the presence of a major insurance company in the city, and a tendency for those who have actually visited tunnel systems to consistently exaggerate their size by threefold when they emerge. And even at a professional level, when you cannot see beneath the surface, it's always safer to say tunnels could be there than to guarantee they are not.
A further complication is that by no means all deep holes that open up are attributable to man-made workings. Natural 'solution features' are also encountered, although these are often activated or intercepted by man-made activities.
To separate tunnel fact from tunnel fiction, we need to consider the scale of underground passageways we know about and why they are there.
People living elsewhere in the country, when reading about the bus in the hole, could be forgiven for concluding that Norwich was a problem location for underground workings. I have seen a regional map showing geological problems of East Anglia with a large orange blob covering the whole of the city. Even the City Engineers used to have a map of Norwich on display with particular streets awarded a red blob to show where a subsidence event had occurred, which you could imagine may have had unwelcome consequences for people wanting to sell their houses.
Geological factors do not normally align themselves with particular streets or postcodes. In practice, holes opening up relating to underground workings, while deep, tend to be only a few metres across and very localised, usually affecting only one part of a plot let alone a whole road, most of which may be perfectly sound. For this reason it is very difficult to carry out a high enough intensity of ground investigation in advance to be sure about things, or to have enough detail to produce meaningful maps showing risk areas. The potential horizontal length of workings is often overestimated and directions of trend are extrapolated from inadequate evidence.
High-tech methods of investigating the ground using radar or resistivity have been tried for detecting voids at depth, but even on flat and open ground (rare in the built-up area) these are rarely successful other than for shallow archaeological work. The problem is that relatively small variations in the ground at shallow depths tend to mask the signal from large variations at greater depths, so deep tunnels can easily be missed.
One of the characteristics of chalk in Norwich is that it is relatively soft and easily dug, but at the same time it is able to stand unsupported.
All of Norwich is underlain by chalk, but only in parts of the city is it easily accessible by digging from the surface. These areas are generally on the sides of the river valleys between the river sediments on the valley bottom and the covering crag and glacial strata at a higher level. This approximates to an elevation above sea level of between about 5 and 20 metres.
Chalk has been extracted for lime necessary for construction and other purposes for many centuries in Norwich. The buildings and city walls contain vast amounts of material which must have come from somewhere locally. The easiest way is to dig from an open pit in the valley side, and there are many places in the city where this has been done, from medieval right through to Victorian times, with some pits still working in the twentieth century.
However, there is a limit to how deep you can go (because of water and the need to haul the chalk out) and to how far back into the hillside you can dig (because of overburden and land boundaries). Chalk will stand unsupported, so you can see how tunnels started to be dug back into the hillside, and these would be extended over time.
The earlier hand-dug tunnels tended to be more haphazard than the later ones, which were often sized to allow a horse-drawn cart to remove the chalk, with some grid-like Victorian tunnel systems evidently using a system of trucks on rails hauled by rope. Typical lengths range from a few metres to tens of metres, but never up into the kilometres scale!
Besides tunnels for chalk mining, there are other types of underground opening, including numerous cellars, crypts and undercrofts originally used for storage, and often with brick-lined arches looking as if they should lead somewhere. It is noticeable how these are concentrated in the areas of the city centre where the buildings are founded on chalk. Some pre-existing tunnels were adapted as air-raid shelters in the run-up to World War Two.
In the 1980s, some deep long tunnels were dug by hand though the chalk and lined with concrete segments to provide stormwater drainage sewers and service ducts.
In most areas of the city where the chalk is accessible on the valley sides, limited tunnel systems have been found, often associated with chalk pits. These include the slopes above Rosary Road and Barrack Street, below Ber Street and the city end of Earlham Road, Pottergate area, Bracondale and Magdalen Road areas. Further out of the city, there are known mine workings off Dereham Road, Newmarket Road and Ipswich Road.
In some cases (e.g. Earlham Road and Rosary Road) accurate plans exist of workings, but in most cases only isolated recorded information where tunnels have been exposed or entrances found.
The hazards associated with underground voids are not unique to Norwich and are reasonably well understood. Some of our tunnels have been filled in, others are still standing open and thus pose a risk of collapse. Some were evidently abandoned at the time of digging when they passed from chalk into zones of sand and the roof fell in.
The most serious problems today occur when stability is disturbed by external factors, especially from construction works, or due to water leakage. These are implicated in most of the dramatic incidents of the recent past. The moral of the tale is to arrange for the ground to be properly checked before building in any of these susceptible areas, and, as always to aim to keep your drains in good condition. With this sensible risk management, we can afford to relax a bit.
Be cool and calm about tunnels, and be a bit sceptical about the more far-fetched tunnel tales.
© Matthew Williams
PICTURE 1: Bus in hole, Earlham Road 1988 © nosher.net
PICTURE 2: Tunnel below west Norwich (from www.ukgeohazards.info)
PICTURE 3: Ground penetrating radar trace (generic)
PICTURE 4: Tunnel entrances at Rosary Caves (George Swain)
PICTURE 5: Old plan of workings dated 1823 (as published in Norfolk Archaeology Vol 38, Fig 2)