Article - Issue 7, February 2001
Extensive geological records show how problems encountered during the construction of the Severn Tunnel might have been avoided J. D. Davis FREng
Sir Alan Muir Wood FRS FREng FICE
The Severn Tunnel is notable for the extent of its geological record, from which it is now evident how the Great Spring (discussed by G. M. David, Ingenia, November 2000) might have been avoided during the tunnel’s construction.
Mr G. M. David retells the story of the flooding of the Severn Tunnel during construction. The most vivid account of these events is to be found in the book by Thomas Walker (Severn Tunnel – its construction and difficulties, first published in 1888, with several subsequent editions). Thomas Walker was contractor for the work from 1879 to completion. His book contains a number of illustrations, including the plan and section used by Mr David; the book is particularly highly regarded on account of the detailed accounts of the ground encountered. For example, he provides a longitudinal section of the tunnel, with associated shafts and headings, which indicates the nature and inclination of the rocks recorded during construction. If hazards in tunnelling such as the Great Spring are to be understood, they must be put into a geological context.
The Severn estuary in the vicinity of the tunnel follows a fault system associated with heavily folded strata. To each side of the fault system, the rocks to the depth of the tunnel generally comprise Triassic sandstones, siltstones and claystones (the nomenclature a little changed since Walker’s time) overlying, unconformably, Carboniferous (Coal Measures) Pennant sandstones, with siltstones and thin coal seams. During investigations for the Second Severn Crossing, close by and straddling the tunnel, it was found that the Trias dip gently towards the S–SE, while the Carboniferous strata dip at 10–20o towards the SE.
The problem at the Great Spring, however, is that a local anticlinal fold brings the underlying sandstone (loosely termed ‘Millstone Grit’ but not precisely coeval with the Millstone Grit of northern England) and limestone to the level of the tunnel. The Great Spring flows from an irregular solution channel leached in the limestone, the conjectural source of the water the Maddern Brook about 2 km west of the tunnel at the point where it struck the spring. The water from the spring has been described as seasonally variable, fresh and clear. It would be possible at the present day to contain the spring but the work would be expensive and difficult to undertake without major interruption to traffic.
Contrary to the statement by Mr David, once the geological structure is understood, it is seen as not difficult to find a route for a road tunnel to avoid such a feature – and indeed road tunnels were seriously considered for the Second Severn Crossing, with differences in operational costs a major reason for choosing a bridge.
The explanation of the high tidal range in the Severn estuary also calls for some clarification. The ‘funnel-like shape’ is only one factor for the tidal magnification. The local system in the open sea and, most importantly, the overall dimensions of the estuary, which are such as to set up a resonant tidal standing wave, are the most vital factors.
Alan Muir Wood FREng FRS
The rate of demand for oil will soon exceed the rate at which it can be produced. Engineers have a responsibility to develop energy-saving solutions to head off this impending crisis.
The BBC are to be congratulated for the programme on the future of oil production broadcast on Wednesday 8 November 2000. It was the first time that this critically important subject – of worldwide importance – was given a public hearing.
From time to time there have been reassuring messages that we have so far only consumed about half of the world’s total oil reserves during the past century. Even at the present high levels of consumption, there is still sufficient oil in the ground to last for at least forty more years. This is perfectly true but on its own it can be misleading. As the BBC programme showed, what is most important is the maximum rate at which it can be produced in a world of increasing oil demand. It is this which has not been given widespread publicity until now, even though reliable information has been available for a considerable time.
For many years the rate of new oil discovery has not been keeping pace with the increasing rate of consumption and the depletion of existing oil fields. Despite very heavy investment in exploration and production and also some dramatic technological developments, no new major oil provinces have been discovered for more than thirty years. Most of the new oil is offshore, is expensive to produce and is in comparatively small fields. Although further technological developments may improve the situation to some extent, they alone are unlikely to be sufficient to prevent a long and well established trend from continuing, a trend towards a physically limited maximum rate of worldwide production of conventional oil. For some years oil companies and independent experts have been in broad agreement that this peak of production is likely to be reached at some time in the second or third decade of the present century. Thereafter they expect it to decline at something like 3% per annum, in an environment in which demand continues to increase, particularly in China, India, other Asian countries and in South America. Also, before the peak is reached, in the absence of readily available cheap alternatives of sufficient quantity, shortages are likely to develop, producing price increases and periods of economic recession. Once the peak is passed, irreversible price increases will be inevitable to keep supply and demand in balance, as the world for the first time in history experiences a shortage of what has become an essential commodity. In the short term there is no readily available substitute to relieve the pressure on prices other than natural gas and its liquid derivatives.
Even for this relief to be of practical use, there needs to be greater urgent investment both in process and distribution facilities for road transport fuels, and also for power plant development and production.
As the BBC programme pointed out, we are not so much running out of oil as running out of time. What the programme omitted to mention was the fact that the existing car population will be replaced within the next fifteen years by a new generation of cars. Given urgent action, using known and proven technologies, the new cars could have at least 70 or 80 miles per gallon instead of the present 30 or 40. But for that to happen, governments worldwide would need to make that standard of fuel economy in all new car production mandatory from an early date.
The possible consequences of a worldwide fuel shortage are so serious that not to do everything possible to prevent it would be unforgiveable. The engineering profession, which has the know-how to buy time, will be failing in its public responsibility if it does not exercise all its influence to ensure that all of the necessary developments are given immediate priority. Doubling miles per litre in a new generation of cars is the best way to buy the greatest amount of time. The profession must do all in its power to make that happen.
J. D. Davis FREng