A Selection of Technical Reports Submitted to the Aberfan Tribunal
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Welsh Office A Selection of Technical Reports Submitted to The Aberfan Tribunal i London : Her Majesty's Stationery Office 1969 Printed image digitised by the University of Southampton Library Digitisation Unit A Selection of Technical Reports submitted to the Aberfan Tribunal Foreword In our Report* published last year we said that as soon as we were constituted as a Tribunal we arranged for a series of scientific investi- gations to be started under the general direction of Professor A.W. Bishop, London University. The National Coal Board brought in expert members of its own staff and also independent experts and the Aberfan Parents' and Residents' Association likewise engaged independent expert advisers. In addition certain other investigations were put in hand on behalf of some of the parties who appeared before the Tribunal. In view of their scientific interest, we thought it right to arrange for the separate publication of seven of the reports submitted by these experts to the Tribunal, and they are reproduced here with exactly the same text as was originally submitted. Most of the original drawings were unsuitable for reproduction within the size of this publication and many have been re-drawn. In some cases there was sufficient similarity between the drawings used by different experts to make it possible to use one drawing here without detracting from the information given in the original report. Certain of the written reports submitted to the Tribunal were required simply as formal evidence and these have not been reproduced in this publication. The task of preparing this volume for publication has been performed by Mr A.D.M. Penman with assistance from the authors of the reports and others. We acknowledge our debt to him and to all who helped him. EDMUND DAVIES HAROLD HARDING VERNON LAWRENCE R. LLOYD THOMAS, Secretary November 1968 *H.L 316 H.C. 553 Printed image digitised by the University of Southampton Library Digitisation Unit Contents Item 1. Geotechnica! investigation into the Causes and Cir- cumstances of the Disaster of 21 st October 1966 by A.W. Bishop, J.N. Hutchinson, A.D.M. Penman and H.E. Evans p. 1 Item 2. Report on the stability of Aberfan Tip No. 7 byJ.KT.L Nash p. 83 Item 3. inquiry into the Aberfan Disaster by G.M.J. Williams p. 91 Item 4. Geological Report on the Aberfan Tip Disaster of 21 St October 1966 by A.W. Woodland p. 119 Item 5. Geological Report on the Tipping Site and its Environs at Merthyr Vale and Aberfan by L.R.Moore p. 147 Item 6. Report on Mining Subsidence by K. Warden and R.J. Piggott p. 187 Item 7. Meteorological conditions relating to the Aberfan Inquiry by A. Bleasdale p. 207 Item 8. Interpretation and analysis of photography covering the Aberfan area carried out by Fairey Surveys Ltd. (Bound separately) Printed image digitised by the University of Southampton Library Digitisation Unit ITEM 1 Geotechnical Investigation into the Causes and Circumstances of the Disaster of 21st October, 1966 Alan W. Bishop MA PhD DSc(Eng) MICE, Professor of Soil Mechanics in the University of London, Imperial College John IM. Hutchinson BSc PhD AMICE FGS, Senior Lecturer in Soil Mechanics, Imperial College, University of London Arthur D. M. Penman MSc AMICE, Member of the Council of the Institution of Civil Engineers, Principal Scientific Officer at the Building Research Station Haydn E. Evans BSc PhD FGS, Lecturer in Civil Engineering, University College of Swansea, University of Wales Contents 1 Introduction 2 Some earlier slips in South Wales and their relevance 3 Geology of the site in the immediate vicinity of the tip and soil properties of the natural strata 4 Ground water conditions 5 Properties of tip material 6 Pore pressures observed in tip material 7 History of Tip 7 8 Discussion of mechanism of failure 9 Conclusions Appendices 1 Summary of paper'Landslides in South Wales' by Professor George Knox 2 Press reports of Abercynon slip 3 Details of soil tests, piezometer readings, etc 4 Stability analyses 5 Model tests 6 Evaluation of quantities involved in the history of Tip 7 Recommendations References Printed image digitised by the University of Southampton Library Digitisation Unit Printed image digitised by the University of Southampton Library Digitisation Unit 1 introduction In preparing this Report we have attempted to give a concise sum- mary of those aspects of our investigation which appear to be directly relevant to the four questions posed by the Chairman at the opening session of the Tribunal: (1) What exactly happened ? (2) Why did it happen ? (3) Need it have happened ? (4) What lessons are to be learnt from what happened at Aberfan ? Details of tests are given in appendices, together with information which appears at this stage to be of marginal relevance to the four questions, but which may be of value to future studies in this field. In this latter connection we would draw attention to the present lack of information on the engineering properties and behaviour of tip materials and on the seasonal variations in water pressure within tips and the strata on which they are founded. It will be one of our recommendations that some of the observations and investigations begun during this present Inquiry should be continued, under the sponsorship of the National Coal Board, the Science Research Council or the National Environmental Research Council, by the Institute of Geological Sciences, the Building Research Station and appropriate University Departments of Civil Engineering and Mining Engineering. A rigorous scientific investigation involving precise calculations of the factor of safety of the structure has been particularly difficult in this case. The structure with which the Inquiry is concerned, though com- parable in magnitude with our major road or reservoir embankments (the height from toe to crest being about 200 ft) is a colliery rubbish tip, and the stages of its construction and its shape immediately prior to the slip of October 1966 are not well documented. The internal water pressures in the foundation and the tip were not measured and have since been substantially modified. Most of the relevant material from the tip has moved many hundreds of yards down the mountainside. Our conclusions therefore depend not only on the measurement of the properties of the foundation strata and the tip material, but on a careful examination of the behaviour of the tip in the years preceding the ultimate collapse. In this examination we have enjoyed the co- operation of the Ordnance Survey, the Royal Air Force, Hunting Surveys Ltd, the Department of Photogrammetry and Surveying of University College London, and the Meteorological Office. The pro- gramme of boreholes has been planned jointly with Dr A.W. Woodland, of the Institute of Geological Sciences, and co-ordinated with work initiated by the National Coal Board. Borehole logs have been agreed by the geologists of the l.G.S. and the N.C.B. Some 80 boreholes have been completed and are shown on Fig. 1.1 (folded drawings are con- tained in the wallet). The National Coal Board have co-operated most willingly in the pro- vision of offices, heavy plant to facilitate the site investigation, and where necessary photographic and surveying services, often at very short notice. Some difficulties have arisen in carrying out our investi- gation due to safety requirements and due to the problems involved in handling heavy plant and carrying out large-scale earth-moving opera- tions under the prevailing weather conditions. Very significant data about the nature of the slip under the southern section of the tip, only began to be available to us on 8th February 1967 and the area imme- diately uphill of the spring revealed by the October 1966 slip has not been cleared at the date of writing this Report. In examining the data on which this Report is based it has become apparent to us that all the events contributing to the disaster cannot be associated with a single cause. Several factors were involved, which separately could have caused a slip, though not necessarily a disastrous one. Our conclusions depend on the general agreement of data from a Printed image digitised by the University of Southampton Library Digitisation Unit number of sources, and are not dependent on the precise measurement is, by itself, no indication that it will not collapse the following day A discussion of the engineering concept of 'factor of safety is outside the scope of this Report, but is presented most lucidly in Chapters 11, 12 and 13 of a short book by Sir Alfred Pugsley entitled The Safety of Structures (1966). 2 Some earlier slips in South Wales and their relevance It is relevant to the four questions posed by the Chairman of the Tribunal to examine the background of earlier slips in the Welsh valleys against which the present disaster is to be seen. An exhaustive survey has not been attempted, but the following information is considered to be useful. (I) A paper on 'Landslides in South Wales Valleys' by Professor George Knox, FGS, JVIIME, read and discussed in 1927 This paper was read and discussed in Cardiff on 29th March 1927, with further remarks and discussion on 28th April 1927. This is pub- lished in full in the Proceedings of the South Wales institute of Engineers, Vol. 43, pp. 161-247 and 257-290. A critical summary is given in Appendix 1. This paper is of importance because of the great interest which it aroused at the time in landslides, including slides involving colliery refuse. Though Knox's understanding of the physical mechanism and applied mechanics of the slide is now considered inadequate, he correctly emphasised the over-riding importance of water in causing landslides.