Engineering Geological Mapping for Civil

Q. Jl Engng Geol. 1974 Vol 7 pp. 223-256 18 figs. Printed in Northern Ireland ENGINEERINGGEOLOGICAL MAPPING FOR CIVIL ENGINEERINGPRACTICE IN THE UNITED KINGDOM William Robert Dearman & Peter GeorgeFookes * Department of Geology, University of Newcastle-upon-Tyne. t 3 Hartley Down, Purley, Surrey. SUMMARY A brief review of past British practice in engineering geological mapping is given. The paper follows the recommendations given in the Report on the Preparation of Maps and Plans in Terms of Engineering Geology prepared by a Working Party of the Engineering Group of the Geological Society. A major division is made into small-scale maps generalised in content and made for regional and planning purposes, and large-scale plans relatively detailed in content and made for specific engineering purposes. Examples from present practice are illustrated and discussed. These include engineering geological maps, and engineering geological plans made at the reconnaissance, site investigation and construction stages of a project. Geotechnical plans, presenting a single engineering design parameter or a combination of a limited number of parameters, are illustrated for both the reconnaissance and site investigation stages of actual en gineerin gP ro'ects~1 .. After considering research in engineering geological mapping, likely future trends and desirable developments are discussed. It is considered that increasing use will be made of (i) engineering geomorphological mapping at the reconnaissance stage of investigation; (ii) site investigation mapping as part of conventional site investigations; (iii) geotechnical plan mapping for specific investigations and designs; (iv) engineering geological maps and plans specifically for use in urban situations by planners concerned with urban development. Introduction The art of preparing engineering geological maps and plans specifically for civil engineering purposes and the appreciation of the value and use of such maps and plans is in its formative period in the United Kingdom at the present time. There appear to be two main problems. The first relates to the development of the skills and techniques required to produce and interpret maps and plans. Secondly, perhaps more difficult, those that relate to the development of an appreciation of the situation in which they are required and of their value in the assessment and design of a project. Downloaded from http://pubs.geoscienceworld.org/qjegh/article-pdf/7/3/223/4847328/qjegh_007_003_0223.pdf by guest on 29 September 2021 224 W.R. DEARMAN & P. G. FOOKES The aim of this paper is, therefore, to highlight these two problems by reviewing from case histories the current state of practice with special reference both to conditions in Britain and to British practice applied abroad. The case histories are drawn from a variety of situations with which the authors have actually been involved, those of which they have knowledge or those that have been published. Inevitably ther~ must be a shortfall in the completeness of coverage. There are major divisions of engineering geological maps in terms of scale, purpose and content (Fookes 1969, Anon. 1972, 1975). Engineering geological maps, produced on a regional scale, 1:10000 (6 in to 1 mile) or smaller, necessarily generalized and essentially are produced for planning and similar purposes. Engineering geologicalplans, on the other hand, are produced for specific engineering purposes on a large scale either during site investigation or during the construction stage of a project. Britain is superbly endowed with 1:10560 coverage of geological ('pure' lithological and stratigraphical) maps, which have been the continuing work of the Institute of Geological Sciences for the past 140 years. To a large extent the availability of these maps has offset the need in this country for regional photogeological maps and regional zonation maps which have been very successfully developed over the past three decades in countries where no primary 1:63360 or similar scale geological maps exist. Many engineering geological maps developed on the Continent, in America, or in the developing countries have stemmed from original engineering photogeological work or from specially commissioned ground surveys in areas where no geological maps existed. Britain, with perhaps two exceptions, does not possess any national engineering geological maps. However, a significant suite of engineering geological plans is being developed by site experience, and these range from quick reconnaissance mapping to detailed in-construction recording of data. There are also being developed, largely by research, methods for the production of geotechnical maps and plans. These are defined (Anon. 1972) as maps and plans in terms of index properties or other engineering parameters, whereas engineering geological maps and plans include both geological maps with added engineering geological information and maps, or more likely plans, in which information is presented in terms of descriptive engineering geological rock or soil classifications. The differences between geological maps produced for geological purposes and geotechnical maps produced for engineering re- quirements are illustrated in the selected case histories which follow. Underlying the production of both types of map is a fundamental understanding and appreciation of geology. It is the contention of this paper that maps and plans should be produced by engineering geologists, or if they are the product of teamwork then an engineering geologist should have a leading role to play. In his training the engineer receives little more than a basic knowledge of geology and often only very limited specialized instruction in the subject. He may not understand the geological processes which led to the formation of a particular deposit that he is considering. On the other hand, the pure geologist may, for example, have some diffi- culty in understanding effective stress concepts or rock moduli. Permeability, strength, stability and other important engineering characters can be radically affected by minor geological features, the significance of which can be missed both by the engineer and by the geologist by virtue of their limited knowledge of one anothers' disciplines. Therefore, the engineering geologist should be conversant with, and experienced in, all the geological principles of mapping and must have a sound knowledge of soil and rock mechanics, Downloaded from http://pubs.geoscienceworld.org/qjegh/article-pdf/7/3/223/4847328/qjegh_007_003_0223.pdf by guest on 29 September 2021 ENGINEERING GEOLOGICAL MAPPING 225 foundation design, and, to a certain extent, the general fundamentals of civil engineering practice. Subsurface maps which have long been in use in subsidence mining and other problems form a significant part of engineering geological mapping. They have been deliberately excluded from this paper as they are to a great extent a separate specialisation. Further information can be found, for example, in the Subsidence Engineers Handbook (Anon 1973). Review of Past Practice Basic principles of modern geological mapping date back, in this country, to the turn of the nineteenth century, to the time when William Smith, engineer and mineral surveyor, had already produced the first true geological map. The unique aspect of Smith's approach was his clear recognition of the order of superposition of strata, how different strata could be recognized, and the general continuity of particular horizons. But he was also concerned with the practical applications of his work, and this is made clear in a prospectus for a book and accompanying map, issued in 1801. Using the proposed book and map, which were never published, "The Canal Engineer will be enabled to choose his stratum, find the most appropriate materials, avoid slippery ground, or remedy the evil" (Sheppard 1920, p. 110), a suitable brief for a modern engineering geological map. Since then there have been many examples of the influence of geological conditions on the planning, design and construction of major civil engineering works, but from about 1850 onwards geology became more and more neglected in civil engineering practice (Glossop 1969). Engineers lost interest in the science of geology which could not provide them with data on which to base their design calculations. But despite this, the foundation conditions in many dams were determined and recorded both in plan and elevation (Deacon 1896, Sandeman 1901, Lapworth 1911, Peach 1929). Simpson (1938)reviewed the problems associated with selected tunnelling projects and dam sites, and provided maps of the geological conditions encountered. Factors influencing the likely engineering behaviour of rocks, rock type, strength, durability, water-bearing capacity, were clearly recognized and in many instances quantified. It is this lack of quantification that makes even fairly recent examples, notable for wealth of detail and clear discrimination between different geological conditions, very valuable geological records, but little more. On the construction record of the Fernworthy dam on the Dartmoor granite, S.W. England, the limits of homogeneous units are given (Fig. 1), and in the description of the engineering works Kennard & Lee (1947) clearly relate the ground conditions in the excavation for the cut-off trench to the geological conditions. They also recount the engineering solution to the diffi- culties encountered, for example, with the clayey mineral lode, but what is lacking is

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