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owe a great deal to Prof. Taylor, at one time Secretary of the Society, with whose death in 1955 soil mcchanics lost one of its most thoughtful and careful research workers.

In the stability of clay slopes there are problems which are still unsolved, but in recent years a number of field inves-tigations of decisivc importance have contributed towards a rational understanding of this subject, some aspects of which did not seem to be susceptible to scientific analysis 25 years ago. Even the notorious landslides in quick clays are beginning to yicld up their secrets, as we can see from a paper just published by the Norwegian Geotechnical Ins-titute. This contribution is an object lesson to us all, and especially to those who are inclined to adopt the attitude that certain classes of landslips are incomprehensible acts of naturę. We may not yet understand all that is happening, but to deny the possibility of a rational explanation is to dcstroy the chief purpose of our work.

There are, of course, many other branches of soil mechanics in which a similar degree of success has been attained, in giving quantitative expression to engineering problems. And it is this fact which gives rise to the second great achievenient of the past quarter of a century. I refer to the wide accep-tance of soil mechanics by the civil engineering profession and by the universities. In 1936 there was only one fuli Professor-ship : that held by Terzaghi in Vienna. Now there are about twenty and one can hardly think of any civil engineering school in which the subject is not taught. As for the profession it is probably true to say that almost every Government department, consulting Office and contractor’s organisation include on their Staff one or morę soils engineers; or, at the very least, have ready access to expert advice.

This is a change indeed. Up till the time of ourfirst Confer-ence, soil mechanics was still largely in the hands of the few men who were themselves actively creating the subject in its modern form. Now we are reaching a stage of matu-rity and it is doubtful if any further spectacular numerical growth is either likely or desirable. Already for every one pioneer at the frontiers of knowledge, there are many engineers engaged in the no less important work of consolidating and applying the discoveries in practice. There is nothing wrong in this State of affairs, which is perfectly natural and inevitable. But the time has come to pause for a moment and consider whether any dangers may lie ahead.

Dangers in the futurę

In think that there are two. The first arises from the situ-ation that design is now frequently carried out by those who have littlc if any direct experience of research and of the painstaking care, one rnight almost say the loving care, with which all the factors in a given problem have to be studied if a correct solution is to be obtained. In a word, I refer to the danger of what rnight be called *’ handbook engineering. It is not sufficient merely to receive some results from a testing laboratory and then to use them in one‘s calculations. In addition it is necessary to deve!op an intimate knowledge of the site and, above all, an understanding of soils and rocks. Such an understanding, however, cannot be gained simply from lectures or by reading reports. I would therefore suggest that every young engineer who shows any aptitude for observation and deductive reasoning should be given the opportunity to take part in fuli scalę tests and to record anything and everything that rnight be of interest; and, finally, if he has discovered something of value, he should be encouraged to publish his findings.

There is nothing particularly novel in this scheme but I would like to see it adopted as a matter of policy on all major works. In this way we can keep alive the spirit of enquiry which is the best antidote to the handbook ” attitude of mind. In this way, also, the site itself becomes a laboratory in which significant discoveries may be madę. Not all senior engineers are willing or perhaps able to take this view. But engineering is not only a matter of getting the work finished in the shortest possible time and in the most economica! manner. These are important consider-ations, obviously, but if they are allowed to dominate all else, the intellectual and creative excitement of the work will dic. And if this should happen civil engineering will cease to be a profession and will become a job for technically-minded business men, content to apply existing techniques without contributing to the storę of knowledge or to the proper training of their successors.

The second danger which can be foreseen and which we must strive to avoid can be expressed in the simple word “ complacency ’. When I entered the world of soil mechanics, at the Building Research Station, under Dr Cooling, our work was largely concerned with the investigation of failures. Within a few years we enquired into the failure of two earlh dams, several foundations, a sheet pile quay, three or four retaining w-alls and numerous slips in clay banks. And I have no doubt that others working at that or earlier periods had similar experience. But today the position is diflerent. Site investigations and tests are carried out for practically every design of any importance; and consequently the number of failures has been drastically reduced.

This is a great improvement, but it is to be notieed that some engineers are beginning to forget the old State of affairs. Very few earth dams, for instance, have failed during the past fifteen years. Therefore these engineers are beginning to feel complacent and to wonder if their soil mechanics consultant is perhaps being too cautious. And, what is much worse, the soil mechanics consultant may also be led to the same feeling. We usually design with factors of safety against complete failure of only 1-5 or 1-4 and sometimes even less. Yet the failure of a dam during construction always has appaling economic consequences and after completion, when the reservoir is fuli, a collapse is likely to have catastrophic results.

In no other branch of engineering are such Iow factors of safety used. On what possible grounds therefore can we afford to encroach yet further into this smali margin of safety ? I have heard it said that the methods of analysis are conservative. Yet. the most complete investigation so far madę of a slip in a clay slope shows the analysis to be almost exact, provided the soil properties and porę pressures are accurately known. Thus it would have to be argued that the measurement of soil properties or the prediction of porę pressures are unduly conservative. Yet recent observations of porę pressures in the upstream slope of an earth dam indicate that these pressures are likely to be rather greater than those used in current methods of design, and there are several cases on record of high porę pressures being set up during construction which, of course, reduce the shear strength of the soil. Hence, whether it is recognised or not, the assumption is being madę that the actual soil properties are better than those measured in the laboratory. What evidence exists for this assumption ? Simply that there have been no recent failures in major earth dams. The conclusion can only be that sooner or later, if this tendency is pushed a little further, such a failure will occur. And the same reasoning can be applied to other works involving soil mechanics problems.

By all means let us question every step in design and in soil testing; and indeed such questioning is vitally necessary. But let this be on a rational basis and not by a so-called intuitive approach. No one va!ues engineering experience and imaginative design morę than I do. But 1 am also intim-atcly acquainted with engineering history, and the lesson to be learnt from such a study is that the great engineers have used, not abused, the most advanced knowledge of their

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