Issues

The study of soil structure has made great progress in recent years, because of the development of the Scanning Electron Microscope and the realization that it may be possible to relate soil structure to soil properties in the classical manner of materials science (e.g. Barden 1972). One of the persistent problems that harass soil structure investigators is that of preparing an undisturbed sample, and being reasonably certain that it stays undisturbed at all stages of the investigation. This had led to the development of various methods of setting the soil sample, usually by introducing some material into the pores which provides a firm matrix for the soil particles (Tovey 1973). It appears that there are instances where this has been done by nature, and primary mineral soil structures have been effectively preserved by the secondary deposition of minerals from groundwater; this is particularly true in the case of loess soils.

The structure of loess is of particular interest from the geotechnical point of view because of the collapse phenomenon which is observed in loess soils. It was suggested long ago by Terzaghi (1950) that as mineral particles become smaller the proportion of platelet shapes increases; thus loess could be composed essentially of quartz plates in a very meta-stable structure. This was in fact proposed by Krinsley & Smalley (1973) as a consequence of their hypothesis on the operation of a cleavage mechanism in quartz particles below a certain critical size. Thus, if concretions in loess preserve the soil structure they

The level of sophistication required by a slope monitoring system should be related to the importance of the slope. Dam abutments, bridge foundations, slopes under expensive engineering installations and slopes which threaten people’s homes justify the installation of the more sophisticated monitoring systems mentioned in order to give the earliest possible warning of instability. Most highway slopes and mining slopes, on the other hand, are too extensive to monitor effectively in this way except at prohibitive cost. In such cases regular inspection of the slopes on a daily or weekly basis may be the most practical and effective method. The inspection should ideally be done by the same person who should note on a standard form the condition of the slopes (e.g. seepage, dangerously loose material) and the presence and extension of any tension cracks.

Once instability has been detected a monitoring system should be designed and installed. In many cases an extended system of surface control points is probably the most effective method of monitoring for the money expended. In practice, control points are often lost due to interference, natural hazards or because it becomes dangerous to approach them, so that it is as well to allow for up to a 50 per cent casualty rate. In addition the mode of failure of a rock slope may change from that predicted (new cracks may open up) so that the original monitoring system could become ineffective. Until a specific problem is delineated, the principle should be to monitor a large

The discussion of the Working Party Report on the preparation of maps and plans in terms of engineering geology (Q. Jl Engng Geol., 5, 293–382) by the London (UK) Section of the Association of Engineering Geologists (Q. Jl Engng Geol., 7,) confirms the general interest that publication of the report has aroused. Four of the speakers had been members of the Working Party, and many of the points raised had either been discussed in committee by members of the Working Party or were embodied in the comments received when an early version of the report was circulated and later discussed at the informal meeting held at the Institute of Civil Engineers in March 1971.

A great deal of effort was needed to produce both the final manuscript and the drawings of all the many illustrations in a form fit for publication. Revision in three years, as suggested, is a daunting prospect, and perhaps a decade would be a more appropriate trial period for the practical application of the recommendations made in the report.

It is pertinent here to comment on some specific aspects of engineering geological mapping raised in the A.E.G. discussion. The terms of reference of the Working Party were: (a) to consider the need for engineering geology maps; (b) to make proposals for the presentation of relevant information on such maps; and (c) to study methods of obtaining basic data required for their presentation. The need for engineering geological maps was dealt with, albeit rather briefly as Mr.