Alan McGown

Genetic influences on the properties of tills

Summary Tills are a distinctive geological and engineering group of soils which are characterized by a wide range of behavioural patterns and a high degree of variability. As a means of identifying the various geological types of tills, a classification system based on their modes of formation, transportation and deposition has been developed. To accommodate the inherent variability of tills, sub-division of these classifications has been carried out on the basis of their gradational characteristics. Further, the fabric features contributing to variability in engineering behaviour are identified. From this, extended classifications are tentatively suggested to aid the prediction of the engineering properties of tills and the selection of the most appropriate sampling and testing techniques for them.

Fissure patterns and slope failures in till at Hurlford, Ayrshire

Summary The present investigations have shown that the till at Hurlford, Ayrshire, is fissured and that a very definite fissure pattern exists. The nature of the fissures and influence of weathering, overburden pressures and stress release have been established and new scalar property classification especially designed for tills have been successfully employed. A close relationship between fissure fabric, slope stability and directional shear strength characteristics has been found and the mechanism of slope failure at Hurlford has been identified to be the opening and softening of fissures in the till. The three-dimensional shear strength anisotropy of the Hurlford soil has been shown to be outside of the bounds of applicability of conventional sample size—shear strength relationships which assume random fissure patterns in clays.

Uniaxial strength testing of woven and nonwoven geotextiles

Abstract This paper describes the different design approaches using geotextiles and presents two types of uniaxial tensile testing: constant rate of strain and rapid loading creep tests. Tests are conducted using a woven geotextile and two nonwoven geotextiles. The effects of the width to length ratio of the test specimens, test temperature and strain rate are examined. Tests are also conducted with the specimen in air and when confined in-soil. The results illustrate that the load-extension behaviour of the geotextiles tested depends on the test conditions and hence the need to standardise these conditions is established. The relevance of test types and conditions to design and quality control is also discussed.

The classification for engineering purposes of tills from moraines and associated landforms

Summary This paper makes a critical review of some of the many previous attempts at identifying and classifying tills from morainicand associated landforms and comments on the deficiencies of these as a basis for an engineering classification system. A new approach to the classification of till is suggested, which it is claimed, is suitable for both engineering and some geological purposes. This new system is then applied to a number of tills from the West Highlands of Scotland and some reported gradings of tills from other countries.

Technical Developments in the Study of Particulate Matter in Glacial Tills

Some techniques in the study of the structural organization of glacial tills are described. The first of these employs a modified version of Harrisons method of studying mesofabrics (clasts 64-2 mm). A lightweight goniometer, suitable for expedition work, is described and some field results presented from a subglacial environment. A dry-brushing technique designed to accelerate fabric analysis and render it less selective is also described. Results of examinations of three-dimensional microfabrics of the same tills using the scanning electron microscope are also presented. The methods, used in combination, increase the diagnostic and analytical potential of till fabric analysis.

STRATIGRAPHY AND PROPERTIES OF THE CLYDE ALLUVIUM

Summary The paper outlines the geological history of the Clyde Valley and attempts to relate this to the nature and distribution of the sediments within it. Because of the position of the valley within a glaciated region in which isostatic recovery has occurred, the infill deposits are very complex, with fluvio-glacial, lacustrine, marine, brackish and freshwater deposits overlying glacial and pre-glacial materials. The soils within the succession have been identified, classified in terms of their basic physical properties and their likely engineering behaviours have been indicated. The engineering significance of the nature of the various strata within the stratigraphical sequence is then discussed.

Review of Session Evaluation of Material Properties

When dealing with reinforced soil structures, knowledge of the behaviour of the different components of the structure is essential. This is gained by testing the materials either separately or in combination, using standard test methods and specially developed testing techniques. Due to the importance of this subject, it is not surprising that most of the published work deals with various aspects of material testing. Accordingly, and perhaps ironically, designers are now faced with the problem of choosing suitable tests from which the relevant design parameters are to be determined. This session deals with the wider aspect of these problems by examining materials evaluation which of course includes testing.

Review of Session Analytical Techniques and Design Methods

The chairman, Rowe, began the session by posing a number of questions that require answers when considering analysis and design and which the various speakers would address during the session. They included the following: 1. Should we be trying to estimate deformations using limit equilibrium analyses? 2. What is the effect of reinforcement extensibility? 3. Is strain-softening of the soils important? 4. How valid is it to find the level of reinforcement required without considering the interaction between it and the soil? 5. What is the Factor of Safety in our designs? 6. How does one account for construction technique in design? 7. How useful is the finite element method in analysis of reinforced soil? 8. Are any of the existing analytical methods providing the needed answers?

Review of Session Construction Methods and Economics

Jones opened this session with a general appreciation of the many factors affecting the economics of geosynthetic construction. Apart from regional market force aspects, which in many cases control cost and methodology, he pointed out that working practices may be the dominant cost factor. The actual cost of the reinforcement is in most cases a small proportion of the total cost of the structure and as such may have little influence on overall economics. He then discussed the concept of using an “Ecology Audit” to establish comparable base costs for different forms of reinforced soil. An ecology audit sums the overall cost to society of a particular form of construction especially with respect to the energy used throughout the process. It therefore avoids to a great extent market factors and other artificial variables. Such an audit indicates that geosynthetic reinforcement is a cost effective means of construction. Despite this economic advantage designers are still hesitant to use this form of construction due primarily to lack of experience. Therefore the keys to market growth are Specifications and Codes of Practice for geosynthetic reinforced soil as such standards relieve the “tension” from the design situation by providing set terms of reference. Work is progressing in a number of countries on these standards.

Review of Session Case Histories and Full Scale Trials

Data obtained from instrumented full scale polymer reinforced structures are of immense value in understanding the behaviour and verifying the design concepts for such structures. However, there are two major difficulties facing researchers, firstly shortage of funds for full instrumentation of trial structures and secondly lack of an agreed list of relevant case history data and a common format for their presentation. Consequently, as Christopher pointed out, of an estimated 200 slopes and walls constructed in the USA and Canada using polymeric materials, data on only 46 projects could be collected. Of these only 6 projects were fully instrumented. He presented these data in four comprehensive tables covering general information; design information; actual properties of the reinforcements and design methodology; and project instrumentation and monitoring information. This work represents a substantial step forward and workers in this field should be encouraged to extend such information so that a comprehensive data bank of case histories is constructed. It is important however, that when consulting such data the user is aware of all the parameters and assumptions used in the design.