The geotechnical properties of some till deposits occurring along the coastal areas of eastern England

Abstract

Deposits of till are found along the coastal areas of eastern England from Northumberland to East Anglia. The geotechnical properties of tills from three areas, namely, north Norfolk, Holderness and Teesside have been investigated. The dominant clay minerals in the fine fraction of the tills are kaolinite and illite. As would be expected, quartz is the other dominant mineral in the fine fraction of the tills.

Deposits of till occur in the Anglian and Devensian stages of the Quaternary succession in Norfolk. All these tills are matrix-dominated, with clay generally forming less than a third of the matrix. They are either firm or stiff with low or intermediate plasticity and have relatively low values of shear strength. The tills are either inactive or have normal activity and all have low sensitivity. Their consolidation properties are characteristic of stiff clays.

The glacial deposits of Holderness consist primarily of tills. Except for the oldest of these tills, which is Wolstonian, the others are of Devensian age. The fine fraction usually constitutes up to 60–80% of the deposits. These clays have a low plasticity. The tills of Holderness have a low sensitivity and a relatively low unconfined shear strength. Their values of shear strength are reduced from peak values, to residual values primarily by a reduction in the cohesion parameter. Like the tills from the other two areas, the pore water pressures on testing in, consolidated, undrained conditions rose rapidly to a peak which was followed by a gradual falling off as failure was approached.

The glacial deposits of the Teesside area are of Late Devensian age. These lodgement tills were products of successive ice sheets. The tills are thickest in the north and west of the area, and are characteristically unsorted and matrix dominated. They are of low to intermediate plasticity, and vary in consistency from firm to hard, generally being stiff to very stiff. The results of triaxial tests indicate a reasonably wide range of strength.

Introduction

During Pleistocene times, a maximum of approximately 30% of the land surface of the Earth was covered by ice. As a consequence, glacial deposits are of particular importance to geotechnical engineers and engineering geologists in northern Europe, northern Asia and North America. Also of geotechnical significance is the fact that tills represent one of the most variable of sedimentary deposits. This variation is brought about by the variety of materials of which tills may be composed, by the various means by which the materials have been incorporated into the ice, by the way in which they were transported and what happened to them during transport, and to how they were deposited. Hence, they can range from extremely dense non-plastic tills to weak plastic clay tills.

Till has been recently defined by Hambrey (1994) as a poorly sorted mixture of clay, silt, sand, gravel, cobble and boulder-sized material deposited directly from glacier ice. As such, Hughes et al. (1998) correctly pointed out that in terms of soil mechanics, till is a non-textbook material, in that it is characteristically neither clay nor sand and does not conform to the depositional models upon which, much of soil mechanics is based. Furthermore, lack of appreciation of the effects of depositional and post-depositional processes on the geotechnical properties of tills can lead to engineering difficulties.

In the past, tills often have been regarded as heavily overconsolidated deposits, and indeed many tills are. For example, Klohn, 1965, Radhakrishna and Klym, 1974 have described such tills from North America. Heavily overconsolidated tills, being relatively incompressible, undergo relatively little settlement when loaded. However, not all tills behave in this way, and less stiff tills can deform when loaded with heavy structures associated with many industrial plants, bridge piers and the like. Therefore, the more geotechnical data there are available on till deposits the better, as this can only lead to more effective understanding of their engineering performance.

In eastern England, the maximum extension of ice during the Pleistocene occurred during the Anglian (Lower Illinoian) stage, when it spread southwards almost to the Thames estuary (Fig. 1). The extension of ice during Late Devensian (Wisconsin) times was restricted to northern England, with a lobe extending along the east coast southwards to northwest Norfolk (Catt, 1991). Accordingly, only a very small area of East Anglia was glaciated during Late Devensian times. In addition, Late Devensian glacial activity removed nearly all traces of Anglian deposits from those areas covered by this later ice. Most of the tills deposited in eastern England are regarded as lodgement tills. Lodgement tills are derived from rock debris carried at the base of a glacier and are the predominant type of till which occurs in glaciated lowland areas. Generally, because of glacial abrasion and grinding, the proportion of silt and clay in lodgement till is relatively high.

Glacial deposits, notably tills, occur extensively along the eastern coastal areas of England, from Northumberland in the north extending southwards into East Anglia (Fig. 1). The tills frequently are well exposed in cliff sections, as in north Norfolk and Holderness, two of the study areas. Unfortunately, the till deposits around the Tees estuary, the third study area, are not well exposed over this flat and rather featureless area. Furthermore, large parts of this industrial area are built over. With the exception of some of the tills in Northumberland which have been investigated by Eyles and Sladen, 1981, Robertson et al., 1994, Hughes et al., 1998, Clarke et al., 1998, little investigative work on the geotechnical properties of the tills of the east coast of England has been reported in the literature. Accordingly, the three areas referred to were chosen from which to collect samples. Their geotechnical properties were determined and compared with those of the tills from Northumberland.

The samples from north Norfolk and Holderness were obtained mainly from coastal exposures, except that some were taken from abandoned clay pits. They were all block samples, which were wrapped and sealed before being packed in containers. The samples from Teesside were obtained from a dozen boreholes by a light cable and tool rig, the samples being retrieved by U100 sampling tubes. All the boreholes were less than 16 m in depth, except one which extended to 26.5 m.

In Norfolk, glacial deposits overlie earlier sediments of marine, estuarine and freshwater origin. The area was invaded more than once by ice sheets but just how many advances and interglacial stages occurred, has been keenly debated (Mitchell et al., 1973, Lewis, 1999). Hence, the stratigraphical succession has not been worked out to the satisfaction of everyone. This is to be expected because of the complicated nature of some of the stratigraphy of the glacial deposits in this area. By comparison, the glacial deposits of the other two areas are simpler in terms of their stratigraphy.

The glacial deposits of the north of Norfolk (Fig. 2) rest on the Cromer Forest Bed Formation, the Weybourne Crag or directly on the Chalk. The earliest till deposits belong to the Cromer Till, which occurs in the lower part of the Anglian stage. The Anglian stage extended from 500,000 to 350,000 years bp. Mitchell et al. (1973) recognised three divisions of the Cromer Till, namely, the First, Second and Third Cromer Tills, although this subdivision is not universally agreed. The Third Cromer Till includes the Contorted Drift (Reid, 1882), which occurs around Trimingham. Around Weybourne, the till deposits include much chalky material and consequently have been referred to as the Chalky Boulder Clay by Baden-Powell (1948). According to Mitchell et al. (1973), the Chalky Boulder Clay marks the end of the Anglian stage. No further till deposits occur in north Norfolk until Late Devensian times when the Hunstanton Till was laid down. It occurs in small patches in northwest Norfolk.

The till deposits of Holderness are well displayed along the coast from Bridlington to Dimlington (Fig. 3). Four units have been recognised, namely, the Basement Till (referred to as the Bridlington Till by Lewis, 1999), the Skipsea Till, the Withernsea Till and the Hessle Till. The Basement Till was considered to have been deposited during the Wolstonian (late Illinoian) glaciation by Mitchell et al. (1973). This stage occurred between 300,000 and 175,000 years bp. The other tills are of Late Devensian age, being deposited between 18,000 and 13,000 years ago (Penny et al., 1969. However, Madgett and Catt (1978) concluded, on the basis of colour, mineralogical composition and particle size distribution, that the Hessle Till was not a discrete till unit but was composed of both weathered Skipsea Till and weathered Withernsea Till.

Two tills have been recognised in the Teesside area, i.e. the Lower Boulder Clay and the Upper Boulder Clay (Fig. 4). These two tills are separated by sands (Agar, 1954). Smith (l981) maintained that the two tills had significantly different erratic suites and clast fabrics. This tripartite division also has been recognised in the glacial deposits of east Northumberland, Durham and northeast Yorkshire (Hughes et al., 1998). These tills are of Late Devensian age (Thomas, l999).