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The Engineering Properties of Glacial Tills

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The Engineering Properties of Glacial Tills Cite this article Research Article Keywords: geology/geotechnical Clarke BG Paper 1800020 engineering/site investigation The engineering properties of glacial tills. Received 23/03/2018; Accepted 15/06/2018 Geotechnical Research, https://doi.org/10.1680/jgere.18.00020 Published with permission by the ICE under the CC-BY 4.0 license. (http://creativecommons.org/licenses/by/4.0/) Geotechnical Research The engineering properties of glacial tills Barry G. Clarke PhD, FGS, FICE, CEng, Eur Ing Professor of Civil Engineering Geotechnics, School of Civil Engineering, University of Leeds, Leeds, UK ([email protected]) (Orcid:0000-0001-9493-9200) Glacial tills are a product of the glacial processes of erosion, transportation and deposition and could have been subjected to several glacial cycles and periglacial processes to the extent that they are complex, hazardous soils that are spatially variable in composition, structure, fabric and properties, making them very difficult to sample, test and classify. An overview of the formation of glacial tills and their properties shows that they are composite soils which should be classified according to their lithology, their mode of deposition to link the glacial processes with the facies characteristics and their engineering behaviour. This enables representative design properties to be assigned using frameworks developed for composite soils. Notation properties. Hence, tills should be categorised according to a A activity lithological classification (e.g. BS EN ISO 14688-1:2002 Cc coefficient of curvature +A1:2013 (BSI, 2002)) as well as a genetic classification (e.g. Cc-g global compression index Giles et al., 2017), to link the glacial processes with the facies Cu coefficient of uniformity characteristics, and according to a classification scheme based on cu undrained shear strength their engineering behaviour (e.g. Clarke, 2017). This paper d10 particle size of 10% of the particle size distribution explains the reasons for this by providing an overview of the e, f constants formation of subglacial tills and the impact that it has on their eg global void ratio characteristics, and techniques that could be used to generate eg100 global void ratio at an effective stress of 100 kPa design properties. The spatial variability of tills means that it is ei intergranular void ratio difficult to assign representative characteristic design values. eL void ratio at the liquid limit Using knowledge of the formation of tills and composite soil em matrix void ratio behaviour, a ground investigation strategy is introduced to IL liquid limit improve the quality of the results. kh coefficient of hydraulic conductivity mv coefficient of volume compressibility Formation pa atmospheric pressure Over the years, glacial geologists have developed classification ϒd dry density schemes for till based on the modes of transport (i.e. subglacial, s 0 v effective vertical stress glacial or supraglacial) and deposition (e.g. deformation, f0 angle of friction lodgement, melt-out and comminution) (e.g. Benn and Evans, 2010). The current view is that glacial tills are a result of Introduction (a) deformation (glaciotectonite), (b) a combination of deposition Glacial tills, which are extensive throughout the temperate and deformation (subglacial traction till) or (c) deposition alone zone, are complex, hazardous soils that are spatially variable in (melt-out till). Glaciotectonite and subtraction tills are generally composition, structure, fabric and properties, making them very much denser than melt-out tills since those deposits are subject to difficult to sample, test and classify (Clarke, 2017; Griffiths and shear as well as gravitational forces during deposition. Glacial Martins, 2017). They are a product of the glacial processes of tills may be the result of several periods of glacial and periglacial erosion, transportation and deposition and could have been activities creating complex structures that are difficult to construct subjected to several glacial cycles and periglacial processes to the from borehole information. extent that interpretation of investigations can be challenging. Glaciers sliding over the substrate, formed of solid and superficial There are four categories used in the classification of soils for geology, initially deform the substrate to create glaciotectonite engineering purposes: very coarse-grained, coarse-grained, fine- which retains some of the original structure of the parent substrate grained and organic soils. Glacial soils can be composed of one or (e.g. Banham, 1977; Benn and Evans, 2010; Pedersen, 1988). more of these categories such that their engineering behaviour is Tectonic features are developed including brittle shear planes, not consistent with the engineering classification (Clarke, 2017). faults and ductile folds. Extensive shear strain can produce All glacial tills are subject to gravitational forces during laminations, with distinctly different soils between the deposition, and many are also subject to shear. Thus, it is possible laminations. Pods of stiffer material are generated because of the to have tills with the same composition but distinctly different variation in stiffness in the substrate and because of the local 1 Downloaded by [ University of Leeds] on [03/08/18]. Published with permission by the ICE under the CC-BY license Geotechnical Research The engineering properties of glacial tills Clarke Layer of sands and gravels Multiple layers Lens of water-bearing or laminated clay of glacial till sands and gravels Lens of laminated clay Lens of weak clay Dropstones Laminated clays Bedrock Rafted bedrock Structural features within till Sand and gravel infill Boulder beds Figure 1. Features of subglacial tills highlighting the challenge of creating a three-dimensional image of glacial tills because of structural features associated with deformation and the difficulty of identifying bedrock due to rafted rock and boulder beds, lens and layers of weaker clays/water-bearing sands and gravels and dropstones (after Clarke (2017)) temperature and pore pressure regimes. Glacial movement, with substrate exceeds the frictional drag of the glacier. Subglacial time, will start to erode the substrate by either abrasion (Hallet, traction tills are usually very dense with a low water content 1979) or quarrying (Iverson, 2012), the amount depending on the because of the combination of the pressures of the ice and shear. glacier velocity, the temperature and pressure at the base of the They are sometimes described as overconsolidated because they glacier, the interface friction between the ice and the bed, a are dense rather than being as result of the geotechnical process of function of the bed roughness and the strength of the underlying consolidation. They are often fissile with potential slip planes, sediment. Further deformation leads to increasing disaggregation of the sediment and abrasion, thus breaking down the sediment Undrained shear strength: kPa until it is eventually completely homogenised (Elson, 1961). 0 100 200 300 400 500 600 700 800 Progressive crushing and abrasion result in a fractal particle size 0 distribution with a dimension of 2·65 (Iverson et al., 1996). The 5 eroded material forms glacial debris, which can be transported 10 OCR = 20 considerable distances by ice before it is deposited as subglacial 15 traction till or melt-out till depending on whether the ice is 20 advancing or retreating. Depth: m 25 30 A vertical profile (Figure 1) through the substrate may show a 35 sequence ranging from undisturbed sediment at depth to OCR = 1 OCR = 5 OCR = 10 40 Strength = 4·9 SPTN σ 0·8 completely homogenised till at the top with no distinct boundaries Triaxial strength cu = 0·3 v’OCR between the layers. If the substrate is a glacial soil, then the 45 deformed substrate can contain remnants of previous glaciations, Figure 2. An example of a comparison of the variation in the treacherous pockets of water-bearing sands and gravels and undrained shear strength (estimated from standard penetration softer clays highlighted by Ansted (1888), for example. tests (SPTN) and overconsolidation ratio (OCR)) with depth highlighting the difficulty of selecting a design profile because of Subglacial traction till results from the lodgement of glacial debris the effects of composition and fabric on matrix-dominated till. This example is from north-eastern England (after Clarke (2017)); beneath a glacier by pressure melting or other mechanical examples from other parts of the UK can be found in the paper by processes (e.g. Dreimanis, 1989) against an obstruction or when Culshaw et al. (2017) the frictional resistance between a clast and the underlying 2 Downloaded by [ University of Leeds] on [03/08/18]. Published with permission by the ICE under the CC-BY license Geotechnical Research The engineering properties of glacial tills Clarke suggesting shear during deposition. These tills have bimodal or subglacial drainage and pore pressure (Table 1) as they control the multimodal particle size distributions with distinct rock flour and rheology and strength of glacier beds and the glacial motion. It is gravel ranges. Cobbles and boulders are aligned with the direction now accepted that till undergoes deformation (at low effective of the ice flow. pressures) and lodgement and ploughing (at high effective pressures) (e.g. Brown et al., 1987; Evans et al., 2006; Hart, Melt-out till is formed of englacial and supraglacial debris 1995; Hart and Boulton, 1991; van de Meer et al., 2003). The deposited from stagnant or slow-moving ice without further effective pressure can vary laterally, creating areas of low and transport or deformation (Benn and Evans, 2010). The clast high effective stress that change with time (e.g. Alley, 1993; content reflects high-level transport in which particles retain their MacAyeal et al., 1995; Piotrowski et al., 2004; Stokes et al., angularity. Melt-out till is generally poorly consolidated because it 2007). Distinct till fabric is created (e.g. Andrews, 1971; Benn, has not been subjected to high pressures or shear. Therefore, it has 1995; Carr and Rose, 2003; Dowdeswell and Sharp, 1986; Hart, a relatively low density compared to other tills.
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