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Plate 4.1 Herringbone drainage system being installed in cut slope of glacial till.4.4 Geotechnics of landslides in ‘mineral’ soilsThe overburden deposits in Ireland are predominantly glacially derived soils which have been deposited incomplex geological conditions. There are also areas of recent fluvial deposits in our rivers and estuaries, andsoft lacustrine soils in our lakes. The recent alluvial and lacustrine soils are deposited in low lying areas and donot in themselves give rise to landslides unless fill is placed on the surface or material dredged or excavated.There are local areas of overconsolidated clay soils which were laid down in interglacial conditions. These claysoils would typically have significantly lower angles of shearing resistance than the more typical glacial tillsand, where encountered, may require special attention.Glacial soils can be deposited in a number of ways, generally in a complex depositional environment, and theirmethod of deposition and the variability of the deposits can have an important bearing on the risk of landslidesoccurring. Glacial deposits include, for example:-lodgement tillsenglacial tillsglaciofluvial depositsglaciolacustrine deposits.Lodgement and englacial tillsIrish lodgement and englacial tills generally have relatively high angles of shearing resistance (φr) and low if anyeffective cohesion intercept (cr), and are generally well graded with sufficient fines to make them appear to bea ‘cohesive’ soil in the short term. These tills are colloquially called boulder clays, although boulders may notalways be present. Typically the angle of shearing resistance of Irish lodgement and englacial tills would bebetween 30 o and 35 o (Hanrahan 1977, Farrell & Wall, 1990) and have residual shear strength parameters closeto the peak φr values (Loughman, 1979). A survey of 150 year old railway slopes on the southwest region of theIrish rail network recorded an average slope inclination of 38 o for slopes with an average height of about 5m(Jennings, 2003). About 90% of all cut slopes surveyed were at a slope angle greater than 30 o . Very steepslopes can be cut into these soils in the short term, however this short term stability arises from soil suctionforces which will dissipate with time and give rise to slope failure. There is little evidence of cementationbetween particles which would give a cohesion intercept. Glacial soils are formed in very complex geologicalconditions and this does result in significant variations within deposits. As a consequence of this, free drainingsand and gravel layers frequently occur within what would otherwise be considered boulder clay, giving rise tothe risk of internal erosion as discussed previously. The normal long term stable slope angles adopted inpractice in these lodgement and englacial tills is 2H to 1V, with herringbone drains used to intercept water flowas required to prevent internal erosion as discussed above.26