Geotechnical Properties of Irish Glacial and Interglacial Soils

The infrastructural developments and building works undertaken in Ireland over the last thirty years or so since Professor Eamon Hanrahan published his seminal work on Irish Glacial till have given considerable data on the properties and behaviour of these soils which are ubiquitous in this country. Added to this, research has given valuable understanding of the geotechnical properties and recent geological studies have revealed new insight into the chronology and conditions during the Pleistocene epoch when these soils where formed.

The recent developments in the understanding of the conditions prevalent during the ice age are reviewed and the locations where interglacial soils have been encountered will be discussed. The geotechnical properties of one interglacial deposit are presented.

The presentation will focus of the properties of unsorted ‘boulder clay’ type soils which are the most common type of deposit and which offer particular challenges due to the difficulty in obtaining good quality samples. Information from research and from practical experience is reviewed to document the current state of knowledge into the properties of these soils. This review will discuss a variety of issues, including the variation of the permeability of boulder clay type soils with relevant geotechnical properties, which is relevant to the design of flood protection embankments and other applications, and the difficulties of determining their stiffness and consolidation characteristics for foundation and settlement analyses. The experience gained from a large database of pile testing in Ireland is used to show that the relatively crude SPT test can give useful design information. The effective stress parameters of these soils are discussed in relation to the design of cut and embankment slopes to meet Eurocode requirements and the results of laboratory earthworks testing are compared with on-site experience.

The Irish Sea till, which is to be found along the south east and south coast of Ireland, differs from the boulder clay type soils in both grading and effective stress parameters. The geotechnical properties of some of these soils, for example the Macamore Clay, are particularly relevant to coastal regions where sea erosion gives rise to slope failures. The effective stress properties of these soils are discussed in relation to slope stability issues.