to download report - Geological Survey of Ireland

Steep slopes in glacial tillMichael LongMuch of Dublin is underlain by competent lodgement till know locally as Dublin Boulder Clay, (DBC) (Skipperet al. 2005). Local experience (Long et al., 2003) confirmed that steep excavations, up to 8 m or so, couldstand unsupported for periods of at least three to four months. Since for many developments temporary supportis only required for short periods engineers have been attempting to use this natural property of the soil for thepurposes of deep excavation construction in order to avoid costly retaining walls or soil nailed support systems.Work is ongoing at UCD in order to understand the mechanical behaviour of the DBC in these situations.Initially it was thought that the soil possessed a high effective cohesion (c’) or some cementation bondingbetween the particles. Laboratory triaxial testing and scanning electron microscopy studies on high quality(triple tube rotary cored) soil samples have confirmed that neither of these factors is significant. Instead it hasbeen concluded that the temporary stability of these steep slopes is controlled by near surface negative porewater pressures (suctions) induced by stress relief due to soil excavation. The effect of sand and gravel lenseswithin the till in reducing or eliminating the suction were found to be very significant.These findings have been confirmed by measurements of suctions during the construction of the northern cutand cover section of the Dublin Port tunnel (Long et al., 2004) and by back analysis of the behaviour of thesteep cuts using the finite element method. This latter work has been carried out by the Geotechnical ConsultingGroup, London (GCG) assisted by UCD (Menkiti et al. 2004).Strength of peat at low effective stresses.Eric Farrell and Martin CarneyRecent landslide events in peats have highlighted the difficulty in predicting the relevant shear strength parametersfor such soils. The permeability of peats is such that it is questionable if undrained shear strength parametersare relevant, particularly as different values of c uare obtained when using different size vanes in in-situ tests.The effective stress parameters determined in laboratory tests generally indicate cr≈ 0, however it is difficult tocarry out such tests at low effective stresses as the membrane forces and other equipment effects can becomesignificant. Furthermore, different values of the effective stress parameters are obtained with different testmethods.The objective of this testing programme is to develop an entirely new test method to determine the effectivestress parameters of peat, particularly an assessment of cr. Peat is known to have high values of φ’ but thiswould not be expected to be a significant contribution to strength where σ nr≈ 0. This new approach will involvetesting relatively large block samples of peat in conditions where the boundary effects are minimal. A videoextensiometer will also be used in the test to enable the deformation pattern of the peat to be studied using asit approaches failure. These tests will give valuable information on the strength of peat at effective stress levelscomparable to those that exist in raised and blanket bogs, which are known to be susceptible to bog bursts.Landslides. A problem for the future?Daragh McDonaghThe main topics covered in this study were:- the economic significance of landslides, landslide types andprocesses, landslide triggering mechanisms, principles of hazard reduction and risk assessment and decisionmaking under certainty and uncertainty of landslide activity.The author reviewed landslides in Ireland in general, noting the greater occurrence in the west and south onupland blanket bogs, during autumn and winter months. The author analysed the different causes of landsidetriggering with particular reference to the two most recent landslides; in Pollatomish, Co. Mayo and in Derrybrien,Co. Galway. An in-depth study of the landslide in Pollatomish was carried out, the primary cause of which wasintense rainfall over a nine hour period.The socio-economic significance of landslides is emphasised because landslide losses continue to grow ashuman development expands into unstable hillside areas under the pressures of increasing population. Asignificant proportion of world landslide losses involves transportation - highways, railways, rivers and pipelines.The nation most severely affected by landslides is Japan, which suffers estimated total (direct plus indirect)87