Now showing 1 - 7 of 7
  • Publication
    Evaluation of peat strength for stability assessments
    (Thomas Telford Ltd., 2013-04-03) ;
    n this paper guidance is given for the assessment of peat strength for stability assessments based on laboratory undrained simple shear tests. When considering the stability of peat, these tests will yield a conservative estimation of the in situ strength of the peat mass. The study was motivated by recent interest in renewable energy developments in upland peat areas. The results of more than 111 simple shear tests from 16 sites in Ireland, Scotland and the Netherlands were studied. It was found that the strength of peat is strongly influenced by its stress history, and also varies as a function of the water content and degree of decomposition (fibre content). The normally consolidated normalised strength ratio (s u/σ′v) from simple shear tests of peat was found to be approximately 0·4, which is towards the lower bound of previously published data for peat. Comparisons of strengths derived from simple shear and field vane tests showed that the ratio of the strength derived from the two tests was influenced by the degree of decomposition, and that previously published correction factors for field vane strengths are inappropriate. Guidance is given for engineers working on future schemes on upland peat areas.
  • Publication
    Development of a direct simple shear apparatus for peat
    This paper discusses the design and development of a new direct simple shear (DSS) apparatus for testing peat soils. The apparatus has been designed to test peat at low effective stresses, representative of its in-situ condition and allow the deformation of the specimen to be monitored. This device uses Particle Image Velocimetry (PIV) image analysis techniques to monitor the side of the peat specimen and provide an insight into the behaviour of peat during shearing. A set of comparative tests on remoulded clay have been conducted with another widely used DSS apparatus and has shown both to yield similar undrained strength ratios (su/σ'vc) for a range of stress levels. Application of the apparatus to peat soils is demonstrated by a set of tests on a high water content blanket bog peat. Analysis of these tests using the PIV technique reveals the complex shear strain and volumetric strain behaviour of peat undergoing shearing. Identification of partial slippage of a specimen is also demonstrated through these analyses.
  • Publication
    In situ strength characterisation of peat and organic soil using full-flow penetrometers
    Full-flow penetrometers have been shown to overcome problems experienced with the cone penetrometer when measuring resistance in very soft peat and organic soil, and give a much more uniform measure of resistance than the cone in fibrous peat. However, at present there is no guidance on the interpretation of strength parameters in these soils using the T-bar and ball. This paper examines the results of tests using these devices at two research sites in the Netherlands in conjunction with high-quality Sherbrooke sampling for laboratory testing. In fibrous peat, the T-bar and ball provided a more uniform measure of resistance with a lower degree of scatter than the cone. The in situ testing results have been compared with the laboratory tests to assess the range of resistance factors relating penetration resistance to the undrained shear strength (su) and have been shown to occupy a lower range of values than the cone penetrometer. However, penetration tests in these soils are likely to be influenced by partial drainage effects and this should be considered during testing and the subsequent interpretation of results. Recommendations are made for the use of full-flow penetrometers to obtain strength parameters in these soils.
      967Scopus© Citations 34
  • Publication
    An investigation of two peat slope failures in the Wicklow mountains
    (Royal Irish Academy, 2010-12) ;
    Although peat slope failures have occurred in Ireland for many thousands of years their causal factors and the triggering mechanisms involved are poorly understood. A particular barrier to quantitative assessment of the risk of failures is the lack of knowledge of the geotechnical properties of peat and its role in failures. In order to advance the understanding of these issues case history data is invaluable. This paper describes a case study from the Wicklow mountains where a desk and remote sensing based study was used to identify peat slope failures. Subsequently detailed field studies were carried out at two failure locations. It was found that although the full causal factors at the time of failure are unknown, a common factor was that the failure took place in a zone of highly decomposed and relatively low fibre content peat. A revision of the standard test method for fibre content for use in peat soils is proposed.
      2357Scopus© Citations 6
  • Publication
    Predictions of settlement in peat soils
    (Geological Society of London, 2013-08-08) ;
    Predictions from laboratory tests of the compression behaviour of peat from 14 sites are compared with full-scale field loading at five sites. Data presented confirm the heterogeneous nature of the deposits. However, for typical engineering works, calculations based on laboratory test data are likely to give reasonable predictions of the magnitude of immediate and primary compression. Standard (20 mm thickness) samples may give misleading data on time for primary consolidation. Thicker samples (e.g. 50 mm) should be used. Sampling by conventional samplers, as used for mineral soils, can cause densification of the peat, resulting in underestimation of actual settlement. Block samples or sample tubes with serrated cutting edges are recommended for peat soils. It was found that the data presented follow the Cα/Cc law of compressibility. There is also is some evidence to suggest that the H2 scaling law may be applicable. Good correlations were found between vertical yield stress (pvy′) and compression index (Cc) and index parameters such as water content (wi). Conventional staged construction with surcharge loading may be successfully applied to peat soils as long as adequate drainage exists to permit consolidation over reasonable time intervals.
      1955Scopus© Citations 35
  • Publication
    Peat slope failure in Ireland
    (Geological Society of London, 2008-02) ; ;
    Recent peat failures in Ireland in the autumn of 2003 at Pollatomish, County Mayo and Derrybrien, County Galway have focused attention on such events. However, peat failures are not a recent phenomenon with possible evidence of peat failures in Ireland having been identified as far back as the Early Bronze Age. This paper summarises the issues surrounding peat failures in Ireland that would be of interest to an engineer\engineering geologist assessing this geohazard. The distribution of peat throughout Ireland, its formation, and its typical characteristic properties are discussed. A review of historical failures shows that there is a relationship between run out distance and failure volume and that the majority of the failures are clustered at slope angles between 4º and 8º. It seems that the risk of fatalities from peat slides is relatively low. The likely casual factors attributed to peat failures are presented using examples of failure, including the recent failures at Pollatomish and Derrybrien both of which have been investigated by the authors. Particular attention is paid to shear strength properties of peat and the applicability of traditional soil mechanics. Given the uncertainties which exist about peat strength, a cautious approach to slope stability assessment is advocated together with identification of potential causal factors to mitigate against this geohazard.
      1859Scopus© Citations 59
  • Publication
    In situ testing of peat – a review and update on recent developments
    (Southeast Asian Geotechnical Society, 2012-12) ;
    This paper reviews the techniques used and some recent developments on in situ testing of peat for the purposes of the design and surveillance of engineering structures. Geophysical techniques, especially ground penetrating radar, are now being used extensively in peatlands. All geotechnical in-situ tests in peat can be influenced by partial drainage and therefore can give misleading results if not used carefully to well established guidelines and if not interpreted correctly. There is therefore a benefit in multi-measurement tests (e.g. CPTU and piezoball) which give additional information to help assess the drainage condition. There seems promise in the use of the pore pressure measurements for both CPTU and piezoball for the purposes of profiling peat decomposition and possibly shear strength assessment. Field vane testing will frequently give misleading results. Other standard geotechnical techniques may only be useful when used in conjunction with locally derived empirical correlations. Pore pressure measurements in peat may be influenced by the presence of gas in the deposits.