Detailed Slope Stability Analysis and Assessment of the Original Carsington Earth Embankment Dam Failure in the UK


Dr. Costas I. Sachpazis

Civil & Geotechnical Engineer (BEng (Hons) Civil Eng. UK, Dipl. Geol, M.Sc.Eng UK, Ph.D. NTUA (?.?.?.), Post-Doc. UK, Gr.m.ICE).
Associate Professor, Lab of Soil Mechanics, Department of Environmental Engineering, Geotechnical Engineering Division, Technological Educational Institute of Western Macedonia. Killa 50100, Kozani, Greece.
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A 1225 m long, 35 m high zone earth filled embankment was being constructed from 1981 to 1984 from a British Regional Water Authority to regulate flows in the River Derwent in England. The Carsington Dam was planned to be one of the largest earth filled dams in Britain. Its reservoir capacity was 35 million m3 and the watertight element was Rolled Clay Core with an upstream extension of boot shaped and shoulders of compacted mudstone with horizontal drainage layers of crushed limestone about 4 metres apart and a cut-off grout curtain (Davey and Eccles, 1983).
The downstream slope was 1:2.5 and the upstream slope 1:3. Fill placing began in May 1982 and took three summers, with winter shutdowns. In August 1983 a small berm was placed at the upstream toe to compensate for a faster rate of construction. Earth filling restarted in April 1984 and was one metre below the final crest level on 4 June 1984 when the upstream slope slipped (Skempton, 1985). Observations of pore pressure and settlement were made during construction at four sections and horizontal displacements were observed from August 1983. The Carsington Dam was almost completed on 1984. However, at the beginning of June 1984, a 400-m length of the upstream shoulder of the embankment dam slipped some 11 m and failed. At the time of the failure, embankment construction was virtually complete with the dam approaching its maximum height of 35 m. Horizontal drainage blankets were incorporated in both the upstream and the downstream shale fill shoulders. Piezometers had been installed and pore pressures were being monitored in the foundation, in the clay core, and in the shoulder fill. The failure surface passed through the boot shaped rolled clay core and a relatively thin layer of surface clay in the foundation of the dam. Investigation of the events at Carsington has made important contributions to the fundamental understanding of the behaviour of large earthworks of this type (Vaughan et al., 1989; Dounias et al., 1996).
The objective of this research is to evaluate a detailed slope stability assessment of the Carsington Earth Embankment Dam in the UK used to retain mine tailings. By using and applying advanced geotechnical engineering analysis tools and modelling techniques the Carsington Earth Embankment Dam, which is considered a particular geotechnical structure, is analysed. In the current detailed slope stability analyses the total and effective stress state soil properties / parameters were used, and the most critical slip circle centre according to Fellenius - Jumikis method was initially determined. Subsequently, the Carsington Earth Embankment Dam and its foundation was analysed and examined against failure by slope instability. Considerations of loading conditions which may result to instability for all likely combinations of reservoir and tailwater levels, seepage conditions, both after and during construction were made, and hence three construction and / or loading conditions were examined in particular:
• The right after Construction Condition,
• The Steady Seepage Condition, and
• The Rapid Drawdown Condition in the reservoir
In this context, the slope stability at the three above mentioned discrete loading cases of the Carsington Earth Embankment Dam was analysed and presented, and certain valuable conclusions concerning the overall stability conditions of the Earth Embankment Dam during its original construction are deduced in this research paper.
In addition, for comparison reasons, a Slope Stability Analysis of the Carsington Earth Embankment Dam during loading case (a) using Taylor’s curves was performed. Furthermore, the Shear Strength Reduction (S.S.R.) Analysis Method, based on the Finite Element Analysis technique (F.E.A.), was executed, for purposes of verification of the Global Slope Stability Analysis of the whole Carsington Earth Embankment Dam for the Loading case (a), i.e. right after construction condition of the Dam but prior to its filling with water, which proved that the results between the Shear Strength Reduction (S.S.R.) Analysis Method and the Limit Equilibrium Analysis Method (LEM) based on the method of slices are comparable and similar. Finally, the reasons why the Fellenius - Jumikis method is inaccurate were examined, analysed and explained, as well as the technical lessons learned from this large scale Earth Embankment Dam body failure were pointed out.

Keywords: Slope Stability Analysis; Earth Embankment Dams; Slope Failure; Embankment Loading Conditions; Soil Properties / Parameters; Critical Slip Circle Centre Determination; Steady Seepage Condition; Rapid Drawdown Condition; Shear Strength Reduction Analysis Method; Fellenius - Jumikis Method; Computer Aided Slope Stability Analysis & Design.

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