Embankment dam behaviour – prediction of arching and cracking potential
Formulator: George Dounias (EDAFOS)
Internal erosion in embankment dams is one of the most common problems that makes for a large proportion of reported dam incidents. One of the recognized mechanisms for the initiation of internal erosion is cracking due to excessive arching in dams with a central core. The issue has been studied extensively in the past but it has never lost its appeal as one of the major design considerations.
The stresses in a central dam core can reduce due to “arching”, in which settlement of the core occurs relative to the embankment shoulders. Side shear then develops on the boundary between the core and shoulders, load is transferred from the core to the shoulders, and the total stresses in the core are reduced. The potential effects of arching increase as a core becomes narrower and exacerbated with the presence of a core trench.
Hydraulic fracture occurs when the seepage pressure exceeds the minimum principle total stress (approximately) by an amount greater than the tensile strength (which is usually negligible). Cracking may lead to internal erosion if it is not prevented by a filter downstream of the crack. The internal erosion is the subject of a recent Bulletin by ICOLD.
Closed form solutions have been developed for the reduction in stress in a central core due to arching but nowadays numerical studies (e.g. finite elements) are used. The analyses show that minor principal stresses at the end of construction are usually less than the eventual seepage pressure which will be applied by the reservoir. In most cases though the total stresses are increased considerably during impoundment and hydraulic fracture is averted.
The above mechanisms will be investigated on the relatively well documented case of Mornos Dam, Greece, in operation since 1978. The Mornos is an earth dam 139 m high with central clay core. The central core is made of alluvial clay and of weathered flysch mudrocks. The core is founded on the bedrock with a wide trench through the 10m deep river gravels. The shoulders are made of well compacted river gravels. The dam is fairly well instrumented and data exist for its 35 years of operation, including the reservoir fluctuations, the development of pore water pressure, deformations and vertical total stress.
Numerical analysis in 2 dimensions (plane strain) will be used in order to estimate the overall stress and strain development at three phases.
Phase 1, End of construction: Prediction of pwp, total stresses and construction settlements – Arching and cracking potential estimation.
Phase 2, After first impoundment: Prediction of pore water pressures, total stresses and deformations – Arching and cracking potential estimation.
Phase 3, After many reservoir fluctuations over 37 years: Prediction of evolution of pore water pressures, total stresses and deformations – Arching and cracking potential estimation.
The main goal of this theme is to investigate the ability to accurately predict the embankment behaviour and in particular the arching potential and the risk of cracking. Through the presented analyses, the best practices regarding the modelling of embankment dams will be investigated.
The instructions and data can be downloaded from the following link