The shear strength parameters of the "Athenian Schist" are estimated through the back analysis of a slope failure. The shape of the failure surface for the particular geological formation is also examined and relevant proposals are made.
Les paramètres de la resistance de cisaillement du Schiste Athenien sont evalues au moyen d' une analyse retrogressive (back analysis) de la rupture du talus. La forme des surfaces de rupture pour la formation geologique consideree est egalement examinee et des propositions appropriees sont faites.
Die Scharfestigkeitsparameter den "Schiefern von Athen" werden aus der Böschungsgleitungsrueckanalyse gerechnet. Fuer die vorhandene geologische Formation wird zusatzlich die Gestaltung der Gleitungsflache studiert und werden die verhaltnismaβige Antrage vorgeschlagen.
The term "Athenian Schist" concerns to the complex geological formation which exists in the greater Athens area. It consists of clayey schists, sandstones, marls, flints, quarzites and limestones, in many varieties and combinations. Sometimes, due to the feeble metamorphism, chloritic and sericitic schists as well as phyllites can be found in great extent. The most characteristic feature of this geological formation is the quick alternation of the aforementioned lithological phases, within rather short distances, resulting in its highly anisotropic behaviour. The anisotropy is stressed by the intense folding of the formation, the different degree of fracturing and weathering from place to place and the presence of water. Thus, the estimation of the shear strength parameters of the "Athenian Schist" presents many difficulties. Borehole sampling and laboratory testing in the Athenian Schist usually fail to provide accurate values of the shear strength parameters of the whole rock mass, even in cases where the formation is more or less uniform. On the other hand, there is very little experience in estimating the behaviour of the formation based on the evaluation of results of in-situ tests, like pressure meter and dilatometer. Hence, the design of slopes in the "Athenian Schist" is mainly based on empirical estimations and modelling of the formation's behaviour, which, in most cases, hardly approximate the actual in-situ conditions. An attempt is being made in this paper to estimate the shear strength parameters of the "Athenian Schist" through back analysis of a slope failure, taking into account the effect of the shape of the failure surface.
The Strefi Hill is one of the seven hills encountered in the central Athens area, consisting of "Athenian Schist" and marly limestone, overlying the schist (Figure 1). More specifically, in the area of the failed slope the following stratigraphy is observed: * Surface weathering mantle of the schist, 0.80 1.50 m thick. * "Athenian Schist" in the form of grey-white to yellow-white very fractured and weathered clayey schist to phyllite. The formation presents very dense schistosity (up to 70 layers per meter were measured) and is very folded. The great number and the irregular shape of the folds give an indication of the intense tectonism of the formation. The surfaces of the discontinuities of the schist are covered by red-yellow to brown-yellow oxides. Pieces of the formation are easily extracted by hand from its mass leaving a talcy feeling, indicative of the phyllite weathering. At the east end of the slope (left side looking to the slope, see Figure 2) traces of water were observed and it is considered that it was coming from the sewage of the neighbouring house. Hence, it can be concluded that the schist is permeable, but this permeability is owed solely to the discontinuities and depends on their conductivity.