Erosion of a cut in stratified sandstone and siltstone over a ten year period resulted in a surface profile conforming to the zero stress contour calculated in the initial excavation. This paper shows stress contours calculated using a two-dimensional finite-element analysis and the configuration of the eroded cut, demonstrating that the calculated zero stress contour may be used to predict the equilibrium surface of the excavation.
L' erosion d' un versant d' excavation au couches greseuses et pelitiques, pendant la duree de 10 ans, a produit une surface de section correspondante à la ligne de tension egale zero, qui a se calcule pour l' excavation primitive. A cette etude se presentent les lignes des tensions egales qui ont calcule par la methode des elements finis à deux dimensions et se conclut par la forme du versant erode, que la ligne de tension egale calculatrice, peut utiliser pour la prevision de la surface d' equilibre d' excavation.
Die Erosion an einer von Flysch besetzten Abtragsböschung, hat in dem Zeitintervall von 10 Jahre ein Hangschnitt geschaft, das die Iso-spannungslinie null der Abtragsböschung entspricht. In der vorliegenden Arbeit werden die Spannungslinien dargestellt, die mit Hilfe der zweidimensionalen Berechnung der Finite-Elemente Methode bestimmt wurden. Von der Form der erodierten Abtragsböschung, wird die Schlussfolgerung gezogen, dass die berechnete Spannungslinie null, fuer die Vorbestimmung der standsicheren Abtragsflache benutzt werden kann.
Unstable regions in rock slopes may be identified through negative minor principal stresses (Bukovansky-Piercy, 1975, Kalkani, 1975) because of the comparative weakness of rock under tension. In case of inclined stratified rock, such as sedimentary formations, the stress condition at the cut surface is determined by the relevant properties of the sequence of layers. A two-dimensional finite-element method incorporating the plane strain assumption and uniformely stressed triangular or quadrilateral elements has been employed for the analysis. Contours of constant stress in the new excavation were derived from these calculations for 2:1 slopes cut in sandstone and siltstone strata. The actual contour of the surface of the cut was measured 10 years after excavation, and it is shown that regions where the rock was in tension have eroded away.
Two cases of excavation for road construction are examined in this study. The road was excavated in a series of Flysch near the Kastraki Hydroelectric project in Greece. The Flysch for this area consists of interbedded sandstones and siltstones of the middle Eocene to the early Miocene. The rock types appearing at the road are weathered siltstone overlying sandstone, which in turn overlies siltstone. The weathered siltstone is light grey and calcareous and contains thin layers of fine grained sandstone. When considerably weathered it is characterized as overburden. The sandstone is grey, possessing a dence texture and contains a few scattered layers of grey shale. The siltstone is grey with fine seams of sandstone, and becomes light grey and friable when exposed. The road excavation cut the flysch series in all directions, and an upslope and a downslope section selected for study are shown in Figures 1 and 2 respectively. In both cases, the cut was at a 2:1 (vertical: horizontal) slope. During the first year after excavation was complete the slope retained its original shape, but shortly afterwards it begun to approach a more stable condition.
The excavation stresses were calculated with a two-dimensional finite-element stress analysis. The minor principal stress plots are presented in Figures 3 and 4, which correspond to Figures 1 and 2 respectively. The elevations and distances in Figures 3 and 4 are given in fractions of slope height H from the toe of the cut, while the minor stress σ3 distribution is given in ratios σ3/γh where γ is the average unit weight of the rock. The following properties are used based on values of the same rock at the nearby hydro-electric project: Poisson's ratio 0.20, Modulus of elasticity 1.40E06, 1.60E06, and 2.00E06 in tons per square meter for overburden, sandstone and siltstone respectively; average unit weight 2.60 tons per cubic meter; and ambient stress coefficient equal to 1.00. The material layers are considered parallel to the ground surface. The top layer is overburden, the middle layer is sandstone and lower layer is siltstone. Excavation in Figure 3 results in a narrow tensile stress region along the cut face which widens at the lower siltstone region. In Figure 4, tensile stress regions are formed with smaller widths at the sandstone and larger widths at the siltstone. Cross sections in Figures 5 and 6 show the actual profiles of the slope cuts corresponding to the photographs.of Figures 1 and 2. The excavated slope has reached an equilibrium and the minor principal stress at the surface is zero. This final configuration is compared with the zero stress line of Figures 3 and 4.