A case of slope failure in marls of expansive nature with clayey gouge materials along discontinuities is presented. Due to the failure of a plane and a wedge during construction, the stability conditions anticipated during design were compared with those prevailing during excavation. It was proved that the main factors affecting the instability were the seepage forces of rainfall water flowing into the excavation area and the conventional and overloaded blasts. The need of technical assistance during construction is emphasized as an important part of the design.
On etudie un cas de glissement d'un talus en marnes expansives avec des discontinuites argileuses. On a pu montrer que les causes principales agissant sur l'instabilite etaient les filtrations de l'eau provenant de la pluie et l'action des explosifs. On en deduit l'interêt de l'assistance technique pendant l'execution des travaux.
Es wird ein Fall von Böschungsbruch in schwellfahigen Mergeln mit tonigen Kluftbestegen vorgestellt. Aufgrund eines ebenen und Keilbruches wahrend der Bauarbeiten wurden die Standsicherheitsbedingungen, die wahrend der Projektierung erwartet wurden, mit den bei der Ausfuehrung herrschenden verglichen. Es wurde nachgewiesen, dass die fuer die Unsicherheit hauptverantwortlichen Faktoren die Strömungskrafte des Regenwassers im Aushubsbereich sowie die konventionellen, zu stark geladenen Sprengungen waren. Auf die Notwendigkeit einer technischen Assistenz wahrend des Baus als einen wichtigen Bestandteil des Projektes wird hingewiesen.
The design of an excavation of 25 × 20 m length and 25 m depth was required to set up a crushing plant of marls for a cement manufacturing company in Olazagutia (Navarra, North East Spain). The site was located on a hill side area formed by stratified marls dipping from 30 to 40 degrees southward. Site investigations consisted on structural analysis of discontinuities, borehole drilling, RQD determination, weathering grades, point load tests (PLT), and permeability assessment and sampling. Because of the proximity to a quarry composed by similar materials it was possible to carry out a back analysis of the stability of fallen rocks. Laboratory tests mainly consisted on shear box tests along discontinuities from borehole core Samples and undisturbed block samples. Comprensive strength, slate durability tests (SDT) and other index tests were also performed, besides X-Ray mineralogy determination. The rock material consisted of softs calcareous marls, grey coloured of very fine grained, with 70% of C03 Ca, 14% of montmorillonite, 8% of illite, 5% of quartz and 2% of clorite. The major discontinuities were the stratification planes, shear joints and tension joints. Discontinuity roughness varies from smooth to undulating and some of them, particularly the stratification planes, showed clayey gouge material of 1 - 2 cm thick. SDT results carried out according to the suggested methods of ISRM indicated a high durability index. However, it was observed the longer periods of time and increasing number of cycles than those recommended by ISRM lead to very low durability index. This low - index indicate much better the swelling mineral composition of the marls. Shear strength parameters are illustrated in figure 1 together with shear strength back analysis and shear box tests results. Acceptable agreement between both results were found. It is remarkable the low values of the shear strength along discontinuities filled with clay material, even lower than those obtained with the same clay but in remoulded state. For further discussion of the geotechnical properties of this material it is referred to Gonzalez de Vallejo and Berzal (1976).
It was requested to undertake the design of the excavation not only under economic considerations but under safety conditions, therefore a conservative criteria was adopted in the selection of the shear strength parameters for design purposes. An angle of internal friction of 30 degrees was taken for discontinuities free of clayey gouge materials and 25 degrees for those with clay fillings. In both cases a cohesion or 20 KN/m2 was chosen. No water table were intercepted during the site investigations neither regional hidrogeological evidences. No ne of both detected the presence of a high water table in the area; therefore it was supposed a drained slope condition. A short term condition was considered be cause, once the excavation were finished, the room between the slopes and the walls of the plant (Fig. 6) had to be filled with rock material, therefore a factor of safety 1.2 was taken for design. The stability analysis was carried out using John (1971) and Hoek and Bray (1974)- approaches to the problem. Based on economic and construction reasons, two solutions were considered. The first one consisted to excavate the North, South and East slopes to an angle of 80 degrees and the West slope to 70 degrees. In this case, the North slope should require reinforcement to set back the factor of safety of plane Pl, which was 0.80 (Fig. 2 and table 1), to the factor of safety stablished by the design.
