A slide occurred in weathered rock near residential buildings located in Caracas, Venezuela. The purpose of the paper is to describe by means of this case history the technical problems and decisions that engineers have had to face to arrive at a reasonably weighted distribution of factors associated with the occurrence of slides. Factors such as orientation of discontinuities, fill weight and water pressure influence were evaluated in terms of Mohr-Coulomb and strength displacement theories. It is concluded that it is possible to arrive at a weighted distribution of factors inducing failure. However, this distribution is strongly dependent on subjective engineering judgment.
Un glissement s'est produit dans des roches alterees près d'edifices residentiels au sud-est de Caracas, au Venezuela. Cet article a pour but de decrire, en se rapportant à ce cas specifique, les problèmes que les ingenieurs ont à confronter et les decisions techniques qu'ils ont à prendre afin d'arriver à une determination ponderee valable des facteurs lies au declenchement des glissements. Des facteurs tels que l'orientation des discontinuites et l'influence du poids de remblais et de la pression d'eau ont ete evalues d'apres la theorie de Mohr-Coulomb et celles du deplacement de la resistance. On en tire la conclusion qu'il est possible d'arriver à une distribution ponderee des facteurs qui declenchent un glissement - neanmoins cette distribution depend en grande partie du jugement subjectif de l'ingenieur.
Ein Hangrutsch hat in verwitterten Felsen in der Nahe von Wohnhausern in Caracas, Venezuela, stattgefunden. Die Absicht dieses Aufsatzes ist es, durch diesen Fall zu einer richtig bewerteten Verteilung aller mit der Rutschung verbundenen Faktoren zu gelangen. Streichen und Fallen der Trennflachen und Einfluß der Auffuellung und des Grundwassers wurden nach Mohr-Coulomb und Festigkeitsverformungskriterien ausgewertet. Man kommt zur Folgerung, daß es zwar möglich ist, allen Faktoren Werte zuzuordnen, aber die Verteilung ist stark von subjektiver Ingenieurbeurteilung abhangig.
Urban developments in hilly areas of southern Caracas, Venezuela, are every year subjected to rockslides which cause importance loss of properties, and expensive repair work. It also originates lawsuits betweenowners,owners and builders, owners and real state agencies. The urgent need for the repair work to be accomplished, makes impossible to wait for a legal verdict. This verdict usually takes many years due to the complexity of determining legal responsibilities of the parties involved, such as urban developers, earthwork contractors, builders geotechnical engineers real state agencies and county or city authorities. Due to this fact affected owners and contractors have search for a way to solve the problem of determining responsibilities in order to avoid sending the case to court. It has been therefore a recent practice to establish an agreement between parties in which they agree to hire an independent board of consulting engineers with the following purpose: "to determine causes of failure and express by means of a percentage the influence of each factor in the occurrence of the rockslide". The results of the technical report would be the base to determine responsibilities and to distribute the purpose of this paper is to describe by means of a case history, the technical problems and decisions that engineers have to face in order to arrive at a reasonable weighted distribution of factors causing the rockslide.
Natural slopes are composed of weathered quartz-micaceous schists and sericite quartz-phyllites. Foliation planes are very well developed with a strike parallel to the strike of the slope and dip of 30 - 40° towards the free surface. Joints strike approximately normal and parallel to the slope close to a 90° dip, representing released surfaces for the slide that occurred along foliation surfaces. Reinforced concrete buildings were built between 1973 and 1975 on foundations embedded in weathered rock. Above the weathered rock existed a 10 m high fill that was placed approximately thirteen years prior to the construction of the buildings. Results of subsoil investigation made in 1973 indicated that the fill has N values, determined from Standard Penetration Tests, ranging from 7 to 20. Once all the apartments were sold, out, owners decided to build new water tanks locating all- necessary pipes on the surface in order to keep control of water leakages. Cracking in walls, broken pipes and fill settlements were observed and repaired from 1975 to 1981. The slide occurred on July 25, 1981 severely damaging parking lots and backyard areas of two residential buildings. Fig. 1 shows geometry of slope before failure.
Oriented monolithic samples of the weathered sericite- quartz-phyllites of the failure surface were taken to be tested in a direct shear box. Foliation planes are usually closed but during the geological investigation it was noticed that most of the samples broke apart very easily exhibiting a very low tensile strength in the direction normal to foliation surfaces.