INTRODUCTION

Caracas, capital of Venezuela, is located on a narrow valley surrounded by hills composed of weathered metamorphic rocks. The pressing need for housing has made developers to use the hilly areas and many cuts and fills have been constructed without an adequate understanding of the weathered rock behavior. Many landslides occur in these areas every year causing important economical losses and some loss of human lives.

By the beginning of 1980, two cuts located within 500m. distance failed 15 year after construction. These cuts exhibited simple geometry, uncovered failure surface and the same lithology. An investigation was carried out that included field measurements, laboratory tests and analytical studies with the objectives of:

  1. To evaluate theories for open joint peak strength.

  2. To determine applicability of direct shear test on foliation surfaces for slope design.

  3. To ascertain the applicability of ultimate strength in the design of slopes in weathered phyllites.

  4. To develop a failure criterion if known theories can not be used to predict failure in weathered phyllites. This criterion should try to define the movements compatible with stability, especially long-term stability. According to Bernaix (1974) this is a wide area for reflection and experiment which has remained almost untouched.

GEOLOGY

Two slides with simple geometry were studied during this investigation. The slides occurred in road cuts in Las Mesetas urban development. Both sites presented the same lithology characterized by weathered sericite- quartz phyllites and quartz-micaceous schists belonging to Las Brisas Formation, Caracas Group, of Triassic-Jurassic age. In both slides the failure occurred along. foliation planes. Joints are not continuous and poorly developed. Incipient crenulation cleavage can be also observed in samples of phyllites. The shape of the failure surface was measured by a geological compass, clinometer, and a metric tape. Zones of maximum curvature of the foliation waviness were approximated to straight lines. The geometry of the cuts before failure are shown in Fig. 1.

The slip surfaces of the slides consisted of weathered phyllite which typically has a light gray color, smooth surfaces and unctuous talc-like structure.

Foliation planes observed in outcrops are usually closed but during the geological investigation it was noticed that some samples broke apart very easily exhibiting a very low tensile strength in the direction normal to foliation surfaces. Other samples could be broken apart if enough hand pressure was applied perpendicular to foliation. Finally, there were pieces of the foliated rock that could not be broken along foliation surfaces by hand. These observations lead us to believe that closed foliation surfaces were composed of unfractured areas and fractured areas in contact. The ratio of these areas obviously determines the strength of a particular specimen which varies from location to location. In view of this fact, the chances of taking samples with equal peak strength properties are very low. The ratio of unfractured to fractured areas depends on the geological history and mainly on the stress deformation characteristics of any particular location within the folded metamorphic rock.

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