Geotechnical properties of tertiary sediments and failure mechanisms of a sea cliff in the State of Rio Grande do Norte State, North East Brazil

摘要

Part of the coastline of Rio Grande do Norte State is bordered by an escarpment of about 90° and 20 to 40 m high. This cliff is composed by beds of tertiary sediments of sandstone and conglomerate (Barreiras Formation). Six kilometers of sea cliff was selected as the subject of this study, with view to the changes brought about by the tourism industry, especially concerning the land use with the building of hotels and restaurants, in the last decade. This paper describes an investigation of failure mechanisms that has caused coastal cliff erosion in this area. Laboratory studies were carried out on twenty disturbed samples obtained along the cliff. Direct shearing and unconfined compression tests on undisturbed block samples were carried out to obtain geotechnical properties of the sediments. The effects of the saturation on the cohesion and the friction angle were also investigated. Field works were conducted to identify the causes of slopes failures. The result of the tests showed that the sediments consist mainly of clay sand with low plasticity. An expressive loss of strength when the soil is saturated was also noted. Three types of rupture were observed in the area of study: gullies due to superficial run off at crest of the cliff, sliding at the upper and middle parts of the slope and fall of material due to undermining of the base of the cliff.