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The role of weathering in the initiation and mobility of a rare complex avalanche at the Nigeria–Cameroon border, West Africa

Research paper by Ogbonnaya Igwe

Indexed on: 04 Feb '14Published on: 04 Feb '14Published in: Landslides



Abstract

A catastrophic rock debris avalanche on one of the highlands demarcating Cameroon and Nigeria, 3 days after a transient period of heavy rainfall in November 2010, killed two people and damaged economic trees, plants and farmlands. Detailed field investigation and sampling were undertaken by a team from the International Programme on Landslide to analyse the slope movement. The investigation was in two phases—5 days and then 7 months after the event to study the effect of alternating dry and rainy seasons on the geotechnical properties of the materials associated with the slope failures. It was also aimed at assessing the effect of time on the morphology of the slip plane as a new approach to understanding the development of potential failure planes and reactivated landslides. Available evidence showed that present failure planes form mini-deposition axes which may become future slip zones as they are still steep enough to accelerate instability. The research also traced the pattern of rock weathering and joints evolution in the area and found that the systematic weathering of feldspar in the basement aggravated slope instability by creating zones of weakness characterized by structures that aid fragmentation. Soils were thin (<1 m) in some slopes and deep (>2 m) in others and may represent the differential effect of weathering and erosion on the mountains which are important in analyzing the mechanism and mobility of the failed masses. Samples collected were subjected to various geotechnical laboratory tests such as unconsolidated undrained triaxial, consistency, particle size and permeability tests.