LANDSLIDE PROCESSES AND LANDSLIDE SUSCEPTIBILITY ANALYSIS FROM AN UPLAND WATERSHED - A CASE-STUDY FROM ST-ANDREW, JAMAICA, WEST-INDIES

Rainfall-induced landslides are common geomorphic events on the steepe r slopes of Upper St. Andrew and cause severe environmental, engineeri ng, social and economic repercussions in the area. A geotechnical stud y was initiated to identify (a) past, present and potential landslide types and sites for landslides (b) to examine the nature of geological , geomorphological and geotechnical conditions under which failures ca n take place and (c) the potential landslide hazard in the area. 886 f ailures were mapped more than 80% of which were located in soils. A to tal of 1.55 km2 (9.80% of the total area) had failed. Approximately 59 failures per km2 were mapped, with the highest density within clastic sedimentary units and weathered granodiorite. 789 failure were mapped on slopes between 20-degrees - 45-degrees, with no failure on slope l ess than 10-degrees. 481 failures were mapped along twelve major roadw ays (46.6 km of road), with approximately 10 failures/km of road. Anal ysis of eighteen geotechnical units within nine geological formations revealed that the highest susceptibility for failure was within Old La ndslide Deposits, with granular nonplastic to low plasticity soils, an d on slopes between 20-degrees and 30-degrees. The lowest susceptibili ty was found within White Limestones and Recent Alluvium and Terrace D eposits. Granular, nonplastic to low plasticity soils are generally mo re susceptible than fine soils, especially on slopes between 30-degree s and 45-degrees, while granular and expansive clay soils demonstrate similar susceptibilities, but on slopes between 30-degrees and 45-degr ees and 20-degrees and 45-degrees, respectively. Clastic sedimentary r ocks are more susceptible that igneous lithologies. An increase in slo pe angle increases the susceptibility to failures, but only on slopes up to 45-degrees. An increase in the degree of weathering of soils and bedrock and fracturing of bedrock, as well as road construction incre ases the landslide susceptibility in the area.