S. BHALAI – Landslide susceptibility of Portland, Jamaica
Table 7. Weighting values of landslide susceptibility applied to the classes of the causative factors
(inducing an increase in mass and volume), or shaking during an earthquake, triggers movement of weathering blanket or fractured rock on slopes. On the steeper slopes exceeding 50º, it was observed that the weathering blanket is easily eroded and generally does not accumulate significantly. Landslides on these slopes are generally comprised of fractured rock material that fails when pore water pressure increases during wet conditions.
High landslide susceptibility computed for east- facing slopes possibly represent a higher degree of tropical weathering on these faces. Easterly slopes (northeast, east and southeast) are exposed to longer durations of insolation and accelerated decomposition of component material compared to slopes facing other directions. The orientation of the bedrock on these slopes with high susceptibility (including those facing northwest) may also explain these findings. The Palaeogene rock types generally dip away from the Blue Mountain inlier towards the northeast and northwest azimuths.
blanket overburden. However, the susceptibility at the highest elevation class range was high and may represent movement of dislocated fractured rocks on the steep upper slopes of the Blue Mountains.
The increase in landslide susceptibility with slope gradient is consistent with observations in other eastern parishes such as St. Mary and St. Thomas. As slopes steepened over 50º, susceptibility decreased. Weathered material accumulates on slopes, but tends to fail when the shear strength is reached as the downward pull of gravitational forces overcomes the cohesiveness of the material. The addition of moisture by rainfall
Five geotechnical groups displayed the highest landslide susceptibility; Yellow Limestone, Shale, Gibraltar White, Cretaceous Volcanics and Colluvium. The Yellow Limestone, Shale and Cretaceous Volcanics all decompose to clay-rich residues. Clays generally behave unpredictably when wet and show high propensity to failure. The Gibraltar White group also decomposes to clay-rich sediments which under moist conditions, and will fail especially when located on poorly drained slopes. Generally the rocks of the Yellow Limestone, Shale, Gibraltar White and Cretaceous Volcanics when fractured, fail easily. Colluvial deposits have residual strength and occasionally are partly consolidated. These fail easily especially when the angle of repose was not attained when the slipped material was deposited. These deposits
contain labile minerals (amphiboles,
Table 8. Prediction capability of the landslide susceptibility model of Portland. This is tested against areas of known landslide susceptibility, based on field experience
AREAS Moore Town to Holland Mountain Maidstone to Rock Hall Passley Gardens Breastworks London (near Moore Town) Friday (near Ginger House)
LANDSLIDE SUSCEPTIBILITY (based on field experience)
Moderately High to Very High Moderately High Negligible/Low to Moderate
Moderately High High High to Very High
MODEL PREDICTION Moderately High to High Moderately High to High Negligible/Low to Moderate Moderate to Moderately High High High to Very High