Decomposed dolerite dykes contain high clay content display high plasticity. Moreover, the material is generally of low permeability than decomposed granite which is found in many slopes.
The presence of decomposed dolerite dykes in a direction parallel to slope surface may cause slope failure after heavy and prolonged rainfall. Owing to the low permeability of decomposed dolerite dykes, perched water table would be developed above the dykes and this essentially increase the positive pore water pressure which reduces the shear strength of soils.
The presence of decomposed dolerite dykes in a direction perpendicular to slope surface may also cause slope failure after severe rainstorm. The low permeability of the material tends to act as a dam building up the local groundwater level for water coming down from upper slopes. The increase of water level behind the “dam” causes an increase in pore water pressure so as to reduce the strength of soils in slope, leading to subsequent failure.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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