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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.
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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