physiographic divisions of india(3)

 Field characters of Deccan trap basalt flows.

The field characters of the basalt flow such as thickness , lateral extent , and form with absence or presences of gas cavities . compact and amygdaloidal basalt have different field characters.

1)    COMPACT BASALT- It occurs as thick, extensive flows , by regular tabular form and top and bottom to parallel plane surfaces  lateral extent up to 20 km, thickness is 45to 70m.

Engineering significance –  1) Heart and bottom portions

      Well jointed, therefore deep weathering and deeper foundation , possibility of leakage below the dam and hence treatment is necessary.

 2)Top portion of the flow – Some thickness of the top portion may become vesicular amygdaloidal and hydrothermally altered  hence un jointed , therefore suitable from dam foundation and tunneling point of view.

 2)AMYGDALOIDAL BASALT-Shows much variation in field characters

1)    a) Plane amygdaloidal basalt - It occurs as flows , by regular tabular form and top and bottom to parallel plane surfaces  lateral extent up to 1 km, thickness is up to 10m. Un jointed

2)    Thin irregular amygdaloidal basalt - It occurs as flows , by irregular form and top and bottom  not parallel plane surfaces  lateral extent up to 40m, thickness is up to 1.5m. Un jointed

3)    Thick irregular amygdaloidal basalt - It occurs as flows , by irregular form and top and bottom  not parallel plane surfaces  lateral extent up to 400m, thickness is up to  6m. Un jointed

Engineering significance-Fresh amygdaloidal basalt are always un jointed . Vesicules in them are either filled by secondary minerals  or even if unfilled . They are not interconnected  therefore they are suitable for dam foundation and for tunneling. Excellent construction material as rubble for masonry metal for concrete and as a substitute for sand after crushing.