Subject Engineering Geology

Topic Introduction

Difficulty High

The property of Rocks which defines water bearing capacity is porosity and permeability. The details are as under.

The rate of ground water flow is controlled by the properties of rock: Porosity and permeability.

i). Porosity:

a. Porosity is the percentage of the volume of the rock that is open space (pore space).

b. This determines the amount of water that a rock can contain.

c. In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation. (Fig 1.1)

d. Well rounded coarse grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit well together (Fig 1.2)

e. Poorly sorted sediments usually have lower porosity because the fine grained fragments tend to fill up the open spaces. (Fig 1.3)

f. Since cements tend to fill in the pore space, highly cemented sediment rocks have lower porosity (Fig 1.4)

ii). Permeability

a. Permeability is a measure of degree to which the pore spaces are interconnected, and the size of the interconnections. Low porosity usually results in low permeability, but high porosity doesn’t necessarily imply high permeability.