Groundwater and permeability

GROUNDWATER

• Ground water by definition is the freely moving water that occurs below the ground water table.
• The ground water is the level below which the geological formation is fully saturated.
• Ground water is considered as the major part of the sub-surface water.
• Water in the rock mass is the integrated part of hydrological cycle.
• The ground water may travel through a considerable distance in a rock mass hence it is important to consider the overall ground water pattern when potential water problem is to be analyzed.
• Water leakage into tunnel and underground opening can greatly effect the working condition. It may lead to the delay of construction work for several month.
• Close to the surface the rock mass is more jointed and the joints are more open in the surface than deeper in rock mass so most leakage occur in the part which is close to the surface.
• However the most serious leakage occurs in deeper part of the tunnel.

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PERMEABILITY AND HYDRAULIC CONDUCTIVITY

i) Permeability may be defined as the ability of the porous(rock or unconsolidated material) to allow the passage of fluid through it.

ii) The permeability of a medium depends upon the porosity but also the shape of pores in the medium and their connectedness.
iii) High permeability will allow the rapid movement of fluid through rock mass.
iv) Permeability is effected by the pressure in the rock.
v) Rock having permeability >100md may be considered as exploitable hydrocarbon reservoir with stimulation.
vi) Rock having permeability <100md may be considered as rocks with conform seals.

HYDRAULIC CONDUCTIVITY
It is the property of the rock which describes the ease with which a fluid can move through pores , spaces and fracture. It depends upon: a) Permeability b)Degree of saturation c) Density and viscosity of fluid

PROBLEM CAUSED BY GROUNDWATER ON ROCK MASS

Water when enters into a joint or fissure present in a rock mass causes many problems. Some of them are as follows:
i) Chemical reaction : some minerals react chemically with water and such reaction may create problem.
ii) Sulphides oxidation also takes place.
iii) Drilling becomes more difficult.
iv) Pumping may become very costly.
v) Stability is often reduced as a result strength of rock is reduced.
vi) High moisture increases the unit weight of the rock and give more problem.
vii) When water enter into the cracks or fracture, water pressure increases due to which shear strength of the rock decreases thereby causing in stability.

REFERENCES:

i) https://opentextbc.ca/geology/chapter/14-1-groundwater-and-aquifers/#:~:text=Permeability%20is%20the%20most%20important,the%20water%20for%20our%20purposes.

ii) https://link.springer.com/chapter/10.1007/978-3-642-88109-1_23

Groundwater and permeability