Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 161

that which is borne through grain-to-grain contact is called the ‘effective stress’ ; the effective

stress, obtained indirectly by subtracting the neutral stress from the total stress, is significant

in the mobilisation of shear strength.

3. Permeability is the property of a porous medium such as soil, by virtue of which water or any
   other fluid can flow through the medium.


4. Darcy’s law (Q = k. i. A) is valid for the flow of water through most soils except in the case of very
   coarse gravelly ones. The macroscopic velocity obtained by dividing that total discharge by the
   total area of cross-section is called ‘superficial velocity’. In contrast to this, the microscopic veloc-
   ity obtained by considering the actual pore space available for flow is referred to as the ‘seepage
   velocity’.


5. Energy may be expressed in the form of three distinct energy heads, i.e., the pressure head, the
   elevation head, and the velocity head. The direct of flow is determined by the difference in total
   head between two points.


6. The constant head permeameter and the variable head permeameter are used in the laboratory
   for the determination of the coefficient of permeability of a soil.
   Pumping tests are used in the field for the same purpose, using the principles of well hydraulics.


7. Permeant properties, such as viscosity and unit weight, and soil properties, such as grain-size,
   void ratio, degree of saturation, and presence of entrapped air, affect permeability.


8. The overall permeability of a layered deposit depends not only on the strata thicknesses and
   their permeabilities, but also on the direction of flow that is being considered. It can be shown
   that the permeability of such a deposit in the horizontal direction is always greater than that in
   the vertical direction.


9. The phenomenon of ‘capillary rise’ of moisture in soil has certain important effects such as satu-
   ration of soil even above the ground water table, desiccation of clay soils and increase in the
   effective stress in the capillary zone.

References

1. Alam Singh and B.C. Punmia: Soil Mechanics and Foundations, Standard Book House, Delhi-6.


2. A.W. Bishop: The Measurement of Pore pressure in the Triaxial Test, Pore pressure and Suction
   in soils, Butterworths, London, 1961.


3. A.W. Bishop, I. Alpan, E.E. Blight and I.B. Donald: Factors controlling the strength of Partly
   Saturated Cohesive Soils, Proc. ASCE Research conference on shear strength of cohesive soils,
   Boulder, Colorado, USA, 1960.


4. H. Darcy: Les fontaines pulaliques de la ville de Dijon, Paris : Dijon, 1856.


5. J. Dupuit: Etudes théoretiques et pratiques sur la mouvement des eaux dans les canaux découvert
   et a travers les terrains perméables, 2nd edition, Paris, Dunod, 1863.


6. A Hazen: Some Physical Properties of Sand and Gravels with Special Reference to Their Use in
   Filtration, Massachusetts State Board of Health, 24th Annual Report, 1892.


7. A Hazen: Discussion of ‘Dams on Sand Foundations’, by A.C. Koenig, Transactions, ASCE, 1911.


8. IS : 2720 (Part XVII)—1986 : Methods of test for soils – Laboratory Determination of Permeability.


9. IS : 2720 (Part XXXVI)—1987 : Methods of test for soils—Laboratory Determination of Perme-
   ability of Granular Soils (constant head).


10. A.R. Jumikis: Soil Mechanics, D. Van Nostrand Co., Princeton, NJ, USA, 1962.