Geotechnical Engineering

DHARM

602 GEOTECHNICAL ENGINEERING

5. Fellenius’ method is based on circular slip surfaces and plastic equilibrium of the soil mass
   within the slip surface. Terzaghi’s theory is based on composite rupture surface (logarithmic
   spiral and plane) and is the most popular.


6. As the footing is loaded to failure, the soil first reaches ‘local shear’ and then ‘general shear’.
   Local shear occurs when the soil in a zone becomes plastic. General shear occurs when all the
   soil along a slip surface is at failure. In loose sand, local shear occurs at a much lower stress than
   does general shear. In dense sand, local shear occurs at a stress only slightly less than that
   which causes general shear.


7. Shape effect causes the bearing capacity of isolated square, circular and rectangular footings to
   be somewhat different from that for continuous footings; in general, the capacity of these will be
   about 20 to 30% more.
   Skempton’s theory relates to the bearing capacity of rectangular footings in pure clay. Brinch
   Hansen’s general bearing capacity equation takes into account the size and shape effects, depth
   effect and the effect of inclined loads in any kind of soil.


8. Plate load tests and penetration tests are semiempirical approaches, which reflect field experi-
   ence; as such, theoretical methods should be used in conjunction with these empirical approaches,
   wherever feasible.
   The ‘Standard Penetration Number, has been correlated to φ, bearing capacity factors and allow-
   able bearing pressure for specified settlements; this approach is more suited to cohesionless
   soils.


9. Bearing capacity and settlement of a footing on sand are related both to footing size and depth of
   embedment and to soil properties. The capacity increases significantly with increase in size of
   footing and depth of embedment. Settlement increases somewhat with size.
   Bearing capacity of a footing on clay is practically independent of size of footing. Even the
   depth of embedment causes the capacity to increase just by the difference between gross pres-
   sure and net pressure. As such, the benefit of the depth of embedment it considered marginal,
   and only the net allowable capacity itself is used for design purposes.

References

1. Abbet: American Civil Engineering Practice. Vol. I.


2. Alam Singh and B.C. Punmia: Soil Mechanics and Foundations, Standard Book House, Delhi-6,
   1970.


3. A.L. Balla: Bearing Capacity of Foundations, Proceedings, American Society of Civil Engineers,
   Vol. 88, 1962.


4. A.L. Bell: The Lateral Pressure and Resistance of Clay, and the Supporting Power of Clay Founda-
   tion, Proc. Institution of Civil Engineers, London, 1915.


5. Bharat Singh and Shamsher Prakash: Soil Mechanics and Foundation Engineering, Nem Chand
   & Bros., Roorkee, India, 1963.


6. Joseph E. Bowles: Foundation Analysis and Design, McGraw Hill Book Co., NY, USA, 1968


7. A Casagrande and R.E. Fadum: Application of Soil Mechanics in Designing Building Founda-
   tions, Transaction, ASCE, Vol. 109, 1944.


8. W. Fellenius: Erdstatische Berechnungen mit Reibung und Kohäsion, Adhäsion, und uncer
   Annahme Kreiszylindrischer Gleitflächen, Rev. ed., Ernst, Berlin, 1939.


9. J. Brinch Hansen: A General Formula for Bearing Capacity, Danish Geotechnical Institute Bul-
   letin, No. 11, 1961.