DHARM620 GEOTECHNICAL ENGINEERING
maximum load. Empirical equations for bearing capacity in terms of N-value may be used
(Chapter 14).
(ii)Allowable bearing pressure based on tolerable settlement: Empirical equation given
by Terzaghi and Peck may be used in terms of N-values—for the net allowable bearing pres-
sure.
The value may be modified by using the linear relationship with permissible settle-
ment, if it is desired for a different value of the permissible settlement.
If Df/b > 1, the value is obtained by multiplying by the factor (1 + Df /b).
The allowable bearing pressure is taken as the smaller of (i) and (ii) finally.Cohesive soils
The ultimate bearing capacity of cohesive soils depends primarily on their shear strength (or
consistency). This may be determined by any one of the following:
(i)Standard penetration tests: For conservative design of small jobs, the correlation
between standard penetration value, consistency and allowable bearing, capacity given by
Terzaghi and Peck (Chapter 14) may be used.
(ii)Unconfined compression tests: For medium jobs, the shear strength obtained from
unconfined compression tests should be used. Skempton’s equation for bearing capacity is
used in which cohesion is taken as half the unconfined compression strength.
(iii)Triaxial tests: For large jobs, the shear strength may be determined from triaxial
tests on undisturbed samples. The shear parameters are obtained by plotting the data from
triaxial tests. Drainage conditions in the field are to be simulated in the laboratory and careful
interpretation of the results is required.
Silts
Silt is often a poor foundation soil and should be avoided for supporting footings. Apparent
cohesion, exhibited by moist silt disappears on immersion. Plate load tests at about the ground
water level are advocated in this case.
Compacted fills
Bearing capacity for compacted fills must be determined both before and after compaction.
Organic Soils
Organic soils are not suitable for supporting footings. Highly organic soils settle unduly even
under their own weight, both by consolidation and by decay or decomposition of the organic
matter.
Rocks
Generally speaking, rocks can withstand pressures greater than concrete can do. Rocks with
fissures. folds, faults and bedding planes are exceptions to this. Shales may become clay or silt
on soaking. Weathered rocks are treacherous and lose strength on wetting.
15.4.4Settlement of Footings
If the allowable bearing pressure is determined based on the smaller value from the two crite-
ria—shear strength and permissible settlement—footings on granular soils do not suffer detri-
mental settlement.