Geotechnical Engineering

DHARM

282 GEOTECHNICAL ENGINEERING

A for a perfectly elastic material may be shown to be 1/3. This may also be written in the

form:

∆u = B. ∆σ 3 + A(∆σ 1 – ∆σ 3 ) ...(Eq. 8.51)

where A = A.B.

If ∆u is considered to be the sum of two components ∆ud and ∆uc,

∆uc = B. ∆σ 3

and ∆ud = A(∆σ 1 – ∆σ 3 )

For the conventional triaxial test at constant cell pressure, during the application of the

deviator stress, ∆σ 3 = 0 and ∆σ 1 = (σ 1 – σ 3 ). Taking B as unity for full saturation, Eq. 8.50 for

this case of UU-test will reduce to

∆u = ∆σ 3 + A(σ 1 – σ 3 ) ...(Eq. 8.52)

A and hence A can be easily determined from the conventional triaxial compression

test of UU type.*

For CU tests where drainage is permitted during the application of cell pressure, ∆uc is

zero, and the corresponding value of ∆u is given by

∆u = A(σ 1 – σ 3 ) ...(Eq. 8.53)

A-factor may be as high as 2 to 3 for saturated fine sand in loose condition, and as low as

• 0.5 for heavily preconsolidated clay.
  Uses and applications of the pore pressure parameters
  Skempton’s pore pressure parameters are very useful in field problems involving the predic-
  tion of pore pressures induced consequent to known changes of total stress.
  One classic example is the construction of an earth embankment or an earth dam over a
  soft day deposit. If the rate of construction is such that pore water pressure induced in the
  foundation soil cannot get dissipated, undrained condition prevails. If the pore pressure devel-
  oped is excessive, the shear strength of the foundation soil which is dependent upon the effec-
  tive stress decreases, thereby endangering the stability of the foundation. Prediction of the
  pore pressure changes with increase in the total stresses consequent to the increase in height
  of the embankment/dam may be done using the pore pressure parameters. The stability of the
  structure may thus be ensured.
  The construction engineer may suggest a suitable rate of construction in stages to that
  the excess pore pressures can be kept under control to ensure stability during and after con-
  struction.

*8.10 STRESS–PATH APPROACH

A ‘‘Stress–Path’’ is a curve or a straight line which is the locus of a series of stress points

depicting the changes in stress in a test specimen or in a soil element in-situ, during loading or

unloading, engineered as in a triaxial test in the former case or caused by forces of nature as in

*However it is better to use a value of B appropriate to the pressure range in the deviator part

of the test.