Compressibility and Pore Pressure Parameters - Unsaturated Soil Mechanics in Engineering Practice - page 806

Figure 15.25 Pore-air pressure parameter B a as function of
degree of saturation for Peorian loess (after Campbell, 1973).

Figure 15.23 Pore-water pressure parameter B w as function of
degree of saturation for Peorian loess (after Campbell, 1973).

show that the B w pore pressure parameter increases rapidly
at a degree of saturation equal to 80% for Peorian loess and
90% for Champaign till. These degrees of saturation corre-
spond to conditions near-optimum water content for each of
the compacted soils. The B w parameter approaches 1.0 as
the degree of saturation approaches 100%.
The B a pore-air pressure parameter also increases with an
increase in degree of saturation for Peorian loess and Cham-
paign till specimens, as shown in Figs. 15.25 and 15.26.
The pore-water pressure approaches the pore-air pressure at
saturation and air may exist in the form of occluded air bub-
bles. The pore-air pressure-measuring device likely begins to
measure the pore-water pressure as saturation is approached.
As a result, B a approaches a value of 1 at saturation, as
exhibited in Fig. 15.26 for Champaign till.

Specimens compacted near and above optimum water con-
tent exhibit a high, immediate pore-water pressure response
to the application of an isotropic pressure increment (i.e., a
steeper curve). At initial water contents well above optimum
water content the pore pressure response curves approxi-
mate a straight line with a slope angle approaching 45 ◦ .In
this case, the B w pore pressure parameter is constant and
approaches unity. In other words, at saturation an increment
in total stress, dσ 3 , is transferred entirely to the pore pressure
(i.e., du w = dσ 3 ). It is common practice in laboratory tests
to ensure the saturation of the soil specimen by confirming
that B w is approaching 1.0.
Figures 15.23 and 15.24 present the B w pore-water pres-
sure parameter as a function of degree of saturation for
Peorian loess and Champaign till, respectively. The plots

Figure 15.24 Pore-water pressure parameter B w as function of
degree of saturation for Champaign till (after Campbell, 1973).

Figure 15.26 Pore pressure parameter B a as function of degree
of saturation for Champaign till (after Campbell, 1973).