Compressibility and Pore Pressure Parameters - Unsaturated Soil Mechanics in Engineering Practice - page 803

Figure 15.16 Effects of coefficient of volume change on pore pressure predictions using Hilf
(1948) analysis.

pressure). An average value for the volume change coeffi-
cient m v is often used over the range of total stress change.
Several curves relating the generated pore-water pressure
to the applied total stress were computed using several val-
ues for m ν . The results are presented in Fig. 15.16. The
coefficient of compressibility m v was assumed to be con-
stant for each curve, regardless of the total stress change,
σ y . Theoretically, the value of m v should be varied along
each curve of u w versus σ y to take the total stress level
into consideration. The shape of the relationship between
u w and σ y varies significantly as the average value of m v is
changed and the initial variables (i.e., S 0 , n 0 , and

curves relating pore pressure and total stress while the ini-
tial degree of saturation S 0 is varied. The average coeffi-
cient of volume change m v is maintained at 8 10 − 5 kPa − 1

for all curves. The results indicate that the pore pressure
versus total stress relationship becomes steeper as the ini-
tial degree of saturation increases. In other words, the pore
pressure responds faster at higher degrees of saturation. A
comparison between Figs. 15.16 and 15.17 suggests that
the coefficient of volume change and the initial degree of
saturation are both important variables affecting the pore
pressure response.

×

u a 0 )are
kept constant. Higher soil compressibility values result in
higher pore pressure responses as a result of an increase in
total stress. This can be seen by the steeper u w versus σ y
curve for more compressible soils (Fig. 15.16).
Reasonably close agreement between the piezometric
measurements and the predicted pore-water pressures have
been obtained when using Hilf's analysis provided appro-
priate soil compressibility values are used in the analysis.
Close agreement is achieved in spite of the crude approx-
imations applied and the average value for m ν . It would
appear that the use of an average coefficient of volume
change seems justified.
The shape of the relationship between pore pressure and
total stress is strongly influenced by the initial volume-
mass properties of the soil. Figure 15.17 presents several

15.6.2 Graphical Procedure for Hilf's Analysis
Hilf (1948) outlined a graphical procedure which used a
nomograph for the solution of the pore pressure versus total
stress applied. The objective of the graphical procedure was
a plot of the relationship between pore pressure and total
stress.
The pore pressure response combines the oedometer test
data and Hilf's equation. The results of a conventional
oedometer test on the soil can be plotted as shown in
Fig. 15.18. The consolidation test results are plotted as
the change in porosity n [i.e., e/( 1

+ e) ] versus the

change in effective stress σ y − u w . Hilf's equation
(i.e., Eq. 15.59) can also be plotted on the same graph. The
ordinate is the change in porosity n , while the abscissa
is the change in pore-water pressure u w (Fig. 15.18).