Geotechnical Engineering

DHARM

146 GEOTECHNICAL ENGINEERING

The left-hand side of Eq. 5.41 is nothing but the slope m of this plot ; thus, we have two

values m 1 and m 2 for the l.h.s. of this equation for the two values of head h 01 and h 02 for the

two stages.

Hence, we have:
m 1 = 2k/n ()hh 01 + c
m 2 = 2k/n()hh 02 + c
The solution of these two simultaneous equations would yield the values for h 0 and k.
This two-stage test is also termed ‘Capillarity-permeability test’ as it affords a proce-
dure for the determination of the permeability in addition to the capillary head.

5.9.8 Vertical Capillarity Test

There is little choice between the horizontal capillarity test and the vertical capillarity test,
which will be described in this sub-section. However, the horizontal capillarity test is gener-
ally preferred.

The set-up for the vertical capillarity test is shown in Fig. 5.20 :

hhcc

zz

YY

X

Soil sample

Line of saturation

hhcc

(h –z)(hcc–z)

Fig. 5.20 Vertical capillarity test (After Taylor, 1948)

The saturation proceeds by capillary action vertically upward into the tube:

At point X:

Both elevation head and pressure head are zero.

At point Y:

Elevation head = z

Pressure head = – hc

Total head = (z – hc)

∴ Hydraulic gradient = ()hz

z

c−

The corresponding differential equation for the speed of saturation is obtained by using
Darcy’s law: