Geotechnical Engineering

DHARM

144 GEOTECHNICAL ENGINEERING

If a dry and powdered soil sample is thoroughly mixed and packed into a glass tube with
a screen at one end and a vented stopper at the other and then the tube is immersed in a
shallow depth of water in a horizontal position, the water is sucked into the soil by capillary
action. The distance to which the sample gets saturated may be expressed as a function of
time.

hh 00

xx

hhcc

Rubber tube

for air vent

Glass tube

Filter of coarse sand

X

Y

Screen

Water Surface

Fig. 5.18 Horizontal capillarity test (After Taylor, 1948)
In this situation, the menisci are developed to the maximum curvature possible for the
void sizes in the sample; the corresponding capillary head constant for the soil at a given void
ratio. The pressure difference between either end of the line of saturation or the menisci in the
pore water is the capillary tension at all times.

Let the area of cross-section of the tube be A, and the porosity of the sample be n. If the
line of saturation has proceeded a distance x, the hydraulic head expended may be formed as
follows:

At point X:
Elevation head: – h 0 (water surface is the datum assumed)
Pressure head: + h 0
Total head = – h 0 + h 0 = 0
At point Y:
Elevation head = – h 0
Pressure head = – hc
Total head = – (h 0 + hc)
Therefore, the head expended from X to Y = 0 + (h 0 + hc) = (h 0 + hc)
If we imagine that standpipes could be inserted at X and Y, water would rise to the
elevations shown in the figure, and the difference between these elevations would be (h 0 + hc).

If a fine-grained sample is placed just below water surface, h 0 may be negligible com-
pared to hc. When the size of the tube is of appreciable magnitude, h 0 varies for different points
and the head lost is greater at the bottom than it is at the top of the sample; however, it may be
made almost constant for all points in the cross-section if the tube is revolved about its axis as
saturation proceeds: