Geotechnical News • December 2015
53
WASTE GEOTECHNICS
Changes in volumetric moisture
content and matric suction
Typical measured changes in volu-
metric moisture content and matric
suction as a function of time illustrated
in Figure 4 demonstrate the distinct
pattern of the capillary barrier effect.
Remarkable time delay was system-
atically observed in the readings of
moisture and suction in capillary bar-
rier profiles, when compared to their
silt counterparts exposed to the same
rainfall intensity. Even more compel-
ling confirmation was acquired upon
inspecting time lapse videos, show-
ing hydraulic impedance of wetting
front propagation at the interface in
the capillary barrier profiles illustrated
in Figure 5. Results at higher rainfall
intensities suggested lesser efficiency
of the capillary barrier system in the
form of shorter time delay for the
wetting front propagation, and higher
overall infiltration.
Conclusions
Surface runoff can be the most critical
component of the water budget that
directly influences the amount of infil-
tration into soil covers systems, thus
controlling their design. Yet little do
we know about prediction of such an
important component of the water bal-
ance equation. A laboratory program
was initiated to address and mitigate
this issue. The chief focus of said
program was to quantify the rainfall
runoff phenomenon in a controlled
laboratory environment for different
cover types, saturation state scenarios,
and different rainfall intensity settings.
In brief, a number of points stood out
among the results of the laboratory
programs. Runoff rates in saturated
profiles behaved in much the same
way as we expect them to, and proved
to be dependent solely on the applied
rainfall intensity and the saturated
hydraulic conductivity of the soil. This
is part and parcel of the fundamen-
tal understanding of saturated soils
mechanics. Water can only infiltrate
at a maximum rate i.e. the saturated
hydraulic conductivity of the soil.
Hence, when introducing a rainfall
intensity that exceeds that limit, a por-
tion of the applied water infiltrate and
the remainder converts into runoff.
By this account, the two primary
parameters controlling runoff onset in
saturated profiles are established.
By the same token, that upper bound
of infiltration also exists in unsaturated
soils in the form of the infiltration
capacity function suggested by Horton
(1939). The logic behind this runs
as follows: as long as the introduced
rainfall does not exceed the infiltra-
tion capacity function, the entire
applied rainfall infiltrates into the soil.
Conversely, once the applied rainfall
exceeds the infiltration capacity of the
soil, runoff transpires. This has been
observed consistently in all unsatu-
rated profiles, where initially the entire
applied rainfall infiltrated, and then the
infiltration rate decreased non-linearly
with time in a comparable manner to
the infiltration capacity function.
The splendor of capillary barrier
covers was also demonstrated as a
part of the results. Clearly proving
how capillary barrier profiles exhibit
higher runoff volumes compared to
low hydraulic conductivity profiles
exposed to the exact same conditions.
This is attained by limiting the down-
ward infiltration considerably due to
the contrast in hydraulic properties
of the capillary barrier materials, and
therefore lowering moisture storage in
the coarse layer. Moreover, the capil-
Figure 5. The wetting front propagation at the same point in time in the silt
profile (left) and the capillary barrier profile (right) when exposed to the
same rainfall intensity.
Figure 4. Typical variation in volumetric moisture content and matric suction
profiles in the unsaturated silt (left) and unsaturated capillary barrier profiles
(right) at 40 mm/hr rainfall intensity.
