discharge capacity of prefabricated vertical drains confined in clay

MIURA AND CHAI D Discharge Capacity of Prefabricated Vertical Drains Confined in Clay
y
x
Confining
pressure,
F=Bσ
Tsinα
Tensile force, T
α
Core
B
Deformed filter
(circular arc)
Figure 7.
Calculation of the reduction of the drainage area.
By varying the value of α (Figure 7) (and therefore, the radius of the arc), balance
between the confining pressure and the mobilized tensile force in the filter can be obtained
(trial and error). The reduction of the cross-sectional area of the drainage channel
is a function of the filter deformation behavior and the geometry of the drainage channel.
Based on the test results, the relationships between confining pressure and the reduction
of the cross-sectional area of the drainage channels were calculated and are
shown in Figures 8a to 8d for PVDs A to D, respectively. Figure 8 indicates that under
a confining pressure of 49 kPa, deformation (including creep) of the filter reduces the
cross-sectional area of the drainage channel by approximately 5% for PVDs A and C,
and approximately 20% for PVDs B and D. Comparing the amount of reduction after
one day, one month, and one year (extrapolated), it can be seen that under a confining
pressure of 49 kPa, most of the cross-sectional area reduction is due to short-term deformation
and the creep deformation is small. The PVD D filter is strong, but the perimeter
length of the filter sleeve is longer than the perimeter length of the core, and the
core is weaker. In particular, the extra length of the filter sleeve contributed approximately
15% to the reduction of the cross-sectional area of the drainage channel. The
test results indicate that PVD filter deformation is not a dominating factor for discharge
capacity reduction.
For PVDs A and C, before terminating the clay-confined tests, hydraulic shockswere
applied by firmly stepping on the inlet hose or rapidly varying the head difference. By
doing so, the fine particles (or bio-films) were partially pumped out of the drainage
channels of the PVD core. Then, the discharge capacities were recovered to a value corresponding
to that of approximately one week of elapsed time (Figures 4a and 4c). This
indirectly indicates that clogging of the PVD core is the main mechanism reducing the
discharge capacity with time. It was observed that, during application of the hydraulic
shocks, flocculated fine particles were forced out of the PVD drainage channels and deposited
on the wall of the outlet hose. Also, after the discharge capacity test, the filter
was analyzed using electron-microscope photographs. There were bio-films on the
drainage-channel side of the filter (Chai and Miura 1999). Air bubbles trapped in the
drainage channels were also considered a possible cause of the discharge capacity re-
128 GEOSYNTHETICS INTERNATIONAL S 2000, VOL. 7, NO. 2