discharge capacity of prefabricated vertical drains confined in clay

MIURA AND CHAI D Discharge Capacity of Prefabricated Vertical Drains Confined in Clay
3. Place the remolded clay (at a water content approximately equal to the liquid limit)
into the mold layer-by-layer until the desired height is attained.
4. Install the top pedestal and connect the PVD specimen to the outlet water flow system.
Fix the rubber membrane to the top pedestal.
5. Apply a suction of approximately 10 kPa to the specimen and remove the mold.
6. Apply the confining pressure and gradually release the suction.
7. Set the desired hydraulic gradient.
8. Allow the clay to consolidate under a given confining pressure. After the clay consolidates,
measure the discharge capacity.
Four types of PVDs commonly used in Japan were tested. The four PVDs have different
core structures (Figure 2) and filters (Table 1). Both rubber membrane-confined
(approximately 0.9 mm thick) and clay-confined tests were conducted. To simulate
long-term field behavior, the clay-confined tests were approximately three months long
(except for one test). A micro pump was used to recirculate the water (tap water, not
de-aired) during the tests. The tests were conducted in an air-conditioned room with the
temperature typically within the range of 18 to 26_C. The pump motor was not placed
in the water to ensure that it would have no effect on the water temperature. Most tests
were conducted at a confining pressure, σ, of 49 kPa. Assuming an earth pressure coefficient
of 0.5, 49 kPa represents the horizontal earth pressure under an embankment
approximately 5 m high, or the initial horizontal earth pressure at a depth of approximately
10 m in the ground. Remolded Ariake clay, which consisted of 57.0% clay,
41.7% silt, and 1.3% sand particles, was used (liquid limit of 105.0% and a plastic limit
of 42.8%).
For the test apparatus used (Figure 1), an amount of head loss is associated with the
hose connecting the inlet and outlet water tanks, which should be considered when interpreting
the test results. To calibrate the head loss, a test without a PVD specimen and
soil column was conducted. In this case, the water flowed from the inlet water tank to
the outlet water tank through the hoses and cylindrical chamber only. The head difference,
∆H, between the inlet and outlet water tanks was varied, and the corresponding
amount of water flow, Q, was measured. The Q versus ∆h relationship for this test device
is plotted in Figure 3. This curve was used to correct the test results according to
the following procedure:
Table 1.
Physical properties of the prefabricated vertical drain (PVD) filters.
Property
PVD filter
PVD A PVD B PVD C PVD D
Mass/unit area (g/m 2 ) 50 37 50 154
Thickness (mm) 0.21 0.12 0.21 0.44
Polymer Polyester Polyolefin Polyester Polypropylene
Manufacturing
process
Continuous filament
(heat-bonded)
Short fibre
(heat-bonded)
Continuous filament
(heat-bonded)
Short fibre
(heat-bonded)
122 GEOSYNTHETICS INTERNATIONAL S 2000, VOL. 7, NO. 2