Proposed criteria for discharge capacity of prefabricated vertical drains

Modified triaxial tests and ASTM-based discharge capacity tests were carried out to obtain specifications of discharge capacity. The test results indicate that, at straight condition, ASTM-based discharge capacity decreases with increasing lateral pressure, time and hydraulic gradient. With lateral pressure application, the filter jacket of a prefabricated vertical drain (PVD) is pressed into the channel system of the core, thereby reducing the flow area. Furthermore, the discharge capacity decreases at a high hydraulic gradient due to loss of flow energy as a result of turbulent flow. Moreover, discharge capacity has been investigated by simulating the different possible drain deformations in the field. The average percentage reductions of the discharge capacity from the straight condition to various PVD deformed conditions have been found as follows: 26, 32, 33, 43, 48 and 78% for conditions of 10% bent, 20% bent, 90 degrees twisted, 180 degrees twisted, 20% bent with one clamp and 30% bent with two clamps, respectively. Subsequently, reduction factors for discharge capacity have been formulated. Moreover, a simple method is proposed to evaluate specifications of discharge capacity with some established relationship considering the length of PVD, spacing, time required for consolidation and the magnitude of the horizontal coefficient of consolidation. Copyright (C) 1996 Elsevier Science Ltd.