SEEPAGE ANALYSIS OF OFFSHORE BUNDS

Abstract

The concept of developing an offshore dumping site as an alternative to the fast depleting landfill sites has been proposed by the Singapore government. The main feature of the scheme is the construction of perimeter bund enclosures from the seabed to a level several meters above the sea. This is done to create a storage site for waste disposal. The proposed idea offer a relatively straightforward and affordable disposal option in view of the high demands for land. However, the disposal of waste in an off shore environment present a number of problems which must be adequately addressed in the planning, design, construction, operation and closure phases. The pollution of the marine environment is potentially the most serious concern. Clearly, the design choice of the perimeter bund cross section plays a significant role in the control of contaminant migration. From the study, it was found that a clay liner offers great benefits. In the short term, the existing tidal variation in the off shore environment produces a form of "hydraulic trap" by providing a hydraulic barrier towards retarding the migration of contaminant from the waste pond. This is caused by the inward flow into the bund towards the waste pond generated by the tidal variation. It was also shown that a combination of keeping the waste pond water level lower than the open sea and having a clay liner as part of the bund design will significantly enhanced the effects of the "hydraulic trap". The development of a simplified seepage chart for rapid estimation of seepage magnitude of bunds with a finite tailwater such as offshore bunds was successfully accomplished with the application of the Dupuit's theory of a hypothetical dam with vertical faces. The effectiveness of a clay liner in the control of contaminant migration is dependent on the contaminant concentration. When the concentration is increased, the migration time will decrease. If the hydraulic conductivity of the clay liner is close to the hydraulic conductivity of the sand bund (e.g. 1 x 10-7 m/s for the clay liner and 1 x 10-5 m/s for the sand bund), the overall migration mechanism will be dominated by the migration process in the sand bund which is governed by the advection process. As the hydraulic conductivity of the clay liner decreases, the predominant migration mechanism will switch from advection to diffusion. For the case of an unlined sand bund, it was shown that the migration time of the contaminant is not affected by contaminant concentration. However, for a clay lined bund, when the concentration is increased it was found that the migration time is shorter.