The paper presents a simple method for the evaluation of laboratory discharge capacity of Plastic Board Drain (PD) which has been widely used as vertical drain for the improvement of soft ground. The effects of confining pressure and local bending deformation on the discharge capacity values of various drain types were investigated in detail. It was found from constant head permeability tests that the discharge capacity is significantly decreased due to the increase in confining pressure and the local bending strain. It was also found that the flow inside the PD is not laminar and hence Darcy's law cannot be applied for the evaluation of permeability. Lastly, a simple design method to obtain the required discharge capacity of PD has been proposed.