*Place an impermeable liner on the bed with 150 mm overlap on sides. Lay the perforated underdrain pipe, Pack 50 mm diameter clear stone to 75 mm above top of underdrain, top with 100 mm of choker layer; | *Place an impermeable liner on the bed with 150 mm overlap on sides. Lay the perforated underdrain pipe, Pack 50 mm diameter clear stone to 75 mm above top of underdrain, top with 100 mm of choker layer; |