A clay specimen, 25 mm thick, has been tested in an oedometer apparatus with two way rainage, and it is observed that 50% of the consolidation settlement occurs in 1 hour. A ayer of the same clay is observed to settle 10 mm in 10 years and after many years to settle (total primary consolidation) by 35 mm. Determine the thickness of the clay layer if it drains only from upper surface