From the results of an oedometer test on an undisturbed sample of clay taken at a depth of 10 m from the ground surface, the maximum vertical preconsolidation pressure (s'vmax or s'c) was found to be 560 kPa. The clay layer has a saturated unit weight of 20 kN/m3 and is overlain by a 3 m thick silty sand layer that has a saturated unit weight of 18 kN/m3. The groundwater table is located at the ground surface. Calculate the overconsolidation ratio (OCR) of the clay layer at 10 m depth.