A culvert is to be constructed in a bed of sand (e = 0.5) for drainage purposes. The foof of the culvert will be located 3 m below ground surface. Currently the groundwater level is at ground surface. But, after installation of the culvert, the groundwater level is expected to drop 2 m below ground surface. Calculated the change in vertical effective sress on the roof of the culvert after installation. You can assume the sand above the groundwater level is saturated.