ketanco
Civil/Environmental
- Aug 7, 2013
- 28
In six minute solution for geaotechnical, problem 51
you place a very large area of fill on a clay layer of 30' thick. water table is at ground surface at first.
they ask the consolidation settlement of clay due to the loading by the fill surcharge and also they are saying that the water table is being lowered to 12' below ground at the same time so there is a stress increase for it too of course..
here is my question... for calculating the final effective stress at the center of clay layer, (which is at 15' depth), i took the dry density of clay above water table (since water is lowered by 12' feet now) so 12 feet of dry clay plus 3 feet of bouyant clay plus surcharge. thats what i used to calculate the final effective stress.
but in the solution, when calculating final effective stress, they only took the initial stress (which is buoyant unit weight clay times 15 feet), plus new surcharge from fill, plus the increase of stress due to lowering of water by 12' so they added (15-3)x62.4 to the surcharge and found final effective stress that way.
but why can we not take the dry clay that is now above water table, plus wet clay of 3 feet plus the surcharge when calculating final stress? there is about 10 percent difference in final stress amounts between these two approach. they dont consider the dry density at all. also isnt their approach of just deducting water of 12 feet wrong? because that 12 feet was not entirely water. it contained soil. they treat that 12 feet as if it was entirely water that way. or am i wrong?
you place a very large area of fill on a clay layer of 30' thick. water table is at ground surface at first.
they ask the consolidation settlement of clay due to the loading by the fill surcharge and also they are saying that the water table is being lowered to 12' below ground at the same time so there is a stress increase for it too of course..
here is my question... for calculating the final effective stress at the center of clay layer, (which is at 15' depth), i took the dry density of clay above water table (since water is lowered by 12' feet now) so 12 feet of dry clay plus 3 feet of bouyant clay plus surcharge. thats what i used to calculate the final effective stress.
but in the solution, when calculating final effective stress, they only took the initial stress (which is buoyant unit weight clay times 15 feet), plus new surcharge from fill, plus the increase of stress due to lowering of water by 12' so they added (15-3)x62.4 to the surcharge and found final effective stress that way.
but why can we not take the dry clay that is now above water table, plus wet clay of 3 feet plus the surcharge when calculating final stress? there is about 10 percent difference in final stress amounts between these two approach. they dont consider the dry density at all. also isnt their approach of just deducting water of 12 feet wrong? because that 12 feet was not entirely water. it contained soil. they treat that 12 feet as if it was entirely water that way. or am i wrong?
