glacialclint
Geotechnical
- Feb 26, 2010
- 4
Sorry for the long post with my question. I've got some background in my question so basic soil mechanics won't need repeating.
It is common to use granular backfill behind retaining walls instead of cohesive backfill to reduce wall pressures. Reasoning being that a higher friction angle results in lower lateral coefficients and cohesive strength is typically ignored in design (at least in local firms to Iowa). My question concerns the geometry of the granular backfill behind a wall that is required to reduce the lateral pressures to those of granular material (when granular backfill is placed between the wall and existing cohesive soils). The two commonly reported slopes found in local geotechnical firms' standard sections are either 1:1 and 1:2 (Hor:Vert)extending up and away from the wall base. These slopes are imaginary lines where granular backfill is between the line and the wall and cohesive soil is allowed on the other side. I was wondering if anyone knew the theory behind a 1:2 slope, if anyone had seen any alternative recommendations, or if anyone had any interesting ideas/background on this topic. Again, we are looking at standard section verbiage here where no triax or plane strain testing has been completed to determine phi.
My thoughts are the 1:1 slope appears to be conservatively derived from a friction angle of 0 for cohesive soils and a failure wedge inclination of 45 + phi/2. But then by that math, a 1:2 would need a phi = 37 degrees, not conservative and not typically applicable to a cohesive soil wedge...
Thanks in advance for any help.
It is common to use granular backfill behind retaining walls instead of cohesive backfill to reduce wall pressures. Reasoning being that a higher friction angle results in lower lateral coefficients and cohesive strength is typically ignored in design (at least in local firms to Iowa). My question concerns the geometry of the granular backfill behind a wall that is required to reduce the lateral pressures to those of granular material (when granular backfill is placed between the wall and existing cohesive soils). The two commonly reported slopes found in local geotechnical firms' standard sections are either 1:1 and 1:2 (Hor:Vert)extending up and away from the wall base. These slopes are imaginary lines where granular backfill is between the line and the wall and cohesive soil is allowed on the other side. I was wondering if anyone knew the theory behind a 1:2 slope, if anyone had seen any alternative recommendations, or if anyone had any interesting ideas/background on this topic. Again, we are looking at standard section verbiage here where no triax or plane strain testing has been completed to determine phi.
My thoughts are the 1:1 slope appears to be conservatively derived from a friction angle of 0 for cohesive soils and a failure wedge inclination of 45 + phi/2. But then by that math, a 1:2 would need a phi = 37 degrees, not conservative and not typically applicable to a cohesive soil wedge...
Thanks in advance for any help.