4.3 Net versus gross pressure: design soil pressures The beating-capacity equations are based on gross soil pressure qut,which is everything above the foundation level. Settlements are caused only by net increases in pressure over the existing overburden pressure. If the allowable pressure is based on the beating- capacity equations,the pressure is a gross pressure. If the allowable pressure is based on settlement considerations,it is a net pressure. If enough soil mass is removed to equalize the building mass and float the building on to the soil with little or no settlement.This is called a floating foundation
4.3 Net versus gross pressure: design soil pressures ▪ The beating-capacity equations are based on gross soil pressure qult, which is everything above the foundation level. ▪ Settlements are caused only by net increases in pressure over the existing overburden pressure. ▪ If the allowable pressure is based on the beatingcapacity equations, the pressure is a gross pressure. ▪ If the allowable pressure is based on settlement considerations, it is a net pressure. ▪ If enough soil mass is removed to equalize the building mass and float the building on to the soil with little or no settlement. This is called a floating foundation
4.4 Erosion problems for structures adjacent to flowing water Bridge pier,abutments,bases for retaining walls, and footings for other structures adjacent to or located in flowing water must be located at a depth such that erosion or scour does not undercut the soil and cause a failure. Scour occurs principally during floods,but some scour occur at other times;in either case a scour-out pit or depression)in the stream bed may result. Scour is accelerated if the foundation creates channel obstruction
4.4 Erosion problems for structures adjacent to flowing water ▪ Bridge pier, abutments, bases for retaining walls, and footings for other structures adjacent to or located in flowing water must be located at a depth such that erosion or scour does not undercut the soil and cause a failure. ▪ Scour occurs principally during floods, but some scour occur at other times; in either case a scour-out pit ( or depression) in the stream bed may result. ▪ Scour is accelerated if the foundation creates channel obstruction