Chapter 4 Factors to consider in foundation design
Chapter 4 Factors to consider in foundation design
4.1 Footing depth and spacing Footing should be carried below: The frost line -Zones of high volume change due to moisture fluctuation Topsoil or organic material ■Peat and muck Unconsolidated material 园 超
4.1 Footing depth and spacing ▪ Footing should be carried below: ▪ The frost line ▪ Zones of high volume change due to moisture fluctuation ▪ Topsoil or organic material ▪ Peat and muck ▪ Unconsolidated material
4.1 Footing depth and spacing When footings are to be placed adjacent to an existing structure,the line from the base of the new footing to the bottom edge of the existing footing should be 45 or less with the horizontal plane. From this requirement it follows that the distance m should be greater than the difference in elevation of the two footings,Z. If the new footing is lower than the exiting footing,there is a possibility that the soil may flow laterally from beneath the existing footing. This may increase the amount of excavation somewhat but,more importantly,may result in settlement cracks in the existing building
4.1 Footing depth and spacing ▪ When footings are to be placed adjacent to an existing structure, the line from the base of the new footing to the bottom edge of the existing footing should be 45°or less with the horizontal plane. ▪ From this requirement it follows that the distance m should be greater than the difference in elevation of the two footings, zf . ▪ If the new footing is lower than the exiting footing, there is a possibility that the soil may flow laterally from beneath the existing footing. ▪ This may increase the amount of excavation somewhat but, more importantly, may result in settlement cracks in the existing building
4.1 Footing depth and spacing If the soil is a sand one cannot excavate to a depth greater than that of the existing foundation. The excavation depth of a o-c soil is limited by the equation: 2c 9o 2 (SF)yK (SF)Y
4.1 Footing depth and spacing ▪ If the soil is a sand one cannot excavate to a depth greater than that of the existing foundation. ▪ The excavation depth of a φ-c soil is limited by the equation: ( ) (SF) q SF K c zf 2 0 = −
4.2 Displaced soil effects Soil is always displaced by installing a foundation. In the case of spread footings the displacement is the volume of the footing pad and negligible amount from the column resting on the footing. In the cases of the spread footing with basement and the mat foundation we should consider loss of a part of ysD when computing the net pressure increase from building
4.2 Displaced soil effects ▪ Soil is always displaced by installing a foundation. ▪ In the case of spread footings the displacement is the volume of the footing pad and negligible amount from the column resting on the footing. ▪ In the cases of the spread footing with basement and the mat foundation we should consider loss of a part of γsD when computing the net pressure increase from building
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
4.5 Corrosion protection In polluted ground areas there can be corrosion problems with metal foundation members as well as with concrete. Concrete is normally resistant to corrosion; however,if sulfates are present,it may be necessary to use sulfate-resistant concrete. It may occasionally be necessary to use air- entrained concrete for foundation member. Steel piles are not suggested for use in sea water unless coated and not allowed to undergo wet-dry cycle. 阳喝
4.5 Corrosion protection ▪ In polluted ground areas there can be corrosion problems with metal foundation members as well as with concrete. ▪ Concrete is normally resistant to corrosion; however, if sulfates are present, it may be necessary to use sulfate-resistant concrete. ▪ It may occasionally be necessary to use airentrained concrete for foundation member. ▪ Steel piles are not suggested for use in sea water unless coated and not allowed to undergo wet-dry cycle
46 Water fable fluctuation -A lowered water table increases the effective pressure and may cause additional settlements. -A raised water table may create problems due to: 3 刷 超
4.6 Water table fluctuation ▪ A lowered water table increases the effective pressure and may cause additional settlements. ▪ A raised water table may create problems due to: ▪ ①～③
4.7 Foundations in sand and silt deposits Foundation on sand and silt will require consideration of the following: ■①~④ Foundation on silt or sand deposits may consist of spread footings,mats,or piles,depending on the density,thickness,and cost of densifying the deposit,and on the building loads. Rapid or immediate settlements occur on noncohesive silt or sand deposits
4.7 Foundations in sand and silt deposits ▪ Foundation on sand and silt will require consideration of the following: ▪ ①～④ ▪ Foundation on silt or sand deposits may consist of spread footings, mats, or piles, depending on the density, thickness, and cost of densifying the deposit, and on the building loads. ▪ Rapid or immediate settlements occur on noncohesive silt or sand deposits
