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Factors to consider in foundation design

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Mushtaq Zaib
Mushtaq Zaib

Factors to consider in foundation design include: footing depth and spacing, location of spread footings, soil pressures, erosion risks for structures near water, corrosion protection, water table fluctuations, and properties of soil types like sand, silt, loess, clays, and expansive soils. Designs must account for frost depth, moisture changes, unsuitable subsurface materials, adjacent existing structures, net versus gross pressures, predicted scour depths, corrosion risks, drainage, and consolidation settlements.

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Factors To Consider In Foundation Design Notes These notes are prepared with the help of books written by Braja M. Das, Craig, Joseph e. Bowles, Wayne C. Teng, class notes and other relevant materials
1. Footing Depth and Spacing 2. Location consideration for Spread footings 3. Displaced Soil Effect 4. Net versus Gross Soil Pressure 5. Erosion Problems for structures adjacent to flowing water 6. Corrosion Protections 7. Water Table fluctuation 8. Foundation Sand and Silt 9. Foundation on Clays and Clayey Silt 10. Foundations On Loess And Other Collapsible Soils 11. Foundations On Clays and Clayey Silts. Factors To Consider In Foundation Design
1. Footing Depth and Spacing Factors To Consider In Foundation Design - Footing Depth and Spacing • The frost line • Zones of high volume change due to moisture fluctuations • Topsoil or organic material • Peat and muck • Unconsolidated material such as abandoned (or closed) garbage dumps and similar filled in areas.
b) Location consideration for Spread footings Factors To Consider In Foundation Design -Location consideration for Spread footings
b) Location consideration for Spread footings Factors To Consider In Foundation Design -Location consideration for Spread footings • If the new footing is lower than the existing footing • σ1 ≈ g zf + q0 ---(01) • σ3= 0 = σ1 K - 2c √K ---(02) • = g zf K+ q0 - 2c √K --- (03) • Solving for excavation depth zf (and using Safety Factor), we obtained • Zf = {2c / [(SF) g √K]} – {q0 / (SF) g} --- (04)
Potential settlement or instability from loss of overburden pressure. Factors To Consider In Foundation Design -Location consideration for Spread footings If the new footing is adjacent to the existing footing
Net versus Gross Soil Pressure (Design soil pressures) • The bearing capacity equations are based on gross soil pressure qult, which is everything above the foundation level. If the allowable pressure is based on the bearing capacity equations, the pressure is a gross pressure. • Settlements are caused only by net increase in pressure over the existing overburden pressure. If the allowable pressure is based on settlement consideration, it is a net pressure. • NOTE :- YOU CAN ADD DERIVATION OF NET / GROSS PRESSURES B4 MIDTERM Factors To Consider In Foundation Design - Net versus Gross Soil Pressure (Design soil pressures) -
Erosion / Scouring Problems for structures adjacent to flowing water • Bridge piers, 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 • An accurate prediction of scour depth is necessary so as to use the shortest possible pile lengths Factors To Consider In Foundation Design -Erosion / Scouring Problems for structures adjacent to flowing water Approaches to avoid Scouring  Scour is accelerated if the foundation creates channel obstruction; To reduce scour the foundation should create a minimum obstruction to normal stream flow patterns  Determine the foundation types  Estimate the probable depth of scour ,effects, etc.  Estimate the cost of foundations for normal and various scour conditions  Determine the cost versus risk and revise the design accordingly
Corrosion Protections • In polluted ground areas such as old sanitary landfills, shorelines, near sewer outfalls line from older industrial plants, or backwater areas where water stands over dead vegetation, there can be corrosion problems with metal foundation members as well as 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 members. • Use of treated timber piling instead of metal piling may be required where the soil has a pH much above 9.5 or below 4.0 Factors To Consider In Foundation Design – Corrosion Protections
Water Table fluctuation • A lowered water table increases the effective pressure and may cause additional settlements. A raised water table may create problems for the owner from the following; • Floating the structure ( making it unstable or tilting it) • Reducing the effective pressure (causing excessive settlement) • Creating a wet basement if the basement walls are not watertight. Factors To Consider In Foundation Design – Water Table Fluctuations Approaches to Water Table Fluctuations By introducing some type of drainage (water does not accumulate around the building walls or produce hydrostatic uplift beneath the basement). Use of drain tile around the basement perimeter (common for residential dwellings and some larger buildings). A sloping basement excavation that is backfilled with granular materials to the required horizontal level in combination with a well (called a sump pit) at the low point that is fitted with a pump (a sump pump system) can be used.
Just Discussion………………. Details from the notes already given  Bearing capacity.  Densification of loose deposits to control settlement.  Placing the footing at a sufficient depth that the soil beneath the footing is confined. If silt or sand is not confined, it will roll out from the footing perimeter with a loss of density and bearing capacity. Wind and water may erode sand or silt from beneath a footing that is too near the ground surface.  Uncontaminated glacial silt deposits can have a large capillary rise because of the small particle sizes. Factors To Consider In Foundation Design – Sand and Silt Deposits
Just Discussion in Class ………………. Details from the notes given Factors To Consider In Foundation Design FOUNDATIONS ON LOESS AND OTHER COLLAPSIBLE SOILS Collapsible soils are generally wind-blown (aeolian) deposits of silts, dune sands, and volcanic ash. Typically they are loose but stable, with contact points well-cemented with a water soluble bonding agent, so that certain conditions of load + wetting produce a collapse of the soil structure with a resulting large settlement. Loess is the predominating collapsible soil that engineers are confronted with.
Just Discussion in Class ………………. Details from the notes given Factors To Consider In Foundation Design FOUNDATIONS ON EXPANSIVE SOILS • Expansive soils undergo volume changes upon wetting and drying. • In general, all clayey soils tend to shrink on drying and expand when the degree of saturation S increases. Usually, the lower the shrinkage limit and the wider the range of the plasticity index Ip, the more likely is volume change to occur
Just Discussion in Class ………………. Details from the notes given Clays and clayey silts may range from very soft, normally consolidated, to very stiff, highly over-consolidated deposits. Major problems are often associated with the very soft to soft, deposits from both bearing-capacity considerations and consolidation settlements. Factors To Consider In Foundation Design - Foundations on Clays and Clayey Silts

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Factors to consider in foundation design

  • 1. Factors To Consider In Foundation Design Notes These notes are prepared with the help of books written by Braja M. Das, Craig, Joseph e. Bowles, Wayne C. Teng, class notes and other relevant materials
  • 2. 1. Footing Depth and Spacing 2. Location consideration for Spread footings 3. Displaced Soil Effect 4. Net versus Gross Soil Pressure 5. Erosion Problems for structures adjacent to flowing water 6. Corrosion Protections 7. Water Table fluctuation 8. Foundation Sand and Silt 9. Foundation on Clays and Clayey Silt 10. Foundations On Loess And Other Collapsible Soils 11. Foundations On Clays and Clayey Silts. Factors To Consider In Foundation Design
  • 3. 1. Footing Depth and Spacing Factors To Consider In Foundation Design - Footing Depth and Spacing • The frost line • Zones of high volume change due to moisture fluctuations • Topsoil or organic material • Peat and muck • Unconsolidated material such as abandoned (or closed) garbage dumps and similar filled in areas.
  • 4. b) Location consideration for Spread footings Factors To Consider In Foundation Design -Location consideration for Spread footings
  • 5. b) Location consideration for Spread footings Factors To Consider In Foundation Design -Location consideration for Spread footings • If the new footing is lower than the existing footing • σ1 ≈ g zf + q0 ---(01) • σ3= 0 = σ1 K - 2c √K ---(02) • = g zf K+ q0 - 2c √K --- (03) • Solving for excavation depth zf (and using Safety Factor), we obtained • Zf = {2c / [(SF) g √K]} – {q0 / (SF) g} --- (04)
  • 6. Potential settlement or instability from loss of overburden pressure. Factors To Consider In Foundation Design -Location consideration for Spread footings If the new footing is adjacent to the existing footing
  • 7. Net versus Gross Soil Pressure (Design soil pressures) • The bearing capacity equations are based on gross soil pressure qult, which is everything above the foundation level. If the allowable pressure is based on the bearing capacity equations, the pressure is a gross pressure. • Settlements are caused only by net increase in pressure over the existing overburden pressure. If the allowable pressure is based on settlement consideration, it is a net pressure. • NOTE :- YOU CAN ADD DERIVATION OF NET / GROSS PRESSURES B4 MIDTERM Factors To Consider In Foundation Design - Net versus Gross Soil Pressure (Design soil pressures) -
  • 8. Erosion / Scouring Problems for structures adjacent to flowing water • Bridge piers, 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 • An accurate prediction of scour depth is necessary so as to use the shortest possible pile lengths Factors To Consider In Foundation Design -Erosion / Scouring Problems for structures adjacent to flowing water Approaches to avoid Scouring  Scour is accelerated if the foundation creates channel obstruction; To reduce scour the foundation should create a minimum obstruction to normal stream flow patterns  Determine the foundation types  Estimate the probable depth of scour ,effects, etc.  Estimate the cost of foundations for normal and various scour conditions  Determine the cost versus risk and revise the design accordingly
  • 9. Corrosion Protections • In polluted ground areas such as old sanitary landfills, shorelines, near sewer outfalls line from older industrial plants, or backwater areas where water stands over dead vegetation, there can be corrosion problems with metal foundation members as well as 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 members. • Use of treated timber piling instead of metal piling may be required where the soil has a pH much above 9.5 or below 4.0 Factors To Consider In Foundation Design – Corrosion Protections
  • 10. Water Table fluctuation • A lowered water table increases the effective pressure and may cause additional settlements. A raised water table may create problems for the owner from the following; • Floating the structure ( making it unstable or tilting it) • Reducing the effective pressure (causing excessive settlement) • Creating a wet basement if the basement walls are not watertight. Factors To Consider In Foundation Design – Water Table Fluctuations Approaches to Water Table Fluctuations By introducing some type of drainage (water does not accumulate around the building walls or produce hydrostatic uplift beneath the basement). Use of drain tile around the basement perimeter (common for residential dwellings and some larger buildings). A sloping basement excavation that is backfilled with granular materials to the required horizontal level in combination with a well (called a sump pit) at the low point that is fitted with a pump (a sump pump system) can be used.
  • 11. Just Discussion………………. Details from the notes already given  Bearing capacity.  Densification of loose deposits to control settlement.  Placing the footing at a sufficient depth that the soil beneath the footing is confined. If silt or sand is not confined, it will roll out from the footing perimeter with a loss of density and bearing capacity. Wind and water may erode sand or silt from beneath a footing that is too near the ground surface.  Uncontaminated glacial silt deposits can have a large capillary rise because of the small particle sizes. Factors To Consider In Foundation Design – Sand and Silt Deposits
  • 12. Just Discussion in Class ………………. Details from the notes given Factors To Consider In Foundation Design FOUNDATIONS ON LOESS AND OTHER COLLAPSIBLE SOILS Collapsible soils are generally wind-blown (aeolian) deposits of silts, dune sands, and volcanic ash. Typically they are loose but stable, with contact points well-cemented with a water soluble bonding agent, so that certain conditions of load + wetting produce a collapse of the soil structure with a resulting large settlement. Loess is the predominating collapsible soil that engineers are confronted with.
  • 13. Just Discussion in Class ………………. Details from the notes given Factors To Consider In Foundation Design FOUNDATIONS ON EXPANSIVE SOILS • Expansive soils undergo volume changes upon wetting and drying. • In general, all clayey soils tend to shrink on drying and expand when the degree of saturation S increases. Usually, the lower the shrinkage limit and the wider the range of the plasticity index Ip, the more likely is volume change to occur
  • 14. Just Discussion in Class ………………. Details from the notes given Clays and clayey silts may range from very soft, normally consolidated, to very stiff, highly over-consolidated deposits. Major problems are often associated with the very soft to soft, deposits from both bearing-capacity considerations and consolidation settlements. Factors To Consider In Foundation Design - Foundations on Clays and Clayey Silts