Presentation on theme: "foundations are generally grouped into two categories:"— Presentation transcript:
1 foundations are generally grouped into two categories: Bearing Capacityfoundations are designed to transmit load from the structure they support to the soilfoundations are generally grouped into two categories:A. Shallow FoundationsB Deep Foundations
2 Shallow Foundationsthe most common (and cheapest) type of shallow foundations areSPREAD FOOTINGSsquare spread footings to support individual columns (also circular)McCarthy, 6th Ed.
3 Strip Footings to support wall loads McCarthy, 6th Ed.Rectangular and Trapezoidal Footings for two columns (combined footing) or machine baseMcCarthy, 6th Ed.
4 RAFT or MAT Foundations McCarthy, 6th Ed.To lower the bearing pressure and reduce differential settlement on soils with low bearing capacity or erratic or variable conditions
5 FLOATING FoundationsMcCarthy, 6th Ed.where deep deposits of compressible, cohesive soil are present and piles are impracticalbuilding’s substructure is a combination mat and caisson to create a rigid boxweight of earth displaced by foundation is equal to total weight of structure, thereby minimizing settlement from consolidation
6 Deep Foundationsused when soil near surface has poor load-bearing capacitythey transmit load through weak soil strata (overburden) to stronger, load-bearing stratum (eg., bedrock, dense sand and gravel, etc.)loose soilbedrock
7 Types of Deep Foundations PIERSwhere load-bearing stratum no more than 5 m deepnot used much any moreMcCarthy, 6th Ed.CAISSONSwhere over-burden no more than m thickreplacing piersMcCarthy, 6th Ed.
8 deep over-burden more than 8 - 9 m thick PILESdeep over-burden more than m thickVarious types and placement methodsCraig, 6th Ed.
9 Structural Requirements Factor of Safety against General Shear Failure of supporting soil is normally required to be in the range 2.5 – 3.0Tolerable amount of settlement; in particular, differential settlement should not cause significant damage to structure nor interfere with functionSecondary to these, during construction, there should be no adverse affect on adjacent structures or services
10 Ultimate Bearing Capacity, qf The least pressure that would cause shear failure of supporting soil immediately below and adjacent to a foundationCraig, 6th Ed.
11 modes of failure: General Shear Failure on low compressibility (dense or stiff) soilsplastic equilibrium throughout support and adjacent soil massesheaving on both sides of foundationfinal slip (movement of soil) on one side only causing structure to tilt
12 Local Shear Failureon highly compressible soilsonly partial development of plastic equilibriumonly slight heaving on sidessignificant compression of soil under footing but no tilting
13 Punching Shear Failure on loose, uncompacted soilsvertical shearing around edges of footinghigh compression of soil under footing, hence large settlementsno heaving, no tilting
14 Terzaghi’s Theory strip footing of infinite length and width B Craig, 6th Ed.strip footing of infinite length and width Buniform surcharge, q0 on surface of isotropic, homogeneous soilRankine active wedge, ABC: forces ¯Passive zones, ADE (¬) & BGF (®)
16 the more general case is a footing at depth D Craig, 6th Ed.Neglecting the shear strength of the soil above depth D implies that this soil is a surcharge: q0 = gDTerzaghi’s general equation:qf = 0.5gBNg + cNc + gDNqSoil Self WeightShear StrengthSurchargeContribution of:
17 Bearing Capacity Factors Ng, Nc and Nq are bearing capacity factors and are derived from various sourcesCraig, 6th Ed.
18 General Shear Failure of Footings (Ultimate Bearing Capacity) theory was developed for strip footingsto adapt to square, circular and rectangular shapes, Terzaghi & Peck developed shape factors here which are still widely used today:FOOTING TYPESγScStrip1.0Square0.81.2Circular1.6Rectangular
19 Allowable Bearing Capacity the allowable bearing capacity, qa is the value used in the design of footing sizein North America, a factor of safety against general shear failure, F is applied to the ultimate bearing capacity, qf:in Britain, F is not applied to the surcharge:
20 Skempton’s Nc Valuesif undrained shear strength parameters are used for the design then a special case arises:since fu = 0, Nq = 1 and:values of Nc are acquired from Skempton’s Chart®Craig, 6th Ed.