Presentation on theme: "Bearing Capacity f foundations are designed to transmit load from the structure they support to the soil oundations are generally grouped into two categories:"— Presentation transcript:
Bearing Capacity f foundations are designed to transmit load from the structure they support to the soil oundations are generally grouped into two categories: A. Shallow Foundations B. Deep Foundations
Shallow Foundations the most common (and cheapest) type of shallow foundations are the most common (and cheapest) type of shallow foundations are SPREAD FOOTINGS SPREAD FOOTINGS square spread footings to support individual columns (also circular) McCarthy, 6 th Ed.
Strip Footings to support wall loads Strip Footings to support wall loads Rectangular and Trapezoidal Footings for two columns (combined footing) or machine base Rectangular and Trapezoidal Footings for two columns (combined footing) or machine base McCarthy, 6 th Ed.
RAFT or MAT Foundations To lower the bearing pressure and reduce differential settlement on soils with low bearing capacity or erratic or variable conditions McCarthy, 6 th Ed.
FLOATING Foundations where deep deposits of compressible, cohesive soil are present and piles are impractical buildings substructure is a combination mat and caisson to create a rigid box weight of earth displaced by foundation is equal to total weight of structure, thereby minimizing settlement from consolidation McCarthy, 6 th Ed.
Deep Foundations used when soil near surface has poor load-bearing capacity they transmit load through weak soil strata (overburden) to stronger, load- bearing stratum (eg., bedrock, dense sand and gravel, etc.) loose soil bedrock
Types of Deep Foundations where load-bearing stratum no more than 5 m deep not used much any more PIERS McCarthy, 6 th Ed. CAISSONS where over- burden no more than m thick where over- burden no more than m thick replacing piers replacing piers
PILES deep over-burden more than m thick Various types and placement methods Craig, 6 th Ed.
Structural Requirements 1.F actor of Safety against General Shear Failure of supporting soil is normally required to be in the range 2.5 – T olerable amount of settlement; in particular, differential settlement should not cause significant damage to structure nor interfere with function 3.S econdary to these, during construction, there should be no adverse affect on adjacent structures or services
Ultimate Bearing Capacity, q f The least pressure that would cause shear failure of supporting soil immediately below and adjacent to a foundation Craig, 6 th Ed.
modes of failure: on low compressibility (dense or stiff) soils plastic equilibrium throughout support and adjacent soil masses heaving on both sides of foundation final slip (movement of soil) on one side only causing structure to tilt General Shear Failure
on highly compressible soils only partial development of plastic equilibrium only slight heaving on sides significant compression of soil under footing but no tilting Local Shear Failure
on loose, uncompacted soils vertical shearing around edges of footing high compression of soil under footing, hence large settlements no heaving, no tilting Punching Shear Failure
Terzaghis Theory strip footing of infinite length and width B uniform surcharge, q0 on surface of isotropic, homogeneous soil Rankine active wedge, ABC: forces Passive zones, ADE () & BGF () Craig, 6 th Ed.
transition between : ACD & BCG (zones or radial shear or slip fans) transition between : ACD & BCG (zones or radial shear or slip fans) above EDCGF: plastic equilibrium above EDCGF: plastic equilibrium below EDCGF: elastic equilibrium below EDCGF: elastic equilibrium Craig, 6 th Ed.
Neglecting the shear strength of the soil above depth D implies that this soil is a surcharge: q0 = D Terzaghis general equation: the more general case is a footing at depth D the more general case is a footing at depth D q f = 0.5 BN + cN c + DN q Contribution of: Soil Self Weight Shear Strength Surcharge Craig, 6 th Ed.
Bearing Capacity Factors N, Nc and Nq are bearing capacity factors and are derived from various sources Craig, 6 th Ed.
General Shear Failure of Footings (Ultimate Bearing Capacity) FOOTING TYPE SγSγ ScSc Strip1.0 Square Circular Rectangular theory was developed for strip footings to adapt to square, circular and rectangular shapes, Terzaghi & Peck developed shape factors here which are still widely used today:
Allowable Bearing Capacity the allowable bearing capacity, qa is the value used in the design of footing size in 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: in Britain, F is not applied to the surcharge:
Skemptons N c Values if undrained shear strength parameters are used for the design then a special case arises: since u = 0, Nq = 1 and: values of Nc are acquired from Skemptons Chart Craig, 6 th Ed.