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**foundations are generally grouped into two categories:**

Bearing Capacity foundations are designed to transmit load from the structure they support to the soil foundations are generally grouped into two categories: A. Shallow Foundations B Deep Foundations

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Shallow Foundations the most common (and cheapest) type of shallow foundations are SPREAD FOOTINGS square spread footings to support individual columns (also circular) McCarthy, 6th Ed.

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**Strip Footings to support wall loads**

McCarthy, 6th Ed. Rectangular and Trapezoidal Footings for two columns (combined footing) or machine base McCarthy, 6th Ed.

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**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

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FLOATING Foundations McCarthy, 6th Ed. where deep deposits of compressible, cohesive soil are present and piles are impractical building’s 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

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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

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**Types of Deep Foundations**

PIERS where load-bearing stratum no more than 5 m deep not used much any more McCarthy, 6th Ed. CAISSONS where over-burden no more than m thick replacing piers McCarthy, 6th Ed.

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**deep over-burden more than 8 - 9 m thick**

PILES deep over-burden more than m thick Various types and placement methods Craig, 6th Ed.

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**Structural Requirements**

Factor of Safety against General Shear Failure of supporting soil is normally required to be in the range 2.5 – 3.0 Tolerable amount of settlement; in particular, differential settlement should not cause significant damage to structure nor interfere with function Secondary to these, during construction, there should be no adverse affect on adjacent structures or services

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**Ultimate Bearing Capacity, qf**

The least pressure that would cause shear failure of supporting soil immediately below and adjacent to a foundation Craig, 6th Ed.

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**modes of failure: General Shear 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

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Local 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

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**Punching 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

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**Terzaghi’s Theory strip footing of infinite length and width B**

Craig, 6th Ed. 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 (®)

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**above EDCGF: plastic equilibrium below EDCGF: elastic equilibrium**

Craig, 6th Ed. transition between ¯ & «: ACD & BCG (zones or radial shear or slip fans) above EDCGF: plastic equilibrium below EDCGF: elastic equilibrium

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**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 = gD Terzaghi’s general equation: qf = 0.5gBNg + cNc + gDNq Soil Self Weight Shear Strength Surcharge Contribution of:

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**Bearing Capacity Factors**

Ng, Nc and Nq are bearing capacity factors and are derived from various sources Craig, 6th Ed.

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**General Shear Failure of Footings (Ultimate Bearing Capacity)**

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: FOOTING TYPE Sγ Sc Strip 1.0 Square 0.8 1.2 Circular 1.6 Rectangular

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**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:

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Skempton’s Nc Values if 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.

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