Soil Mechanics II

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Soil Mechanics-II
GAMBELLA UNIVERSITY
By Fentahun A.
MSc. In Geotechnical Eng.
Lecture notes
Bearing Capacity of Soil
INTRODUCTION
 A foundation, often constructed from concrete, steel or
wood, is a structure designed to transfer loads from a
superstructure to the soil underneath the superstructure.
 The foundation should be designed such that
The soil below does not fail in shear &
Settlement is within the safe limits.
 In general, foundations are categorized into two groups,
namely, shallow and deep foundations.
 Shallow foundations are comprised of footings, while deep
foundations include piles that are used when the soil near
the ground surface has no enough strength to stand the
applied loading.
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 Total overburden pressure,
q0 It is the intensity of total
overburden pressure due to the
weight of both soil and
water at the base level of the
foundation.
 Effective overburden
pressure, q’0 It is the effective
overburden pressure at the base
level of the foundation
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Terminology’s
 Ultimate bearing capacity of soil, qu It is the maximum
bearing capacity of soil at which the soil fails by shear
 Net ultimate bearing capacity, qnu It is the bearing
capacity in excess of the effective overburden pressure
expressed as
 Gross allowable bearing pressure, qa
 Net allowable bearing pressure, qna
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Cont.…

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General shear failure
on low compressibility (dense or stiff) soils
plastic equilibrium throughout support and adjacent soil masses
final slip (movement of soil) on one side only causing structure to tilt
Well defined failure surface
Local shear failure
on highly compressible soils
only partial development of plastic equilibrium
Significant compression of soil under footing but no tilting
Failure surface first developed right below the foundation and
slowly extends outwards with load increments
Principal Modes of Shear Failure 6
Punching shear failure
Common in fairly lose sand or soft clay
Failure surface does not extend beyond the zone right beneath the
foundation
Cont.…
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Cont.…
Ultimate Bearing Capacity Equations
Terzaghi’s Bearing Capacity Equation
 Terzaghi’s Bearing Capacity assumes strip footing of
infinite length and width B and a uniform surcharge, q0
on surface of isotropic, homogeneous soil
• The failure mechanism in a C, Ǿ soil for Terzaghi's
bearing capacity solution is shown above.

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Assumptions for Terzaghi's Method
Depth of foundation is less than or equal to its width.
No sliding occurs between foundation and soil (rough foundation).
Soil beneath foundation is homogeneous semi infinite mass.
Soil strength is governed by Mohr-Coulomb model.
General shear failure mode is the governing mode.
No soil consolidation occurs.
Foundation is very rigid relative to the soil.
Applied load is compressive and applied vertically to the centroid of
the foundation.
No applied moments present.
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Cont.… 10
 Considering this all assumptions Terzaghi bearing capacity is
given by.
Cont.…
Figure : Terzaghi's bearing capacity coefficients.
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Cont.…
Meyerhof’s Bearing Capacity Equation
 Meyerhof (1951) developed a bearing capacity equation by
extending Terzaghi's failure mechanism and taking into
account the effects of footing shape, load inclination and
footing depth by adding the corresponding factors of s, d,
and i. For a rectangular footing of L by B (L > B) and
inclined load:
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Figure : Meyerhof’s bearing capacity coefficients.
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Cont.… 14
Cont.…
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Cont.…
Hansen’s Bearing Capacity Equation
 Hansen (1961) extended Meyerhof’s solutions by considering
the effects of sloping ground surface and tilted base (Fig. 2.5)
as well as modification of Ng and other factors.
 For a rectangular footing of L by B (L > B) and inclined
ground surface, base and load:
 Equation 2.9 is sometimes referred to as the general bearing
capacity equation. In the special case of a horizontal ground
surface,
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Figure : Identification of items in Hansen’s bearing capacity equation.
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Figure : Hansen’s bearing capacity coefficients.
Cont.…
A Comparative Summary of BC Equations
 Terzaghi’s equations were and are still widely used, perhaps
because they are somewhat simpler than Meyerhof’s and
Hansen’s.
 However, Terzaghi’s equations have the following major
drawbacks:
Shape, depth and inclination factors are not considered.
Terzaghi’s equations are suitable for a concentrically
loaded horizontal footing but are not suitable for
eccentrically (for example, columns with moment or titled
forces) loaded footings that are very common in practice.
The equations are generally conservative than Meyerhof’s
and Hansen’s.
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Effects of Groundwater Table on Bearing Capacity
 For all the bearing capacity equations, you will have to make
some adjustments for the groundwater condition.
 The term γD the vertical stress of the soil above the base
of the footing and the last term γB the vertical stress of a
soil mass of thickness B, below the base of the footing
needs some adjustments.
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A strip footing of width 3 m is to be placed at a depth of 0.5m below
the ground surface on a sandy silt having a bulk and saturated unit
weight of 18.6 kN/m3 and 20 kN/m3 respectively .The shear strength
parameters of the soil are c’= 8 kPa and ϕ’=22o. Determine the net
ultimate and allowable capacity, when the
a) GWT is 3 m below the ground surface.
b) b) GWT is at ground surface. Take FS=3 (use Terzaghi’s eqn)
Example 1
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Cont.…

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Cont.…
A square footing of width 3 m is to be placed at a depth of 1 m below
the ground surface on a sandy silt having a bulk and saturated unit
weight of 18.6 kN/m3 and 20 kN/m3 respectively. The footing carries a
load P inclined 15 degree to the vertical at its center. The shear strength
parameters of the soil are c’= 8 kPa and ϕ’=22o. Determine the net
ultimate and allowable capacity, when the
a) GWT is 3 m below the ground surface.
b) GWT is at ground surface. Take FS=3 (use Meyerhof's Eqn)
Example 2 26
Cont.…
Thank you!!!
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