Transcript shallowfoundationbyindrajitmitra01-140414015530

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Shallow foundations are those that transmit
structural loads to the near surface soils.
According to the Terzaghi, a foundation is
shallow foundation if its depth is equal to or less
than its width i.e d ≤ w.
For most of the residential buildings or
buildings with moderate height or multistoreyed
building on soil with sufficient strength, shallow
foundation
is
used
from
economical
consideration.
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Near surface soil should be strong enough
Foundation structures should be able to
sustain the applied loads without exceeding
the safe bearing capacity of the soil.
 The settlement of the structure should be
should be within the tolerable limits.
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When the upper soil layer is highly
compressible and too weak
In the case of Expansive soils
In case of Bridge abutments and piers
because of soil erosion at the ground
surface
Soils such as loess are collapsible in
nature
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 Spread
footing: A spread footing is one which
supports either one wall or one column.
Spread footing may be of the following types –
 Strip footing
 Pad footing
Fig: Pad Footing
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Combined footing: When a spread footing
supports the load of more than one column or
wall.
Fig: Combined Footings
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Strap footing: : A strap footing comprises of
two or more footings of individual columns,
connected by a beam, called a strap.
Fig: Strap Footings
Raft foundation: A raft foundation is a
combined footing that covers the entire area
beneath a structure and supports all the walls and
columns.
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Fig- Raft foundations
 Requirements for the raft foundations:
 The allowable soil pressure is low, or the building loads are heavy
 Use of spread footings would cover more than one-half of the area
 Soil is sufficiently erratic so that the differential settlement difficult to control
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Bearing capacity of soil
Ground water table
Depth of frost action
Depth of volume change due to presence of
expansive soils
Local erosion of soil due to flowing water
Underground defects such as root holes,
cavities, mine shafts, etc.
excavation, ditch, pond, water course, filled
up ground
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The distribution of soil pressure under a footing is a function of the
type of soil, the relative rigidity of the soil and the footing, and the
depth of foundation at level of contact between footing and soil.
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A RISING WATER TABLE HAVE FOLLOWING ADVERSE EFFECTS :
1) Appreciable reduction in the bearing capacity
2) Development of uplift pressure
3) Possible ground heave due to the reduction of the
effective stresses caused by the increasing pore water
pressures.
4) Expansion of the heavily compacted fills under the
foundation
5) Appreciable settlements of the poorly compacted fills
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 Soil stiffness is generally measured in the terms of Modulus of sub-
grade reaction (K-value).
Where, p = load intensity corresponding to settlement of plate (30cm x 30cm)
of 0.125 cm.
 TABLE: K-VALUE CHANGES WITH SOIL CHARACTERISTICS
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 Foundation Size Effect on Modulus of Sub grade Reaction in
Clayey Soil :
 Foundation Size Effect on Modulus of Subgrade Reaction In
Sandy Soils:
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Factors influencing Bearing Capacity:
Type of soil
III. Unit weight of soil
II. Surcharge load
IV. Depth of foundation
V. Mode of failure
VI. Size of footing
VII. Shape of footing
VIII. Depth of water table
IX. Eccentricity in footing load
X. Inclination of footing load
XI. Inclination of ground
XII. Inclination of base of foundation
I.
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 General shear failure: Seen in dense and stiff soil.
Fig: Fig: General shear failure
 Local shear failure: Seen in relatively loose and soft soil.
Fig:
Fig: Local shear failure
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 Punching shear failure: Seen in loose , soft soil and at deeper elevations.
Fig- punching shear failure
TERZAGHI’S BEARING CAPACITY THEORY:
According to Terzaghi the equation for ultimate bearing capacity for a strip footing
is obtained as follows, ultimate bearing capacity
qf = cNC + γDNq +0.5 γBNγ
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Circular footing :
qf = 1.3 cNc + γDNq +0.3 γBNγ
Square footing:
qf = 1.3 cNc + γDNq +0.4 γBNγ
Rectangular footing:
qf = (1+0.3 B/L)cNc + γDNq + (1-0.2 B/L)0.5γBNγ
Effect of Water Table fluctuation :
Ultimate bearing capacity with the effect of water table is given by,
qqf= =
cNcN
γBN
C + γDN
q RW1
γ RW2R
+ γDN
R+0.5
+0.5
γBN
f
C
q
W1
γ
W2
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CASE 1:
Where, ZW1 is the depth of water table from ground level.
CASE 2:
Where, ZW2 is the depth of water table from foundation level.
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 General shear failure:
qf = c Nc sc dc ic + q (Nq-1) sq dq iq + 0.5γ B Nγ sγ dγ iγ W
 Local shear failure:
qf = ⅔ c N'c sc dc ic + q (N

q-1) sq dq iq +
0.5γ B N'γ sγ dγ iγ W
Shape factors for different shapes of footings:
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Depth factors:
Inclination factor :
Values of W :
1. Water table remain at or below a depth of (Df + B), then W = 1.
2. Water table located at depth Df or likely to rise above the base then,
W = 0.5
3. If Df < Dw < (Df + B), then W be obtained by linear interpolation
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The total settlement of a footing in clay may be considered to three
components (Skempton and Bjerrum, 1957)
 Immediate
Settlement:
Values for influence factors, If :
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Primary Consolidation: The primary consolidation
settlement Sc is given by the following formula:
Sc =
Values of 𝜆 for different types of soil :
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Secondary consolidation: Secondary consolidation
settlement is more important in the case of organic and highlycompressible inorganic clays which is given by,
Ss =
Cα = Secondary Compression Index =
Fig: void ratio vs. time (log scale)
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1) Effect of Depth of Foundation:
Corrected settlement = Scorrected = Sc x Depth factor
Fig: Fox’s correction curves for settlements of flexible
Rectangular footings of BxL at depth D
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2) Effect
of the rigidity of foundation:
 Rigidity factor =
= 0.8

TABLE: Permissible uniform and differential settlement and tilt for footings
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 LOADING SYSTEMS: There are two loading set-up :
Fig: set up for gravity loading platform
Fig: set up for reaction loading platform
 DETERMINATION OF SETTLEMENT:
According to Terzaghi and Peck (1948):
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 According to Bond (1961):
 Table: Values of index n for different soils:
 DETERMINATION OF BEARING CAPACITY:
 Bearing capacity can be obtained from the load settlement curve
that can be plotted from settlement data.
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o
Fig : Load- settlement curves
obtained from test
From the corrected load settlement curves (given below)the
ultimate bearing capacity in case of dense cohesionless soils or
cohesive soils can be obtained without difficulty (curves D and
B ) as the failure is well defined.
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 Fig : Corrected Load–Settlement curve (in log-log scale)
The bearing capacity of sands and gravels increases with the size of
footings.
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 The following conclusions can be drawn , they are  Shallow foundations are used when the soil has sufficient strength
within a short depth below the ground level.
 Terzaghi’s equation is generally used for computation of bearing
capacity of soil.
 For design purpose, it is usually necessary to investigate both the
bearing capacity of soil and the settlement of a footing.
 Plate load test is used to determine the ultimate bearing capacity
and settlement of a footing in field.
 There are another tests like S.P.T and C.P.T also used to determine
ultimate bearing capacity.
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IS 6403: 1981 (Reaffirmed 2002): Code of practice for determination of breaking
capacity of shallow foundations
IS:1888:1982 (Reaffirmed 1995) : Method of load test on soils
IS 1080 - 1985 (Reaffirmed 1997): Code of practice for design and construction of
shallow foundations in soils (other than raft, ring and shell).
IS 2950 (Part1) -1981 (Reaffirmed 1998): Code of practice for design and
construction of raft foundations - part 1 design.
IS 8009 (Part 1) - 1976 (Re affirmed 1998): Code of practice for calculation of
settlements of foundations part-1(swallow foundations subjected to symmetrical static
vertical loads).
IS 8009 (Part 2) - 1980 (Re affirmed 1995): Code of practice for calculation of
settlements of foundations part-2(deep foundations subjected to symmetrical static
vertical loading).
IS 9214 - 1979 (Re affirmed 1997): Method of determination of modulus of
subgrade reaction (k-value) of soils in field.
Soil mechanics and foundation: Punmia, Jain and Jain.
NPTEL – Advanced foundation engineering.
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