Regarding Types of Foundation, Methods, Uses of different types of foundation at different soil properties. Methods of construction of different types of foundation, Codal Provisions etc.
2. Shallow Foundations
• Shallow Foundations versus Deep
Foundations
Foundations
Shallow
Foundations
Deep
Foundations
Spread
Footings
Mat
Foundations
Driven
Piles
Drilled
Shafts
Auger Cast
Piles
3. Types of Foundations
Shallow Foundations:
Founded near to the finished ground surface;
where the founding depth (Df) is less than
the width of the footing. It includes footings,
mat or raft foundation.
Pile Foundations:
Long and slender structural members which
transfer the load to deeper soil or rock of
high bearing capacity avoiding shallow soil
of low bearing capacity.
4. Shallow Foundations
• Usually the more economical option
• As a general rule, consider deep foundations
only when shallow foundations do not give
satisfactory design
• Types of Shallow foundations
• Spread footings (square, circular, rectangular)
• Combined Footings
• Continuous Footings
• Mat or Raft Foundations
19. MODES OF SHEAR FAILURE :
Vesic (1973) classified shear failure of soil
under a foundation base into three categories
depending on the type of soil & location of
foundation.
1) General Shear failure.
2) Local Shear failure.
3) Punching Shear failure
23. Effect of water table on Bearing
Capacity
:
* Terzaghi’s equation is based on the assumption that
water table is located at a great depth .
* If the water table is located close to foundation ; the
equation needs modification.
24. i) Water table is above the base of footing -
*
The effective surcharge is reduced as the
effective weight below water table is
equal to submerged unit weight.
q = Dw.r + x.γsub
put x = Df - Dw
q = γsub
Df + ( γ- γsub
)Dw
25. Thus,
qu = c’
Nc + [γsub
Df +(γ - γsub
)Dw] Nq + 0.5 γsub
BNr
When, Dw =0
(when water table is at ground level)
qu =c’
Nc + γsub
Nc + 0.5 γsub
BNr
& when x = 0
(when water table is at foundation level)
qu = c’
Nc + γ Df Nq + 0.5 γsub
BNr
26. ii) Water table at depth y below base :
* Unit weight in term is γ = γsub
+ y ( γ – γsub
)
B
Thus,
qu = c’
Nc + γ Df Nq + 0.5B γ Nr
iii) When y = B ; W.T. at B below base of footing.
qu = c’
Nc + γ Df Nq + 0.5 B γ Nr
Hence when ground water table is at b ≥ B, the equation is not
affected.
Surcharge term
is not affected
ii)
27. Basic Definitions :
1) Ultimate Bearing Capacity (qu) :
The ultimate bearing capacity is the gross pressure at
the base of the foundation at which soil fails in shear.
2) Net ultimate Bearing Capacity (qnu) :
It is the net increase in pressure at the base of
foundation that cause shear failure of the soil.
Thus, qnu = qu – γDf (overburden pressure)
28. 3) Net Safe Bearing Capacity (qns) :
It is the net soil pressure which can be safely
applied to the soil considering only shear failure.
Thus, qns = qnu /FOS
FOS - Factor of safety usually taken as 2 -3
4) Gross Safe Bearing Capacity (qs) :
It is the maximum pressure which the soil can carry
safely without shear failure. qs = qnu
/ FOS + γ Df
29. 5)Net Safe Settlement Pressure (qnp) :
It is the net pressure which the soil can carry without
exceeding allowable settlement.
6) Net Allowable Bearing Pressure (qna ):
It is the net bearing pressure which can be used for design of
foundation.
Thus,
qna = qns ; if qnp > qns
qna = qnp ; if qns > qnp
It is also known as Allowable Soil Pressure (ASP).
30. UNDRAINED CONDITION ( clay )
• Total Stress Analysis
• Total Stress Parameters
Cu (undrained cohesion)
φu (undrained internal friction angle)
• Unconfined compression strength test (UCS) ,
Unconsolidated Undrained Triaxial Test (UU)
Field Vane Shear Test
f u c 1
u c
a 1
q = S N + D
S N
q = + D
F.S.
γ
γ
34. Hansen’s Bearing Capacity Equation
qu = cNcScdcic + qNqSqdqiq + 0.5 γ BNrSrdr ir
where,
Nc,Nq, & Nr are Hansen’s B.C factors which are
some what smaller than Terzaghi’s B.C. factors.
Sc.Sq &Sr are shape factors which are
independent of angle of shearing resistance;
dc,dq, & dr are depth factors ;
ic, iq & ir are inclination factors
35. The same form of equation has been adopted
by I.S. 6403 –1971 & may be used for general form
as
qnu = cNcScdcic + q-(Nq-1)Sqdqiq
+ 0.5 γ BNrSrdr irW ’
36. Basic criteria of foundation design
1. Sufficient Factor of Safety (> 2)
in BEARING CAPACITY
2. No excess settlement,
especially differential
settlement
Clay depend on both Bearing
capacity & Settlement
Sand depend on both Settlement
governs
usually
38. The Long Term Response
(constant load)
Despite the low permeability, the soil will drain,
over months, or maybe years
reduction in void ratio, e
Settlement!
∆Vsoil = ∆Vwater
sc= ∆Vw = 1D consolidation
39. In Summary
Consolidation in saturated, low
permeability soils, under external
loading, is about building up the
effective vertical stress by
dissipating the initial excess water
pressure in the soil through
drainage
- or shedding of load back to
the soil skeleton, so that σσvv′′
= σσvv
40. A consolidated soil is a
physically improved soil!
Less void space
Greater contact between soil
particles
Stronger and stiffer!
42. Time, t, after load application
but before equilibrium
⇒ Intermediate level of pore water
pressure
⇒ u = 0 at drainage surfaces, any
time!
⇒ soil drains though top or bottom
face of sample
⇒ Maximum path length?
44. Information from testing
a) Each “stage”
a) Time rate of consolidation
b) All loading stages
a) Compressibility of the soil
b) Pre-consolidation history of the soil
c) Unloading stages
a) “rebound” or “swelling” index
45. “coefficient of consolidation”
Is estimated from plots of settlement against
EITHER:
1. SQRT(time)
2. LOG10 (time)
AND using Terzaghi’s 1D Consolidation theory
wv
v
γm
k
c =
48. SETTLEMENT OF FOUNDATIONS
a) Settlement under loads
1. Elastic settlement (Si):
Elastic or immediate settlement takes place during or immediately
after the construction of the structure. It is also known as the
distortion settlement as it is due to distortions within foundation soil.
2. Consolidation settlement (Sc):
Consolidation settlement occurs due to gradual expulsion of water
from the voids in the soil. It is determined using Terzaghi's theory of
consolidation.
49. Thus,
Total settlement (S) = Si+ Sc + Ss
33. Secondary consolidation settlement. Secondary consolidation settlement (S(Sss ):):
The settlement essentially occurs after completionThe settlement essentially occurs after completion
of the primary consolidation (after excessof the primary consolidation (after excess
pore water pr=0) .pore water pr=0) .
The secondary consolidation is insignificant forThe secondary consolidation is insignificant for
inorganic soilsinorganic soils..
50. b) Settlement due to other causes
1. Structural collapse of soil.
2. Underground erosion.
3. Lowering of water table. .
4. Thermal changes.
5. Subsidence etc.
52. Consolidation Test AnalysisConsolidation Test Analysis
… consolidation test data plotted as (a) percent consolidation
versus effective stress, and (b) void ratio versus effective
stress.
53. Consolidation Test AnalysisConsolidation Test Analysis
… consolidation test data semi-log plots: (a) percent
consolidation versus effective stress, and (b) void ratio versus
effective stress.
54. Compression and recompressionCompression and recompression
indicesindices
log σv’
voidratio
1
Cc
Cc ~ compression index
Cr ~ recompression index
(or swelling index)
1
Cr
1
C
r
55. • Normally consolidated Clays (NCC) :-
A normally consolidated soil is one which had not been
subjected to a pressure greater than the present existing
pressure.
• Over consolidated clays (OCC): -
A soil is said to over consolidated if it had been subjected in
the past to a pressure in excess of the present pressure.
56. • NCC and OCC are not different types of soils but these are
conditions in which a soil exists.
• Pre-consolidation Pressure-
The maximum pressure to which an over-consolidated soil
had been subjected in the past is known as pre-consolidation
pressure ( σc)
• When a soil specimen is taken from a natural deposit, the
weight of overlying material is removed.. Thus the specimen is
generally pre-consolidated, even though to a small extent.
57. • Pre-consolidation stress (pressure)
The maximum effective stress which has been applied
to an element of soil
• Over - consolidated
A soil is called over-consolidated (OC) if:
• Normally consolidated
A soil is called normally consolidated (NC) if:
Terminology
Current Effective Stress Pre-consolidation
Stress
<
Current Effective Stress Pre-consolidation
Stress
=
59. 1) Coefficient of compressibility ( av)
( slope of e - σ curve, units – m 2
/kN )
av = -de/dσ
= -∆e/ ∆σ
2) Coefficient of volume change ( mv)
mv = - (∆v / v o)/ ∆σ in which, vo = initial volume,
∆v = change in volume
∆ σ = change in effective stress
= -(∆e / 1+ eo)/ ∆ σ
for1- D consolidation s, ∆v = ∆H
mv = - (∆H / Ho) / ∆σ
also mv = av / (1+ eo ) in which, eo= initial void ratio.
∆e = change in void ratio.
Ho = initial thickness.
∆H = change in thickness.
60. 3) Compression index ( Cc) is equal to the slope of the
linear portion of the void ratio versus
log σ plot.
Cc = - ∆ e/ log 10
(σ 0
+ ∆ σ) / σ0
in which, σ0
= initial effective stress.
∆σ = change in effective stress.
Empirical relationship ( Terzaghi & Peck)
a) for undisturbed soils Cc = 0.009 ( w L
- 10 )
b) for remoulded soils Cc = 0.007 (wL
- 10 )
c) Also Cc = 0.54 ( eo – 0.35 )
Cc = 0.0054 ( 2.6 wo- 35 )
61. Settlement computations
eo, σvo’, Cc,
Cr, σp’, mv
-oedometer
test
∆σ=q
q kPa
H
Two different ways to estimate the
consolidation settlement:
(a) using mv
(b) using e-log σv’ plot
settlement = mv ∆σ H
H
e
e
settlement
o+
∆
=
1
next slide
62. Settlement computations
~ computing ∆e using e-log σv’ plot
'
''
log
vo
vo
cCe
σ
σσ ∆+
=∆
initial
σvo’
eo
σvo’+ ∆σ
∆e
If the clay is normally consolidated,
the entire loading path is along the VCL.
63. Settlement
computations
~ computing ∆e using e-log σv’ plot
'
''
log
vo
vo
rCe
σ
σσ ∆+
=∆
σvo’
initial
eo
σvo’+ ∆σ
If the clay is over-consolidated, and remains so
by the end of consolidation,
∆e
VC
L
note the use of Cr
67. FINAL SETTLEMENT OF SOIL DEPOSIT
IN THE FIELD
• Let Ho = initial thickness of clay deposit.
Consider a small element of thickness Δz at depth z.
∆σ = effective pressure increment causing
settlement.
Then, ∆H = mv Ho (∆σ )
Representing the final settlement as ∆sf &
taking Ho = ∆z
Thus, total settlement of the complete layer,
-
1
)........(2i)z(i)(i)mv( ∆∂∆∑=
=
n
i
Sf
68. ELASTIC SETTLEMENT OF
FOUNDATION ON COHESIVE SOILS
The vertical settlement under uniformly distributed
flexible area is,
Si = q B 1- μ2
/Es I
where
q -uniformly distributed load.
B - characteristic length of loaded area,
Es - modulus of elasticity of the soil.
μ - poisson's ratio.
I - influence factor dependent upon elastic
properties of base & shape at base.
Alternatively, the value of [1- μ2
/Es] I can be
determined from the plate load test.
69. ALLOWABLE SETTLEMENT
* The allowable maximum settlement depends upon the type
of soil, the type of foundation & the structural framing system.
* The maximum settlement ranging from 20 mm to 300 mm
is generally permitted for various structures.
* IS 1904-1978 gives values of the maximum & differential
settlements of different type of building.
70. Sand & hard
Clay
Plastic clay
Max.Settle. Diff.Settl Angular
distortion
Max.Settle Diff.
Settle.
Angular
distortion
Isolated
foundation
i) steel struct
ii) RCC struct
50mm
50mm
0.0033L
0.0015L
1/300
1/666
50mm
75mm
0.0033L
0.0015L
1/300
1/666
Raft
foundation
i) steel struct
ii) Rcc struct.
75mm
75mm
0.0033L
0.002L
1/300
1/500
100mm
100mm
0.0033L
0.002L
1/300
1/500
Theoretically, no damage is done to the superstructure
if the soil settles uniformly.
However, settlements exceeding 150mm may cause
trouble to utilities such as water pipe lines, sewers,
telephone lines & also is access from streets.
71. Pile Foundations
Pile material.
1.Timber. 2.Concrete
3.Steel. 4.Composite piles
Effect on the soil.
1.Driven piles 2.Bored piles.
Classification according to
Load transfer mechanism: Loads carried
1.End bearing pile Axial compression
2.Friction piles Axial tension/lateral load
3.Combination of friction and end bearing piles
74. Theory of the Rock
2
4
2
1
45tan
2
2
D
π
A
AqQ
πDLA
qαβf
AfQ
QQQ
tip
tiptip
s
uc
sfriction
tipfrictionult
N
qNq uctip
=
×=
=
=
×=
+=
+=
=
φφ
φ
