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1. FOUNDATIONS &
BEARING
CAPACITY
Geotechnical and Foundation Engineering
Faisal Raza
1

2. Foundations
• The lowest part of a structure generally is
referred to as the foundation.
• Its function is to transfer the load of the
structure to the soil on which it is resting. A
properly designed foundation transfers the
load throughout the soil without
overstressing the soil.
• Overstressing the soil can result in either
excessive settlement or shear failure of the
soil, both of which cause damage to the
structure.
2

3. Foundations
in Civil
Engineering
• The foundation is the structural element
that connects a structure to the ground
• These elements are made of concrete, steel,
wood, etc
• Foundations can be divided into two broad
categories
• Shallow foundations and deep foundations
• Shallow foundations transmit the structural
loads to the near-surface soils
• Deep foundations transmit some or all the
loads to deeper soils.
3

4. Types
Foundations
Shallow
Spread
Mat
Deep
Piles
Anchors
Drilled Shafts
Depending on the structure and soil encountered,
various types of foundations are used.
4

5. Spread
Footing
• A spread footing is simply an enlargement
of a load-bearing wall or column that makes
it possible to spread the load of the
structure over a larger area of the soil
5

6. 6

7. Use of a strap footing
with
a grade beam to
support exterior
columns
when construction
cannot extend beyond
the property line.
7

8. Mat / Raft
Footing
• In soil with low load-bearing capacity, the size of the spread
footings required is impracticably large. In that case, it is
more economical to construct the entire structure over a
reinforced concrete pad.
•The structural load is so high, or soil conditions are so poor that spread
footings would be exceptionally large. As a rule of thumb, if spread
footing would cover more than 50 % of the building footprint area, a mat
or some type of deep foundation will usually be more economical.
If the soil is very erratic and prone to excessive differential
settlements. The structural continuity and flexural strength of a mat
will bridge over these irregularities.
The uplift loads are larger than spread footings can accommodate.
The greater weight and continuity of a mat may provide sufficient
resistance.
The bottom of structure is located below the GWT, so water proofing is
an important concern. Because mats are monolithic, they are much
easier to waterproof. The weight of mat also helps resist hydrostatic uplift
forces from the groundwater.
8

9. MAT FOOTING
9

10. Deep
Foundations
• Pile and drilled shaft foundations are used for
heavier structures when great depth is
required to support the load.
• Piles are structural members made of timber,
concrete, or steel that transmit the load of the
superstructure to the lower layers of the soil.
• According to how they transmit their load into
the subsoil, piles can be divided into two
categories:
• Friction piles and End-bearing piles.
• In the case of friction piles, the
superstructure load is resisted by the shear
stresses generated along the surface of the
pile.
• In the end-bearing pile, the load carried by the
pile is transmitted at its tip to a firm stratum.
10

11. Deep
Foundations
• In the case of drilled shafts, a shaft is
drilled into the subsoil and then filled with
concrete. A metal casing may be used while
the shaft is being drilled.
• The casing may be left in place or may be
withdrawn during the placing of concrete.
• Generally, the diameter of a drilled shaft is
much larger than that of a pile.
• The distinction between piles and drilled
shafts becomes hazy at an approximate
diameter of 1 m (3 ft), and the definitions
and nomenclature are inaccurate.
11

12. Rule of
Thumb
• In a more general sense, shallow
foundations are foundations that have a
depth-of-embedment-to-width ratio of
approximately less than four.
• When the depth-of-embedment-to-width
ratio of a foundation is greater than four, it
may be classified as a deep foundation.
12

13. Foundations
13

14. BEARING
PRESSURE
14

15. Bearing
Pressure
• Bearing pressure is the contact force per unit
area along the bottom of the foundation.
• The bearing pressure is not necessarily
distributed evenly. Analytical studies and field
measurements indicate that actual distribution
depends on several factors, including the
following:
• Eccentricity, if any, of the applied load
• Magnitude of the applied moment, if any
• Structural rigidity of the foundation
• Stress-strain properties of the soil
• Roughness of the bottom of the foundation
15

16. Distribution of
bearing pressure
Real footings are close to being perfectly rigid,
so the bearing pressure distribution is not
uniform. However, bearing capacity and
settlement analyses based on such a
distribution would be very complex, so it is
customary to assume that the pressure beneath
concentric vertical loads is uniform across the
base of the footing, as shown. The error
introduced by this simplification is not
significant.
16

17. Computation
of bearing
pressure
• where
• q = bearing pressure
• P = vertical column load
• Wf = weight of foundation, including
the weight of soil above the
foundation, if any
• A = base area of foundation
• uD = pore water at bottom of
foundation (i.e. at a depth D
below the ground surface
GWT
D
P
17

18. Bearing
pressure –
Continuous
Footings
18

19. Example
• The 5 ft square footing
shown in Figure supports a
column load of 100 k.
Compute the bearing
pressure.
19

20. Example
A 0.70 m wide continuous footing
supports a wall load of 110 kN/m.
The bottom of this footing is at a
depth of 0.50 m below the adjacent
ground surface and the soil has a
unit weight of 17.5 kN/m3. The
groundwater table is at a depth of
10 m below the ground surface.
Compute the bearing pressure.
20

21. Net bearing pressure
• An alternative way to define bearing pressure is the net bearing pressure,
q′, which is the difference between the gross bearing pressure, q, and the
initial vertical effective stress, s′zo, at depth D. In other words, q′ is a
measure of the increase in vertical effective stress at depth D.
21

22. Example
• The mat foundation in Fig.
below is to be 50 m wide, 70 m
long, and 1.8 m thick. The sum
of the column and wall loads is
805 MN. Compute the average
bearing pressure, then compare
it with the initial vertical
effective stress in the soil
immediately below the mat.
Use conc = 23.6 kN/m3.
22

23. BEARING
CAPACITY OF
SOILS
23

24. Definitions
• Ultimate bearing capacity
• The value of the average contact pressure between
the foundation and the soil which will produce shear
failure in the soil.
• Safe bearing capacity
• The maximum value of contact pressure to which
the soil can be subjected without risk of shear
failure. This is based solely on the strength of the
soil and is simply the ultimate bearing capacity
divided by a suitable factor of safety.
• Allowable bearing pressure
• The maximum allowable net loading intensity on the
soil allowing for both shear and settlement effects.
24

25. Definitions
• Bearing Capacity
• Bearing capacity is the power of foundation soil to
hold the forces from the superstructure without
undergoing shear failure or excessive settlement.
• Total Overburden Pressure q0
• The pressure due to the weight of both soil and
water at the base level of the foundation
• Effective Overburden Pressure q'0
• q'0 is the effective overburden pressure at the base
level of the foundation.
25

26. Definitions
• Ultimate Bearing Capacity (qu)
• It is the maximum pressure that a foundation soil
can withstand without undergoing shear failure.
• Net ultimate Bearing Capacity (qnu)
• qnu is the bearing capacity in excess of the
effective overburden pressure q’o expressed as
• qn = qf – q’o
• Gross Allowable Bearing Pressure (qa)
• Net Allowable Bearing Pressure, (qna)
26

27. Bearing
Capacity
• Spread footings transmit the applied
structural loads to the near-surface soils.
• In the process of doing so, they induce both
compressive and shear stresses in these
soils. The magnitudes of these stresses
depend largely on the bearing pressure and
the size of the footing.
• If the bearing pressure is large enough, these
shear stresses may exceed the shear strength
of the soils, resulting in a bearing capacity
failure.
• Researchers have identified three types of
bearing capacity failures
• General shear failure, Local shear failure, and
Punching shear failure.
27

28. General Shear Failure
• General shear failure is the most common mode for spread
footings. It occurs in soils that are relatively
incompressible and reasonably strong, and in saturated,
normally consolidated clays that are loaded rapidly enough
that the undrained condition prevails.
• The failure surface is well-defined, and failure occurs quite
suddenly, as illustrated by the load-displacement curve. A
clearly formed bulge appears on the ground surface
adjacent to the spread footing.
• Although bulges may appear on both sides of the footing,
ultimate failure occurs on one side only, and it is often
accompanied by rotation of the footing.
28

29. Punching Shear Failure
• The opposite extreme is punching shear failure. This mode
of failure occurs in very loose sands, in a thin crust of
strong soil underlain by very weak soil, or in weak clays
loaded under slow, drained conditions.
• The high compressibility of such soil profiles causes large
settlements and poorly defined vertical shear surfaces.
Little or no bulging occurs at the ground surface and
failure develops gradually, as illustrated by the ever-
increasing load depicted in the load- settlement curve.
29

30. Local Shear Failure
• The local shear failure is an intermediate case. The
shear surfaces are well-defined under the spread
footing, and then become vague near the ground
surface.
• A small bulge may occur, but considerable settlement,
perhaps on the order of half the footing width, is
necessary before a clear shear surface forms near the
ground. Even then, a sudden failure does not occur, as
happens in the general shear case. The footing just
continues to sink ever deeper into the ground.
30

31. Failures’
Summary
31

32. Failures’
Summary
• Shallow foundations (D/B less than about 2)
can fail in any of the three modes, depending
on the relative density.
• However, deep foundations (D/B greater than
about 4) are always governed by punching
shear.
• Although these test results apply only to
circular foundations in Vesic’s sand and
cannot necessarily be generalized to other
soils, it does give a general relationship
between the mode of failure, relative density,
and the D/B ratio.
32

33. Failures’
Summary
• The following guidelines are helpful:
• Spread footings in undrained cohesive soils are
governed by the general shear case.
• Spread footings in dense cohesionless soils are
governed by the general shear case. In this context,
a dense sand is one with a relative density, D r,
greater than about 67%.
• Spread footings on loose to medium dense
cohesionless soils (30%<Dr<67%) are probably
governed by local shear.
• Spread footings on very loose cohesionless soils
(Dr<30%) are probably governed by punching shear.
33

34. Failures’
Summary
34

35. Failures’
Summary
35

36. EVALUATION OF
BEARING CAPACITY
36

37. Approaches
• Assessments of the performance of real foundations,
including full- scale load tests
• Full- scale load tests, which consist of constructing real spread footings
and loading them to failure, are the most precise way to evaluate
bearing capacity. However, such tests are expensive, and thus are
rarely, if ever, performed as a part of routine design.
• Load tests on model footings
• Model footing tests have been used to research bearing capacity, mostly
because the cost of these tests is far below that of full - scale tests.
Unfortunately, model tests have their limitations, especially when
conducted in sands, because of uncertainties in applying the proper
scaling factors. However, the advent of centrifuge model tests has
partially overcome this problem.
37

38. Approaches
• Bearing capacity theories
• The dominant way to assess bearing capacity of spread
footings is to use bearing capacity theories. In a typical
bearing capacity theory, the shape of the failure surface
is defined in advance and then equilibrium is considered
to evaluate the stresses and strengths along this surface.
• Detailed numerical analyses, such as those
using the finite element method (FEM)
38

39. Bearing
Capacity
Analysis
39

40. Terzaghi’s
Bearing
Capacity –
Assumptions
• The depth of the footing is less than or equal to its width
(D ≤ B).
• The bottom of the footing is sufficiently rough that no
sliding occurs between the footing and the soil.
• The soil beneath the footing is a homogeneous semi-
infinite mass (i.e., the soil extends for a great distance
below the footing and the soil properties are uniform
throughout).
• The shear strength of the soil is described by the formula
𝝉 = c′ + 𝝈′ tan 𝝋′.
• The general shear mode of failure governs.
• No consolidation of the soil occurs (i.e., settlement of the
footing is due only to the shearing and lateral movement
of the soil).
• The footing is very rigid in comparison to the soil.
• The soil between the ground surface and a depth D has
no shear strength and serves only as a surcharge load.
• The applied load is compressive and applied vertically
through the centroid of the footing and no applied
moment loads are present.
40

41. Terzaghi’s
Bearing
Capacity
41

42. Terzaghi’s
Bearing
Capacity
• Terzaghi considered three zones in the soil.
Immediately beneath the footing is a wedge zone
that remains intact and moves downward with the
footing. Next, a radial shear zone extends from
each side of the wedge, where he took the shape of
the shear planes to be logarithmic spirals. Finally,
the outer portion is the passive zone or linear
shear zone in which the soil shears along planar
surfaces.
• Since Terzaghi neglected the 𝝉 of soils b/w the
ground surface and a depth D, the shear surface
stops at this depth and the overlying soil has been
replaced with the surcharge pressure s′zD.
• Terzaghi developed his theory for continuous
footings. This is the simplest case because it is a
2D problem. He then extended it to square and
round footings by adding empirical coefficients
obtained from model tests.
42

43. Terzaghi’s
Bearing
Capacity
43

44. Terzaghi’s
Bearing
Capacity
44

45. Terzaghi’s
Bearing
Capacity
• In the case of local shear failure, we may
assume that
45

46. Terzaghi’s
Bearing
Capacity
46

47. Terzaghi’s
Bearing
Capacity
47

48. Effect of
GWT
• The presence of shallow groundwater
affects shear strength in two ways: the
reduction of apparent cohesion, and the
increase in pore water pressure.
• Both of these affect bearing capacity, and
thus need to be considered.
48

49. Effect of
GWT
Apparent
Cohesion
• Sometimes soil samples obtained from the exploratory
borings are not saturated, especially if the site is in
an arid or semi- arid area. These soils have additional
shear strength due to the presence of apparent
cohesion. However, this additional strength will
disappear if the moisture content increases. Water
may come from landscape irrigation, rainwater
infiltration, leaking pipes, rising groundwater, or other
sources. Therefore, we do not rely on the strength due
to apparent cohesion.
• To remove the apparent cohesion effects and simulate
the “worst case” condition, geotechnical engineers
usually wet the samples in the lab prior to testing.
However, even with these precautions, the cohesion
measured in the laboratory test may still include some
apparent cohesion. Therefore, we often perform
bearing capacity computations using a cohesion value
less than that measured in the laboratory.
49

50. Effect of
GWT
Pore Water
Pressure
• If there is enough water in the soil to develop
a groundwater table, and this GWT is within
the potential shear zone, then pore water
pressures will be present, the effective stress
and shear strength along the failure surface
will be smaller, and the nominal unit bearing
capacity will be reduced. We must consider
this effect when conducting bearing capacity
computations.
• When exploring the subsurface conditions, we
determine the current location of the GWT and
the worst-case (highest) location that might
reasonably be expected during the life of the
proposed structure.
50

51. Effect of
GWT
51

52. Effect of
GWT
• Case I: If the GWT is located at a distance D
above the bottom of the foundation, the
magnitude of q in the second term of the
bearing capacity equation should be
calculated as
𝒒 = 𝜸 𝑫𝒇 − 𝑫 + 𝜸′𝑫
where 𝜸′ = 𝜸𝒔𝒂𝒕 − 𝜸𝒘 = effective unit weight of
soil. Also, the unit weight of soil, g, that appears
in the third term of the bearing capacity
equations should be replaced by 𝜸′
• Case II: If the GWT coincides with the bottom
of the foundation, the magnitude of q is equal
to 𝜸Df. However, the unit weight, g, in the third
term of the bearing capacity equations should
be replaced by 𝜸′
52

53. Effect of
GWT
• Case III: When the GWT is at a depth D
below the bottom of the foundation,
𝒒 = 𝜸𝑫𝒇. The magnitude of g in the third
term of the bearing capacity equations
should be replaced by 𝜸av.
53

54. Effect of
GWT
54

55. Example – 1
A square footing is to be constructed. The GWT
is at a depth of 50 ft below the GL. Compute the
nominal unit bearing capacity and the column
load required to produce a bearing capacity
failure.
55

56. Example – 2
The proposed continuous footing will support the
exterior w all of a new industrial building. The
underlying soil is an undrained clay, and the GWT is
below the bottom of the footing. Compute the
nominal unit bearing capacity and compute the wall
load required to cause a bearing capacity failure.
56

57. Bearing
Capacity
Developments
• Skempton (1951)
• Meyerhof (1953)
• Brinch Hanson (1961)
• De Beer and Ladanyi (1961)
• Meyerhof (1963)
• Brinch Hanson (1970)
• Vesic (1973, 1975)
57

58. Vesić’s
Bearing
Capacity
Formulas
• Vesic´ retained Terzaghi’s basic format and
added the following additional factors:
• sc, sq, s𝜸 = shape factors
• dc, dq, d𝜸 = depth factors
• ic, iq, i𝜸 = load inclination factors
• bc, bq, b𝜸 = base inclination factors
• gc, gq, g𝜸 = ground inclination factors
58

59. Vesić’s
Bearing
Capacity
Formulas
59

60. Vesić’s
Bearing
Capacity –
Shape &
Depth
Factors
• For continuous footings, B/L is small, so sc, sq, and
s𝜸 are close to 1. This means the shape factors may
be ignored when analyzing continuous footings.
• For relatively shallow footings (D/B≤1), use k = D/B.
For deeper footings (D/B>1), use
k = tan-1(D/B) with the tan-1 term is expressed in
radians. Note that this produces a discontinuous
function at D/B=1.
60

61. Vesić’s
Bearing
Capacity –
Inclination
Factors
• For loads
inclined in the
B direction:
• For loads
inclined in the L
direction:
61

62. Vesić’s
Bearing
Capacity –
Base
Inclination
Factors
• The vast majority of footings are built with
horizontal bases. However, if the applied load is
inclined at a large angle from the vertical, it may be
better to incline the base of the footing to the same
angle so the applied load acts perpendicular to the
base. However, keep in mind that such footings may
be difficult to construct.
• If the base of the footing is level, which is the usual
case, all of the b factors become equal to 1 and may
be ignored.
62

63. Vesić’s
Bearing
Capacity –
Ground
Inclination
Factors
• Footings located near the top of a slope have a
lower bearing capacity than those on level
ground.
• If the ground surface is level (β = 0), the g factors
become equal to 1 and may be ignored.
63

64. Vesić’s
Bearing
Capacity –
Bearing
Capacity
Factors
• Most other authorities also accept above
equations, or others that produce very similar
results. However, there is much more
disagreement regarding the proper value of N𝜸.
Relatively small changes in the geometry of the
failure surface below the footing can create
significant differences in N𝜸, especially in soils
with high friction angles. Vesic recommended the
following formula
64

65. Shape,
Depth, and
Inclination
Factors
65

66. Shape,
Depth, and
Inclination
Factors
66

67. Shape,
Depth, and
Inclination
Factors
67

68. N𝜸
Computations
68

69. N𝜸
Computations
69

70. N𝜸
Computations
70

71. Rule of
Thumb
71

72. ECCENTRICALLY
LOADED
FOUNDATIONS
72

73. Eccentrically
Loaded
Foundations
In several instances, as with the
base of a retaining wall,
foundations are subjected to
moments in addition to the
vertical load, as shown in Figure
4.17a. In such cases, the
distribution of pressure by the
foundation on the soil is not
uniform. The nominal distribution
of pressure is
73

74. Eccentrically
Loaded
Foundations
Figure 4.17b shows a force system
equivalent to that shown in Figure
4.17a. The distance is the
eccentricity.
74

75. Eccentrically
Loaded
Foundations
• Note that, in these equations, when the
eccentricity e becomes B/6, qmin is zero.
• For e>B/6, qmin will be -ve, which means
tension will develop. Because soil cannot
take any tension, there will then be a
separation between the foundation and the
soil underlying it.
• The nature of the pressure distribution on
the soil will be as shown in Figure 4.17a.
• The value of qmax is then
75

76. Eccentrically
Loaded
Foundations
76

77. Bearing Capacity — One-Way
Eccentricity
• Step 1 Determine the effective dimensions
of the foundation (Figure 4.19a):
B’ = effective width = B-2e
L’ = effective length = L
• Note that if e were in the direction of the
length of the foundation, the value of L’ = L -
2e. The value of B’ would equal B. The
smaller of the two dimensions (L’ and B’) is
the effective width of the foundation.
77
Effective Area Method (Meyerhoff, 1953)

78. Bearing Capacity — One-Way
Eccentricity
• Step 2 Use following for the ultimate
bearing capacity
• Step 3 The total ultimate load that the
foundation can sustain is
78
Effective Area Method (Meyerhoff, 1953)

79. Bearing Capacity — One-Way
Eccentricity
• Step 2 Use following for the ultimate
bearing capacity
• Step 3 The total ultimate load that the
foundation can sustain is
79
Effective Area Method (Meyerhoff, 1953)

80. Bearing
Capacity —
One-Way
Eccentricity
80
Prakash and Saran Theory

81. Bearing
Capacity —
One-Way
Eccentricity
81
Prakash and Saran Theory

82. Bearing
Capacity —
One-Way
Eccentricity
82
Prakash and Saran Theory

83. Bearing
Capacity —
One-Way
Eccentricity
83
Prakash and Saran Theory

84. Self Study Reduction Factor Method
84

85. BEARING CAPACITY—TWO-WAY ECCENTRICITY
85

86. Bearing
Capacity—
Two-Way
Eccentricity
86

87. Bearing
Capacity—
Two-Way
Eccentricity
87

88. Bearing
Capacity—
Two-Way
Eccentricity
88

89. Bearing
Capacity—
Two-Way
Eccentricity
89

90. Factor of
Safety
90

91. BEARING
CAPACITY
ON LAYERED
SOILS
91

92. IN-SITU
TESTING
92

93. PLATE LOAD
TEST
93

94. Why in-situ
Testing?
• Very Soft or Sensitive Clays
• Difficult/Expensive to Get Sample
• Stony Soils
• Damage samplers
• Sands and Gravels
• Expensive, difficult, little disturbance causes loss of
‘memory’
94

95. Tests
• Plate Load Test
• Standard Penetration Test (SPT)
• Cone Penetration Test (CPT)
• Dynamic Cone Penetration (DCPT)
• Vane Shear Test (VST)
• Dilatometer Test (DMT)
• Pressure-meter Test (PMT)
• Many more
95

96. Plate Load
Test (PLT)
• Most reliable method of obtaining the ultimate
bearing capacity of soil. Test would directly
give the bearing capacity if the load test is on
a full-size footing; however, this is not usually
done since an enormous load would have to
be applied.
• A compressive stress is applied to the soil
through rigid plates and the deflections are
measured for various stress values
• A graph is plotted between the measured
deflection (settlement) and applied load
• The pressure corresponding to the limiting
settlement is obtained from this graph
96

97. PLATE LOAD TEST (PLT)
97

98. Plate Load
Test (PLT)
98

99. Plate Load
Test (PLT)
• Apply load on small plates of diameters
from 30-75cm. These sizes are usually too
small to extrapolate to full-size footing
• The following two main factors make the
extrapolation questionable.
• The test gives information about the soil only to a
depth of twice the diameter of the bearing plate.
• The test takes into account only part of the effect of
time. The test is usually completed only in hours,
while foundation soils (especially clayey soils) take
years to consolidate.
99

100. Plate Load
Test (PLT)
100

101. Plate Load
Test (PLT)
101

102. Plate Load
Test (PLT)
Advantages
• Applicable to soils and rocks
• Relatively undisturbed conditions
• Can perform in soils difficult to sample &
test in the laboratory (gravelly soils, tills,
loess, etc.)
• Tests larger volume of soil than nearly all
laboratory tests –
• Particularly important in stiff, fissured clays
102

103. Plate Load
Test (PLT)
Limitations
• Time-consuming & expensive
• Limited depth until development of
borehole plate load tests
• It is essentially a short duration test, and
hence the test does not give the ultimate
settlement, practically in the case of
cohesive soil which consolidates after much
longer duration.
• The test data will be unreliable if plate
settlement is restricted by presence of a
boulder under the plate.
• Scale effects when evaluating Modulus
103

104. Plate Load
Test (PLT)
Limitations
104

105. Plate Load
Test (PLT)
Calculations
of Bearing
Capacity
105
CLAYEY SOILS
For clayey soils it is common to note that the
‘BNγ’ term in the equation for the ultimate
bearing capacity is zero, so that one might say
that qu is independent of width of footing and
therefore the ultimate bearing capacity of
proposed foundation is given by Equation below.
qu(foundation)= qu(plate)
C-ϕ SOILS
𝒒𝒖(𝒇)
= 𝒒𝑢 𝑝 ×
𝐵𝑓
𝐵𝑝
• The use of the equation-3 is recommended
only when the
Bf
Bp
ratio is up to about 3 or 4

106. Plate Load
Test (PLT)
Calculations
of
Settlements
106
For clayey soils
𝑆𝑓 = 𝑆𝑝 ×
𝐵𝑓
𝐵𝑝
For Sandy soils
Sf = 𝑆𝑝
𝟐Bf
Bf + Bp
2

107. STANDARD
PENETRATION
TEST
107

108. Standard
Penetration
Test
108

109. Standard
Penetration
Test
109

110. Standard
Penetration
Test
110

111. Standard
Penetration
Test
• Most frequently used In-situ test to measure
the shear strength of soil
• More useful for cohesionless soils
• SPT is conducted in a borehole using
standard equipment consisting of a
• A Standard Weight
• A split spoon sampler
• A mechanism for lifting and dropping the
standard weight, and
• A Set of connecting rods to reach the desired
depth
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112. Standard
Penetration
Test
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113. Standard
Penetration
Test
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114. Standard
Penetration
Test
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115. Standard
Penetration
Test
Procedure
• The bore hole is drilled to the desired depth
• The drilling tools are removed and the sampler is
lowered to the bottom of the hole
• The sampler is driven into the soil by a drop
hammer weighing 63.5kg mass falling through a
height of 750mm (30 inch)
• The sampler is driven by 450mm (18 inch) and the
number of hammer blows (N) required to drive
each 150mm (6 inch) are recorded
• The number of blows (N) recorded for the first
150mm are disregarded whereas the number of
blows recorded for last two 150mm intervals are
added to give the standard penetration number (N)
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116. Standard
Penetration
Test
Influences
• Influences on SPT N-values
• Variations in the test apparatus and procedures
• Disturbance created by bore hole
• Soil type and properties into which sampler is
driven
• Effective stress level
• Apply corrections for these influences
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117. Standard
Penetration
Test
Corrections
• Corrected N Values
(N1)60 = Nfield * CE * CN * CB * CS * CR
• (N1)60 = Corrected N value (including overburden
correction)
• Nfield = N value measured in the field
• CE = correction for hammer energy
• CN = correction for overburden
• CB= correction for Borehole size
• CS= correction for sampler used (smooth vs lined)
• CR = correction for Rod length
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118. Standard
Penetration
Test
Corrections
CE
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119. Standard
Penetration
Test
Corrections
CN, CB, CS, CR
• CN = correction for overburden
• Normalized with respect to atmospheric pressure
CN =√ (
Pa
σvo′
) ≤ 1.7 − 2.0
• CB= correction for Borehole size
• Bore size 2.5 to 4.5 in 1.00
• Bore size up to 6 in 1.05
• Bore size up to 8 in 1.15
• CS= correction for sampler used
• Smooth Sampler (or with liner) 1.00
• Without Liner 1.1-1.3
• CR = correction for Rod length
• 30 to 100 ft 1.0
• 20 to 30 ft 0.95
• 13 to 20 ft 0.85
• 10 to 13 ft 0.75
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120. Standard
Penetration
Test
Variations
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121. Standard
Penetration
Test
Readings
121
N Relative Density Φ’
0-4 Very Loose <28
4-10 Loose 28-30
10-30 Medium Dense 30-36
30-50 Dense 36-41
>50 Very Dense >41
Consistency N
qu
(kPa)
Very Soft < 2 < 25
Soft 2 – 4 25 to 50
Medium Stiff 4 – 8 50 to 100
Stiff 8 - 15 100 to 200
Very Stiff 15 - 30 200 to 400
Hard > 30 > 400

122. Standard
Penetration
Test
Readings
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123. Standard
Penetration
Test
Readings
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124. Standard
Penetration
Test
Readings
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125. Standard
Penetration
Test
Readings
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126. Standard
Penetration
Test
Readings
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127. Standard
Penetration
Test
Readings
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128. Standard
Penetration
Test
Readings
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129. Standard
Penetration
Test
Readings
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130. Standard
Penetration
Test
Readings
130
Terzaghi et al.
(1996) Method

131. Self Study
• Cone Penetration Test
• Vane Shear Test
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132. THANKS
132