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Bearing Capacity of Shallow
Foundation
In situ TestsIn situ TestsIn situ TestsIn situ Tests
Several in-situ tests define the stratigraphy
and obtain direct measurements of soil
properties and geotechnical parameters.
The common tests include:
Standard penetration test (SPT)
Cone penetration test(CPT)
Vane shear test(VST)
Borehole Shear Test (BST)
Piezocone test (CPTu)
Flat dilatometer test (DMT)
Pressuremeter test (PMT)
Plate Load test (PLT)
Bearing Capacity of Shallow Foundation SPT
Standard Penetration Test, SPT
SPT is the most widely used test procedure to determine
the properties of in-situ soils
63.5 kg
0.76 m
Drill rod
0.15 m
0.15 m
0.15 m
Number of blows = N1
Number of blows = N2
Number of blows = N3
Standard penetration resistance (SPT N) = N2 + N3
Number of blows for the first 150 mm
penetration is disregarded due to the
disturbance likely to exist at the bottom
of the drill hole
The test can be conducted at every 1m
vertical intervals
Various correlations have been developed to determine soil
strength parameters (c, φ, q) from N
Standard Penetration Test, SPT
SPT (Manual operation)
Standard Penetration Test (SPT)
140 lb (63.5 kg) Hammer
30in (76 cm) free fall
Drive sampler over 18 inches
Record no. of blows per each 6 inch
penetration
SPT blow count=blows for 2nd 6 inch
penetration + blows for 3rd 6inch penetration
Standard Split Spoon Sampler
(SPT)
Thick wall (0.25in) cylinder
Sampling tube is split along the length
Hammered into the ground
Standard Split
Spoon Sampler
Standard
Penetration Test
(SPT)
Types of SPT Hammers
SPT: Automatic Trip
Hammer
Bearing Capacity of Shallow Foundation SPT
Bearing Capacity of Shallow Foundation SPT
Bearing Capacity of Shallow Foundation SPT
Bearing Capacity of Shallow Foundation SPT
Bearing Capacity of Shallow Foundation SPT
Bearing Capacity of Shallow Foundation SPT
1. Points to note
If number of blows for blows for 150 mm
drive exceeds 50, it is taken as refusal and
the test is discontinued
Standardization of SPT
The validity of the SPT has been the
subject of study and research by many
authors for the last many years.
The basic conclusion is that the best
results are difficult to reproduce.
Some of the important factors that affect
reproducibility are:
Variation in the height of fall of the drop
weight (hammer) during the test
The number of turns of rope around the
cathead, and the condition of the manila
rope
Length and diameter of drill rod
Diameter of bore hole
Overburden pressure
Corrections to SPT Blow Counts
Factors affecting SPT blow count:
Hammer Efficiency
Borehole diameter
Type of sampler
Rod length
Corrections to the Observed SPT
Value
• Three types of corrections are normally
applied to the observed N values. They are:
• 1. Hammer efficiency correction (Eh)
• 2. Drillrod, sampler and borehole corrections
(Cd, Cs and Cb)
• 3. Correction due to overburden pressure (CN)
1. Hammer Efficiency Correction,
Eh
• Different types of hammers are in use for driving
the drill rods. Two types are normally used in
USA.
1. Donut with two turns of manila rope on the
cathead with a hammer efficiency Eh = 0.45.
2. Safety with two turns of manila rope on the
cathead with a hammer efficiency as follows:
Rope-pulley or cathead = 0.7 to 0.8;
Trip or automatic hammer = 0.8 to 1.0.
2. Drill Rod, Sampler and Borehole
Corrections
• Correction factors are used for correcting the
effects of length of drill rods, use of split
spoon
• sampler with or without liner, and size of bore
holes. The various correction factors are
(Bowles,1996).
Bearing Capacity of Shallow Foundation SPT
Drill Rod, Sampler and Borehole
Corrections
Sampler correction factor, Cs
Without liner CS = 1.00
With liner,
Dense sand, clay = 0.80
Loose sand = 0.90
Over burden pressure correction
for granular soils
CN =1 for cohesive soils at any
depth
This is most commonly used equation to
calculate CN
Other Empirical relationships for CN (Note: σ′σ′σ′σ′o
is in kN/m2)
Source CN
Liao and Whitman
(1960)
Skempton (1986)
Seed et al. (1975)
Peck et al. (1974)
o
.
σ′′′′
1
789
o
. σ′′′′++++ 0101
2





 ′′′′
−−−−
695
2511
.
log. oσ
2
52
1912
770
m/kN.for
log.
o
o
≥≥≥≥′′′′






′′′′
σ
σ
N60 0r Ncor (N corrected)
Bearing Capacity of Shallow Foundation SPT
qu for cohesionless soil from SPT
After knowing value of angle of internal
friction, use bearing capacity equation
Peck et al. 1974
is a proportionality factor. Bowles (1996)
suggested the value of 12 for this
Peck et al. 1974
qu for cohesive soil from SPT
After knowing value of qu (Unconfined
compressive strength), use Skempton Equation.
Skempton’s Bearing Capacity Factor NC
Skempton (1951) for Saturated clay soils and strip
foundations suggested
cfunu
fcu
CNDqq
DCNq
=−=
+=
γ
γ
C is half of unconfined compressive strength
(Recall unconfined compression test)
Bearing Capacity from SPT
Bearing Capacity from SPT
The standard penetration test (SPT) is
performed during the advancement of a soil
boring to obtain an approximate measure of
the dynamic soil resistance, as well as a
disturbed drive sample (split barrel type).
The test was introduced by the Raymond Pile
Company in 1902 and remains today as the
most common in-situ test worldwide.
The procedures for the SPT are detailed in
ASTM D 1586 and AASHTO T-206.
Notes on SPT
The SPT involves the driving of a hollow thick-
walled tube into the ground and measuring the
number of blows to advance the split-barrel
sampler a vertical distance of 300 mm (1 foot).
A drop weight system is used for the pounding
where a 63.5-kg (140-lb) hammer repeatedly
falls from 0.76 m (30 inches) to achieve three
successive increments of 150-mm (6-inches)
each.
The first increment is recorded as a seating,
while the number of blows to advance the
second and third increments are summed to
give the N-value ("blow count") or SPT-
resistance (reported in blows/0.3 m or blows
per foot). If the sampler cannot be driven 450
mm, the number of blows per each 150-mm
increment and per each partial increment is
recorded on the boring log.
For partial increments, the depth of
penetration is recorded in addition to the
number of blows.
The test can be performed in a wide variety of
soil types, as well as weak rocks, yet is not
particularly useful in the characterization of
gravel deposits nor soft clays.
The fact that the test provides both a sample
and a number is useful, yet problematic, as
one cannot do two things well at the same
time.
The SPT is conducted at the bottom of a soil
boring that has been prepared using either
flight augers or rotary wash drilling
methods. At regular depth intervals, the
drilling process is interrupted to perform the
SPT.
Generally, tests are taken every 0.76 m (2.5
feet) at depths shallower than 3 meters (10
feet) and at intervals of 1.5 m (5.0 feet)
thereafter.
The head of water in the borehole must be
maintained at or above the ambient
groundwater level to avoid inflow of water
and borehole instability.
In current U.S. practice, three types of drop
hammers (donut, safety, and automatic) and
four types of drill rods (N, NW, A, and AW) are
used in the conduct of the SPT. The test in fact
is highly-dependent upon the equipment used
and operator performing the test. Most
important factor is the energy efficiency of the
system.
The theoretical energy of a free-fall system
with the specified mass and drop height is 48
kg-m (350 ft-lb), but the actual energy is less
due to frictional losses and eccentric loading. A
rotating cathead and rope system is commonly
used and their efficiency depends on
numerous factors including: type of hammer,
number of rope turns, conditions of the
sheaves and rotating cathead (e.g.,
lubricated, rusted, bent, new, old), age of
the rope, actual drop height, vertical
plumbness, weather and moisture
conditions (e.g., wet, dry, freezing), and
other variables.
Trends in recent times are towards the use
of automated systems for lifting and
dropping the mass in order to minimize
these factors.
Bearing Capacity of Shallow Foundation SPT

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Bearing Capacity of Shallow Foundation SPT

  • 1. Bearing Capacity of Shallow Foundation
  • 2. In situ TestsIn situ TestsIn situ TestsIn situ Tests
  • 3. Several in-situ tests define the stratigraphy and obtain direct measurements of soil properties and geotechnical parameters. The common tests include: Standard penetration test (SPT) Cone penetration test(CPT) Vane shear test(VST) Borehole Shear Test (BST) Piezocone test (CPTu) Flat dilatometer test (DMT) Pressuremeter test (PMT) Plate Load test (PLT)
  • 5. Standard Penetration Test, SPT SPT is the most widely used test procedure to determine the properties of in-situ soils 63.5 kg 0.76 m Drill rod 0.15 m 0.15 m 0.15 m Number of blows = N1 Number of blows = N2 Number of blows = N3 Standard penetration resistance (SPT N) = N2 + N3 Number of blows for the first 150 mm penetration is disregarded due to the disturbance likely to exist at the bottom of the drill hole The test can be conducted at every 1m vertical intervals Various correlations have been developed to determine soil strength parameters (c, φ, q) from N
  • 6. Standard Penetration Test, SPT SPT (Manual operation)
  • 7. Standard Penetration Test (SPT) 140 lb (63.5 kg) Hammer 30in (76 cm) free fall Drive sampler over 18 inches Record no. of blows per each 6 inch penetration SPT blow count=blows for 2nd 6 inch penetration + blows for 3rd 6inch penetration
  • 8. Standard Split Spoon Sampler (SPT) Thick wall (0.25in) cylinder Sampling tube is split along the length Hammered into the ground
  • 11. Types of SPT Hammers
  • 19. 1. Points to note If number of blows for blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued
  • 20. Standardization of SPT The validity of the SPT has been the subject of study and research by many authors for the last many years. The basic conclusion is that the best results are difficult to reproduce. Some of the important factors that affect reproducibility are:
  • 21. Variation in the height of fall of the drop weight (hammer) during the test The number of turns of rope around the cathead, and the condition of the manila rope Length and diameter of drill rod Diameter of bore hole Overburden pressure
  • 22. Corrections to SPT Blow Counts Factors affecting SPT blow count: Hammer Efficiency Borehole diameter Type of sampler Rod length
  • 23. Corrections to the Observed SPT Value • Three types of corrections are normally applied to the observed N values. They are: • 1. Hammer efficiency correction (Eh) • 2. Drillrod, sampler and borehole corrections (Cd, Cs and Cb) • 3. Correction due to overburden pressure (CN)
  • 24. 1. Hammer Efficiency Correction, Eh • Different types of hammers are in use for driving the drill rods. Two types are normally used in USA. 1. Donut with two turns of manila rope on the cathead with a hammer efficiency Eh = 0.45. 2. Safety with two turns of manila rope on the cathead with a hammer efficiency as follows: Rope-pulley or cathead = 0.7 to 0.8; Trip or automatic hammer = 0.8 to 1.0.
  • 25. 2. Drill Rod, Sampler and Borehole Corrections • Correction factors are used for correcting the effects of length of drill rods, use of split spoon • sampler with or without liner, and size of bore holes. The various correction factors are (Bowles,1996).
  • 27. Drill Rod, Sampler and Borehole Corrections Sampler correction factor, Cs Without liner CS = 1.00 With liner, Dense sand, clay = 0.80 Loose sand = 0.90
  • 28. Over burden pressure correction for granular soils CN =1 for cohesive soils at any depth This is most commonly used equation to calculate CN
  • 29. Other Empirical relationships for CN (Note: σ′σ′σ′σ′o is in kN/m2) Source CN Liao and Whitman (1960) Skempton (1986) Seed et al. (1975) Peck et al. (1974) o . σ′′′′ 1 789 o . σ′′′′++++ 0101 2       ′′′′ −−−− 695 2511 . log. oσ 2 52 1912 770 m/kN.for log. o o ≥≥≥≥′′′′       ′′′′ σ σ
  • 30. N60 0r Ncor (N corrected)
  • 32. qu for cohesionless soil from SPT After knowing value of angle of internal friction, use bearing capacity equation
  • 33. Peck et al. 1974
  • 34. is a proportionality factor. Bowles (1996) suggested the value of 12 for this Peck et al. 1974
  • 35. qu for cohesive soil from SPT After knowing value of qu (Unconfined compressive strength), use Skempton Equation.
  • 36. Skempton’s Bearing Capacity Factor NC Skempton (1951) for Saturated clay soils and strip foundations suggested cfunu fcu CNDqq DCNq =−= += γ γ C is half of unconfined compressive strength (Recall unconfined compression test)
  • 39. The standard penetration test (SPT) is performed during the advancement of a soil boring to obtain an approximate measure of the dynamic soil resistance, as well as a disturbed drive sample (split barrel type). The test was introduced by the Raymond Pile Company in 1902 and remains today as the most common in-situ test worldwide. The procedures for the SPT are detailed in ASTM D 1586 and AASHTO T-206. Notes on SPT
  • 40. The SPT involves the driving of a hollow thick- walled tube into the ground and measuring the number of blows to advance the split-barrel sampler a vertical distance of 300 mm (1 foot). A drop weight system is used for the pounding where a 63.5-kg (140-lb) hammer repeatedly falls from 0.76 m (30 inches) to achieve three successive increments of 150-mm (6-inches) each.
  • 41. The first increment is recorded as a seating, while the number of blows to advance the second and third increments are summed to give the N-value ("blow count") or SPT- resistance (reported in blows/0.3 m or blows per foot). If the sampler cannot be driven 450 mm, the number of blows per each 150-mm increment and per each partial increment is recorded on the boring log.
  • 42. For partial increments, the depth of penetration is recorded in addition to the number of blows. The test can be performed in a wide variety of soil types, as well as weak rocks, yet is not particularly useful in the characterization of gravel deposits nor soft clays. The fact that the test provides both a sample and a number is useful, yet problematic, as one cannot do two things well at the same time.
  • 43. The SPT is conducted at the bottom of a soil boring that has been prepared using either flight augers or rotary wash drilling methods. At regular depth intervals, the drilling process is interrupted to perform the SPT. Generally, tests are taken every 0.76 m (2.5 feet) at depths shallower than 3 meters (10 feet) and at intervals of 1.5 m (5.0 feet) thereafter.
  • 44. The head of water in the borehole must be maintained at or above the ambient groundwater level to avoid inflow of water and borehole instability. In current U.S. practice, three types of drop hammers (donut, safety, and automatic) and four types of drill rods (N, NW, A, and AW) are used in the conduct of the SPT. The test in fact is highly-dependent upon the equipment used and operator performing the test. Most important factor is the energy efficiency of the system.
  • 45. The theoretical energy of a free-fall system with the specified mass and drop height is 48 kg-m (350 ft-lb), but the actual energy is less due to frictional losses and eccentric loading. A rotating cathead and rope system is commonly used and their efficiency depends on numerous factors including: type of hammer, number of rope turns, conditions of the
  • 46. sheaves and rotating cathead (e.g., lubricated, rusted, bent, new, old), age of the rope, actual drop height, vertical plumbness, weather and moisture conditions (e.g., wet, dry, freezing), and other variables. Trends in recent times are towards the use of automated systems for lifting and dropping the mass in order to minimize these factors.