1. ECGD 4122 – Foundation Engineering
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
2nd Semester 2008/2009
ECGD 4122 – Foundation Engineering
Lecture 2

2. Revision of Soil Mechanics
Soil Composition
Soil Classification
Groundwater
Stress (Total vs. Effective)
Settlement
Strength

3. Soil: A 3-Phase Material
Air
Water
Solid

4. The Mineral Skeleton
Solid Particles
Volume
Voids (air or water)

5. Three Phase Diagram Air Water Solid Idealization: Mineral Skeleton

6. Fully Saturated Soils
Water
Solid
Mineral Skeleton
Fully Saturated

7. Dry Soils
Air
Solid
Dry Soil
Mineral Skeleton

8. Partly Saturated Soils
Partially Saturated Soils
Solid
Air
Water
Mineral Skeleton
Partly Saturated Soils

9. Three Phase System Air Water Solid Volume Weight Va Wa~0 Vv Vw Ww WTVT Vs Ws
Volume
Weight

10. Weight Relationships Weight Components: Weight of Solids = Ws
Weight of Water = Ww
Weight of Air ~ 0

11. Volumetric Relationships
Volume Components:
Volume of Solids = Vs
Volume of Water = Vw
Volume of Air = Va
Volume of Voids = Va + Vw = Vv

12. Volumetric Relationships
Volume Components:
Volume of Solids = Vs
Volume of Water = Vw
Volume of Air = Va
Volume of Voids = Va + Vw = Vv

13. Specific Gravity Unit weight of Water, w
w = 1.0 g/cm3 (strictly accurate at 4° C)
w = 62.4 pcf
w = 9.81 kN/m3

14. Specific Gravity, Gs Iron 7.86 Aluminum 2.55-2.80 Lead 11.34
Mercury 13.55
Granite 2.69
Marble 2.69
Quartz 2.60
Feldspar

15. Specific Gravity, Gs

16. Example: Volumetric Ratios
Determine void ratio, porosity and degree of saturation of a soil core sample
Data:
Weight of soil sample = 1013g
Vol. of soil sample = 585.0cm3
Specific Gravity, Gs = 2.65
Dry weight of soil = 904.0g

17. Example Air Water Solid Wa~0 134.9cm3 W =1.00 243.9cm3 109.0g
Volumes
Weights

18. Example Air Water Solid 134.9cm3 W =1.00 243.9cm3 109.0cm3 585.0cm3
Volumes
s =2.65
341.1cm3
109.0cm3
243.9cm3
134.9cm3
W =1.00

19. Soil Unit weight (lb/ft3 or kN/m3)
Bulk (or Total) Unit weight
 = WT / VT
Dry unit weight
d = Ws / VT
Buoyant (submerged) unit weight
b = - w

20. Typical Unit weights

21. Fine-Grained vs. Coarse-Grained Soils
U.S. Standard Sieve - No. 200
inches
0.074 mm
“No. 200” means...

22. Sieve Analysis (Mechanical Analysis)
This procedure is suitable for coarse grained soils
e.g. No.10 sieve …. has 10 apertures per linear inch

23. Hydrometer Analysis
Also called Sedimentation Analysis
Stoke’s Law

24. Grain Size Distribution Curves

25. Soil Plasticity
Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity.
Albert Atterberg, Swedish Soil Scientist ( )…..series of tests for evaluating soil plasticity
Arthur Casagrande adopted these tests for geotechnical engineering purposes

26. Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid
Consistency of fine-grained soil varies in proportion to the water content
Shrinkage limit
Plastic limit
Liquid limit
solid
semi-solid
plastic
liquid
Plasticity
Index
(cheese)
(pea soup)
(pea nut butter)
(hard candy)

27. Liquid Limit (LL or wL) Empirical Definition
The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device

28. Engineering Characterization of Soils
Soil Properties that Control its Engineering Behavior
Particle Size
coarse-grained
fine-grained
Particle/Grain Size Distribution
Particle Shape
Soil Plasticity

29. Clay Morphology Scanning Electron Microscope (SEM)
Shows that clay particles consist of stacks of plate-like layers

30. Soil Consistency Limits
Albert Atterberg ( ) Swedish Soil Scientist
….. Developed series of tests for evaluating consistency limits of soil (1911)
Arthur Casagrande ( )
……Adopted these tests for geotechnical engineering purposes

31. Arthur Casagrande ( )
Joined Karl Terzaghi at MIT in as his graduate student
Research project funded by Bureau of Public Roads
After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard
Soil Plasticity and Soil Classification (1932)

32. Casagrande Apparatus

33. Casagrande Apparatus

34. Casagrande Apparatus

35. Liquid Limit Determination

36. Plastic Limit (PL, wP)
The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches

37. Plastic Limit (PL, wP)

38. Plasticity Index ( PI, IP )
PI = LL – PL
or IP=wL-wP
Note: These are water contents, but the percentage sign is not typically shown.

39. Plasticity Chart

40. USCS Classification Chart

41. USCS Classification Chart

42. Plasticity Chart

43. U = porewater pressure = wZw
Groundwater
U = porewater pressure = wZw

44. Stresses in Soil MassesP X X
Area = A
 = P/A
Soil Unit
Assume the soil is fully saturated, all voids are filled with water.

45. ′ =  - u where, ′ = effective stress
From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil.
Therefore, we may define
effective stress = total stress minus pore pressure
′ =  - u where, ′ = effective stress
 = total stress
u = pore pressure

46. ′ =  - u Effective Stress
The effective stress is the force carried by the soil skeleton divided by the total area of the surface.
The effective stress controls certain aspects of soil behavior, notably, compression & strength.

47. ′z =  iHi - u Effective Stress Calculations where,
H = layer thickness
sat = saturated unit weight
U = pore pressure = w Zw
When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.

48. Geostatic Stresses

49. Compressibility & Settlement
Settlement requirements often control the design of foundations
This chapter provides a general overview of principles involved in settlement analysis
The subject will be dealt with in greater detail in Chapter 7.

50. Increase in Vertical Effective Stress
Due to a Placement of a fill
Due to an external load

51. }z f′ }z f′ Consolidation z0′ z′ z0′ z0′ z0′ z′ Before AfterH
z0′
}z f′
z0′
z′
Voids
 e
Vv = eVs
Vv = (e -  e)Vs
Voids
Solids
Solids
Before Vs Vs
After

52. Before Loading 
Point, P  u0 0

53. Immediately After Loading
Point, P 
u0+u
0 + 

54. Shortly after Loading No settlement Long after Loading
0 + 
Long after Loading
Settlement Complete 
u0+u  u0
0 + 

55. Settlement Distortion Settlement (Immediate)
Consolidation (Time Dependent)
Secondary Compression
Time
Settlement

56. Laboratory Consolidation Test

57. Consolidation Test

58. Test Results

59. Consolidation Plot

60. Idealized
Data
Test Results

61. Compression Index and Recompression Index

62. Compression Ratio and Recompression Ratio

63. Normally and Over-Consolidated Soils
….. Normally consolidated
….. Over consolidated
….. Under consolidated

64. Over-Consolidation Margin & Over-consolidation Ratio

65. Typical Range of OC Margins

66. Compressibility of Sand and Gravels (Table 3.7)

67. Example 3

68. Settlement Predictions N.C. Clays

69. Settlement Predictions O.C. Clays…… Case I

70. Settlement Predictions O.C. Clays…… Case II

71. Example 4

72. Example 4

73. Example 5

74. Example 5

75. Failure due to inadequate strength at shear interface
Slope Failure in Soils
Failure due to inadequate strength at shear interface

76. Shear Failure in Soils

77. Shear Failure in Soils

78. Bearing Capacity Failure

79. Transcosna Grain Elevator Canada (Oct. 18, 1913)
West side of foundation sank 24-ft

80. Shear Strength of Soils
Soil derives its shear strength from two sources:
Cohesion between particles (stress independent component)
Cementation between sand grains
Electrostatic attraction between clay particles
Frictional resistance between particles (stress dependent component)

81. Shear Strength of Soils; Cohesion
Dry sand with no cementation
Dry sand with some cementation
Soft clay
Stiff clay

82. Shear Strength of Soils; Internal Friction

83. Mohr-Coulomb Failure Criterion
Shear Strength,S
 =  C
Normal Stress,  = 

84. Shear Strength is controlled by Effective Stress, '
Slope Surface
Potential Failure Surface

85. Mohr-Coulomb Failure Criterion

86. Typical  Values

87. Effect of Pore Water on Shear Strength
Pore water pressure
Total Stress, versus Effective Stress,
Shear Strength in terms of effective stress

88. Apparent Cohesion Moist beach sand has apparent cohesion
Negative pore water pressures

89. Measuring Shear Strength
Laboratory
Direct shear test
Unconfined compression test
Triaxial compression test
Field
Vane shear test

90. Direct Shear Test ASTM D-3080; AASHTO T 236

91. Direct Shear Test

92. Direct Shear Test

93. Direct Shear Test Device

94. Direct Shear Test Device

95. Direct Shear Test Data
Shear stress

96. Direct Shear Test Data Volume change

97. Peak vs. Ultimate Strength

98. Example: Direct Shear Test
Given:
A direct shear test conducted on a soil sample yielded the following results:
Normal Stress,  (psi)
Max. Shear Stress, S (psi)
10.0
6.5
25.0
11.0
40.0
17.5
Required:
Determine shear strength parameters of the soil

99. Example 6

100. Drained versus Undrained Conditions ….
Before loading
After loading

101. Drained versus Undrained Conditions ….
Before loading
After loading

104. Soil Shear Strength under Drained and Undrained Conditions ….
Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drain
Sands drain fast; therefore under most loading conditions drained conditions exist in sands
Exceptions: pile driving, earthquake loading in fine sands

105. Soil Shear Strength under Drained and Undrained Conditions ….
In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quickly
Therefore, in clays the short-term shear strength may correspond to undrained conditions
Even in clays, long-term shear strength is estimated assuming drained conditions

106. Shear Strength in terms of Total Stress
Shear Strength in terms of effective stress
Shear strength in terms of total stress
u at hydrostatic value

107. Long-term Stability
Potential Failure Surface
Slope Surface

108. Short-term Stability
Potential Failure Surface
Slope Surface

109. Shear Strength in terms of Total Stress;  = 0 condition
For cohesive soils under saturated conditions,  = 0.

110. Mohr-Coulomb Failure Criterion
Shear Strength,S
 = 0 C
Normal Stress, 

111. Mohr’s Circles
3=0 1
Direct Shear
Uniaxial Compression

112. Mohr’s Circles  1  3=0 1 Uniaxial Compression Max. shear plane
Horiz. plane  1

113. Mohr’s Circles
3=0 1
Uniaxial Compression

114. Unconfined Compression Test ASTM D-2166; AASHTO 208
3 = 0 1
For clay soils
Cylindrical specimen
No confining stress
(i.e. 3 = 0)
Axial stress = 1

115. Unconfined Compression Test Data

116. Unconfined Compression Test

117. Example: Unconfined Compression Test
Given:
An unconfined compression test conducted on a soil sample yielded the results shown in the table.
Required:
Determine undrained shear strength, Su of the soil

118. Example: Unconfined Compression Test

119. Example: Unconfined Compression Test
Su= 21.7psi = 3128 psf
qu= 43.45psi=6257 psf

120. Triaxial Compression Test
Unconfined compression test is used when  = 0 assumption is valid
Triaxial compression is a more generalized version
Sample is first compressed isotropically and then sheared by axial loading 1 3

121. Triaxial Compression Test1 3
Load applied in 2 stages
confining pressure, 3
dev. stress,  = 1 - 3

122. Triaxial Compression Test

123. Triaxial Compression Test

124. Triaxial Compression Test for Undisturbed Soils 

125. Drainage during Triaxial Compression Test

126. Triaxial Compression Tests
Unconsolidated Undrained (UU-Test); Also called “Undrained” Test
Consolidated Undrained Test (CU- Test)
Consolidated Drained (CD-Test); Also called “Drained Test”

127. Triaxial Compression Tests ASTM Standards
ASTM D2850: Unconsolidated Undrained Triaxial Test for Cohesive Soils
ASTM D4767: Consolidated Undrained Triaxial Test for Cohesive Soils

128. Triaxial Compression Tests AASHTO Standards
AASHTO T-296: Unconsolidated Undrained Triaxial Test for Cohesive Soils
AASHTO T-297 : Consolidated Undrained Triaxial Test for Cohesive Soils

129. Consolidated Undrained Triaxial Test for Undisturbed Soils