# ECGD 4122 – Foundation Engineering

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

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

Soil: A 3-Phase Material
Air Water Solid

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

Three Phase Diagram Air Water Solid Idealization: Mineral Skeleton

Fully Saturated Soils Water Solid Mineral Skeleton Fully Saturated

Dry Soils Air Solid Dry Soil Mineral Skeleton

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

Three Phase System Air Water Solid Volume Weight Va Wa~0 Vv Vw Ww WT
VT Vs Ws Volume Weight

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

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

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

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

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

Specific Gravity, Gs

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

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

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

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

Typical Unit weights

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

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

Hydrometer Analysis Also called Sedimentation Analysis Stoke’s Law

Grain Size Distribution Curves

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

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)

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

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

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

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

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)

Casagrande Apparatus

Casagrande Apparatus

Casagrande Apparatus

Liquid Limit Determination

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

Plastic Limit (PL, wP)

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

Plasticity Chart

USCS Classification Chart

USCS Classification Chart

Plasticity Chart

U = porewater pressure = wZw
Groundwater U = porewater pressure = wZw

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

′ =  - 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

′ =  - 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.

′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.

Geostatic Stresses

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.

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

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

Point, P u0+u 0 + 

0 +  Long after Loading Settlement Complete u0+u u0 0 + 

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

Laboratory Consolidation Test

Consolidation Test

Test Results

Consolidation Plot

Idealized Data Test Results

Compression Index and Recompression Index

Compression Ratio and Recompression Ratio

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

Over-Consolidation Margin & Over-consolidation Ratio

Typical Range of OC Margins

Compressibility of Sand and Gravels (Table 3.7)

Example 3

Settlement Predictions N.C. Clays

Settlement Predictions O.C. Clays…… Case I

Settlement Predictions O.C. Clays…… Case II

Example 4

Example 4

Example 5

Example 5

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

Shear Failure in Soils

Shear Failure in Soils

Bearing Capacity Failure

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

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)

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

Shear Strength of Soils; Internal Friction

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

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

Mohr-Coulomb Failure Criterion

Typical  Values

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

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

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

Direct Shear Test ASTM D-3080; AASHTO T 236

Direct Shear Test

Direct Shear Test

Direct Shear Test Device

Direct Shear Test Device

Direct Shear Test Data Shear stress

Direct Shear Test Data Volume change

Peak vs. Ultimate Strength

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

Example 6

Drained versus Undrained Conditions ….

Drained versus Undrained Conditions ….

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

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

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

Long-term Stability Potential Failure Surface Slope Surface

Short-term Stability Potential Failure Surface Slope Surface

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

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

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

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

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

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

Unconfined Compression Test Data

Unconfined Compression Test

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

Example: Unconfined Compression Test

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

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

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

Triaxial Compression Test

Triaxial Compression Test

Triaxial Compression Test for Undisturbed Soils

Drainage during Triaxial Compression Test

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

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

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

Consolidated Undrained Triaxial Test for Undisturbed Soils

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