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Soils and Foundations Reference Manual Vol. I

Civil Engineering Resources

Soils and Foundations
Reference Manual Vol. I
Naresh C. Samtani*, PE, PhD
Edward A. Nowatzki*, PE, PhD

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Soils and Foundations
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Preface

This manual addresses topics ranging from fundamental concepts in soil mechanics to the practical design of various geotechnical features ranging from earthworks (e.g., slopes) to foundations (e.g., spread footings, driven piles, drilled shafts and earth retaining structures). In the literature each of these topics has developed its own identity in terms of the terminology and symbols. Since most of the information presented in this manual appears in other FHWA publications, textbooks and publications, the authors faced a dilemma on the regarding terminology and symbols as well as other issues. Following is a brief discussion on such issues.

Pressure versus Stress The terms “pressure” and “stress” both have units of force per unit area (e.g., pounds per square foot). In soil mechanics “pressure” generally refers to an applied load distributed over an area or to the pressure due to the self-weight of the soil mass. “Stress,” on the other hand, generally refers to the condition induced at a point within the soil mass by the application of an external load or pressure. For example, “overburden pressure,” which is due to the self weight of the soil, induces “geostatic stresses” within the soil mass. Induced stresses cause strains which ultimately result in measurable deformations that may affect the behavior of the structural element that is applying the load or pressure. For example, in the case of a shallow foundation, depending upon the magnitude and direction of the applied loading and the geometry of the footing, the pressure distribution at the base of the footing can be uniform, linearly varying, or non-linearly varying. In order to avoid confusion, the terms “pressure” and “stress” will be used interchangeably in this manual. In cases where the distinction is important, clarification will be provided by use of the terms “applied” or “induced.”

TOC

1.0 INTRODUCTION . 1-1
1.1 PURPOSE AND SCOPE 1-1
1.2 SOILS AND FOUNDATIONS FOR HIGHWAY FACILITIES 1-5
1.3 ORGANIZATION OF MANUAL .. 1-10
1.4 REFERENCES . 1-12
1.4.1 Primary FHWA References 1-12
1.4.2 Other Primary References 1-13

2.0 STRESS AND STRAIN IN SOILS. 2-1
2.1 BASIC WEIGHT-VOLUME RELATIONSHIPS .. 2-3
2.1.1 Volum e Ratios . 2-3
2.1.2 Weight Ratios 2-4
2.1.3 Weight-Volume Ratios (Unit Weights) and Specific Gravity .. 2-5
2.1.4 Determination and Use of Basic Weight-Volume Relations . 2-7
2.1.5 Size of Grains in the Solid Phase. 2-9
2.1.6 Shape of Grains in Solid Phase.. 2-13
2.1.6.1 Bulky Shape . 2-13
2.1.6.2 Platy Shape .. 2-15
2.1.7 Effect of Water on Physical States of Soils . 2-15
2.2 PRINCIPLE OF EFFECITVE STRESS 2-19
2.3 OVERBURDEN PRESSURE. 2-19
2.4 VERTICAL STRESS DISTRIBUTION IN SOIL DUE TO EXTERNAL LOADINGS . 2-22
2.4.1 Uniformly Loaded Continuous (Strip) and Square Footings . 2-24
2.4.2 Approximate (2:1) Stress Distribution Concept 2-26
2.5 REPRESENTATION OF IMPOSED PRESSURES ON THE po DIAGRAM . 2-26
2.6 LOAD-DEFORMATION PROCESS IN SOILS . 2-30
2.6.1 Time Dependent Load-Deformation (Consolidation) Process . 2-31
2.6.2 Comparison of Drainage Rates between Coarse-Grained and Fine- Grained Soil . 2-34
2.7 LATERAL STRESSES IN FOUNDATION SOILS .. 2-34
2.7.1 Effect of Shear Strength of Soils on Lateral Pressures.. 2-36
2.8 STRENGTH OF SOILS TO RESIST IMPOSED STRESSES . 2-38
2.8.1 Basic Concept of Shearing Resistance and Shearing Strength.. 2-38
2.9 STRENGTH OF SOILS RELATED TO LATERAL EARTH PRESSURES 2-42
2.9.1 Distribution of Lateral Earth and Water Pressures .. 2-45
2.9.2 Deformations Associated with Lateral Pressures . 2-47
2.10 UNSATURATED SOIL MECHANICS 2-47

3.0 SUBSURFACE EXPLORATIONS 3-1
3.01 Prim ary References . 3-3
3.1 PREPARING FOR SUBSURFACE EXPLORATION 3-3
3.1.1 Soil Formations and Landforms 3-6
3.1.1.1 Residual Soils 3-7
3.1.1.2 Transported Soils . 3-9
3.1.1.3 Area Concept of Explorations Based on Landforms . 3-9
3.2 FIELD RECONNAISSANCE 3-13
3.3 SUBSURFACE EXPLORATION PROGRAM 3-15
3.4 SAMPLING TECHNIQUES AND TOOLS. 3-17
3.5 BORING METHODS . 3-20
3.5.1 Auger Borings . 3-20
3.5.2 Wash-type Borings 3-26
3.5.3 Coring in Rocks . 3-26
3.6 SAMPLING METHODS .. 3-28
3.6.1 Disturbed Sampling of Soil.. 3-28
3.6.2 Undisturbed Sampling of Soil. 3-28
3.6.3 Thin-Walled (Shelby) Tube Sampling 3-35
3.6.4 Undisturbed Sampling of Rock (Rock Coring) . 3-38
3.6.4.1 Core Barrels . 3-38
3.6.4.2 Coring Bits 3-40
3.6.4.3 Drilling Fluid .. 3-43
3.6.5 Observations During Rock Core Drilling . 3-43
3.6.5.1 Drilling Rate/Time 3-43
3.6.5.2 Core Photographs.. 3-43
3.6.5.3 Rock Classification .. 3-44
3.6.5.4 Recovery 3-44
3.6.5.5 Rock Quality Designation (RQD). 3-44
3.6.5.6 Drilling Fluid Recovery . 3-47
3.6.5.7 Core Handling and Labeling 3-48
3.6.5.8 Care and Preservation of Rock Samples 3-50
3.6.6 Geologic Mapping 3-50
3.7 STANDARD PENETRATION TEST (SPT) .. 3-51
3.7.1 Energy Efficiency of Hammers . 3-54
3.7.2 Effect of Overburden Stress on N-values . 3-56
3.7.3 Correlation of SPT N-Values with Basic Soil Characteristics.. 3-58
3.7.3.1 Applicability of SPTs in Gravelly Soils 3-58
3.7.4 SPT T est Errors .. 3-59

3.8 LOG OF BOREHOLE INFORMATION (“BORING LOGS”).. 3-62
3.8.1 Borin g Log Format 3-62
3.8.2 Duties of the Logger . 3-62
3.9 CONE PENETRATION TESTING (CPT).. 3-67
3.9.1 Equipment Description and Operation .. 3-68
3.9.2 The Standard Cone Penetration Test (CPT) 3-69
3.9.3 The Piezo-cone Penetration Test (CPTu) . 3-71
3.9.4 The Seismic Piezocone Penetration Test (SCPTu) .. 3-71
3.9.5 Test Procedures .. 3-71
3.9.6 CPT Profiles . 3-72
3.9.7 CPT Profile Interpretation 3-72
3.10 DILATOMETER TEST (DMT) 3-75
3.11 PRESSUREMETER TEST (PMT).. 3-76
3.12 VANE SHEAR TEST (VST).. 3-77
3.13 GROUNDW ATER MEASUREMENTS.. 3-78
3.13.1 Information on Existing Wells 3-78
3.13.2 Open Borings .. 3-79
3.13.3 Observation Wells. 3-79
3.13.4 Water Level Measurements . 3-81
3.13.4.1 Chalked Tape 3-81
3.13.4.2 Tape with a Float 3-82
3.13.4.3 Electric Water-Level Indicator 3-82
3.13.4.4 Data Loggers . 3-82
3.14 GUIDELINES FOR MINIMUM SUBSURFACE EXPLORATION .. 3-83
3.14.1 Recommendations for Sampling Depth Intervals in Soils 3-87
3.14.2 Recommendations for Sampling Depth Intervals in Rocks . 3-89
3.14.3 Recommendations for Water Level Monitoring in Borings 3-89
3.15 GEOPHYSICAL TESTS .. 3-90
3.15.1 Types of Geophysical Tests. 3-90
3.15.2 Advantages and Disadvantages of Geophysical Tests 3-94
3.15.2.1 Advantages of Geophysical Tests .. 3-94
3.15.2.2 Disadvantages of Geophysical Tests 3-95
3.15.3 Examples of Uses of Geophysical Tests 3-95

4.0 ENGINEERING DESCRIPTION, CLASSIFICATION AND CHARACTERISTICS OF SOILS AND ROCKS .. 4-1
4.01 Primary References . 4-3
4.1 SOIL DESCRIPTION 4-3
4.1.1 Consistency and Apparent Density. 4-4
4.1.2 Water Content (Moisture) .. 4-6
4.1.3 Color .. 4-6
4.1.4 Type of Soil. 4-6
4.1.4.1 Coarse-Grained Soils (Gravel and Sand) . 4-7
4.1.4.2 Fine-Grained Soils . 4-9
4.1.4.3 Highly Organic Soils .. 4-10
4.1.4.4 Minor Soil Type(s) 4-11
4.1.4.5 Inclusions .. 4-12
4.1.4.6 Other Descriptors .. 4-12
4.1.4.7 Layered Soils .. 4-12
4.1.4.8 Geological Name .. 4-13
4.2 SOIL CLASSIFICATION 4-14
4.2.1 Unified Soil Classification System (USCS) 4-14
4.2.1.1 Classification of Coarse-Grained Soils .. 4-18
4.2.1.2 Classification of Fine-Grained Soils 4-21
4.2.2 AASHTO Soil Classification System . 4-26
4.3 ENGINEERING CHARACTERISTICS OF SOILS.. 4-31
4.3.1 Engineering Characteristics of Coarse-Grained Soils (Sands and Gravels).. 4-31
4.3.2 Engineering Characteristics of Fine-Grained Soils (Inorganic Clays) .. 4-31
4.3.3 Engineering Characteristics of Fine-Grained Soils (Inorganic Silts) . 4-31
4.3.4 Engineering Characteristics of Organic Soils 4-32
4.4 PRACTICAL ASPECTS OF ENGINEERING CHARACTERISTICS OF COARSE-GRAINED SOILS . 4-32
4.5 PRACTICAL ASPECTS OF ENGINEERING CHARACTERISTICS OF FINE-GRAINED SOILS .. 4-33
4.6 DESCRIPTION OF ROCK . 4-34
4.6.1 Rock Type . 4-34
4.6.2 Color 4-36
4.6.3 Grain Size and Shape.. 4-36
4.6.4 Stratifica tion/Foliation 4-37
4.6.5 Mineral Composition .. 4-37
4.6.6 Weathering and Alteration 4-37
4.6.7 Strength .. 4-37
4.6.8 Hardness . 4-39
4.6.9 Rock Discontinuity .. 4-39
4.6.10 Fracture Description 4-41
4.6.11 Rock Mass Classification.. 4-42
4.7 SUBSURFACE PROFILE DEVELOPMENT . 4-45
4.7.1 Use of Historical Data in Development of Subsurface Profile .. 4-48

5.0 LABORATORY TESTING FOR GEOTECHNICAL DESIGN AND CONSTRUCTION .. 5-1
5.01 Prim ary References . 5-1
5.1 QUALITY ASSURANCE FOR LABORATORY TESTING 5-2
5.1.1 Sample Tracking .. 5-2
5.1.2 Sample Storage . 5-2
5.1.3 Sample Handling.. 5-4
5.1.4 Effects of Sample Disturbance.. 5-4
5.1.5 Specim en Selection . 5-4
5.2 LABORATORY TESTING FOR SOILS .. 5-5
5.3 LABORATORY INDEX TESTS FOR SOILS 5-10
5.3.1 General 5-10
5.3.2 Moisture Content .. 5-10
5.3.3 Unit Weight.. 5-11
5.3.4 Particle Size Distribution.. 5-12
5.3.4.1 Sand Equivalent . 5-16
5.3.5 Atterberg Limits. 5-16
5.3.5.1 Significance of the “A-line” and “U-line” on Plasticity Chart 5-19
5.3.6 Specific Gravity . 5-19
5.3.7 Organic Content . 5-20
5.3.8 Electro Chemical Classification Tests 5-21
5.3.9 Laboratory Classification.. 5-21
5.4 CONSOLIDATION TESTING 5-22
5.4.1 Process of Consolidation .. 5-22
5.4.2 Consolidation Testing . 5-23
5.4.3 Procedures . 5-24
5.4.4 Presentation and Understanding the Consolidation Test Results . 5-26
5.4.5 Comments on the Consolidation Tests .. 5-31
5.4.6 Useful Correlations between Consolidation Parameters and Index Values . 5-33
5.4.6.1 Compression Index, Cc .. 5-33
5.4.6.2 Recompression Index, Cr .. 5-35
5.4.6.3 Coefficient of Vertical Consolidation, cv.. 5-35
5.4.6.4 Coefficient of Secondary Compression, Cα . 5-37
5.5 SHEAR STRENGTH OF SOILS . 5-38
5.5.1 Concept of Frictional and Cohesive Strengths .. 5-38
5.5.1.1 Strength Due to Friction 5-38
5.5.1.2 Strength Due to Cohesion . 5-40
5.5.1.3 Simplified Expression for Shear Strength of Soils .. 5-42
5.5.2 Strength Testing of Soils in the Laboratory. 5-42
5.5.2.1 Unconfined Compression (UC) Tests .. 5-44
5.5.2.2 Triaxial Tests .. 5-45
5.5.2.3 Direct Shear Tests .. 5-49
5.5.3 Factors Affecting Strength Testing Results . 5-51
5.5.4 Comparison of Laboratory and Field Strengths. 5-52
5.5.5 Selection of Design Shear Strength . 5-53
5.5.6 Correlations of Shear Strength Parameters with Index Parameters . 5-53
5.5.6.1 Undrained Shear Strength of Cohesive Soils.. 5-53
5.5.6.2 Drained and Effective Shear Strength of Cohesive Soils . 5-55
5.5.6.3 Shear Strength of Cohesionless Soils . 5-56
5.6 PERMEABILITY . 5-58
5.6.1 General 5-58
5.6.2 Equipm ent . 5-58
5.6.3 Procedures . 5-61
5.6.4 Useful Correlations of Permeability with Index Values 5-62
5.7 VOLUME CHANGE PHENOMENA DUE TO LOADING AND MOISTURE 5-65
5.7.1 Swell Potential of Clays 5-65
5.7.1.1 Evaluation of Expansion (Swell) Potential .. 5-67
5.7.2 Collapse Potential of Soils .. 5-68
5.7.3 Expansion of Soils due to Frost Action 5-70
5.8 COMPACTION CHARACTERISTICS OF SOIL . 5-72
5.8.1 Concept of Compaction 5-72
5.8.2 Test Procedures . 5-73
5.8.3 Implication of Laboratory Tests on Field Compaction Specifications . 5-76
5.8.4 Engineering Characteristics of Compacted Soils.. 5-81
5.8.4.1 Effect of Increase in Moisture Content on Shear Strength of Compacted Soils .. 5-83
5.9 ELASTIC PROPERTIES OF SOILS . 5-83
5.10 COMMON SENSE GUIDELINES FOR LABORATORY TESTING OF SOILS . 5-84
5.11 LABORATORY TESTS FOR ROCK 5-87
5.11.1 Introduction . 5-87
5.11.2 Point-Load Strength Test .. 5-89
5.11.3 Unconfined Compressive Strength of Intact Rock Core . 5-91
5.11.4 Elastic Modulus of Intact Rock Core . 5-91
5.11.5 Laboratory Direct Shear Test . 5-92
5.12 ELASTIC PROPERTIES OF ROCKS .. 5-94
5.12.1 Elastic Modulus of Rock Mass . 5-95
5.13 COMMON SENSE GUIDELINES FOR LABORATORY TESTING OF ROCKS .. 5-96
5.14 PRACTICAL ASPECTS FOR LABORATORY TESTING.. 5-98
5.15 VARIABILITY OF MEASURED PROPERTIES .. 5-99

6.0 SLOPE STABILITY 6-1
6.01 Prim ary Reference .. 6-1
6.1 EFFECTS OF WATER ON SLOPE STABILITY . 6-3
6.2 DESIGN FACTOR OF SAFETY 6-5
6.3 INFINITE SLOPE ANALYSIS 6-5
6.3.1 Infinite Slopes in Dry Cohesionless Soils 6-5
6.3.2 Infinite Slopes in c-φ Soils with Parallel Seepage.. 6-6
6.4 CIRCULAR ARC FAILURE 6-9
6.4.1 Simple Rule of Thumb for Factor of Safety 6-10
6.4.2 Stability Analysis Methods (General). 6-11
6.4.3 Ordinary Method of Slices – Step-By-Step Computation Procedure . 6-13
6.4.4 Recom mended Stability Methods . 6-20
6.4.5 Remarks on Safety Factor . 6-22
6.5 CRITICAL FAILURE SURFACE . 6-22
6.6 DESIGN (STABILITY) CHARTS.. 6-24
6.6.1 Historical Background 6-25
6.6.2 Taylor’s Stability Charts 6-25
6.6.2.1 Determination of the Factor of Safety for a Slope 6-30
6.6.3 Janbu’s Stability Charts . 6-33
6.7 SLIDING BLOCK FAILURE 6-43
6.7.1 Sliding Block – Hand Method of Analysis.. 6-44
6.7.1.1 Computation of Forces - Simple Sliding Block Analysis . 6-45
6.7.2 Computation of Forces - Complicated Sliding Block Analysis. 6-47
6.8 SLOPE STABLITY ANALYSIS USING COMPUTER PROGRAMS .. 6-48
6.9 IMPROVING THE STABILITY OF EMBANKMENTS 6-50
6.9.1 Embankment Stability Design Solutions .. 6-50
6.9.2 Design Approach for Reinforced Soil Slopes . 6-50
6.9.2.1 Preliminary Feasibility Design of RSS .. 6-53
6.10 IMPROVING THE STABILITY OF CUT SLOPES 6-57
6.10.1 Deep Seated Failure .. 6-57
6.10.2 Shallow Surface Failures 6-58
6.10.3 Factor of Safety - Cut Slopes 6-59

7.0 APPROACH ROADWAY DEFORMATIONS . 7-1
7.1 TYPICAL APPROACH ROADWAY DEFORMATION PROBLEMS 7-1
7.2 INTERNAL DERFROMTION WITHIN EMBANKMENTS . 7-6
7.2.1 General Considerations for Select Structural Backfill.. 7-6
7.2.2 General Considerations for Drainage Aggregate. 7-6
7.2.3 Use of Geosynthetics to Control Internal Deformations .. 7-10
7.3 EXTERNAL DEFORMATION IN FOUNDATION SOILS BELOW EMBANKMENTS .. 7-10
7.3.1 Procedure for Estimating Stress Distribution in Foundation Soils under Fills . 7-11
7.4 COMPUTATION OF IMMEDIATE SETTLEMENT.. 7-15
7.4.1 Modified Hough’s Method for Estimating Immediate Settlements. 7-16
7.4.1.1 Comments on the Computed Settlement of Embankments . 7-19
7.5 COMPUTATION OF CONSOLIDATION (LONG-TERM) SETTLEMENTS 7-19
7.5.1 Correction of Laboratory One-Dimensional Consolidation Curves 7-21
7.5.2 Computation of Primary Consolidation Settlements .. 7-24
7.5.2.1 Normally Consolidated Soils .. 7-24
7.5.2.2 Overconsolidated (Preconsolidated) Soils 7-26
7.5.2.3 Underconsolidated Soils 7-27
7.5.3 Consolidation Rates (Time Rate of Consolidation Settlement) . 7-28
7.5.3.1 Percent Consolidation . 7-30
7.5.3.2 Step-by-step Procedure to Determine Amount and Time for Consolidation .. 7-33
7.5.4 Secondary Compression of Cohesive Soils . 7-38
7.6 LATERAL SQUEEZE OF FOUNDATION SOILS .. 7-40
7.6.1 Threshold Condition for Lateral Squeeze. 7-41
7.6.2 Calculation of the Safety Factor against Lateral Squeeze 7-42
7.6.3 Estimation of Horizontal Movement of Abutments. 7-43
7.7 DESIGN SOLUTIONS - DEFORMATION PROBLEMS 7-43
7.7.1 Reducing the Amount of Settlement 7-44
7.7.1.1 Category 1 - Increasing the Resistance .. 7-44
7.7.1.2 Category 2 - Reducing the Load 7-44
7.7.2 Reducing Settlement Time 7-44
7.7.2.1 Surcharge Treatment 7-45
7.7.2.2 Vertical Drains 7-46
7.7.3 Design Solutions to Prevent Abutment Tilting .. 7-47
7.8 PRACTICAL ASPECTS OF EMBANKMENT SETTLEMENT .. 7-48
7.9 CONSTRUCTION MONITORING AND QUALITY ASSURANCE 7-48
7.9.1 Embankment Construction Monitoring by Instrumentation 7-49
7.9.1.1 Inspector’s Visual Observation.. 7-49
7.9.1.2 Types of Instrumentation .. 7-50
7.9.1.3 Typical Locations for Instruments 7-51

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