Manual for Design and Construction Monitoring of Soil Nail Walls

250 Pages·12.035 MB·250\250

us.D efxnmenr
of Transportdim
kderol Hishwoy
Administration
MANUAL FOR DESIGN &
CONSTRUCTION
MONITORING OF SOIL
NAIL WALLS
Revised October 1998
TABLE OF CONTENTS
.
CHAPTER 1 INTRODUCTION AND APPLICATION CRITERIA
1.1 Purpose and Scope of Manual ..................................................................... 1
1.2 Limitations ............................................................................................ 3
1.3 The Soil Nailing Concept ........................................................................... 4
1.4 Advantages of Soil Nailing ......................................................................... 7
1.5 Limitations of Soil Nailing ......................................................................... 9
1.6 Application Criteria ................................................................................ 10
1.6.1 Retaining Structures in Cuts ............................................................ 10
1.6.2 End Slope Removal Under Existing Bridge Abutment. ............................. 10
1.6.3 Repairs and Reconstruction of Existing Retaining Structures ...................... 11
1.7 Ground Conditions Best Suited for Soil Nailing ............................................... 12
1.8 Ground Conditions Not Well Suited for Soil Nailing. ........................................ 13
1.9 Cost Data ............................................................................................ 15
1.10 Acknowledgments ................................................................................ 16
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CHAPTER 2 DESCRIPTION OF SOIL NAILING AND BASIC MECHANISMS
2.1 Background .......................................................................................... 30
2.2 Construction Sequence ............................................................................. 33
2.3 Construction Equipment and Materials ......................................................... 39
2.4 Behavior of Soil Nail Walls., ..................................................................... 44
2.4.1 Fundamental Mechanism and Potential Failure Modes of a Soil Nail
wall ............................................................................................... 44
2.4.2 Nail-Ground Interaction ................................................................. 45
2.4.3 Nail-Ground-Facing Interaction ........................................................ 48
2.4.4 Distribution of Nail Forces ............................................................. 49
2.4.5 Face Loading Magnitude and Distribution ............................................ 50
2.4.6 Deformation Behavior ................................................................... 53
2.4.7 Role of Nail Bending and Shear ....................................................... 54
2.5 Comparison with MSE and Tieback Walls. .................................................... 58
2.5.1 Tieback Walls ............................................................................ 58
2.5.2 MSE Walls ................................................................................ 62
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CHAPTER 3 SITE INVESTIGATION AND TESTING
3.1 Ground Characterization ........................................................................... 66
3.2 Estimating Nail Pullout Resistance for Design ................................................. 73
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CHAPTER 4 SOIL NAIL WALL DESIGN
4.1 Introduction .......................................................................................... 76
4.1.1 Limit States ............................................................................... 77
4.1.2 Design Approaches ...................................................................... 77
4.2 Soil Nail Wall Stability Considerations ......................................................... 79
4.2.1 Basic Concepts ........................................................................... 79
4.2.2 Internal Stability - Nail .................................................................. 82
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4.2.3 Internal Stability .N ail Head ........................................................... 85
4.2.4 External Stability ......................................................................... 85
4.3 Design Method Evaluation ........................................................................ .89
4.3.1 “Earth Pressure” Method ................................................................ 89
4.3.2 “Slip Surface” Method .................................................................. 92
4.3.3 Current Soil Nailing Design Methods ................................................9. 4
4.3.4 Recommended Design Method ......................................................... 95
4.4 Layout and Dimensioning ......................................................................... 97
4.4.1 Wall Location and Dimensions., ....................................................... 97
4.4.2 Preliminary Nail Layouts ............................................................... 98
4.4.3 Caltrans Design Practice ............................................................... 100
4.4.4 Schnabel Foundation Company Design Practice ................................... 100
4.5 Nail Head Strength.,. ............................................................................. 103
4.5.1 Nail Head Failure Modes .............................................................. 103
4.5.2 Flexural Strength of the Facing ....................................................... 104
4.5.3 Punching Shear Strength of the Facing .............................................. 107
4.5.4 Headed-Studs Tensile Strength ....................................................... 110
4.5.5 Selecting Nominal Nail Head Strengths. ............................................ 112
4.6 Real World Design Process ..................................................................... 115
4.7 Soil Nail Wall Design ............................................................................ 118
4.7.1 Service Load Design (SLD). .......................................................... 118
4.7.2 Load and Resistance Factor Design (LRFD) ....................................... 132
4.8 Corrosion Protection .............................................................................. 143
4.8.1 Nail Tendon Corrosion Protection ...................................................1 43
4.8.2 Nail Head Corrosion Protection ...................................................... 143
4.9 Wall Drainage ..................................................................................... 146
4.10 Special Design Considerations. ............................................................... 149
4.10.1 Heterogeneous Soil Profiles ......................................................... 149
4.10.2 Surcharge Loading ....................................................................1 51
4.10.3 Bridge Abutments ..................................................................... 151
4.10.4 Stepped Structures ..................................................................... 153
4.10.5 Composite Structures. ................................................................ 153
4.10.6 Structures with Variable Nail lengths .............................................. 157
4.10.7 Structures with Variable Nail Inclinations. ........................................ 159
4.10.8 Structures with Variable Nail Orientations. ....................................... 159
4.10.9 Ground Water Seepage Forces or Water Table Close to Wall Base ............ 161
4.10.10 Infinite Slope Condition ............................................................ 161
4.10.11 Performance Under Seismic Loading ............................................. 161
4.10.12 Frost Protection ...................................................................... 163
4.10.13 Expansive Soils ...................................................................... 164
4.10.14 Residual Soils ........................................................................ 164
4.10.15 Structures with Externally Loaded Wall Facings ................................ 164
4.10.16 Design of Support for Facing Dead Load., ...................................... 166
4.10.17 End of Wall Transitions. ........................................................... 170
4.10.18 CIP Concrete Form Connection to Shotcrete Facing ........................... 170
4.11 Aesthetic Issues ................................................................................. 172
4.12 Simplified Design Charts for Preliminary Design of Cut Slope Wall .................... 173
...
111
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CHAPTER 5 WORKED DESIGN EXAMPLES
.............................................................
5.1 Design Example 1 .C utslope Wall 185
...........................................................
5.1.1 Service Load Design (SLD) 185
......................................................
5.1.1.1 Static Loading Condition 185
...................................................
5.1.1.2 Seismic Loading Condition 208
.......................................
5.1.2 Load and Resistance Factor Design (LRFD) 221
......................................................
5.1.2.1 Static Loading Condition 221
...................................................
5.1.2.2 Seismic Loading Condition 244
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CHAPTER 6 SOIL NAIL WALL PERFORMANCE MONITORING
.....................................................................................
6.1 Introduction.. $258
............................................
6.2 Soil Nail Wall Performance Monitoring Methods 259
................................................
6.3 Soil Nail Wall Performance Monitoring Plan 262
..................................................................
6.4 Parameters To Be Monitored., 263
........................................
6.5 Soil Nail Wall Performance Monitoring Instruments 264
..................................................................
6.5.1 Slope Inclinometers., 264
...........................................................................
6.5.2 Survey Points 265
...............................................................
6.5.3 Soil Nail Strain Gages., 265
.........................................................
6.5.4 Load Cells at the Nail Head., 266
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CHAPTER 7 SHOTCRETE
......................................................................................
7.1 Introduction., 268
..................................................................................
7.1.1 General. 268
..........................................
7.1.2 The Function of Shotcrete in Soil Nailing 268
............................................................................
7.1.3 Definitions., 269
.....................................................................
7.1.4 Types of Shotcrete 270
..............................................
7.1.5 Development of Shotcrete in the USA 270
..............................................................................
7.2 Shotcrete Equipment 273
...............................................................................
7.3 Shotcrete Materials 274
..............................................................................
7.4 Shotcrete Properties 277
.............................................
7.4.1 Factors Determining Shotcrete Quality 277
...............................................
7.4.2 Common Specification Requirements 277
..........................
7.4.3 Shotcrete Mixture Proportioning and Air Entrainment. 277
....................................................................
7.5 Proper Shooting Techniques. 282
...............................................................................
7.5.1 Nozzling., 282
..................................................................
7.5.2 Minimizing Rebound 282
...............................................
7.5.3 Shooting Reinforcement in Shotcrete. 287
.........................................................
7.6 Shotcrete Quality Control and Testing 288
...........................................................................
7.6.1 Basic Purpose 288
.................................................................
7.6.2 Shotcrete Submittals., 288
...........................................................................
7.6.3 Qualifications 288
....................................................................
7.6.3.1 Contractors., 288
.......................................................................
7.6.3.2 Nozzlemen 288
............................................................
7.6.4 Pre-Construction Testing., 289
..................................................
7.6.5 Quality Control During Production 289
...........................................................................
7.6.5.1 Testing 289
....................................................
7.6.5.2 Inspection and Acceptance 290
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7.7 Shotcrete Application .T emporary Shotcrete Facings ....................................... 292
7.7.1 Finishing ................................................................................. 292
7.7.2 Tolerances .............................................................................. 292
7.7.3 Joints ..................................................................................... 292
7.7.4 Protection and Curing .................................................................. 292
7.8 Potential Problems With Construction Facing Shotcrete In Soil Nailing Applications 294
7.8.1 General Durability ..................................................................... 294
7.8.2 Potential Problems With the Shotcrete .............................................. 294
7.9 Additional Aspects Of Shotcreting For Permanent Shotcrete Faqing ..................... 295
7.9.1 General ................................................................................... 295
7.9.2 Comparison of Shotcrete and Conventional CIP Concrete ........................ 295
7.9.3 Durability and Defects ................................................................. 295
7.9.4 Materials ................................................................................. 298
7.9.5 Shotcrete Application-Permanent Shotcrete Facings .............................. 299
7.9.5.1 Protection and Curing ......................................................... 299
7.9.5.2 Shooting Reinforcement in Permanent Facings., ........................... 300
7.9.5.3 Surface Preparation ............................................................ 300
7.9.5.4 Finishing ......................................................................... 301
7.9.5.5 Tolerances ........................................................................ 301
..............................................................................
7.9.5.6 Joints 301
7.9.5.7 In Situ Density and Permeability ............................................. 302
7.10 Steel Fiber Reinforced Shotcrete (SFRSC) .................................................. 303
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CHAPTER 8 CONTRACTING METHODS. PLANS. AND SPECIFICATIONS
8.1 Contractor Prequalification ...................................................................... 306
8.2 Contracting Methods and Definitions .......................................................... 307
8.2.1 Contract Procurement with Owner-Design .......................................... 307
8.2.1.1 Contracts with Owner Design and Performance Specification ........... 308
8.2.1.2 Contracts with Owner Design and ProceduralPrescriptive
Specifications ........................................................................... 310
8.2.2 Contractor Design/Build ("Turnkey") Contracting Methods ..................... 311
8.3 Plan Preparation .................................................................................. 316
8.3.1 Conceptual Plans for Desigfluild Contracting .................................... 316
8.3.2 Final Plans .............................................................................. 318
8.4 Example Plan Details ............................................................................. 319
8.5 Guide Construction Specifications ............................................................. 320
8.5.1 Guide Specifications for Owner-Design ............................................. 320
8.5.2 Guide Specification for Contractor Desigfluild Solicitation ................... 320
REFERENCES ............................................................................................. 321
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CHAPTER 1. INTRODUCTION AND APPLICATION CRITERIA
1.1 Purpose and Scope of Manual
The specific purpose of this manual is to introduce the concept of soil nailing use into American
transportation construction practice and to provide guidance for selecting, designing, and
specifying soil nailing for those many applications to which it is technically suited and
economically attractive. A comprehensive review of current design and construction methods
has been made and the results compiled into a guideline procedure. The focus of this document
is primarily on design methods and procedures for permanent soil nailing.
Permanent soil nailing systems are generally considered to have a service life of 75-100 years.
However, soil nailing is commonly used for temporary applications. The service life of
temporary earth support works is based on the time needed to support the ground while the
permanent works are installed. This document has adopted the AASHTO guidance which
considers temporary works to be those that are to be removed on completion of the permanent
works. The time period for temporary works is commonly stated to be 18-36 months but may
be shorter or longer based on actual project conditions.
The intent of presenting the guideline procedure is to ensure that agencies adopting permanent
soil nail wall design and construction follow a safe, rational procedure from site investigation
through construction. Close attention has been given to the presentation of suggested general
specifications and plan details. Contract documents such as these, which provide the transition
from design analyses to field construction, frequently decide the success or failure of new design
concepts. Every effort has been made in the sample specifications to give all experienced nailing
contractors an opportunity to use innovative methods or equipment in construction. Such
specifications are needed to encourage contractors to seek cost-effective improvements to current
soil nailing methods.
Engineers responsible for design and construction of public works usually need long-term
monitoring of new techniques pioneered in the private sector before they can be confidently
incorporated in permanent public projects. The long-term performance of soil nail walls has
been proven after 20 years of use in Europe and the United States. This manual is intended to
permit engineers to rationally and confidently specify permanent soil nailing in cost-effective
situations. Implementation of permanent soil nailing is consistent with national efforts to
upgrade the safety and efficiency of the transportation system in the most cost-effective manner
possible. The goal of this FHWA demonstration project is to assist US.t ransportation agencies
in implementing the safe and cost-effective use of permanent soil nail designs as alternate bid
items to the standard wall systems presently used to retain steep excavation cut slopes.
The scope of this manual includes:
Chapter 1 A brief description of the soil nailing concept and a discussion of the
advantages, limitations, and recommended applications of the technique.
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Chapter 2 A description of the history of the use of soil nailing in both North
America and Europe, of the method of construction (including
construction equipment and materials), and of the behavior of soil nail
walls as determined from both experimental programs and monitoring of
in-service walls.
Chapter 3 Recommended methods of site investigation and testing.
Chapter 4 Recommended design procedures.
Chapter 5 Worked Design Example including simplified design charts for the
preliminary design of cut slope walls.
Chapter 6 Wall performance monitoring recommendations.
Chapter 7 Discussion on the practice and quality control of shotcrete application in
soil nailing.
Chapter 8 Discussion of contracting procedures and guidance on the preparation of
plans and specifications.
Appendices A-D Guide Specifications and Example Plans for Soil Nail Wall Construction
and Performance Monitoring.
Appendix E Quality Control Checklist for soil nail design and construction.
Appendix F Presentation of procedures for determining the capacity of nail head
connectors and wall facings, including demonstration calculations.
Appendix G Worked design example 5.2 for cut-slope walls.
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1.2 Soil Nail Description
A soil nail is a structural element which provides load-transfer to the ground in excavation
reinforcement applications. The "nail" may simply consist of a steel tendon, but most commonly
the tendon is encapsulated in a cement grouted body to provide corrosion protection and
improved load-transfer to the ground.
' w
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1.3 The Soil Nailing Concept
The basic concept of soil nailing is to reinforce and strengthen the existing ground by installing
closely-spaced steel bars, called “nails”, into a slope or excavation as construction proceeds from
the “top down.” This process creates a reinforced section that is itself stable and able to retain
the ground behind it. As with mechanically stabilized earth (MSE) walls, the reinforcements are
passive and develop their reinforcing action through nail-ground interactions as the ground
deforms during and following construction. Nails work predominantly in tension but are
considered by some to work also in bendingkhear in certain circumstances. Consideration of
shearhending contributions is not included in the recommended design methods presented later
in this manual. The effect of the nail reinforcement is to improve stability by (a) increasing the
normal force and hence the soil shear resistance along potential slip surfaces in frictional soils;
and (b) reducing the driving force along potential slip surfaces in both frictional and cohesive
soils. A construction facing is also usually required and is typically shotcrete reinforced by
welded wire mesh.
There are three main categories of in-situ reinforcement techniques used to stabilize soil slopes
and support excavations. These are nailing, reticulated micro-piling, and doweling.
In soil nailing (figure 1. la), the reinforcement is installed horizontally or sub-horizontally
(approximately parallel to the direction of major tensile straining in the soil) so that it contributes
to the support of the soil partially by directly resisting the destabilizing forces and partially by
increasing the normal loads (and hence the shear strength) on potential sliding surfaces.
Reticulated micro-piles are steeply inclined in the soil at various angles both perpendicular and
parallel to the face (figure 1.lb). The overall aim is similar to soil nailing, namely to provide a
stable block of reinforced soil which supports the unreinforced soil by acting like a gravity
retaining structure. In this technique, the soil is held together by the multiplicity of
reinforcement members acting to resist bending and shearing forces. [ 11 [2]
Soil doweling is applied to reduce or halt downslope movements on well-defined shear surfaces
(figure 1.1~).T he slopes treated by doweling are typically much flatter than those in soil nailing
or reticulated micro-pile applications. Gudehus [3] has shown that the most efficient way to
mechanically increase the shearing resistance on a weakened shear surface through the soil is to
use relatively large diameter piles which combine a large surface area with high bending
stiffness. Thus, the diameter of a soil dowel is generally greater than that of a soil nail or micro-
pile.
This design manual is concerned only with soil nailing. Furthermore, the method of analysis
presented herein addresses tension only as the resisting element for excavation support systems
and slope stabilization. The reinforced soil body becomes the primary structural element. The
reinforced zone performs as a homogenous and resistant unit to support the unreinforced soil
behind it in a manner similar to a gravity wall.
Since the introduction of soil nailing, its application has extended to a wide variety of ground
types ranging from soils to weathered and unweathered rock. While the term “ground” nail might
be a more suitable generic term, “soil” nail has become established as the commonly accepted
generic terminology. Therefore, to be consistent with established practice, the term “soil nail” is
used in this manual as a generic term that applies to nails installed in all types of ground,
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