HomeMy WebLinkAboutRR-41 WSDOT GeotechGeotechnical Design Manual
M 46-03.16
February 2022
Environmental and Regional Operations
Construction Division
Geotechnical Office
RR-41 – CA received 8/22/25, Exhibit 41
RR-41 – CA received 8/22/25, Exhibit 41
Geotechnical Design Manual M 46-03.16 Page 3
February 2022
Contents
Chapter 1 Geotechnical Operations and Administration . . . . . . . . . . . . . . . . . . . . . . . .1-1
1.1 Scope of Geotechnical Design, Construction, and Maintenance Support ..................1-1
1.2 Role of Offices Providing In-House Geotechnical Design, Construction, and
Maintenance Support ..............................................................1-3
1.3 Geotechnical Support within the WSDOT Project Management Process (PMP)..........1-10
1.4 Geotechnical Report Review Process, Certification and Approval Requirements .........1-14
1.5 Reports Produced by Consultants or other Agencies for WSDOT ......................1-18
1.6 Geotechnical Consultant Administration............................................1-19
1.7 Geotechnical Information Provided to Bidders ......................................1-21
1.8 Sample Retention and Chain of Custody ............................................1-23
1.9 Geotechnical Design Policies and their Basis ........................................1-23
1.10 Geotechnical Construction Support Policies ........................................1-25
1.11 Geotechnical Construction Submittal Review Policies ................................1-27
1.12 References .....................................................................1-29
Appendix 1-A Preliminary Geotechnical Engineering Services Scope of Work ......1-A-1
Appendix 1-B Geotechnical Engineering Services Scope of Work for PS&E-Level
Design .......................................................1-B-1
Chapter 2 Project Geotechnical Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-1
2.1 Overview ........................................................................2-1
2.2 Preliminary Project Planning ........................................................2-1
2.3 Development of the Subsurface Exploration Plan ......................................2-8
2.4 Development of the Laboratory Testing Plan .......................................2-15
2.5 References .....................................................................2-16
Appendix 2-A Field Exploration Request Form .................................2-A-1
RR-41 – CA received 8/22/25, Exhibit 41
Contents
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February 2022
Chapter 3 Field Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1
3-1 Overview ........................................................................3-1
3-2 Activities and Policies – Before Exploration ...........................................3-1
3-3 Activities and Policies – During Exploration ...........................................3-7
3-4 Activities and Policies – After Exploration ..........................................3-10
3-5 Standard Penetration Test (SPT) Calibration ........................................3-11
3-6 References .....................................................................3-12
3-7 Appendices .....................................................................3-12
Appendix 3-A Daily Drill Report Form ..........................................3-13
Appendix 3-B Portable Penetrometer Test Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
Appendix 3-C Field Investigation Best Management Practices for Erosion and
Spill Prevention.................................................3-21
Appendix 3-D Department of Natural Resources Memorandum of Understanding:
Drilling Operations – State Owned Aquatic Lands ...................3-23
Appendix 3-E Geotechnical Field Investigation and Contaminated Drilling Waste
Management Procedures ........................................3-29
Appendix 3-F State Materials Laboratory Sample- Handling Policy for
Contaminated and Potentially Contaminated Samples . . . . . . . . . . . . . . . 3-30
Appendix 3-G WSDOT Well Tracking Database Information to be Tracked .........3-33
Chapter 4 Soil and Rock Classification and Logging . . . . . . . . . . . . . . . . . . . . . . . . . . .4-1
4.1 Overview ........................................................................4-1
4.2 Soil Classification ..................................................................4-1
4.3 Rock Classification ................................................................4-9
4.4 References .....................................................................4-17
Appendix 4-A Test Boring Legend ............................................4-A-1
RR-41 – CA received 8/22/25, Exhibit 41
Contents
Geotechnical Design Manual M 46-03.16 Page 5
February 2022
Chapter 5 Engineering Properties of Soil and Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-1
5.1 Overview ........................................................................5-1
5.2 The Geologic Stratum as the Basis for Property Characterization ........................5-1
5.3 Influence of Existing and Future Conditions on Soil and Rock Properties..................5-2
5.4 Methods of Determining Soil and Rock Properties .....................................5-2
5.5 In-Situ Field Testing ...............................................................5-3
5.6 Laboratory Testing of Soil and Rock .................................................5-6
5.7 Back-Analysis Based on Known Performance or Failure ................................5-9
5.8 Engineering Properties of Soil .....................................................5-11
5.9 Engineering Properties of Rock....................................................5-14
5.10 Determination and Use of Soil Cohesion ...........................................5-15
5.11 Final Selection of Design Values ...................................................5-16
5.12 Selection of Design Properties for Engineered Materials ..............................5-21
5.13 Properties of Predominant Geologic Units in Washington ............................5-24
5.14 Application of the Observational Method to Adjust Design Properties .................5-42
5.15 References .....................................................................5-42
Chapter 6 Seismic Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-1
6-1 Seismic Design Responsibility and Policy .............................................6-1
6-2 Geotechnical Seismic Design Considerations ..........................................6-7
6-3 Seismic Hazard and Site Ground Motion Response Requirements .....................6-17
6-4 Seismic Geologic Hazards ........................................................6-27
6-5 Input for Structural Design .......................................................6-48
6-6 References .....................................................................6-57
6-7 Appendices .....................................................................6-63
Appendix 6-A Site Specific Seismic Hazard and Site Response ....................6-A-1
Appendix 6-B High Resolution Seismic Acceleration Maps .......................6-B-1
Chapter 7 Slope Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-1
7.1 Overview ........................................................................7-1
7.2 Development of Design Parameters and Other Input Data for Slope Stability Analysis .....7-1
7.3 Design Requirements ..............................................................7-2
7.4 Resistance Factors and Safety Factors for Slope Stability Analysis .......................7-4
7.5 References .......................................................................7-5
RR-41 – CA received 8/22/25, Exhibit 41
Contents
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February 2022
Chapter 8 Foundation Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1
8.1 Overview ........................................................................8-1
8.2 Overall Design Process for Structure Foundations .....................................8-1
8.3 Data Needed for Foundation Design .................................................8-5
8.4 Foundation Selection Considerations ................................................8-9
8.5 Overview of LRFD for Foundations ................................................8-10
8.6 LRFD Loads, Load Groups and Limit States to be Considered .........................8-11
8.7 Resistance Factors for Foundation Design – Design Parameters .......................8-18
8.8 Resistance Factors for Foundation Design – Service Limit States ......................8-19
8.9 Resistance Factors for Foundation Design – Strength Limit States .....................8-19
8.10 Resistance Factors for Foundation Design – Extreme Event Limit States................8-19
8.11 Spread Footing Design ...........................................................8-20
8.12 Driven Pile Foundation Design ....................................................8-27
8.13 Drilled Shaft Foundation Design ...................................................8-41
8.14 Micropiles ......................................................................8-46
8.15 Proprietary Foundation Systems ..................................................8-46
8.16 Detention Vaults ................................................................8-46
8.17 References .....................................................................8-48
Appendix 8-A Approved Proprietary Foundation Systems (Vacant)................8-A-1
Appendix 8-B Approved AASHTO LRFD Bridge Design Specifications Drill Shaft
Design Provisions ..............................................8-B-1
Chapter 9 Embankments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9-1
9-1 Overview and Data Needed ........................................................9-1
9-2 Design Considerations .............................................................9-6
9-3 Stability Mitigation ..............................................................9-15
9-4 Settlement Mitigation ............................................................9-28
9-5 Construction Considerations and PS&E Development ................................9-31
9-6 References .....................................................................9-39
Appendix 9-A Examples Illustrating Staged Fill Construction Design...............9-A-1
RR-41 – CA received 8/22/25, Exhibit 41
Contents
Geotechnical Design Manual M 46-03.16 Page 7
February 2022
Chapter 10 Soil Cut Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1
10.1 Overview and Data Acquisition ...................................................10-1
10.2 Overall Design Considerations ....................................................10-5
10.3 Soil Cut Design..................................................................10-6
10.4 Use of Excavated Materials ......................................................10-11
10.5 Special Considerations for Loess .................................................10-12
10.6 PS&E Considerations ...........................................................10-19
10.7 References ....................................................................10-20
Appendix 10-A Washington State Department of Transportation Loess Slope
Design Checklist ..............................................10-A-1
Chapter 11 Ground Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1
11.1 Overview ......................................................................11-1
11.2 Development of Design Parameters and Other Input Data for Ground Improvement
Analysis ........................................................................11-2
11.3 Design Requirements ............................................................11-3
11.4 References .....................................................................11-3
Chapter 12 Rock Cut Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12-1
12.1 Overview ......................................................................12-1
12.2 Development of Design Parameters and Other Input Data for Rock Cut Stability Analysis 12-1
12.3 Design Requirements ............................................................12-1
12.4 References .....................................................................12-1
Chapter 13 Landslide Analysis and Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13-1
13.1 Overview ......................................................................13-1
13.2 Development of Design Parameters and Other Input Data for Landslide Analysis ........13-1
13.3 Design Requirements ............................................................13-1
13.4 References .....................................................................13-1
Chapter 14 Unstable Rockslope Analysis and Mitigation . . . . . . . . . . . . . . . . . . . . . . .14-1
14.1 Overview ......................................................................14-1
14.2 Development of Design Parameters and Other Input Data for Unstable Rockslope
Analysis ........................................................................14-1
14.3 Design Requirements ............................................................14-1
14.4 References .....................................................................14-1
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Contents
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February 2022
Chapter 15 Abutments, Retaining Walls, and Reinforced Slopes . . . . . . . . . . . . . . . . .15-1
15-1 Introduction and Design Standards ................................................15-1
15-2 Overview of Wall Classifications and Design Process for Walls ........................15-2
15-3 Required Information ............................................................15-3
15-4 General Design Requirements ....................................................15-11
15-5 Wall Type Specific Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-23
15-6 Standard Plan Walls ............................................................15-57
15-7 Temporary Cut Slopes and Shoring ...............................................15-58
15-8 References ....................................................................15-70
15-9 Appendices ....................................................................15-74
Appendix 15-A Preapproved Proprietary Wall and Reinforced Slope General
Design Requirements and Responsibilities .......................15-A-1
Appendix 15-B Preapproved Proprietary Wall/Reinforced Slope Design and
Construction Review Checklist .................................15-B-1
Appendix 15-C Wall/Reinforced Slope Systems Evaluation: Submittal Requirements 15-C-1
Appendix 15-D Preapproved Proprietary Wall Systems ..........................15-D-1
Appendix 15-E MSE Wall Design Using the Stiffness Method ....................15-E-1
Appendix 15-F Description of Typical Temporary Shoring Systems and Selection
Considerations ...............................................15-F-1
Appendix 15-G Testing and Acceptance Protocols for Tiebacks in Clay ............15-G-1
Appendix 15-H Preapproved Wall Appendix: Specific Requirements and Details for
Hilfiker Welded Wire Faced Walls ..............................15-H-1
Appendix 15-I Vacant .......................................................15-I-1
Appendix 15-J Preapproved Wall Appendix: Specific Requirements and Details
for Reinforced Earth (RECO) Concrete Panel Walls .................15-J-1
Appendix 15-K Preapproved Wall Appendix: Specific Requirements and Details
for Tensar ARES Walls ........................................15-K-1
Appendix 15-L Preapproved Wall Appendix: Specific Requirements and Details
for Tensar MESA Walls .........................................15-L-1
Appendix 15-M Preapproved Wall Appendix: Specific Requirements and Details
for Tensar Welded Wire Form Walls ............................15-M-1
Appendix 15-N Preapproved Wall Appendix: Specific Requirements and Details
for SSL Concrete Panel Walls...................................15-N-1
Appendix 15-O Preapproved Wall Appendix: Specific Requirements and Details
for Landmark Reinforced Soil Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15-O-1
RR-41 – CA received 8/22/25, Exhibit 41
Contents
Geotechnical Design Manual M 46-03.16 Page 9
February 2022
Appendix 15-P Preapproved Wall Appendix: Specific Requirements and Details
for Allan Block Walls ..........................................15-P-1
Appendix 15-Q Preapproved Wall Appendix: Specific Requirements and Details
for Redi-Rock Positive Connection Walls ........................15-Q-1
Appendix 15-R Preapproved Wall Appendix: Specific Requirements and Details
for Lock and Load Walls .......................................15-R-1
Appendix 15-S Preapproved Wall Appendix: Specific Requirements and Details
for KeyGrid Walls .............................................15-S-1
Appendix 15-T Preapproved Wall Appendix: Specific Requirements and Details
for Basalite GEOWALL ........................................15-T-1
Chapter 16 Geosynthetic Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16-1
16.1 Overview ......................................................................16-1
16.2 Development of Design Parameters for Geosynthetic Application .....................16-1
16.3 Design Requirements ............................................................16-2
16.4 References .....................................................................16-2
Chapter 17 Foundation Design for Signals, Signs, Noise Barriers, Culverts,
and Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17-1
17.1 General ........................................................................17-1
17.2 Foundation Design Requirements for Cantilever Signals, Strain Poles, Cantilever Signs,
Sign Bridges, and Luminaires - General .............................................17-4
17.3 Noise Barriers..................................................................17-11
17.4 Culverts .......................................................................17-16
17.5 Buildings ......................................................................17-17
17.6 References ....................................................................17-20
Chapter 18 Geotechnical Design for Marine Structure Foundations . . . . . . . . . . . . . .18-1
18.1 Overview ......................................................................18-1
18.2 Design Philosophy ...............................................................18-1
18.3 Load and Resistance Factors for Marine Structures Subject to Ship Impact..............18-1
18.4 References .....................................................................18-1
Chapter 19 Infiltration Facility Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19-1
19.1 Overview ......................................................................19-1
19.2 Geotechnical Investigation and Design for Infiltration Facilities ........................19-1
19.3 References .....................................................................19-1
RR-41 – CA received 8/22/25, Exhibit 41
Contents
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Chapter 20 Unstable Slope Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20-1
20.1 Overview ......................................................................20-1
20.2 References .....................................................................20-1
Chapter 21 Materials Source Investigation and Report . . . . . . . . . . . . . . . . . . . . . . . . .21-1
21.1 Overview ......................................................................21-1
21.2 Material Source Geotechnical Investigation .........................................21-1
21.3 Materials Source Report..........................................................21-5
Chapter 22 Geotechnical Project Development, Reports, and Support for
Design-Build Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22-1
22-1 Overview ......................................................................22-1
22-2 Definitions .....................................................................22-2
22-3 Purpose and Content of the Geotechnical Reports Included in the Contract Documents ..22-6
22-4 Geotechnical and Other Reference Documents ....................................22-11
22-5 Geotechnical RFP Development ..................................................22-13
22-6 Geotechnical Investigation During RFP Advertisement ..............................22-13
22-7 Geotechnical Support for Design-Build Projects During RFP Advertisement and
Post-Award....................................................................22-14
22-8 References ....................................................................22-15
22-9 Appendices ....................................................................22-15
Appendix 22-A Example Supplemental Geotechnical Boring Program ITP Language .22-A-1
Chapter 23 Geotechnical Reporting and Documentation . . . . . . . . . . . . . . . . . . . . . . . .23-1
23.1 Overview and General Requirements ..............................................23-1
23.2 Report Certification and General Format ...........................................23-1
23.3 Geotechnical Office Report Content Requirements ..................................23-7
23.4 Information to Be Provided in the Geotechnical Design File ..........................23-21
23.5 Consultant Geotechnical Reports and Documentation Produced on Behalf of WSDOT ..23-24
23.6 Summary of Geotechnical Conditions .............................................23-25
Appendix 23-A PS&E Review Checklist ........................................23-A-1
Appendix 23-B Typical Design Cross-Section for a Deep Foundation ..............23-B-1
RR-41 – CA received 8/22/25, Exhibit 41
Chapter 7 Slope Stability Analysis
7 .1 Overview
Slope stability analysis is used in a wide variety of geotechnical engineering problems,
including, but not limited to, the following:
• Determination of stable cut and fill slopes
• Assessment of overall stability of retaining walls, including global and compound
stability (includes permanent systems and temporary shoring systems)
• Assessment of overall stability of shallow and deep foundations for structures
located on slopes or over potentially unstable soils, including the determination
of lateral forces applied to foundations and walls due to potentially unstable slopes
• Stability assessment of landslides (mechanisms of failure, and determination
of design properties through back-analysis), and design of mitigation techniques
to improve stability
• Evaluation of instability due to liquefaction
Types of slope stability analyses include rotational slope failure, translational failure,
irregular surfaces of sliding, and infinite slope failure. Stability analysis techniques
specific to rock slopes, other than highly fractured rock masses that can in effect be
treated as soil, are described in Chapter 12. Detailed stability assessment of landslides
is described in Chapter 13.
7 .2 Development of Design Parameters and Other Input Data for Slope
Stability Analysis
The input data needed for slope stability analysis is described in Chapter 2 for site
investigation considerations, Chapters 9 and 10 for fills and cuts, and Chapter 13 for
landslides. Chapter 5 provides requirements for the assessment of design property
input parameters.
Detailed assessment of soil and rock stratigraphy is critical to the proper assessment
of slope stability, and is in itself a direct input parameter for slope stability analysis.
It is important to define any thin weak layers present, the presence of slickensides,
etc., as these fine details of the stratigraphy could control the stability of the slope
in question. Knowledge of the geologic nature of the strata present at the site
and knowledge of past performance of such strata may also be critical factors in
the assessment of slope stability. See Chapter 5 for additional requirements and
discussion regarding the determination and characterization of geologic strata and the
determination of ESU’s for design purposes.
Whether long-term or short-term stability is in view, and which will control the
stability of the slope, will affect the selection of soil and rock shear strength parameters
used as input in the analysis. For short-term stability analysis, undrained shear strength
parameters should be obtained. For long-term stability analysis, drained shear strength
parameters should be obtained. For assessing the stability of landslides, residual shear
strength parameters will be needed, since the soil has in such has typically deformed
WSDOT Geotechnical Design Manual M 46-03.08 Page 7-1
October 2013
RR-41 – CA received 8/22/25, Exhibit 41
enough to reach a residual value. For highly overconsolidated clays, such as the
Seattle clays (e.g., Lawton Formation), if the slope is relatively free to deform after
the cut is made or is otherwise unloaded, even if a structure such as a wall is placed
to retain the slope after that deformation has already occurred, residual shear strength
parameters should be obtained and used for the stability analysis. See Chapter 5 for
requirements on the development of shear strength parameters.
Detailed assessment of the groundwater regime within and beneath the slope/landslide
mass is also critical. Detailed pieziometric data at multiple locations and depths within
and below the slope will likely be needed, depending on the geologic complexity
of the stratigraphy and groundwater conditions. Potential seepage at the face of the
slope must be assessed and addressed. In some cases, detailed flow net analysis
may be needed. If seepage does exit at the slope face, the potential for soil piping
should also be assessed as a slope stability failure mechanism, especially in highly
erodable silts and sands. If groundwater varies seasonally, long-term monitoring
of the groundwater levels in the soil should be conducted. If groundwater levels tend
to be responsive to significant rainfall events, the long-term groundwater monitoring
should be continuous, and on-site rainfall data collection should also be considered.
7 .3 Design Requirements
Limit equilibrium methods shall be used to assess slope stability. The Modified
Bishop, simplified Janbu, Spencer, or other widely accepted slope stability analysis
methods should be used for rotational, translational and irregular surface failure
mechanisms. Each limit equilibrium method varies with regard to assumptions used
and how stability is determined. Therefore, a minimum of two limit equilibrium
methods should be used and compared to one another to ensure that the the level
of safety in the slope is accurately assessed. In cases where the stability failure
mechanisms anticipated are not well modeled by limit equilibrium techniques,
or if deformation analysis of the slope is required, more sophisticated analysis
techniques (e.g., finite difference methods such as is used by the computer program
FLAC) may be used in addition to the limit equilibrium methodologies. Since these
more sophisticated methods are quite sensitive to the quality of the input data and
the details of the model setup, including the selection of constitutive models used
to represent the material properties and behavior, limit equilibrium methods should
also be used in such cases, and input parameters should be measured or assessed
from back-analysis techniques whenever possible. If the differences in the results are
significant, the reasons for the differences shall be assessed with consideration to any
available field observations to assess the correctness of the design model used. If the
reasons for the differences cannot be assessed, and if the FLAC model provides a less
conservative result than the limit equilibrium based methods, the limit equilibrium
based methods shall govern the design.
If the potential slope failure mechanism is anticipated to be relatively shallow and
parallel to the slope face, with or without seepage affects, an infinite slope analysis
should be conducted. Typically, slope heights of 15 to 20 feet or more are required
to have this type of failure mechanism. For infinite slopes consisting of cohesionless
soils that are either above the water table or that are fully submerged, the factor
of safety for slope stability is determined as follows:
Slope Stability Analysis Chapter 7
Page 7-2 WSDOT Geotechnical Design Manual M 46-03.08
October 2013
RR-41 – CA received 8/22/25, Exhibit 41
Tan φ FS = (7-1) Tanβ
Where:
φ = the angle of internal friction for the soil
β = the slope angle relative to the horizontal
For infinite slopes that have seepage at the slope face, the factor of safety for slope
stability is determined as follows: γb Tan φ FS = (7-2) γs Tanβ
Where:
γb = the buoyant unit weight of the soil
γs = the saturated unit weight of the soil
Considering that the buoyant unit weight is roughly one-half of the saturated unit
weight, seepage on the slope face can reduce the factor of safety by a factor of two, a
condition which should obviously be avoided through some type of drainage if at all
possible; otherwise much flatter slopes will be needed. When using the infinite slope
method, if the FS is near or below 1.0 to 1.15, severe erosion or shallow slumping
Slope Stability Analysis Geotechnical Design Manual M 46-03
Chapter 7-4 December 2006
Slope Stability AnalysisDetailed assessment of the groundwater regime within and beneath the slope is also critical. Detailed pieziometric data at multiple locations and depths within and below the slope will likely be needed, depending on the geologic complexity of the stratigraphy and groundwater conditions. Potential seepage at the face of the slope must be assessed and addressed. In some cases, detailed flow net analysis may be needed. If seepage does exit the slope face, the potential for soil piping should also be assessed as a slope stability failure mechanism, especially in highly erodable silts and sands. If groundwater varies seasonally, long-term monitoring of the groundwater levels in the soil should be conducted. If groundwater levels tend to be responsive to significant rainfall events, the long-term groundwater monitoring should be continuous.7 .3 Design RequirementsLimit equilibrium methodologies shall be used to assess slope stability. The Modified Bishop, simplified Janbu, Spencer, or other widely accepted slope stability analysis methods should be used for rotational and irregular surface failure mechanisms. In cases where the stability failure mechanisms anticipated are not well modeled by limit equilibrium techniques, or if deformation analysis of the slope is required, more sophisticated analysis techniques (e.g., finite difference methodologies such as is used by the computer program FLAC) may be used in addition to the limit equilibrium methodologies. Since these more sophisticated methodologies are quite sensitive to the quality of the input data and the details of the model setup, including the selection of constitutive models used to represent the material properties and behavior, limit equilibrium methods should also be used in such cases. If the differences in the results are significant, engineering judgment should be applied in conjunction with any available field observations to assess the correctness of the design model used.If the potential slope failure mechanism is anticipated to be relatively shallow and parallel to the slope face, with or without seepage affects, an infinite slope analysis should be conducted. Typically, slope heights of 15 to 20 ft or more are required to have this type of failure mechanism. For infinite slopes consisting of cohesionless soils which are either above the water table or which are fully submerged, the factor of safety for slope stability is determined as follows:
Tan
TanFS
Tan
TanFS
s
b
(7-1)
where,
φ = the angle of internal friction for the soil
β = the slope angle relative to the horizontal
For infinite slopes that have seepage at the slope face, the factor of safety for slope stability is determined
as follows:
(7-2)
where,
γb = the buoyant unit weight of the soil
γs = the saturated unit weight of the soil
Tan
TanFS
Tan
TanFS
s
b
Slope Stability Analysis Geotechnical Design Manual M 46-03
Chapter 7-4 December 2006
Slope Stability AnalysisDetailed assessment of the groundwater regime within and beneath the slope is also critical. Detailed pieziometric data at multiple locations and depths within and below the slope will likely be needed, depending on the geologic complexity of the stratigraphy and groundwater conditions. Potential seepage at the face of the slope must be assessed and addressed. In some cases, detailed flow net analysis may be needed. If seepage does exit the slope face, the potential for soil piping should also be assessed as a slope stability failure mechanism, especially in highly erodable silts and sands. If groundwater varies seasonally, long-term monitoring of the groundwater levels in the soil should be conducted. If groundwater levels tend to be responsive to significant rainfall events, the long-term groundwater monitoring should be continuous.7 .3 Design RequirementsLimit equilibrium methodologies shall be used to assess slope stability. The Modified Bishop, simplified Janbu, Spencer, or other widely accepted slope stability analysis methods should be used for rotational and irregular surface failure mechanisms. In cases where the stability failure mechanisms anticipated are not well modeled by limit equilibrium techniques, or if deformation analysis of the slope is required, more sophisticated analysis techniques (e.g., finite difference methodologies such as is used by the computer program FLAC) may be used in addition to the limit equilibrium methodologies. Since these more sophisticated methodologies are quite sensitive to the quality of the input data and the details of the model setup, including the selection of constitutive models used to represent the material properties and behavior, limit equilibrium methods should also be used in such cases. If the differences in the results are significant, engineering judgment should be applied in conjunction with any available field observations to assess the correctness of the design model used.If the potential slope failure mechanism is anticipated to be relatively shallow and parallel to the slope face, with or without seepage affects, an infinite slope analysis should be conducted. Typically, slope heights of 15 to 20 ft or more are required to have this type of failure mechanism. For infinite slopes consisting of cohesionless soils which are either above the water table or which are fully submerged, the factor of safety for slope stability is determined as follows:
Tan
TanFS
Tan
TanFS
s
b
(7-1)
where,
φ = the angle of internal friction for the soil
β = the slope angle relative to the horizontal
For infinite slopes that have seepage at the slope face, the factor of safety for slope stability is determined
as follows:
(7-2)
where,
γb = the buoyant unit weight of the soil
γs = the saturated unit weight of the soil
Tan
TanFS
Tan
TanFS
s
b
is likely. Vegetation on the slope can help to reduce this problem, as the vegetation
roots add cohesion to the surficial soil, improving stability. Note that conducting
an infinite slope analysis does not preclude the need to check for deeper slope failure
mechanisms, such as would be assessed by the Modified Bishop or similar methods
listed above.
Translational (block) or noncircular searches are generally more appropriate for
modeling thin weak layers or suspected planes of weakness, and for modeling stability
of long natural slopes or of geologic strata with pronounced shear strength anisotropy
(e.g., due to layered/bedded macrostructure or pre-existing fracture patterns). If there
is a disparately strong unit either below or above a thin weak unit, the user must ensure
that the modeled failure plane lies within the suspected weak unit so that the most
critical failure surface is modeled as accurately as possible. Circular searches for these
types of conditions should generally be avoided as they do not generally model the
most critical failure surface.
For very simplified cases, design charts to assess slope stability are available.
Examples of simplified design charts are provided in NAVFAC DM-7 (US Department
of Defense, 2005). These charts are for a c-φ soil, and apply only to relatively uniform
soil conditions within and below the cut slope. They do not apply to fills over relatively
soft ground, as well as to cuts in primarily cohesive soils. Since these charts are for a
c-φ soil, a small cohesion will be needed to perform the calculation. If these charts are
to be used, it is recommended that a cohesion of 50 to 100 psf be used in combination
with the soil friction angle obtained from SPT correlation for relatively clean sands
and gravels. For silty to very silty sands and gravels, the cohesion could be increased
to 100 to 200 psf, but with the friction angle from SPT correlation (see Chapter 5)
reduced by 2 to 3 degrees, if it is not feasible to obtain undisturbed soil samples
suitable for laboratory testing to measure the soil shear strength directly. This should
be considered general guidance, and good engineering judgment should be applied
when selecting soil parameters for this type of an analysis. Simplified design charts
shall only be used for final design of non-critical slopes that are approximately 10 feet
Chapter 7 Slope Stability Analysis
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in height or less and that are consistent with the simplified assumptions used by the
design chart. Simplified design charts may be used as applicable for larger slopes for
preliminary design.
The detailed guidance for slope stability analysis provided by Abramson, et al. (1996)
should be used.
For additional design requirements for temporary slopes, including application of the
applicable WAC’s, see Sections 15.7 and 9.5.5.
7 .4 Resistance Factors and Safety Factors for Slope Stability Analysis
For overall stability analysis of walls and structure foundations, design shall be
consistent with Chapters 6, 8 and 15 and the AASHTO LRFD Bridge Design
Specifications. For slopes adjacent to but not directly supporting structures, a
maximum resistance factor of 0.75 should be used. For foundations on slopes that
support structures such as bridges and retaining walls, a maximum resistance factor
of 0.65 should be used. This reduced resistance factor also applies if the slope is not
directly supporting the structure, but if slope failure occurred, it could impact and
damage the structure. Exceptions to this could include minor walls that have a minimal
impact on the stability of the existing slope, in which the 0.75 resistance factor may
be used. Since these resistance factors are combined with a load factor of 1.0 (overall
stability is assessed as a service limit state only), these resistance factors of 0.75 and
0.65 are equivalent to a safety factor of 1.3 and 1.5, respectively.
For general slope stability analysis of permanent cuts, fills, and landslide repairs, a
minimum safety factor of 1.25 should be used. Larger safety factors should be used
if there is significant uncertainty in the analysis input parameters. The Monte Carlo
simulation features now available in some slope stability computer programs may be
used for this purpose, from which a probability of failure can be determined, provided
a coefficient of variation for each of the input parameters can be ascertained. For
considerations regarding the statistical characterization of input parameters, see Allen,
et al. (2005). For minimum safety factors and resistance factors for temporary cuts, see
Section 15.7.
For seismic analysis, if seismic analysis is conducted (see Chapter 6 for policies on
this issue), a maximum resistance factor of 0.9 should be used for slopes involving or
adjacent to walls and structure foundations. This is equivalent to a safety factor of 1.1.
For other slopes (cuts, fills, and landslide repairs), a minimum safety factor of 1.05
shall be used.
Slope Stability Analysis Chapter 7
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Conditions Probability of Failure, Pf
Unacceptable in most cases > 0.1
Temporary structures with no potential life loss and low repair cost 0.1
Slope of riverbank at docks, no alternative docks, pier shutdown threatens 0.01 to 0.02
operations
Low consequences of failure, repairs when time permits, repair cost less than 0.01
cost to go to lower Pf
Existing large cut on interstate highway 0.01 to 0.02
New large cut (i.e., to be constructed) on interstate highway 0.01 or less
Acceptable in most cases except if lives may be lost 0.001
Acceptable for all slopes 0.0001
Unnecessarily low 0.00001
Slope Stability – Probability of Failure (Adapted From Santamarina, et al ., 1992)
Table 7-1
7 .5 References
Abramson, L., Boyce, G., Lee, T., and Sharma, S., 1996, Slope Stability and
Stabilization Methods, Wiley, ISBN 0471106224.
Allen, T., Nowak, A., and Bathurst, R., 2005, Calibration to Determine Load and
Resistance Factors for Geotechnical and Structural Design. TRB Circular E-C079,
83 pp.
Santamarina, J. C., Altschaeffl, A. G., and Chameau, J. L., 1992, “Reliability of Slopes:
Incorporating Qualitative Information,” Transportation Research Board, TRR 1343,
Washington, D.C., pp. 1-5.
US Department of Defense, 2005, Soil Mechanics, Unified Facilities Criteria (UFC),
UFC 3-220-10N,
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Slope Stability Analysis Chapter 7
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