Revised AASHTO Manual on Subsurface Investigations

Glenn J. Rix

Previous Edition

Evolutionary Change in the Past 30 Years

• Geotechnical uncertainty and risk• Developments in in-situ test

methods and their interpretation to estimate soil and rock properties

• Rock mass characterization• Geotechnical reports for

alternative project delivery methods

• Geotechnical instrumentation• Geotechnical data management

Geotechnical Uncertainty and Risk

• Technical risks– Inability to satisfy desired 

performance requirements for one or more limit states

• Financial risks– Claims, change orders, cost and 

schedule overruns attributed to differing subsurface conditions

• Load and resistance factor design

Geophysical Methods

• New methods– Surface wave methods– Electrical resistivity imaging– Ground penetrating radar

• New applications– Seismic site classification via Vs,30

In Situ Testing

• Increased standardization of the SPT

• Widespread use of CPT• Development of methods to

estimate engineering characteristics and properties of soils directly from in situ tests

Drilling and Sampling of Soil and Rock

• Direct-push sampling techniques– Continuous hydraulic systems– Continuous sonic drilling– Vibracore

• Measurement while drilling (MWD)

Laboratory Testing of Soil and Rock

• Quantitative assessment of sample disturbance– X‐ray radiography– Observed changes in vertical strain and void ratio during reconsolidation

Interpretation of Soil and Rock Properties

• Soil properties– Soil type– Unit weight– Preconsolidation or yield 

stress– Drained and undrained 

strength properties– Lateral stress state– Modulus– Coefficient of consolidation– Hydraulic conducivity

• Rock mass properties– Rock Mass Rating– Geological Strength Index– Strength estimates

• Hoek et al. (2002)• Barton (2002)

Reporting Geotechnical Information

• Alternative project-delivery methods– Construction manager at 

risk– Design‐build– Negotiated general 

contractor

Geotechnical Instrumentation

• Sensor technologies• Communication

technologies• Automated data

collection• Data storage and

information management• Field installation

methodsDunnicliff (1982)

Management of Geotechnical Data

• Unprecedented amount of data is available, but is usually not readily accessible

• Tools such as DIGGS offer a standardized data transfer protocol to help manage large amounts of data

Other New Developments

• Increased emphasis on quality assurance/quality control

• Increased emphasis on health and safety

• Increased use of outsourcing for subsurface investigations

• Technology transfer strategies

Objectives of the Revised Manual

• Develop a concise*, comprehensive document that will be invaluable for planning, executing, and using subsurface investigations for transportation projects

• Define a reasonable minimum standard of practice for a modern geotechnical site investigation

• Enable geoprofessionals to develop cost-effective design solutions while optimizing life-cycle costs, ensuring public safety and environmental sustainability, minimizing contract disputes and cost overruns, and accelerating construction

* The 1988 edition of the manual is 391 pages vs. 373 pages for the revised version

Intended Users

• State transportation agencies who are responsible for:– developing the scope of subsurface investigations,– selecting and managing qualified consultants and contractors to perform 

the investigation, and– understanding the results of the investigation

• Consultants and contractors who are responsible for:– executing a sound site investigation program– using the results to develop a ground model for the project– reporting the results in a manner that facilitates peer review, 

communication with stakeholders, and potential future uses of the information

Project Team

• Geosyntec Consultants– Glenn Rix– Njoroge Wainaina– Bob Bachus– Ali Ebrahimi– Rodolfo Sancio– Brooke Faite– Maria Limas‐Suarez– Laura Leighton (Technical 

Editor)

• Georgia Institute of Technology– Paul Mayne

Organization of the Manual

Topic Updated Manual 1988 Manual

Chapter 1 - Introduction Chapter 1 - Introduction

Planning the Investigation

Chapter 2 – Geotechnical Uncertainty and Risk

Chapter 2 – Subsurface Data Requirements Chapter 5 – Geologic Constraints

Chapter 3 – Subsurface Investigation Processes

Chapter 2 – Subsurface Data Requirements Chapter 3 – Conduct of Investigations Chapter 4 – Field Mapping

Organization of the Manual

Topic Updated Manual 1988 Manual

Executing the Investigation

Chapter 4 – Geophysical Investigations

Chapter 6 – Engineering Geophysics

Chapter 5 – In Situ Testing of Soil and Rock

Appendix B – In Situ Borehole Testing Appendix C – In Situ Testing Procedures

Chapter 6 – Drilling and Sampling of Soil and Rock

Chapter 7 – Subsurface Exploration Appendix A – Drilling, Sampling, and Installation Procedures

Chapter 7 – Hydrogeologic Characterization

Chapter 8 – Hydrogeology Appendix B - In Situ Borehole Testing

Chapter 8 – Laboratory Testing of Soil and Rock

Chapter 9 – Laboratory Testing of Soil and Rock Appendix D – Laboratory Testing Procedures – Soils and Rock

Organization of the Manual

Topic Updated Manual 1988 Manual

Interpreting the Results of the Investigation

Chapter 9 – Interpretation of Soil Properties

Appendix E – Materials Classification

Chapter 10 – Interpretation of Rock Mass Properties

Appendix E.6 – Classification of Rock

Reporting and Presenting the Results of the Investigation

Chapter 11 – Compiling, Reporting, and Presenting Geotechnical Information

Chapter 10 – Compilation and Presentation of Geotechnical Information

Organization of the Manual

Topic Updated Manual 1988 Manual

Supplemental Investigative Information

Appendix A – Geotechnical Instrumentation

Appendix G - Instrumentation

Appendix B – Applications of Geotechnical Instrumentation

Not included

Appendix C – Evaluation of Existing Bridge Foundations for Reuse

Not included

Supplemental Administrative Information

Appendix D – Management of Geotechnical Data

Not included

Appendix E – Quality Assurance Systems

Not included

Appendix F – Health and Safety Not included Appendix G – Contracting Subsurface Investigations

Chapter 7.3 – Contracts and Specifications

Appendix H – Technology Transfer Strategies

Not included

Chapter 2: Geotechnical Uncertainty and Risk

• Uncertainties regarding subsurface soil and rock conditions are a significant contributor to technical and financial risks.

• Broadly, the purpose of a subsurface investigation is to gather sufficient information about soil and rock conditions to aid in identifying geotechnical-related risks and reducing them to tolerable levels.

• Subsurface investigation strategies for reducing geotechnical risks are presented.

Chapter 3: Subsurface Investigation Processes

• Identifying the types of data required to address the anticipated geotechnical risks and performance issues

• Selecting the most appropriate investigation equipment for the anticipated site conditions

• Selecting the appropriate scope and methods for:– geophysical testing– in situ testing– drilling and sampling– evaluating groundwater conditions– laboratory testing

Chapter 4: Geophysical Methods

• Surface geophysical methods– Seismic

• Refraction• Reflection• Surface wave

– Electrical and electromagnetic• Resistivity• Time‐domain and frequency‐domain electromagnetic• Ground penetrating radar

– Potential field• Microgravity• Magnetometry• Self‐potential

Chapter 4: Geophysical Methods

• Borehole geophysical methods– Seismic

• Crosshole• Downhole (e.g., seismic CPT)

– In‐hole logging• Mechanical• Electrical and electromagnetic• Nuclear• Optical and acoustic televiewer• Seismic logging (e.g., P‐S suspension logging)

Chapter 5: In Situ Testing of Soil and Rock

Chapter 5: In Situ Testing of Soil and Rock

• Borehole test methods– Standard penetration test– Vane shear test– Pressuremeter test

• Direct-push test methods– Cone penetration test– Flat plate dilatometer test

• In situ test methods for rock– Plate load test– Flat jack test

Chapter 6: Drilling and Sampling of Soil and Rock

• Field equipment• Methods for advancing boreholes

– Measuring (or monitoring) while drilling

• Soil sampling• Rock coring methods• Logging borings • Boring closure

Chapter 7: Hydrogeologic Characterization

• Groundwater levels and pressures– Existing information sources – Geotechnical borings – Monitoring wells– Piezometers– Geophysical testing

• Aquifer characteristics– Hydraulic conductivity– Porosity– Permeability– Transmissivity– Storage coefficient (confined aquifers) or specific yield (unconfined aquifers)

Chapter 8: Laboratory Testing of Soil and Rock

• Quality assurance– Sample tracking, transportation, storage and handling– Quantitative assessment of sample disturbance

• Index properties• Soil classification• Compaction• Hydraulic conductivity• Consolidation• Shear strength• Dynamic properties• Tests for subgrade soils and unbound bases• Laboratory tests for rock

Chapter 9: Evaluation of Soil Properties

• Subsurface stratigraphy• Soil classification• Unit weight ( )• Preconsolidation stress or

effective yield stress( = OCR∙ )

• Shear strength ( , , )• Lateral stress state ( )• Modulus ( , )• Coefficient of consolidation ( )

Chapter 10: Evaluation of Rock Mass Properties

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ROCK STRUCTURE

INTACT or MASSIVE rock with few widely spaced discontinuities

BLOCKY - well interlocked undisturbed rock mass composed of cubical blocks with 3 sets of intersecting discontinuities

VERY BLOCKY - interlocked partially-disturbed rock mass with multi-faceted blocks having 4+ joint sets

DISTURBED-BLOCKY- SEAMY: folded with angular blocks formed by many intersecting joint sets with bedding planes or schistocity

DISINTEGRATED ROCK: Poorly interlocked and heavily broken with mix of angular & rounded pieces

LAMINATED-SHEARED: lack of blockiness due to close spacing of weak schistocity or shear planes

Geological Strength Index (GSI)

90

80

70

60

50

40

30

20

10

Not applicable

Not applicable

Decreasing Interlo

cking of Rock Pieces

Decreasing Surface Quality of Discontinuities

Rock Mass Strength ‐ Hoek ‐ Brown Model (Hoek 2007) ‐ Example Calculation for Marble

PROBLEM DATA Calculated GSI Parameters GSI = 45 Equivalent RMR= 50 qu (MPa) = 37 mb/mi Reduction = 0.140 mi = 9 s (Rock Mass) = 0.00222 GWT depth(m) = 2 mb (Rock Mass) = 1.262 (kN/m3) = 25 a (exponent) = 0.508 Depth (m) = 5 D (disturbance) = 0

Effective Principal Stresses MIT Stress Space MOHR-COULOMB CRITERION Depth 3 =v 1' uo 3' 1' q p' Ratio Secant Incremental Parameters

z (m) (kPa) (kPa) (kPa) (kPa) (kPa) (kPa) (kPa) q/p' ' c', kPa ' ID (degrees) (degrees)

0 0 1966 0 0 1966 983 983 1.000 90.01 25 2345 0 25 2345 1160 1185 0.979 78.2 253 61.2 A2 50 2677 0 50 2677 1314 1364 0.963 74.4 276 59.3 B3 75 2863 10 65 2863 1399 1464 0.955 72.8 295 58.1 C4 100 3040 20 80 3040 1480 1560 0.948 71.5 309 57.3 D5 125 3209 29 96 3209 1556 1652 0.942 70.4 323 56.6 E

MIT Parameters: q = (1'- 3')/2 Mean = 291 58.5 p' = (1'+ 3')/2 Values (kPa) (deg)

Chapter 11: Compiling and Reporting Geotechnical Information

• Factual information– Preexisting data resources– Remote sensing– Geophysical information– In situ testing– Hydrogeologic information– Laboratory testing– Instrumentation

• Interpretive information– Performance criteria– Ground model– Design recommendations– Construction considerations– Recommendations for 

geotechnical instrumentation and monitoring

– Information for LRFD

Chapter 11: Compiling and Reporting Geotechnical Information

• Geotechnical data reports• Geotechnical baseline reports• Geotechnical design memoranda

Complementary Resources

• AASHTO Load and Resistance Factor Design Bridge Design Specifications (AASHTO, 2017)

• FHWA Geotechnical Engineering Circular No. 5 (FHWA, 2017)

• NHI Courses on Soils and Foundations (FHWA, 2006)• U.S. Army Corps of Engineers Engineer Manual 1110-1-

1804 on Geotechnical Investigations (USACE, 2001)• ASTM guides and standards

Availability

• National Academies Press– https://www.nap.edu/catal

og/25379/manual‐on‐subsurface‐investigations

• Currently under review by AASHTO

Acknowledgements

• American Association of State Highway and Transportation Officials

• National Cooperative Highway Research Program• NCHRP Project Managers

– Mr. David Reynaud– Dr. Waseem Dekelbab

• NCHRP Project Panel