Guidelines for Forensic Investigation of Geotechnical Failures
Dr. V.V.S. Rao, Ph.D, Nagadi Consultants, Delhi
Introduction
Forensic analysis in geotechnical engineering involves scientific and legalistic investigations and deductions to detect the causes as well as the process of distress in a structure, which are attributed to geotechnical origin. Cases of remedied installations where the analysis and evaluation of adopted remedial measures with regard to their effectiveness and economy may be subjected to judicial scrutiny also fall under this purview. The normally adopted standard procedures of testing, analysis, design and construction are not adequate in the forensic analysis in majority of cases. The test parameters and design assumptions will have to be representative of the actual conditions encountered at site. The forensic geotechnical engineer (who is different than the expert witness) should be able to justify the selection of these parameters in a court of law. Hence he has to be not only thorough in his field, but should also be familiar with legal procedures. This paper presents principles of planning and executing a forensic investigation.
Scope
While investigating any distress, the engineer should meticulously follow a well planned program. The scope of work would broadly be under the following heads:
a. Compulsory tasks
- Survey and documentation of the distress
- Scrutiny of all design documents
- Review results of original geotechnical investigations, their analysis
- Study the field reports of construction
- Interview persons involved in planning, design, construction and performance monitoring, etc.
b. Optional tasks
- Perform additional investigations
- Develop and conduct special tests
- Non-destructive testing of structural element
c. Analyze all data and evaluate
- The distress history
- Causes of distress
- Identify the shortcomings in the original investigation and analysis
d. Report
- Authority and scope
- History
- Summary of original documents
- Data collected
- Interviews
- Meteorological information
- Earthquake
- Investigations performed , their methodology and their results
- Analysis
- Conclusions.
Types of Distress
The visual distress which commonly occurs in normal structures and the most probable causes are listed in Table 1:
Table 1: Types of distress
Apart from the above causes, most important causes would be inadequate and/or inappropriate soil investigation, selection of design parameters and use of inappropriate theories.
Diagnostic Tests
After identifying the cause of distress, the following questions arise:
- Has the distress fully occurred and if not, how much more can be expected? Quantify.
- What were the precise causes for distress?
- Whether the soil underwent same stress-deformation history as was anticipated? If not, what was the actual history at site?
- The effect and efficacy of remedial measures on the soil PLUS structure behaviour.
To answer these questions, detailed tests have to be conducted both in the field and in the Laboratory. The choice of tests will normally be from among the following tests depending upon the problem.
A. Field Tests
Figure: Cone Penetration Test
- Borehole investigations including SPTs to a depth deeper than the influence zone
- Cone penetration tests
- Load tests
- Special tests like, pressure meter tests, vane shear tests, seismic or dynamic tests.
Figure: Vane shear test diagram
Figure: In-situ vane shear test in progress.
B. Laboratory Tests
- Triaxial shear tests; to simulate the actual field conditions, these tests should be done on stress increment basis on partially saturated sample. The effect of fluctuation in degree of saturation on the deformation behaviour of the soil should also be investigated. In case of clays, the field stress history is also to be considered
- Repeated cyclic shear tests, in cases like water towers, bridges, etc.
- Large deformation tests, to assess the residual strength and magnitude of final deformations in cases of slopes, etc..
- Compaction tests with different compaction energies
- Permeability tests
In case of residual and wind deposited soils, the effect of change of soil structure due to loads, and both static and moving ones. In all cases it is advisable to conduct regular borehole investigations and evaluate the sub-soil profile.
Analysis
After collecting all data detailed analysis can be done to evaluate the design parameters. Use of empirical relationships should be avoided, unless their validity in the particular site is established. The analysis should be based on:
-
limit conditions
- partial factors of safety
- equilibrium state vis-à-vis flow state
- liquefaction potential
- critical void ratio in compacted fills
With these design parameters the load - deformation history of the soil plus structure combine can be reconstructed. This process will lead to identification of the causes of distress. A suitable and economically viable remedy can then follow.
Legal Issues
In the entire process of investigation, the forensic engineer should be careful to ensure that all the experimental and analytical procedures as well as the selected parameters for tests and analysis fully conform to the field conditions. The report should be comprehensive and intelligible to a legal person also. It is advisable to avoid "hi-tech" terminology and strong verbs like-should, must, etc. As far as possible, it is better to avoid too many details in the main text. At the same time, the report should have sufficient details for the client to give a comprehensive brief to the executing agency.
One should realize that the association of the engineer with the project is based on the principle of-"contract of skill". Hence the consultant should ensure competent and reliable advice to the client. It is also imperative for the consultant to explicitly detail the risks that might be involved or expected in using the conclusions and recommendations.
The consultant should be aware of the importance and implications of the following guidelines (ref: Guidelines for the provision of Geotechnical Information in Construction Contracts- the Institution of Engineers, Australia, and 1987.):
Facts: These have to be true and should not be erroneous.
-
Exploration locations
- Samples and cores available for inspection
- Lithological descriptions of soils and rocks
- Measured water tables
- Test results
Interpretations: The skill of the engineer is judged here
-
Borelogs
- Inferred stratigraphy between boreholes
- Properties of various layers for use in the analysis
- Seismic interpretations yielding velocity and layer depths
Opinion: May or may not be disputable
- Assumptions
- Judgement based on facts and interpretations
Negligence: Obviously, very serious
Overall, it is emphasized that application of standard of skill and care is expected from a professional, irrespective of quantum of remuneration. However, the liability of the consultant is also limited as the owner pays for the "skill and service" and not for "insurance".
REFERENCES
Leonards, Gerald A., "Investigation of failures", Journal of Geotechnical Engineering Division, ASCE, GT 2, Feb. 1982.
Task Committee on Guidelines for Failure Investigation, "Guidelines for Failure Investigation", ASCE,1989.
Green, D.C., "Principles for providing Geotechnical Data in Construction Contracts", Conference on Dams,
Queenstown, Tasmania, 1988, (also in Ancold Bulletin No. 81.)
Robert W Day, "Forensic Geotechnical and Foundation Engineering" Mc Graw Hill, 1998.
Note: Images shown are not part of the original paper.
ABOUT THE AUTHOR
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Dr. V V S Rao, conducted research in the field of soil dynamics initially at Roorkee, under Dr. Jaikrishna and later under Dr. H. Lorenz at Technical University, West Berlin. He was on the faculty of the Indian Institute of Technology, Delhi for a tenure of 4 years where he organised summer schools and conducted various research programmes. In 1971 he started practising as a Professional Consulting Engineer. He has been a Consultant for over 15000 projects all over the country and abroad. He is a member of BIS Committees and has drafted a few standards. In the Nagadi Organisation, he exercises the overall technical control on recommendations of every project executed from all the branches. |
institutions and voluntary organisations in recognition of his pioneering contributions in the field of engineering consultancy. He has published several papers and delivered lectures in workshops and conferences.
Presently
- Chairman (1997-2001) of Technical Committee on Forensic Geotechnical Engineering (TC 40) of the International Society of Soil Mechanics and Geotechnical Engineering.
- Chairman, Technical Committee on Professional Practice of the Indian Geotechnical Society.
- Past Chairman of Technical Committee on Professional Practice (TC 20) constituted by the International Society of Soil Mechanics and Foundation Engineering.
- Past President Association of Consulting Civil Engineers (India).
- Past Chairman Indian Geotechnical Society, Madras Chapter.