The document discusses managing geotechnical risk on major infrastructure projects. It emphasizes that geotechnical engineering involves uncertainty due to variable ground conditions and limitations of site investigations. It advocates using the observational method, which involves ongoing monitoring during construction and allowing design modifications based on field observations. The greater the involvement of the geotechnical engineer in all project phases, through tools like instrumentation and full-time site presence, the less conservative the initial design needs to be and the better geotechnical risks can be managed to avoid failures. The Heathrow tunnel collapse case study highlights issues that can arise without adequate geotechnical input and oversight during construction.

1. The management of geotechnical
risk on major infrastructure projects
Chris Bridges
23 October 2014
AGS Queensland Symposium 2014

2. Geotechnical Risk
Key messages:
• Geotechnical engineering = uncertainty
• Geotechnical uncertainty can be managed through
ongoing input through construction
• The greater the involvement of the geotechnical
engineer the less conservative the design needs to
be
23 October 2014

3. 01
Geotechnical Risks

4. Geotechnical Risks
• Geological – e.g. cavities, soft soils, groundwater
• Manmade – e.g. fills, mine workings
• Engineered - e.g. walls, slopes, foundations, tunnels
• Inherent – e.g. people and processes
23 October 2014

5. The challenge of Geotechnical Engineering
Non-geotechnical engineers believe that you can:
• drill a hole into the ground;
• send the soil samples obtained from the hole
through a laboratory with standard apparatus;
• collect the figures;
• introduce them into equations; and
• compute the result.
Terzaghi, 1936
23 October 2014

6. The challenge of Geotechnical Engineering
But:
• Ground conditions are highly variable.
• Its impossible to obtain a complete picture of the ground
conditions.
• Predictions of ground behaviour made during design are
approximations at best.
• It is easy for inexperienced designers using routine
design procedures to miss critical failure mechanisms.
• We need to explain this uncertainty to our Clients!
23 October 2014

7. Geotechnical Investigations
23 October 2014

8. Geotechnical Investigations
23 October 2014
U50 Tube
50mm (ID) x 500mm – 0.00098m3

9. Geotechnical Investigations
23 October 2014

10. 23 October 2014

11. Geotechnical Investigations
23 October 2014

12. 23 October 2014
INB drilling accounted for
only about 10m3 of
soil/rock from 70,000m3
excavated (0.015%)
Geotechnical Investigations

13. 23 October 2014
Φ’=30°
Φ’=39°
Φ’=18°
Parameter Selection

14. 23 October 2014
Example – slope stability FOS
Φ (°) FOS
18 1.2
30 1.75
38 1.9
Vary from Φ = 18° to 38°
Parameter Selection

15. Design Method
23 October 2014
0 2 4 6 8 10 12
Bad (<50)
Poor (50-75)
Fair (75-85)
Good (85-95)
Excellent (95-105)
Good (105-115)
Fair (115-125)
Poor (125-150)
Bad (>150)
Number of Predictions
AccuracyofPrediction(%)
Driven Steel Pile
Total Capacity (kN)
Base Capacity (kN)
Shaft Capacity (kN)
Under estimate
Over estimate
Shaft - 88% Poor / Bad
Base - 63% Poor / Bad
Total - 88% Poor / Bad

16. Design Method
23 October 2014
0 2 4 6 8 10
Bad (<50)
Poor (50-75)
Fair (75-85)
Good (85-95)
Excellent (95-105)
Good (105-115)
Fair (115-125)
Poor (125-150)
Bad (>150)
Number of Predictions
AccuracyofPrediction(%)
Jet Grouted Pile
Total Pile Capacity (kN)
Base Capacity (kN)
Shaft Capacity (kN)
Over estimate
Shaft - 69% Poor / Bad
Base - 81% Poor / Bad
Total - 81% Poor / Bad
Under estimate

17. Design Method
23 October 2014
0 2 4 6 8 10 12 14
Bad (<50)
Poor (50-75)
Fair (75-85)
Good (85-95)
Excellent (95-105)
Good (105-115)
Fair (115-125)
Poor (125-150)
Bad (>150)
Number of Predictions
AccuracyofPrediction(%)
Spread Footing on Sand
Bearing Capacity
Settlement
Under estimate
Bearing - 66% Poor / Bad
Settlement - 90% Poor / Bad
Over estimate

18. Design Method
23 October 2014
0 2 4 6 8 10 12 14 16
Bad (<50)
Poor (50-75)
Fair (75-85)
Good (85-95)
Excellent (95-105)
Good (105-115)
Fair (115-125)
Poor (125-150)
Bad (>150)
Number of Predictions
AccuracyofPrediction(%)
Embankment Collapse Height
Muar Embankment
Prediction Competition
MIT Embankment
Prediction Competition
Non-conservative
Conservative
60% Poor / Bad –
majority on the
conservative side of
prediction

19. Design Method
• Sieffert & Bay-Gress (2000) Comparison of European bearing capacity calculation methods for
shallow foundations Proc. Instn Civ. Engrs Geotech. Engng, 143, 65-74
23 October 2014
734kN
1297kN

20. 23 October 2014
380
38
50
480
36°

21. Morgenstern (2000)
• It is rare for the geotechnical engineer to rely on
quantitative prediction to meet his objectives.
• Risk must be managed to overcome the limitations
of site characterization, knowledge of material
properties, other unknowns and the vagaries of
construction practice.
• It is essential that the geotechnical engineer
maintain an ongoing awareness of factors that
contribute to unsuccessful performance and
introduce this awareness into comprehensive risk
management tools.
23 October 2014

22. 02
Managing the
Geotechnical Risks

23. Managing Construction Risks
The design must consider:
• the contractors capability
• the designers site presence
23 October 2014

24. The Observational Method
The Observational Method in geotechnical engineering is a
process which enables the designer to:
• continuously re-evaluate design assumptions and
predictions; and
• modify “the design” during construction based on
observations and/or take remedial actions where
required.
23 October 2014

25. Observational Method - Summary
23 October 2014

26. Ground Model – Regional Geology
23 October 2014
Investigate the existing ground conditions through on site
ground investigation, laboratory testing, site reconnaissance
and desk study

27. 23 October 2014
Ground Model - Focused Geotechnical
Investigations

28. Design
Design - best approximation using available data
• predict movements / forces / porewater pressure
• establish “trigger levels”
e.g. Trigger level – 70% of design
Design level – 100% of design
Allowable level – permitted maximum movement
(limit to 120% of design)
23 October 2014

29. Construction Control
Set up instrumentation/monitoring plan (plus install
and monitor instrumentation):
• design and install instrumentation to capture
movements at critical locations;
• establish frequency of monitoring; and
• have a plan should the monitoring indicate you
have greater movement than predicted.
Clear lines of responsibility & decision making
23 October 2014

30. 23 October 2014

31. 23 October 2014

32. Review
• Observe ground condition encountered and ground
reaction (full time site presence)
• Obtain and back-analyse monitoring data in order to refine
the geotechnical design parameters by “matching” the
theoretical behaviour with the measured performance –
predict future performance
• Continuously review monitoring results to assess
predictions and modify construction if required and take
remedial action if necessary
23 October 2014

33. Geotechnical Risk Management
Inner Northern Busway
• Alliance
• Permanent & temporary works design
• Embedded in the Design and Construction Teams
• Determined required resources
• Geotechnical Designer had an equal seat at the table
• Full-time presence on site including senior professionals
available locally
• Geotechnical Team controlled instrumentation, monitoring &
specified software
23 October 2014

34. 23 October 2014

35. 23 October 2014
CITY HALL
KING GEORGE SQUARE

36. 23 October 2014

37. 23 October 2014

38. Heathrow Tunnel Collapse
23 October 2014
NATM Tunnel
Construction
collapse -
20/21 October 1994

39. Heathrow Tunnel Collapse
The role of geotechnical sub-consultant (temporary tunnel
support):
• Geotechnical sub-consultant put forward 3 staff – only 1
accepted (24/7 Operation)
• Provided with 3 juniors by the contractor who were not
NATM experienced
• Did not certify construction of the works they designed
• Inadequate computer software system for processing
the monitoring data
23 October 2014

40. Heathrow Tunnel Collapse
23 October 2014
Concourse tunnel to be
constructed followed by
up-line platform and
then down-line platform

41. Heathrow Tunnel Collapse
23 October 2014
Settlement at Camborne
House exceeded predicted
settlement (9mm) after only
concourse tunnel completed
by about 20mm.

42. 23 October 2014

43. 23 October 2014

44. 23 October 2014

45. Heathrow Tunnel Collapse - conclusions
• lack of on-site NATM authority at Heathrow
• lack of experience among the field engineers, the tunnelling
foremen and the crews – design did not take this into
account – poor workmanship an issue
• poor quality of the monitoring instrumentation data
• lack of full time geologists within an NATM supervision team
• Contractors geotechnical sub-contractor not kept in the loop
• limited instrumentation data available
• serious omissions in the installed instrumentation regime
and,
• "inadequate" computer software system for processing the
instrumentation data but,
• there was still enough data available to see what was
happening in the two weeks leading to the collapse.
23 October 2014

46. Heathrow Tunnel Collapse
• Contractor fined £1.2M + £100k costs (contract value £60M)
• Geotechnical consultant fined £500k + £100k costs (contract
value £1M) – did not pay!
• Recovery took nearly two years and cost around £150M -
nearly three times the cost of the original contract
23 October 2014

47. Geotechnical Risk Management
Inner Northern Busway
• Alliance
• Permanent & temporary
works design
• Involved in tender
• Embedded in the Design and
Construction Teams
• Determined required
resources
• Geotechnical Designer had
an equal seat at the table
Heathrow Tunnel
• D & C
• Temporary works design
• Early involvement with
Contractor
• Embedded in Construction
Team
• Geotechnical Designers
resources rejected
23 October 2014

48. Geotechnical Risk Managment
Inner Northern Busway
• Full-time presence on site
including senior professionals
available locally
• Geotechnical Team controlled
instrumentation, monitoring &
specified software
Heathrow Tunnel
• Part-time presence on site –
junior non-specialist support
• Contractor controlled
monitoring
• Software not up to task
• Contractor inexperienced
23 October 2014

49. Conclusions
Geotechnical prediction can be difficult due to uncertainties
in the ground model, parameter selection and design
methods adopted.
Therefore, for larger infrastructure projects
• Geotechnical risk has to be managed during construction
through the use of the Observational Method, and
• The Geotechnical designer must have an independent
(and supported) voice in the construction team
23 October 2014