CIVL 411 Shotcrete Presentation 2017 (for distribution).pdf

Presentation

1. TEMPORARY
EXCAVATION SUPPORT SHOTCRETE USE IN LOCAL PRACTICE Presented to CIVL 411
March 6, 2017
Steven Fofonoff, M.Eng., P.Eng.

2. Outline •
Review of shoring and shoring types
•
Introduction to anchored shotcreteshoring
•
Design considerations
•
Example simple design calculation
•
Construction sequence
•
Case Studies

3. Shoring •
What is Shoring?
•
the process of supporting a building, vessel, structure, or tre nch
with shores (props) when in danger of collapse or during repair s or
alterations (from Wikipedia)
•
Shoring types
•
Anchored Retaining Walls
•
Reinforced shotcrete
•
Secant pile, jet-grout, diaphragm walls
•
Sheet pile walls
•
Soldier pile with lagging (Timber or Shotcrete)
•
Cantilevered Retaining Walls
•
Pipe pile supported walls
•
Sheet piles walls
•
Soldier pile walls with lagging

4. Anchored Shotcrete Shoring •
What is Anchored Shotcrete?
•
An anchored reinforced concrete membrane (diaphragm) tied back
into the earth

5. Anchored Shotcrete Shoring

6. Anchored Shotcrete Shoring: A Brief Introduction •
Originally use was for tunnel support
•
In 1960’s, adopted for use in excavations by E. Mason
•
By 1970’s used in over 30 excavations and patented in
US and Canada
•
After seeing the successful use by Mason, designers and
contractors started to emulate the practice (mid 1970’s)
•
Since then methods modified by designers to meet project
needs and based on experience
•
From Shoring Practices in Greater Vancouver, British Columbia, Naesgaard, MacLeod, and Inglis, In proceedings of 48th Canadian
Geotechnical Conference, Vancouver, B.C., September 1995

7. Anchored Shotcrete •
Why do we use anchored shotcrete?
•
Proven to work
•
Most common method in Greater Vancouver
•
Contractors are familiar with the method
•
Economical
•
Lower material cost, relatively fast
•
Flexible
•
Highly adaptable to geometry
•
Can be used in tight spaces
•
Allows for construction at property line
•
With permission to encroach on neighbours property

8. Design of Anchored Shotcrete Shoring •
Design of anchor length, type, and spacing to support the
excavation
•
Requires making an estimate of earth pressure how it changes as
the excavation proceeds

9. Design Considerations •
Geology!!
•
Soil Type (frictional/cohesive)
•
Geometry (Height)
•
Site Constraints
•
Property lines – encroachment
•
Neigbouring buildings, services, and utilities
•
Surcharge Loads (Concrete Trucks, Cranes, Etc.)
•
Anticipated Impacts (On-Site and Off-Site)

10. External Loads Design Life?
Other Design Considerations

11. Well points installed to temporarily
dewater shoring face
Development of sand boils inside
excavationOther Construction Considerations: Dewatering

12. A large void developed over the weekend from the piping
of fines behind the shotcrete wall
Pre-Glacial Sands (Void)

13. Potential Failure Modes to Consider •
Internal Failure –Anchor Failure
•
Steel tendon failure (load exceeds tensile strength / damage)
•
Grout to ground bond failure
•
Grout to tendon failure
•
External Stability
•
Global Stability Failure (circular, wedge, compound)
•
Bearing Capacity
•
Base Heave

14. Design Earth Pressure •
Depends on…
•
Soil Type
•
Friction Angle
•
Cohesion
•
Groundwater Level / Dewatering
•
Excavation Sequence

15. Estimating Lateral Earth Pressure for Design
(Peck, 1969)

16. Apparent Earth Pressure Diagrams
SandSoft to Medium ClayStiff Clay
2
=
L 0.65?*-
a
2
=
L 0.2?* PK0.4?*
2
=
L?*
1F
4?
?*
(Peck, 1969)

17. Example Basic Design Calculation •
Assume an excavation in dense partially cemented sand
•
Excavation is 30 feet deep
•
Anchor spacing is 6 feet horizontally and vertically
•
This is typical for dense sand or glacial till site in Vancouve r

18. Design Calculation –determining anchor design
loads •
Assume an Apparent Uniform Pressure Distribution(Pa)
Pa = 0.65KaγH; Ka=(1-sinΦ)/(1+sinΦ)
Pa = 0.65(0.27)125lbs/ft^3 x H (height)
Pa = 22*H
•
Load Calculation
•
assume 30’ depth, with spacing at 6’ horizontally and verticall y
F = Pressure x Tributary Area
F = 22(30’) x 30’(6’)
F = 118,800 lbs ≈ 120 kips
•
Determine Anchor Design Load
Anchor Design Load = Total Load / # of Anchor Rows
(Number of anchor rows would be 4 in this case_
Anchor Design Load = 120 kips / 4 Rows = 30 kips Horizontal
Anchor Design Load (installed at 15°) = 30 kips / cos (15°) = 31 kips

19. Anchor Design •
Choosing an Anchor Bar Type –considerations
•
Application (Permanent or Temporary)
•
Soil Type (Competent, Collapsing)
•
Groundwater Level
•
Capacity - Load / Testing Requirements
•
Design (<0.6Pu Permanent, <0.7Pu Temporary)
•
Test Load (<0.8Pu)
•
Required Anchor Capacity
•
31 Kips Design (from previous calculation)
•
Test Load = 1.3xDL = 40 kips
•
Test Load < 0.8Pu
•
So 40 kips x 1.25 = 50 kips
•
Refer to table from anchor supplier

21. Anchor Design
•
Choosing an Anchor Length
•
Calculate Required Bond Length
•
Based on friction along area of drill hole
•
Factors; normal stress, adhesion, mobilized friction
•
Drilling Techniques
•
Must be beyond active failure plan
•
Calculate Required Free Length (within active wedge)
•
Assuming level ground behind wall
measured from bottom of excavation
•
Generally works out to be 1H:2V

22. Design Example -Typical Shoring Section

23. Building an Anchored Shotcrete Wall

24. Typical Construction Sequence –First Lift

25. Soil berm removed; preparing for
shotcrete application
Temporary soil berm in place to
ensure stability
First Lift

26. Subsequent Lifts

27. Drilling anchors with an air track drill
for the next row; berm in place
Underpinning of a building
Drilling / Underpinning

28. Large “floaters” being prepared for
blasting
Continuous trimming in competent
sandstone

29. Bottom of the excavation Construction of the underground
parkade

30. Field Review •
Confirm inferred ground conditions
•
Review contractor sequence / methods
•
Review reinforcement placement and shotcrete
application
•
Anchor Test Review

31. Anchor Testing -Basic •
Confirm design load is achieved
•
Typical test of grout to ground bond is 1.33 to 1.5 times desig n load
for temporary applications
•
Confirm free length is achieved
•
Some elastic elongation should be observed
•
Test load is typically held for 2 minutes for proof test

32. Anchor Testing –PTI (1996) •
Proof Test Procedure
•
Anchor is incrementally loaded to Test load, held, and then ret urned to
the alignment load.
•
Creep – anchor held at test load for 10 min.
•
Acceptance Criteria
•
Movement between 1 and 10 min should be less than 1 mm
•
If criteria exceed test should be held to 60 min and log cycle between 6 min
and 60 min should be less than 1 mm
•
Free Length – elastic elongation
•
Theoretical Elongation can be determined from PL/EA
•
Acceptance Criteria
•
Minimum 80% design free length
•
Maximum 100% design free length plus 50%

33. Load Displacement Plot –Insufficient Free Length

34. Measured Elongation vs. Theoretical –Insufficient Free Length

35. Anchor Creep vs. Time (Acceptable)

36. Anchor Creep vs. Time (Failure)

37. Special case: what if you can’t encroach? •
Most often encroachment rights are required to place
anchors on adjacent private or city property.
•
It is not always possible to achieve this.
•
Alternative designs could include:
•
Internally Braced Excavations
•
Struts
•
Rakers
•
Corner Bracing

38. RakerSystem
1. Vertical shoring elements installed
2. Internal shotcrete waler
3. Bulk excavation
1. Rakers welded to wall and placed
in footings
2. Footings constructed
3. Hold down anchors installed

39. Building footing and drain pipes
installed
Soil berm left in pl ace until hold down
anchors are installed

42. Case History -UV Disinfection Plant –
Coquitlam Watershed

43. UV Plant -Shoring Considerations •
Large, temporary excavation approximately 18 m (~60
feet) deep
•
Permanent excavation was to be up to 9 m in height;
permanent retaining wall required
•
Considerations
•
Sloping ground behind
•
Water Table / Saturated Sands
•
Dewatering
•
Monitoring
•
Seismic design for permanent wall
•
Corrosion
•
Wall finish

44. UV Plant –Typical Shoring Section

45. UV Plant –Global Stability Check

46. Continuous Flight Auger Method Typical Secant Pile Cut-off Wall Layout

47. Secant Piles Exposed Beneath
Temporary Shotcrete
Drilling Anchors Through Secant Piles

48. Instrumentation –Vibrating Wire Piezometers

49. UV Plant –NW Wall Monitoring

50. UV Plant –Permanent Shoring Complete

51. Permanent Architectural Concrete in
Place
Reinforcing Being Placed for
Permanent Retaining Wall

52. UV Plant –Permanent Shoring Complete

53. Case History

54. Shoring Considerations •
Large, deep excavation –approximately 18 m
•
Adjacent to Canada Line Piers and Cambieand Marine
Station
•
Low tolerance for wall movement
•
Extensive monitoring required
•
Translink required minimal impact to facilities during construct ion
•
We demonstrated minimal settlement and displacement would occur,
using finite element modelling

55. Marine Gateway Shoring –Excavation Underway

56. Marine Gateway Shoring –Partially Shored

57. Marine Gateway Shoring –View From Excavation

58. Marine Gateway Shoring –Numerical Modelling -Settlement

59. Marine Gateway Shoring –Numerical Modelling –Horizontal Displac ement

60. Marine Gateway Shoring –Final Depth Achieved

61. Marine Gateway Shoring Inclinometer Results

62. Overview •
Anchored shotcrete shoring is the most common method of
supporting excavations in the Greater Vancouver area
•
Adaptable to project demands and soil conditions
•
Design assumptions (soil conditions) must be verified by
observation and testing during construction
•
Monitoring provides ongoing feedback of overall excavation /
shoring performance – early detection of unexpected behavior
allows design to be modified during construction if necessary
•
Thanks for listening – any questions?