1. Critical Levels for Monitoring of Ground
Anchor System for Deep Excavation Project
T. S. Chua, Meinhardt Infrastructure Pte Ltd
S.S. Marican, Land Transport Authority
T.W. Kok, Andrew, Meinhardt Infrastructure Pte Ltd
K. Tani, Taisei Corporation
C424 of KPE
2. Ground anchors
Permanent Temporary
Design life more than 2
years
Design life less than 2
years
FOSstruct = 2 FOSgeo = 3 FOSstrut =1.6 FOSgeo = 2.5
Non-removable:
Anchor tendons left-in
Removable:
Anchor tendons removed after use
2
4. Instrumentation and Monitoring
verification of design and activating contingency measures
• Critical levels = Alert Level (AL) and
Work Suspension level (WSL)
• WSL = Allowable level or design level
• WSL = Structural capacity or geotechnical
capacity; or its weakest links i.e. waler
• Action plans
• AL = 70% of WSL
• Exceed AL, close monitoring
• Exceed WSL, work suspension
AL WSL
70% 100%
4
Close
monitoring
Regular
Monitoring
Monitoring frequency
5. Ground anchors – Instrumentation and Monitoring
Historical practices Current practices
Numerical analysis has been used,
Pre-load less than WL
Numerical analysis routine
Pre-load = 110% WL ( BS8081 ) Pre-load = design pre-load
Instrument readings as feedback Instrument readings as Critical Level
Inconsistency acceptable
In-consistency leads to stop work
order
i.e. DL = WL, WSL = WL
Preload = 110% WL >WSL
5
6. Current issues
• What is the right pre-load?
• Should we follow BS8081:Ground Anchorages?
• How to manage the conflicts with the Code?
• Is design level a critical level?
• Should work stop if the field data hits design level?
• The best way to answer these questions is
to go through a case study, C424 of KPE…
6
10. Sequence of excavation simulated in the analysis
Excavation to 1st level Install 1st strut, excavation to 2nd level
Install 2nd strut, excavation to 3rd level Install 3rd anchor, excavation to 4th level
Install 4th anchor, excavation to 5th level Install 5th anchor, excavation to final level
10
11. Deformation of
retaining wall (Plaxis)
80.0
85.0
90.0
95.0
100.0
105.0
110.0
115.0
-10 10 30 50 70 90 110 130 150 170 190
Horizontal Displacement [mm]
ReducedLevel[m]..
Exc to belowS1
Exc to belowS2
Exc to belowGA3
Exc to belowGA4
Exc to belowGA5
Exc to FL of tunnel
Construct tunnel
backfill to below
GA5 & remove GA5
backfill to below
GA4 & remove GA4
backfill to below
GA3 & remove GA3
backfill to belowS2
& remove S2
backfill to belowS1
& remove S1
backfill to GL
Strut 1, RL112.0m
Strut 2, RL107.5m
Ground Anchor 3,
RL104.0m
Ground Anchor 4,
RL101.0m
Ground Anchor 5,
RL98.5m
Formationlevel
RL95.2m
11
16. Design of ground anchors
Structural check Geotechnical check
N = (WL X Fs) / (UTS x Rd) Lfix = (WL x FG) / (Π x D X fs)
N = required number of strands, 2,4
6... Number of unit anchor = N/2
WL = Working load of anchor, kN
Fs = factor of safety, 1.6
UTS = Ultimate tensile strength, kN
Rd = reduction factor due to bend
Lfix = required fixed length, m
WL = working load of each unit anchor, kN
FG = Safety factor for geotechnical, 2.5
D = diameter of anchor, m
fs = unit skin friction, kN/m2
Rd
16
17. All ground anchors are subjected to acceptance test
Passing criteria:
Apparent free length, Lapp = (AEδ/∆P)
Upper limit Lower limit
Lapp < Ltf +Le +0.5Ltb
or
Lapp< 1.10Ltf + Le
Whichever is larger
Lapp> 0.8Ltf +Le
17
18. Pre-load
load transferred to the anchor head immediately on completion of
stressing operation
How high to Pre-load?
• Sufficient to ensure that the anchorage resistance under
SLS conditions will be mobilised with acceptable head
displacement
• Too low – wall movement may be too large, uneven
distribution of loads
• Too high – not economical as WL will be higher, may hits
WSL
Currently with monitoring based on Critical levels,
works could be suspended
18
19. Pre-load affects behaviour of ground anchor
Type of ground anchor also affects the amount of pre-load
δ=PL/AE
Llong=24m > Lshort = 16m
19
∆δ =constant
∆P= ƒ(1/L)
U-turn system
20. Load distributions if the pre-load is too low
Pre-load to 70%
Ref. Lgth Elong
at
70%
Elong
at
100%
Addn
elong
Inc in
load
Max
load
FOS
m mm mm mm kN kN
1 24 75 107 32 44 165 1.8
2 22 69 98 29 48 169 1.7
3 20 63 89 27 52 174 1.7
4 18 56 80 24 58 180 1.6
5 16 50 71 21 65 187 1.6
Pre-load to 50%
Ref Lgth Elong
at
70%
Elong
at
100%
Addn
elong
Inc in
load
Max load FOS
m mm mm mm kN kN
1 24 54 107 55 73 160 1.9
2 22 49 98 49 79 166 1.8
3 20 45 89 45 87 174 1.7
4 18 40 80 40 97 184 1.6
5 16 36 71 36 109 196 1.5
20
21. What if the pre-load is too high?
• Design engineer assumed a certain pre-load
(70% WL), but …
• Contractor installed based on
recommendation by BS8081, (110% WL)?
• i.e. Actual pre-load higher than design pre-
load
21
24. How to overcome these problems?
• Pre-load to design pre-load
• Avoid strict interpretation of BS8081:1989
• Adopt most adverse combinations
– Max. and min. pre-load should be selected
when analyzing the temporary earth retaining
system (TERS)
• Select appropriate design parameters
– Use worst credible parameters; or moderately
conservative parameters with appropriate
safety margin
– If use most probable parameters, ensure that
there are spare capacity i.e. WSL >DL
24
Ciria C580
34. Conclusions
• ‘Critical Level’ is effective and efficient
• Pre-load affects the behaviour of ground anchor
• U-turn ground anchors
• Affected by pre-load
• Important to check internal structural capacity
• BS8081:1989, Ground Anchorages
• Strict interpretation on it’s recommendation could results in
conflict i.e. ground anchor load will breach Critical Level
• Drafted in 1989, the Code need to be re-look
• Pre-load
• Removable ground anchor system
• Case study: C424 of KPE
• Holistic approach, using conservative design parameters and
close monitoring using Critical Levels, is the key to ensure
adequacy and safety of TERS.
34
