Exploring the limits of the Observational Method 1. Background – key requirements for OM
2. New Wembley Stadium – raising the arch (pile group behaviour)
3. Limehouse Basin
Exploring the limits of the Observational Method
– a step too far? (retaining wall behaviour)
4. Earthworks asset management – the weakest link? (degradation of clay fill embankments)
Tony O'Brien BGA/CFMS Seminar, Paris - December 2011
Key factors for Observational Method implementation
Key requirements for OM Peck (1969) outlined 8 requirements !
Adverse factor
– Exploration (or Ground Investigation) – Assessment of variations in conditions - most probable and most unfavourable [now - use of "progressive modification"] – Design basis – Key observations and predictions (most probable)
Adequate warning when approaching a ULS?
Progressive collapse
Failure of one component, leads to rapid failure of overall system
Lack of stakeholder support
All parties in project need to be actively involved and supportive
Unable to obtain critical observations reliably
Control of works dependant on obtaining pertinent data and acting on it
Implementation of contingency measures is too slow
The contingency plans need to be fully developed and able to be implemented within the available timescale
Contract conditions
Appropriate?
– Key observations and predictions (most unfavourable) – Design modifications for every foreseeable scenario – Make observations and evaluate actual conditions
Comment
Brittle failure
– Modify design based on observations
Page 1
New Wembley Stadium - raising the iconic arch
Demolition and site preparation
Retaining walls, shear cores and arch fabrication
Raising the arch
! arch: 133m high, 315m span ! longest single span roof structure in the world
Page 2
! key concern: arch buckling " pile group deformation critical
! risk management " use of OM (OM rare for pile groups)
Raising the arch Concerns !
!
Lifting mechanism – side elevation Risk management - use of OM
if any pile group was to move excessively, due to its slenderness, the arch may buckle no case histories exist for pile groups of this size subjected to complex load combinations vertical, horizontal moment and torsion
!
complex for piled foundations
!
use of non-linear boundary element analysis
Turning Struts
Arch at Original Position
Turning Struts .
Jacking Points
Jacking Points
Arch Stillage Base
– allowable movement dependent on load combination !
– pile group displacement
instrumentation and observation of pile groups
!
consideration of failure/deformation mechanisms
!
contingencies
Jacking Points
– kentledge – tie backs to shear cores
Piled Foundations
Original Position of Arch
180000 Vertical
! many load cases – 13 different angles – 9 per angle
x (+ve east)
! lack of case history data Turning Struts
– lateral/moment loading
Jacking Bases
Horizontal
160000
Moment
Moment / Force (kNm / kN)
Western Arch Base
+ve x rotation
y (+ve north)
Restraining Lines
Restraining Lines
Challenges example of complex loads, eastern arch base
Lifting mechanism – plan view Eastern Arch Base
Arch in Final Position
Arch in Vertical Position
– structural forces
!
Torsion
140000 120000 100000 80000 60000 40000 20000 0 0
Pile group configurations
10
20
30
40
50
60
70
Arch Angle (degrees)
! temporary base pile groups vary from 6 to 12, 1.5m dia piles ! pile length varied from 10 to 42m
Page 3
80
90
100
110
120
Shear modulus vs. Depth
Site geology and topography Old Concourse
Seismic cone and self-boring pressuremeter Old Concourse
Old Wembley Pitch Area
South Way
Shear Modulus at Small Strain (MN/m2)
Chiltern Marylebone Railway Line
0
BH201A/201B DH202 60
BH205
WS7
BHC1
BHB2
BH6
BH204
BHB3 BH203
Depth below Top of London Clay (m)
BH3
Made Ground IVa IIIb IIIa
Reduced Level (m AOD)
50
IIIb
IVb IIIc IIIa IIa
40
IIIb IIIa IIb IIa
IIIa IIa/b
30
Ib
A
Ib
dw
w ater flo
B B
IIIa
Ia Gro un
IIb
20
IIIb
Unweathered London Clay
Ib Ia
A
B
B
A
A
Unit B Unit A
Ib
IIa
Weathered London Clay
IIIb IIIa
10
Lambeth Formation 0
Upnor Sands
! site history
! deep weathering of London Clay
– influence of old stadium
– 8 to 10m
! topography
! hence, may be differences in London Clay behaviour across site and c.f. central London
– on top of hill, adjacent rail cutting
Analysis Type
Lateral pile test results
Vertical Horiz. Stiffness Stiffness (MN/m2) (MN/m2)
100
200
300
400
0 5
Weathered London Clay
SBP
10
SCPT
Unweathered London Clay
15
Unit B
20 25
Unit A
30 35
Lambeth Group
40
Lateral test pile results
232
171
! initial tangent stiffness, assumed anisotropic
400
– vertical - seismic cone
300
– horizontal - SBP
200
Back analysis using linear elastic model
100
500
0
Stiffness at FOS = 1.7
0
20
40
60
80
Lateral Movem ent (mm )
118
23
Eh drops by 19x during loading ! #Eh >> #Ev
Stiffness at Failure
60
9
Horizontal Load (kN)
Initial Linear Stiffness
Horizontal Load (kN)
500
Test Data
400 300 200
Non-linear 100
Single pile testing 0
Non-linear analysis essential for large lateral loads
0
20
40
L ateral M o vem en t (m m )
Page 4
60
80
The use of the observational method Identifying threshold limits
application to piled foundations
Load combination A
1. 121 Load combinations for each pile group
Load combination B
! Displacements
Movement
Red Limit
! Structural forces (BM/SF/Axial) in piles [CRITICAL]
2. Red Limit
Amber Limit
a) Full load combination (Horizontal force/overturning moment/etc) x Factor ! Failure of a pile in group b) (Dominant load only) x Factor ! Failure of a pile in group
Angle of Arch
Allowable movement dependent on load combination !
Worst of a) and b) " Red Limit (two-thirds of ultimate structural capacity)
Identifying threshold limits
Predicted x Rotation for eastern arch base 25
3. Amber Limit
Red Limit
20
x rotation (mdegs)
4 Criteria ! a) Predicted pile group deformation - most likely load combinations + plausible ground stiffness variations (± 25%) b) Sufficient "distance" between Amber + Red to facilitate timely implementation of contingencies c) Sufficiently beyond "expected" deformation to avoid regular breaches of Amber limit
Amber Limit
15 10 5 0 0 -5
d) Deformation monitoring accuracy
20
40
60
80
Repute Non-Linear
Repute Linear Run 1
Repute Linear Run 2
Repute Linear Run 3
-10 Angle of Arch (degrees)
Non-linear analysis - essential
Page 5
100
120
Monitoring strategy
Raising the arch Jacking initially applied 14MN at each JP
! Vary load at each jack to maintain arch alignment
! primary system – precise levelling (± 0.1mm) and surveying (± 1.5mm)
! torsion to pile group
– had to measure small movements accurately – Survey 3 months before arch raising – Initial 6 weeks survey accuracy improved, ± 5mm ! ± 1.5mm
! secondary system – electrolevel beams, selected pile groups
A critical risk "brittle" failure of corner pile
Risk mitigation
Movement
(combined tension and shear) due to torsional loading
Unacceptable and unsafe, "brittle" ; no time for contingency
OM contingency measures (pre-designed)
Unacceptable but safe trend, "ductile" ; use contingency
! apply large "kentledge", put corner pile into compression ! ductile mechanism ! other contingencies D u c t ile S tru c tu ra l f a ilu r e
– install and pre-stress tie backs F o rc e
– modify jacking forces
Angle of Arch
! if adverse trend
! brittle mechanism
– pile specific capacity checks for current (rather than critical) load combination
– occur at small displacement – no warning – unacceptable
Page 6
B rittle S tru c tu ra l fa ilu re
Construction of the arch
Pin at eastern arch base
Monitoring results
– initial elastic stiffness: SBP and Seismic Cone – calibration vs single pile tests – scale up to 1.5m dia and model Pile Group
!
monotonic loading – good prediction
!
load reversal – underpredict (but anticipated!)
!
monitoring data overload? – 72 graphs per arch lift increment
Page 7
East Arch Base
predictions: non-linear (hyperbolic) analyses
14
Horizontal y Movement (mm)
!
12
Surveyed Non-Linear
10 8 6 4 2 0 -2
0
20
40
60
80
-4
Angle of Arch (degrees)
100
120
Predicted and observed horizontal movement in the x direction of eastern arch base
Predicted and observed z rotation of eastern arch base
40 30
7
20 6
z rotation (mdegs)
Horizontal x Movement (mm)
8
5 4 3 2 Surveyed Non-Linear Repute
1
10 0 -10 0
20
40
40
60
80
100
100
120
-30 -40 Surveyed Non-Linear Repute
-60 20
80
-50
0 0
60
-20
-70
120
Angle of Arch (degrees)
Angle of Arch (degrees)
Arch roll up phase 3 - OM successfully implemented
Limehouse Basin ! old project (early 1990's, but important lessons on "limits" of OM
Page 8
Limehouse Basin
Limehouse Basin
! OM introduced - eliminate the need for mid-height props?
! load on cofferdam – mainly groundwater pressure (control of water levels in fill?)
! failure mode – excessive bending of sheet pile wall
! critical measurements – complex due to stiffness contrast between N. wall ("stiff" steel tubes) and S. wall ("soft" sheet piles) – (absolute wall movement not wall convergence)
Granular fill London Clay Woolwich and Reading Beds
Progressive modification ! soft prop trial - gap at "safe" wall displacement limit ! risks associated with OM ! too high Top prop Sheet pile wall
! OM did facilitate project benefits – Phase 2 - construction sequence changed
Soft prop trial
Original
Gap closed in 7 to 10 days. Inadequate time for base slab construction
After OM
Exc. to mid-height prop
Excavate to base slab level
Install mid-height prop
Install mid-height prop
Exc. to base slab level – Phase 3 - reduction of sheet pile wall embedment, from 4m to 0.5m reduced sheet pile damage, hard driving
Lambeth Group
reduced risk of declutching of sheet piles
Page 9
Earthworks asset management Potential for long term (decades) application of OM?
! most of UK rail network built >100 years ago ! embankments - end tipping of clay fill ! increasing problems of delayed failure and excessive track deformation
Deep Seated Delayed Failure of Railway Embankment (6m high Grass Covered Slope)
Track Deformation. Seasonal Movement of Railway Embankment (Mature Tree Covered Slope)
‘Fatigue’ failure of high PI clay fills
Field observations indicate embankment deformation critically influenced by
Consequence of seasonal changes in pore water pressure Toe is critical area
Discontinuous shear surface
! Climate ! Vegetation
Development of shear surface due to shrink swell progressive failure
– eg. High water demand trees on slope or grass covered slope
P e a k
P o s t
r u p t u r e R e s i d u a l
High plasticity clay, post-peak strain softening NB Mechanism observed in both numerical and centrifuge modelling and in field Development of Reidel Shear Zones
Page 10
Long term Observational Method applications - Challenges
Can OM lead to more cost-effective stabilisation? Complexity of ground movements
Normal seasonal movements vs. Movements prior to failure. Can we differentiate?
! site access often difficult ! local environmental constraints
Speed of contingency measure implementation vs. speed of failure (days/weeks)
! duration and cost of monitoring (decades) ! organisational/human challenges ! communication and control over long term
Pound Green Tree removal during instrument installation
Crest Extensometer
! potential to save money vs. cost of OM
Numerical modelling shown wide range of movements may be acceptable, depending on local conditions
Exploring the limits of the Observational Method
Exploring the limits of the Observational Method
Conclusions
Conclusions
1. OM for pile groups - raising the Wembley Arch
2. Limehouse Basin - a step too far
! !
– pile groups, intrinsically stiff structures
– OM benefits outweighed by risks
– monitoring system, reliable measurement of small movements
– BUT
– threshold limits (amber/red), depend on load combination
– introduction of OM created opportunities:-
– simple contingency, rapidly implemented
– improved construction sequence – reduced wall embedment
challenging application! !
OM successful for managing risk during a unique task
Page 11
hence, cost and time savings still achieved!
Exploring the limits of the Observational Method
Overall
Conclusions 3. Earthworks asset management - the weakest link? Long term application of OM
Conclusions Reliability of key measurements
Mode/magnitude of deformation
– delayed failure of clay embankments
Speed of response
Type of failure mechanism
– potential need for very long term application (decades)
"Team" communication
Team organisation/interfaces
– prime challenge is human rather than technical ie. Ability of any organisation to apply OM over many years
(OM successful for cutting stabilisation, short term during construction)
Page 12
Human vs Technical Issues
