Australian Geomechanics
Society Mini-symposium
Sydney October 2005
Soil Loads on Cut and Cover
Tunnels Under High Fills
Doug Jenkins
Interactive Design Services
Overview:
Influence of high fills on structural
actions
 Design of foundation structures for high
vertical loads
 Longitudinal differential settlement
 Longitudinal loads
 Examples
 Conclusions

Influence of high fills on structural actions
 Axial loads
 High fill = high axial load
 Effect of foundation stiffness on axial load
Influence of high fills on structural actions

Bending moments and shear forces
Effect of foundation stiffness on axial load
 Lower bound stiffness may not be conservative

Influence of high fills on structural actions

Bending moments and shear forces
Effect of arch profile on arch actions
 Morwell Tunnels - alternative cross sections

Influence of high fills on structural actions

Bending moments
Morwell Tunnels
Compa rison of Arch S ha pe s
Be nding Mome nts
100
Bending Moment; kNm
0
-100
-200
-300
Actual
Circular
Elliptical
-400
-500
-600
-700
-800
-8
-6
-4
-2
0
X-Axis
2
4
6
8
Influence of high fills on structural actions

Shear Forces
Morwell Tunnels
Compa rison of Arch S ha pe s
S he a r Force s
Shear Force; kNm
1000
500
Actual
Circular
Elliptical
0
-500
-1000
-8
-6
-4
-2
0
X-Axis
2
4
6
8
Influence of high fills on structural actions





Use finite element analysis.
Consider the effect of boundary conditions in
the model.
Consider the effect of upper and lower bound
stiffness values in the foundation materials
Model the footing structure accurately
Optimise the arch shape for the fill height
Essential Features of Finite Element
Model
Include foundation layers.
 Add arch fill in layers, modelling the
actual construction sequence.
 Allow slip at the arch/soil interface.
 Non-linear Mohr-Coulomb soil model.

Design of foundation structures for high
vertical loads


Morwell Conveyor Tunnels
Alternative foundation treatments
Design of foundation structures for high
vertical loads


Detail of finite element mesh
Alternative simplified model
Design of foundation structures for high
vertical loads
Design actions in actual raft under maximum
load
Type A Raft
1000
500
Base Forces; kN,m

Moment
Shear
Axial
0
-500
-1000
-3
-2
-1
0
1
2
3
Design of foundation structures for high
vertical loads
Comparison of bending moments in flat and
tapered rafts
Bending Moment
Flat raft vs. Actual
1000
500
Bending moments; kNm

0
Flat raft
Actual raft
-500
-1000
-1500
-2000
-3
-2
-1
0
1
2
3
Design of foundation structures for high
vertical loads

Alternative raft models: bending moments
Bending Moment
Alternative analysis methods
600
500
Raft Moments; kNm
400
300
R E P L FE A
S tra nd7 FE A
S tra nd7 simplifie d
200
100
0
-100
-200
-3
-2
-1
0
1
2
3
Design of foundation structures for high
vertical loads

Alternative raft models: axial forces
Axial Load
Alternative analysis methods
0
-100
Axial Force; kN
-200
-300
R E P L FE A
S tra nd7 FE A
S tra nd7 S implifie d
-400
-500
-600
-700
-800
-900
-3
-2
-1
0
1
2
3
Design of foundation structures for high
vertical loads

Larger spans
Design of foundation structures for high
vertical loads

Larger spans
Longitudinal Differential Settlement
Long Section - Morwell Tunnels
Longitudinal Differential Settlement


Morwell conveyor tunnels analysis
Deep brown coal deposits + uncompacted fill
Vertical Settlement
Horizontal Movements
Longitudinal Differential Settlement
Settlement and Longitudinal Movement
Minimum stiffness all layers
0.2
-0.5
-1
0
-1.5
-0.1
-2
-2.5
-0.2
-100
0
100
DX
DY
Settlement and Longitudinal Movement
Maximum differential stiffness
200
0
X-Axis
0.1
-0.1
0.05
-0.2
-0.3
0
-0.4
-0.05
-0.5
-0.6
-0.1
-0.7
-0.8
-0.15
-200
-100
0
X-Axis
100
200
Horizontal Movement, DX, m
-200
Settlement DY, m
Settlement DY, m
0.1
Horizontal Movement, DX, m
0
DX
DY
Longitudinal Differential Settlement
Longitudinal Loads




Steep fill slopes above the structure
Segmental structures on steep slopes
Longitudinal spreading of foundations under
high fills
Longitudinal differential settlement effects on
continuous structures
Longitudinal Loads
Longitudinal Loads





Use wider precast elements
Provide shear connectors between adjacent
elements and/or strengthen crown beam
Provide movement joints in the crown beam
and footings
Design for variation in axial load
Detail robust crown joints
Examples - Morwell Conveyor Tunnels
Examples - Mt Arthur, Olympic Dam
Examples - Spain
Conclusions




High axial loads, possibly high bending
moments and shear forces
Consider upper and lower bound foundation
stiffness
Design raft foundations for soil-structure
interaction
Consider longitudinal effects:
– Differential settlement
– Foundation spreading
– Longitudinal loads due to fill slopes and/or inclined
structure