Australian Geomechanics Society Mini-symposium Sydney October 2005 Soil Loads on Cut and Cover Tunnels Under High Fills Doug Jenkins Interactive Design Services

Overview: l Influence of high fills on structural actions l Design of foundation structures for high vertical loads l Longitudinal differential settlement l Longitudinal loads l Examples l Conclusions

Influence of high fills on structural actions l Axial loads  High fill = high axial load  Effect of foundation stiffness on axial load

Influence of high fills on structural actions l 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 l Bending moments and shear forces  Effect of arch profile on arch actions  Morwell Tunnels - alternative cross sections

Influence of high fills on structural actions l Bending moments

Influence of high fills on structural actions l Shear Forces

Influence of high fills on structural actions l Use finite element analysis. l Consider the effect of boundary conditions in the model. l Consider the effect of upper and lower bound stiffness values in the foundation materials l Model the footing structure accurately l Optimise the arch shape for the fill height

Essential Features of Finite Element Model l Include foundation layers. l Add arch fill in layers, modelling the actual construction sequence. l Allow slip at the arch/soil interface. l Non-linear Mohr-Coulomb soil model.

Design of foundation structures for high vertical loads l Morwell Conveyor Tunnels l Alternative foundation treatments

Design of foundation structures for high vertical loads l Detail of finite element mesh l Alternative simplified model

Design of foundation structures for high vertical loads l Design actions in actual raft under maximum load

Design of foundation structures for high vertical loads l Comparison of bending moments in flat and tapered rafts

Design of foundation structures for high vertical loads l Alternative raft models: bending moments

Design of foundation structures for high vertical loads l Alternative raft models: axial forces

Design of foundation structures for high vertical loads l Larger spans

Design of foundation structures for high vertical loads l Larger spans

Longitudinal Differential Settlement Long Section - Morwell Tunnels

Longitudinal Differential Settlement l Morwell conveyor tunnels analysis l Deep brown coal deposits + uncompacted fill

Vertical Settlement

Horizontal Movements

Longitudinal Differential Settlement

Longitudinal Loads l Steep fill slopes above the structure l Segmental structures on steep slopes l Longitudinal spreading of foundations under high fills l Longitudinal differential settlement effects on continuous structures

Longitudinal Loads

l Use wider precast elements l Provide shear connectors between adjacent elements and/or strengthen crown beam l Provide movement joints in the crown beam and footings l Design for variation in axial load l Detail robust crown joints

Examples - Morwell Conveyor Tunnels

Examples - Mt Arthur, Olympic Dam

Examples - Spain

Conclusions l High axial loads, possibly high bending moments and shear forces l Consider upper and lower bound foundation stiffness l Design raft foundations for soil-structure interaction l Consider longitudinal effects: –Differential settlement –Foundation spreading –Longitudinal loads due to fill slopes and/or inclined structure