The assessment of ageing tunnels requires a deeper understanding of the longterm
behaviour of twin tunnels, whilst lack of permeability data limits the accuracy
of long-term predictions. This thesis therefore investigates long-term twin-tunnel
behaviour through finite-element parametric analyses, and provides additional permeability
data through laboratory studies.
Permeability tests are performed on fissured London Clay, exploring the effect
of isotropic stress cycles on the permeability of fissures. A model explaining
the permeability- stress relationship is proposed to explain irrecoverable changes
observed in fissure permeability, and is formulated mathematically for numerical
Laboratory investigations are performed on grout from the London Underground
tunnels, investigating permeability, porosity, microstructure and composition.
A deterioration process is proposed to explain observations, consisting of acid
attack and leaching. The deterioration had appeared to transform the grout from
impermeable to permeable relative to the soil. The change in grout permeability
with time would strongly influence long-term movements.
The long-term behaviour of single tunnels is investigated in a finite-element
parametric study. A new method is formulated to predict long-term horizontal and
vertical surface displacements after excavation of a single tunnel, and incorporates
an improved measure of relative soil-lining permeability. The study also predicts
significant surface movements during the consolidation period, contradicting the
lack of further building damage observed in the field.
A further parametric study also investigates the long-term behaviour of twin
tunnels. Key interaction mechanisms are identified, leading to the postulation of
the long-term interaction behaviour under different tunnelling conditions. Longterm
interaction is found to be complex and significant, and should be accounted
for in numerical simulations.