Structural problems in TT10

concretecakeUrban and Civil

Nov 29, 2013 (3 years and 6 months ago)


Structural problems in TT10

This problem is of particular
importance in TT10 because there
is strong evidence of recent and
ongoing movement. It should
however be noted that the same
basic failure mechanisms are
present in other similar designs of

The movement observed in
the floor of the tunnel is due to

Heave occurs when the
surrounding rock swells due to
the removal of the constraining
rock to form the tunnel or
when the rock swells in the
presence of water.

The use of a very thin (22cm)
unreinforced invert slab means
there is no resistance to the
swelling ground.

The cracking usually consists
of fine crazing of the concrete
and finally a clear longitudinal
crack that follows features
such as the central drain or the
bolt holes used to fix the beam

Any subsequent leaks from
either the machine or natural
ground water then further
aggravate the situation.



Progression of the
failure mechanism

Crazing patterns

Longitudinal cracking

The resulting stress induced in the lining
results in compression failure and flaking
of the concrete at the interface between
the tunnel wall and floor.

The picture below shows a lab test
where the top surface of a beam has
failed in compression. The flaking
concrete is very similar to that found
in the TT10.

Concrete flaking from the crown of the tunnel due to the compression caused as the structure flexes.

Cracking in the crown of the tunnel

Some of the resulting panels of concrete created by
the cracking show signs of radial movement further
weakening the structure

In order to stabilise the tunnel there are two basic solutions:

1. Short term

prop the centre of the tunnel to stop further movement

2. Long term

replace the floor slab increasing its depth and adding reinforcement, Strengthen
the main tunnel lining

Initial estimates suggest that 15 to 20 m of invert will need to be replaced, however there are
other areas yet to be assessed. Some repair will also be required to the main tunnel lining.

Current status


In order to complete the technical assessment the following actions are required:

1) Install convergence monitzoring

2) Detailed level survey of the tunnel floor and scan of the lining.

3) Concrete samples to be tested for strength

4) Trial hole to be excavated to establish the construction of the lining at the wall floor interface
and identify the type of waterproofing used.

5) Check the lining thickness

6) Camera inspection of central drain

7) Assess the structural capacity and prepare remedial messures

8) Define the extent of the area to be repaired

Preparation for the works

We are currently sourcing the specialist equipment required for the works, such as:

Concrete pump

Electrically operated excavator

Ventilation equipment

Rock bolting specialist

Initial planning is being prepared to establish a method of work which will minimise the need to
access TT10 by either SPS or PS tunnels. Possible scenarios include bringing concrete in by the
ventilation shaft at building 806 and or opening the PGCN1 shaft at the entrance to BA1