for structural tunnel analysis to prioritise TOTEX expenditure
by Julian Britton I.Eng MICE
Hydraulic overloading causing liner displacement - Courtesy
of Wessex Water
has a tremendous legacy of tunnel construction over the last 150
years, from Joseph Bazalgette’s brick sewers in London, through
to reinforced concrete designs based upon early engineers such
as François Hennebique. Wessex Water has a considerable
cumulative metreage of tunnels constructed over the last 150
years, up to 5 metres in diameter, serving over a million
customers in Bristol, Bath and the
Poole/Bournemouth/Christchurch conurbation. As part of a
structural survey programme, Wessex Water looked for a
predominantly non-destructive testing (NDT) method of analysing
the structural capabilities of the tunnels and understanding
their longevity. In 2016 Wessex Water became the first UK WaSC
to use the IKT MAC System (Mécanique d’Auscultation de Conduits)
for such an assessment of the structural condition, of a
man-entry concrete sewer in Bournemouth, and its host geology.
The MAC system
The system was devised
initially by Eau de Paris to target the repair of deteriorating
large diameter sewers, recognising that both the stability of
the pipe and of the surrounding soil needs to be understood to
determine where remediation is required. The technology was
further developed by IKT (Institute for Underground
Infrastructure, Germany) over three years, to the point where it
was first deployed for commercial use in Spring 2015. Its first
major deployment was for Hamburg Wasser to survey 450m of an
oval masonry sewer (DN 1700/2000).
With the advent of a
device such as the MAC Extensometer, the stiffness of
constructional materials can be determined, aided by coring and
unconfined compressive testing, to establish a more thorough
non-destructive test regime, augmented by finite element
analysis, that will allow understanding of the tunnel lining
capabilities under given loads and in a particular geology.
The MAC System comprises
a powerful hydraulic pressure cylinder, which forces two bearing
plates, simultaneously against opposite walls of the tunnel,
automatically controlled, with pressures ranging from 10 to 100
KN. Fine sensors (Extensometers), measure the deformation
arising directly in the area of the pressure plates and at a
distance of approximately one metre in front and behind these
points to record the three-dimensional deformation. Despite the
high forces applied, the automatic control of the pressure
cylinder prevents any overloading and damage to the tunnel
walls. The actual deformations measured are, at most 0.5mm
2 : Diagrammatic MAC layout - Courtesy of Wessex Water
The rate of loading is
approximately 5KN/s. The deformation of the horizontal diameter
is limited to a minimum of 0.025% or 500μm. The inventor, Dr.
Olivier Thepot, states the entire system is of variable geometry
and can be adapted to all shapes of sewers and tunnels. Three
measurements are obtained, the force pushing on the bearing
plates, the displacement measured by the main rod and the
displacement measured by the 3-dimensional rod.
This data is collected
and analysed, instantaneously displaying a graph on the data
logger showing the force/displacement curve across the pipe, and
the damping of the displacement in the longitudinal direction.
From readings, it is possible to calculate two parameters; the
global stiffness which is equal to the ratio between the force
and the main displacement and the damping factor, which is equal
to the ratio between the 3-dimentional displacement and the main
Cores are then taken
through the tunnel lining to determine the mechanical properties
of the structure. These cores are crushed for unconfined
compressive strength (UCS) in terms of N/mm2. In addition, it is
imperative to establish the external geology/lithology via a
borehole, with a range of in situ and laboratory tests. When the
geometry of the pipe (shape and thickness) is known, it is
possible to back calculate the Young’s modulus of the soil (ESoil)
and the Young’s modulus of the structure (EMaterial).
Working with the Institut
Für Geotechnik (IGtH) der Universität Hannover through Wessex
Water’s partners IKT in Germany, a Finite Element Model is then
IKT developed the system
for use in sewers from DN 1200 to DN 3000 and it now works in a
half-automatic mode to improve safety at work. At certain limits
of force and displacement, hydraulic power shuts off.
Furthermore wireless data transmission makes testing in a sewer
much easier. The benefits of the MAC system are:
Identifies the weak point
in the pipe-soil-system.
Determines whether the
pipe or the surrounding soil is the cause of weakness.
Equipment is portable and
can be assembled via man-access chambers.
Core drilling in the
sewer wall and bores in the surrounding soil are reduced to a
Determines whether repair
refurbishment and subsequent checking of its effectiveness.
Success of repair can be
Changes in stability can
be investigated over long periods.
Considered by the client
to be cost-effective.
Analysis of ‘Flexilok’
tunnel in a deep saturated sand geology
The MAC system was used
for the first time in the UK by Wessex Water (YTL E&C) in the
summer of 2016, at Braidley Road, Bournemouth.
The tunnel in question
was constructed in 1971 through a dense sand geology at a depth
of 19m. The geology of Bournemouth is typified by wind-blown,
coarse and fine hygroscopic silver sands, which are, usually
well cemented, but can be prone to degrees of migration when
saturated. The Flexilok tunnel lining is an unbolted ring of
concrete segments best suited to the over consolidated London
Clays which ‘squeeze on’ to the extrados of the newly
constructed ring when the shield moves forward, and exposes the
annular over cut, represented by the thickness of the shield.
At some point over the last few years, the Braidley Road tunnel
has been hydraulically surcharged and the water pressure has
forced the segments out, over a 70m length, mid-way along the
tunnel (Figure 1). For the first time the MAC Extensometer has
given Wessex Water the ability to understand the structural
capability of the tunnel lining, which was assumed would vary
along its 220m length, between shafts. By iteratively analysing
the tunnel in discrete lengths, Wessex Water was able to
understand the potential longevity of the tunnel (Figure 3).
3 : Extensometer in tunnel - Courtesy of Wessex Water
MAC conclusions and least
cost tunnel repair
Of the total 220m length
of tunnel that was measured, the visually defective range
exhibited by the segment deformation between a chainage of
100-140m (Figure 4) and to compare to what was thought to be
competent lining, Wessex Water measured from 70–100m and a
second range upstream of the defects at 140-170m. In actual
fact, the external stiffness in the two base line zones
indicated a lower stiffness value, proving loose sand, whilst
the deformation zone was in either compact or very compact
It was concluded that the
grouting at springing level in the deformed zone provided
adequate lateral support as viewed by an endoscope survey where
possible, and the remainder of the tunnel exhibited the
propensity for failure. Although this was not expected, it does
indicate the worth of the system in understanding the likelihood
of future collapse.
Figure 4 :
Pipe-soil stiffness (Kg) with zoning  - Stiffness
results linearly along the tunnel length
A lining solution was
designed utilising 1050mm ID liners SRM Type 1 ‘Fully
Deteriorated’ conditions and a liner thickness was devised at
35mm, cross-referenced to the American standard ASTM F1216 :09
(Now ASTM F1216 :16). The linings were procured from Amiantit
and manufactures in single piece from their Polish factory.
The scheme was completed
to programme over a six week period, including a plasticised
intergrout between host lining and extrados of the Type 1
shells, by the contractor Matt Durbin Associates Ltd, at a total
cost of £365k (Figure 5).
- Typical defect
Figure 5 : GRP
Type 1 Shell installation
In summer 2017, the MAC
Extensometer will be used on a similar section of tunnel known
as the Southern Foul Water Interceptor (SWFI) in Bristol;
constructed with the same linings in 1973. Although that tunnel
was constructed in part through the Redcliffe Sandstones and
Mercia Mudstones, which should offer a high degree of lateral
support to the tunnel, there was a collapse at the Tannery in
1998, as it is routed adjacent to the New Cut; a diversion
channel constructed in 1817 for the River Avon.
This system allows the
owners of tunnels, whether for sewerage, railway, cables,
whatever, to target the available maintenance or capital funds,
on the most deserving assets. It also means reduced
frequentative man entry, which mitigates concerns of health and
safety, and satisfies the insurers of such structures, that they
are insurable, and premiums are balanced and appropriate.
Fundamentally, the MAC
system allows perceived structural problems to be analysed,
along with structural elements which are not showing signs of
and publishers would like to thank Julian Britton,
Trenchless Technology Manager with YTL Engineering &
Construction, Wessex Water, for providing the above article
thanks the following for their input: Dr Dec Downey of
Trenchless Opportunities Ltd, UK, Roland Waniek, Managing
Director of IKT Institute for Underground Infrastructure,
Germany, Kevin Jefferson of Amiantit Pipe Systems Ltd,
Norway, and Matt Durbin of Matt Durbin Associates Ltd, UK.