Problem
The structure is a cut and cover reinforced concrete box that acts as a
subway link between Walthamstow Central and the Waltham Forrest Bus
Station. The surface area of the internal structure is approximately
1500m².The structure was commissioned as part of the pre PPP contracts.
The chosen design solution was a Bentonite matt. This failed to be
effective and the contractor spent a considerable sum chasing leaks
around the subway. The situation turned contractual and work was
stopped. USIL (now Triton Norway (TN)), along with Metronet proposed the
MPS system to rectify the situation and achieve the LUL cat 1 standard
level three requirements. The structure was due to be fitted out imminently
but were on stand down due to the water ingress adding to the contractual
pressure.
Challenges
There was no set process for obtaining acceptance across the LUL network of a product such as the MPS system. With great
enthusiasm and assistance from both LUL and Metronet a way forward was agreed. The MPS system had to go through the
normal LUL and Metronet CDS and site approvals systems to be allowed on site, prove compliance with the UK and EU directives
on both Low Voltage Safety and EMC compliance as well as the extra over Metronet signalling requirements, demonstrate the
anodic properties of the system and that there was no direct connection to the re-bar along with an extra over requirement to
achieve a level 4 (Totally Dry) standard requirement set by LUL at 90% Relative Humidity within the structure
Working within the LUL environment meant that all staff had to be certified and accredited. Due to the nature of access to this
structure and the Fit out contractors work taking priority, the main works were carried out in a 5 hour night shift pattern this left
an effective productive rate of 4 hours per night.
The break through of the structure into the station area had not taken place which left the structure classified as a confined
space; this limited the men and materials that could be used. In conjunction with this and the section 12 fire classification
Specialist dust suppression equipment had to be used contributing to the lost hour of productive time.
An advantage of the MPS system is that it increases the effectiveness of the bond between applied materials and the original
structure by drawing some of the material into the capillaries of the structure. This proved a challenge in achieving the RH as
more water was been added by the Fit out contractor in their work.
TN make a statement of success criteria that the inside surface of the structure must be vapour open. This was not possible
with this installation as ceramic glazed tile are to be used in the public areas. This is designed into the capacity of the system.
TN are able to give assistance with compatible materials unfortunately a tanking material Ronafix (Vapour Closed) was used
by the fit out contractor without the knowledge of TN this caused the system to slow down and draw extra current. A third pulse
box was installed to cope with this extra requirement.
Contract
The cost of the trial was not bourn fully by Metronet however they did contribute to the cost, as this is the first installation in the
UK and LUL environment. Taking into account all the restrictions and obligations, If this project had been assessed as a normal
commercial project. The MPS system is not an off the shelf product at a standard rate. Each structure has its own set of
conditions and environment and therefore its own cost. The MPS system is a unique innovative first world technology with a
simple installation technique. However the largest component to cost is the cost of labour and the access to site. There is no need
to undertake expensive excavations as the MPS system is applied within the inside/dry environment of the structure. Hence the
placement of the MPS is slightly more expensive than a traditional temporary solution but much less that a traditional permanent
solution and for this we give a structural life time guarantee on the MPS equipment.
Outcome
The MPS system has achieved DTI approval and we now have the CE mark. We are also compliant in terms of EMC to the extra
over Metronet signalling system and infra structure requirements. Areas of the structure that have been unaffected are now
showing that the target 90%RH has been achieved or bettered. Final sign off by Metronet was achieved on 26th September 2007.
This means that we are now in a position to pursue feasibility studies and design concept work on other assets and stations.
During June and July 2015, Triton Norway was engaged by TFL to relocate the control gear of the AOP system at Walthamstow
Central. At this time, the system had been powered on and working without maintenance for 9 years. It had kept the Walthamstow
underpass dry and operational for all this time and since this is a hidden system, LUL had forgotten it was even there.
AOP system
Hydrotech the original company terminated in 2011, Triton Norway remains as the authority on AOP as the system was
developed by the Triton Norway team. The latest generation of AOP is now called AOP (Advanced Osmotic Puls). The AOP
System is a revolutionary technology based on an accepted theory of electro osmosis. It gives new life to existing brick, masonry
and concrete structures avoiding expensive reconstruction and has a broad field of application in new concrete structures. The
“state of the art” methods in use in the building industry today can be characterised as temporary solutions to a problem for which
there is no apparent cost effective permanent solution. The AOP System provides a permanent solution. The AOP System is
utilised for transporting water encapsulated within the capillaries out of structures, as well as permanently preventing the
penetration of water into structures. A control unit produces a low voltage electrical charge, which is passed through electrodes in
the form of probes or wires strategically placed within portions of the walls and/or floors which are wet.
