The key challenges and background
Following the completion of pre-demolition piling and construction of the buttress wall at London Bridge station, structural settlement continued once these works had been completed. This resulted in a total settlement of the 200-year-old masonry arch, which supports the live railway, of circa 30mm with an annual rate of 10mm.
Due to the ongoing settlement, significant additional ground investigations were undertaken over two phases to understand the material below the masonry arch footing and to install extensometers and piezometers into the ground below. This would help engineers to understand where ground movements were occurring and any change in the pore pressure.
Historically, settlement in other parts of the project settlement had abated following the completion of works; at Arch E929 settlement rates were not reducing and exceeded the acceptable limit.
Whilst following the observational approach to reviewing structural monitoring results within the area of settlement and the additional investigations, it was decided that some form of ground treatment was required to stiffen a very loose stratum. A very loose deposit of sand had been proved toward one section of the buttress foundation. Video footage of the material showed it comprised of sand with some gravel with limited silt content.
Treating the loose gravels was both a logistical and technical challenge. Access was limited to one side of the masonry footing which had a height restriction. This was combined with a treatment which could not create differential settlement rates to surrounding structures or increase load paths to the London clay containing the Jubilee Line Tunnel Portal.
How did permeation grouting overcome this problem?
Permeation grouting was selected to address the issue of the cohesion-less sand and gravel with limited silt content. The technique was assessed as being appropriate for the ground conditions following a review of the ground conditions. A treatment area below the foundation was specified by the designer encompassing the entire length of section of buttress and for the full depth of the River Terrace Gravels (RTD).
The grouting overcame the problem by improving the stiffness of the in-situ loose granular deposit without creating significant disturbance of the material under the existing foundation. Multiple treatments at relatively low injection pressures mitigated against the risk of structural instability, which was determined by real time monitoring of the structure and ground throughout the grouting works.
The particle size distribution of the RTD sitting below the arch foundation and above the London clay was used to select the most appropriate grout. A 12µm grout was selected to be injected through sleeved, grout injection pipes into the targeted strata.
TaM pipes were installed into drill holes, the position, inclination and length of which being determined by the profile of the foundation to ensure the correct distribution of grout within the treatment area.
Over 2,000 grout injections were executed with individually designed parameters for each specific TaM port; volume, flow rate and pressure of each injection was controlled by computer piloted pumps until the grouting design criteria was met.
Following the completion of works, ongoing movements within the RTD and structure was below the acceptable criteria.
Long and short term importance to the project
The resolution of Arch E929 settlement and associated loose materials was imperative in both the short and long term.
In the short term it allowed key stakeholder areas to be completed, thus enabling the migration of station staff into their final position from temporary retail locations.
A key client deliverable for Network Rail was the successful opening of retail units to release revenue streams.
In the long term, ongoing settlements of the structure would have resulted in changes in geometry of the masonry arch with associated cracking and potentially differential settlements between adjacent arches – a risk to occupied spaces below the arch.
The Network Rail tracks above the arches rely on the masonry piers for stability, any settlements of the piers would be translated to the tracks. Therefore, significant settlement would have a detrimental effect on the Network Rail tracks and this would be presented in potential track faults. Any remedial ballasting of the track to maintain levels could lead to a change in platform gauge.
The platforms are also supported on the over site slab which is also supported on the masonry arches, further movements would result in settlement of the concrete platform structures leading to misalignment and cracking of the concrete and platform finishes.
Station stairs and escalators are supported from the Arch E929 masonry buttress. Any future movement of the masonry structure could lead to excessive deflections within the stairs and escalators outside of their serviceability limits. This could result in finishes cracking and escalator operational tolerances being exceeded with associated breakdowns.
Outcomes and benefits to the overall project delivery
The main benefits of the permeation grouting was that it allowed the main project delivery works to be undertaken with minimal disturbance, whilst ensuring ongoing stability of the masonry arch above the treatment zone during the works.
Although limited by head room and access to the treatment zone, the specialist equipment mobilised to install the TaMs allowed other works to be carried out in adjacent areas. As the drilling and grouting works were self-contained within the arch above the treatment zone, other critical project works were not delayed as they may well have been if a more aggressive treatment had been selected.
A key benefit of the permeation grouting scheme was that grout could be delivered to the treatment zone in a controlled manner talking cognisance of the real-time monitoring alarms. Injection data was recorded, analysed and presented graphically for daily review by the team. This data was reviewed by the team, together with that provided by the real-time monitoring, to develop the dynamic response to the observed reaction of the structure.
Although some small settlement was considered during the installation works, the works were completed with only very small amounts of heave being recorded by the structural monitoring.
Successful project delivery was reliant on the permeation grouting resolving the ongoing settlement issues within the arch whilst achieving the requirements to stiffen the loose granular material within the river terrace deposits, which was validated after six months of monitoring and grout volumes used reflecting calculated ones.