Balfour Beatty Rail
Design, Geotechnical, Construction, Ground Modelling, Permanent Works
The site at Scorrier is in a known historic mining area over which a railway was constructed. As a result of a number of historic surface collapses and surface subsidence, the existing rail-line was subject to periodic monitoring, and a rail glanding system was put in place. However, these remedial works were not felt to provide sufficient support in the event of further collapse events or large-scale surface settlements, and the Client requested that further remedial measures be put in place.
We were tasked with producing a cost effective and stable solution to a combination of geo-hazards arising from the historic mine workings. The design created by Crouch Waterfall used a combination of new and innovative techniques to create a truly unique and bespoke solution to the problem. Our approach has lead to significant reductions in the cost of GI, a very short construction and installation period, and cost savings over more traditional solutions.
In May 2017, Crouch Waterfall were commissioned by BBR to undertake:
- A geotechnical desktop study and risk assessment of historic mining works, which included the creation of a 3D ground model of the site plus the concept design of a number of remediation options;
- The design of an intrusive ground investigation aimed to determine the ground conditions and to allow detailed design of permanent remedial works to take place;
- The design of the permanent remediation option selected.
There had been numerous efforts to establish the risks to the rail over a considerable period of time using a variety of techniques, non had been conclusive. Our proposed modelling exercise sought to combine all of the existing information into a single unified 3D model of the site. Data was acquired from topographic surveys, historic mining records, previous ground investigations, ground penetrating radar, anecdotal evidence and the evidence presented by surface features. The 3D model was then used as a means to establish gaps in the GI required to fully define the subsurface risks. By specifically targeting information gaps in the 3D model, we significantly reduced the scope of GI work required. Once updated with the new GI information, and the gaps in the model had been filled, the model was used to define and assess the risks to the operational railway from mining features. From this risk assessment an options appraisal was undertaken, and a preferred solution taken forward.
Over September 2017 – March 2018 Crouch Waterfall undertook the design of the permanent ground stabilisation and remediation works which comprised:
- Structural design of a ground bearing slab-and-beam structure, designed to take full railway loading spanning over a length which is determined from a modelled collapse of the adjacent mine workings;
- The design of reinforced sub-ballast platform and transition ramps, designed to improve track foundations and provide longitudinal, transversal and vertical transition transfer in stiffness from the track to the subgrade or beam & slab structure;
- The design of ground improvement measures consisting of geo-polymer injections, designed to support the beam & slab, to improve the overall bearing capacity and settlement characteristics of the ground, to fill small voids, and to protect against any lateral loss of support as a result of a mine-shaft collapse.
A maintenance programme was included as part of the design works to determine if any, the ‘loss of effectiveness’ of the improved ground and consequently the loss of support of the structures above. This maintenance programme aims to anticipate any future potential issues and guarantee the design life of the ground improvement to 120 years.
On Site were responsible for the polymer grout injection, with both Ground Stiffness Surveys and Geotechnical Engineering Ltd undertaking the ground investigation works.