Bradley Lane Park and Ride Bilston, West Midlands, UK by AECOM
Shortlisted Brownfield Awards Category 11 - Best Infrastructure Project
Historical mineworkings blight the UK. In many cases urbanisation developed alongside industry leading to the current situation where brownfield locations ripe for development potentially carry with them significant hazards.
The AECOM Team have facilitated the revitalisation of a brownfield through the use of detailed desk-based research and community/industry engagement at initial and early stages. The community has been provided with vital infrastructure to help drive forward and develop the local and regional economy once more.
‘The completed project has since won the praise of the Coal Authority which is now using it as a model for construction on other former mines.’ – Express and Star Feb 7 2020.
West Midlands Metro is a light-rail/tram operating between Birmingham and Wolverhampton. The infrastructure is owned by Transport for West Midlands (TfWM), the public body responsible for co-ordinating transport services in the West Midlands Metropolitan County. In 2013 TfWM identified limited provision of ‘park and ride’ facilities along the route which were regularly operating over capacity. Expansion of the park and ride provision along the line was clearly required to meet demand.
Options were identified for expanding provision at existing sites and locations where new facilities could be provided. Bradley Lane was identified, partly driven by residential developments underway in the surrounding locality, and the availability of adjacent open space.
Bradley Lane Metro ‘Stop’ is located between the towns of Wednesbury and Bilston, within an area of the West Midlands known as ‘The Black Country’. The area is named as such due to its extensive history of mining and heavy industry. Indeed, the location of the ‘Stop’ is close to where the great Staffordshire ironmaster John ‘Iron-Mad’ Wilkinson built the first blast furnace (known as the Mother Furnace) in 1766. Thus, the area was instrumental in the beginning of the industrial revolution and the Site’s historical significance to the larger widespread industrialisation of the western world should not be underestimated.
However as with many areas of the Black Country, the Site’s historical past blighted its future as the unknown risk associated with the legacy of shallow mine workings had previously prevented redevelopment.
An initial concept for the scheme was developed by AECOM covering an area of 0.8ha, comprised a 196-space car park including dedicated disabled bays, accessed via a new access road from Bradley Lane, to the east.
The infrastructure was to be constructed on part of Walsall Council owned playing fields located at the toe of embankment which the Metro line and stop are situated on. This required the provision of a DDA (The Disability Discrimination Act) compliant access from the car park along the existing 4m high earthwork to the Metro stop.
However, the scheme was put on hold for several years until 2017. At this stage AECOM recommenced to take the scheme forward through the planning process to construction. Following initial consultation, the scheme was amended to incorporate access from Belmont Street to the west of the Site. This also required the car park itself to be translocated west of its initial concept position.
The car park’s proposed position now placed it on land which had previously suffered from mineshaft collapse and was likely to be underlain by abandoned workings. AECOM’s project team would have to overcome these issues in order to reach a successful outcome.
Coal Mining Legacy
Part of the work undertaken by AECOM for the planning submission was the preparation of a detailed Coal Mining Risk Assessment (CMRA). This was required due to the significant abandoned underground mining legacy identified at desk study stage, which in summary comprised:
The famous Staffordshire ‘Thick Coal’ seam at c.15m depth, historically mined across the region (Image 1),
‘Heathen Coal’ seam at c.30m depth,
Ten untreated mine shafts recorded,
Failure of mineshafts across the Site in the 1990’s,
Image 1: Site Cross Section – supplied by others
The CMRA provided the means for AECOM to provide the LPA with legacy coal mining information; and an assessment of its potential impact on land stability, for the proposed end use, including proposed mitigation measures.
The South Staffordshire Thick Coal was worked up until the 1920’s in the locale and was recorded to be in excess of 8m thick. In addition, it (the coal) was often worked and re-worked as techniques and methods improved allowing the removal of relic coal left by the mining of this seam (Room and Pillar workings). Indeed, the economic value of the Thick Coal at the Site was such that in the 1990’s rejected reclamation proposals included the opencast working of the Site to win the remaining coal.
Furthermore, and quite spectacularly, the seam is known to be prone to spontaneous combustion and boreholes at the Site had recorded encountering ‘burnt shale’. Additionally, the immediate environs of the Site are known locally as ‘Fiery Holes’ (Image 2) with one of the local public houses named as such.
Finally, historical ground subsidence attributable to settlement caused by collapsing relic mine workings had been recorded locally leading to the demolition of nearby Council housing.
Image 2: The Fiery Holes
Amongst the mining legacy features were the significant number of recorded mine shafts and the shallow presence of the Thick Coal. Given the shallow depth of the Thick Coal and several coal, ironstone and fireclay seams below, coupled with the industrial legacy at the Site it was of little surprise that many shafts were recorded at the Site.
The Coal Authority are a statutory consultee body for planning process in areas deemed ‘High Risk Development Areas’ within the UK coalfield. AECOM’s engineering mitigation measures put forward comprised
The location and treatment by grouting (injection of PFA/OPC slurry under pressure) of remaining untreated shafts;
The installation of a geo-grid reinforcement below the car park and access road in order to provide a costs effective method to limit any settlement that may become apparent at ground level.
Working closely with the Coal Authority; AECOM undertook further work to further limit the risk posed by any historical mining. The finalised CMRA was subsequently accepted by The Coal Authority with all Stakeholders agreeing to the following measures being implemented:
The location and treatment of remaining untreated shafts;
The installation of grout plugs formed over shafts; creating a reinforced ‘mattress’ of material to limit the risk of any shaft collapse migrating to the surface and to avoid costly excavation and installation of reinforced concrete caps as are typically advised in similar scenarios;
The pressure grouting of Thick Coal and Heathen Coal seams below car park, access road and ramp/stairs (Image 3);
Installation of geo grid over shafts/shaft groups.
A tendering process lead by AECOM on behalf of TfWM utilising an existing client framework and mini competition was completed during the autumn of 2018.
The works were split into two phases with the first being the mine stabilisation portion of the job. The appointed Principal Contractor appointed Forkers Ltd to undertake this first phase of the project. AECOM undertook supervision of the works throughout including a full-time presence during sensitive parts of the mine stabilisation works, which included night-time working.
Before commencing the works several specialist safety measures were required to be installed and commissioned. Firstly, due to the risk from mine gases being displaced during the works, a gas monitor was installed in the cellar of a public house located immediately north of the Site. The gas monitor recorded continuous real time data remotely. In addition, a monitoring system was installed to measure displacements on the existing in use permanent way and supporting earthworks.
These monitoring stations provided continuous real time data, monitored by staff remotely using digital devices 24/7. A red/amber/green (RAG) based action plan was agreed with all stakeholders and implemented to ensure appropriate measures were undertaken at the right time should trigger levels be exceeded.
As noted, the Thick Coal seam has a history of spontaneous combustion. In order to mitigate any further potential for this to occur AECOM specified that drill holes did not remain open and were to be either grouted immediately or capped temporarily to stop oxygen entering the workings.
Image 3 Grout treatment plan
Treatment of the mine workings was undertaken by drilling and grouting to a maximum depth of 35m BGL (below the Heathen Coal) to prevent any future instability risks to the proposed development caused by void migration. Stabilisation was carried out by completing a 4.25m centred treatment grid over the proposed development, with the grid extending beyond the car park and access road footprints into adjacent playing fields to mitigate any subsidence that may occur from workings immediately adjacent to the Site.
Treatment under the rail embankment to ensure treatment of workings beneath the proposed access steps and ramp structure from the car park was undertaken at night (Image 4) during 4-hour closure periods of the Metro. This required angled drilling up to 20 degrees from the vertical.
Image 4 Instrumented and monitored night working during Metro closure
Completion of a robust sustainable remedial design
Throughout the works extensive broken ground was encountered, which was interpreted to represent workings in the Thick Coal. Associated with this, drilling also encountered areas of red ‘burnt shale’ evidencing historic combustion of this seam.
The ten untreated mine shafts were investigated by probe drilling. Nine other ground anomalies were also located and treated during the work. During peak activity up to eight drilling rigs were on site as well as a substantial grout mixing plant.
Summary of the work completed:
Mine Workings Treatment 1,637 no. perimeter, primary & secondary boreholes drilled & grouted (55,392m drilled);
4,540 Tonnes 5:5:1 (PFA:Sand:OPC) grout Injected;
Shaft Location and Treatment 3,179 no. probe holes (17,078m drilled);
19 Mineshafts/Anomalies located, drilled & treated, depths of between 12m to 34m BGL;
Installation of 19 Grout Caps over the located mineshafts and anomalies. Grout caps were completed over each location by drilling a close grid of grout holes to stabilise the materials above the shaft column;
Drilling & grouting was carried out in an agreed sequence with down-dip perimeter holes being completed first.
Mine shaft treatment was carried out by drilling a 90mm hole to prove natural strata below the base of the shaft and grouting through the drill rods in 3m ascending stages. A second treatment hole with staged grout injection was completed to each shaft to ensure full validation of treatment.
Shaft treatment carried out from robust drilling safety platforms.
Continuous monitoring and recording of injected grout quantity and pressure together with extensive quality control and testing of mixed grout.
As part of the project team’s commitment to educating the local community of the history and significance of the Site geology students from Birmingham University were invited to undertake a visit (Image 5).
Image 5: Undergraduate students on Site
During the visit AECOM presented the history of the Site and the legacy of the wider Black Country area and how that had impacted the design of the mine stabilisation and civil engineering works of the scheme. This passing on of knowledge was seen as a key benefit of the close working relationship of all parties engaged.
Following the completion of the remedial mining works, Fitzgerald Ltd were commissioned during late summer by TfWM to complete the remainder of the Civil Engineering works and see the contract through to scheme delivery.
AECOM compiled a Completion Report in accordance with CIRIA best practice guidance, and this was submitted as a detailed record of works undertaken. This report detailed variations from the anticipated ground model encountered during the work and the treatment completed, including detailed drilling and grouting records.
The works were completed and opened to the public by Andy Street, the Mayor of the West Midlands, and other Council dignitaries in February 2020 (Image 6).
The facility now provides a key location for commuters and shoppers to access Birmingham and Wolverhampton City centres and the surrounding area. This in turn cuts down vehicular traffic and helps move the local economy.
Image 6: Opening of completed Park & Ride by local dignitaries alongside AECOM Team