Stanlow Abbey was not a lucky place. Built on the Mersey’s south bank, the last decade of the thirteenth century brought flooding, storm damage and fire. No surprise then that its resident monks craved calmer waters, finding them 40 miles away in north Lancashire. There, alongside the River Calder, they established new domestic ranges, outbuildings and a fine church, enclosing the site with crenellated walls and successfully exploiting local resources to flourish financially.

It was a seat of genuine industry, built on crops, wool, iron and coal. But the Dissolution of the Monasteries did for it, prompting piecemeal destruction over the centuries that followed. Today the remains are listed – still significant local landmarks although somewhat overshadowed by a more recent structure of even greater scale just 70 yards beyond the surviving gatehouse.

Engineering at a price

1845 saw Royal Assent for an Act that incorporated the Bolton Blackburn Clitheroe & West Yorkshire Railway (BBC&WYR) – known later as the Blackburn Railway to save time and breath. Planned as a 45-mile connection between Bolton and the North Western Railway at Long Preston, its southern section was completed in stages, opening to Chatburn – two miles north of Clitheroe – on 22 June 1850. The through route would not be advanced for another 30 years, joining the then-Midland line at Hellifield and creating a strategically important link from Manchester to Glasgow via the Settle and Carlisle.

Engineered by Terrence Wolfe Flanagan, the BBC&WYR boasted two substantial structures: the 2,015-yard Sough Tunnel and a viaduct of 48 arches across the Calder valley. This stands alongside the abbey ruins and, to help harmonise it with them, Flanagan inserted brick screens beneath two of the spans, decorated with large Gothic arches.

Reputedly comprising seven million bricks (it’s not known who counted them), Whalley Viaduct carries its two-track railway for 660 yards, crossing the river at a height of 70 feet and climbing to the south on a gradient of 1:82. It commands the landscape thereabouts. Contractors Nowell Hattersley & Shaw were given the job of erecting it, taking three years and earning £40,000 for their troubles. With the end in sight, two spans collapsed on 6 October 1849, killing three men and injuring a fourth. Blame was attributed to heavy rains – percolating through the brickwork to prevent the mortar from setting – compounded by the premature removal of the arches’ centring.

Flaws and imperfections

Defects have manifested themselves throughout the viaduct’s history, with the first recorded strengthening work taking place in 1884. Today, the undulating line of the parapet tells its own story. Timber piles support the piers: numbers 13 and 14 (in the river), 46 and 47 were underpinned as part of two projects, the first taking place in the early 1940s. The northernmost arch was also rebuilt. And evident throughout are patch repairs and recasing. Unsurprisingly then, Network Rail has a long-term monitoring plan in place, with sensors located at various points along the structure tied to the usual system of alerts and alarms.

Routine inspections late in 2010 recorded significant defects in Span 34 – towards the northern end – including flattening to the west ring face of the arch barrel and a full-width transverse crack. It was immediately clear that Network Rail Infrastructure Projects would need to fast-track another intervention, with Birse Rail assigned the works under the civils framework agreement.

To better understand how the structure was behaving, Datum Monitoring Services installed crack and rotation sensors around the affected span, together with two extensometers alongside the east and west faces of Pier 34. Six cores were drilled through the pier to confirm its composition: solid brickwork sitting on a sandstone raft above the timber piles. Some evidence of hay bales was also apparent.

Over the spring months, movement in the arch appeared to be a function of thermal expansion – sometimes as much as 5mm in a day – but recovery was only partial so it was gradually rotating towards the north, with the crack acting as a hinge. The east-elevation spandrel face was also being pushed outwards. Subsequent 3D monitoring suggested that the deformation was actually being driven by settlement of the pier, at a rate of around 5mm per month during wetter periods. Computer modelling techniques confirmed this through analysis of the gathered data.

Ground investigations, performed by BAM Ritchies, recorded a locally high water table – almost reaching ground level – the seasonal rise and fall of which could have hastened the timber piles’ deterioration through repeated cycles of wetting and drying. Additional insight was obtained using a Cone Penetration Testing truck provided by Lankelma. Sinking a borehole for a piezometer revealed that disturbance of the groundwater regime accelerated the vertical movement of the pier, adding to the future challenges confronting the project team. The rock head was encountered 6.5-8.15m below ground level, varying across the pier.

A limited menu

Timescales and the need for guaranteed longevity helped to define the way forward. Construction methodology was another critical factor, given the possibility of transitory instability. Whatever the permanent solution, it would have to be delivered sensitively in light of the structure’s condition. A two-phased approach was therefore adopted, first stabilising the arch with a concrete ring before then installing piles onto which the pier’s load would be transferred.

Birse Rail appointed Donaldson Associates to develop the Form B design. This identified complex sequencing to mitigate the risks involved, although it was recognised that there would be some “evolution” as practical experience filled knowledge gaps. The solution reflected the viaduct’s assigned capacity of RA8 at 45mph with a heavy axle weight restriction of 20mph.

Constraints came in a number of forms, the viaduct’s Grade II listed status being just one. But Ribble Valley Borough Council adopted a helpfully pragmatic position in granting consent. More significantly, the availability of only one wheels-free possession late in November 2011 – needed for initial shotcreting – created a fixed point in time around which the programme would have to be planned and executed.

Welcome to Whalley

Site mobilisation occurred in October with the establishment of a compound in a field alongside the viaduct. Span 34 was the other side of a lane – the west elevation being accessed through a narrow parcel of land in which the northern part of the structure stands, necessitating a section of wall to be taken down. Standard Scaffolding Specialists used a school playing field to reach the east elevation, erecting a protective screen between the worksite and very-adjacent bungalows. All this demanded land access agreements. An 11kV power cable, inconveniently passing beneath the affected arch, had to be diverted further south.

Using MEWPs (Mobile Elevating Work Platforms), initial stabilisation work involved pinning, grouting and stitching to spans 33, 34 and 35. Additional monitoring was also installed. Two rows of staggered needle beams were then inserted above each of the two piers to form springers for the new 475mm concrete arch and its reinforcements, to sit beneath the existing barrel. Located at 490mm centres, each hole was core drilled, the beam inserted and fully grouted up before work began on the next, the overall process following a carefully developed sequence. This paved the way for the first spray-concrete operation during November’s crucial 29-hour possession. Recognising that, at this stage, the additional dead load of the planned arch could prompt further deterioration, it was decided only to form an intermediate ‘holding’ ring of 150mm.

Concurrently, a trial pile was driven and load tested, 3.5m from the west elevation, to confirm the design suitability and proposed methodology. After a seven-day curing period, work proceeded with the structural piling, undertaken by main subcontractor APB Group – a predetermined sequence again being adopted, although this differed to that specified in the design as a consequence of the adverse ground conditions. There are 14 piles per pier; seven per elevation – each was poured in 450mm permanent steel casings, it being impractical to subsequently remove them as would generally be the case due to the inevitable disturbance it would cause.

After each opposing pair was sunk, the brickwork was cored, a beam installed and the hole grouted at the earliest opportunity to start transferring the load, the intention being to reduce any settlement. This required supporting stools to be secured between the piles and the beam.

Room for manoeuvre

With the piles at 1.2 metre centres, the piling rig’s lateral operation became increasingly restricted as more needle beams were inserted on a ‘do one, miss two’ rotation. Mounting the coring drill suffered from similar constraints. End to end, this critical process took about four weeks during which time three wet months worth of settlement – about 15mm – was recorded. With the site closing down for Christmas, it was vital to have confidence that everything was fully supported and stabilised, with contractors remaining on standby to respond to any alarms.

The pier element of the design was concluded with the installation of intermediate needle beams which would load the reinforced concrete boots around the piers’ bases once they were cast. This allowed the remaining arch reinforcement and shotcreting to be progressed – a job that APB Group subcontracted to BAM Ritchies. In terms of eccentric loading, the impact on the piers is limited, with the new arch adding around 10% to the dead load. To soften its visual impact, the elevation sections were faced in brick, with corbelled brickwork detail beneath connecting the new with the old.

Years of movement had opened some of the mortar joints between the parapet and string course. As a final act, the parapets were pinned to ensure gravity couldn’t take the initiative if further movement occurred. Two expansion joints were introduced on each elevation, relieving some of the compressive forces focussed at this location. By early April the physical works were done; the site shut down on the 24th.

Up against it

Continuing monitoring of the installed strain gauges show the load is being progressively transferred to the piles. Any arch movement is now exclusively thermal, expanding and contracting as would be expected. And there are no signs of this new hard spot pushing problems elsewhere.

Costing a little under £2 million, this scheme met no-one’s definition of cheap and cheerful. But what’s reflected in that price is the severity of the problem, the no-choice solution, the logistical complexity and the challenges thrown up by dynamic conditions to which the team had to respond. Viewed in that context, a successful and entirely non-disruptive scheme has emerged here from demanding circumstances. No mean feat.

Whalley Abbey will remain the only ruin locally.

Many thanks to Dan Wilcock and Mark Billington of Network Rail, and Birse Rail’s Gavin Collins and Graham Gallagher for their help with this article.