Plate 1 - View to North, Late 1992
Plate 2 - View to South, Late 1992
Lancaster & Carlisle Line Bridge Number 109, Tebay
- Reconstruction of Up Side Walkway
Looking at one of the summer 1998 Railway Modellers, I noticed the rather nice OO layout of Tebay in the steam era by the Skipton club. Had that model been post April 1993, then there would have been a detail that few purists, if any, would notice. Obvious to anyone travelling north on the part of the Lancaster and Carlisle Line covered by the model, is the crossing of the River Lune by Bridge No.108, a 3 arch stone viaduct immediately north of the former Tebay Station. This is well reproduced in the model and widely known for the fishing in the pool adjacent. A little further north lies the small stone arch of Br No.109, at 32m 660yds. Sometime in the early 1970’s, an Up side walkway was provided in the form of a steel beam laid across the abutments and clad with in-situ concrete. Within 20 years corrosion on the beam had fractured the concrete, as shown in Plate 1 and Plate 2.
Plate 1 - View to North, Late 1992 |
Plate 2 - View to South, Late 1992 |
The replacement design was produced by the BR Bridge Design unit at the Regional Civil Engineer's office in Birmingham. The preferred design at the Preston Area office was to have the new walkway and parapet seated on piles which would have been placed in weekend possessions preceding the removal works and placing of the new beams. However, much to the surprise and general consternation of the Area staff, the design adopted, although using less weekends, was generally felt to be less durable and more complicated.
The scheme consisted of excavating the embankment to a depth of 3 metres below rail level behind both abutments, placing a dry mix concrete bed at the bottom of each hole onto which was placed a precast caisson. The caissons are hollow concrete boxes and once placed, concrete was pumped into them to give sufficient dead load to support everything under operational conditions. Each caisson was capped with a p.c. concrete bearing to carry each end of the new walkway and parapet units. The extract from the scheme drawing, Figure 1, shows a part elevation of north east corner of the bridge showing one end of the new walkway and parapet beam, seated on the new buried caisson foundations. Figure 2 and Figure 3 show the cross section and long section of the foundation units with walkway and parapet beams.
Figure 1 - Part Section on North East Bank Seat - preliminary drawing
Figure 2 - Section through foundation units and beams
Figure 3 - Long section through south foundation units and beams
A works train was prepared during the preceding week at Carnforth Yard and consisted of the two new caissons and caps loaded on 2 brills, the parapet and walkway beams on a pike, dry mix concrete, handrails and other sundries. A wagon of fresh ballast in anticipation of some losses during the work with fifty new plain sleepers, were also in the train. The train was marshalled at Carnforth adding the concrete components which came by rail from the BR concrete works at Taunton. The golden age of railways where rail was the natural form of transport for bridge components. Unfortunately, the wagons from Taunton had arrived at Carnforth with the beams the wrong way round and so a local crane was quickly hired to turn them round in time for the weekend. Better to be wise before the event! As is often the case with structures works, access scaffolding was required and this was constructed also during the previous week below the Up side of the bridge.
The work was carried out during a 32 hour possession and isolation of the overhead line electrification on the weekend of 24th April 1993. During this period, Preston to Carlisle services were diverted via the Settle and Carlisle line, Oxenholme to Preston or Carlisle services were replaced by buses and Penrith and Carlisle were linked by a shuttle train service. This work coincided with the waterproofing of Br 164 Lowther Viaduct near Penrith, thus making maximum use of the block and diversionary services.
Both Up and Down contact wires were slued over the Down cess for crane operations and the S & T cables on the old walkway were fixed to the Up cess rail web. Although the weight of the existing beam was quite small, the combination of lifting radii, weights of new components, most notably the beams, and access meant that it was not possible to use a single crane and so two road mobile cranes were used for the beam lifts, one at each end. The foundation units weighed 8.2 tonnes and the larger parapet beam, 17.2 tonnes.
The north crane was brought to site along the railway, travelling some 1½ miles from Scotty Bridge, No. 112 (33m 1475yds), north of the M6 motorway crossing, Bridge 109a. AWS ramps were temporarily removed so the crane could straddle the track with wheels in the four foot and the six foot. The south crane came on to the site from Tebay Yard immediately to the south of the works.
The first crane was rigged on the Up line immediately to the north of the bridge, the works train brought in and the wagon with the new beams left on the bridge, thus allowing the south crane to be brought in and rigged. A standing instruction on this part of the L&C is that no vehicles may be left uncoupled from a loco and therefore this wagon on the bridge had to be restrained with scotches in addition to the wheel brakes.
The pair of cranes together first placed the new beams on timber cradles in the six foot and Up four foot before lifting the old walkway out and onto the vacated wagon. At this point, both cranes set back, without complete derigging, in order that the dismantled walkway wagon could be propelled off the worksite and the materials wagon unloaded. The masonry above the arch was prepared for the new beams before work began on excavating the caissons, the most problematic of all the project stages.
The excavation of 3m deep holes immediately adjacent to the running line and in an embankment was seen from the outset by Area Civil Engineer's staff as a potential problem. The logical engineering procedure for excavation is to place trench sheeting and this was confirmed as appropriate from trial pits dug some weeks beforehand, showing the soil to be an ash, probably from locomotive fireboxes. When driving of the sheets started on the night, it quickly became obvious that glacial boulder clay formed the lower embankment strata and the sheets could not be driven deep enough. A decision was made to dispense with the trench support and to batter the excavation such as to reduce collapse risk. Thus the excavation for each caisson had three sloped sides but the side adjacent to the track had to remain almost vertical. A makeshift trench support was arranged on this vertical face but this stage potentially could have put the job into serious overrun as extra time had to be taken in order that safety and quality of work were not compromised. A risk at the design stage in order to reduce possessions nearly backfired and certainly made me worry!
Once the concrete blinding had been placed in the base of the excavations, the caisson foundation units were lifted into position and 30m3 of ready mixed concrete was pumped from the rough road below the Down side with the concrete pump supply boom high in the air over the electrification wires. The lifting was completed with caps to the filled caissons, walkway and parapet beams. Plate 3 shows beam lifting in progress. Various sundry items such as handrails, S&T cable troughing and OLE repositioning completed the works.
Plate 3 - Placing the New Walkway Beam by tandem lift
In retrospect, the design proved to be a bone of contention, not only because of the use of caissons in preference to piles, but also niggling details, such as the S&T cables not being in a integral trough and the way they had to be draped over the steps at each end of the walkway, as Plate 4 shows. These aside, as can be seen in Plate 5, the finished work satisfactorily addressed the problems of the old beam.
Plate 4 - New View to North
Plate 5 - New East Elevation
Next time you are travelling on the L&C north of Oxenholme, take a look out of the Up side window just north of Tebay and look for a singular concrete walkway and parapet topped with a handrail. Problem is that at 110mph line speed, less than a blink and you’ve passed it!
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