Principal Inspection Report
Principal Inspection Report
Ashted Tunnel – Functional Location: WZ-001-019
Birmingham & Fazeley Canal (Digbeth Branch)
Inspection date: 18th October 2006
Executive Summary
Ashted Tunnel is a 102m long, brick lined, one way working tunnel with one towpath. The B4114 dual carriageway crosses over the tunnel line but is supported independently from the tunnel lining by a concrete raft on bored piles.
Extensive repairs where undertaken in 1984/85 with a new concrete surface to the towpath and general repointing to the mortar joints below normal water level.
The South Portal has been rebuilt in the past but the date of construction is unknown. The tunnel is in reasonable condition but with some open construction joints at both ends of the tunnel and areas of recessed mortar joints.
The old wooden handrail to the towpath is low in height and loose in parts. It is recommended that this handrail is replaced on health and safety grounds in the short term.
Introduction
The original detailed survey of the tunnel was undertaken by the former Mining Section in March 1984 (above and below normal water level). This original survey is used as a benchmark for subsequent inspections and the detailed tunnel sheets are updated following each inspection.
The Principal Inspection was undertaken from a workboat by Chris Reynard and Jon Muir – Tunnels & Confined Spaces Team, Leeds on 18th October 2006.
The tunnel was classed as a low risk confined space for the inspection due to the nature of the tactile tunnel inspection.
A walk over the surface above the tunnel was undertaken to check for any changes.
[ Addendum: Below water inspection undertaken by Chris Reynard and Jon Muir – Tunnels & Confined Spaces Team, Leeds on 7th January 2009. Lucas Brown and Simon from West Midlands waterway were also in attendance and undertook a below water level P.I. on the lock at the north end of the tunnel (upstream end).
The tunnel was classed as a low risk confined space for the inspection due to the nature of the tactile tunnel inspection. The maximum water depth was recorded as 430mm (reduced water level achieved by drawing down the pound between the locks – water level reduced by approximately 1.55m) and there was no significant silt deposits on the invert. A good view of the invert was obtained through clear water, the lower side walls being largely exposed.
Weather – cold with light snow – canal iced over.
Previous dewatered inspection on 21/05/1991. ]
The weather on the day of inspection was dry with mild temperature but overcast.
The previous Principal Inspection was undertaken on 15th January 2002.
The previous Intermediate Inspection was undertaken on 25th October 2005. No major deterioration was noted.
The tunnel is inspected on a standard five year principal inspection interval as recommended by the Tunnels & Confined Spaces team.
The dewatered inspection was last undertaken in 1991 and is now overdue.
Description
Ashted Tunnel is a brick lined arch shaped tunnel with brick footings to the side walls and dished brick invert. The tunnel portals are also constructed in brickwork.
The tunnel is a one way working tunnel with one towpath and operates on a line of sight procedure although there is a lock immediately adjacent to the tunnel at the north end.
The tunnel was constructed at some time between 1783 and 1790 with John Smeaton as the engineer for the canal. There has been no detailed historical search of information relating to the tunnel.
The tunnel is 102m in length and approx. 3.8m wide at normal water level including the towpath (towpath varies in width between 1.24m – 1.50m). There is between 1.1 – 1.5m water depth below normal water level and 2.48 – 2.77m headroom above normal water level.
There is one former shaft at ch.28m from the south end (probably an old loading shaft from former factories above tunnel). Probing has revealed that the shaft is almost certainly fully backfilled with no evidence of stagings or a cap (from ground investigation for surface development in 1988).
The tunnel was constructed by cut and cover techniques between the South Portal and at least ch.37m from the South Portal.
Fill material extends down to at least shoulder level of the tunnel.
The depth of cover over the tunnel is shallow with approx. 4m depth of maximum cover.
The tunnel is located in an urban area in Birmingham and the B4114 (Jennens Road) dual carriageway crosses over the tunnel.
The tunnel previously carried a road crossing (Ashted Row) and was also assessed as a road bridge up to 1973 when a relieving bridge was constructed. The new dual carriageway road crossing was constructed in 1991 and is not carried by the tunnel lining; it is independently supported on a concrete raft with bored piles.
There is good access to both ends of the tunnel.
The geology surrounding the tunnel briefly consists of Bromsgrove Sandstone overlain by superficial glacial deposits consisting mainly of sands and gravels.
There was a site investigation of the tunnel in 1973 and a ground investigation in 1988 for a proposed surface development.
In 1984/85 extensive repairs were undertaken with re-pointing of the mortar joints below water level and occasional brick patch repairs below water level. Also at this time the towpath was raised and surfaced in concrete.
The South Portal has been rebuilt but the date of construction is unknown.
The canal tunnel is orientated in a general north – south direction with the towpath on the east side.
There are new tunnel safety signs at both portals, these are not of the standard BW Tunnel Safety Standards design but are adequate.
There are no nearest telephone location signs.
Observations
Tunnel (above normal water level)
The tunnel remains in reasonable condition with brickwork lining largely intact.
The tunnel was generally dry on the day of inspection however there are some minor areas of slight dry calcite deposits on the tunnel lining indicating previous water seepage. There is one damp area of lining with very occasional dripping water infiltration at ch.70 – 76m, the water is dripping onto the wooden handrail which has caused the top rail to become rotten here.
There is a series of open circumferential construction joints at both ends of the tunnel (between ch.0m – 20m & ch.85m – 102m approx.). The bricks are occasionally perished along the construction joint edges. There are also some areas of open mortar joints surrounding the open construction joints.
The shape of the tunnel arch remains good with occasional as built slight misalignment and bulges. There are some occasional boat rubbing marks on the offside shoulder but generally the profile is adequate for narrow boat passage.
The internal dimensions were re-measured at three locations:-
Ch.12m – Tunnel 3.83m wide at water level including 1.24m wide towpath.
Tunnel 2.48m high from water level to crown.
Ch.50m - Tunnel 3.85m wide at water level including 1.50m wide towpath.
Tunnel 2.71m high from water level to crown.
Ch.97m – Tunnel 3.83m wide at water level including 1.30m wide towpath.
Tunnel 2.77m high from water level to crown.
The profile splays outwards slightly at the north end beyond ch.99m.
In addition the height from water level to crown of tunnel was measured at 2.61m at ch.45m.
The South Portal (ch.0m) has been re-built at some time and the rebuilding works extend to ch.4m inside the tunnel. The new blue engineering brick lining is all intact and well mortared.
The original brick lining in the remainder of the tunnel is also generally intact with occasional open pocket holes in the lower side walls (some holes filled with loose bricks) and occasional slightly perished bricks.
There are areas of recessed mortar joints throughout the tunnel.
Deterioration of these recessed joints and of the isolated perished brickwork continues at a slow rate only.
There is a hairline circumferential crack around the arch ring brickwork at the north end (ch.101m in the crown). This is a long standing defect and there is no further deterioration or movement.
The condition of the tunnel lining below normal water was not checked due to the limited access from the workboat in this narrow tunnel. However the lining below water level was extensively repaired in 1984 with general repointing repairs and there is no evidence of slumped brickwork in the offside wall above water level (slumped brickwork above could indicate missing brickwork/undermining below). The lining remained in reasonable condition on the last de-watered inspection in 1991 but a further dewatered inspection to check on this is now overdue.
[ Addendum 8/01/09: Tunnel (below normal water level)
The lining remains largely intact and in reasonable to good condition with only occasional slightly perished brickwork. The repointing repairs remain intact and there are occasional brick patch repairs on the offside and extensive brickwork repairs to the lower wall on the towpath side. The repairs on the towpath side appear to be placed to protect the original timber foundation which is exposed between ch.33 – 37.5m and is slightly rotten resulting in some slight undercutting of the towpath wall.
There is a short brickwork repair to the invert between ch.81.4 – 82.5m with some concrete chamfering to both sides of the repair.
The water level was reduced in the tunnel by approximately 1.55m leaving a reduced water depth of 430mm at ch.15m and 230mm at ch.42m, i.e. invert is not level, some of invert exposed towards north end of tunnel (i.e. upstream).
Some missing front skin brickwork was noted on the approaches to both ends of the tunnel (offside wall at south end and immediately below rotten timber curbing on towpath side at north end). ]
Shaft
The brick ring to the shaft (probable former loading shaft) at ch.28m sits slightly proud of the tunnel lining (as built) but the brickwork seal is all intact and tight. There are some dry calcite deposits covering the brick ring to the shaft indicating previous water seepage either through the shaft fill material or around the old shaft lining. There is currently no evidence of the shaft on the surface, i.e. no depressions on the surface.
Towpath
The towpath was repaired in 1984 at the same time as the below water level repointing repairs. The towpath is surfaced in concrete which is in good condition with minimal standing water.
The old wooden handrail to the towpath is generally in reasonable condition but with poor areas. The handrail is low, measured at 0.75m from towpath level to top of top rail. Handrails should be 1.1m high to comply with BW Tunnel Safety Standards.
The bottom rail to the handrail is missing at both ends of the tunnel, between ch.0m – 26m & ch.72 – 102m. The top rail is rotten between ch.74m – 79m caused by occasional dripping water infiltration through the lining around this area. The wooden stanchions are loose at ch.15m and between ch.65m – 69m where the handrail is pushed outwards.
The towpath is quite well frequented by pedestrians, joggers and cyclists as was witnessed during the inspection period. This is no doubt due to the close proximity of the tunnel to Birmingham city centre.
There are a series of strip lights down the centre of the crown throughout the tunnel which provide good illumination of the towpath and tunnel in general. These lights are maintained by Birmingham City Council and were all working on the day of inspection.
Portals
The South Portal is a new brick portal but the date of construction is unknown.
The portal is intact, well mortared and in good condition and comprises English bond brickwork with a four skin arch ring to the tunnel (with a rounded inner skin), concrete coping course to the top of the wall, corbelled string course (three courses deep) above the arch ring and buttresses to both ends of the wall, the portal wall is also battered below string course level on either side of the tunnel.
There is some general vegetation/tree growth above the portal. Since the last Principal Inspection one tree to the right hand side has been cut back but the tree near the centre of the portal remains as before.
The retaining wall to the side of the portal wall on the towpath side was previously noted as in poor condition and unstable on the last Principal Inspection this has now been temporarily supported with scaffolding and is fenced off. The support works were undertaken by Birmingham City Council who is responsible for this retaining wall.
The North Portal is constructed in brickwork and is situated immediately downstream of Ashted Lock. The portal is in reasonable condition with isolated perished bricks and areas of recessed mortar joints in the upper wall with vegetation growth through the wall. There is a one and a half brick thick arch ring to the tunnel (with rounded inner edge) and a parapet wall with rounded brick coping course to the top of the wall.
There is an existing displacement crack between the portal wall and the retaining wall constructed at a right angle to the portal on the towpath side. This crack is filled with an old rubber/bitumen type expansion filler and shows no signs of further movement.
The retaining wall continues round the edge of the towpath around the bottom gate of Ashted Lock. There are some vertical cracks in the retaining wall but the wall is in reasonable condition.
Tunnel safety features & signs
There are new Tunnel Safety signs at each portal. These signs are not of the usual design complying with BW Tunnel Safety Standards but they are adequate and contain the appropriate information.
There are no nearest public telephone location signs at the tunnel.
There is good access to both ends of the tunnel.
[ Addendum 8/01/09: New tunnel information and safety signs have been installed at both ends of the tunnel although the information on the length of the tunnel is incorrect. ]
Surface
The surface remains unchanged since the last Principal Inspection with no surface development.
It was noted that the rough ground above the south end of the tunnel had been used as a lorry park in the past but this activity has now ceased and the ground is fenced off and has become slightly overgrown with vegetation.
Discussion
The tunnel remains in reasonable condition but there are health and safety concerns over the low and in some areas poor condition of the old wooden handrail to the towpath.
It is recommended that the handrail is replaced with a new 1.1m high handrail in the short term particularly as the towpath appears to be well used.
The reason for the construction of a new portal wall at the south end is not known but there used to be a large retaining wall above the new South Portal to an old industrial bakery building above.
The North Portal remains largely unchanged from the last Principal Inspection and still requires vegetation clearance and long term pointing repairs.
The retaining wall to the towpath side of the North Portal contains some vertical movement cracks. Ownership of this wall requires determining, if it is BW responsibility then installation and measurement of demec points across the cracks is recommended to check for further movement. However, any slight movement in this wall is unlikely to affect the tunnel.
The main defect in the tunnel is the presence of open construction joints at both ends of the tunnel (between ch.0m – 20m & ch.85m – 102m approx.) with some areas of recessed mortar joints around the construction joints. The brickwork to the edge of the open joints continues to weather at a slow rate and is not of immediate concern, however long term pointing repairs are recommended for general maintenance of the lining.
The lining below water level required extensive repointing repairs and occasional brick patch repairs in 1984 and checks for further deterioration should be undertaken by means of a dewatered inspection.
[ Addendum 8/01/09: No new repair works identified. ]
Assessment
Name:Ashted Tunnel
Functional Location:WZ-001-019
Chainage Code:WZ-00057
National Grid Reference:408014 287536 (centre of tunnel)
407986 287578 (north portal)
408041 287495 (south portal)
Location:Nechells Green, Birmingham
Length of tunnel:102m
Maximum cover:4m
(intrados crown to ground level)
Minimum headroom:2.4 m
(above normal water level)
Design of tunnel:S1 (Single one towpath)
Main construction material:BRK (Brick)
Recommendations
Recommend replacement of the low wooden handrail in the short term with new post and rail handrail to 1.1m height above the towpath surface level. This is a health and safety related recommendation to comply with BW Tunnel Safety Standards.
Recommend a full dewatered inspection is undertaken before the next Principal Inspection to check on the lining below water level. The last dewatered inspection was undertaken in 1991 and therefore the dewatered inspection is now overdue. The lining below water level required extensive repointing repairs in 1984 and checks for deterioration are now required.