Southampton Tunnel - what lies beneath?

ok I confess - this is technically about the Big Railway but I know that more knowledgeable experts will read this if I post it here.
The trackbed of Southampton Tunnel is being lowered in connection with gauge improvements related to freight traffic to and from the port, but it is an almost forgotten fact that there is actually the remains of another tunnel UNDER the rail tunnel dating from around 1790.
The Salisbury and Southampton canal constructed a 550 yard tunnel which was hardly used and soon fell into disuse and became blocked at both ends. When the LSWR built the rail tunnel it crossed the canal obliquely in the middle and only one foot above the canal tunnel roof. To prevent future rail tunnel collapse, spoil from the rail workings were thrown into the canal tunnel to create a rubble heap up to the canal tunnel ceiling. However this measure proved partially satisfactory due to saturation of the clay and to prevent rail tunnel subsidence the canal tunnel was re-entered and a series of galleries constructed to effectively strengthen the canal tunnel and thereby safeguard the rail tunnel.
As far as I can tell there has only been one minor collapse of the canal tunnel since the 1850s and that much of it remains hidden underground and virtually forgotten.
it occurrs to me that there must be maintenance access to the old tunnel from somewhere - possibly within Southampton Rail tunnel itself. Does anyone have any knowledge of this? Or photographs, or documented descriptions of what remains down there?
For those raile enthusiasts with an affinity for canals the canal's western portal was about 100 yards north of the rail tunnel (just the other side of the BBC studios) and the eastern portal was to the south of the rail tunnel where East Park is today. There is still a Canal Walk near Debenhams for those wondering why it is there!
Any info out there?

 

i already knew why Debenhams were their

Thats very interesting information...can you tell us where the canal went from and to? presumably not as far as Salisbury if it didnt last very long...

 

With pleasure Arthur. It's a fascinating story, not least because it is virtually forgotten even in Southampton, and there are a lot of rail connections too.
The first canal to be built was the Andover and Redbridge completed around 1790. Very few traces exist because the Redbridge to Andover railway line was built on much of the former canal bed in the 1850s. However north of Romseynear Timsbury where the River Test flows down several channels there is still around 2 miles of the old waterway which can be seen on an OS map. The Sprat and WInkle railway at that point was two miles to the west, diverging from the Salisbury Railway at Mottisfont Junction. After the success of this canal, work began on the Salisbury and Southampton Canal in 1796. This used the Andover Canal as far as Kimbridge then struck westwards. Construction reached a cutting at Alderbury where a tunnel was to be built across Alderbury Common. While this tunnel was being built a horse drawn railway took goods from the barges into trucks and over Alderbury Common to the turnpike road (now the A36) and on to Salisbury. In the other direction the canal at the mouth of the Test at Redbridge struck eastwards towards Millbrook, avoiding Southampton Water which was back then just a shallow marshy silty estuary (this was long before the SR dredged drained and and constructed the Western Docks over 100 years later). The Southampton spur was driven through Freemantle and Millbrook as far as Blechynden Hill (where the Civic Centre now stands, and an 800 yard tunnel was dug. However due to poor construction and poor materials the tunnel was shortened to around 550 yards. At its eastern portal in what is now Palmerston Park, the canal divided. One branch went south to where Gods House Tower museum stands today; the other branch went to Southampton's original docks which were on the Itchen at Northam, and a link to the Itchen Navigation. The Southampton Canal lingered on but like many railways years later, was crippled by its construction debt The Napoleonic Wars reduced trade and by the time of Trafalgar in 1805 the canal was all but derelict.
The route was filled in to prevent the level canal becoming a source for cholera, and the tunnel was blocked and forgotten about until the London and Southampton Railway arrived some 45 years later.
I stumbled across all this by chance and only realised what a great untold story there was, and possibly one with its counterparts in many southern english towns and cities.
I suppose that to Canal enthusiasts, the rail pioneers and engineers were held in similar regard to the way Dr Beeching and Ernest Marples are held here!

 

I would hope that the people concerned are aware of the canal beneath the tunnel and that the lowering will not break through the invert of the tunnel?
I suppose it is being relaid in some form of slab track.

 

Indeed. I was really hoping that an NR civil engineer working on the tunnel would read this and tell us what is to be seen today. I would suspect there must be an inspection cover somewhere in the tunnel guarding a passageway down to the lower workings.