How were tunnels originally surveyed

[Robsignals]

I often take a bus along the road above Clayton Tunnel through the South Downs on the Brighton Line spotting the ventilation shafts, presume they were all construction headings originally. Apart from the enormous effort in digging it I'm impressed that with the equipment available they got all the levels and direction right, any ideas how? Marking the route is simple enough by line of sight fixing shaft centres on the way, how then was that translated into digging the shafts, getting the different depths right and both tunnel headings in the right direction at each one.

Bringing the spoil up was a major effort in itself, suppose it was simply spread around nearby - the ground looks very mounded in places. The A23 was widened at the south end 20 years ago just fitting round a shaft that had a large retaining wall built for it, road foundations can't be far above the tunnel roof!

 

[krus_aragon]

Skipping surveying and going straight to construction, the tunnel would be dug outwards from the base of the shafts as well as each end in order to complete the tunnel quicker.

I remember a story my father told me of taking a bus of school kids to Penrhyn Castle near Bangor. He fooled a few kids into thinking that this ventilation shaft of the Llandegai tunnel was actually Penrhyn Castle!

 

[Tomnick]

At the shafts, the heading would be transferred from ground level down to the bottom of the shaft, across the width of the shaft, to give them something to work towards. In shallow shafts, this could be done reasonably well using conventional surveying methods (and a theodolite), otherwise it would have been a case of dropping two plumb lines down. Working from the two ends would have been much easier - it'd be far more practical to determine an accurate heading initially (with a lot more visible!).

 

[swt_passenger]

It is possibly worth noting that the civil engineers had already developed most of the surveying methods 100 years earlier when building canal tunnels; so nothing much new was needed to adapt for rail - they would have just added the possibility of gradients within the tunnel.

 

[brompton rail]

I think we can safely assume that the Romans had these same engineering skills otherwise their fresh water collection systems would not have been so effective!

 

[pablo]

Theodolite, tape and water level, pencil and notebook.
Logarithmic and offset tables did help.
Chinese would use an abacus.

 

[Robsignals]

It is possibly worth noting that the civil engineers had already developed most of the surveying methods 100 years earlier when building canal tunnels; so nothing much new was needed to adapt for rail - they would have just added the possibility of gradients within the tunnel.

Aren't all canal tunnels meant to be straight? I've been through some that definitely aren't, they may have flooded at least a channel at an early stage to get the level right. Did they dig narrow headings? Once linked-up the main headings could be adjusted, sure that was and is still done when headings are a few hundred metres from meeting.

 

[LE Greys]

Aren't all canal tunnels meant to be straight? I've been through some that definitely aren't, they may have flooded at least a channel at an early stage to get the level right. Did they dig narrow headings? Once linked-up the main headings could be adjusted, sure that was and is still done when headings are a few hundred metres from meeting.

There's at least one railway tunnel with a kink in it as well. Kinghorn Tunnel on the Fife Coast was dug by two contractors, starting at opposite ends, who couldn't agree on the heading. They met in the middle, but at different angles, resulting in the kink.

 

[Robsignals]

At the shafts, the heading would be transferred from ground level down to the bottom of the shaft, across the width of the shaft, to give them something to work towards. In shallow shafts, this could be done reasonably well using conventional surveying methods (and a theodolite), otherwise it would have been a case of dropping two plumb lines down. Working from the two ends would have been much easier - it'd be far more practical to determine an accurate heading initially (with a lot more visible!).

Yes, but how exactly? Were precise datum points set either side of a shaft, were they built into the top of the shaft and are they still visible, did they set the centre of the tunnel and then mark the sides? Any markers are vulnerable to accidental or deliberate tampering, were watchmen appointed to gaurd them?

 

[Tomnick]

I don't know how they marked the location of the various (surveying) stations back then, but nowadays it's common to find them marked by a pin or nail set into the surface (often the joint between two kerbstones) or on top of a stake hammered into the ground. It's possible that they might get damaged (wilfully, accidentally or as works progress), but this should be obvious (and most setting-out will have some sort of check built in which would catch it anyway) and can be readily put right using the other stations that hopefully will have remained untouched.

In the case of a tunnel shaft, they might (at surface level) have been located some distance apart (and away from the shaft itself) to give a more accurate heading. This would, I suppose, be transferred at the centreline of the shaft to give the full diameter of the shaft to play with - whether this marked the centreline of the tunnel itself or an offset.

I must confess that, although I've done a fair bit of surveying and setting-out in my previous life, none of it has involved transferring anything (other than a simple level) down any sort of deep hole!

 

[3141]

Aren't all canal tunnels meant to be straight? I've been through some that definitely aren't, they may have flooded at least a channel at an early stage to get the level right. Did they dig narrow headings? Once linked-up the main headings could be adjusted, sure that was and is still done when headings are a few hundred metres from meeting.

Standedge (canal) Tunnel has a kink in it as a result of an error over the direction taken by tunnellers working from each end.

Presumably, as tunnellers improved their techniques, they would take into account the advantages of including a curve in a tunnel if that would shorten the distance to be dug and bring the line (water or rail) to a suitable level in relation to where it needed to be at each end.

 

[LE Greys]

Standedge (canal) Tunnel has a kink in it as a result of an error over the direction taken by tunnellers working from each end.

Presumably, as tunnellers improved their techniques, they would take into account the advantages of including a curve in a tunnel if that would shorten the distance to be dug and bring the line (water or rail) to a suitable level in relation to where it needed to be at each end.

At one point, it was considered difficult and expensive to put bends in tunnels. Note that every tunnel between King's Cross and York is ruler-straight. It's most noticeable between Hadley Wood and Potter's Bar, where the line runs dead-straight through one tunnel, curves round, then dead-straight through the next tunnel, then a slight curve through the station, then dead-straight through the third tunnel. There are curves north and south as well. I've never had a cab ride through there, but from one reverse-formed 225 set, I've been able to sight through the tunnels and it's astonishingly precise.

They obviously resolved this later on.

 

[michael769]

When tunneling you can never be 100% sure you won't hit bad ground. I'd be surprised if some tunnels did not have kinks or bends that were added deliberately to avoid faults or other issues.

 

[Liam]

Isn't there some myth about the original of the Blackwall tunnel being built with kinks in it to stop horses from getting scared when the saw the daylight at the other end?

 

[theboywho]

It is my understanding (I could be wrong )that OS datum points are no longer being maintained.

 

[Robsignals]

It is my understanding (I could be wrong )that OS datum points are no longer being maintained.

They don't need most of them but last I heard were keeping some as reference points.
--- old post above --- --- new post below ---

At one point, it was considered difficult and expensive to put bends in tunnels. Note that every tunnel between King's Cross and York is ruler-straight. It's most noticeable between Hadley Wood and Potter's Bar, where the line runs dead-straight through one tunnel, curves round, then dead-straight through the next tunnel, then a slight curve through the station, then dead-straight through the third tunnel. There are curves north and south as well. I've never had a cab ride through there, but from one reverse-formed 225 set, I've been able to sight through the tunnels and it's astonishingly precise.

They obviously resolved this later on.

It's always going to be easier to build straight, the shortest distance between portals will be a straight line but may need quite sharp curves just beyond. A curve will need a wider cross section to fit the coaches round, on the Waterloo - Portsmouth line Buriton tunnel between Petersfield and Rowlands Castle is curved and the new trains have a lower differential speed limit than the old slammers due to the longer coaches.

 

[michael769]

They don't need most of them but last I heard were keeping some as reference points.

About 190 very high acurracy points (known as the Fundamental Bench Marks) are still maintined and used for calibration and monitoring.

The remainder (approx 500,000) were 'abandoned' in 2000.

 

[rob w]

Have always wondered why Oxted tunnel straight for 75% and then has an S bend.

 

[ANorthernGuard]

Cowburn Tunnel certainly rises and dips slightly in the middle and Totley went through "The Cathedral" (A large natural Cavern)

 

[Wyvern]

Cowburn Tunnel is at the summit of the line. Nevertheless, when the headings met the error was less than an inch.

The old Thurgoland Tunnel on the GCR line into Sheffield had to curve quite sharply with a gradient. It was laid out in a series of straight segments which must have made gauging pretty compliicated.

 

[Tiny Tim]

Before tunnel shields were in common use, tunnels were often bored outwards from the various shafts and both ends using a small 'header'. This allowed considerable wiggle room when opening up the full diameter bore. Mistakes weren't uncommon, but only really major cock-ups would have affected the final line of the tunnel. By the time the deep level tubes were being built (using shields) the technology was fairly accurate, although the Central Line did suffer from inaccurate tunnelling, resulting in it's restricted gauge.

 

[ANorthernGuard]

Cowburn Tunnel is at the summit of the line. Nevertheless, when the headings met the error was less than an inch.

The old Thurgoland Tunnel on the GCR line into Sheffield had to curve quite sharply with a gradient. It was laid out in a series of straight segments which must have made gauging pretty compliicated.

if you sit in the back and look at the rear tunnel entrance you see it vanish, appear, then vanish again, not for long mind but there is a definite rise/dip

 

[pablo]

I wonder how the Swiss managed with their spiral tunnels?
Brigg-Domosodola and Chur-St Moritz come to mind.

 

[PaulLothian]

It is possibly worth noting that the civil engineers had already developed most of the surveying methods 100 years earlier when building canal tunnels; so nothing much new was needed to adapt for rail - they would have just added the possibility of gradients within the tunnel.

I have read that early canal surveyors also had, on the longest canal tunnels, to allow for the curvature of the earth. If an online calculator has worked it out right for me, you would have to be looking from a height of nearly 2 metres to see waterlevel at the far end of a straight 5-kilometre canal tunnel!

 

[DXMachina]

Since the angle of gravity is effectively true vertical, wouldnt an adjustment for curvature be simple to achieve by hanging a plumb weight and ensuring the cut was always at 90 degrees to it?

 

[Bushy]

Aren't all canal tunnels meant to be straight? I've been through some that definitely aren't, they may have flooded at least a channel at an early stage to get the level right. Did they dig narrow headings? Once linked-up the main headings could be adjusted, sure that was and is still done when headings are a few hundred metres from meeting.

The bends in many canal tunnels are due to ground movement. Quite a few of the UK canals were built in mining areas and subsidence is a common problem. Some bends were however due to construction tolerances.

The three tunnels at the northern end of the Trent and Mersey have impressive kinks.

I have read that early canal surveyors also had, on the longest canal tunnels, to allow for the curvature of the earth. If an online calculator has worked it out right for me, you would have to be looking from a height of nearly 2 metres to see waterlevel at the far end of a straight 5-kilometre canal tunnel!

There is only one UK canal tunnel exceeding 5 km, Standedge at 5.2, which has three parallel railway tunnels. During construction, Telford, who was employed to restart the abandoned construction, was concerned that the two headings would miss each other.

The next longest was Strood Tunnel on the Thames and Medway Canal at 3.5 km, built in 1824. In 1844 had a single track railway, built partly on the towpath and partly on trestles into the bed of the canal. A short time later the tunnel was sold to South Eastern Railway, who filled in the canal and built a twin track railway. SER and its successors have suffered the problems of falls of the chalk roof until 2004/5 when the remainder of the tunnel was lined.

There were several between 2.8 and 3.5 km that are not in use but after Standedge the next longest still in use is Blisworth at 2.8 km.

Regards

Bushy