Pontcysyllte Aqueduct

We awoke to heavy rain and high winds.  Looking up through the houdini hatches proved difficult as they were covered in leaves.  However there was a break in the weather at 10am with no wind and a blue sky.  A decision was made to top up the water tank from the adjacent CRT water point and then cross Pontcysyllte Aqueduct whilst the weather held.

Pontcysyllte means ‘The bridge at Cysyllte’ in welsh.  The southern side of the aqueduct is Frontcysyllte and the northern side Trevor.  The Dee Valley runs roughly west to east here and the aqueduct was built to cross the valley utilising the most suitable natural crossing point. 

The original plan was for the canal to cross the Dee Valley and continue a northern route to Chester.  A feeder was constructed between Llangollen and Trevor to supply the canal with water.  What actually happened was the rapid expansion of railways made the proposed canal from Trevor to Chester financially unviable.  However the feeder to Llangollen was subsequently widened for use by narrowboats.

We had previously crossed Pontcysyllte Aqueduct in 2001 during our first one week canal holiday.  On that occasion there was little time to closely examine the construction of the aqueduct.  Living on Waiouru gives us more time to enjoy the sights.

The aqueduct consists of a cast iron trough supported on 18 hollow masonry columns.  The columns taper ( like a four sided pyramid).  It’s 307 metres long and at it’s highest point, 38 metres above the river.

I found the method of fixing the masonry rather interesting.  A mixture of lime, water and bulls blood was used.  Thomas Telford was the civil engineer but the principal canal engineer was Jessop.  Telford had previously built something similar, albeit on a smaller scale.  Using cast iron arches saved on weight which meant the columns could be hollow and the span of the arches greater. 

The cast iron trough that carries the water is not bolted to the iron girders underneath.   Instead lugs on the girders hold the trough in place.  I was interested to note the sides of the trough a made from many sections of cast iron plate that are bolted together through the flange.

You may have noted in the above photos that the angle of the flange joints are not vertical.  They follow the angle of the curve in the arch below.  I assumed this was to give additional strength to the structure but have since read it’s purely decorative.  The joins in the flange were sealed with a combination of Welsh flannel, white lead and iron particles.

The footpath is suspended above the trough.  This allows water to pass around the hull of boats. 

The lack of a safety rail on the off-side makes for a great view from the boat.

This next photo gives an idea of the cast iron arch frames.  There are four think skeletal cast iron frames to each arch.

  

For a 210 year old structure the design is revolutionary being very light.  It’s also very stable because it carries a static load.  A road or rail bridge has a static load and a ‘live load’.  When there are no trains on the bridge the load is static.  But when a train starts to cross the bridge there is an impact load and then a live load on the bridge.  The aqueduct carries water so the static and live loads are the same.  When a narrowboat crosses the aqueduct it displaces it’s own weight in water (Archimedes in the bath) so the boat doesn’t add anything to the load.