Earlier this year the team at Geoinvestigate North West carried out an interesting borehole site investigation in Irlam to the west of Manchester. A house on the north side of the Liverpool and Manchester railway embankment had been subject to ongoing subsidence for many years and though it was suspected peat might be the problem there was no hard evidence to confirm this other than the severe and long term nature of the damage which suggested that something unusual was present in the ground below the building.
A little bit of historical digging – (call it a Phase 1 desk study by Geoinvestigate if you want) carried out for free by Geoinvestigate confirmed that the site was located on the north side of the world famous Liverpool and Manchester railway line and peat was likely to be the problem. Opened in 1830 this railway was the worlds first ticketed and timetabled public transport system which did not use animal traction power but steam locomotives – the first and most notable loco to use the line being the “Rocket”. Up to then pulley, cable systems or horse drawn carts on rails or tracks had been used.
It was also the site of mankind’s first railway accident and since then there has always been an uneasy relationship between the travelling British Public, its love for the railways and concerns about railway safety.
Engineering Geology of Chat Moss
The Phase 1 desk study of the geological plans of the area confirmed that the site was located in what is known as Chat Moss, a treacherous area of bog and marsh with a bad reputation and a place where travellers who dared to venture could disappear without a trace.
Originally the bog covered an area of about 10.6 square miles (27.5 km2). The bog is believed to be about 7000 years old but may be older with the accumulation of peat possibly commencing at the end of the last Ice Age, about 10,000 years ago. The maximum depth of peat ranges from 24 feet (7 m) to 30 feet (9 m) but thins towards the peripheries of the bog.
The peat is underlain by Devensian Glaciofluvial Sheet Deposits comprising Sand and Gravel. These deposits were formed up to 2 million years ago in the Quaternary Period in a local environment previously dominated by ice age conditions with Ice Age glaciers scouring the landscape and depositing moraines of till with outwash sand and gravel deposits from seasonal and post glacial meltwaters.
The base of the superficial soils are underlain by the Wilmslow Sandstone Formation of the Triassic laid down in a hot dry environment comprising loess, dune and evaporite deposits when Manchester was much closer to the equator and much warmer and drier than it is today – mores the pity!.
A great deal of Chat Moss was reclaimed with drainage channels in the 19th Century and even today in the places where moorland still exists drainage works are still required to stop the area reverting to bog. Much of Chat Moss is now developed and the former peat bog makes up 30 per cent of the City of Salford in Greater Manchester. Parts of the peat bog which remain relatively undisturbed were designated Sites of Special Scientific Interest in 1989 and European Union Special Area of Conservation, known as Manchester Mosses. Commercial peat cutting is still carried out in the vicinity of Irlam.
Engineering the Railway Crossing
It was known by the railways engineers – led by the iconic George Stephenson and his trusted and highly capable assistant John Locke that the creation of a stable track bed across this 4 .75 miles (7.6 km) of treacherous, deep peat bog was going to be one of the most difficult challenges faced by the constructors of the line in the late 1820s when work started. Stephenson’s solution was to “float” the line on a bed of bound heather and branches topped with tar and covered with rubble stone.
As it was found impossible to drain the bog at Chat Moss Stephenson acting upon the suggestion of one of his men on site (believed to be Robert Stannard) began constructing a large number of wooden and heather hurdles. These were sunk into the bog using stones and earth until they could provide a solid foundation. At one point it was reported that tipping went on solidly for weeks until a reliable foundation had been created. This is one of the earliest recorded uses of Geo-mats in a major civil engineering project.
Even to this day the recently refurbished track bed across Chat Moss still floats on the hurdles that Stephenson’s workers laid in the 1800s even though the line now supports locomotives 25 times the weight of the Rocket, which hauled the first experimental train over the Moss in January 1830.
Today if you stand near the lineside you can feel the ground move as a train passes – a reminder that the embankment floats over what still remains shaky and unstable ground.
The line opened on 15 September 1830. However, the festivities of the opening day were marred when William Huskisson, the popular Member of Parliament for Liverpool, was killed in front of the many illustrious dignitaries attending the opening that day including the Duke of Wellington.
Accounts of the accident suggest that as there was an interval between the delayed passing of the trains carrying difnatories in opposite directions from Manchester and Liverpool. Huskisson being on the train pulled by the locomotive “Northumbrian” which had stopped to await the arrival of Rocket seized the opportunity to alight and stroll alongside the train. He then caught Wellington’s eye through the Duke’s carriage window. As the two were politically estranged, it was a golden opportunity to commence a reconciliation. The Duke inclined his head, someone opened the carriage door, and the two swapped pleasantries.
Then, people noticed Rocket approaching on the Northern line and shouted a warning. The Austrian ambassador was bodily pulled into the carriage, but Huskisson panicked. He tried to climb into the carriage, but he gripped the open door, which swung back, causing him to lose his grip. He fell between the two tracks, but the Rocket ran over his leg which was fouling the rail, shattering it. He is said to have uttered the tragic words “I have met my death — God forgive me!”
Not only was the crossing of Chat Moss a major construction feat but the line as a whole being 35 miles (56 km) line was a remarkable engineering achievement for its time, beginning with the 2,250 yards (2,057 km) Wapping Tunnel beneath Liverpool from the south end of Liverpool Docks to Edge Hill. This was the world’s first tunnel to be bored under a city. Following this was a 2-mile (3.2 km) long cutting up to 70 feet (21.3 m) deep through rock at Olive Mount, and a nine arch viaduct (each arch of 50 feet (15.2 m) span), over the Sankey Brook Valley, around 70 feet (21.3 m) high.
Given the lines great engineering accomplishments and history and its association with George Stephenson, the Rocket, the Duke of Wellington and railway safety you can understand why some railway enthusiasts are today calling for the line to be made into a World Heritage Site.
Recent Borehole Site investigation
As expected from the desk study the borehole investigation carried out by Geoinvestigate encountered peat. The log of the borehole is presented below.
Peat or clayey peat extended to a depth of 2.20m with very loose and loose or soft organic clayey sand and sandy clay extending to 3.90m. The horizon of stiff slightly gravelly clay with roots at 3.90m may correspond with the end of the last Ice Age. The laminated clay below may be varved glacial lake deposits.
The peat at 1.70m thickness was not as deep as expected possibly because the area as depicted in Henry Pyall’s picture of the Crossing of Chat Moss may have been subject to partial excavation by peat cutting many years before the house was built and/or the site was located towards the southern fringe of Chat Moss where the peat became thinner.
However the peat and weak organic soil below it was sufficiently deep and compressible to cause serious subsidence and structural damage to the building over a considerable period of time. The solution was to underpin the building on sleaved piles extending below 3.90m.
Geoinvestigate North West provides borehole site investigation services to Wrexham, Stoke-on-Trent, Bolton, Crewe, Salford and Stockport. Please let us quote for your next borehole site investigation in Salford or Stockport, Chester or Oldham.