A 40ft-deep crater has appeared in middle of Mancunian Way blocking a major route around the city centre.
The sinkhole opened up around midday on Friday 14th August in central Manchester after torrential rain hit the city, with almost half of August’s rain falling in only 6 hours(1). United Utilities said the section of Mancunian Way affected by the hole could be shut for at least two weeks(2).
Engineers have been monitoring and assessing the affected area to ensure no further collapse occurs. Further work to stabilise the ground is being carried out after a full assessment was completed and proved that area is safe for inspection.(2)
The sinkhole is believed to have been caused by either a collapsed sewer or water pipe beneath the area.(3) However, experts have also highlighted the possibility of the sediment under the road becoming weaker due to the wet weather and localised flooding.
Manchester City Council and United Utilities has as yet not been able to confirm the cause of the sinkhole, though a radar survey of the ground beneath has been undertaken which should help identify the root of the problem.
On basis of the information available and the collection of historical maps held by Groundsure, our experts have also formulated their own hypotheses.
The area where the incident took place has been identified to be underlain by superficial deposits of both Devensian Till and Alluvium. These two formations mainly consist of silt, sand, clay and gravel and are usually deposited directly by a glacier or rivers. Although these formations are not the most susceptible to suffer collapse in case of heavy rains, it does indicate the presence of historic watercourses running beneath the site and in the nearby areas. The Alluvium deposits here in particular are associated with an elevated potential for subsidence due to their compressibility and through running sand issues. Heavy rain could exacerbate these issues by mobilising deposits previously bound in the rock matrix, and lead to voids beneath the concrete of the Mancunian Way.
An alternative theory which has been reported in the media is that of a historical forgotten culvert running beneath the site. Many large cities are crossed by the routes of former rivers driven underground in order to make room for development. Historic mapping held by Groundsure indicates a possible route and reason for this potential culvert. In 1851, the River Medlock is noted to the north of the site, with a long forgotten pond in Ardwick Green to the south and the lost river of Corn Brook further south. A pond of that size would not have been stagnant water, but fed from existing watercourses, and the most likely source would have been either the River Medlock or Corn Brook, depending on the flow and topography of the land at the time, with the outflow being piped to the other. The pond appears to have been constructed for private use in the late 1700s, and detailed plans of the supporting infrastructure are unsurprisingly unavailable.
The map below shows the location of the watercourses and Ardwick Green pond, with a possible route for the culverts and the location of the sinkhole also marked:
This is only one of the many cases of sinkholes documented in the last few years in the U.K. On 21 February 2014, The Met Office announced that the winter storms of December 2013 to January 2014 were the wettest period of the time since 1910. This is suspected to be the trigger for the increased incidence of sinkholes and collapse subsidence in the south and south-east of the country.(4)
Some areas throughout the UK are prone to suffer these issues, especially the ones underlain by Carboniferous limestones, notably the Mendips, parts of Wales, the Peak District, and the northern Pennines including the Yorkshire Dales.Whether this sinkhole was caused by the weather, natural geology, historical engineering, a combination of all of them or other causes, the increase in extreme weather conditions means we expect to see plenty more sinkholes across the country over the coming years.
(1) BBC News, 2015. Mancunian Way Hole: Warning not to approach road collapse. Available at: http://www.bbc.co.uk/news/uk-england-manchester-33946097 [Accessed 18th August 2015]