Sinkhole caused by collapsed Chalk Mine at Pinner Wood School, Harrow, Greater London

A large sinkhole opened in the car park of Pinner Wood School, Harrow, forcing its immediate temporary closure. Geotechnical surveys and site investigations have been commissioned to determine the cause of the collapse and assess the stability of the land.

Key points:

•    July 2015 an 11ft wide and 6ft deep hole suddenly appeared in the car park of Pinner Wood School; a primary school with over 450 pupils. 
•    Preliminary investigations determined the hole to be caused by collapse of a disused chalk mine shaft. 
•    The school was temporarily closed with classes relocated, pending the results of geotechnical surveys assessing the stability of the land beneath the school. 
•    The surveys revealed the presence of a previously uncharted chalk mine dating back to the 19th century approximately 65ft below the school buildings. 
•    The school remains closed with a strong possibility of permanent closure due to the presence of these chalk mines compromising the ground stability.

 Image: Left – Pinner Wood School. Middle – hole in school car park. Right – Chalk mine revealed from survey beneath school. Photo credit - SWNS (The Mirror)  

Image: Left – Pinner Wood School. Middle – hole in school car park. Right – Chalk mine revealed from survey beneath school. Photo credit - SWNS (The Mirror)
 

The geology of greater London is largely dominated by the Thames group (Clays, Silts and Gravels) and the White Chalk Subgroup. The rapid expansion and development of London in the past demanded abundant building materials such as bricks, tiles and cement. In order to facilitate this need, extensive mining of chalk and clays was performed leaving the area of Greater London littered with abandoned mines, many of which are unrecorded and awaiting discovery. 

The chalk mine beneath the Pinner Wood School was most likely from the 19th Century; a period where strict mining regulations were not (well-) enforced and it was common for mines to have no abandonment plans. Laser imaging surveys find the mine 65ft beneath the school to be in a state of breakdown with there being evidence for two of the tunnels having suffered from partial roof collapses. The settling of roof material into these voids allows the propagation of voids to the surface which could result in further holes and even collapse of structures. This of course poses a significant risk to any staff and pupils that may be in the school should it reopened. The recent discovery of this new data is has called for meetings between school administrators and Harrow Council members to deliberate the permanent closure of the original Pinner Wood School site due to the serious risk of future ground instability.

Reference:

http://metro.co.uk/2017/03/24/primary-school-car-park-crumbles-after-chalk-mine-collapse-6531615/