Historical chalk mines below Pinner Wood School

The unknown location of old historic chalk mines, shafts and adits beneath the streets, properties and structures of London present serious issues for the local communities. A major concern for these communities is that collapses of these workings could occur at any time and without any prior warning. When they do occur those who are affected want to know the full extent of the problem; is it localised or extensive? This paper examines the ground investigations and surveys of historical chalk mine workings beneath a community primary school in Pinner, Middlesex and the specific use and analysis of subsurface laser scanners to identify the precise location and extent of the previously unknown and unmapped workings.Following a ground surface collapse in 2015, intrusive ground investigations identified the presence of a loosely backfilled chalk mine shaft, extending to some 24m below ground level (bgl). Further investigations included the use of dynamic probes, window sample boreholes and hollow stem auger boreholes. Rotary boreholes, with measurement of penetration rate, percentage flush return, applied load on the drill head and rotary speed, were used to provider greater clarity regarding the ground conditions. These intrusive works were supported by site wide geophysical investigations, utilizing Ground Penetrating Radar (GPR) and Electromagnetic (EM) mapping. Specialist Geospatial Engineers, Geoterra deployed their geo-referenced Cavity Auto Scanning Laser System (C-ALS) to carry out multiple subsurface void laser scan surveys and then processed the data using their bespoke Cavity Profiler software.The use of subsurface laser scanning has enabled large sections of open mine workings to be quickly and accurately mapped, in three dimensions, saving significant time and cost in the process of mapping the extent of the former chalk mine workings. Laser scanning also reduced the health and safety risks associated with intrusive investigation methodologies as well as being far more sustainable, compared to the use of heavy plant and associated materials required for intrusive investigation. Analysis of the further ground investigation and survey data by Peter Brett Associates has enabled informative decisions to be made with regards to effective remediation solutions and mitigation measures.