A large hole opened up overnight in the rear yard of a mid-terrace property in the Spinney Hills area of Leicester. The house was originally built around 1900, with an extension added more recently. The 4.50m diameter, cylindrical straight sided hole led to a claim for subsidence damage.
Ground Engineering was appointed by the Loss Adjuster acting on behalf of the householders Insurance Company, to advise on the situation and provide guidance and a solution to the problem. An investigation, including historical and mining desk study and physical investigation was undertaken to provide information on the ground both inside and outside the area of collapse, in order to establish the reason for the collapse and allow remedial measures to be formulated.
Initially, the crater was filled and a stable drilling platform constructed to enable a restricted access cable percussive boring rig to sink a 30m borehole through the centre of the area. Similar boreholes were sunk adjacent but outside the area of collapse.
The investigation confirmed that the collapse had resulted from a former coal prospecting shaft dating to the late 1700s. The investigation found several baulks of timber, some of which were rotten and believed to represent the former capping structure to the shaft.
Since the considerable depth of fill material contained a void ratio of 20%, grouting was ruled out based on the substantial amount of grout required. Ground Engineering recommended a remedial solution of a capping structure supported by the competent Mercia Mudstone surrounding the shaft to support the property and adjacent house.
