Clovercroft Mill was to be one of Lancashire’s latest redevelopments, but with initial survey results indicating loose materials thought to be the remains of old mine works, the development of the mill seemed in doubt.
To locate and define the extent of mine workings on the Clovercroft mill site, a former mill undergoing extensive redevelopment.
The previous site investigation undertaken by a third party concluded that there were voids in the boreholes drilled in the factory building and loose materials thought to be collapsed mine workings north east of the site. These findings meant it was deemed necessary to carry out further investigative works to confirm the extent of the mine works.
Intrusive surveying would have required a large number of boreholes to be drilled to determine the extent and presence of the mine workings, a timely and costly exercise.
With extensive experience and expertise in non-intrusive geophysical surveying, Met Consultancy Group (Met) recommended an integrated geophysical survey which was able to provide a more cost effective characterisation of the sites sub-surface conditions.
The survey area consisted of a single story factory building and its surrounding grounds. The factory building had three main rooms:
• Show room, with a carpeted floor and partitioned walls which joined the original mill building
• Main factory building with concrete floors
• Second smaller room to the north of the main factory floor.
Previous experience has indicated that combined technology provides the best and most accurate results, especially where ground and surrounding materials differ. The team decided to use two geophysical techniques to ascertain the depth and exact location of the mine workings.
A microgravity survey would be used to ascertain the mine workings and voids at a depth greater than 5 metres and the electromagnetic (EM) survey would look at shallower depths. The microgravity survey was conducted using a Scintrex CG5 microgravity meter. Gravity data was captured from grids with 4 metre centres. Corner points of the grid were set out using a total station. A base station was established inside the single story building on a level stable concrete step. The station was revisited at intervals of 1.5 hours throughout the survey. Eleven percent of readings were repeated with minimal repeat errors highlighted. This demonstrated a high level of data integrity and certainty in the results produced.
A Geonics EM31 turbo was used for the electromagnetic part of the survey. Readings were taken at 10cm intervals along survey transects spaced 1 metre apart.
Microgravity Results and Analysis
Gravity results were reduced to the Bouguer Anomaly using in house software and had a first order polynomial surface removed from the data to account for the local dip of underlying geology. The survey successfully located areas of low density within the site boundary. These are shown in blue on the figure overleaf. Highlighted areas show negative anomalies, some indicated voided areas, and others were interpreted as loosely compacted material.
The microgravity survey successfully defined the extent of the voiding and other loose material in the area and was able to identify other anomalies as possible mine workings. The microgravity investigation also identified a void in an area not covered by the previous site investigation. Another high amplitude anomaly lying at the edge of the survey area was identified however it was not possible to categorise. This could have been caused by a change in the dip of the underlying strata or a void located to the west of the site. As indicated by the initial site investigation, the mine workings were located to the north east of the single story building.
Electromagnetic Results and Analysis
The EM survey identified two anomalies with anomalously high conductivity and phase angle values which were attributed to metallic surface features. Several areas of low conductivity and phase angle were also identified and attributed to areas of concrete visible on the surface. A linear feature of non-metallic origin was also identified to the north east of the site. No anomalies indicative of subsurface voids were detected by the EM survey, supporting the findings of the original third party report that voiding was only present at depth.
The combined surveys successfully delineated the extent of the mine workings at depth , and after these were remediated by grouting, allowed the project to move forward safe in the knowledge that no shallow workings were present.