Safe as houses? – subsidence
Historical mining often results in subsidence. A housing estate in Hatfield, UK, faced this issue until remedial work corrected the problem. Michael Forrest reports.
In the early decades of the 20th century, the area surrounding the Briars Lane suburb of Hatfield, UK, was the site of chalk mines – shallow underground workings in the geological horizon known as the Cretaceous Upper Chalk (shown in green on the map below). The only record of the mine is a single kiln marked on a historic Ordnance Survey map.
Between six and 20m below the surface is a labyrinth of shafts and horizontal workings from which the chalk was extracted. There appears to be no specific mined horizon, nor any directional trend, and therefore the assumption is that the miners followed, probably by colour and softness, a particularly pure chalk.
The workings were typically two metres wide and a maximum of 3.5m high. Up to four levels were tunneled in any one location but their orientation appeared completely random. Mapping of the workings was made doubly difficult by frequent roof collapses. As the workings were artisanal and unmechanised there are no existing plans of the tunnels and stopes, nor of the amount of material mined. The last workings were in the 1920s.
A decade later the area above the chalk became the site of council housing, but it was not until 1978 that the first subsidence was recorded in an area 3.6x4.6m. In 2001, a six metre-deep crownhole developed, collapsing a footpath and damaging a property. The risk of further collapses required action, highlighted by the evacuation of some houses and a school in 2005.
The local authorities, Welwyn Hatfield Borough Council and Hertfordshire County Council, together with English Partnerships, the national regeneration agency, began remedial work. The objective was to provide a solution that would allow safe occupancy of the site, ensure minimal future ground movements, remove site blight and restore public confidence, while minimising disruption to the community.
It is usual in civil engineering projects for the work to be specified by an overseeing agency and let to a contractor to carry out the work and take the operating risk. In this case, the risk remained with the client allowing the best technical solution to be developed and implemented by three UK companies – Hyder Consulting, the designer and contract supervisor; the contractor BAM Ritchies, and their consultant RJM Ground Solutions.
The first challenge was to ascertain the extent of the chalk workings and volume of voids. An extensive phased ground investigation was begun in August 2006 with dynamic probing to 20m. The probes comprise a steel rod driven into the ground to measure the penetration rate for each calibrated blow. The lower the resistance, the greater the penetration. Although crude, the technique allowed rapid progress with 1,589 holes completed in a few weeks with minimal interference to residents and traffic.
In addition to the dynamic probes, 82 boreholes were drilled in locations identified by the probes for a more detailed view. The boreholes were used to insert CAL-S downhole lasers and CCTV. Microgravity surveys completed the underground picture. The survey work revealed extensive voids where mining had taken place and also areas where subsidence had weakened the ground. These required separate solutions, bulk infilling of the voids and compaction grouting of the weak areas. Again minimum disruption to residents and their property was the prime consideration.
Normally cement is mixed on site with a bulking agent such as pulverised fly ash (waste from power stations), however, the Upper Chalk is an aquifer and therefore leached metals within the ash may have contaminated water. The chosen filler was ground limestone, which is chemically similar to chalk. A foaming agent from a mobile plant was also used to infill the voids and combined with the ready-mixed cement brought to the site to avoid the associated noise and dust.
In the compaction grouting a three-metre square grid over targeted features was the operational level. With bulk infilling, the objective was to fill the void without over pressuring and to ensure adequate flow was achieved to reach distant parts. Down hole CCTV and dynamic probing were used to monitor the void filling.
A thin cement mix was pumped out at high pressure as the borehole probe was gradually withdrawn from the base of the hole. An inflatable packer system ensured that the grout remained in the hole. A Lutz meter was used to monitor the flow and reconciled against input to the grout plant. The danger of infilling and grouting is that the disturbance related to drilling and pumping of the cement mix may cause ground movement. To monitor this, the consortium carried out structural leveling surveys and installed automated tiltmeters on buildings. These instruments were linked to the Internet and could also provide immediate notification of movement via a mobile telephone. This treatment work commenced in August 2007, with site works completed by the following March.
To ensure confidence in the remedial work a series of validation exercises was carried out. The results of these exercises and data from the initial ground investigation survey were entered into a Rockworks computer database.
Accessing the data allowed a continuous review of progress and enabled real-time discussion of the effectiveness of the works onsite. Of specific note was the integration of the results of 375,000 dynamic probe blow counts and borehole records that provided an interpretation of the voids and disturbed ground. These data were validated against the input of grout and foamed concrete measured by the Lutz meters.
The success of the treatment was checked against drilling records of any large voids and excess grout intake during pumping, both in total and vertically in the hole. Another 251 dynamic probes were taken post-treatment. Although the technical aims of the project were met, a greater satisfaction resulted from the reaction of the residents who were allowed to move back to the area in August 2008. Since then, properties have been bought and sold consigning the blight over Briars Lane to history.
This article is based on the MinSouth British Tunneling Society joint meeting held at the Institution of Civil Engineers earlier this year. The presentation was given by Chris Milne, Geotechnical Consultant, Hyder Consulting UK Ltd; Andrew O’Donovan, Senior Geotechnical Engineer, BAM Ritchies; and John Rigby-Jones, Director, RJM Ground Solutions Ltd.