Drilling and Grouting: Bulk Infill Stabilisation of sub-surface voids
Shallow voids pose a risk to surface building and sub-surface infrastructure because collapse of the ground overlying the voids can result in voids migrating to the surface, leading to crown-hole development. This poses a risk to both foundations, structures themselves, and below-ground infrastructure. One option for remediation is bulk filling of these voids by drilling and grouting.
The depths and thickness of the void(s) should be identified by desk study and subsequent detailed Ground Investigation supervised by a suitably qualified Geotechnical Engineer/Engineering Geologist. Generally, if there is more than 10x the extracted thickness (i.e. the original height of the mining void) of rock cover, then there is low risk of the void migrating to the surface. If the rock is heavily fractured, or vulnerable to weathering, then greater cover may be required.
Treatment to mining voids in the UK requires the permission of the Coal Authority. Special Publication 32 “Construction over Abandoned Mine Workings’, 1984 provides information and guidance for engineers and geologists who engage in mine treatment. The application requires details of the buildings, the design of the grouting strategy including the hole spacing and grout mixes and the method of drilling (including flushing details).
- Identify the correct treatment strategy: This is highly dependent on the subsurface geology i.e. presence of faults, thickness of superficials, influence of ground water, dip of strata, surface topography, depth to coal seam. Undertake any further exploratory drilling and testing if further information is required.
- Determine treatment area: Treatment is required around any proposed structure or high load bearing ground that has less than 10 x thickness of rock above the top of the worked coal seam.
- Determine off-set around building. Void migration to the surface can spread laterally, so it is necessary to allow for this angle of draw. Generally, an angle of 15 degrees is taken in rock, and a function of the angle of shearing resistance in soil. Generally, an off-set of 2-8m will be required.
- Determine primary treatment grid: Holes for drilling and grouting are generally spaced on a square grid. Where workings are generally absent or if there is uncertainty about their extent, a wide hole spacing of 6m is often used as a primary grid.
- Where voids greater than 500mm are encountered, it may be necessary to install a perimeter around the treatment area to prevent grout loss outside the intended treatment area. These would normally be drilled at 1.5m centres and be filled with gravel prior to grouting with a more viscous mix, usually 5:1 PFA/OPC mix.
- Drilling holes: Each of the holes within the treatment grid will then be drilled and grouted. The hole diameter is usually 75mm for infill holes and 100mm for perimeter holes or holes where gravel is required. . The drilling process involves rotary percussive rigs, firstly installing casing seated into rockhead, then drilling to 1m below the base of the seam.
- Grout injection: Commencing at the lowest dip side, grout is injected through a flexible tube called a tremie from the base of the drilled hole to the surface. The grout mixture is commonly 10:1 ratio of PFA/OPC. Where grout take is very high, sand may be added to the mix. Where voids greater than 500mm are encountered, gravel may be added.
- Depending on various factors including grout take, nature of backfill, collapse of overlying ground, it may be necessary to drill a secondary grid, at closer hole spacings. Normally, this would be at 4.25 or 3m centres.
Testing & Reporting
- Pressure testing: 2% of the total number of holes drilled on a site will usually be pressure tested. A gauge pressure of 10kN/m2 per metre of overburden pressure (up to a maximum of 200 kN/m2) is typically used. The test is considered a pass if the pressure holds for 2-3m which indicates no further grout can be injected.
- Material Testing: Grout cubes will be taken at regular intervals, which are crushed in the laboratory to ensure compliance. The required grout strength is in the order of 0.7 – 1N/mm2.
- Reporting: A completion report is usually prepared for inclusion in the Health and Safety File, which is also submitted to the Coal Authority for their records. This will include a plan of the treatment and any perimeter holes, drilling logs, grouting logs and any material test results.
Drilling and Grouting Case Studies
The Avenue, Whitley Bay
As part of the enabling works, a series of grouting was undertaken to stabilise unrecorded workings in the shallow coal seams below the site. As part of the works and deploying its Boart Longyear rig, Aarsleff Ground Engineering drilled 90 No. 75mm dia. holes including the temporary casings and mixing and injecting 300T of 6/1 PFA/OCP Cement grout. Read more (PDF)
Shakespeare School, Leeds
Aarsleff drilled 109No. vertical bores on a radial grid including casing under the school building with its Boart and Klemm 806 rigs. Aarsleff then mixed and injected OPC/PFA grout to the 4500m2 area ensuring that any surface instability due to the shallow workings, was eliminated. Read more (PDF)
Saxton Gardens, Leeds
Aarsleff Ground Engineering utilised its JS130 rig to drill 72No. vertical and inclined holes to 18m past the existing coal seam & filled with grout. Due to many areas of the site being close to the foundations of existing building walls and underground services, it was necessary to work with extreme caution and assess all risks prior to drilling. Read more (PDF)
Covering an area of 5615m2, Aarsleff utilised three rotary percussive drill rigs for probe drilling and artificial consolidation of the site in the guise of a JS130 excavator with a drill mast attached and two Boart Longyear DB 102’s. Specifically, they drilled 530 No holes to probe the High Main Coal. There’s a coal seam down at around 32m. We have one crew undertaking the probing for the shafts, whilst the other two crews are undertaking the drilling. Aarsleff then mix and injected an estimated 2700T of site batched grout using PFA/cement to consolidate the suspected abandoned mine working. For treatment of the old mineshaft, Aarsleff drilled a temporary liner to its base and injected grout under pressure in ascending stages to fully consolidate. Further test bores were constructed, and pressure grouted to prove the construction. Finally, Aarsleff constructed the standard accepted Coal Authority mineshaft cap with an assumed shaft cap of maximum 6.40m diameter. Read more (PDF)
For more information about Aarsleff Ground Engineering or to send in Drilling and Groutingenquiry, call 01636 611140 and speak to a member of our expert team today, or drop us an email