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Report on the British Tunnelling Society meeting Filling the void: Shaft stabilisation at Strood Tunnel, held at the Institution of Civil Engineers, London, on 19 May 2005, by Philip Wall, Thomas Greig and Charles McAnally.

This presentation to the British Tunnelling Society (BTS) focused on remedial works to Shaft No 4 of the Strood and Higham lining project in north Kent.

The presenters were Network Rail tunnel engineer Philip Wall, Halcrow Group senior design engineer Thomas Greig and Keller Ground Engineering director of special projects, geotechnical division, Charles McAnally.

Built between 1821-24 as canal tunnels, both the Strood and Higham structures were built using a number of headings excavated from the tunnel portals and a number of working shafts (GE September 2004).

In 2000, a 12m diameter hole, tapering to a depth of 11m, appeared in an apple orchard (Figure 1). There was no noticeable effect on the brick-lined tunnel 30m below, but the assumption was that a construction shaft had collapsed.

Under a separate instruction within the main tunnel relining works contract, Costain and its designer Halcrow were asked to reinstate the ground to its original profie, while ensuring no additional load to the tunnel lining, and no encroachment on the tunnel profile.

Twelve known working shafts were used for material hoisting, climbing access and ventilation purposes when the tunnels were built, but there may be other shafts at uncorroborated locations which may have been backfilled later.

There are five shafts in the Higham tunnel. Four were filled with Wilkit foam following the recent tunnel lining works, and one was filled with foam concrete before work began.

Strood tunnel has seven known shafts. S1, S2 and S3 were filled with Wilkit foam following the relining works during August 2004.

S5, S6 and S7 were filled with foam concrete in the late 1990s due to risks of instability.

Collapse of shaft S4 was believed to be the cause of the surface settlement, and although the exact location of the shaft was not known, the frequency of the other shafts indicated there was one in this area.

However, there were known to be dissolution features in the area, along with disused flint mines.

The first phase of the study was to investigate the suspected shaft by 3D tomography using four bored holes on each side of the collapse to send and receive the seismic signals (Figure 2).

Variations in seismic velocity were related to physical parameters such as density and the area of the collapse and the tunnel clearly showed up, along with a number of other zones that showed a similar seismic responses.

Data from a laser survey revealed a circular shape in the tunnel crown in the soot cake covering the lining, although nothing was visible within the tunnel by eye (Figure 3).

The 8m diameter of this circular shape was far too large to be the shaft diameter, but it was considered likely that this detail, and the collapse, were caused by a shaft.

However, a dissolution feature could also have been involved in the collapse.

Core holes from tunnel level were taken to intercept the shaft from two sides, and to investigate one of the outlying voids. The logs picked up brickwork on one side of the shaft, but not the other, and it was felt that the cores must have intercepted part of the collapsed brick domed cap that was assumed to be in the open shaft.

The area between the brick lining and the chalk appeared to be full of low to medium density chalk fill with brick fragments, although it was uncertain if this was back'll or collapse material.

A third core hole intercepted some solution features in the form of open structured chalk with sand infill but no open voids.

This went some way to explaining the low velocity features shown from the tomographic survey, but did not discount the chance of open voids.

It was concluded there really was a shaft, and the collapse had been as a result of a failure of the brick cap at the chalk rockhead.

The shaft diameter was estimated to be between 4.2m and 4.9m, calculated from the volume of the surface void, although this appears larger than historical information suggests.

There were dissolution features in the surrounding geology, possibly with voids, and there were likely to be voids within the collapsed material. There was also loose material between the lining extrados (the outer curve of the tunnel crown) and the ground, again, potentially with voids.

A detailed review of the possible remedial works options and methodology resulted in a solution consisting of six main stages (see box).

The project benefited from recent experience gained on the nearby CTRL 320 Thames Tunnel crossing, where treatment in various grades of chalk was conducted in advance of tunnel cross-passage construction. This established parameters for chalk treatment using micro'ne grouts.

At Strood, there was limited site investigation, but the chalk was known to be Grade A and treatment was therefore designed to improve the strength of the chalk and fill any voids around the tunnel by pressure grouting.

It was known that groundwater level was around tunnel invert level but the methods chosen for treatment allowed for control of water in the event of discharge.

To minimise loading to the tunnel lining, the design required the treatment be phased to provide sequential filling. There was a requirement to maintain half tunnel access for the main tunnel relining operations, which necessitated operational flexiility for the drilling and grouting works to meet the tight programme.

A Wombat rig was used for the drilling works inside the tunnel.

This was operated from track level, with only man access required at the borehole entry position for which a lightweight scaffold was used.

Throughout the treatment works, real time monitoring of the tunnel lining was conducted using electrolevel arrays on each side of the shaft, with convergence monitoring at routine intervals to beyond the treatment zone. Trigger levels were set at 50% of permitted movement levels, but in the event no movement of concern was detected.

The local tunnel lining of 450mm brickwork was carefully inspected before work started and found to be in reasonable condition. It was visually monitored throughout the £350,000 contract, which was carried out in stages over seven to eight weeks.

Reinstatement of the surface with topsoil to a depth to match the surrounding area was completed slightly ahead of programme, allowing the overall Strood and Higham project to be successfully completed in January 2005.

The shaft treatment works were a critical aspect of the main reinstatement project, and success was achieved through close technical and logistical cooperation by all parties.

Discussion Jack Knight from Haswell wondered how common the problem of buried shafts was. At a previous project in Corby, the shaft was found using resistivity from the tunnel rather than ground penetrating radar (GPR).

He also wondered why a 4.5m to 4.9m diameter shaft was there, as they were usually about 3m diameter. Knight suggested it might have been sunk and timbered, ending in a square opening with joints at the junction, and that soot and dust had covering the joints. A circular junction did not ring true - could the panel comment on the methods used to 'nd the shaft, and was over-coring considered to find the stress in the lining?

The panel said GPR was attempted but found nothing conclusive. Over-coring was not considered. A visual examination of the area revealed no signs of stress - soot falling off was one of the 'rst signs of stress. The square junction might help explain the volumes, but not the perfect circular image.

Consultant David Hartwell suggested an irrigation line across the orchard may have set off the collapse by moving silty/ sandy material around the old cap. He also asked about grouting pressures and displacements at different stages.

Charles McAnally confirmed contact grouting was done at 0.35 bar at the lining. Chalk treatment was undertaken at 5bar and permeation grouting at 10 to 15bar, although with very small volumes. In the final stage compaction grouting pressures of 0.5 to 1.0 bar/m depth were used with a stiff grout (75 to 125mm slump). Only small movements were recorded.

Peter Town of Oxford Hydrotechnics asked if water testing was used to determine the voiding within the permeation grouting area and was the 'meanability' theory used. For surface grouting, what determined the limit of 29t for the first hole, he asked.

McAnally confirmed that two types of water testing were used in the tunnel. The first was within the chalk, to give an idea of grouting performance and an observation was made on the permeability reduction.

The second type was in the lower shaft section where permeation grouting was carried out, and sleeve water tests were used to indicate permeability both before and after treatment.

These mainly relied on dynamic probes as the final part of the testing programme.

The first set of holes had a lower pressure limit, set at about 0.5bar/m depth, with secondary holes at 1bar/m depth but with higher slump grout. This limited the grout take and treatment was undertaken in 1m lifts as each level achieved the set pressure.

Overall treatment was about 25% of the volume to be filled.

Alan Auld of Alan Auld Associates questioned if the 0.5 bar pressure could cause hydrofracturing and affect the surface.

McAnally said the pressure was higher than overburden but the grout was going into the shaft, resulting in a localised infilling with no breakout into the chalk.

During permeation grouting there was a combination of permeation and hyrdrofracture, but monitoring and reference to the electrolevels was critical to control. High pressure grouting was only undertaken with small volumes.

Barry New of Geotechnical Consulting Group asked what the permitted movement of the brickwork was and how the number was derived. Also, the tomographic projections appeared to detail what was known, but he wondered if they revealed anything new.

Thomas Greig confirmed the tomographic images shown in the presentation were for one velocity range only, and other images detailed more information. A trigger value of 5mm and a 'stop work' value of 10mm were adopted, along with visual inspections of the lining as the best indicator of any overstressing, based on a beamspring model analysis. It was also con'rmed that the values were validated against what had been seen in other tunnels.

Shaft remediation

Stage 1 Install steel channels to the tunnel lining face using rock bolts through to the chalk to provide additional tunnel support during all temporary works.

Four rows of 3m long bolts (64, 25mm diameter) were installed in 27mm cored holes.

Stage 2 Undertake cavity contact grouting of potential voids between the tunnel walls and external geology to improve the lining capacity and to secure the shaft directly above the tunnel.

Grout barriers were formed at each end of the treatment zone using a viscous grout with thixotropic stabiliser (Conbex 653) to control grout loss and allow treatment to half the tunnel at a time.

Holes were cored 500mm into the chalk at 1.5m centres and grouting conducted in four 'pours' to balance loadings. A relatively viscous OPC/ bentonite grout was used, with low pressures (0.35bar). An average gap of about 100mm between the lining and the ground was found at most borehole positions and this was also treated.

Stage 3 Undertake permeation grouting of the bottom 6m of the shaft to reduce the risk of direct load transfer to the tunnel lining. Plastic tubes-Ó-manchette (42mm diameter, with sleeves spaced at 330mm) were used at 1.2m centres, with grouting conducted upwards, away from the tunnel in three phases:

primary holes with OPC grout (70% of treatment)

secondary holes with microfine grout (25% of treatment)

alternate sleeves with microfine as a proving operation (5% of treatment) Pre- and post-treatment sleeve water testing indicated a permeability improvement, from 10 -5 m/s to 10 -7 m/s.

Further testing within this zone by dynamic probe was carried out after compaction grouting to the shaft infill (stage 5). Analysis of grout injections showed treatment volumes to be in the order of 20% of the chalk infill at the base of the shaft, which was consistent with predictions.

Stage 4 Grout a 6m annulus of the surrounding chalk from the tunnel to increase ground stiffness and fill voids. Radial holes at 1.5m centres were drilled using water flush, with grouting conducted in 3m stages such that loading to the tunnel was evenly balanced, with pressures restricted to 5bar.

Treatment was conducted with microfine grout (Rheochem 650) using high speed colloidal grout mixers. Prior experience demonstrated the requirement for ' ner cement for such treatment. In all, some 2050m 3 of chalk was treated, with grout take at 2.1% of chalk volume.

Stage 5 Fill the 11m deep and 12m diameter void at surface level with foam concrete in five layers. The lowest pour had a higher strength (3N/mm 2) to support subsequent plant loading. The upper 2m was ' lled with imported chalk and used as a working platform for the subsequent compaction grouting.

Stage 6 Compaction grouting of a 13m height of collapsed material, overlapping the stage 2 works by 1m, to eliminate future settlement. Treatment was conducted through seven cased/auger boreholes, working on a primary/secondary/tertiary approach.

The primary boreholes were injected using lower pressures (0.5bar/m depth) to limit loading to the shaft infill. Pressures for secondary boreholes increased to 1bar/m depth, to achieve improved penetration and consolidation of the chalk infill. The tertiary borehole acted as a proving hole. Analysis of grout takes showed treatment was achieved with grout consumption at 25% of the shaft volume.

Before compaction grouting, a series of dynamic probes were carried out to verify the shaft location, to provide pre-treatment strengths, and to provide correlation with design. Following treatment, probes were taken within the compaction grouted zone and within the previously grouted stage 2 permeation zone, demonstrating satisfactory levels of consolidation with increased probe resistance.

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