Civil: Farnworth Tunnel Stabilisation

Farnworth Tunnel Stabilisation

Location: Lancashire, UK

Located approximately 2 miles south of Bolton in Lancashire 10 miles North of Manchester The Tunnel is to be driven and widened along the existing tunnel line.

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Site

Located approximately 2 miles south of Bolton in Lancashire 10 miles North of Manchester The Tunnel is to be driven and widened along the existing tunnel line.

The Project

Minova were called to site to find a solution for the control and stabilisation of the weak sandy strata above the TBM shield, after a roof collapse in which the sand had gone up to the surface the cavity was backfilled with aggregate and cement pumped into the cavity to consolidate the area.

Solutions

After discussions with the client it was agreed to implement a strategy to inject Carbofill be- hind, and in advance of the TBM Shield to pressurise the moving sand to prevent further  roof falls this was achieved by injecting through a plastic lance with a SK90 4/1 ratio pump and injection gun, until the Carbofill was visible and leaching from behind the TBM shield under pressure.

Result

Advance rates increased with the introduction of the system, the injection works continued almost daily to control and consolidate the strata a proactive procedure was implemented by management to inject and consolidate any ground movement that occurred.

Excavation after roof Collapse

The tunnel advanced into an area of fine sand where a major roof collapse occurred.The management decided it was no longer safe to advance the tunnel in the present roof condi- tions, a decision was taken to excavate down to the shield, 12mt steel piles were driven down into the sand either side of the shield and the area excavated down to the TBM, ex- tension plates were then welded to the shield to give more cover at the face as the hydrau- lic extension plates could be advanced whilst advancing the tunnel this allowed the shield to be tight to the face at all times.

Solution

Minova approached site management and suggested installing 6mt spile bolts in 2mt lengths drilled and injected with Wilkit ‘T’ into the sand to consolidate the area to allow the TBM to re-enter into competent strata when tunnelling commenced. The management agreed to our suggestion and the work commenced.

Result

The TBM reā€entered the excavation without any cavities forming.

Ventilation Shaft ground reinforcement

On the tunnel drive line there was a ventilation shaft, core samples taken from this area indicated a large volume of sand it was decided to spile the area before the TBM entered this area

Result

The area was completed and the TBM advanced through Ventilation shaft with no incident, Minova remained on site until the TBM advanced under the A666 dual carriageway.