References
la Duchère tunnel - Lyons Northern Ring Road
FRANCE, 1995
Consolidating a collapsed tunnel via compaction grouting from the surface and horizontal jet while progressing.
Under the La Duchère district, the Lyons "Boulevard Périphéri-que Nord", or Northern Ring Road, crosses the crystalline hills of the Monts du Lyonnais via two motorway tunnels, with 12-m openings. Excavation is performed in weathered gneiss using the standard method of extraction with explosives.
At the western end of the South pipe nearest the hillside, because of extremely weathered ground covered by 15 to 20 m of clayey-sandy colluvium, the screw and discharged concrete support structure was reinforced by arches every 1.5 m. However, at PM 201, the tunnel face crumbled and collapsed soil resurfaced, thus creating a 15 to 20 m-diameter and 4 to 5 m deep depression.
The tunnel was partially banked up and distorted arches propped up by backfill-supported HEB 220 sections. Then, after a survey was carried out from the surface and the neighbouring North tunnel under construction, ground treatment was performed including:
- reconsolidating collapsed ground from the surface via the compaction grouting method, with a drill mesh of about 3 x 3 m.
- building an umbrella vault while moving forward from the tunnel, using 4 slightly divergent (about 11°) concentric rings of 12 m-long jet grouting columns, reinforced by 89 mm-diameter and 12.5 mm-thick pipes. In each conical ring, the columns are spaced out every 0.75 m (for an 80 cm-diameter in collapsed parts) and cones fit together in order to ensure permanent dual protection of superposed columns.
Horizontal jet works
Until about PM 200, where existing distorted arches have to be taken down, the back of the umbrella vault is supported by the last newly-installed arch and its front is supported by an old, previously-installed arch. From PM 200, where there are no more existing arches, the back of the umbrella vault is supported by the last newly-installed arch and its front is supported by the tunnel face. Problems arose when producing the reinforced columns, because of extremely variable soil, from hard gneiss to a mass of decompressed fallen clayey rocks, and within the congestion of existing arches and props.
These problems were solved thanks to using a drill with a long horizontal pole enabling to bore without cutting rods and equipped with an arm handling same-length heavy framework pipes. Thus, hard ground was able to be crossed by down the hole hammer, then performing jet treatment (single jet process) without taking down rods, by replacing deep drill by the jet monitor, and even reboring in fresh jet spoil, via framework pipes. As most drilling uplifts slightly, the pipe must be inserted with low-pressure slurry injection and final " key " injection, after sealing the pipe circumference, is required to make up for loss of slurry due to gravity during various column execution stages.
Compaction grouting works
Compaction grouting from the surface was performed first, via 2 drilling rigs working at one shift.
All boring was performed while recording boring parameters and mortar injection pressure was limited to 15 bars in the lower area (for the last 5 metres under the tunnel and in front of the tunnel face) and pushed to a maximum of 40 bars in the uppermost 10 meters of treatment. Amounts of mortar injected, per boring operation, usually varied from 1 to 5 m3. However, some drilling used between 10 and 25 m3. Pressure and amount injected were also systematically recorded.
Works results
Following such various treatment, the difficult removal of spoil and distorted arches, as well as their replacement by new arches, was performed witholding major problems and the tunnel was able to progress again after only a two-month stoppage.
Techniques
Compaction grouting is a process used for improving the engineering characteristics of a soil, through the high-pressure injection of stiff mortar through cased boreholes.
Jet grouting is a construction process that uses a high-pressure jet of fluid (generally 20 – 40 MPa) to break up and loosen the soil at depth in a borehole and to mix it with a self-hardening grout to form columns, panels and other structures in the ground. The parameters for the jet-grouting process and the desired final strength of the treated soil depend on a number of characteristics, such as the soil type, the technique used and the objective to be reached. In granular soils, the high-pressure jet breaks up the grains through erosion, while in a cohesive soil, such as clay, the jet breaks the mass up into small particles. High pressure is needed to produce the kinetic energy required for the jet through a small-diameter nozzle. Waste material from the process (a mix of soil, water and binder) is recovered at the surface before being taken away for disposal.
Ouvrages
Soletanche Bachy has developed a number of techniques for use in road structures; cut and cover tunnels, viaduct etc.
In the tunnelling field, Soletanche Bachy’s subsidiaries Soletanche Bachy Tunnels and CSM Bessac specialise in management of tunnel sites and also, for the latter, the design and construction of tunnel boring machines.
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