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Hydrofracturing tests used for first time in UK

ROUND UP

The Channel Tunnel Rail Link involves a huge variety of techniques used to deal with complex geotechnical engineering challenges, some of which are specific to building a high speed railway line.Over the next few pages, just some of the many geotechnical firms involved in the project give some of the highlights of their work.

SOIL MECHANICS carried out what is believed to the first hydrofracturing and hydrojacking testing in chalk in the UK along the North Downs Tunnel route.

Hydrofracturing involves injecting fluid into a sealed-off section of a borehole to induce and propagate hydraulic fractures in rock. Pressure data can then be used to determine the stress regime.

Hydrojacking is used to estimate the minimum principal stress in heavily fractured rock with a high conductivity/permeability. The idea is to determine the pressure at which a set of fractures opens and becomes fully 'jacked'. A cyclic step rate test, similar to the Lugeon test, extended to higher pressure, is used.

The investigation aimed to provide additional geotechnical information for design of the tunnel lining support system, particularly the magnitude and orientation of the insitu stress regime in the rock and the likely behaviour of the chalk under long-term unloading conditions after tunnelling.

Three rotary boreholes were drilled into the chalk using Geobor S wireline methods with air mist flush to produce chalk cores. Undisturbed samples for strength and creep testing were taken from the core and verticality surveys carried out in two of the boreholes using an inclinometer. A CCTV survey was also carried out after drilling in all three by Robertson Geolog.

The hydrofracture and hydrojacking tests and impression packer tests were carried out by specialist MeSy GEO-MeßSysteme using its MeSy wireline technique.

Stress measurements were carried out using a straddle packer tool lowered with a mobile winch on a borehole logging cable. The wireline testing approach also allowed control of pressure and fracture growth because of the high system stiffness and the possibility of on-line downhole pressure recording.

Test sections were selected on the basis of core inspection and the video camera log. Once in position, a 1m test section was sealed off by inflating the packers to a pressure of 4MPa. Two or three injection cycles were carried out using water to induce fracturing or open any existing fractures. A jacking test was then carried out. Once all the tests were finished, the impression packer and a magnetic single shot orientation device was run into the hole to obtain fracture orientation data. CCTV logging was then used to see the effects of the testing.

Nineteen hydrofracturing and impression packer test results were successfully produced from the three boreholes. Tests yielded consistent pressure data and the impression packer tests showed that mainly vertical to sub-vertical fractures with similar orientation were initiated or stimulated.

Numerical modelling showed that the influence of local topography was limited to shallow depths and the vertical overburden stress could be taken as a principal stress. Analyses neglected the ambient pore pressure (most of the tests were conducted above the groundwater table).

Results indicated a marked anisotropy between the maximum and minimum horizontal stresses at 75% and 150% of vertical stress respectively. These did not change significantly over the depth range of the tunnel.

The stresses most critical to tunnel lining design are radial.

Luckily, the maximum stress was found to be in the direction of the tunnel which has a negligible effect on the tunnel design. As such, main contractor Eurolink JV was able to make dramatic savings on the costs of the secondary lining.

Magnet extensometers were installed in two of the boreholes and a standpipe piezometer in the other. The extensometers enabled monitoring of ground movements as the tunnel approached and passed, giving additional geotechnical parameters for use in design.

Andy Suckling, principal engineer, Structural Soils and Peter Gee, associate director, Soil Mechanics.

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