Grouting with silica sol in the Törnskog Tunnel, Grouting design for silica sol in full production
This report describes a full-scale grouting test in the Törnskog Tunnel. The tunnel is part of the Nordlänken project north of Stockholm, which has two separated tunnel tubes. The test was conducted during production of the tunnel during spring 2005 in chainage from 12012 to 12112. A new concept of grouting was designed and executed based on the grouting material silica sol with the brand name Meyco MP320. Silica sol has been succesfully tested before in field tests in Hallandåsen 2003 and in Äspö HRL 2004. This test is the first test with silica sol in full production of a tunnel in hard rock. The initiative was taken by the National Road Administration, Oden Anläggningsentreprenad and AB BESAB.
Design of the grouting procedure were based on a standard tests for characterisation of the rock mass. With use of a pareto-distribution curve, the hydraulic apertures of the fractures could be computed, further the hydraulic apertures was used to estimate the inflows to the tunnel. The demands set on the allowable leakage into the tunnel resulted that hydraulic apertures down to 14 μm needed to be sealed. Cement can penetrate and seal apertures down to 0.05 - 0.1 mm and therefore it was necessary to use a grout material that has a better penetrability, like silica sol. The grout, named Meyco MP320, was chosen and the design parameters were established: grout pressure, gel time, grouting time. The desired penetration was 1.2 m in order to limit unnecessary spread of the grout. The design was different than the traditional grouting methods with demands set on the effective grouting time at a prescribed pressure. All grout fans were carefully monitored using hydraulic tests, before and after grouting as well as control of the actual grouting. Last, the result of the grouting was complemented with a drip characterisation.
Two types of grout fans were used. One with the above mentioned grout procedure and one based on the grouting procedure normally used in the Törnskogs Tunnel, but with silica sol instead of cement. The first type fan was used when the rock quality was poor. The other when the rock quality was better. The test with these type fans with silica sol were conducted in one tunnel tube, T201, the other tube was grouted with cement following the standard procedure used in the project. This enabled a comparison of the sealing effect between the cement grouted tunnel tube and silica sol grouted tube.
The results of the grouting showed that 8 out of totally 9 grouting fans were successful. The designed fans were used in five cases were the rock mass were presumed to be of poor quality, that is with larger fracture zones present. The objective for this field test was to limit the inflow of water to tunnel corresponding to value of the transmissivity of 3.2ּ10-7 m2/s for the grouted section of 100 m. After grouting the achieved transmissivity was calculated to be 6.2ּ10-7 m2/s. The achieved value of the transmissivity is rather close to the desired; the achieved value considers all fans, including the unsuccessful one. No significant conclusion could be made with the comparison of the sealing effect using cement or silica sol as a grouting agent. The conclusion is that the technique of grouting is very important and more effort needs to be done regarding the grouting time.