STOCKHOLM BYPASS | PROJECT


Table 2: Permissible values for control parameters to verify the validity of the conditions assumed in the design of the temporary rock support Control parameter


Limit value RMRbase RMRbase :


: Transition part of the fault zone RMRbase: Centre part of the fault zone


Rock mass and rock support properties Water inflow


JRC (Joint Roughness Coefficient) Ja


RMRbase ≥40 0> RMRbase ≥29


(rock mass/rock support) is performed by measuring the tunnel deformation


4 l/min, 0.2 Lugeon 6


8


Length of discontinuities > 10 m or persistent through the middle bench over a distance of 4m from the tunnel front


Rock cover >10m


Water leakage or water measurement in every drilled hole Relatively planar joints


No joint-wall contact after a shear deformation of 10cm


Mapped length of sub-horizontal fractures which is judged to affect the bearing capacity of the bench


Drill hole Joint infilling consisting of consolidated clay. Control of the response of the interaction system Comment


for the rock mass quality. The average value for 10m is used for the assessment


The RMR-index is used as control parameter


PROBE AND CORE DRILLING


Stille says: “If you look at the topographical map you see we have the hills and we have the valleys. And the valleys are under water – under Lake Mälaren and its branches. We have altogether three lake passes to tunnel. And those parts are not only under water, but are also the parts where we would expect to have some sort of regional faults and poor rock. “The exact geology quality is, of course, fairly difficult


to predict in advance but in a general way we can predict that we will have a failure zone in these areas.” After the topographical studies and the geophysical


surveys there was a lot of probe drilling undertaken “to see what was really there,” he says. “It turned out that the rock cover was not super-large everywhere. The maximum cover was 70 or 80 metres, and at the lake passages it was only between 15 and 20 metres at the deepest points of the lake – and we could not be certain that we were sampling at the deepest points. In some cases, by tunnel entrances, there was no rock cover at all, only soil. “So, then we did core drilling, to get the cores from


the areas where we suspected that we had poor rock.” The cores showed that in those fractured areas, under


the lake, there were sometimes large voids. “These were really important. They were not


representative of the whole – mostly we had very good rock – but they were a problem that needed to be solved,” he says, as wide tunnels were planned.


“In areas where we had little rock cover, or where


we had these regional fault zones with very poor rock, we added final support with cast-in-place lining. So, we excavated first, then installed rockbolts, then a shotcrete lining, and, finally, a cast-in-place lining,” he says. “Of course, we needed space for all that, so in those areas the tunnel was wider, around 20 metres.”


GROUTING Stille says: “The most difficult part was going through the fracture zones under the lakes with heavily weathered rock mass.” Water ingress would of course have been potentially


disastrous. “That is one of the reasons why we chose drill and


blast. It was not only because the rock is of good quality; it is also because we thought that in those areas we could secure the water leakages, prior to excavation, with cement grouting,” he explains. “The grouting was performed through drilling long


grout holes, ahead of the tunnel in a fan shape around its circumference, and then injecting grout with a grain size of between 20 and 30 micrometres. The grout spreads around the holes of course. “The main reason we did the grouting was to make


sure that we had a good transfer of loads from the rock without a lot of deformations. We wanted to have good contact between tunnel and rock.


Above: Geometry for the main tunnels in the subsea passage between Sätra and Kungshatt April 2025 | 17


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