STOCKHOLM BYPASS | PROJECT


Control parameter


Table 3: Control programme and observational values for the temporary rock support behaviour Limit value


Visual observation of cracks in the shotcrete installed at the tunnel front


Observational Limit = No cracks appear; Alarm Limit 1 = 3m; Alarm Limit 2 = 6m


Comment


The Observational Limit is based on the design assumption that no cracks are expected in the installed shotcrete under stable conditions.


For Alarm Limit 1, the level of deformation implies that relatively large cracks have started to occur in the shotcrete.


Alarm Limit 2 is set to account for the continuously growing cracks in the shotcrete. This may indicate creeping behaviour and unstable conditions during the excavation.


Visual observation in the tunnel


Identification of cracks in the shotcrete/ Mapping


from the tunnel front. In case the observational or alarm limits stated below are exceeded, additional mapping must be performed


Inclinometer measurement in the transition part of the fault zone


Observational Limit = 7mm; Alarm Limit 1 = 12mm; Alarm Limit 2 = 17mm


The Observational Limit is based on the assumption that 30% of the total deformations has occurred when the inclinometer is installed. Inclination changes are converted to deformations.


If Alarm Limit 1 is reached, the yield strength of the pipe umbrella support is considered to be exceeded.


If Alarm Limit 2 is reached, a plastic hinge is assumed to be formed in the pipe umbrella support.


Inclinometer measurement in the centre part of the fault zone


Observational Limit = 20mm; Alarm Limit 1 = 30mm; Alarm Limit 2 = 40mm


The Observational Limit is based on the assumption that 30% of the total deformations has occurred when the inclinometer is installed and approximately 5% of the deformations occurs 10m in front of the tunnel front. Inclination changes are converted to deformations.


A plastic hinge formation is observed in the calculations when the observational limit is reached.


Alarm Limit 1 and Alarm Limit 2 are set after the value for the convergence measurement limit.


Deformation measurement and extensometer


Observational Limit = 10mm; Alarm Limit 1 = 15mm; Alarm Limit 2 = 20mm


The Observational Limit is based on a stress level in the pipe umbrella support equal to the 70% of the yield strength (characteristic value).


If Alarm Limit 1 is reached, the yield strength of the pipe umbrella support is considered to be exceeded at some extent.


If Alarm Limit 2 is reached, a plastic hinge is assumed to be formed in the pipe umbrella support.


Convergence measurement in the transition part of the fault zone


Observational Limit = 5mm; Alarm Limit 1 = 10mm; Alarm Limit 2 = 15mm


that approximately 50% of the total calculated deformation has occurred when the measurement point is installed (6m from the tunnel front).


If Alarm Limit 1 is reached, there is a risk for cracks in the shotcrete.


Alarm Limit 2 is set at a level which may indicate that the rock mass behaviour differs from the estimated variation in the performed assessment.


Convergence measurement in the centre part of the fault zone


Observational Limit = 20mm; Alarm Limit 1 = 30mm; Alarm Limit 2 = 40mm


The Observational Limit is based on the assumption that approximately 50% of the total calculated deformation has occurred when the measurement point is installed (6m from the tunnel front).


If Alarm Limit 1 is reached, cracking in the shotcrete has occurred.


rock mass behaviour differs from the estimated variation in the performed assessment in combination with load variation.


Alarm Limit 2 is set at a level that may indicate that the The Observational Limit is based on the assumption Cracks in the shotcrete will be mapped up to 20m back


be sufficient. In the fractured rock zones the spacing between the rockbolts need to be smaller, so from 2 metres centre to centre they go down to 1.5 metres, in some cases. “We identified some parts that needed more design


work, more support, and more extensive grouting; and those we treated separately,” he adds.


“We had also had some parts where we didn’t have


any rock cover, and in those cases we had choices. Open cut was the simplest and cheapest, but some areas of the Bypass go under an environmentally sensitive area where that was not possible. In those areas we could do some work but we were not allowed to make any permanent changes to the surface. And, it was


April 2025 | 19


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