MINED TUNNELING | TECHNICAL


Above, figure 2: Exposed TTC box during the excavation of the first two galleries, with the two strengthening patches on the unit joints visible


Existing Units (from South to North) 1604, 1608 and


1612 (see Figure 1) were expected to be impacted by the mined gallery excavations. Connection plates (patches) were installed at the


external faces of the TTC box following the removal of the soil cover, to minimize the differential movements of these units during the ongoing excavation works (see Figure 2). The station had to remain operational during the


construction works. To ensure the smooth operation of the railway, real-time monitoring of the unit joint movements was utilized with the stringent alert level of 3mm differential displacement between the joints of adjacent units.


GROUND CONDITIONS The soil deposits in the project area are a result of glacial depositional systems resulting in a complex distribution of heavily overconsolidated hard plastic glacial till layers, separated by interstadial stratified deposits of very stiff to hard clays and very dense, non-plastic, silt, and sand. The upper groundwater is about 3m to 6m below


ground surface, meaning that the excavation of the galleries extended below the groundwater level. Groundwater ingress could occur from perched groundwater, surface water flow, and from more permeable sand/silt layers within the plastic clayey deposits.


Table 1: Geotechnical Properties Measurement Property


Young’s Modulus (secant) Elastic Modulus


Poisson’s Ratio (drained) Effective Friction Angle Effective Cohesion


Undrained Shear Strength Unit Weight


In Situ K0


Symbol E50


Eur v’


ϕ΄ c’


Su γ


k0 Unit


[MPa] [MPa] [-] [o


Fill 10


30 0.30 ]28 -


[kN/m3 [MPa]


]


Non-Plastic Till


40 120


0.25 38


-


Plastic Till


65 255


0.25 37


450 0.53


222323 1.0


Interstadial Sand


110 420 0.25 44


[kPa] 0000 [kPa]


- 21 0.6-0.9 0.4-0.5


Winter 2023


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