SOFT GROUND TUNNELLING | TECHNICAL
Above: Soil simulation in the 3D FE model: the upper layer includes the embankment fill and the construction backfill at the sides of the existing tunnel, and the lower lever is the glacial till
soil elements; and, the non-linear Concrete Damaged Plasticity model as used for the tunnel lining. All other structural members were assumed linear elastic. Sensitivity analyses were conducted to account for the
variability in these materials, with special focus on the poor-quality backfill on the sides of the existing tunnel and the impact of K0. Another critical aspect of the numerical analysis was
the assessment of the tunnel’s fire performance. A heat- transfer analysis was conducted to simulate the effects of a 1200°C fire event maintained for 120 minutes. As a result, a 60mm sacrificial layer was accounted for in the minimum thickness of the final lining.
CONSTRUCTION MONITORING AND EXECUTION Once the design was finalised, the construction phase began under strict monitoring protocols to ensure that ground movements remained within the allowable limits. The team installed automated total stations and inclinometers to monitor both surface and subsurface settlements in real-time. A critical part of the monitoring strategy involved observing the surface of the highway, where 21 lanes of live traffic were present.
A contract requirement was to perform a six-month
baseline reading for the highway surface, the existing tunnel and the retaining walls. Monitoring data were collected every five minutes,
ensuring that even the slightest movements could be detected and analysed immediately. This approach allowed the project team to adjust tunnelling operations if necessary, preventing any disruption to traffic or damage to the highway infrastructure. In some areas, additional measures were taken, such as closing off individual lanes to minimise risk during critical phases of the excavation. The construction activity that initially caused the
largest settlements was the shaft construction in the median of the highway rather than the tunnel excavation. Also, the 800mm pipe installation was closely monitored during installation. It was decided to excavate one tunnel at a time..
By focusing on the right-side tunnel first, which was farther away from the existing rail line tunnel, the team could closely monitor the interaction between the tunnelling operations and the nearby infrastructure. Once the first tunnel was completed, the second tunnel was excavated, ensuring that the project proceeded in a controlled and safe manner.
Table 1: Ground design parameters Parameters
Unit weight, γ
Primary Compression Modulus, E50 Elastic Modulus, Eur
Units kN/m3 MPa MPa
kPa -
Fill 21 10 35
Effective friction angle, φ’ o35 Effective cohesion, c’ k0
0 0.5
Till 22 35
150 33 5
0.5
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