During construction, being able to temporarily suspend traffic or trains above an excavation can mitigate those risks which carry the highest consequences for safety. However, in many cases, this will not be possible, which means there is no room for the occurrence of substantial settlement, sinkholes or damage. Any miscalculation could result in accidents or in the worst-case scenario, loss of life, not to mention major disruption to the travelling public.
The design and construction of tunnels under live loads is challenging and requires careful consideration to allow unhindered operation of existing rail lines and roads. Close coordination with regional municipalities and transportation departments is a must.
The successful approach to such projects consists of a thorough review of existing data to develop tunnelling configurations and methodologies in order to devise a preferred tunnelling strategy; this includes traffic patterns and operational risks during all periods of the proposed construction-time windows When tunnelling under live loads with low cover, the type, thickness and strength of elements that constitute the road structure are just as important as the geology of the ground below. In the case of highways, for example, the design of the road base, thickness of asphalt and strength of the concrete slab (if one exists) are among the many items the design team needs to consider.
On the other hand, for railroads, the design of track slab, the characteristics of the compacted soil below it, maximum weight and frequency of trains, are key factors to study.
Ground improvement through grouting is a good option in these circumstances. An experienced specialist can design the optimal grouting solution. In addition to using the appropriate grout type, consideration must be given to parameters such as pressure, flow and volume that have been tested in a similar geology in order to mitigate the risk of grout damaging the road base or causing channel out (‘frac-out’) to the surface. In many cases, existing boreholes and water monitoring wells complicate this process by becoming a path of least resistance to the ground surface.
Establishing a well-examined baseline and continuous monitoring of road surface (and subsurface) movements is essential to obtain a technical understanding of ground response, and to optimise tunnelling parameters. Typically, robotic theodolites, wireless data loggers and web-based software are used to constantly survey the area and send out timely alerts to the project team if any significant movement occurs. Monitoring will provide real-time assessments on the selection of pre-support systems, the timing of installations, and the requirements for ground stabilisation. Some of the more common methods of tunnelling under live loads include:
Box jacking: this is a tunnelling technique that involves jacking a typically rectangular structure simultaneously with excavation. Prefabricated tunnel sections are advanced horizontally using high capacity hydraulic jacks. This concept requires a jacking frame and equipment to be set up at one end of the tunnel alignment and the delivery of large concrete segments to the site. The benefit of jacking a precast box is having initial and final ground support immediately after excavation. This allows excavation of the ground and construction of the tunnel lining to be performed safely and simultaneously. Risks involved in box jacking include more difficult steering and the chance the box deviates from the design alignment. In addition, the short distance between the top-of-box and the road base (or rail slab) could destabilise the existing structure and cause deformation or settlement.
Umbrella roof/wall system: this method supports the ground ahead of the excavation by installing – and possibly grouting – a series of pipes around the excavation profile. Adequate cover above the excavation profile is required for this technique to work. The method allows the tunnelling crew to observe ground response every few metres and to customise the design of the support as they go, within pre-planned parameters.
This approach results in speeding up the advance rate when conditions are more favourable or increasing the amount of support when ground is less stable than initially anticipated.
A specific complexity that needs to be studied in the design of support systems under live loads is the constant punching load and induced vibration applied by moving vehicles. These loads and vibrations can weaken the ground during construction, induce additional settlement and de-stabilise the tunnel support system in the long term. An experienced design team can help mitigate these serious risks.
