Tunnel Convergence — What is it and Why Should you Care?
During the construction of a tunnel, the excavated areas will deform — or “converge” — over time due to the weight of its surroundings. One way to think about this is to consider the geology of an area pre-construction as being in a state of equilibrium. The boring/excavation process creates instability; large quantities of soil, rock, and debris are displaced, and the surrounding environment will react to the sudden change in order to find a new “equilibrium.”
Several different factors can affect how much a tunnel deforms such as the stress field in relation to the tunnel’s shape, construction materials, project depth, nearby structures, soil moisture, and other complex geological conditions. Most of the deformation occurs around the excavated area, which is also known as the “tunnel face.” This means that the amount that a tunnel converges could pose a safety risk to on-site workers and a stability risk to the project. This is why it is especially important to monitor tunnel convergence early on during the construction process.
Post-construction, convergence monitoring can still be utilized as a risk management system by detecting structural movements, cracks, and even potential geohazards. Early warning detection is a major factor when it comes to risk reduction. Additionally, observing how the finished tunnel performs is useful when planning future projects as this information can be used to improve later tunnel designs and processes.
Trimble Business Center™ (TBC) office software has new tunnel convergence features that cater to a unique workflow used by surveyors who work in tunnels and underground mines. As the tunneling industry continues to grow, Trimble is also looking to further support these surveyors’ needs and influence the monitoring workflow as a whole.
Total stations are used to collect positional data of target points within a tunnel face. The use of robotic total stations have greatly improved the monitoring workflow by automatically providing real-time 3D positions. This is especially helpful in cases where sending a survey crew to frequently capture data isn’t viable.
Trimble Access™ (TA) Monitoring field software removes the need to manually point and measure every convergence point. Instead, a surveyor can program the total station to automatically collect the specified number of monitoring targets and rounds, which reduces the overall field survey time and allows the crew to focus on other survey tasks.
Using the Trimble SX12 scanning total station or S Series total stations, TA Monitoring allows for streamlined data collection while also providing simple data transfer to TBC. Users can then use TBC for data management, processing, and reporting. This consolidates how much software you will need for creating user deliverables. This reduces costs in terms of buying software licenses. If you are already using TA and TBC, the Monitoring modules are simple add-ons. Additionally, less time will be spent training employees to use an array of different applications, thus increasing productivity.
Together, TA, the SX12, and TBC are a complete system for tunnel surveying and convergence monitoring. So, existing users can benefit from this additional functionality and new users can get all the workflows they need with these same products.