Monitoring:

A crucial part of NATM tunneling (Sequential Excavation Method) as well as of other underground construction work is monitoring. Assumptions made in the design stage have to be verified or modified during the execution of the project.

With today’s laser-based optical tunnel convergence monitoring, an accuracy of between 1 mm and 3 mm (0.04" to 0.12") can be achieved and depends on site conditions like machinery activity as well as on human factors.
Tunnel monitoring together with surface monitoring provide important information about ground behavior and can be compared to anticipated deformations (from e.g. FEM modeling).Movement trends are more important than the actual deformation as different soil (or rock) conditions will show different values. Once the support is placed and has reached its preliminary strength, deformations should slow down and eventually cease.

A large number of instruments have been developed in recent years. They can be grouped by their installation method and include bore-hole placed inclinometers, extensometers to monitor lateral and vertical ground movement, observation wells, flow-meters and piezometers to monitor ground water conditions. Tiltmeters, seismographs, convergence gage points and crack monitors can be located on the surface or in and on buildings to monitor the impact of the construction work on their location.

In-situ measurement provides a comprehensive picture of the ground behavior and allows construction methods to be adjusted to meet criteria and values set during design.

Pressure cells which are oil filled compressible metal pads, have frequently been installed in recent projects. They allow monitoring the load distribution inside the shotcrete lining of NATM tunnels as well as the interaction between lining and surrounding ground and anchor loads. Anticipated and calculated (FEM) loading can be compared to insitu measurements as it was done at the Heathrow Trial Tunnel project in London, UK, the DART Cityplace Station in Dallas, Texas, USA and the Metro Santiago, Chile.

At the Russia Wharf NATM tunneling project for Boston’s Silverline concrete, ground and pile pressure cells have been installed in several cross sections. Pile pressure cells monitor the load wooden foundation piles cut by tunneling operations will transfer onto the primary shotcrete liner once the ground thaws. During the construction period a beam of artificial frozen ground supports the building’s footings which are founded on hundreds of wooden friction piles.

 

On Site Documentation:

Extensive documentation of all construction activities, construction quality as well as monitoring results and any construction related design changes is an important part of Dr. Sauer Group’s on site performance.

Custom field reports are set up to track all construction activities and ground conditions. A customized monitoring data evaluation system is installed and maintained through out the project. Concise monitoring reports which compile readings of the various instruments are prepared in order to have an overall picture.

Documentation can prevent unwarranted cost overruns, keep track of extras and become part of the permanent project documents.