Surveyor Donovan Pearce using the surveying machinery that helps us plot the course of our tunnel boring machine.
When completed, our Mineral Transport System (MTS) will be the longest tunnel in the United Kingdom, stretching 37km from Wilton in Teesside to our Woodsmith mine site, just south of Whitby.
We’re excavating our state-of-the-art MTS that will transport polyhalite via a conveyor belt for processing and shipping using a tunnel boring machine (TBM). The machine cuts away at the rock then grouts and lines the tunnel wall before shifting forward.
Our MTS will be highly efficient, while also minimising our impact above ground, negating the need for truck journeys round the clock or an overland rail system, both of which would impact negatively on the landscape and environment.
But how does the TBM know where its heading? Sat Navs do not work that far beneath the surface and there are no signs directing the driver of the TBM towards its destination.
Planning
The tunnel alignment has been designed specifically to try and allow for the straightest possible route between Wilton and Woodsmith.
Considerations were made to position the alignment through the North York Moors such that it did not pass directly under any residential properties. The next consideration was to make sure the tunnel is constructed within the best possible geology, avoiding the vast amount of iron stone workings known to be located in the area. After these two considerations had been made, a 3D alignment representing the tunnel axis was produced which is used within a navigation system to advance the tunnel.
The navigation system determines the TBM position automatically and in real time to show the TBM operator the position and tendencies of the TBM cutter head with regard to the 3D tunnel axis.
Surveying
A series of surface survey control points are also established using Ordnance Survey National Grid and satellite positioning system data to check the tunnel position and ensure the TBM proceeds along the 3D tunnel axis in the correct orientation.
A robotic total station, a surveyor’s tool that measures angles and distances, is used to transfer the surface survey control points underground and specialist survey equipment, called a gyrotheodolite, is used to correct for any underground drift due to atmospheric influence on the instruments.
At the Lockwood Beck and Ladycross access shafts, which will provide ventilation and emergency egress, the surface survey control is transferred down the shaft using lasers and wire lines. The ‘closeness’ between the calculated co-ordinates from the tunnel to the coordinates transferred down the shaft gives us a measure for accuracy in the survey, and to adjust for the rest of the tunnel drive.