How many times on jobs do we find that the geotechnical information provided did not reflect the actual ground conditions encountered? No one wants to hear the ‘latent ground condition’ phase used either, because these would be grounds to create an ‘extra situation’ on a contract.
If we are lucky, the difference in the geotech is picked up at the earliest stage, when excavating the shafts. If we are unlucky, it is established during a microtunnelling crossing and according to Murphy’s Law, it will be directly under the very asset that prompted the use of a microtunnel to begin with.
Traditional microtunnelling techniques rely on the jacking of a product pipe to propel the microtunnel head. In this instance, when the ground substantially changes to the point where the contractor can no longer proceed with the line, inevitability there will be a need to dig up the head.
For this reason, there is a lot to be said for installing a pilot line that can be retracted prior to jacking. This gives the contractor the ability to test the ground conditions before they are committed to jacking pipe, which is essentially the point of no return.
In the past, pilot installations were predominantly installed via a displacement method. For this method to be successful, the ground must be displaceable. Contractors also need to be wary of the effect of the displacement of the ground on the surrounding assets.
AXIS pilot advantage
The Vermeer AXIS machine has a distinct advantage in the way it installs its pilot line. The system is designed to cut and extract the ground as it proceeds, and in doing so, has little to no influence on the ground directly surrounding the installation. By extracting the ground, rather than displacing the ground, microtunnellers can visually inspect the condition that is being excavated at the face.
Because the pilot line can be retracted, it allows for different bits to be effectively trialled during the pilot stage. This allows the contractor to assess the best bit for the prevailing ground conditions. With the pilot now complete, the contractor can commence jacking the final product pipe with a far greater likelihood of a successful final installation as the ground conditions are now predominantly known.
When it’s all said and done, we are all looking to reduce the risk on our trenchless projects. Before your next microtunnelling project, please consider the use of a pilot shot to confirm the ground and minimise your risk.
About Stuart Harrison
Stuart Harrison is the Managing Director of Edge Underground, where he specialises in on-grade microtunnelling installations with millimetre accuracy. He is also the inventor of the AXIS guided boring system, and he is constantly working to improve the effectiveness of this and other trenchless systems used in the installation of gravity sewers.