One of the most critical factors in a microtunnelling project is the geology of the pipeline route as unexpected ground conditions is one of the main causes of failure during installation. Depending on the geology – such as soil type, groundwater conditions, and the presence of rock and debris – it can significantly impact on the project parameters, design and construction method used, as well as the methodology, costs and expected timeframe needed to complete the project.

The types of soil involved, the groundwater conditions, and the presence of large boulders or debris within the fill are all-important factors in determining the method.

Unless contractors are fully aware of the ground conditions they are likely to come across during the drill, there is a high risk of using the wrong equipment, resulting in potentially expensive recovery operations and pipe failure. 

Geotechnical surveys should be completed before the project begins and the results should be provided to the contractor, however, it is not uncommon for the results to be incorrect. This is of no fault of anyone as geotechnical inspections are like looking for a needle in a haystack. A small core sample of the ground is taken, and what is seen may not reflect the conditions even a short distance away. Unlike a core sample, when installing a pipeline, it’s passing through every metre ground from one point to another, and you’re going to find any changes to the geology along.

The main problem with unknown ground conditions is that the drill head needs to be selected to best match the conditions, otherwise there is a high chance that drilling will need to stop part way through the drill if there is a significant change in geology. However, as most microtunnelling machines are not designed with the ability to retract, if ground conditions change, a 911 hole will need to be dug from above the drill head in order for it to be swapped out – at additional cost and time for the project.

The other downside to this is that if the area is known to have, or the geotechnical information shows, difficult ground conditions, it may be difficult for project managers to find a contractor who is willing to do the job due to the high risk of problems occurring. 

Ground conditions and how they react

Some of the different conditions that contractors can come across include:

  • Above and below the water table
  • Changing ground
  • Coal
  • Fractured rock
  • Georgia granite
  • Rock limestone
  • Sand
  • Shale

Looking at some of the different installation techniques often used by microtunnelling contractors, they are suitable for different ground conditions:

  • Guided boring is most suited to soft to very soft clays, silts and organic deposits; medium to very stiff clays and silts; hard clays and highly weather shales; medium to dense sands above the water table; gravels and cobbles less than 50-100mm in diameter; weathered rocks, marls, chalks and firmly cemented soils; and slightly weather to unweather rocks. It has a bit more difficulty in but can be used for very loose to loose sands above the water table; and soils with significant cobbles, boulders, and obstructions larger than 100-150mm diameter. This technique should not generally suited to medium to dense sands below the water table. 
  • Auger boring is suited to soft to very soft clays, silts and organic deposits; medium to very stiff clays and silts; very loose to loose sands above the water table; medium to dense sands above the water table; gravels and cobbles less than 50-100mm in diameter; and soils with significant cobbles, boulders, and obstructions larger than 100-150mm diameter. It is not as suited but can still be used with hard clays and highly weather shales;  weathered rocks, marls, chalks and firmly cemented soils; and slightly weather to unweather rocks. Like guided boring, it is not generally suited for medium to dense sands below the water table. 
  • Pipe jacking is suitable to use in medium to very stiff clays and silts; hard clays and highly weather shales; gravels and cobbles less than 50-100mm in diameter; and medium to dense sands above the water table. It can be used, with some difficulty, in soft to very soft clays, silts and organic deposits; very loose to loose sands above the water table; soils with significant cobbles, boulders, and obstructions larger than 100-150mm diameter; and weathered rocks, marls, chalks and firmly cemented soils;. It should generally not be used in medium to dense sands below the water table; and slightly weathered to unweathered rocks.  

A versatile and retractable machine

The AXIS laser guided boring system is the only machine on the market designed with the risks of changing ground conditions in mind. Unlike other machines that can only move forward, the AXIS is designed to retract. This feature allows the contractor to perform a pilot line, inspect the ground conditions and select the best drill head before committing to the final jacking. It also allows the contractor to stop the machine part way through the drill, retract and change the drill head if conditions do change.

This allows us to take on jobs that no other contractor wants to, or can, take on, and complete jobs successfully and with minimal risk.

For instance, we took over one job from another contractor because the geotechnical information said the ground was sandy clay, but instead it was wet sand with clay bits, and as they were using a vacuum style machine which isn’t suited to wet ground conditions as the drill head drops. After some delays, we were called in, completed a pilot line, selected the best drill head and pipe for the conditions and completed the job successfully.

We’ve also taken on jobs no one else wanted due to the difficult conditions, including those where rock is present, a condition that is usually avoided by microtunnelling contractors. A pilot line on one project showed that there was more rock with seams of clay and partial fracturing than the geotechnical survey indicated. Such ground has a high risk of wedging, but we were confident in our skills and our equipment, and completed the job successfully.

No matter the selected trenchless method, having as much knowledge about the ground conditions as possible will enable project managers to select the best technique for the conditions, as well as ensure the contractor can use the right equipment to have the greatest chance of success.