The rise in trenchless construction with its advantages of not impacting roads, railways and other infrastructure while underground piping is installed, has led to investigations for expanding its use. Some soil conditions have been considered unsuitable for trenchless construction due to the fine grain size. In other cases, a trenchless construction project may already be in progress only to encounter unexpected ground conditions that hamper the progress of the installation.
Jet grouting is a trenchless technique that is enabling construction companies to stabilize underground soil conditions so that piping can be installed using standard trenchless construction techniques.
What is Channeling?
Channeling or tunneling is the construction activity of installing an underground pipe without open trench excavation. Horizontal directional drilling (HDD), pipe jacking, microtunnelling and auger boring are all trenchless techniques used to create underground channels and install piping.
The problem with channeling is that you may encounter soil conditions that are difficult to manage. There is a risk of the channel collapsing or the ground surface subsiding due to the instability below the surface. These obstacles would have prevented trenchless construction activities in the past, but the development of jet grouting has enabled construction companies to overcome them.
Jet grouting is a trenchless technique that can be used to install a solid column or inclusion of concreted soil into the unstable surrounding environment. Standard trenchless construction can then be carried out to install the piping inside this column of concreted soil.
What is Jet Grouting?
The simplest form of jet grouting consists of two nozzles diametrically opposite from each other just above the drilling tool. High-pressure grout is pumped through these nozzles while the drill string rotates at speeds of 15 to 25 RPM. At the same time, the drill is pulled back towards the entrance pit at a rate of 100 to 500 mm/min. The principle is that the grout mixes with the surrounding soil, forming a hardened cylindrical shape around the drill string.
Excess soil is lifted to the surface by the upward movement of the drill.
Two further technological developments from this simple form of jet grouting involve the use of air, or water and air. The compressed air surrounding the grout minimizes the reflection of excess soil back into the core and makes it easier to move that soil to the surface by the presence of air in the mixture.
Water can be used to greatly increase the diameter of the grout cylinder so that less passes are needed to create the same structural effect in the ground.
Where has jet grouting been used for trenchless channeling?
In a project to install an underground chamber for heat exchangers associated with the Nestle Inc., headquarters in Vevey, Switzerland, it was necessary to connect the chamber with the bottom of the lake next to the building by two 500mm diameter pipes. These pipes would supply and discharge water used for heat exchange.
A long history of construction activity and various fills that had been laid in the ground made the installation of the water pipes complex. The layers of fill were bordered by very soft soil and would have to be navigated in the construction. In addition, it was essential to carry out the activity with minimal vibration and noise impact on the occupants of the office building. Environmental concerns related to disturbances to the bottom of the lake also had to be managed. For all these reasons, trenchless construction techniques were chosen with the use of jet grouting to stabilize the ground before installing the pipes.
The jet grouted inclusion had to be more than 600mm thick due to the width required to lay the pipes and grout them in place. Large diameter jet grouting can be done with the use of water to increase the penetration of the grout into the soil, or multiple passes can be done in order to create a combined diameter that meets the requirements.
Another practical example of jet grouting for trenchless channeling was the installation of an underground sewer line in Franklin County Ohio. The project was executed using Tunnel Machine Boring (TBM), but problems were encountered when the tunnel passed through a section of fine gravel. After exploring the options to stabilize the ground so that the TBM could continue, it was decided that jet grouting was the best way forward.
The jet grouting parameters were specified on the basis of the strength needed for the TBM to mine through the inclusion. Test columns were built in order to check the effectiveness of jet grouting on the specific gravel type and the strength that could be achieved. After testing, adjustments had to be made to the air pressure and flow in order to minimize the disturbance to the gravel. An accelerator was also used to cause the cement to set very quickly and prevent defects in the grouted column. In the end, a column with a width of 6.9 m and 92 m long was installed using jet grouting before the MBT activities could continue.