When working on a trenchless project to install underground piping – the one thing you don’t want to happen is for the soil to collapse into the tunnel. If soil collapses at the tunnel face, it may cause subsidence at ground level, leading to project delays and negative environmental consequences.
Before beginning a microtunneling project, you should get familiar with the following two terms:
What is Soil Pressure?
Soil pressure is a measure of the tendency of the soil in the face of the excavation to collapse. It differs depending on the type of soil and the level of groundwater present. The presence of groundwater adds to soil pressure and contributes to the tendency to collapse. This is called groundwater pressure.
What is Tunnel Face Pressure?
Face pressure is the soil pressure at the face of the microtunnel. This is the current soil that is being excavated that may have a tendency to collapse into the tunnel.
What is Microtunneling?
Microtunneling is a trenchless method of construction using pipe jacking to penetrate the soil. Remote guidance systems provide direction control, while a shield balances the soil pressure.
Soil Pressure Balancing with Tunnel Boring Machines
Mechanical Balancing
Mechanical balancing uses a physical device called an Earth Pressure Balance Shield (EPBS) to counter the soil pressure. The cutting head of the machine excavates the tunnel face, dropping the cuttings into an excavation chamber. The EPBS closes the rear of the excavation chamber.
Inside the chamber, the cuttings mix into a paste, and the hydraulic pressure of the machine transfers to the paste. A screw conveyor removes cuttings from the excavation chamber. Pressure sensors in the excavation chamber enable operators to control the speed of the screw conveyor to keep the pressure balanced.
This method is ideal for soft, cohesive soils where the tunnel is above the groundwater table.
Air Pressure Balancing
During microtunneling, boulders enter the excavation chamber that are too large for the screw conveyor to remove. Personnel enter the machine to dislodge the boulders and remove them.
Entry may also be required for mechanical failures or inspections. Under these conditions, operators pump compressed air into the excavation chamber to provide the soil pressure balance. They monitor the pressure sensors in the excavation chamber and adjust the air pressure accordingly.
Using air to balance the soil pressure keeps personnel safe while working in the tunnel.
Slurry Balancing
When soil conditions are very soft or groundwater pressure is high, the cuttings paste is insufficient to balance the soil pressure.
A positive pressure must be created by pumping a slurry mix into the excavation chamber. Slurry is a mixture of bentonite or a clay/water mix. Pumps supply slurry at sufficient pressure to push into the soil face, thus providing stability and resisting the groundwater pressure. Cuttings from the face, therefore, contain soil, water and slurry.
The cuttings/slurry mix is drawn out of the excavation chamber as the TBM progresses forward through the soil. Sophisticated slurry recovery systems at ground level remove the soil cuttings and recycle the slurry back to the excavation chamber.
Mixed Balancing
Some manufacturers have developed mixed shields, which use slurry and air pressure to balance the soil pressure. By having separate compartments for slurry and air, they are able to control the pressure in the excavation chamber more precisely and can operate at even higher groundwater pressures.
Pitfalls
Tunnel boring machines (TBMs) use pipe jacking to progress the segments of pipe through the soil. These pipe segments themselves provide support for the soil around the tunnel. It is only at the face itself where there is a constant risk of collapse if the soil pressure balance is not maintained.
However, there is an area of weakness. The cutting head and shield have a slightly larger diameter than the pipe segments following behind. This can leave a narrow air gap between the pipe segments and the soil. A collapse can occur against the pipe, leading to subsidence at the surface. If any joints in the pipe segments are inadequately sealed, soil and groundwater can actually enter the pipe, causing damage to the mechanical equipment and safety risks for employees.
Other Applications of Soil Pressure Balancing
Other trenchless methods are also subject to issues regarding soil pressure balance. Horizontal directional drilling uses a slurry system to remove cuttings from the bore. The slurry pressure and flow must be strong enough to create the flow of material to the surface. However, if the slurry pressure is too high, it will break down the tunnel walls, leading to a collapse. Possible ground subsidence will result, which can end the project in costly repairs, damaging environmental consequences and delays.
The composition of the slurry mix is another important factor. Slurry mixtures are designed for specific ground conditions to prevent the slurry from penetrating too far into the soil, causing to instability.
Soil pressure balancing is a critical component of trenchless construction and getting it wrong can lead to severe consequences. However, the equipment and tools are available to manage soil and water pressure during construction and properly used, will lead to a successful project.