Utility tunneling is a trenchless construction method which requires workers to enter the borehole area. It is similar to pipe jacking. However, this technique involves liner plates to provide a shield for workers and to secure the tunnel. The process is relatively straightforward.
Excavating Soil
The first step in utility tunneling is to excavate the soil. The excavation process depends on the type of soil removed and the conditions of the site. A project engineer assigned to the job reviews the geographical data to determine the best method of removal.
The first and simplest excavation method is hand mining. Generally, workers use this excavation method for shorter tunnels as it is time-consuming. Crews work with pneumatic hand tools, picks or shovels to remove the soil. Placement of wood or metal support beams keeps the newly made tunnel from collapsing in on workers as they continue excavating the area. Due to the nature of this work, the minimum diameter of these lines is 48 inches. Hand mining occurs when soil conditions vary, or large obstructions such as boulders make it challenging to use other tunneling methods.
A second option is partial-face mechanical tunneling. This method uses an automated cutting tool with an open face that allows the operator to precisely control the excavation and adjust as ground conditions change. While partial-face machines do have shielding to protect operators during use, they do not have a closed system offering pressure balance support. Support beam placement is necessary during excavation.
A final excavation option is the tunnel boring machine (TBM). This tool uses rotary or disk cutting tool to bore through various soil types. Unlike hand mining and partial-face machine tunneling, tunnel boring machinery offers support to prevent collapse while tunneling. This excavation method produces a smooth tunnel wall with limited disturbance to the surrounding ground. The disadvantage to using a TBM is the upfront cost and its restriction to circular tunneling projects.
Line and Grade Control
While excavating the tunnel, workers must ensure that the grade, or angle, on the excavation and direction, are on track. To control the line and degree of the hole, workers utilize laser systems and theodolite. The theodolite is a surveying instrument which measures the horizontal and vertical angles of the tunnel.
Employing laser systems allows workers to see variation in the set alignment. While the laser system is helpful in alerting miners to a variation issue, any significant change in temperature can cause a false alert as the LED lights may read this as a variation. Also, workers digging a curved tunnel may need to use a gyroscope to help calculate the curvature.
Spoil Removal
During tunnel excavation, removal of the excess soil is essential. There are several different methods that companies employ for this process.
One popular method is vacuum extraction. As the name implies, workers insert a wide pipe into the tunnel and suck up loose debris into a vacuum truck. Often the suction nozzle may be toothed to help cut into the dirt. Operators must be aware that debris may get lodged in the teeth. When tunneling near utilities already in place, vacuum extraction can help workers see the existing utility lines before they inadvertently damage them.
In some cases, workers use wheeled carts to remove the spoils. While digging out the tunnel, workers pile the spoil into a wheelbarrow or other movable cart. The soil is then pushed out of the tube and into a dump truck or stacked away from the excavation.
Most often, companies use a belt or chain conveyors to remove the spoils from the tunnel. This removal method is typically found when utilizing mechanical excavation tools. Tunnel boring machinery, for example, has this technology built into its structure to pull the excess soil away from the bore and out of the tunnel.
Liner Installation
After workers complete the tunnel excavation, they install a liner. This liner works to support the tunnel and protect the utilities that run through it. Liner construction can be either steel or reinforced concrete. However, steel liners are more popular due to their higher strength to weight ratio.
As workers line the structure, workers bring the new liner in through the existing structure. By coming through the existing structure, it makes it easier to connect the pieces together.
Excavation of a utility tunnel is straightforward but time-consuming process. Workers secure the liners to one another as the shaft is complete. Determining which method of excavation is best depends on the analysis of the ground conditions as well as budgetary constraints on the project.