Pipe jacking and utility tunneling methods are types of trenchless construction methods, which involves the requirement of workers to enter the bore hole.
Following are the types of trenchless construction methods as shown in figure-1.
Fig.1: Trenchless Construction Methods
We will discuss pipe jacking and utility tunneling methods in this article.
Pipe Jacking Method in Trenchless Construction
Pipe jacking method involves the installation of a pipe (prefabricated) through the ground. The driving takes place from a drive shaft to the reception shaft. The drive shaft consists of jacks, that moves the pipe.
During the execution of the process, the hydraulic jack makes use of thrust power to move the pipe forward into the ground. The rams of the jacks are retracted after installation of each pipe. This facilitates the preparation of next pipe in a position to repeat the same procedure.
Fig.2: A Real Site Pipe Jacking Work
The soil is transported out of the jacket pipe as well as the driven shaft, as the excavation proceeds. This is done either manually or mechanically.
As mentioned above, the pipe jacking is a worker’s entry method, which involves workers to be there at both excavation and the soil removal processes inside the pipe. So, for personnel safety and health of workers, a minimum pipe diameter of 42 inches minimum is required at all conditions.
Slurry systems, vacuum extraction systems, wheeled carts or skips, auger systems are some of the modes by which the soil is conveyed.
The process of soil excavation is mainly conducted based on the soil conditions. A collapse of soil during excavation is not recommended. So, if a pre-check of the soil conditions show any possibility of collapse, it is recommended to undergo soil stabilization first and then proceed with excavation.
Mechanical excavation within a shield or using tunnel boring machine are some of the methods used for excavation of soil. Even hand mining can also be employed based on the size of the project.
The working space should be adequate so that it facilitates proper storage, the handling and to occupy the shaft. The shaft size mainly varies with the type of excavation equipment that is used and the type of jacking. In a safer side, a working space of 10 to 25 feet longer than the length of pipe has to be prepared. In terms width, 4 to 10 feet wider than the pipe diameter would be best.
As too many workers are employed, a care on unanticipated problems has to kept in mind throughout the fulfillment of the project. The first main concern is the subsurface soil behavior. If the water level is not above the pipe invert, the sandy clay is the best soil for such projects.
The decision on best excavation method for the site under consideration will help in avoiding many problems at the site. This decision is taken based on certain factors like groundwater level, unanticipated obstructions like boulders, change in soil conditions if any.
Other factors that are to be considered are the design features of the shaft. It must be a good design to withstand the large thrust. It is always told to avoid over excavation, which can cause voids. These voids if arises have to be fixed with the help of grouting techniques.
Utility Tunneling Method in Trenchless Construction
Utility tunneling is a method with the same purpose of pipe jacking. The only difference is the lining used or the support structure used. The pipe jacking uses special steel supporting structure that is mainly meant for this. But the UT make use of a temporary support like concrete liner plates, wood lagging systems, or steel ribs.
The procedure of utility tunneling method in trenchless construction mainly involves installation of a liner that acts as the continuous supporting structure at the front cutting face, where the soil is removed by excavation.
The tunnel construction takes place between two accessible shafts. The procedure can be explained in the following steps:
1. Soil Excavation
There are mainly three methods of excavation carried out, which is chosen based on the soil conditions. The first method is hand mining, which is the simplest method of all the methods available for excavation. This method makes use of picks, shovels or any pneumatic hand tools.
This method proceeds with the help of a protective shield, that provides face stability during excavation. The method is simple and is helpful when the site consists of varying soil conditions. But this method is time-consuming.
Another method is open face mechanical excavation, that is quite faster than the hand mining, as it uses mechanical devices. Here also, shields are provided, with power excavation devices. The shields provide access to the front face if any adjustments have to made, which cannot be done manually under unexpected situations.
The third method is Tunnel Boring Machine (TBM), which employs rotary cutter or disk cutters that are driven either hydraulically or electrically. The most improved version of TBM makes use of pressure chamber. This method has high cost and has limited access. This method is restricted in circular tunnels.
2. Removal of Spoil
Based on space, the method of excavation conducted and the total tunnel length, the best spoil removal system is chosen. The most common method used are mentioned below:
- Slurry systems
- Auger systems
- Vacuum extraction systems
- Belt or chain conveyors
- Wheeled cart or skips
- Positive displacements pumps
3. Installation of Segmental Liner
After the above excavation, removal of the spoil, the liner sections are brought through the already set lining. The new liner is connected to the existing ones. The liner plates are steel or reinforced concrete made which are prefabricated ones.
The steel plates are of higher demand compare to concrete plates, due to a higher strength to weight ratio.
4. Control of line and grade
The control is mainly done with the help of theodolite and the laser systems. The current position of the tunnel systems can be estimated with the help of theodolite.
The laser system shows any kind of variation in already set alignment. This makes use of led lights. But one of the drawbacks is that it shows variation with temperature changes. The gyroscope is a higher sophisticated control device used mainly in curved tunneling.
During the excavation, the change in direction is carried out by applying different forces to the jacking cylinders. The utility tunneling procedure is labor intensive in nature. But it can be adopted in a site with varying soil conditions.