Trench less –refers to a situation where one is talking about fewer trenches while executing subsurface works. In other words, one is working below the ground surface without exposing the complete network.Sub surface construction activities not only cover new developmental activities but also address the requirements of subsurface network rehabilitation as well as replacements.
  • Under- ground space is vital for the functioning for the modern society.
  • This is where – pipelines for utilities and waste disposal, cables and communication networks and underground transport routes (e.g. metro) – are located.
  • Constructing under the surface or subsurface construction is what the trench less technology techniques are all about.

Classification can be done in either of the following styles:

1. Techniques for creating NEW INSTALLATIONS;
2. Techniques for RENOVATION of existing underground networks;
3. Techniques for making REPLACEMENTS in the existing underground networks.
E.g. Micro tunnelling, Directional Drilling, And Pipe Jacking.

1. Techniques suitable for WATER SUPPLY networks;
2. Techniques suitable for SEWER networks;
3. Techniques suitable for GAS networks;
4. Techniques suitable for TELECOM / POWER networks.

The classification of techniques types primarily depends upon whether a personnel entry is required or not. This can be shown with the help of a flowchart given below.
Trench less construction methods allowing Man Entry

2.3.1 Pipe Jacking
Pipe Jacking is a method for directly installing pipes behind a shield machine by hydraulic or other jacking from a drive shaft such that the pipes form a continuous string in the ground.

Utility Tunnelling
Utility tunnelling is a two stage process in which a temporary ground support system is constructed to permit installation of the product pipe. The temporary tunnel liner is installed as the tunnel is constructed.
Types of Techniques in this class are:

Hand Mining
Hand Mining is accomplished by craftsmen utilizing either pneumatic equipment or simply by picks & shovels. Excavations are normally accomplished inside an articulated or fixed shield.

Open Face Shield

This is a method where a small backhoe or similar cutting equipment is mounted inside a shield. As the
method utilizes support of equipment in place of craftsmen the speed of work is faster than hand mining.

Tunnel Boring Machines

Tunnel Boring Machines are full faced circular mechanized shield machine, steer able and with a rotary cutting head, which may be either jacked or self-propelled. It may be controlled from within the shield or remotely.
Road Header Method

A Road header is either a wheel mounted or track mounted cutting equipment and consists of a boom with spherical ball with teeth at end and is manoeuvrable permitting considerable flexibility within a tunnel. For larger tunnels the machine is mounted on jumbo.
New Austrian Tunnelling Method (NATM)

NATM as is popularly known is a tunnelling method used for tunnelling in rocky strata of fixed or varying texture or hard soil. NATM is based on the principle of allowing the ground surrounding the tunnel to deform enough to mobilize the shear strength of the soil or rock. Frequently this controlled deformation is achieved through wire reinforced shot Crete and steel ribs to form a flexible tunnel lining.


Trench less construction methods - Non-Man Entry Types
These are classified as Horizontal Earth Boring, which are further classified as:

Auger Boring

It is a technique for forming a bore from a drive pit to a reception pit, by means of a rotating cutting head. There are two types of Auger Borings:-
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  • Track Type Auger Boring;
  • Cradle Type Auger Boring.


Compaction Method

It is a technique to form a bore hole by compressing the earth that immediately surrounds the compacting device. The hole is created more due to displacement of soil rather that by removal and therefore the application of this method is restricted to compressible grounds and lesser diameter holes only.
These are sub classified as:
· Push Rod Compaction Method;
· Rotary Compaction Method;
· Percussion Compaction Method.

Pipe Ramming Method

In this technique generally air powered device rams a steel pipe through the soil. The pipe in fact does not create a borehole rather it acts as a hammer to drive the pipe through the ground.


Slurry Method
Slurry method involves the use of a drilling fluid such as water or betonies slurry to aid in the drilling process and spoils removal. The sub grouping is as follows:
· Water Jetting
· Slurry Boring

Micro Tunnelling Method

It is the method of steer able remote controlled pipe jacking to install pipes of internal diameter less than that of permissible for man-entry. In North America the term is used to describe remote control continuous pipe jacking in all diameters.
· Slurry Method
· Auger method

Directional Drilling

It is a steer able method for the installation of pipes, conduits and cables in a shallow arc using a surface launched drilling rig. In particular, the term applies to large scale crossing in which a fluid filled pilot bore is drilled without rotating the drill string, and this is then enlarged by a wash over pipe and back reamer to the size required for the product pipe. The required deviation during pilot boring is provided by the positioning of a bent sub.
Sub classification is as under:
· Maxi (for drive lengths up to 1.5 km).
· Midi (for drive lengths up to 300 m).
· Mini (for drive lengths less than 180 m).
- Fluid Cutting.
- Mechanical Cutting.

For a specific type of work a particular technique is to be adopted, and the decision should be appropriate in terms of many aspects vis-à-vis purpose, soil type, space availability etc.
Following table helps in selection of technique according to the specific aspect.
Above stated table can also be represented in a flowchart kind of system which helps in adopting appropriate technique according to the series of aspect considerations.