"Spotlight on tunnelling"

Here I'm writing an article about the new technique followed in railway tunnelling in Himalayan region and is a most flexible technique with lots of advantages. On that topic I had also delivered seminar on the topic in the forum of "Indian Infrastructure" at Delhi on 21 and 22 feb'17. Hope you all go through that article and please don't miss to advice or suggesting me if anything new in your mind.

Introduction:

Tunnels are important elements of Infrastructure projects, Hydro powers, Transportation, water supply & sewerage system etc. Its construction involves different complexities in terms of different shapes, rock conditions, proper alignment etc.

Himalayas is a young mountain with complex geology and tunneling in Himalayas are suffered by complex geological problems such as rugged topography/ or difficult terrain conditions, Rock cover, Thrust zones, shear zones, Folded rock sequence, Faults, in-situ stresses, water ingress, high level seismicity etc.

 Complexities and Challenges in tunneling may results in increased cost and extended completion period. Engineering challenges are also involved and play a vital role for tunneling in Himalayas such as orientation of tunnels and caverns, construction methodologies, Deficiencies in selection of equipments and many more.

Strategies for handling the challenges of fast track tunneling requires, adequate Geological and Geotechnical investigations, Drifting, proper geophysical investigations, selection of tunneling equipments, Adequate contracting practices, site clearances, construction material source, Financing, Suitable construction agencies, Documentation of experience.

Tunneling Practices and Method Adopted:

Mainly two methods are classified for tunneling:

     (i)           Conventional Method of Tunneling.

   (ii)           Mechanized Method of Tunneling.

Conventional method of tunneling involves:

a)     Drilling & Blasting method

b)     New Austrian Tunneling method

c)      Drainage, Reinforcement, Excavation , Support solution (DRESS)

Mechanized method of Tunneling involves:

a)     Road Headers,

b)     Tunnel Boring Machine (TBM).

The great advances are made in methodology for tunneling all around the globe and in India, considering the complexities and challenges in Himalayan region for Railway tunnels, the most advance approach/techniques are applied in today’s is NATM (NEW AUSTRIAN TUNNELING METHOD)

NATM (NEW AUSTRIAN TUNNELING METHOD):

The New Austrian Tunnelling Method (NATM), also known as Sequential Excavation Method (SEM), is a popular method of modern tunnel design and construction for Railway Tunnels in Himalayan region. The name NATM was intended to distinguish it from the old Austrian tunnelling approach. The fundamental difference between these new methods of tunnelling, as opposed to earlier methods, comes from the economic advantages made available by taking advantage of the inherent geological strength available in the surrounding rock mass to stabilize the tunnel.

NATM/SEM is generally thought to have helped revolutionise the modern tunnelling industry. Many of the Railway tunnels in India have used this excavation technique like Pir Panjal Tunnel constructed by Hindustan construction co. Ltd. Is the Asia’s second longest Tunnel commissioned for rail traffic in Banihal-Qazigund railway line.

BROAD PRINCIPLES OF NATM:

The Main principles of NATM are:

— Mobilization of the strength of rock mass:

— Shotcrete protection

— Measurements

— Primary Lining

— Rock mass classification

— Dynamic Design

EXCAVATION AND SUPPORT CATEGORIES IN ROCK:

Description:

Intact Rock:

— Spot Bolting.

— Occasional Sealing Shotcrete.

— Full Face or Top Heading/Bench excavation.

Stratified Rock:

— Systematic Rock Bolting

— Systematic Shotcrete initial lining.

— Top Heading Excavation.

— Bench Excavation follows distant.

Fractured Rock:

— Systematic Rock Bolting

— Systematic Shotcrete initial lining.

— Top Heading Excavation.

— Bench Excavation follows anytime.

— Description:

Soft Ground -  Shallow Cover:

— Systematic pre-support.

— Systematic Shotcrete initial lining support with early ring closure.

— Top Heading excavation, Bench and Invert excavation.

Soft Ground - Deep Level:

— Systematic Shotcrete support with early ring closure.

— Top Heading Excavation closely followed by Bench/ Invert Excavation.

Advantages of NATM:

·      Flexibility to adopt different excavation geometries and very large cross sections.

— Flexibility to install additional support measures, rock bolts, dowels, steel ribs if required.

— Lower cost requirements for the tunnel equipments at the beginning of the projects.

— Flexibility to monitor deformation and stress redistribution so that necessary precautions can be taken.

— Easy to install a waterproof membrane.

— Easy to install primary support, i.e. Shotcrete.

— TBM (Tunnel Boring Machine) is not an effective method as compared to NATM.

Pre-requisites for NATM :

 i) Balanced contract structure

ii) Qualified Personnel: for ensuring quality of Shotcrete, rock bolts etc.

iii) Better site management- for coping with unseen events, proper application of the observational method, Elimination of human errors.

iv)  Continuous monitoring of Geology.

CONSTRUCTION SEQUENCE:

— The tunnel advance can be achieved using blasting, a partial face boring machine or simply using an excavator, depending on the ground conditions. Generally, the advancement is spatially and timely staggered in the heading, benching and invert.

— Sealing the exposed ground if necessary.

— Mucking

— Installation of lattice girders or mesh reinforcement, and application of Shotcrete. Depending on the quality of the ground the support might be installed first before the spoil is removed.

— Potential installation of second layer of reinforcement and application of Shotcrete.

— Installation of Anchors.

— Construction of inner lining.

SUMMARY OF THE PROCEDURES IN NATM

NATM procedures involves:

— Shotcreting at the Excavated Area (Primary Lining)

— Placing of wiremesh along the Face of the tunnel

— Erection of Lattice Girder/ Steel Ribs along the Face of tunnel.

— Particular type of Rock Bolting.

— Shotcreting the Whole Arrangements (Secondary Lining)

CONCEPTS OF 3D-MONITORING

— 3D- Monitoring is new technique using for taking the right alignment of the tunnel .

— Optical targets are used for determining the coordinates for measurements .

— The coordinates should be checked daily .

— It is necessary for the tunnel as by it we know any displacement and wrong alignments.

CONCLUSIONS:

— NATM – New Austrian Tunnelling Method, is one of the most well known methods of tunnelling in the present time.

— The method, has often been mentioned as a ‘value engineered ’ version of tunnelling due to version of tunnelling due to its use of light, informal support.

— NATM, with its use of modern means of monitoring and surface stabilisation, such as shotcrete and rock bolts, utilizes this effect systematically.

— The concept of NATM is to control deformations and stress rearrangement process in order to obtain a required safety level.

— The main idea is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself.

— The success of execution of NATM is based on three premises:

(i) Thoughtful and skillful design, (ii) Skilled Execution and (iii) Competent supervision and interpretation of observation results.

Most of the tunnels in the Himalayan region in the present scenario adopted this method for the accomplishment of project within the schedule time frame and because of its flexibility and less cost.