INTRODUCTION:

The basic aim of NATM is for getting stable and economic tunnel support systems. This method has been very useful in complex diversified geological condition where forecasting of the rock mass is difficult due to rapidly varying geologic strata.

BROAD PRINCIPLES OF NATM:

NATM broadly based on the following principles:

· Mobilization of the strength of rock mass The method relies on the inherent strength of the rock mass being conserved as the main component of tunnel support. Primary support is directed to enable the rock to support itself.

· Shotcrete protection – Shotcrete is defined as a mixture of cement, aggregate, water and accelerators in correct proportions with maximum size of aggregate less than 10mm projected at high velocity from a spray nozzle on to surface to form a layer of pneumatically applied concrete on that surface. Loosening and excessive rock mass deformation should be minimized by applying a layer 25-50mm of sealing shotcrete immediately after opening of the face.

· Measurements - Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground such as load cells, extensometers and reflectors.

· Primary Lining - The primary lining is thin. It is active support and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock bolts, wire mesh and Lattice girders.

· Closing of invert – Early as far as possible closing the invert so as to complete the arch action and creating a load-bearing ring is important. It is crucial in soft ground tunnels

· Rock mass classification - The participation of expert geologist is very important as the primary support as well as the further designing of supports etc. during the excavation of rock requires the classification of the rock mass.

· Dynamic Design – The deigning is dynamic during the tunnel construction. Every face opening classification of rock is done and the supports are selected accordingly. Also the design is further reinforced based on the deformation as noticed during the monitoring.

Classification of Rock Mass type:

Rock mass encountered during excavation cannot be said to be favorable or unfavorable only on the basis of the type of the rock. The excavation of the rock is depends on the rock class based on several factors such as – compressive strength of rock, water condition, number of cleavages, condition of cleavages, dip and strike of the rock etc. There are various approaches of classification of the rock mass and most predominantly are Rock Quality Designation (RQD), Rock Mass Rating (RMR) and Quality of the Rock Mass (Q) factor of the rock mass.

Rock Quality Designation index (RQD)

The Rock Quality Designation index (RQD) was developed by Deere (Deere et al 1967¹) to provide a quantitative estimate of rock mass quality from drill core logs. RQD is defined as the percentage of intact core pieces longer than 100 mm (4 inches) in the total length of core. The core should be at least NW size (54.7 mm or 2.15 inches in diameter) and should be drilled with a double-tube core barrel.

RMR Value:

RMR value depends upon the following factors:

1. Uniaxial compressive strength of rock material.

2. Rock Quality Designation (RQD).

3. Spacing of discontinuities.

4. Condition of discontinuities.

5. Groundwater conditions.

6. Orientation of discontinuities.

Based on this the rock mass classification as per RMR is as under:

RMR VALUE	100-81	80-61	60-41	41-20	<20
ROCK CLASS	I	II	III	IV	V
DESCRIPTION	Very Good	Good	Fair	Poor	Very Poor

Quality of the rock (Q Factor):

It depends on the following:

i) Block size

ii) Inter block shear

iii) Active stress

iv) Reduction for joint water flow

v) Presence of weakness zones

The Q-value is determined with

Where,

RQD = Rock quality designation

J_n= Joint set number

J_r = Joint roughness number,

J_w = Joint water parameter and

SRF = Stress reduction factor.

Q factor varies from 0.01 to 1000 i.e. from exceptionally poor rock to exceptionally good rock.

Components and Sequence of Execution in NATM:

a) Surveying: Keeping the fact in mind that tunnel is concrete lined, so shape was to be maintained to have good profile and to keep a check on over break. Meticulous survey was done after each blast and Profile was marked properly in the minimum excavation line in hard strata and modified the Drilling pattern. As a result we achieved accuracy for maintaining center line and limiting the over break much below then admissible over break 7%. Theodolite, lazer beam analyser and total stations were used to keep the alignment (Photo 1) in order.

Photo 1: Profile marking and surveying.

b) Drilling Loading and Blasting: Face drilling was done using Twin Boom drill jumbo (Electrically operated). Appropriate drilling pattern need to be selected so that powder factor and pull achieved is maximum. There are many types of drilling pattern but in tunnels two types of drilling pattern are adopted i.e. Burnt Cut and Wedge cut.

Powder factor can be expressed as a quantity of rock broken by a unit weight of explosives.

i) Burnt Cut: A series of parallel holes are drilled closely spaced at right angles to the face. One hole or more at the centre of the face are uncharged. This is called the burn cut. Since all holes are at right angles to the face (Fig.1 and Photo 2), whole placement and alignment are easier than in other types of cuts. The burn cut is particularly suitable for use in massive rock such as granite, basalt etc.

Fig.1: Burnt Cut

Photo 2: Face Drilling.

ii) Wedge Cut: Blasthole are drilled at an angle to the face in a uniform wedge formation (Fig.2) so that the axis of symmetry is at the centre line of the face. Hole placement should be carefully preplanned and the alignment of each hole should be accurately drilled.

Fig.2: Wedge Cut

c) Defuming: It means removing of foul gases by ventilation. Defuming activity shall be carried out immediately after the face blasting. Fresh air will be pumped inside the tunnel from outside using installed ventilation system Ventilation system shall be designed to meet the requirement of fresh air inside tunnel, systems consist of Blower fan and ducts (Photo 3).Blower fans are fixed outside i.e. near portal for pumping fresh air in heading through flexible duct.

Photo 3: Defuming

d) Scaling: It means removal of loose rocks after blasting. Scaling is carried out after Defuming to remove the loose boulders hanging at roof and on vertical faces of the tunnel by using the excavator.

e) Shotcreting 1^stStage: Shotcrete is defined as a mixture of cement, aggregate, water and accelerators in correct proportions with maximum size of aggregate less than 10mm projected at high velocity from a spray nozzle on to surface to form a layer of pneumatically applied concrete on that surface. Shotcreting (Photo 4) will be carried out immediately after the scaling is over as an immediate rock support. Shotcrete mix of required grade shall be prepared in batching plant outside of tunnel and will be transported to the discharge location through transit mixers. The mix is unloaded in to the shotcrete pump from where it is conveyed to the spraying nozzle by means of positive displacement or compressed air. During shotcrete operation operator will ensure that accelerator is passing at the spraying end and spraying nozzle is placed perpendicular to the surface approx. 1.5mtr away during Shotcreting which is essential to minimize wastage/rebound. Approved accelerators will be added in the shotcrete mix at the end of spraying nozzle for quick setting.

Photo 4: Tunnel Profile after Shotcrete and rock bolt.

f) Mucking: All the scattered/spread muck of heading will be collected towards heading prior to start of mucking using the excavator. Water will be sprinkled over the muck to avoid dust formation during mucking. Excavator and haulers of required capacities should be deployed during mucking operation. The excavated muck will be loaded into the dumper by using excavator. The loaded muck will be transported to the approved dumping yard for disposal.

g) Rock anchoring/ bolting: A rock bolt is a long anchor bolt for stabilizing rock excavations. It transfers load from the unstable exterior, to the confined (and much stronger) interior of the rock mass.

i) Resin grouted type and cement type Rock Bolt: Resin grouted type and Cement type rock bolts of required diameter and length should be preferred. Each bolt should have one end chamfered and the other end threaded over a length of 200 mm. The rock bolt will have a bearing plate, washer and nut for tightening to fix the plate perpendicular to the surface. Tightening should be done by suitable torque winch. Resin capsule should be filled 1/3^rd of the hole and remaining should be filled with cement capsule or grouted.

ii) Cement Grouted Anchor bars/ Anchor bolts: Anchor bar/bolt is an intentioned element consisting of a rod embedded in cement/sand grout filled hole; the anchor bar/bolt will have a bearing plate nut for tightening to fix the plate perpendicular to the surface. The drilled hole should be filled with grout constituting cement/sand mix with lower w/c according to the technical specifications. Admixtures for fast setting and shrinkage may be added in grout mix as required. Anchor bar/bolts will be inserted centrally into the grout filled hole prior to initial set of grout.

iii) Self-Drilling anchor (SDA): These are self-drilling type of Rock bolts with sacrificial bit at start, suitable for rapidly collapsing soils. SDA are the solution for the reinforcement of the tunnel circumference and advanced roof support for tunnel excavation in the overburdened and soft fractured rock conditions.

iv) Expansion rock bolts: Swellex type, the rock bolt is inflated after insertion with the water pressure for better anchorage.

h) Wire mesh with Shotcreting 2^nd Stage: Welded wire mesh conforming to the technical specifications should be installed in underground tunnel surface over the first layer of shotcrete as per the geological conditions .The wire mesh should be held in position by providing hilti Pin/ steel rod anchors. As soon as the wiremesh installation is completed final layer of shotcrete should be sprayed. Proper curing of shotcrete should be done for achieving desired strength within specified time period.

i) Steel rib support (For Class IV and V): Steel rib supports should be installed where rock class- IV, V and squeezing rock is encountered. Proper supporting and strengthening should be done as per drawing and technical specifications.

· Forepoling: In tunnel excavation to support the heading roof horizontal reinforced bar of 25 dia. or higher dia. are inserted which forms an umbrella (Fig.3) and ensure the stability of the perimeter and allows for neat excavation cycle.

Fig.3 Forepoling

· Pipe roofing:

Pipe roofing – the technique of pre-reinforcing the ground ahead of the tunnel face to ensure that the excavation can proceed safely until permanent support structures can be installed. Used for crown support for next Excavation cycle ( for Rock Class after III only)

· Lattice Girder:

Lattice girder (Photo 5) shall consist of 25mm or greater dia. reinforcing bars forming an equilateral triangle (or square) of 150mm or greater depth laced together by 12mm dia. or greater dia. reinforcing bars. In order to improve the tunnel cycle time and expedite the rate of progress the alternative is comprising of lattice girders embedded in shotcrete. Due to their light weight they provide major reduction in erection time and thus safety. Its structural compound when fully embedded in the shotcrete is reliable than the steel arches in soft ground. After introduction of lattice girders rate of excavation has improved as compared to support system with steel sets.

Photo 5 Installation of Lattice Girder

Cycle time and sequence of operation in Class (I, II and III)

Sl.No.	Description	Hrs.
1	Survey and profile marking	0.50
2	Drilling	2
3	Charging and Blasting	1
4	Defuming	0.5
5	Scaling/Mucking and bottom cleaning	4.5
6	Rock support /Shotcreting	2.5
7	Miscellaneous	1
Total		12

CONCLUSION:

NATM approach of design and execution of the tunneling in varied geology and especially in soft ground tunneling is advantageous and scientific way of tunneling in comparison to the old /conventional way of tunneling. This system monitors the rock mass deformation and designs the support system with reference to the rock mass type and deformation. The NATM represents the state of art in modern tunneling. Its concept makes NATM more economical than conventional means of tunneling. NATM is a technique suitable for tunneling in all types of rock but, in the case of rock class IV and poor value of rock it is required to provide lattice girder in addition to wire mesh provision, shotcreting and rock bolting provides a uniform load bearing structure in soft rocks. Further the use of Lattice Girder with shotcrete has been found to be more effective as an alternate support system in poor and very poor/soft rock conditions as compared to steel sets for reducing cycle time. The cycle time achieved 12 hrs. For class I, II and III but for the rock classes IV and V the cycle time achieved normally is 16-18 hrs.

REFERENCES:

1. 50 years of NATM Experience reports by Jahre NATM, Publisher ITA – Austria Karlsgasse 5, 1040 Wien, Austriaoffice@ita-aites.at, www.ita-aites.at

2. http://www.dr-sauer.com/technical-info/912

3. http://miningandconstruction.com/construction/completepackageforpiperoofingsolutions-1070/

Received on 10.11.2015 Accepted on 20.12.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 245-250

DOI: 10.5958/2231-3915.2015.00028.0