Author : G.S. Saini & A.K. Dube, B.Singh
ABSTRACT
Various tunnelling problems in the Himalayan rocks at depth are discussed. A study on various tunnel failures in squeezing rocks, have led to a number of conclusions which are briefly discussed in the text. besides two case histories of tunnelling under squeezing conditions.
1.0 Introduction
With the rapid pace of development for harnessing water resources for irrigation and power generation in India, the importance of underground diversion structures and underground powerhouse has increased enormously during the recent years. The construction of these structures is a challenge to engineering skill, when these are located in the weak and fragile rocks of Himalayan region,
Himalayas are young mountains and tectonically active. The rocks of the
Himalayas, as mostly experienced through tunnelling, are incompetent, quasielastic with a number of folds, faults and thrusts of various magnitudes and are heavily charged with sub-terrain water. The chances of existence of the subterrain water head is much more than that of mountain ranges.
The tunnelling at great depth through such a terrain with mixed lithology, varying tectonic behaviour and trapped water reservoir at considerable head becomes a very costly affair and hazardous operation due to the encounter of problems of squeezing, swelling, running ground, sudden ingress of water combined with a poor state of rock, excessive temperature and gases in rocks.
The case histories of some of the problems encountered during the drivage of tunnels at great depth, through Himalayan rocks are discussed briefly, along with the findings of the studies carried out during the constructions.
The studies have led to a number of valuable conclusions of practical significance and have provided data for theoretical analysis. The experiences gained in combating such problems have provided the technical skill to the designers and engineers of future tunnels in the Himalayan region.
2.0 The Himalayas

The evolution of Himalaya is as a result of repeated deformation of the sedimentary successions accumulated in the Tethys Sea. Topographically, the highest and in age, the youngest, the Himalaya is characterised by its distinctive structural architecture and unique sedimentary and tectonic history.
The following four major litho-tectonic physiographic zones have been recognised, (Valdiya, 1980) which are separated from each other by major faults and thrusts (Fig. 1).
(a) Outer Himalayan Siwalik belt
(b) Lesser Himalaya
(c) Great Himalaya
(d) Tethys Himalaya
The majority of the hydro electric projects are located in the Lesser Himalaya due to deep dissected valleys, favourable for high dams, big reservoirs and availability of the water heads for power generation (Fig 2). The lesser
Himalaya, with elevation ranging from 1500 to 3000 m, has strongly folded and multiply thrusted sedimentary and metamorphic rocks incorporating huge bodies of plutonic and volcanic material (of Paleazoic and Precambrain age) in the dissected valleys.
The Himalayan region is within major earthquake region owing to the continuous collision of the plates (Plate Tectonics), and hence is subjected to so high pressure that it is an Intricate problem to understand the implication of rock stresses whenever any underground excavation is done.
The tunnelling problems mainly squeezing, swelling and running ground conditions etc. encountered in the Himalayan region, basically pertain to the supporting of tunnels. The experience with different approaches for supporting the underground openings in the region reveals that the conventional rock-mass classification system (empirical approach) does not lead to realistic assessment of the design parameters. As a matter of fact, many tunnels suffered due to inadequate knowledge about appropriate method for computation of the parameters. The analytical approach has been tried upon as an alternative to classification systems. Some simple and reliable relationships, out of the studies have been driven. The results from these derivations are found nearer to the results obtained through observational approach. However, the accuracy suffers due to lack of knowledge of rock-mass behaviour and in-situ state of stress.
Fig. 1 Geological map of the Himalaya
3.0 Construction Problems for Deep Tunnels

Almost all the major tunnelling projects in the lesser Himalaya have been beset with widely ranged construction problems. Among the most commonly occurring and hazardous tunnelling problems are squeezing, swelling and running ground conditions. The pressures generated under such conditions are capable of causing failure of heavy supports and the containment are major problem. The problems of excessive temperature and gases in tunnels are also reported from the projects in the Himalayan region.
Fig. 2 Tectonic map of the Himalya (Valdiya, 1984)
3.1 Concept of Squeezing and Swelling
The state of stress within the rock mass, at depth, is disturbed on driving the tunnel. The stresses are re-distributed with time to a new state of equilibrium, which is accelerated and further modified by installed support. The changed stresses may or may not exceed the in-situ strength of surrounding rock mass.
As long as the stresses are low and are well within the in situ strength of the rock mass, there are no tunnel stability problems. However, the higher stresses on exceeding the in situ strength of the rock mass initiate the process of progressive failure around the periphery of the tunnel opening. And moves away from the periphery. This zone of failed rock mass is called “broken zone” (fig. 3).
The widening of failed rock mass stops when stresses (tangential stresses) equalise with uniaxial crushing strength of rock mass The failure of a rock mass is associated with volumetric expansion. The failure of rock mass due to over stressing and movement of the failed rock mass into tunnel opening is called squeezing During the interaction of tunnel support with squeezing rock mass, the supports get loaded and such a pressure on the support is called 'squeezing pressure. Swelling pressure on tunnel support is also mobilised due to the volumetric expansion of the rock mass around the tunnel but the volumetric expansion, in this case is caused due to increase of volume by absorption and adsorption of water.
Under many circumstances it has not been possible to distinguish between squeezing and swelling because introduction of small quantity of water increases the tendency toward plastic behaviour in squeezing ground. The squeezing ground can be distinguished from swelling ground on the basis that under the former, the movement is visible as soon as the excavation is made, on other hand, swelling ground is relatively slow process and the time factor is involved. Swelling pressure, due to lack of proper studies is usually converted into squeezing pressure. The tunnel closure under the squeezing condition may continue for long period, sometimes over an year. The total tunnel closures may be as high as 20% of the tunnel size, in exceptional cases and support pressures may be of the order of 20 kg/cm 2 unless flexible supports, are used. Fig.4 shows the closures measured in a tunnel under squeezing in the
Himalaya.

Fig. 3: Development of various zones around a tunnel in Squeezing ground condition
3.2 Running Ground Condition
The running ground condition is water soaked fragmented rocks often containing larger rock fragments embedded in a matrix of finer grained material that has almost no strength and flow into tunnel as slurry. The running ground is quite a common feature found associated with faults, thick shear zone, thrusts and buried fossil valleys in the Himalayan region. The initial pressures generated by running ground may be high to cause failure of very heavy supports. The typical examples are the power tunnels of Beas Sutlej Link and Maneri Bhall Stage-II in
Central Himalaya.
3.3 Excessive Temperature
A very serious problem of excessive temperature (390 - 40'C) was reported recently in the power tunnel, of 3.2m excavated diameter, of Bhabha Hydro
Electric Project, Central Himalaya. The project lies within the zone of Lesser and
Great Himalaya. The most of the hot springs are also located along this zone.
Underground temperature of water, rock and humid tunnel atmosphere that exceed about 32'C present very difficult working conditions. An extensive system of refrigeration or rapid circulation of tunnel air is required to work in tunnel having temperature beyond 32 0 C.
The problem in the above project was tackled by supplying fresh air through efficient ventilation, reducing the pull per blast to 1.0-1.2 m per day and by introducing the alternative rotation of workmen for short period, in order to take care of exhaustion. The same kind of thermal problem is also anticipated in the tunnel of Nathpa Jhakri Project coming up in Central Himalaya.
Fig 4: Closure-Time relation for different rocks under squeezing in Himalaya
3.4 Gases in Rocks
A variety of natural gases namely, carbon di-oxide, methane, Sulpher di-oxide and hydrogen sulphide etc. have been encountered in the tunnels excavated in civil sector. During the excavation of Giri Bata tunnel, the gas was ' encountered and many persons had severe burn injuries due to fire. Other recent example is
Loktak Hydel Project in North-Eastern region of Himalaya. The methane (marsh gas) was encountered in abundant in the tunnel located in the formations of sedimentary environments. Some 15 persons were killed and many had burn injuries of high degree due to explosion and fire near the heading. Similar case is

anticipated in the Ranganadi Project in the same region where the tunnel will pass through Gondwana sandstone (known granite gneisses, schists etc. for coal deposits) apart from
4.0 Tunnelling through Squeezing Condition
Tunnelling through highly fractured and folded rock at depth, produces squeezing condition. The heavy supports get buckled and twisted like pins. The
Table-1 lists some of the tunnels, under squeezing ground conditions in the
Himalayan region. Two case Table 1. Tunnels under squeezing conditions in
Himalayas histories, one each from a completed project and project under construction are briefly discussed.
S. Name of Tunnels. Project, Rock Type Tunnelling Problems
No. Length, and Cross section
1. lchari Chibro Tunnel, Quartzite, Slate and High overbreak. poor
Yamuna Hydel Project Limestone
6.2 Km, 7.0 m dia
strata, Running and
squeezing conditions
2. Chibro Khodri Tunnel, Quartzite, Slate High overbreak, High
Yamuna Hydel Project Limestone. Shale squeezing and swelling
5.6 Km, 7.5 m dia Sandstone & Clays
3. Pandoh Baggl Tunnel, Granite with and schistose bands running ground
Squeezing, Swelling
Beas Sutlej Link
13.0 Km, 7.62 m dia and kaolinised
pockets, Phyllite conditions, Heavy water inflow
4. Giri Bata Tunnel. Slate with boulder High over break, squeezing
High Giri Hydel Project bed. Clays Phyllite.
7.0 Km, 3.66 m dia Shale and Slate bands.
5. Maneri Bhall Stage-I Quartzite, Basic rocks High overbreak, Heavy
Tunnel Limestone, and Slate water Inflow, Squeezing
16.8 Km, 6.0 m dia bands condition
5. Maneri Bhall Stage-II Meta-volcanics, Quartzite, Roof collapse,
Tunnel 16 Km, 6.0 m dia Limestone, Phyllite, and conditions
Graywacke high squeezing and running ground
7. Nathpa Jhakri Tunnel, Gneiss, Schist,
Nathpa Jhakri Project Amphibolites.and Mild
Running ground squeezing and

30.0 Km, 5.0 m dia Granite excessive
running
conditions
8. Bhaba Power Tunnel Granitic gneiss, Running condition,
Bhaba Hydel Project Amphibolite and Mild squeezing
and
8.0 Km, 5.0 m dia Schist Excessive temperature
9. Tenga Tunnel,Tenga Granitic gneiss, Schist, Running condition
Phyllite,
Hydel Project 16.5 km, Quartzite,
Mild
Water inflow,
squeezing and gases 5.0 m dia Shale, Sandstone,
Slate. in rock
10. Loktak Power Tunnel, Sandstone and Shale
Loktak Hydel Project
6.25 Km, 3.65 m dia
Gases inrock, Heavy
water inflow and
excessive pressure
Fig 5: Geological section along alingnment of Chibro-Khodri Tunnel (After GSI)
4.1 Chibro Khodri Tunnel of Yamuna Hydel Stage-II, Part-11
A 5.6 km long tunnel of 7.5 m diameter has been constructed to utilise the 64 m drop between the tail race of Chibro underground powerhouse and Khodri powerhouse for generating 12OMW. Tunnel construction was started from both the ends and through a inclined adit, which was initially excavated with flexible support in soft clays for monitoring the rock mass behaviour in the fault zone. The head race tunnel passes through Nahan, Subathu and Mandhali formations separated by Nahan and Krol thrusts, which are of small diameters in the intra thrust still active and prone to tectonic zone. The following conclusions were movements (fig 5). The tunnel was to pass through th tunnel bands of sandstone, silt stone on down stream side and quartzite and slate on uip stream sides. In between these two formations, an intra thrust zone of about 300 m width bounded by Krol and Nahan thrusts, comprising shear red shale “Sabathu” clays was to be met along the tunnel alignment. During actual excavation of tunnel the red shale were encountered again and again in the intra thrust zone, causing the excavation and supporting problems of tunnel due to squeezing.
The problems had delayed the completion of tunnel nearly six years. Ultimately it was decided to triplicate the major tunnel into three tunnels of small diameters in the intra thrust zone. The following conclusions were drawn out of the studies including systematic instrumentation done in the tunnel (Jethwa, 1981).

(1) The empirical approach does not provide realistic assessment of the support design parameters under squeezing rock condition. The elasto-plastic theory provided a better assessment of rock pressure under these conditions.
(2) Stiffer supports were found to attract higher load under squeezing condition.
A flexible support system consisting of steel arches with loose back-fill caused significant load shedding (fig.6).
(3) The tunnel closures must be considered to find out when it is safe to provide permanent support.
Fig 7: Geological L-section of power Tunnel, Maneri Bhall State-II, (After GSI)
4.2 Maneri Bhali Stage II Tunnel
The Maneri Bhali Hydro electric Scheme Stage-II having an installed capacity of
304MW envisage the construction of 16 Km long tunnel of 6.0 m diameter in standard horse shoe section. The tunnel is being driven -From both the ends and through an intermediate adit, at Dhanerigad in both directions. The tunnel mainly passes through highly jointed and sheared meta-volcanics and quartzites with some patches of limestone, Phyllites, slate, gneisses and greywackes, separated by faults and thrusts (fig 7).
The problems of the sudden ingress of water, running ground, cavity formation and high squeezing ground were encountered one after another within 40 m of reach from chainage at the 776.0 m upstream heading of Dhanerigad adit. The highly crushed, pulverised and thick contact of metabasics and quartzite is in the near vicinity of the heading chainage at 793.0 m with the rock cover varying from
300 m to 400 m .
This zone is acting as a water reservoir and creating high hydrostatic pressure. The metabasics are highly strained due to accumulated tectonic forces. These factors seamed to have given rise to the above problems.
There had been four major collapses Since December,1984 in addition to the severe squeezing problems. These collapses resulted to large number of problems like formation of cavity, inflow of water and materials etc. The inflow material with water was filled up to 70 m from heading more than two times; burring the mucking equipment. A lot of methods including fore-poling of the heading portion, radial umbrella grouting etc. were tried upon to make through the troubled zone, but all in vain. There was virtually no advancement of heading during a period of two and a half years.
In view of failure of endless efforts in tackling the problems, the construction of the by-pass, from the centre line of main HRT at chainage 735.0 m was taken up by multi drift system of excavation in heading and benching.

The cross section was changed from horseshoe to circular shape. The instrumentation studies are being conducted in the troubled zone by
Central Mining Research Station (CMRS) to make observations on squeezing rock condition.
5.0 Design of Deep Tunnels
It has been realised that the deep tunnelling in the fragile and tectonically active rock of Himalayan region poses unaccountable problems. And of all, the major problem is tunnelling through squeezing condition. The prediction of rock mass behaviour, tunnel closure and support pressures under such situation, has remained one of the most difficult task in rock engineering despite rapid stride made in recent years.
5.1
Methodology
The formulation of a methodology for a realistic planning of time and costs of a project, at pre-construction and construction stage is Himalayan tunnelling quite essential.
The essential steps for the pre-construction stage include prediction of (i) squeezing zone, (ii) support pressure and (iii) tunnel closure.
Estimation of time and cost of various possible tunnel alignments and selection of one best alignment and finally detailed investigations for realistic estimates of above. The theories and approaches used for predicting the design and other parameters of rock mass are based on simplifications. It is therefore, necessary to verify and update the analysis and design by field monitoring during the constructing stage.
5.2 Construction Stage Instrumentation
The construction stage monitoring should consist of the following:
(1) Measurement of support pressure
(2) Measurement of tunnel closure
(3) Measurement of size of broken zone.
(4) Estimation of coefficient of volumetric expansion of failed rock mass.

(5) Refinement of predicted values of support pressure and tunnel closure (of pre-
construction stage)
(6) Revision of support-system
The measurement and estimation of first four steps are carried out in the tunnel by implementing a scheme of instrumentation consisting of load cells, contact pressure cells, tape extensometer and bore hole extensometer.
The several sections of about ten tunnels under diverse geo-mining condition have been instrumented by CMRS. A detailed instrumentation conducted in the Giri, Yamuna and Loktak tunnels has established a useful and reliable relationships which are described in the following paras:
5.3 Prediction of Rock Mass Behaviour
Tunnel closures under elastic conditions remain one percent of the tunnel size and it is advisable to provide stiff supports. The tunnel closure under squeezing condition are found as high as 20 percent of tunnel size in the
Himalayan region. Singh et al. (1985) have analysed all the Indian case histories and some case histories, of Barton's et al. (1974) to suggest that a tunnel will experience squeezing condition only when
H > 350 Q 1/3
(1) where H is the height of overburden above tunnel and Q is Barton quality index
(Fig 8).
5.4 Prediction of Support Pressure
Prediction of tunnel support pressure has perhaps remained one of the relatively difficult tasks in rock mechanics. Various classification methods under empirical approach since Terzaghi (1946) to Bieniawski (1979) does not provide reasonably reliable prediction in respect of support pressure for the tunnel under squeezing ground condition. The elasto-plastic theory, on the other hand requires the values of shear parameters, which are difficult to obtain. An effective alternative is available in the form of the empirical method of CMRS which incorporates the effects of thickness of the overburden and the tunnel closures in the Barton’s method. The support pressure in this method is given as: p = f. f ‘. p’
(2)

where f
(3) f ‘
H p'
= the correction factor for overburden
= 1 + (H-320/800
= correction factor for tunnel closure given in Table-2
= thickness of over-burden in meters, and
= support pressure obtained from Barton's method
The concept of ground and support reaction curve, based on elasto-plastic behaviour of rock mass is an important for studying the mechanism of squeezing pressure development. The expressions for estimation of rock pressure and radial deformation after this concept are available but it was not possible to apply the equations in the field because the reliable values of coefficient of volumetric expansion and size of broken zone were not known.
With the help of observational approach, the equations are modified precisely and some new theories and methods are suggested. The contribution made by CMRS in this respects are briefly summarised in the following (Dube, 1979 and Jethwa, 1981).
(1) Graphical method to determine the size of broken zone from observations of bore hole extensometers and tunnel closures.
(2) Theory for immediate support pressure after elasto-plastic theory considering a stress distribution at the broken zone boundary and shear stresses close to the tunnel face.
(3) Values for coefficient of volumetric expansion of failed rock masses.
(4) Semi-empirical theory for- ultimate support pressure.
The above developments for determining the rock pressure and radial deformations around tunnels, can not be discussed in the present paper. Those who are interested to receive the information on the above mentioned works, are welcome to write to the authors.
TABLE 2
Values of correction factor (f’)
Rock condition Support stiffness Tunnel
(%) closure f’
Non-squeezing
Squeezing
Squeezing
Very stiff
Very Stiff
Stiff
<1
1 - 2
2 - 4
1.00
1.80
0.85

Squeezing
Squeezing
Squeezing
Flexible
Very flexible
4 - 6
6 -8
Extremely flexible >8
0.70
1.75
1.80
6.0 CONCLUSION
The deep tunnelling in the fragile and tectonically active, young Himalayan rocks are posing numerous problems and may lead to hazards. Of all the problems, squeezing ground condition is being frequently met in the tunnels at depth.
With the help of instrumentation data, some simple and reliable empirical relations and theories have been developed These may not be perfect but provide guideline for tunnel support design during construction stage.
7.0 REFERENCES
Dube, A.K. 1979. Geomechanical Evaluation of Tunnel Stability Under failing
Rock Conditions in a Himalayan Tunnel. Ph.D. Thesis. Univ. of
Roorkee, Roorkee.India.
Jethwa, J.L. 1981. Evaluation of Rock Pressures in Tunnels Through Squeezing
Ground in Lower Himalayas. Ph.D. Thesis Univ. of Roorkee.
Roorkee. India Valdiya, K.S.1980. Stratigraphy and Correlations of
Lesser Himalayas formations, Hindustan Publishing Corporation.
New Delhi, India.
Varshney, R.S. 1988. Tunnelling in Squeezing Rocks. Int. Symp. on
Tunnelling for Water Resources and Power Project. Jan 188. New
Delhi, India.