CONTRIBUTION TO THEME “UNDERGROUND WORKS”, GEOENG 2000 MELBOURNE , INTERNATIONAL CONFERENCE ON GEOTECHNICAL & GEOLOGICAL ENGINEERING, 19-24 NOVEMBER 2000, AUSTRALIA.

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TUNNELLING AND ROCK DRILLING UNDER HIGH STRESS CONDITIONS AT THE NATHPA-JHAKRI HYDRO PROJECT

Kurosch Thuro1 and Marco Gasparini2

ABSTRACT In the Nathpa-Jhakri Hydroelectric Project - under construction since 1993 - unusual stress conditions

show a severe influence on the drilling and blasting conditions. It is believed, that the in-situ stress field has rotated from a vertical alignment to one that is parallel to the surface of a creeping landslide. For this reason, the tunnel shows a typical deformation pattern with convergence along the downhill spring line and along the uphill wall. Over all, very poor drilling conditions have been recorded along the headrace tunnel, espe-cially in those regions with high rock cover and foliation striking parallel to the tunnel and dipping downhill towards the river. The main sliding and creeping surfaces coincide with this foliation which is encountered in a series of quartz-mica-schists, schistose quartzites and amphibolites. Estimating the true overburden load to account for the foliated nature of the creeping rock mass, the derived stress state explained why drilling velocities have been significantly influenced. Drilling rates were reduced in those areas corresponding to the rotated stress conditions at the tunnel face. In other words, the face was under increased confining stress, requiring more drilling effort to fragment the rock.

PROJECT OVERVIEW

Since 1993, the Nathpa-Jhakri Hydroelectric Pro-ject (NJHP) has been under construction and in-cludes a 60.5 m high concrete gravity dam, an un-derground desilting arrangement, a 27.3 km long headrace tunnel, a 300 m deep surge shaft and an underground power house to generate 1500 MW of power. The project is situated in the higher Himalya in the middle reaches of the river Satluj in the northwestern part of India (Himashal Pradesh) and has been one of the largest civil works in India. Figure 1 provides an overview of the geological and tectonic situation (Schwan, 1980).

The project´s headrace tunnel was drifted through quartzites, gneissose schists, quartz-mica-schists and amphibolites of low to medium grade metamorphism striking parallel to the tunnel axis over long distances and dipping towards the river valley. The Satluj river valley is typically overdeep-ened due to glaciation with steep valley-side slopes.

Due to the evident sliding planes and connected joint systems, many landslides have occured in the vicinity (as reported by Manhas and Kumar, 1996) including several, that have blocked the river at the dam site. These landslides not only influence the surface excavations and access roads but also the underground workings. For example the headrace tunnel was situ-ated close to the valley and therefore the access tunnels and their portals were directly affected by landslide

1 Institute of Geology, Engineering Geology, Swiss Federal Institute of Technology, 8093 Zurich, Switzerland 2 Impregilo spa, Via Bergognone 31, 20144 Milano, Italy

Figure 1 : Geological and tectonic situation of the project area in the Himalaya (after Schwan, 1980).

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related problems. In this sense, the access tunnel of “Daj Khad” had to be given up entirely due to severe stabillity problems, as the portal was located within the fractured and decomposed material of the creeping rock mass. This paper deals with some sections of the Rattanpur and Manglad headings in Lot 2.

STABILITY PROBLEMS OF THE UNDERGROUND WORKS

Due to the inhomogenous rock mass conditions, the tunnel was excavated by drill and blast techniques.

The support consisted partly of wire meshing, shotcrete, steel ribs and rock bolts and partly of steel lining with steel ribs, steel plates and concrete bars with sand or concrete packing.

Figure 3 : Cracks in shotcrete lining in the spring line

at chainage RU 24400. Figure 4 : Twisting, buckling and bending of

150x150 mm steel ribs with steel lining between crown and spring line (Chainage RU 24420-24430).

Along the Manglad and Rattanpur sections, the headrace tunnel was severely affected by stabillity prob-lems, associated with the orientation of the schistosity (Figure 2). In addition to “Daj Khad”, deformations have been recorded in areas with high rock cover. Examples of the type of damage encountered can be seen in Figure 3 and Figure 4. Cracks in the shotcrete developed shortly after installation of the support, but the deformations continuosly increased for a even longer period. As a remit, the support in some sections had to be stiffened by additional wire mesh, steel ribs, shotcrete and rock bolts or welded steel beams.

These squeezing ground conditions have been observed in several tunnel sections (adits of “Manglad Up-stream” and “Rattanpur Upstream / Downstream”) with the following de-formation patterns: • Cracks in the shotcrete progressively

opening up. Sagging of crown is ob-served.

• Continual opening of cracks in shot-crete with subsequent rock and shot-crete falls. Cracking noises audible.

• Rockfalls with pulling out of rock-bolts. Forming of chimneys in the crown / springline area.

• Reapearance of cracks in shotcrete after applying second coat.

• Breakage of steel and concrete lag-ging.

• Buckling, bending and twisting of steel ribs with breakage of steel and concrete lagging. Rock falls off in places where steel lagging has bro-ken.

Cracks inshotcrete lining

Spallingof rock materialand shotcreteBucklingof steel ribs

Sheardeformation

Foliation (quartz-mica-schist)

UPHILL

DOWNHILL

Deformation ofrock mass undercompression / tensionstress

field

Figure 2 : Typical deformation pattern in the heading of Rat-

tanpur Upstream due to the orientation of schistosity.

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• Removal of 150x150 mm ribs due to buckling and twisting. Replacement with heavy sec-tion ribs 300x140 mm. Figure 5 indicates the typical

springline settlements observed during the excavation period be-tween chainage 24500 to 24750 m (heading of “Rattanpur Up-stream”). The two parallel lines show the space between the mean excavation line and the outer tun-nel lining versus absolute eleva-tion. The recorded maximum set-tlements are provided with the period of measurement following excavation indicated below.

The orientation of the schis-tosity was found to be unfavour-able with respect to the axis of the Rattanpur and Manglad headings. In Figure 6 favourable and unfa-vourable orientations of foliation versus tunnel axis are shown. The Manglad and Rattanpur headings plot in the worst area of the stereo net. In these headings the foliation is nearly parallel to the tunnel axis and dipping at an angle of 25° to 60°.

The real rock cover was also believed to be much higher in these cases than the normal over-burden directly above the tunnel alignment. Stini (1950) has de-scribed this phenomen in the late 40´s (Figure 7). Other authors have described similar conditions (Gehring, 1996; Ewy and Cook, 1990).

Figure 8 shows a cross section through the mountain valley and the location of the NJHP headrace tunnel near one of the access tun-nels. Rock slabs (in shaded grey) shown in this picture are believed to be creeping down the slope along the dipping foliation, as as these inclinations are steeper then the expected angle of friction. Furthermore, observations have been made of several large land-slides above the tunnel alignment with visible open cracks in body

Mean excavation line

24500 24550 24600 24650 24700 24750

Tunnel lining

Elev

atio

n[m

]

after 200 days

after 70 days

after80 days

Chainage [m]

1292.2

1292.0

1291.8

1291.6

1291.4

1291.2

1291.0

1290.8

1290.6

1290.4

Springline SettlementRattanpur Upstream

≈0.9m

Steel rib intrados

≈1.1m

Figure 5 : Recorded settlement of the crown / spring line between CH

24500 and CH 24750 versus absolute elevation.

90º70º50º30º

0º

30º

60º

120º

150º180º

Tunnel AxisAssumptions:

Foliated rockJoints not consideredConstant rock cover

Unfavourable Favourable

Orientation of foliation poles

8 7 6 5 4 3 2 1

87

6

5

4

2

3

1

Unfavourable

Favourable

Rattanpur

Manglad

Figure 6 : Favourable and unfavourable orientations with respect to

foliation versus tunnel axis. Poles plottet on a stereo net (after SPAUN, unpublished).

P Real rock pressureα −H Height of total rock cover−h Height of normal rock cover−α − ip angleDγ − R hock weig tϕ − riction angleF on foliation

Pα γ α= × H / sinpresuming >α ϕ

Figure 7 : Influence of inclined schistosity on the in-situ stress condi-

tions of a valley tunnel (after Stini, 1950).

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of the slope. It is likely, that these valley-parallel open cracks and fissures also occure below the mountain ridge, at the head of the slide body, thus providing addi-tional evidence for deep creeping mass movements of the valley slope. Unfortunately the ridge is 2,500 m above the tunnel axis and is in a restricted military area due to its near proxinity to tibetian China. EXCAVATION PERFORMANCE

Determining tunnel stability is a

key issue during preliminary site investigation. However, prediction of drillabillity has also become an important factor in conventional drill and blast tunnelling in hard rock. Drillability is not only decisive for the wear of tools and equipment but - along with the drilling velocity - is a standard factor controlling the progress of excavation works. The estimation of drillability, and its depen-dance on predicted rock conditions, might bear a high proportion of risk with respect to the project costs. Thus, an improved prediction of drilling velocity and bit wear would be desirable. Drillability of a rock mass is determined through various geological and mechanical parameters as detailed by Thuro (1997a; 1997b). In this study, the effects of anisotropy and the related in-situ stress conditions with measured drilling veloc-ity were included.

Figure 9 shows the classification of drilling rates and bit wear for 28 case studies involving different homogeneous rock mass types as derived from 12 tunnel projects. The rock types encountered in the Nathpa-Jhakri Hydroelectric Project are quartzites, (quartz-) mica-schists and amphibolites – all of which plott in the very poor and extremely poor drillability fields. Of special note is the somewhat unusual `poor´ classification for the different (quartz-) mica-schists. In several large sections (especially between the Manglad and Daj valleys) foliation was approximately parallel to the tunnel axis (Figure 6) and likewise parallel to the direction of drilling. These experiences correspond to similar ones encountered by the author, where lower drilling rates (up to 50%) have been recorded when drilling parallel to schistosity compared to

Satluj river

h

H Typical majorrock slide

Creepingrock mass

Stress

field

Creeping movement

α

Foliation (quartz-mica-schists and related rock types)

Tunnel

WE

Figure 8 : Cross section of the headrace tunnel and mountain valley

showing the assumed in-situ stress conditions.

Phyllite Gneiss

very high

high

med miu

low

very low

Dril

ling

velo

city

extre

mel

yhi

gh

high

mod

erat

e

very

high

low

very

low

Bit wear

0

1

2

3

4

5

0 500 1000 1500 2000 2500

normal

poor

very poor

easy

poor

extremely

Drillability

not btained yet

o

percussive drill COP 1440 - 20 kW

[m/bit]

Dril

ling

rate

[m/m

in]

Sandstone

MarbleLimestone & Marl

Conglomerate & Fanglomerate

Quartzite

not obtained yet

Nathpa Jhakri Hydro ProjectQuartzite

Amphibolite

Biotite-Quartz-Mica-Schist

Muscovite-Quartz-Mica-Schist

Muscovite-Schist

Rock types

Anhydrite Silific Dolomite

Figure 9 : Classification diagram enclosing 28 case studies of different homogeneous rock mass types as derived from 12 tunnel projects. NJHP-rock types: Quartzite, amphibolite and quartz-mica-schists.

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drilling rates (up to 50%) have been recorded when drilling parallel to schistosity compared to the rigth-angled case (Thuro, 1997). Figure 10 for example indicates the close relationship be-tween drilling velocity and tensile rock strength. This figure clearly shows, that in the parallel case, tensile strength is relatively high and the corresponding drilling rates are low. Blasting conditions can also be related to drilling rates, and, in cases where the tunnel axis was oriented parallel to the main foliation, poor blasting con-ditions were likewise expected (Thuro and Spaun, 1996).

The expected in-situ stress conditions (i.e. one where the principal stress axis would have rotated to an orientation parallel to the slope surface), was unfortunately not clearly shown in the overcoring stress measurements (Figure 11). These tests were performed in horizontal bore-holes drilled into the left downstream wall (CH 24720) and upstream face (CH 24648). The re-sults show increasing stress values with depth and reorientation of the major principal stress axis towards the dip angle of foliation (approx. 40° at that location).

Using the formula shown in Figure 7, acting rock pressure would roughly be calculated as being 100 MPa (asuming 2,500 m of overburden and a foliation dip angle of 35°-40°). In this case, the deviatoric stresses would be roughly 50-60 MPa. However, this calculation method does not consider cohesive strength, amongst other fac-tors, and provides largely conservative, or over-estimations, of the acting rock stresses. Future studies are planned for this ongoing project to

include numerical modelling of the principal stress state due to rock cover, anisotropy of rock mass and to-pographical features relating to the valley. In this sense, the mechanisms responsible for the trends observed in the in-situ stresses and measurements may be explained and validated through numerical modelling.

Examination of the drilling performance (Figure 12) revealed, that a distinct pattern could be seen in the

25

50

75

100

Dril

ling

rate

[%]

Drilling rate

Tensile strength25

50

75

100

Indi

rect

tens

ilest

reng

th[%

]

Dip angle of foliation90 75 60 45 30 15 0

High tensile stress Low tensile stress

y = a + b·cos xGraph equation

Figure 10 : Drilling rate and tensile strength plotted

against the orientation of foliation for qu-mica-schist.

0

15

30

45

60

75

90

Dip

angl

e[º]

ϕ

0

10

20

30

40

50Pr

inci

pals

ress

[MPa

]

5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5Borehole depth [m]

σ1

σ3

ϕ( *)

σ1

σ3

ϕ

Stress measurements by overcoringRU CH 24720 / CH 24648

Figure 11 : Stress measurements by overcoring per-

formed in “Rattanpur Upstream” (* disturbed, σ1 – ma-jor principal stress, σ3 – minor principal stress).

Manglad up

Manglad down

Rattanpur up

Rattanpur down

1,0 1,5 2,0 2,5 3,0 3,5Drilling rate [m/min]

Rock cover

800 m

300 m

foliation ⊥ tunnelnormal situation

800 m

350 m

foliation tunnelinfluenced by creeping(near surface)

foliation tunnelhigh stress conditions

foliation tunnelinfluenced by creeping(near surface)

Situation

Figure 12 : Drilling rates with mean value and standard deviation in the Manglad and

Rattanpur sections of the headrace tunnel.

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velocity ranges. In the Manglad Upstream adit tunnel, drilling rates were found to be quite good due to fa-vourable foliation orientation (i.e. normal to the tunnel axis). No significant influences were recorded with respect to induced or disturbed stress conditions relating to the foliation and/or creeping slope movements. Downstream, drilling velocities were seen to be significantly lower and were attributed to subsurface effects relating to the creeping slide mass. High stress conditions could be observed in the Rattanpur Upstream driftings, which experienced the lowest mean drilling rates and the widest range of velocities. In sections having undergone large deformations, drilling rates were considerably higher, owing to the release of accu-mulated stresses and the corresponding stress induced fracture damage. These were the experiences at the Rattanpur Downstream drill location, where rock conditions were much poorer.

CONCLUSION

Several observations have been presented with respect to in-situ stress conditions encountered in a Hima-

layan hydroelectricity project. The primary focus was the effect of these stresses on the foliated rock as en-countered in a series of low to medium grade metamorphic rock types, and the influence of the foliation orientation with that of the tunnel axis on observed drilling rates. In sections with foliation parallel to the tunnel, a typical, almost symmetrical deformation pattern, with convergence along the spring line and shear deformation along the left wall, was observed. Drilling rates were lowest in these sections due to higher principal stresses at the tunnel face (i.e. resulting in higher confining pressures). These higher stresses were attributed to the rotation of the principal stress axis due to foliation orientation, surface topography (i.e. glacial valley) and unstable surface slope conditions. Under these conditions, the tunnel face was highly stressed, requiring increased drilling effort to fragment the rock using drilling tools. Only in regions where accumulated stresses were released due to large deformations and stress-induced brittle microcracking, did the observed drilling rates considerably increase.

ACKNOWLEDGEMENTS

The authors would like to thank Impregilo, spa with all collaborators involved and the Nathpa Jhakri

Hyro Project authorities for allowing the use of unpublished project data and for unreserved help during our investigations. Many thanks also to Georg Spaun, Technical University of Munich, Engineering Geology, and Erik Eberhardt, ETH Engineering Geology, for their discussions and contributions.

REFERENCES

Ewy, R.T. & Cook, N.G.W. (1990). “Deformation and fracture around cylindrical openings in rock, IObser-

vations and analysis of deformations.” Int. J. Rock Mech. Min. Sci., Vol. 27, pp. 387 - 407. Gehring, K.-H. (1996). “Design criteria for TBM´s with respect to real rock pressure.” Proceedings Int. Lec-

ture Series TBM Tunnelling Trends, Hagenberg, Austria, 14-15 Dec. 1995, H. Wagner & A. Schulter Eds., Tunnel boring machines. Trends in design & construction of mechanized tunnelling, 263 p., Rotter-dam, Brookfield: Balkema, pp. 43 - 53.

Manhas, G.S. and Kumar, A. (1996). “A major rock slide and its impact on 1500 MW Nathpa Jhakri Hydel Project in northwest Himalya, India”. In Senneset Ed., Landslides, Rotterdam: Balkema pp. 1765 - 1769.

Schwan, W. (1980). “Shortening structures in eastern and northwestern Himalayan rocks”. In P.S. Saklani Ed., Current trends in geology, Vol. 3 (Special Number), New Delhi: Today and Tomorrow´s Printers & Publishers, pp. 1 – 62.

Thuro, K. (1997a): “Drillability prediction - geological influences in hard rock drill and blast tunnelling”. Geol. Rundsch., Vol. 86, pp. 426 - 437.

Thuro, K. (1997b): “Prediction of drillability in hard rock tunnelling by drilling and blasting”. Proceedings of the World Tunnel Congress, Vienna, Austria, 12-17 April 1997, J. Golser, W.J. Hinkel, W. Schubert, Eds., Tunnels for People. Tunnel für Menschen, Vol. 1, 854 p., Rotterdam, Brookfield: Balkema, pp. 103 - 108.

Thuro, K. and Spaun, G. (1996): “Drillability in hard rock drill and blast tunnelling”. – Felsbau, Vol. 14, pp. 103 - 109.