288 Tunnelling in weak rocks Jr = joint roughness number of Barton et al. (1974), f = correction potx

ii The excavation should be by heading and benching method in minor squeezingground and by multiple drift method in severe or very severe squeezing grounds.Drill 10 m advance probe hole

288 Tunnelling in weak rocks

Jr = joint roughness number of Barton et al (1974),

f = correction factor for overburden = 1 + (H−320)/800 ≥ 1,

f′ = correction factor for tunnel closure (Table 5.10) obtained from Fig 5.4,

= 1 in non-squeezing ground,

f′′= correction factor for the time after excavation = log (9.5 t0.25),

H = overburden above crown or tunnel depth below ground level in meters and

t = time in months after excavation

The theoretical support pressures assuming Mohr’s theory for elastic zone also weretoo conservative when compared with the observed support pressures So the same is notrecommended

19.8 STRAIN CRITERION OF SQUEEZING GROUND CONDITION

The experience proved that squeezing occurred when overburden exceeded 350Q1/3m(Singh et al., 1992) One should calculate the corresponding tunnel closure which is asfollows:

(2001) as shown in Fig 19.7 The uniaxial compressive strength of rock mass qcmassmay

be estimated from correlations (equation (8.9) or preferably equation (8.5) of Hoek, 2001)

The tunnel strain (ua/a) may be predicted after knowing the ratio qcmass/P Then, one may

have an idea of the degree of squeezing and the associated problems The tunnel strain

is reduced by the support capacity (pi) Hoek (2001) has plotted theoretical curves and

field data to get the tunnel strain (ua/a) for a given value of qcmass/P and pi/P (Fig 19.8).

0.1 1 10 100 0.1

1

Uniaxial Compressive Strength, MPa

Fig 19.6 Field observations by Cheru et al (1998) from second Freeway, Pinglin and New TienlunHeadrace Tunnels in Taiwan

Strain greater than 10%

Extreme squeezing problems

Strain between 5% and 10%

Very severe squeezing problems

Strain between 2.5% and 5%

Severe squeezing problems

Strain between 1% and 2.5%

Minor squeezing problems

Strain less than 1%

Few support problems

a

0.1 0.2 0.3 0.4 0.5 0.6 0

290 Tunnelling in weak rocks

16

0.25 0.20

0 5

11 14

9 4 13

12 10

0.10 0.05 0.15

1

7 2

3 15

6 8

0.1 0.0

Support Pressure pi/In situ Stress P

Fig 19.8 Influence of internal support pressure piupon deformation of tunnels in weak ground(Numbered points are from case histories) (Hoek, 2001)

Conversely, the support pressure ( pi) may be assessed from Fig 19.8 for a pre-planned

value of tunnel strain for a given overburden pressure P.

Fig 19.6 and experiences in Himalaya suggest that tunnels, in minor to severe ing ground conditions, have been completed successfully but the construction problemsincreased with increasing tunnel strain Tunnelling through very severe squeezing groundcondition was naturally most difficult and must be avoided by changing alignment oftunnel to reduce the overburden

squeez-An educative case history of extreme squeezing ground conditions at Tymfristos tunnel(11 m diameter), Greece has been illustrated by Kontogianni et al (2004) The tunnelclosure was 20 percent The redesigned supports also failed after 6 percent closure Thetunnel cost increased by 10 times The rock mass is claystone and slickensided argillaceous

schist, intensely folded and tectonized (qc= 5–50 MPa) The overburden was only 153 m

It should be realized that re-excavation and installation of the new supports should bedone after closure has stabilized The latter may take several years of monitoring in verysevere squeezing ground conditions

19.9 SUPPORT DESIGN

Fortunately, the steel fiber reinforced shotcrete with embedded ribs has proved to besuccessful in supporting tunnels in the mild to severe squeezing ground conditions TheFig 10.2 may be used for the design of support system The following detailed strategyhas been adopted in squeezing grounds as shown in Fig 19.9

(i) Circular or horseshoe shaped tunnel should be planned in the squeezing groundcondition The tunnel width should preferably be less than 6 m in severe or verysevere squeezing grounds The excavated diameter may be 10 percent more thanthe design diameter

(ii) The excavation should be by heading and benching method in minor squeezingground and by multiple drift method in severe or very severe squeezing grounds.Drill 10 m advance probe hole ahead of the tunnel face to know the rock massquality and drain out ground water if any

(iii) The horizontal drill holes of 3 m length are drilled ahead of the tunnel face and theforepoles of mild steel rods are inserted and welded to the nearest steel ribs Thensmooth blasting is adopted with short length of blast holes (1 m) to cope up withthe low stand-up time

(iv) A steel fiber reinforced shotcrete (SFRS) layer of 2.5 cm thickness is sprayedimmediately to prevent rock loosening Full-column grouted bolts are installed allaround the tunnel including the bottom of tunnel

(v) Steel ribs with struts at the bottom are erected and designed to support the forepoleumbrella and rock support pressure The struts should be strong enough to resisthigh wall support pressures in the squeezing grounds

(vi) The additional layers of SFRS are sprayed after some delay to embed the steel ribs

It will provide lateral stability of ribs and also create a structurally robust lining

Drift

Umbrella of Forepoles Welded

to Steel Ribs

Steel Ribs

292 Tunnelling in weak rocks

(vii) The SFRS should also be sprayed on the floor to cover steel struts and counterheaving tendency of the squeezing ground by withstanding high bottom supportpressures

(viii) The convergence of the tunnel roof and walls should be monitored and plottedwith time In case rate of convergence/closure is not dropping with time, addi-tional SFRS layers need to be sprayed It is a good tunnelling practice if multipleborehole extensometers are installed to know what is happening within the brokenzone particularly in severe or very severe squeezing ground conditions

19.9.1 Precautions in tunnelling

In the cases of big tunnels (10 to 16 m span), the recommendations of Hoek (2001) need

to be followed It is a very challenging task

It may be mentioned that TBM is obviously a failure in squeezing grounds, as it isstruck inside the ground and may have to be abandoned

In very poor ground, stand-up time is only a few hours It is difficult to install supportsystem within the stand-up time So length of blast holes may have to be decreased to

1 m to increase the stand-up time for unsupported span of 1 m In very poor ground, it isdifficult to keep drill holes open for rock bolting SFRS without rock bolt may work well

in such situation Forepoling is difficult here

For a very severe squeezing condition, rock anchors (dowels) may be added on thetunnel face where the face is also squeezing, particularly in the big tunnels This is inaddition to the forepole umbrella A frequent mistake is made in using the large forepolesfor protecting the tunnel face The steel ribs which support the forepoles are loadedadversely, specially in big tunnels Full face tunnelling method may be a failure due toslow progress of tunnelling It is good practice to install forepoles first and then make drillholes for blasting

It may be realized that there is no time to use lengthy software packages and foracademic advice at the tunnel face Spot decisions have to be made on the basis of past

experiences It is, therefore, justified that a tunnel engineer who understands the tunnel

mechanics and has experience should be made sole in charge of supporting the ground and related works.

REFERENCES

Barla, G (2004) Tunnelling Under Squeezing Rock Conditions www.polito.it/ricerca/rockmech/

publcazioni/art-rivista

Barton, N., Lien, R and Lunde, J (1974) Engineering classification of rock masses for the design

of tunnel support Rock Mechanics, Springer-Verlag, 6, 189-236.

Cheru, J C., Yu, C W and Shiao, F Y (1998) Tunnelling in squeezing ground and support

estimation Proc Reg Symp Sedimentary Rock Engineering, Taipei, 192-202.

Daemen, J J K (1975) Tunnel Support Loading Caused by Rock Failure PhD thesis, University

of Minnesota, Minneapolis, U.S.A

Dube, A K (1979) Geomechanical Evaluation of Tunnel Stability under Failing Rock tions in a Himalayan Tunnel PhD thesis, Department of Civil Engineering, University of

Condi-Roorkee, India

Dube, A K., Singh, B and Singh, Bhawani (1986) Study of squeezing pressure phenomenon

in tunnel Part-I and Part-II Tunnelling and Underground Space Technology, 1(1), 35-39

(Part-I) and 41-48 (Part-II), U.S.A

Hoek, E (2001) Big tunnels in bad rock, The 36th Karl Terzaghi lecture Journal of Geotechnical and Geo-environmental Engineering, A.S.C.E., 127(9), 726-740.

Hsu, S C., Chiang, S S and Lai, J R (2004) Failure mechanism of tunnels in weak rock with

interbedded structures, Sinorock 2004 Paper Published in special issue of International Journal of Rock Mech & Mining Sciences, 41, UK.

Jethwa, J L (1981) Evaluation of Rock Pressures in Tunnels through Squeezing Ground in Lower Himalayas PhD thesis, Department of Civil Engineering, University of Roorkee,

India, 272

Kontogianni, V., Tzortzis and Stiros, S (2004) Deformation and failure of the tymfristos tunnel,

Greece J Geotechnical and Geoenvironmental Engineering, ASCE, 30(10), 1004-1013 Labasse, H (1949) Les Pressions de Terrians antour des Puits Revue Universelle des Mines,

92 e Annee, 5-9, V-5, Mars, 78-88

Sakurai, S (1983) Displacement measurements associated with the design of underground

openings Proc Int Symp Field Measurements in Geomechanics, Zurich, 2, 1163-1178.

Shalabi, F I (2005) FE analysis of time-dependent behaviour of tunnelling in squeezing ground

using two different creep models Tunnelling & Underground Space Technology, In Press.

Singh, Bhawani, Jethwa, J L., Dube, A K and Singh, B (1992) Correlation between observed

support pressure and rock mass quality Tunnelling & Underground Space Technology,

Tun-and Stamping Co., Youngstown, Ohio, U.S.A

Yassaghi, A and Salari-Rad, H (2005) Squeezing rock conditions at an igneous contact zone

in the taloun tunnels, Tehran-Shomal freeway, Iran: A case study Int J Rock Mech & Min Sciences, January, 42(1), 95-108.

This Page is Intentionally Left Blank

Case history of tunnel in squeezing ground ∗

“The first sound and the first sign of instability is noted initially by the foreman and the workers at the tunnel face, much before the big thud of collapse is felt in the designer’s office.”

In part II, a 5.6 km long tunnel of 7.5 m diameter has been constructed between Chhibroand Khodri to utilize the discharge from the Chhibro powerhouse A surface powerhouse

of 120 MW capacity is built at Khodri to utilize a drop of 64 m

Tunnel construction in part II was started from both the Chhibro and the Khodri ends.Near Kalawar, a village midway between these two places, a small incline (2 × 2.5 m),called the Kalawar Inspection Gallery, was driven up to the tunnel level to observe thebehavior of rock masses in the fault zone (Fig 20.1) Subsequently, this gallery was used

to construct the main tunnel through this zone by opening two additional headings

20.2 REGIONAL GEOLOGY, TUNNELLING PROBLEMS AND

ALTERNATIVE LAYOUTS

The regional geology of the area was mapped by Auden (1934, 1942) followed by Mehta(1962) and Krishnaswami (1967) Additional information was presented by Shome et al

∗This chapter is reproduced from the paper by Jethwa et al (1980).

Tunnelling in Weak Rocks

B Singh and R K Goel

296 Tunnelling in weak rocks

Limestone, lower Krol Slates, infra Krols Slates

Quartzites, Nagthat Slates, Chandpur Limestone, Bansa Limestone, Dhaira Phyllites & slates - Mandhali Quartzites Bhadraj Sandstone, Nahan Fault

Thrust Drift Drill hole Village, colony

E

Khodri

surface powerhouse F

Construction shaft

at Chhibro

F B

C G

A2 A

Khadar

Kalsi

Dhaira

4×60 MW Underground Powerhouse Chhibro

7.0 m dia finished 6.25 km long HR

5.6 km HR T

5.6 km HR T

Fig 20.1 Regional geology and alternative layout of the Yamuna hydroelectric scheme, stage II,part II

(1973) based on their observations in a few drifts, drill holes and trenches near the villages

of Kalawar and Kala-Amb and some surface features in the region (Fig 20.1)

20.2.1 Tectonic sequence

The following tectonic sequence from north to south was postulated by Auden (1934)between Ichari and Khodri

Simla slatesNummuliticsTons thrustNagthat quartzitesThrust Bound Chandpur series

Jaunsar Syncline Mandhali series

Krol thrustNummuliticsNahan thrustNahan series

20.2.2 Lithology

The Chhibro–Khodri tunnel passes through the following three formations from north tosouth (Shome et al., 1973):

Mandhali series Boulder slates;

(Palaeozoic) Graphitic and quartzitic slates;

Bhadraj quartzite unit of width 5–10 mCrushed quartzites near the Krol thrust

Krol thrustSubathu–Dagshai series 1–3 m thick plastic black clays along the

(Lower miocene) thrust, red and purple shales and siltstones;

Minor grey and green quartzites, 22 m thickblack clays with thin bands of quartzites;

5–10 m thick plastic black clays along theNahan thrust

Nahan thrustNahan series Greenish-grey to grey micaceous sandstone;

(Upper tertiary) Purple siltstone;

Red, purple, grey and occasional mottled blueconcretionary clays

298 Tunnelling in weak rocks

The regional strike of these formations is almost normal to the tunnel alignment withthe dips ranging from 20◦to 60◦in NNW to NNE direction, i.e., towards the upstream

20.2.3 Structural features

The major structural features in this area are the two main boundary faults running fromPunjab to Assam along the foothills of the Himalaya The faults are observed across theriver Tons near Khadar and at a few gully exposures near Kala-Amb and Kalawar Thesewere further explored with the help of a few drill holes, drifts and trenches (Fig 20.1).The dips of the Nahan and the Krol thrusts vary from 27◦to 30◦due N10◦E to N10◦W,and 26◦due N26◦W, respectively The strike is almost normal to the tunnel alignment

20.2.4 Anticipated tunnelling problems and alternate layouts

Krishnaswami (1967) anticipated squeezing problems in the intra-thrust zone and indicatedthat locked-up water was likely to be present in large quantities in the crushed Mandhaliquartzites Subsequently, Krishnaswami and Jalote (1968) attempted either to avoid theintra-thrust zone or to reduce the tunnel length through it, and they proposed severallayouts, as alternatives to a straight tunnel These are shown with costs (of 1968) inTable 20.1 and Fig 20.1

Table 20.1 Alternate layouts proposed for the Yamuna hydroelectric scheme, stage II, part II(Fig 20.1)

Increase in costrelated to layoutone in 1968

1 5.5 km long and 7–7.5 m diameter straight tunnel AE, width of

intra-thrust zone = 800 m

Original

2 5.6 km long and 7–7.5 m diameter tunnel AKg, E with a kink at Kg,

near Kalawar village, width of intra-thrust zone = 230 m

nominal

3 A 51 m high earth and rockfill dam near Kalsi, a 3.0 km long and

7.0 m diameter tunnel GE, intra-thrust zone eliminated

33.8

4 A 2.4 km long and 7.0 m diameter tunnel AB, a 30 m high and

1.6 km long reservoir BC at Kalawar and a 2.8 km long and 7.0 m

diameter tunnel CE, open reservoir across intra-thrust zone

68.4

5 Replacing of open reservoir at Kalawar in layout No.4 by a 1.45 km

long open channel BC, open channel across intra-thrust zone

22.3

6 A 50 m high concrete dam at Dhaira, a 1.2 km long and 8.0 m

diameter tunnel A2B, a 1.2 km long open channel BC at Kalawar

and a 2.8 km long and 7.0 m diameter tunnel CE, open channel

across intra-thrust zone

21.1

Layout No.2, with a kink at Kalawar, was accepted on account of cost considerations.Although the total length of the tunnel along this layout was increased by 0.4 km ascompared to the straight tunnel, the width of the intra-thrust zone was reduced from 800

to 230 m Fig 20.2a shows the original geological cross section along this alignment(Auden, 1942)

20.2.5 Recurrence of intra-thrust zones

In addition to their presence at Kalawar, the Subathu–Dagshai red shales were againintercepted in the tunnel between 1140 and 1300 m from the Chhibro end

A hole drilled at 1180 m (from Chhibro) in the tunnel roof at an inclination of 60◦due

E established the presence of the Krol thrust over the tunnel Finally, Jain et al (1975)presented an ingenious interpretation of the existing geological data and predicted theexistence of a series of tear faults (Fig 20.2b) between Chhibro and Kalawar with a thirdintra-thrust zone between 1861 and 2166 m (from Chhibro) Thus, the total width of theintra-thrust zones was found to be 695 m against an estimated width of 230 m along thetunnel alignment Hence, there is a need for subsurface geological and proper rock massclassification

Considerable tunnelling difficulties were encountered within the intra-thrust zones.The multi-drift method was adopted to prevent frequent rock falls at the face A central pilothad to be excavated by forepoling Heavy steel arches (300 × 140 mm and 150 × 150 mmsections with 20–25 mm thick plates welded on flanges) were erected at 0.25–0.50 mspacing, (see Table 20.4 and Fig 20.11) to cope with high squeezing pressures

20.2.6 Branching of the main tunnel into three small tunnels

The project was delayed by over six years due to the very slow progress of tunnelling(5–6 m per month) through the intra-thrust zones At this rate, it would have taken fiveand a half years to excavate the remaining 695 m (between P and Q, Fig 20.2b) from thetwo ends At this stage, the project authorities considered it wiser to replace the main tunnel

by three smaller tunnels (5.0 m excavated diameter) Consequently, driving of the centraltunnel was started at the end of 1976 and was completed by the middle of 1979 Assumingthat the remaining two small tunnels would be excavated simultaneously during the samelength of time as the central tunnel, the saving in time would be barely six months Thus,branching of the main tunnel into three small tunnels is not proving to be a wise decision.However, simultaneous excavation of the three tunnels could have been quicker

20.2.7 Flooding of the tunnel at Kalawar

In November 1972, the perched water of the rock mass suddenly punctured the imperviouslayer of argillaceous clays along the Krol thrust and rushed in from the tunnel roof at 182 mtowards Chhibro from Kg(the point of inter-section of the Kalawar inspection gallery and

From Under Ground

Power House at Chhibro

??

To be excavated Excavated Excavated

Index

Quartzites, Slates, Lime Stones Intra-thrust zone consisting of crushed red shales, silt stones, clays Sand stone, siltstone, claystone

F -F Kg

Talus Fault Thrust

Intersection of Kalawar inspection gallery and H R T.

To be Excavated Excavated Excavated

2166

?

5600 m 200

Nahan

Nahan

Thrust

Construction Shaft

Construction Shaft

Kg

? ?

Construction Shaft

Construction

N.Th K.Thrust K.Thru

st K.Thrust

1297 m

2377 m? Kg

Fig 20.2 (a) Original geological section along the Chhibro–Khodri tunnel (Auden, 1942); (b) Revised geological section along the Chhibro–Khodri

tunnel (Jain et al., 1975)

Probable Trace of Krol Thrust

Conjunctured Cavity Observed Cavity

Distance from Kg towards Chhibro, m

Index

Water Bearing Crushed Quartzites of Mandhali Series

Black Clays Along Krol Thrust

Red Shales, Silt Stones and Minor Quartzites of Subathu Series

Non - Coring Drill Hole

Shear Zone, Upto 5 cm Gouge

Shear Zone, Upto 50 cm Gouge

Fig 20.3 Geological features causing flooding of tunnel at Kalawar (Shome et al., 1973)

the main tunnel) and flooded the whole tunnel at Kalawar Fig 20.3 illustrates the detailedgeology around Kg(Shome et al., 1973) The rate of inflow was estimated to be 1.2 cusecs(34 liters/s), and 110,000 m3of water were pumped out in three months

20.2.8 Properties of rock masses

The properties of the Subathu red shales and black clays are given in Tables 20.2a and b.The samples were collected from the Kalawar inspection gallery and from gully exposuresnear Kala-Amb

Table 20.2a Properties of red shales and black clays.

Red shales Black clays

General properties:

Density at zero moisture content (g/cm3) 1.86 – 1.88 –Natural moisture content (% by wt) 8.02 9.5 11.7 18.95

Angle of internal friction (degree)

Table 20.2a— Continued

Kalawar inspection gallery Black clays Plate-bearing test 2.7 12.18

3.0 m diam pilot tunnel Black clays Plate-bearing test 1.405 –

3.0 m diam pilot tunnel Red shales Tiwag radial 3.7 (minimum)

in Doon Valley As a result of this earthquake, the town of Dehradun was lifted up by0.13 m relative to Mussoorie Other indications of recent tectonic activity are huge boulders

of quartzites (overall size 5 m) lying in the valley near the drift at Kala-Amb and elongatedspindle-shaped “boudins” of quartzite found embedded in the brecciated, pulverized andgouged material along the Nahan thrust

20.3.2 Measurement of tectonic movement

Agrawal and Gaur (1971) fixed a pillar on the Nahan sandstone and another pillar onthe Subathu clays across the Nahan thrust in the cross-cut from the Kalawar inspectiongallery (Fig 20.4) They measured the relative vertical displacement between the twopillars with the help of a water-tube tiltmeter At the end of three years, they reported thatthe rate of the vertical component of the relative displacement across the Nahan thrust

304 Tunnelling in weak rocks

Subathu Shales and Clays

Naha

n Thr ust

Fig 20.4 Plan of tiltmeter bases for measuring tectonic movement along the Nahan thrust(Agrawal & Gaur, 1971)

varied from 0.4 to 1.0 mm per month However, they conceded that a substantial portion

of this movement might be attributed to the squeezing of the clays and concluded that therate of the vertical component of the tectonic slip across the Nahan thrust was 0.5 mm permonth Subsequently, Jethwa and Singh (1973) reported that the rate of radial closure inthe clays, as measured at the end of two years of excavation, was 1 mm per month in thevertical direction

A single-point rod-type borehole extensometer was installed across the Nahan thrust

in the Kalawar inspection gallery to measure the relative movement between the Nahansandstone and the Subathu red shales (Fig 20.5) Observations, spread over six months,did not show any movement across the Nahan thrust

A conclusion which follows from the above measurements is that squeezing of theclays should not have been ignored while assessing the fault slip

20.3.3 Flexible tunnel lining

Based on the work of Agrawal and Gaur (1971), Jai Krishna et al (1974) suggestedthat the tunnel lining for the intra-thrust zone should be designed to withstand a totalvertical dislocation of 0.5 m expected during the life of the project (100 years) Further,they considered that the total slip would be distributed uniformly along the width of theintra-thrust zone Based on the above assumptions, they proposed a “flexible lining” tocope with the tectonic slip (Fig 20.6) It consisted of circular segments of varying lengths

Nahan Sandstone Inspection

Subathu Clay

Subathu Shales

Nahan Thrust

Krol Thrust

Steel Arches Concrete

Rock

A– Segments of 1.5 m each B– Segments of 3.0 m each C– Segments of 6.0 m each

(a)

(b)

Fig 20.6 Flexible tunnel lining in intra-thrust zone: (a) segmental lining; (b) flexible joint

306 Tunnelling in weak rocks

connected together by flexible joints Contrary to the above assumption, tectonic slip inthick fault gouge may take place along any one plane as suggested by Brace and Byerlee(1967) who explained the mechanism of earthquakes by the “stick-slip” phenomenon

It cannot be proved conclusively from the above that the faults are active Even if this

is so, it may be questionable to provide a tunnel lining on the assumption that the tectonicslip would be uniformly distributed along the entire width of the intra-thrust zone

20.4 TUNNEL CONSTRUCTION AND INSTRUMENTATION IN

THE INTRA-THRUST ZONE AT KALAWAR

20.4.1 Support behavior in Kalawar inspection gallery

Steel ribs for the Kalawar inspection gallery, under a maximum cover of 280 m were first

designed for Terzaghi’s (1946) rock load factor of 1.1 (B + Ht) where B is width and Htisheight of the opening (Table 20.3) It corresponds to squeezing rocks at moderate depths.The water table was observed to be below the tunnel invert but was considered to be abovethe tunnel crown for the purpose of design In order to arrest rib deformations, the rock

load factor was gradually increased to 3.5 (B + Ht), which is equivalent to squeezingrocks at “great depths.”

20.4.2 Tunnel construction

A pilot tunnel of 3.0 m diameter was driven on both sides from Kg In the Subathu redshales, this diameter was enlarged to 9.0 m towards Chhibro from a point 36 m away from

Kg The tunnel was excavated by the multi-drift method The heading was supported

by semi-circular steel arches with temporary invert struts to withstand side pressures(Fig 20.7) The Nahan thrust was exposed in the pilot tunnel at a distance of 40 m from

Kgtowards Khodri, whereas the Krol thrust was exposed at a distance of 190 m from Kgtowards Chhibro The gouge in the 230 m wide intra-thrust zone consisted of soft andplastic black clays over lengths of 16 m and 2 m along the Nahan and the Krol thrusts,respectively, and of crushed, sheared and brecciated red shales and siltstones over a length

of 212 m between the layers of the black clays

20.4.3 Instrumentation

The necessity for tunnel instrumentation was felt in order to evolve a rational tunnelsupport system which could cope with the squeezing ground conditions encountered inthe intra-thrust zone The instrumentation program consisted of measuring: (i) hoop load inthe steel arches by hydraulic load cells (ii) contact pressure at the rock-support interface bycontact pressure cells (iii) “tunnel closure,” defined as reduction in the size of the opening,

by an ordinary steel tape to an accuracy of ±1 mm and (iv) “borehole-extension” (defined

Reach Rock (Terzaghi, 1946) (kg/cm2) Cross section Spacing Deformational behavior

(m) type Vertical (Hp) Horizontal Horizontal Vertical Shape and size (mm) (mm) of supports (visual)

160 to Black 1.1 (B + Ht) 0.3 (Hp + Ht) 1.30 4.48 D-shaped ribs 100 × 75 500 Intolerable rib deformations,

Ht= 2.5 m of vertical legs into the opening

308 Tunnelling in weak rocks

4 - Temporary invert strut

5 - Steel ring support 150x150mm or 300x140mm RSJ at 40cm spacing

6 - Joint of circular support

Fig 20.7 Sequence of excavation and support for the main tunnel through the intra-thrust zone atKalawar

as the relative movement between the tunnel periphery and the interior of the rock mass)

by single-point, rod-type borehole extensometers (depth equal to the diameter of opening)

to an accuracy of ±0.02 mm

These instruments were designed and developed at the Central Mining Research tute, Dhanbad (India) Test sections were established with “loose backfill” and “tightbackfill” in both the red shales and the black clays The loose backfill consisted of a 30 cmthick layer of tunnel muck thrown manually in the hollows around steel arches The tightbackfill consisted of systematically packed PCC (precast cement concrete) blocks

Insti-20.4.4 Test sections

The instruments were installed at the tunnel face soon after excavation Support density,type of backfill and the method of tunnelling were kept unchanged on either side ofthe test sections over a length equal to the tunnel diameter Table 20.4 describes the

Distance Support details (maximum observed value)

pilot shales mild blasttunnel

tunnel mild blast plate on

outer flange

3 2530 As above As above As above As above As above As above As above 824 0.8 0.4 – – 3.772 1.250 0.332

pilot clays mild blasttunnel

5 2631 9.0 m φ As above Heading 300 × 140 275 20 Tight 719 11.50 12.20 – – 5.512 1.620 4.408

tunnel mild blast plates on

both flanges

Notations: φ = Diameter; Pv= Support pressure at roof; PH= Support pressure at sides; Urv= Radial tunnel closure in vertical direction; UrH = Radial tunnel closure in horizontal

direction; Uby= Borehole extension at roof; UbR= Borehole extension at right wall; UbL = Borehole extension at left wall; [* Borehole extension is defined as relative displacement between two points – one located on the tunnel periphery and the other located at a depth equal to tunnel diameter].

310 Tunnelling in weak rocks

L,B B L,C,B L,C

P Contact pressure between rock and rib

Intersection Of Kalawar Inspection Gallery

and H R T

Thrust

Crushed Quartzites, Slates, Lime Stones

Crushed, Brecciated, Sheared Red Shales, Silt Stone

Soft, Plastic Black Clays

Sandstone, Silt Stone, Clay Stone

Kg

Head Race Tunnel

Gallery Inspection 3.0 m Dia Kalawar

N 19 W Thrust Krol

9.0 m Diam (Excavated)

2590 m Thrust

2600 2700

Distance From Chhibro End, m

Kg

Fig 20.9 Support density in the intra-thrust zone at Kalawar

locations of test sections, the size and shape of the opening, details of steel arches, type ofbackfill and results of instrumentation Test section 5 was set up in the black clays (nearthe Nahan thrust) when the 3.0 m diameter pilot tunnel was widened to 9.0 m diameter.Fig 20.8 shows the locations of test sections and Fig 20.9 shows the density of supportsprovided in this zone Typical observations of support pressure and radial tunnel closure,and borehole extension are shown in Fig 20.10

20.4.5 Design of supports

Tight backfill was used to minimize the loosening of the rock mass above the tunnel crown

in order to minimize the risk of flooding (although the loose backfill relieved the rockload) Hence higher support pressures were assumed; for example, 6.0 kg/cm2(0.6 MPa)

in the red shales and 20–22 kg/cm2(2.0 to 2.2 MPa) in the black clays against observedsupport pressure of 3.07 and 12.2 kg/cm2(0.3 to 1.22 MPa), respectively (Fig 20.10, aand f) The support pressure was increased gradually from 6.0 kg/cm2 (0.6 MPa) in themiddle portion of the intra-thrust zone to 22 kg/cm2(2.2 MPa) in the black clays along the

Radial tunnel closure, cm

Radial tunnel closure, cm

4 8

12 0 2 4 6 0 2 4

(a) & (b) Red shales at Kalawar, Test section 1

(c) Red shales at Kalawar (closure at 3m),

Test section 1

(d) & (e) Black clays at Kalawar, Test section 4

(f) Black clays at Kalawar, Test section 5

Fig 20.10 Monitoring of support pressure and radial tunnel closure in red shales and black clays

in different instrumentation test sections at Kalawar

thrusts (Fig 20.9) The support density was reduced gradually to 6.0 kg/cm2(0.6 MPa) oneither side of the intra-thrust zone Subsequent embedment of these supports in concretehas not shown any sign of distress

20.5 TUNNEL CONSTRUCTION AND INSTRUMENTATION IN

INTRA-THRUST ZONE AT CHHIBRO

20.5.1 Tunnel construction

Local geology and construction details of the tunnel through this zone are shown inFig 20.11 In the beginning, the unexpected exposure of the red shales at 1139 m whiletunnelling from Chhibro was considered to be a local occurrence and the support den-sity was kept unchanged With continuation of the red shales, it was realized that asecond intra-thrust zone had been intersected The support pressure beyond 1185 m was

Excessive Precast Concrete Blocks

F1

6 - 150 mm x 150 mm with 250 mm x 25 mm plate welded on outer flange and 125 mm x 25 mm plate welded on inner flange

5 - 150 mm x 150 mm with 250 mm x 25 mm plate welded on outer flange

To Khodri

Mandhali quartzites and slates

Fig 20.11 Geological plan and construction details of head race tunnel through intra-thrust zone at Chhibro