STUDY OF STABILITY ANALYSIS OF UNDERGROUND …icmr.crru.ac.th/Journal/Journal...

Volume 3 Number 2, July-December 2015

76

STUDY OF STABILITY ANALYSIS OF UNDERGROUND STRUCTURE IN HIMALAYAN REGION OF NEPAL

Shyam Sundar Khadka, Ramesh Kumar Maskey

Department of Civil &Geomatics Engineering, Kathmandu University, Nepal

ABSTRACT This study focuses on the

stability analysis of underground structures in Lesser Himalayan Region of Nepal. Squeezing is one of the major stability problems in this region due to the poor rock mass quality and high overburden pressure. When the stress level exceeds the rock strength the tunnel fails. The critical stress is an indicator to design the support system for the tunnel. The conventional support itself is not adequate to control the squeezing problems.

Most of the hydropower tunnels in this region suffered from the squeezing. A detail study of the existing analytical, semi-empirical and empirical method of stability analysis must be conducted. In this study an empirical approach is used for squeezing assessment and support pressure estimation of the tunnel lies in Lesser Himalayan region. KEYWORDS

Himalaya, Stability Problems, Underground Structures

INTRODUCTION Nepal has the longest division

of the Himalaya occupied the central sector of Himalayan arc with about 880 kilometers from east to west and has a width ranging from 150 to 250 kilometers. It lies in a highly seismically vulnerable region by virtue of its proximity to the young Himalayan range and the ongoing neo-tectonic activities in the region. The seismicity of the country is attributed to the location in the sub-duction zone of Indian and Asian tectonic plate. Due to this active tectonic movement, the rock masses in Nepal are fragile and different in their engineering behavior. The major tectonic thrust faults such as main central thrust (MCT) and main boundary thrust (MBT) have significant influence on the high degree of shearing and fracturing to the rock mass. Considerable discrepancies have been found between predicted and actual rock mass conditions, resulting insignifi-cant cost and time overrun for most of the tunneling projects (Panthi & Nilsen, 2006).

In the Lesser Himalaya region of Nepal, tunnel squeezing is common phenomenon as the fault


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zones and weak rocks (e.g., mudstone, shale, slate, phyllite, schists, highly schistose gneiss) that compose the mountains are not capable of withstanding high stress (Panthi, 2006). In general, tunneling through such weak rock mass may cause severe squeezing problems which are directly related to its stability. Hydropower tunnels of four projects Kaligandaki A, Middle Marsyangdi, Modi and Khmiti (Figure 1) respectively have consider-able amount of squeezing occurred while tunneling. These all tunnels lies in Lesser Himalaya and have high overburden pressure with weak geology. Recently, the headrace

tunnel of Chameliya hydroelectric project has faced severe squeezing problem (Figure 2) with its cross section squeezed up to a maximum of 2.3 m (Table 1.) measured at 3+398 chainage, along the 843 m length. This geological difficulty resulted into a huge financial loss because of the necessity of heavily reinforced support in the squeezed section, although, the heavily reinforced concrete ring around the cross section of the tunnel is not a good option. Therefore, the knowledge on squeezeing plays a vital role in designing the support system.

Figure 1. Squeezing at Modi pressure tunnel (left) and Kaligandaki headrace

tunnel (right) (Panthi, 2006). From last decade, the construction of underground structures, like tunnel and caverns, has considerably increased day-by-day in Lesser Himalayan zone. The Lesser Himalayan zone lies between the Main Boundary Thrust (MBT) in the

south and the Main Central Thrust (MCT) in the North (Figure 1.). It contains many major thrusts as well as other types of fault. Tectonically, the zone is made up of low grade meta-sedimentary rock unit.


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Table 1. Tunnel in Lesser Himalayan region of Nepal.

Major existing and proposed

hydropower projects of Nepal are located in this region. Due to the valleys and water available with favorable water head number of hydropower projects are planning to construct and proposed in this region despite with weak geology. Due to few detail studies on ever changing

Himalayan region and geological condition in Nepal, it has been difficult to predict the effect of geology in underground structures. The need of study is not only relevant to underground structure such as tunnel but also helps for sound structural stability of small to large-scale projects.

SN Project Length ,

opening sizes, Shape

Geological rock types Stability Problems

1 Chameliya Hydroelectric Project

4.067 Km 5.2 & 4.2m dia., Horse shoe

shape

slate, phyllite, schists, quartzite, limestone, dolomite, etc

Tunnel squeezing inflow of ground water

2 Kulekhani III Hydroelectric Project

4.294 Km ,3.50 m dia., Horse Shoe shape

sandstone, siltstone, and mudstone Roof collapse

3 Modi Hydroelectric Project

1.5 Km, 15 Sq. m

highly fractured quartzite and highly sheared and highly deformed phyllite green schist

Rock Squeezing as well as severe ground water inflow

4 Middle Marsyangadi Hydroelectric

5.3 Km, 34 Sq.m, horse-shoe shaped

quartzite, phyllite and meta-sandstone Rock squeezing

5 Kali Gandaki Hydroelectric Project

5.95 Km, 60 Sq.m

Headrace tunnel mostly passes through highly deformed phyllite

Tunnel squeezing

6 Khimti Hydroelectric Project

7.9 Km, 14 Sq. m., inverted D-

Shape

banded gneiss and augen mica gneiss Tunnel squeezing


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Figure 2. Squeezing of wall of headrace tunnel (left) and deformation of crown of tunnel (right) of Chameliya hydroelectric project.

Source: Chameliya Hydropower Project. GEOLOGY OF NEPAL

Basically, the geology of Nepal is divided into following five zones: Gangetic plain zone, Sub-

Himalayan zone, Lesser Himalayan zone, Higher Himalayan zone, Tibetan-Tethys zone. Show in Table 2 and geological map in Figure 3.

Table 2. Geomorphic units of Nepal and main types of rocks

(After Upreti, 1999).

Geomorphic Unit Width(km) Altitudes (m) Main rock types

Gangetic plain (Terai zone) 20-50 100-200 Alluvium deposits ,coarse gravel in the

north near the foot of mountains

Siwaliks (Churia Group) 10-50 200-1000 Sandstone, mudstone, shale and

Conglomerate etc

Lesser Himalayan Zone 70-165 1000-5000 Schist, Phyllite, gneiss, quartzite, granite,

marble and dolomite.

Higher Himalayan Zone 10-60 > 5000 Gneisses schists and marbles

Tibetan-Tethys Zone - 2500 - 4000

Gneisses schists and marbles of Higher Himalayan Zone and Tethyan sediments (limestone, shale, sandstone etc)


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Figure 3. Geological map of Nepal (after Upreti and Le Fort, 1999). LH: Lesser Himalaya, HH: Higher Himalaya, TTS: Tibetan-Tethys Sediments,

MBT: Main Boundary Thrust, MCT: Main Central Thrust, MT: Mahabharat Thrust, STDS: South Tibetan Detachment System.

STRESS DISTRIBUTION AROUND UNDERGROUND OPENING

During and after an under-ground opening is excavated the in-

situ stresses are redistributed in the remaining rock mass. The rock mass is homogenous, hydrostatic stress condition, isotropic, dry, linearly elastic and infinite medium.

Figure 4. The stress distribution around circular tunnel in a) elastic rock mass

(Goodman, 1989) b) plastic rock masses (Herget, 1988).


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The stress around the opening with radius, ri, depends on the distance R from the circle centre. The radial stress and tangential stress σr and σθ respectively are given by:

σr = σ ( 1-ri

2/R2) ; σθ = σ ( 1+ri2/R2)

The stress distribution is

shown in Fig. 4 in which tangential stress is twice the magnitude of the isostatic stress (σ1=σ2=σ3=σ) will be induced around the periphery of excavation. The distribution of the tangential stress depends on the mechanical properties of the rock mass. In weak rocks after excavation, if the strength of rock mass is less than the tangential stress there is

plastic deformation around the opening. The plastic zone around the opening cannot take high stress and the stress peak moves outward, from the boundary of the opening into the rock as shown in Figure 4 (b).

When support applied to the opening, stress redistribution takes place and stress also build up in the tunnel support as shown in Figure 5. The support affects the stress condition in two ways: the stress level at opening contour increases and the stress peal level decreases (Goodman, 1989). Support provides confining pressure (σ3) and helps the rock mass to take more stress (Mohr-Coulomb criteria). Support activates only after rock mass pushes it.

Figure 5. The stress distribution around an opening after application of the tunnel support.

CONCEPT ON SQUEEZING

Squeezing is due to failure of a weak rock mass around a tunnel under the influence of high

overburden pressure or tectonic stresses. In 1995, the International Society of Rock Mechanics (ISRM) commission defined the rock squeezing as: “Squeezing of the rock


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is the time dependent large deformation, which occurs around the tunnel, and is essentially associated with creep caused by exceeding a limiting shear stress. Deformation may terminate during construction or continue over long time period.”

The over stressed zone of rock mass fails where tangential stress exceeds the uniaxial compressive stress of the rock mass. The rock mass around the opening is strained under the influence of induced stresses and deformed accordingly. These displacements are elastic in nature and remain generally within 1% of the tunnel radius, in case of competent rock masses. In soft rock, the failure process will travel gradually from the tunnel boundary to deeper region inside the unsupported rock mass. The zone of the failed rock mass is called the “broken zone.”

The tunnel closure may be both instantaneous and time dependent. It is the time dependent

displacement which dominates in fragile rock masses under high overburden, particularly when a broken zone is formed around an opening. Therefore, support system attempts to curb these time -dependent tunnel closure and in turn attracts higher loads (Jethwa, 1981, Dube, Singh & Singh, 1986).

Use of Rock Mass Classifica-tion approach

Classification approaches would be useful tool for estimation of squeezing potential. This approach is based on the rock mass quality Q (Barton et al. 1974) and overburden depth H, Singh et al. (1992) plotted a clear cut demarcation line to differentiate squeezing cases from non-squeezing cases as shown in Figure 6.

For squeezing condition, H >> 350Q1/3 (m)

For non squeezing conditions, H <<350 Q 1/3 (m)

Figure 6. Singh et al. (1992) approach to predict squeezing condition.


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Goel et al. (1995) developed a simple empirical approach based on the rock mass number, N as defined with stress reduction factor (SRF) is equal to 1. Rock mass number, N, is needed because of the problem and

uncertainties in obtaining the correct rating of Barton’s SRF parameter. Considering the overburden depth H, the tunnel span or diameter B, and the rock mass number N (Figure 7).

Figure 7. Criteria for predicting squeezing ground conditions using rock mass

number N.

For squeezing conditions, H >> (275 N0.33)B0.1(m)

For non-squeezing conditions, H<< (275 N0.33)B0.1(m)

It may be added that for both empirical approaches above, the degree of squeezing can be represented by tunnel convergence as follow (Singh &Goel, 1999)

i) Mild squeezing conver-gence 1-3% tunnel diameter

ii) Moderate squeezing con-vergence 3-5% tunnel diameter

iii) High squeezing conver-gence> 5% tunnel diameter

For Squeezing condition, Goel (1995) also estimated the ultimate support pressure with the following equations

Pv (sq) = 10. .

. (1) where, Pv(sq)= short-term support in squeezing ground condition in MPa, f(N)=correction factor for tunnel closure, and H and a =tunnel depth and tunnel radius in meters, respectively.

Similarly, Goel (1995) also estimated tunnel closure for squeezing ground condition with the following equation


84

.

. . (2)

Where, Ua/a= normalized tunnel closure in percentage, K= effective support stiffness in MPa, and H and a = tunnel depth and tunnel radius (half of tunnel width) in meters respectively. SQUEEZING ASSESSMENT OF CHAMELIYA HYDROELEC-TRIC PROJECT

Chameliya Hydro-electric Project (CHP) is an under construc-tion National Priority Project. It is located in Shikhar VDC of Darchula District, Mahakali Zone, Far-western Development Region of Nepal. Project area lies in Lesser Himalayas zone, in the catchment of the Chameliya River. A 4.067 Km long tunnel of 5.2 diameter has been

constructed for generating the 30 MW with gross head of 103.7 m. The main rock types within the project area are Dolomite, Sandstone, Slate, Dolomite intercalated with Slate, talconic Dolomite and Dolomite interbedded with Phyllite. The rock mass in the area are folded and faulted. Two faults are inferred across the tunnel alignment.

CHP has come up with a unique way to tackle with the squeezing problem encountered in the headrace tunnel. There is severe squeezing of headrace tunnel of 843 m length between Adit 2 and 3. The headrace tunnel passes through poor rock mass and the overburden varies between 174 and 279 m (Figure 9). Deformation of steel ribs, cracking of shotcrete, large mud flow and large deformation in crown and sidewalls are significant due to severe squeezing.

Figure 8. Monitoring of Squeezing in headrace tunnel of Chameliya.


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Monitoring the squeezing (Figure 8) is done on monthly basis. The instrument used is total station and method adopted is triangulation

survey. Squeezing treatment is requires between chainage 3+102 to chainage 3+945 m.

Figure 9. Longitudinal profile with geological description of the Chameliya

Hydroelectric Project between Adit 2 and Adit 3 (squeezing portion). Squeezing deformation and

support pressure calculations A circular tunnel with

hydrostatic stress conditions has been considered and support is assumed to act uniformly on the entire perimeter of the tunnel. An empirical method bade on the classification approach is used to calculate the support pressure and squeezing potential. Twenty numbers of sections are measured

and it is found the maximum convergence of 2.3 m of 2.5 m radius tunnel, Table 2. From both Singh et al (1992) and Goel et al (1995) approach squeezing takes place in the rock mass.

For the squeezing ground condition, short -term roof support pressure has been calculated in the squeezed section, Table 3.


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Table 3. Measured convergence of headrace tunnel, calculation of support pressure.

SN

Cha

inag

e

Ove

rbur

den

dept

h (m

)

Q- v

alue

Mea

sure

d C

onve

rgen

ce (m

)

Tunn

el c

losu

re

N-V

alue

Supp

ort

pres

sure

Sque

ezin

g Po

tent

ial

Nor

mal

ize

d tu

nnel

cl

osur

e

P v(S

q)

(Mpa

)

Sing

h et

al

(199

2)

Goe

l et

al(1

995)

Ua/

a

1 3+172 199.7 0.02 0.238 10% 0.14 0.27 93.52 122.37 2%

2 3+190 203.9 0.031 1.326 53% 0.21 0.44 106.41 139.88 2%

3 3+253 220.1 0.031 0.104 4% 0.08 0.77 77.96 101.73 3%

4 3+275 230.7 0.031 0.822 33% 0.1 0.95 75.52 109.51 3%

5 3+296 239.5 0.01 0.650 26% 0.1 1.02 75.52 109.51 3%

6 3+305 243.2 0.01 1.117 45% 0.1 1.05 75.52 109.51 3%

7 3+314 246.3 0.01 0.198 8% 0.1 0.5 75.52 109.51 3%

8 3+398 274.4 0.01 2.319 93% 0.06 2.31 70.11 92.52 4%

9 3+404 275.2 0.008 2.142 86% 0.06 2.33 70.11 92.52 4%

10 3+420 277.1 0.008 1.570 63% 0.06 2.36 70.11 92.52 4%

11 3+439 275.5 0.008 1.752 70% 0.06 2.33 70.11 92.52 4%

12 3+454 274.4 0.008 1.420 57% 0.06 2.31 70.11 92.52 4%

13 3+499 268.0 0.008 0.801 32% 0.06 2.2 70.11 92.52 4%

14 3+543 249.8 0.008 2.090 84% 0.06 1.89 70.11 92.52 3%

16 3+681 210.8 0.01 0.952 38% 0.1 0.82 75.52 109.51 3%

17 3+709 212.5 0.01 2.038 82% 0.04 2.15 55.66 80.93 3%

18 3+733 219.1 0.01 0.630 25% 0.1 0.88 75.52 109.51 3%

19 3+764 230.0 0.015 0.510 20% 0.14 0.71 84.48 122.37 3%

20 3+820 211.4 0.015 0.941 38% 0.14 0.62 84.48 122.37 2%

Approach for obtaining

Ground Reaction Curve The ground reaction curve

(GRC) is quite useful for the designing the supports for tunnel in squeezing ground condition (Daemen, 1975). A GRC shown in

Figure10 has been developed using the equations (1) and (2) based on the empirical approach suggested by Geol et at (1995).

T

C

thHfrwcinHoinpinpre

Table 4. Calcof G

Figure 10. G

CONCLUSIOThe un

he fragile aHimalayan regfrom differenwhich may leconstruction ang is the com

Himalayan regof the tunnels n terms of ti

proper geotenstrumentatio

predict the sock mass

estimation of

0

0.5

1

1.5

2

2.5

3

0 1

Supp

ort P

ress

ure

(Pv)

, MPa

Normaliz

Assumed Ua/a (%)

0.5 1 2 3 4 5

culation for coGRC.

Ground reactio

ON nderground strand tectonicagion of Nepant stability

eads hazards and operation

mmon problemgion of Nepalsuffered and

ime and costechnical stuon data it is dsqueezing be

along thef support pre

2 3 4zed Tunnel closure (Ua/a

Boundary ConditTunnel depth= 50Tunnel radius= 5Rock mass numb

Correction factor

2.7 2.2 1.5 1.2 1

0.8

onstruction

on curve.

ructures in ally active al suffered

problems during the n. Squeez-

m in Lesser l and most

d huge loss t. Lack of

udies and difficult to ehavior of e tunnel, essure and

5 6a),%

tions00 m5 mber =1

Ground Reaction Curve

Pv(Sq)

0.86 0.7

0.475 0.38

0.317 0.25

Volume 3

estimation othe preconstru

The Chameliya hsqueezed up The radius ototal length ois highly approach isidentificationduring theconstruction applicable toanalysis. Mpressure, tuninstrumentatiguidelines fdesign in tconstruction ACKNOWL

The thanks to DGeomatics mandu UniveChameliya htheir help reports and writing this p BIBLIOGRAAydan, O., A

T. potentunneRock 26(2)

Barla, G. 2squee

3 Number 2, July-D

f tunnel closuction stage.

headrace hydroelectric

to maximumof tunnel is 2of 843 m head

squeezed. s good for n and design e initial

of tunnel o general tunneasurement nnel closure ion data givefor the tunnthis region stage.

LEDGEMENauthor giv

Department oEngineering ersity. Also thhydroelectric

in collectininformation

paper.

APHY Akagi, T. and

1993.The ntial of roc

ls; theory andMech. Ro

, pp 137-163. 2001. Tunnelezing rock co

December 2015

87

sure during

tunnel of project is

m of 2.3 m. 2.5 m. The drace tunnel

Empirical squeezing

of support stages of

and also nel stability of support and other

es the clear nel support during the

T es sincere

of Civil & of Kath-

hanks to the project for

ng relevant needed in

Kawamoto, squeezing

cks around d prediction. ck Engng,

lling under onditions. A


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