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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
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.
. . (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
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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
Volume 3 Number 2, July-December 2015
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