Geological and geomechanical heterogeneity in deep hydropower tunnels: A rock burst failure case study

Tunnelling and Underground Space Technology 84 (2019) 507–521

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Geological and geomechanical heterogeneity in deep hydropower tunnels: A rock burst failure case study Abdul Muntaqim Najia,c, Muhammad Zaka Emadb, Hafeezur Rehmana,d, Hankyu Yooa,

T

⁎

a

Department of Civil and Environmental Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan 426-791, Republic of Korea Department of Mining Engineering, University of Engineering and Technology, Lahore, Pakistan c Department of Geological Engineering, Balochistan University of Information Technology Engineering and Management Sciences (BUITEMS), Quetta, Pakistan d Department of Mining Engineering, Balochistan University of Information Technology Engineering and Management Sciences (BUITEMS), Quetta, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Abnormal in-situ stresses Geological anomaly Rock burst Deep tunneling

Tunneling in the Himalayas is full of surprises due to the active state of stresses, along with hostile geological and geomechanical heterogeneities. These sometimes go unnoticed during the design stage, which can result in rock burst type failures, which is the case with a recently completed hydroelectric project in Pakistan. In this project, twin headrace tunnels are excavated by tunnel boring machines (TBMs). Several rock bursts have occurred in the tunnels of the case study project, which are influenced by many factors like in-situ and excavation induced stresses, rock type and its brittleness, bedding orientation, geological anomalies, and geological structures. The Himalayas lay on the most active plate margin zone, and excavation here is more difficult than in the Andes, Alps, or any other mountain belt in the world. The project area is surrounded by the Main Boundary Thrust (MBT), the most active fault in the region, along with local faults that pass through the headrace tunnels. Due to this active stress condition, thrust faulting is common, and a complex structural geological regime is prevalent. Abnormal in-situ stress conditions at a deep depth have caused unique geological anomalies in the unique sedimentary geological settings of the project area. The TBM, which has little flexibility during excavation, has done little to disperse the in-situ stresses near the boundary of the tunnel and has exaggerated the situation. The two rock burst events of January 13, 2016, and May 31, 2015, are selected for this study; they are classified as strain and fault-slip bursts, respectively. Empirical approaches have been used to evaluate the proneness of rock burst occurrence. Numerical simulation has also predicted the actual failure zone well, in such deep excavation. The details documented for these events not only provide a basis for understanding the process of rock burst in the Himalayas but also provide a good reference regarding the occurrence of rock burst in deep civil tunnels excavated in the hard rock.

1. Introduction Rock burst is a dangerous phenomenon, usually caused by the brittle failure of rock in deep excavations and associated with excavation induced seismic events (Kaiser and Cai, 2013). This type of failure releases violent energy that results in damage to openings, equipment, and casualties in severe cases (Mazaira and Konicek, 2015). This happens in hard massive rocks due to redistribution of high in-situ stresses around the openings at deep depth (Ortlepp and Stacey, 1994, Weishen et al., 2011; Li et al., 2012). Generally, violent rock burst occurs in harder and stiffer rocks because they can store more strain energy. Initially, this problem was common in deep mines, and reports of rock burst were made regarding mines in different countries such as India, South Africa, Russia, Europe, United States, Canada, and China (Tang,

⁎

2000, Kaiser and Cai, 2012). Today, however, rock bursting is a wellknown problem faced by engineers and workers during tunneling in civil engineering projects around the world. The majority of rock bursts in civil tunnels are strain type rock bursts, however, when different geological structural planes (faults, and shear zones) are present, fault slip type rock bursts usually occur. In case of strain type rock bursts, Hoek and Marinos (2009) described brittle rock failure in overstressed rock as, when the maximum boundary stress is close to the uniaxial compressive strength of the rock mass. An integrated method was proposed by Zhang et al. (2012) to interpret the effect of high in-situ stresses during the brittle failure that usually occurs in hard rocks. In such rocks, TBM operations are negatively affected by rock bursting, as in the case of Jinping II Hydropower station excavated in marble, due to the lack of flexibility of the TBM in

Corresponding author. E-mail address: [email protected] (H. Yoo).

https://doi.org/10.1016/j.tust.2018.11.009 Received 30 October 2017; Received in revised form 14 November 2018; Accepted 17 November 2018 0886-7798/ © 2018 Elsevier Ltd. All rights reserved.

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Muzaffarabad fault crosses the headrace tunnel. Frequent earthquakes and landslides are common in this area. Recently, the Muzaffarabad earthquake (2005), historically the largest earthquake recorded in the Himalayan range, was caused by a rupture along the Muzaffarabad fault and resulted in more than 75,000 fatalities. The NJHEP has been under construction since 2008 and completed recently in 2018. The project involves the diversion of Neelum River waters through tunnels at Nauseri, about 41 km upstream of Muzaffarabad, and out falling in the Jhelum River at Chatter Kalas in AJ&K, where the powerhouse is located (Fig. 1). Water is conveyed from the Neelum River through 28.5 km long headrace tunnels to the underground hydropower station.

adverse situations (Delisio et al., 2013). For fault slip type rock burst, Jiayou et al. (1989) described rock burst phenomena in headrace tunnels of the Tianshengqiao hydropower station excavated by tunnel boring machine (TBM) with a 10.8 m diameter in the limestone formation. Geologically, this area is located at the Nila anticline and Zhongshanbao syncline. Zhang et al. (2012) found different reasons for four extremely intense rock burst events in deep tunnels of the Jinping II Hydropower station, which has seven parallel tunnels. Two tunnels were excavated using TBMs with a diameter of 12.4 m. Major structural planes that were invisible during excavation caused the rock burst failure. One tunnel section was close to the syncline fold and had a more intense burst issue compared to the others due to local stress concentration. Geological and geomechanical failures are very critical during deep tunnel excavation for the safe and timely completion of a project. Insitu field measurements either in the form of in-situ stresses or geological mapping are very important for revealing different uncertainties. In the Neelum-Jhelum Hydroelectric Project (NJHEP), over coring method was used to measure the abnormal in-situ stresses, and detailed geological mapping was done. Induced stresses due to tunnel excavation were estimated. This paper explains how high abnormal horizontal stresses caused the brittle failure, which resulted in frequent rock bursting in hard sedimentary geological settings of headrace tunnels. Unseen structural and shear planes were identified along with geological anomaly that finally caused fault-slip type rock bursts in deep underground. Rock brittleness and induced stress criteria have been used to evaluate rock burst potential in headrace tunnels and actual failure zone due to rock bursting is numerically evaluated.

2.1. Project layout The project area is divided into three main sites: Nauseri area (Lot C1), Majhoi/Thota area (Lot C2) and Chatter Kalas area (Lot C3) (Figs. 1 and 2). The intake area is close to the town of Nauseri, located at the Neelum River, and the dam is a composite gravity dam 160 m long and 60 m high. The Lot C1 also includes the first 7.4 km of the single headrace tunnel, excavated using drill and blast (D&B). The Lot C2 area includes most of the headrace tunnel, access tunnels, and assemble and disassemble chambers for the TBMs. The headrace tunnel crosses the Jhelum River approximately 180 m below its bed. Finally, Lot C3 contains the powerhouse area, including the transformer hall, surge shaft, penstock/draft tube/busbar tunnels, tailrace tunnel, access tunnels and other housing facilities for operation and maintenance. The headrace tunnel will feed four vertical shaft turbines, with a combined installed capacity of 969 MW. The water will be discharged into the Jhelum River through a 3.54 km long tailrace tunnel.

2. Project description

2.2. Headrace tunnel layout

The Himalayas have great potential for hydropower generation; to harness this energy, deep tunneling here is the most challenging job in the world. The NJHEP is present in the Muzaffarabad district of Azad Jammu and Kashmir (AJ&K), the northeastern area of Pakistan (Fig. 1). The project area is a part of the Himalayan orogenic system, which consists of mighty mountain chains as well as deep and narrow gorges and valleys. Major faults, such as the Main Boundary Thrust (MBT), are located on both the intake and tailrace tunnel areas, while the

The headrace tunnels are excavated by the D&B method and TBM. The length of the headrace tunnel is 28.5 km. Initially, a single headrace tunnel was planned, but due to susceptible instability under high overburden, it was decided to split the single tunnel into twin tunnels under a high overburden. A single tunnel with the length of 8.94 km having a cross sectional area of 104 m2 was excavated by D&B method,

Fig. 1. Location map of the project area. 508

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Fig. 2. Project layout.

11 + 165 m and 12 + 130 m, with an expected invert level at chainage of 10 + 360 m and 9 + 350 m, respectively. Among these structures, faults and shear zones are the most susceptible structures with respect to rock bursting.

while twin tunnels with a length of 19.6 km having a cross sectional area of 52 m2 were excavated by TBM. The TBM twin tunnels are numbered 696 and 697. The TBM 696 is excavating the left tunnel (looking upstream), while 697 is excavating the right tunnel (looking upstream). During the tunnel excavation progress, TBM 697 was the lead TBM. The twin tunnels named the right and left tunnels, had an initial separation distance of 33 m, which was increased to 66 m to avoid inter-tunnel pillar bursting. The project area lay in an active tectonic zone and the headrace tunnels are passing through a stratigraphic sequence of alternative beds with a highly deformed geology having adverse folding and faulting. They pass through a series of northwest-trending anticlines and synclines, which are zones susceptible to stress concentration caused by the active Himalayan orogeny. During geological mapping, different types of structures are present in the area including folds, faults, and shear zones (Fig. 3). Both regional thrust (Himalayan frontal thrust) and locally developed low persistence faults are present in this area. Different local faults (F1–F15) have been identified in the field at different locations. Among these faults, F2 and F3 are present at chainage

3. Parameters susceptible to rock bursting Most of the rock burst events during headrace tunnel excavation were located at 4 different stations relative to TBM as; at TBM face, TBM shield, 5 m work area behind the TBM shield (L1) and 65 m work area behind the TBM Shield, (Jack Mierzejewski and Ashcroft, 2017). Based on in-situ stress measurements and geological mapping reports, the following geological and geomechanical parameters are related to frequent rock burst events: i. Abnormal in-situ stresses ii. Hostile sedimentary geological settings and thrust tectonics in deep excavation iii. Geological anomalies

Fig. 3. NJHEP headrace tunnel geological map. 509

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2000

5

1500

4

High overburden stresses High tectonic stresses 31st May Rockburst event

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2

500

1

0 6000

8000

10000

12000

6000

14000

8000

10000

12000

14000

Tunnel chinage (m)

Tunnel chainage (m)

Fig. 4. Overburden and horizontal to vertical ratio of stresses (K0) along with headrace tunnel chainage.

TF (thrust faulting) condition. The magnitude of total tectonic horizontal stress varies considerably and depends upon the geographic location, geological environment, and distance from the main tectonic fault systems of the Himalaya (Panthi, 2012); it can be calculated from Eq. (1). Studies from nearby the Kohala hydropower project also showed that the area has high horizontal stresses due to active Himalayan faults (Wang and Bao, 2014). In the NJHEP, in-situ stress measurements were initially done at two different locations: one at Jhelum River and the second at the underground powerhouse. Hydro-jacking and hydro-fracturing gave variable results due to the anisotropic nature of the rock and high in-situ stresses (Jack Mierzejewski and Ashcroft, 2017)

3.1. Abnormal in-situ stresses The Himalayan region is the most tectonically active mountain range in the world due to collision between the Indian and Eurasian Plates, and the northward movement of the Indian Plate at a speed of 40–50 mm/y indicates a very complicated geological background in this area and caused many strong and violent ground shaking events especially the 2005 earthquake (Wang and Bao, 2014). Due to the IndoAsian plate movement, the tectonic stresses in this area are very high resulting in elevated horizontal stress along the tunnel chainage as shown in Fig. 4. Tectonic horizontal stresses are oriented along N60-65E in the project area (Fig. 5) (Heidbach et al., 2016). Most data points indicate a

Fig. 5. World stress map. 510

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K0 0

Depth (m)

500

0.0

0.5

1.0

K 0 = 0.3 +

1.5

2.0

2.5

3.0

Additionally, a subsidiary reverse/thrust fault named the Muzaffarabad Fault runs close to the course of the Jhelum River at Thotha, beneath which the project’s twin headrace tunnels pass. Main Sedimentary Lithologies in the project area:

3.5

100 z ( m)

(i) (ii) (iii) (i)

1000 1500 K 0 = 0.5 +

2000

1500 z ( m)

The sandstone (SS) is subdivided into SS1 and SS2. The SS1 is the strongest lithological unit in the headrace tunnel which is thickly bedded, at places massive and blocky and is moderate to closely jointed. At the outcrop, its thickness varies from 5 to 20 m. The SS2 is usually reddish brown and appears to be gradually changing to finer materials including thin siltstone and mudstone beds. On the other hand, during deep excavation, the thickness of the bedding was highly variable, from a few meters to more than 50 m. Laboratory test data at the start of the project indicated a mean Uniaxial Compressive Strength (UCS) of about 86 MPa in this unit, but subsequent tests on samples taken from beds that produced rock bursts were given significantly higher strength values. After performing the UCS test on 30 samples, 77% gave results in the 130–170 MPa range and the remaining 23% were to 230 MPa (Jack Mierzejewski and Ashcroft, 2017). These UCS test results reveal the very high strength of sandstone at depth, which results in the storage of a high amount of strain energy that is finally released in form of the frequent rock bursts events that occurred in this lithological unit.

2500 High tectonic stresses High overburden stresses

3000

Fig. 6. Abnormal measured stresses by over-coring.

σh =

ν × σv + σtec 1−ν

Sandstone Siltstone Mudstone Sandstone

(1)

However, over-coring was done later at different locations in the twin tunnels in sedimentary sandstone beds in the TBM tunnels indicated very high horizontal stress (Fig. 4). The value of K0 was up to 2.9 modified, where the major principal stress was oriented sub-horizontally and nearly perpendicular to the tunnel azimuth. These elevated values of horizontal stress have a big component of tectonic horizontal stress, which results in severe folding and faulting along the tunnel and is a primary reason behind slabbing in general and rock bursting in particular. According to (Brown and Hoek, 1978) vertical stresses increase with depth, but in the case of the NJHEP, horizontal stresses showed some abnormal behavior and exhibit out-of-range values (Fig. 6). This is due to the horizontal tectonic stresses component in Eq. (1) of the active Himalayan range. Therefore, in such conditions, brittle rock failure in the form of rock burst is almost certain and presence of geological structure can cause fault slip rock burst.

(ii) Siltstone Siltstone is strong to medium strength rock unit and intermixed with mudstone and shale. In general, the thickness of siltstone between Nauseri and Thotha ranges from 3 to 6 m, while that between Thotha and Agar Nullah ranges from 0.3 to 2 m. The intact rock strength of this unit is almost 66 MPa and has less potential to store strain energy, resulting in a lesser brittle failure type rock bursts in the headrace tunnels as compared to sandstone.

3.2. Hostile sedimentary geological settings and thrust tectonics in deep excavation

(iii) Mudstone Mudstone is the weakest rock unit of the Murree Formation, which has low strength properties. In general, the thickness of mudstone between Nauseri and Thotha ranges from 1.5 to 4 m. During TBM excavation, it accounted for approximately 9% of the total tunnel length, usually having a thickness of 2 m or less. The intact rock strength of mudstone shows that it has less potential for brittle failure. Engineering properties of these rocks units are shown in Table 1. During TBM excavation, the rock burst events occurred in sandstone and siltstone lithologies mostly. The sandstone was massive, while siltstone was mostly fractured and sheared. In the project, the rock mass class was categorized as Q1 to Q5 classes from best to worst rock mass quality based on Tunneling Quality index (Q) system. These rock qualities have been registered and categorized at the face during construction.

The Himalayan orogenic system is divided into the (i) SubHimalayas, (ii) lesser Himalayas, (iii) Higher Himalayas, (iv) Tethyan Himalayas and (v) Trans Himalayas (Fig. 7) (DiPietro and Pogue, 2004). The headrace tunnel of NJHEP is situated in the Sub-Himalayas which is deepest hydropower tunnel in the region and has unique geological settings with respect to world tunneling. In most of deep hydropower and road tunnels, the rocks units are mostly of metamorphic or igneous origin. The Jinping I and II hydropower project in China has mostly metamorphic rocks in deep civil tunnels (i.e., marble, schist) while the headrace tunnel of the Cheves hydropower project in Peru, and the Lotschberg base tunnel in Switzerland, have a mix of igneous and metamorphic rocks (i.e., granite, granodiorite, and gneiss) (Rojat et al., 2009, Shiyong et al., 2010, Veyrat et al., 2016). However, the entire NJHEP is excavated in sedimentary rocks of the Murree Formation of the Sub-Himalayas, which is of Eocene to Miocene age and the lateral equivalent of the Siwalik Group in India. The Murree Formation comprises intercalated beds of sandstone, siltstone, and mudstone that are folded and tectonized. The weakness zones and local thrust faults are commonly observed and are invariably oriented parallel to the regional bedding strike. The TBM tunnels are driven within the central portion of a zone bounded by major active Himalayan faults that trend sub-perpendicular to the tunnels. The MBT, which is the bounding fault that extends the full length of the Himalayan range, approximately follows the course of the Neelum River at the upstream start of the headrace tunnels at Nauseri.

3.3. Geological anomalies The northward movement of the Indian plate in the Himalayan orogenic system caused several thrust faults in the region, such as the Muzaffarabad fault (Sharma and Shanker, 2001). As mentioned in Sections 2.2 and 3.2, the project geological investigation revealed that this area consists of alternate beds of sedimentary rocks (sandstone, siltstone, and mudstone) experiencing high folding and faulting along with shear zones under high tectonic stresses. Additionally, the bedding planes were normally perpendicular to the tunnel direction which was a 511

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Fig.7. Tectonostratigraphic subdivisions of the Himalaya (DiPietro and Pogue, 2004).

This anomalous behavior of the sedimentary rock of the Murree Formation was further confirmed from the surface exposure of a local thrust fault in drag folded strata (Fig. 9) present near the geologically anomalous region. The over coring in-situ stress measurement program also confirmed a high horizontal stress regime between this chainage. Thus, this anomalous region had caused a series of strain burst failures before the big fault slip rock burst event of May 31, 2015. As mentioned in Section 2.2. F2 and F3 are two local faults that pass close to the invert level in headrace tunnels. Therefore, it can be said that the May 31 event was due to movement along either of local faults or shear zones. The presence of such geological structures remained totally unnoticed during geological mapping, resulting in both life and economic losses.

Table 1 GSI and intact rock parameters of different rock units along the headrace tunnel.

*

Rock type

UCS (MPa)

GSI

Ei (GPa)

SS1 SS2 Siltstone Mudstone

86 47 66 42

65 50 50 50

32 18.8 23.1 12.6

GSI-geological strength index

*

Ei-intact rock elastic modulus.

favorable condition for construction, however, the occurrence of rock burst due to brittle failure of hard sedimentary sandstone was most probable in this area. During the excavation of tunnel 696 near chainage 09+740, there were frequent rock bursts and geological face mapping of this chainage has shown anomalous geological bedding than the expected perpendicular bedding (Fig. 8a). This was later confirmed through the geological modeling of this area that the abnormal stress concentration caused reorientation of perpendicular bedding planes to traverse along the tunnel direction (Fig. 8b) (Bawden, 2015).

4. Instability problems in headrace tunnel During excavation, in-situ stresses are disturbed and readjusted per the initial state of stresses depending on excavation shape and size. Just after the excavation, there is a huge variation in principal stresses at the excavation boundary. In deep excavations, high in-situ stresses usually induce large deformation like squeezing or violent breakouts like rock bursting in weak and hard rocks, respectively (Ortlepp and Stacey

Fig. 8. (a) Geological face mapping (b) horizontal bedding of sedimentary rock units. 512

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Fig. 9. Surface exposure of the local thrust fault near C2 (Bawden, 2015).

Brittle failure

Hardening

Tensile strength

h

gt h

ngt

en

s t re

st

pe

ak

ult

im

at e

str

k P ea

Po

Major principal stress (σ1)

(Rockbursting)

0

Minor principal stress (σ3)

σ0

Fig. 10. Brittle failure around tunnel periphery under low confining stress (σ3).

the pieces are held together by the friction on the surface of the slabs without any cohesion. It is observed that this type of failure can potentially happen under low confining stress (σ3) environments at the boundary of the opening. Hajiabdolmajid et al. (2002) proposed Cohesion Weakening and Friction Strengthening (CWFS) model that captures the brittle failure of rock in deep excavation very well (Fig. 11) and is helpful to understand the actual brittle strain bursting phenomenon in the field. Based on this model, numerical simulation has been done in a later section and breakage zone match well with the actual breakage profile after strain burst event in NJHEP. Generally, circular tunnels with hard rock in an anisotropic stress field typically have their maximum stress concentration in the face region and on the two opposite walls behind the face. When the tangential (boundary) stresses exceed the rock mass strength (σcm), brittle failure will occur. Compared to D&B excavation method, in which desired rock support can be installed at the face, TBM is less flexible because support cannot be installed at the face directly after excavation. Generally, open type TBMs have a short front shield for cutter head and

1994, Barla et al., 2014, Fan et al., 2016, Miao et al., 2016). The headrace tunnel in the NJHEP has a maximum overburden height of 1870 m. Confining stresses at the excavation boundary are very low or equal to zero, resulting in brittle failure at the boundary (Fig. 10), however, the high confining pressure away from the boundary usually causes shear failure. 4.1. Instability due to brittle failure under high stresses The headrace tunnel 696 consists of alternate beds of hard sandstone and siltstone, which are brittle in nature. In brittle rocks, stressdriven failure is common, that often dominates the rock mass behavior around the tunnel. Such failure processes can lead to gradual raveling or to violent, strain bursting modes of instability that create difficult conditions for tunnel construction, whether advanced by TBM or by blasting. This is a progressive brittle failure in the form of breakouts occurs once the stress exceeds the in-situ rock strength at the tunnel wall (Martin, 1995). A distinct characteristic of brittle rock failure around the tunnels is the formation of unstable slabbing zones where 513

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Cohesion

Residual Cohesion

Frictional Strength

Maximum Frictional Strength Initial Cohesion

Plastic Strain

Plastic Strain

Fig. 11. Cohesion weakening and friction strengthening model (Hajiabdolmajid et al., 2002).

excavation on January 10, 2016. On January 13, 2016, a rock burst occurred at chainage 09+702 (Fig. 13). After this event, two more rock burst events occurred in this very hard massive sandstone bed under an overburden of almost 1400 m. Due to rock burst, the sandstone in the crown became highly fractured. The tunnel area was damaged at about 6–7 m from the shield along the tunnel. As the rock mass in this area was massive and intact, and no local fault was found along with high insitu stresses. Therefore, this strain type rock burst occurred due to the brittle failure of hard sandstone under high in-situ stress in a low confining stress (σ3) environment. 4.2. Instability due to fault slip type rock burst On May 31, 2015, an extreme rock burst occurred between chainage 09+706 and 09+793 in the headrace tunnel of TBM 696 (Fig. 14). Three workers died and a few others were injured after this event. Additionally, a large area of crown and walls fell down. Wire-mesh and ring beams were deformed or destroyed, and the TBM equipment was destroyed. After this big event, the rock burst shockwave caused damage to the left wall and crown of the nearby headrace tunnel already excavated by TBM 697. Before this big event, five mediums to slight strain burst events had occurred consecutively (Fig. 14). The details of each event are as follows: On May 21, 2015, a slight rock burst occurred at chainage 09+758, resulting in rockfall above the shield of the cutter head. The second event happened on May 23, 2015, with a moderate rock burst that occurred at chainage 09+742 with a great sound heard from L1. After this event, a large-scale rock mass was scaled, and the completed support tunnel sections obviously descended. The third event happened on May 25, 2015, with a slight rock burst near chainage 09+729, which resulted in the fall of fresh rock beyond the shield. On May 26, 2015, multiple, muffled sounds were heard near chainage 09+722 beyond the shield. The surrounding rocks deformed, and a lot of fresh rock fell down in section L1. It was judged as a slight rock burst. On May 29, 2015, during excavation, a rock burst happened with loud sounds from chainage 9+721 to 9+722, and rock fell down in section L1. The great sound of the rock burst was also heard in TBM 697 at the same time. A rock burst happened again with a great sound after some time. A lot of fresh rock also fell down beyond the shield. This was judged to be a medium rock burst. Finally, on May 31, 2015, at 11:35 PM, a super-strong rock burst occurred with a great sound from chainage 19+706 to 09+793 (Fig. 14) and damaged caused in the tunnel (Fig. 15a and b). The tunnel section between chainage 09+706 to 09+793 has abnormal geological conditions compared to the other sections (Fig. 14) and also mentioned in Section 3.3. This structural anomaly became the main reason for frequent strain bursts during this chainage due to high stress concentration. A shear zone was exposed after the most hazardous rock burst event of May 31, 2015, near the wall of tunnel 696 (Fig. 16). This is very similar to the fault-slip rock burst in the headrace tunnel of the Tianshengqiao II hydropower station (Lee et al., 1996) and drainage tunnels of the Jinping II hydropower station (Zhang et al.,

Fig. 12. GSI chart showing the rock mass brittle degradation process (Kaiser, 2007).

work protection that inhibits the installation of rock bolts and wire mesh behind the face, and large-scale shotcrete operation is not normal (Dammyr, 2016). This problem was confirmed during the excavation of the NJHEP headrace tunnels. The sudden brittle failure usually reduces the rock mass quality due to induced fractures near the boundary of the tunnel, as shown in the Geological Strength Index (GSI) chart (Fig. 12), which shows that the rock mass is degraded after brittle failure from an initially very good quality rock with GSI > 65 to a heavily damaged rock with GSI values as low as 50–35 (Kaiser, 2007). This same degradation process is witnessed in the case of NJHEP, and Q4 rock mass quality was very common after every rock burst event in hard brittle sandstone. In TBM tunnel 696, the presence of hard sandstone caused frequent rock burst events due to its high storage capacity of strain energy and high in-situ stresses. After the May 31, 2015 event, TBM 696 restarted 514

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Fig. 13. TBM 696 tunnel showing strain type rock burst due to brittle failure of hard sedimentary rock.

method for long tunnels in good-quality rock masses. Additionally, the lack of intermediate access points due to the steepness of Himalayan mountain ranges, also makes TBM a potentially viable method of excavation. However, due to the lower flexibility of this method, it is especially important to understand the rock mass behavior well to avoid rock burst failure. Low TBM flexibility has caused considerable difficulty in executing the tunnel excavation, creating serious concerns about tunnel stability and the safety of working personnel. For safe construction, horizontal and vertical relief holes were excavated to release the high stresses. Such relief holes proved insufficient to avoid the May 31, 2015, and January 13, 2016 events. In such tunnel sections where safe TBM tunneling is inevitable, the D & B method should be used to excavate a pilot tunnel to release high stresses in advance. Such pilot tunnels also serve as an advanced geological exploration tunnel. Extreme rock burst can be effectively overcome by using such an excavation scheme (Zheng

2012). Therefore, it is concluded that as the tunnel 696 approached one of such shear zone, causing an un-clamping of this structure which resulted in the most intense and disastrous event of rock burst. 4.3. Instability due to low flexibility of TBM Mechanized tunneling in hard rock has low excavation disturbance to adjacent rock masses, which has been observed by more violent/ intense failures in tunnels compared to conventional tunneling (Ortlepp and Stacey, 1994; Myrvang et al., 1998). As reported (Ortlepp and Stacey, 1994), increased fracturing in the skin of a rock around a blasted tunnel destresses the skin and results in conditions that are less conducive to strain bursting. Thus, the intensity of failure is higher in machine-excavated tunnels; experience has shown that the extent of failure and rock support needs can be greater compared to D&B excavation (Myrvang et al., 1998). TBM is often the preferred excavation

Fig.14. TBM 696 tunnel showing chronological order of the May 31, 2015 event. 515

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Fig. 15. Rock burst damage from the May 31, 2015 event. 1000

5. Empirical rock bursting evaluation According to (Jack Mierzejewski and Ashcroft, 2017), 80% of rock burst events occurred in high strength sandstone, and the remaining events occurred in siltstone. During headrace tunnel excavation, initially no rock bursting occurred in the TBM tunnels, but after 2.3 km, rock bursts commenced. After 4.7 km, the number and magnitude of rock bursts increased dramatically. Rock bursts in the NJHEP have been classified into four main categories; Category: 1 (only sound due to brittle failure in rock mass) Category: 2 (mild damage to rock support) Category: 3 (spalling and slabbing) Category: 4 (violent ejection) The cumulative rock burst count for Category-3 and Category-4 is calculated, which is further updated and modified after (Jack Mierzejewski and Ashcroft, 2017) according to the in-situ-stress ratio (k0) along the headrace chainage (Fig. 17). In the project history, the rock burst on May 31 (Fig. 17) between

5 Category-3 Rockburst Category-4 Rockburst K0

800

4

600 3

K0

Rockburst cumulative count

et al., 2016).

400 2

200

0 7000

8000

9000

10000

11000

1 12000

Tunnel chainage (m) Fig.17. Cumulative rock burst count in abnormal stress area.

chainage 9+000 to 10+000 is regarded the most severe rock burst, and this event is equated to a 2.4 magnitude earthquake. According to face mapping reports, the Q4 rock mass quality of sandstone is

Fig. 16. Fault exposed in TBM tunnel 696. 516

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Sandstone Siltstone Contact

100

TBM697

60

60

Frequency (%)

TBM696

80

40 40

20 16 12

20 8 4 0

0

13+100-10+300

10+300-9+500

9+500-8+700

8+700-7+700

13+100-10+100 11+100-10+500

10+500-9+900

9+900-9+500

Chainage (m)

Chainage (m) Fig.18. Frequency of rock burst in different rock zones.

encountered in this section, which is potentially an unstable class according to the Q system of the project. High horizontal stress due to the active stress state and adverse geological settings paved the way to the release of the stored strained energy under very low confining stress (σ3) near headrace tunnel periphery. The high amount of stored strain energy in hard sandstone that was suddenly released caused brittle failure of the locally parallel striking bedding of sandstone. After analyzing the rock burst record, it can be said that this event occurred in sandstone, siltstone and the contact between sandstone and siltstone (Fig. 18). The following criteria have been used to empirically evaluate this failure phenomenon.

Table 2 Rock brittleness criteria for NJHEP.

5.1. Rock brittleness criteria Wang and Park (2001) explained rock brittleness criterion which is defined by an index of the ratio of uniaxial compressive strength to the tensile strength of rock, that is:

σc σT

(2)

where σc is the uniaxial compressive strength (MPa), and σT is the tensile strength of the rock (MPa). Experimental study and in-situ investigation by (Chunsheng and Zhiyou, 1998) show the following: 1. 2. 3. 4.

Sedimentary rock unit

σc (MPa)

σT (MPa)

B

Failure

1 2 3

Sandstone (SS-I) Sandstone (SS-II) Siltstone

86 46 66

7.72 8.51 6.93

11.3 5.4 9.52

violent rock burst violent rock burst violent rock burst

work, Barton addresses the rock stress problem in competent rock masses. This work was further updated in 1993 (Grimstad, 1993). In stressed rock mass, when stress conditions, compressive strength, and tensile strength of a rock mass are known, then it is possible to predict rock burst in hard rocks. In NJHEP, Data from five different locations were used to correlate the relationship between compressive strength (σc), tensile strength (σt), maximum tangential stress (σθ), and virgin maximum principal stress level (σ1). The rock burst sections in NJHEP satisfied the criteria above and proved successful in estimating the risk at the measurement sites. Table 3 shows the different rock stress conditions when the in-situ stresses and rock strength values are known. Based on SRF relation to stress-strength ratio, 20 was the extreme value of SRF for competent rock mass having rock stress problem. Eq. (3) was used for SRF calculation in this case which was proposed by (Kirsten, 1988).

1. Rock brittleness criteria 2. Induced stress criteria

B=

Sr. no

1.322

H SRF = 0.244K 0.346 ⎛ ⎞ ⎝ σc ⎠ ⎜

⎟

σ 1.43 + 0.176 ⎛ c ⎞ ⎝H⎠

(3)

where σc is uniaxial compressive strength, H is depth (m) and K is horizontal to vertical stress ratio. Table 4 shows the calculated values of different parameters like σc/ σ1, σt/σ1, σɵ/σc, and SRF for five dangerous chainages. Values in Table 4 are in the range of heavy rock burst as mentioned in Table 3. Therefore, these empirical evaluation criteria have validated the occurrence of frequent brittle rock bursts in the headrace tunnels.

B > 40, then no rock burst; B = 40–26.7, then weak rock burst; B = 26.7–14.5, then strong rock burst; and B < 14.5, then violent rock burst.

In NJHEP, three sedimentary rock units are very dangerous and most susceptible to brittle failure: Sandstone (SS-I), Sandstone (SS-II), and Siltstone. When compressive strength and tensile strength are known, then it is empirically possible to predict rock burst failure. The rock brittleness criterion, discussed above, is satisfied for these rock units, and Table 2 shows the brittleness and failure modes of these intact rock units that resulted in frequent brittle failure during the construction of the headrace tunnel.

Table 3 Rock burst estimation from (Barton et al., 1974) and (Grimstad, 1993).

5.2. Induced stress criteria In 1974, Barton provided an engineering classification of rock masses for the design of tunnel support (Barton et al., 1974). In this 517

Sr. no

Rock stress problem

σc/σ1

σt/σ1

σθ/σc

SRF

1 2 3 4 5 6

Low stress, near the surface Medium Stress High Stress Moderate Slabbing Slabbing and Rock burst Heavy rock burst (strainburst)

> 200 200–10 10–5 5–3 3–2 <2

> 13 13–0.66 0.66–0.33 – 0.33–0.16 < 0.16

< 0.01 0.01–0.3 0.3–0.4 0.5–0.65 0.65–1 >1

2.5 1.0 0.5–0.2 – 5–10 10–20

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Table 4 NJHEP rock burst evaluation based on the Barton and Grimstad criteria. Sr. no

Chainage (km)

Overburden (m)

Stress ratio (k)

σc/σ1

σt/σ1

σθ/σc

SRF

Failure

1 2 3 4 5

10 + 875 9 + 750 9 + 570 8 + 760 8 + 245

1090 1210 1220 1550 1795

2.9 2 2 1.25 1

0.75 1.32 1.31 1.64 1.77

0.067 0.118 0.117 0.148 0.159

3.66 1.9 1.92 1.34 1.13

11.19 10.25 10.32 11.89 11.32

Heavy Heavy Heavy Heavy Heavy

6. Numerical simulation of tunnel excavation

Rock Rock Rock Rock Rock

the actual failure zones around the tunnel. The simulated model was run after applying the initial conditions. The numerical modeling has given promising results in low confinement area around the periphery of the tunnel. The CWFS model captured the brittle failure well which developed in the upper right and lower left quadrant of the circle tunnel excavated by TBM (Fig. 19) which is according to the frequent strain burst issues during the construction of headrace tunnel in hard rock unit. Therefore, it is concluded that the recurring issue of strain burst was due to brittle failure of hard sandstone. The model is not symmetric in terms of stresses and the geological conditions are complex. Normally during excavation, stresses are considered in pure compression while in the actual situation, the excavations are subjected to both compression and shear loading (Suorineni et al., 2011; Suorineni et al., 2014). The shear part of loading is not taken in to account usually, in underground excavations which results in unintended consequences like elevated risks of seismicity and rock burst. During the construction of NJHEP, overcoring stress measurement program was conducted, presence very high abnormal horizontal stresses along with shear stresses in the field has caused asymmetric failure in the upper left and lower right quadrant around the tunnel 696 (Fig. 19). A shear stress value of 8.9 MPa was selected for this analysis.

6.1. Numerical modeling of strain burst around tunnel 696 Brittle failure of rocks in deep excavation has been studied by many researchers. The traditional failure models like Mohr-coulomb and Hoek-Brown are inappropriate for brittle failure process and overestimate the rock mass strength (Ortlepp and Stacey, 1994, Eberhardt, 2001, Diederichs et al., 2004). In these models, cohesion and friction angle are mobilized simultaneously. Hajiabdolmajid et al. (2002) proposed a CWFS model which states non simultaneous mobilization of cohesion and friction angle as a function of rock damage or plastic strain (Fig. 11). According to this model, cohesion is mobilized first followed by friction angle. The brittle failure in rock is a gradual process of cohesive strength weakening by tensile cracking at the early stages of loading. This cohesional component of strength is the predominant strength component at the early stage of brittle failure and cohesion loss is the predominant failure process leading to the observed brittle behavior without affecting the frictional component. The final cohesive strength is gradually destroyed by tensile cracking and crack coalescence. The mobilization of the frictional strength component occurs when the cohesive strength component is significantly reduced. Hajiabdolmajid (2003) attributed the very low strength observed around the Mine-by tunnel project to a delayed mobilization of the frictional component unlikely to laboratory compression tests in which the frictional strength reaches its full mobilized capacity with less cohesion loss. Therefore, a very small peak friction angle in combination with an actual peak cohesion value is used for well capturing of notch breakout in hard brittle rocks (Hajiabdolmajid et al., 2002, Corthésy and Leite, 2008). Many case studies have also verified this model (CWFS) to study brittle failure at depth (Edelbro, 2009, Lee et al., 2012, Golchinfar, 2013). The abnormal stress concentration in the complex geological system of in NJHEP caused frequent strain burst events like January 13, 2016 event. Most of these events occurred 9–12 o’clock in the upper left shoulder and 4–7 o’clock in lower right quadrant in circular headrace tunnel. A numerical analysis was conducted based on rock properties determined from testing to study the strain burst type brittle failure around headrace tunnel 696. As this failure occurred mainly in hard sandstone rock. Table 5 shows different sandstone properties which were determined from testing performed during the design stage and during the construction stage. The Peak and residual values for strength parameters were calculated from damage control test. The FLAC2D numerical simulation with CWFS model was adopted for the modeling of tunnel 696 with the diameter of 8.53 m. The actual in-situ stress conditions were adopted based on over coring test results for capturing

6.2. Numerical modeling of fault slip burst around tunnel 696 The presence of small scale faults or shear zones like geological structures is one of the main factors for rockburst occurrence near the excavation boundary. These structures have been present along the headrace tunnel of NJHEP which are discussed in detail in the geological mapping section as shown in Fig. 3. A shear zone type small scale fault plane exposed after the rock burst event of May 31, 2015, along with the side of tunnel 696. Such type of geological structures also caused major rockburst during the construction of Jinping II hydropower project (Feng, 2017). The FLAC2D explicit code has been used to simulate shear zone to find the mechanism of rock burst near the wall of the tunnel which has caused the most intense rock burst event of NJHEP under static and dynamic conditions. FLAC provides an interface element to model fault/shear zone which is a collection of triangular elements with different parameters like shear stiffness (ks), normal stiffness (kn), cohesion (c) and friction angle (ϕ). Sainoki and Mitri (2014) have discussed the mechanical properties of the fault influenced by many factors which are difficult to determine due to varying material accumulation on fault surface and scale effect during laboratory tests. Barton and Choubey (1977) have proposed the relationships (Eqs. (4) and (5)) to calculate kn and shear stiffness ks. Therefore, on the basis of these equations stiffness properties of shear zone have been

Table 5 Stress ratio and material properties for CWFS model. Rock type (sedimentary) Depth (m) Stress ratio (k0) Unit weight (kN/m3) Uniaxial compressive strength (MPa) Elastic modulus (GPa)

burst burst burst burst burst

sandstone 1200 2.4 27 86 32

Peak cohesion (MPa) Residual cohesion (MPa) Peak friction angle (ϕ) Residual friction angle (ϕ) Plastic strain (%) Poisson’s ratio

518

4.9 3.6 0 46 2 0.25

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Fig. 19. Brittle failure zone around the Tunnel using CWFS model.

Fig. 20. Yielded zone state in upper left quadrant of TBM tunnel 696.

Er = intact rock Young’s modulus G = rock mass shear modulus Gr = intact rock shear modulus s = joint spacing

calculated.

kn =

EEr s (Er − E )

(4)

ks =

GGr s (Gr − G )

(5)

Palmström (2000) has suggested different joint spacing values for different rock masses and should be greater than 10 m for massive rock. Therefore, considering the value of joint spacing suggested by Palmstrӧm, the stiffness values of interface element have been calculated: kn = 9.43 × 102 MPa/m and ks = 3.77 × 102 MPa/m. A fictitious value of frictions angle 5° has been adopted after performing sensitivity analysis for different friction angles like 5°,15°,25°, and 35°, along with

where: kn = joint normal stiffness ks = joint shear stiffness E = rock mass Young’s modulus 519

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6. This analysis of NJHEP headrace tunnel provides a valuable reference for understanding the rock burst mechanism in deep tunnels with special reference to the Himalayas.

the very low value of cohesion (Naji et al., 2018). An elasto-plastic constitutive material model based on hard sandstone rock unit parameters, with Mohr-Coulomb criteria, have been used for this analysis. During static analysis fixed boundaries conditions have been used and during dynamic analysis quiet (viscous) boundaries have been used to prevent reflection of the dynamic wave into the model. The static analysis resulted in the concentration of principal stresses near the extreme boundaries of the shear zone which caused shear slip. This shear slip along the shear zone is a dynamic phenomenon which has caused a sudden release of seismic energy stored in rock mass resulting in the violent ejection of failed rock mass along with huge failure zone near the boundary of the tunnel. During dynamic analysis, a synthetic compressive wave has been used as an input near the upper boundary of the shear zone where the principal stresses were concentrated after static analysis. The applied input is in the horizontal direction due to horizontal major principal stress. The yielded zone around tunnel 696, illustrates shear and tensile failures around the boundary of the tunnel as shown in Fig. 20. This yielded area observed after numerical analysis is very similar to the actual damage zone as shown in Fig. 16 as observed by (Bawden, 2015) near the wall of tunnel 696 which has caused this extreme event of rockburst on May 31, 2015. The displacement of rock mass around an underground excavation is a very important factor to its stability. Therefore, the displacement vectors shown in Fig. 20, reflect larger and deeper deformations on the side of the shear zone. These displacement vectors are high in upper left quadrant which has further confirmed the failure zone area where dynamic rockburst event of May 31, 2015, occurred. This rockburst event produced a damage zone with the depth of almost 5 m similar to actual field observation as reported by Bawden (2015).

Acknowledgements This research was supported by the Korea Agency for Infrastructure Technology Advancement under the Ministry of Land, Infrastructure and Transport of the Korean government. (Project Number: 18SCIPB108153-04). The authors (Abdul Muntaqim Naji and Hafeezur Rehman) are extremely thankful to the Higher Education Commission (HEC) of Pakistan for HRDI-UESTPs scholarship. References Barla, G., Barla, M., Bonini, M., Debernardi, D., 2014. Guidelines for TBM tunnelling in squeezing conditions – a case study. Géotechnique Lett. 4 (2), 83–87. Barton, N., Choubey, V., 1977. The shear strength of rock joints in theory and practice. Rock Mech. 10 (1–2), 1–54. Barton, N., Lien, R., Lunde, J., 1974. Engineering classification of rock masses for the design of tunnel support. Rock Mech. 6 (4), 189–236. Bawden, W.F., 2015. Neelum Jhelum Hydroelectric Project Rockburst Investigation Report. Brown, E., Hoek, E., 1978. Trends in relationships between measured in-situ stresses and depth. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. (Elsevier). Chunsheng, Q., Zhiyou, T., 1998. Possibility of rockburst occurrence in Dongguashan copper deposit. Chinese J. Rock Mech. Eng. 17, 917–921. Corthésy, R., Leite, M., 2008. A strain-softening numerical model of core discing and damage. Int. J. Rock Mech. Min. Sci. 45 (3), 329–350. Dammyr, Ø., 2016. Prediction of brittle failure for TBM tunnels in anisotropic rock: a case study from Northern Norway. Rock Mech. Rock Eng. 49 (6), 2131–2153. Delisio, A., Zhao, J., Einstein, H., 2013. Analysis and prediction of TBM performance in blocky rock conditions at the Lötschberg Base Tunnel. Tunn. Undergr. Space Technol. 33, 131–142. Diederichs, M., Kaiser, P., Eberhardt, E., 2004. Damage initiation and propagation in hard rock during tunnelling and the influence of near-face stress rotation. Int. J. Rock Mech. Min. Sci. 41 (5), 785–812. DiPietro, J.A., Pogue, K.R., 2004. Tectonostratigraphic subdivisions of the Himalaya: a view from the west. Tectonics 23 (5). Eberhardt, E., 2001. Numerical modelling of three-dimension stress rotation ahead of an advancing tunnel face. Int. J. Rock Mech. Min. Sci. 38 (4), 499–518. Edelbro, C., 2009. Numerical modelling of observed fallouts in hard rock masses using an instantaneous cohesion-softening friction-hardening model. Tunn. Undergr. Space Technol. 24 (4), 398–409. Fan, Y., Lu, W., Zhou, Y., Yan, P., Leng, Z., Chen, M., 2016. Influence of tunneling methods on the strainburst characteristics during the excavation of deep rock masses. Eng. Geol. 201, 85–95. Feng, X.-T., 2017. Rockburst: Mechanisms, Monitoring, Warning, and Mitigation. Butterworth-Heinemann. Golchinfar, N., 2013. Numerical modeling of brittle rock failure around underground openings under statis and dynamic stress loadings. Laurentian University of Sudbury. Grimstad, E., 1993. Updating the Q-system for NMT. In: Proceedings of the International Symposium on Sprayed Concrete-Modern use of Wet Mix Sprayed Concrete for Underground Support, Fagemes, Oslo, Norwegian Concrete Association, 1993. Hajiabdolmajid, V., Kaiser, P., Martin, C., 2002. Modelling brittle failure of rock. Int. J. Rock Mech. Min. Sci. 39 (6), 731–741. Hajiabdolmajid, V.R., 2003. Mobilization of Strength in Brittle Failure of Rock. Heidbach, O., Rajabi, M., Ziegler, M., Reiter, K., 2016. The World Stress Map database release 2016-global crustal stress pattern vs. absolute plate motion. EGU General Assembly Conference Abstracts. Hoek, E., Marinos, P., 2009. Tunnelling in overstressed rock. ISRM Regional SymposiumEUROCK 2009. International Society for Rock Mechanics. Jack Mierzejewski, Bruce Ashcroft, G.P., 2017. Short-Term Rockburst Prediction in TBM Tunnels. Worl Tunnel Congress. Bergen,Norway, 10. Jiayou, L., Lihui, D., Chengjie, Z., Zebin, W., 1989. The brittle failure of rock around underground openings. ISRM International Symposium, International Society for Rock Mechanics. Kaiser, P., 2007. Rock mechanics challenges and opportunities in underground construction and mining. Keynote lecture, 1st Canada-US Rock Mechanics Symposium, on CD. Kaiser, P., Cai, M., 2013. Critical review of design principles for rock support in burstprone ground–time to rethink! In: Proceedings of the Seventh International Symposium on Ground Support in Mining and Underground Construction, Australian Centre for Geomechanics. Kaiser, P.K., Cai, M., 2012. Design of rock support system under rockburst condition. J. Rock Mech. Geotech. Eng. 4 (3), 215–227. Kirsten, H., 1988. Case histories of groundmass characterization for excavatability. Rock classification systems for engineering purposes, ASTM International. Lee, C., Sijing, W., Zhifu, Y., 1996. Geotechnical aspects of rock tunnelling in China. Tunn. Undergr. Space Technol. 11 (4), 445–454. Lee, K.-H., Lee, I.-M., Shin, Y.-J., 2012. Brittle rock property and damage index

7. Conclusion Stress driven strain burst and shear zone type fault slip rock burst impose very dangerous conditions during deep tunneling. Careful excavation is required in both geologically and geomechanically heterogeneous environment. Rock burst not only poses a great threat to worker safety but also slows the tunneling process. For excavation stability, failure conditions must be anticipated early to minimize losses due to rock bursting. Unseen and delicate shear and fault zones should be carefully studied and evaluated during both the design and construction phase of the project. Such heterogeneous environments require a comprehensive characterization of the rock mass. In this fragile environment, rock bursting is inevitable when tunnels are excavated with less flexible TBM. The main conclusions of this work are as follow: 1. The NJHEP tunnels lay near convergent plate boundaries in the Himalayas, posing a great threat to their stability because tectonic processes have a considerable effect on the major principal stresses. Over-coring in-situ stress measurements showed very abnormal stress conditions along the headrace tunnel. Violent rock bursts occurred where the K0 value was greater than or equal to 2 (K0 ≥ 2). 2. The project area has a unique sedimentary geological setting with brittle sandstone and siltstone rock units that are very susceptible to rock bursting, thus causing the January 13, 2016 event. 3. As the tunnel 696 approached the shear zone type small scale fault, caused a sudden slip (due to unclamping) creating favorable conditions for the most dangerous fault slip rock burst event of May 31, 2015. 4. The area 65 m behind the tunnel is the most susceptible to rock bursting. A robust system of prediction should be incorporated in the construction phase so that such failures must be averted. 5. Horizontal and vertical relief holes were excavated to release the high stresses but such relief holes proved insufficient to avoid the aforementioned strain burst and fault slip burst. In such tunnel sections, different techniques like pilot tunnel excavation should be used to release high stresses. 520

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