Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore

Accepted Manuscript Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore

Kar Winn, Louis Ngai Yuen Wong, Leandro R. Alejano PII: DOI: Article Number: Reference:

S0013-7952(19)30066-3 https://doi.org/10.1016/j.enggeo.2019.105164 105164 ENGEO 105164

To appear in:

Engineering Geology

Received date: Revised date: Accepted date:

10 January 2019 20 May 2019 24 May 2019

Please cite this article as: K. Winn, L.N.Y. Wong and L.R. Alejano, Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore, Engineering Geology, https://doi.org/10.1016/j.enggeo.2019.105164

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ACCEPTED MANUSCRIPT Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore Kar Winn1 , Louis Ngai Yuen Wong 2, 3* Leandro R. Alejano 4 1

Society of Rock Mechanics and Engineering Geology,Singapore, Home: Block 102, #03-275, Aljunied Crescent, Singapore 380102 2

Faculty of Engineering, China University of Geosciences, Wuhan, Hubei, China, 430074 Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China 4 Department of Natural Resources and Environmental Engineering, University of Vigo, Spain

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3

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*Corresponding author: [email protected]

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Abstract:

The first commercial underground rock caverns for hydrocarbon storage in South East Asia, the Jurong Rock Caverns Project (JRC Project), have recently been completed in Singapore.

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The storage capacity of 5 caverns is 1.47 million cubic meters and 3.5 million cubic meters of rocks were excavated in this project. Geologically, it was excavated in low-angled bedded

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meta-sedimentary mudstone, pyroclastic rocks and sandstone. Although traditional archroofed style excavations are typical for tunnel and cavern for stability condition, flat-roofed profiles tend to form in low-angle stratified sedimentary rock mass. In this study, the stability

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of such flat-roofed tunnel and cavern with structurally controlled stability was analyzed using geotechnical classification systems including the stability graph method, analytical Voussoir

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techniques and numerical models. Accordingly, and once presented the main features of the project, characterization of the rock masses is presented. Some classification system approaches providing a convenient but probably over-estimated support and reinforcement

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recommendation are illustrated. Then, basic wedge stability analysis is briefly illustrated. Voussoir analogue techniques are applied to unsupported cavern profiles showing instability, but clear stability for the supported case. Discontinuous models of the final designed cavities are presented, showing a good agreement with indicative measurements on place, and stress distributions associated with the excavation sections are analyzed using Examine2D to show that abutment relaxation was unlikely to occur in JRC Project. The study shows that classification systems are useful tools but application of analytical techniques and numerical models contribute to a better understanding of the excavation and reinforcement process. Key words: low-angle bedded sedimentary rocks, Voussoir beam theory, arched-roofed design, flatroofed design, abutment relaxation, unstable wedge.

ACCEPTED MANUSCRIPT 1. Introduction The design of underground excavations typically proceeds from an initial configuration which could satisfy its function requirements, such as minimum dimensions required, preferred location and orientation, and the need for integration with other elements of the general layout. The draft design should then be assessed by considering the possible occurrence of various undesired instability phenomena. To achieve that, Brady & Brown

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(2006) proposed to follow a logical framework where first, the stress distribution around the excavation is computed, and boundary stresses are compared against in-situ crack initiation

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stress and rock mass tensile strength. When no relevant boundary failure is predicted, it remains to examine the effect of any major discontinuities which will transgress the with

particular

attention

to

stability

problems

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excavation,

the

vicinity

of the

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discontinuity/boundary intersection.

in

In this way, excavations in stratified rock masses, rather common in mining, tend to be largely affected by bedding planes and cross-joints. Bedding planes are characterized by their

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planar geometry and their large persistence from a geometrical point of view, and by their low or null tensile and shear strengths from a geomechanical point of view. In bedded

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medium, cross-joints tend to be associated with bedding planes (Brady & Brown, 2006). These cross joints are less persistent discontinuities normal to bedding, which make the rock mass behave like a brick structure. These properties of stratified rock masses lead to specific

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modes of rock mass response to excavation, which must be considered in the excavation design procedure (Fig. 1a).

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Accordingly, when arch-roofed excavations are developed in stratified rock masses the intersection of bedding planes and cross joints tend to produce wedges of rock subjected to gravity which tend to be kinematically unstable and fall immediately after excavation. This has led to the so called flat roof designs where the roof of the excavation follows a bedding plane (Fig. 1b). Sometimes this form of excavation arises in a natural form due to the falling of rock wedges in the shoulders of the tunnel and cavern when excavating. This type of excavation is however not always stable. In particular, when beds are not too thick, bedding involves the problem of detachment of the immediate roof from the host medium, and its loading and deflection into the excavation under gravity loading (Brady & Brown, 2006). Flat roof excavations have always been common in underground mines when mining a mineral bed located within a bedded sedimentary series. The stability of such flat roof tunnel

ACCEPTED MANUSCRIPT and cavern have been studied since the late 19th century. The first studies addressed the behavior of the immediate roof comparing it to an elastic beam, and showed that even when this beam cracks, typically at its center and abutments, it can still be stable due to the occurrence of a ‘voussoir beam’ spanning the excavation, using the analogy with the voussoir arch considered in bridges and masonry structures (Evans, 1941). It is relevant to remark that in this approach, the abutments are considered to be fixed and no horizontal stress is considered in the analytical approaches. Since these assumptions are not fulfilled in practice,

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practically convenient to ensure stability (Arzúa et al. 2015).

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checking the results with numerical models in order to quantify these aspects tend to be

Various authors have addressed with increasing rigor the instability processes associated with

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a voussoir beam. In this way Beer & Meek (1982) developed a first reasonable approach that was improved by Brady and Brown (1985). More recently, Sofianos (1996) and Diederichs

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and Kaiser (1999) noted some limitations in previous versions and proposed alternative ways of tackling the static indeterminacy of roof bed analysis. It is relevant to highlight that

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whereas the solution procedure by Sofianos (1996) refers to a rigorous mechanics approach of a beam formed by two elements; that by Diederichs and Kaiser (1999) assumes an immediate roof formed by various elements associated with the spacing of cross joints

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normal to bedding and to the excavation section. Additionally, this last method sought to provide a rational approach to its engineering application, and was compared to field data by

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Diederichs and Kaiser and by one of the authors of this paper (Alejano et al., 2008), which makes it reasonably reliable. This is why this approach is used in this study. In relation with this type of failure mechanism, traditional geotechnical classification systems

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for tunnel design such as RMR (Bieniawski, 1976) and Q (Barton et al., 1974) may not be well suited to account this kind of instability mechanisms and sometimes they tend to be conservative. However, the stability graph approach (Potvin, 1988; Nickson, 1992), oriented to mine design and focusing on analyzing the stability of the different planar faces of an excavation (for instance roof stability), is generally more suitable to analyze the stability of planar roofed excavations from an empirical point of view. This is undoubtedly due to the fact that it is based on the study of a large database representing unsupported stopes, including cavities in bedded rock masses. Underground caverns for storage purposes such as JRC Project, which is a mega engineering project in Singapore, are usually designed with an arched roof of either a horseshoe or near

ACCEPTED MANUSCRIPT circular profile. These shapes provide optimal stress distributions when the main stability problem is associated with intact rock failure, which often results in a more stable excavation. However, as pointed out above, an arched roof design may not be appropriate in geological settings consisting of horizontal or low-angled bedded sedimentary rocks, such as that under scrutiny. Adopting an arched roof design in such layered ground conditions involves dealing with wedge rock fall, and auxiliary support and reinforcement would be required. In many cases this involves the design of a wide spanned flat-roofed cavern, where the rock mass

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behaves as a natural structural beam in the roof of the cavern following the approaches

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indicated above.

The main aim of this study is to analyze the stability of JRC Project based on the techniques

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developed for excavation design in bedded medium and to back-analyze the stability of the near-flat-roofed tunnel and cavern profiles unexpectedly encountered in the development of

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JRC Project. It is also relevant to highlight how geological structures play a controlling factor for excavation profiles in this project.

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2. Project background

South East Asia's first commercial underground hydrocarbon storage rock caverns, namely

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JRC Project, had recently completed excavation works in Jurong Island within the south western region of main Singapore Island (Fig. 2a). A total of five caverns of storage capacity

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1.47 mil cubic meters volume had been excavated under Phase 1. There are two levels of excavation namely Level 0 at 132 m below ground level and Level 1 at 100 m below ground level. Storage rock caverns are excavated at level 0 whereas operation tunnels for working

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access / operations and other water curtain galleries for water injection into surrounding rock mass are excavated at level 1. The overall bird’s eye view of Jurong Rock Cavern Project is presented in Fig. 2b. The dimensions of each storage cavern are 340 m in length, 20 m in width and 27 m in height. Excavation is carried out in 3 stages, namely top heading (TH), bench 1 (B1) and bench 2 (B2) with 9 m height in each stage. The total excavated rock volume of the project is about 3.5 mil cubic meters and total linear length of excavation is about 18 km. The JRC Project is a very relevant mega engineering project critical for energy supply to Singapore. Therefore, it has been studied for many years in relation to its geology, geotechnical aspects and feasibility (Zhao et al., 1999). Among the risks faced in this mega project the fact of venturing so deep underground in sedimentary rock for a civil application

ACCEPTED MANUSCRIPT and the unpredictable geological conditions should be highlighted. This study puts forward some of the lessons learned in the JRC construction. 2.1.Geological settings of the area The previous studies (PWD, 1976; DSTA, 2009) on the Geology of Singapore (Fig. 3) only focused on the main Singapore Island since the current study area of JRC Project was still in offshore zone. Later, the reclamation project began in 1993 to merge seven isolated

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southwestern islands into one that came to be called Jurong Island.

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Between 2001 and 2007, three phases of extensive site investigation works were carried out at Selat Banyan basin under JTC Corporation as a developer for storage rock caverns. It

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comprised vertical and sub-horizontal bore holes, in situ testing such as permeability tests, horizontal stress measurement and geophysical surveys works. The engineering properties of

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intact drilled core rock specimens were tested at local and overseas laboratories. During the excavation and construction phase of JRC project (between 2009 and 2014), the required

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engineering properties of intact rocks such as strength properties determination by point load test, unconfined compression test, triaxial compression test (cohesion c, friction angle ), Brazilian tensile test, shear strength of natural rock joint by direct shear test and permeability

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of intact rocks were additionally carried out at local and overseas laboratories. Based on the location and observation of lithology in the study area, the rocks belong to Ayer

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Chawan Facies, Jurong Formation (Kar Winn et al., 2018a). It is composed of well-bedded marine muddy sandstone, pyroclastic rocks and mudstone. Red roundstone conglomerate is common and all beds are tuffaceous. Spilitic lava is present in this Facies. The type section is

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defined as the west coast of Pulau Sakra in the southwestern group of islands (PWD, 1976). The Ayer Chawan facies yielded an U-Pb zircon age of 240.6 ± 1.2 Ma (Middle Triassic) as determined by Laser Ablation Inductively Couple Plasma Mass Spectrometer (LA ICP-MS). The sediments of the Ayer Chawan facies are dominantly fine-grained with laminated bedding and possess only minor current features. These features, together with the occurrence of finely disseminated carbonaceous materials, suggest a low energy environment. The characteristic black coloration points to a reducing environment (DSTA, 2009). Kallang Formation (Fig. 3) is named after the Kallang River Basin, which is located in the south-central part of Singapore, where it is most extensive, but no type area is proposed as the formation is poorly exposed (PWD, 1976; DSTA, 2009). It is found along the coastline and

ACCEPTED MANUSCRIPT extends into the headwaters of the rivers draining in Singapore. It is not found in the project area but its existence is reported in the other parts of Jurong Island. The formation includes the sedimentary deposits of marine, littoral, alluvial, trnsitional and reef members. The age of formation is middle Pleistocene to the present. The working access shafts were constructed through the reclaimed land of the Jurong Islands, south west part of main Singapore Island.

About 15 m to 20 m thick hydraulically filled

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coarse sand were used to reclaim the sea around the southern part of Jurong Island. Underneath the sand are about 30 m thick residual soils of Jurong Formation consisting of

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hard silty clay / clayey silt and dense to very dense silty sand which were derived from chemical and physical weathering of the bedrock. Below the residual soils a roughly 10 m

Jurong Formation (Fig. 4).

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thick layer of weak rocks occurs, which is followed by strong bedrocks of sedimentary It comprises mainly interbedded mudstone, sandstone and

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pyroclastic rock. Limestone was found localized in other access shaft area. Structurally the sedimentary rocks show a broad anticlinal folded nature with gentle dipping

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about 0 to maximum 25 degrees towards 030 degree (NNE). Two major sub-vertical joint sets striking NS and EW were observed in the area. Majority of joint planes were coated with

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quartz and calcite fillings of secondary origin. Minor faults were also observed frequently but they were sealed with quartz and calcite filling along the planes.

An approximately 20 m

thick rhyolite intrusion, which cut across the sedimentary rocks, was observed in both levels.

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Generally the Jurong Formation Rocks in the area have undergone low grade regional metamorphism which gives rise to higher strength as compared with the typical sedimentary

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rocks.

3. Geomechanical basic characterization 3.1. Joint data

The orientations of discontinuities such as sedimentary bedding planes, joints and fault planes were measured and recorded during the course of face geological mapping while excavation. Those recorded dip and dip direction of such discontinuities were analyzed by using “Dips” programme (Rocscience Inc., 2013a). Fig. 5 a – c show the pole plots with overlay contours of different discontinuities such as bedding planes, joint planes and fault planes measured and recorded during geological face mapping from one of the storage rock caverns. From the contour plot analysis, 1 set of

ACCEPTED MANUSCRIPT bedding with varying dip angles, 4 sets of joints and 1 set of faults are grouped as prominent sets in the cavern and are presented in Table 1. 3.2. Intact rock properties Eighty four (84) rock core specimens from JRC Project were tested at the Institute of Rock and Soil Mechanics of Chinese Academy of Science (Wuhan, China) (Li et al., 2012). Two different rock types namely mudstone and pyroclastic rocks were tested under dry and wet

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conditions to explore the water content effect on the strength and deformability.

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In each test, five specimens were tested to obtain the peak strength under each of the four different σ3 (1.25, 5, 10 & 15 MPa). Subsequently, sets of the intact uniaxial compressive

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strength (σci), Hoek-Brown constant (mi), cohesion (c MPa) and friction angles ( degree) were obtained after those data pairs are analyzed by applying the RocData 3.0 program

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(Rocscience, Inc., 2004). The sample plot of Mohr circles to determine the Mohr-Coulomb criterion was presented in Fig. 6. The mean and standard deviation of those results are

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presented in Table 2 together with other rock types. Since wet mudstone is the most often encountered rock material in the project, the Hoek-Brown parameters of this material are

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conservatively used for the design and modelling purposes. The triaxial compressive tests revealed that the cohesion (c) increased and friction angle ( degree) decreased from dry to wet conditions for both tested rock types (Li et al., 2012).

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Based on the Hoek-Brown failure criterion, the H-B constant of mi was found to decrease obviously from dry to wet conditions for both tested rock types. The uniaxial compressive strength (σci) decreased in mudstone from dry to wet conditions but increased in pyroclastic

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rocks. It is interpreted that the bonding strength between individual rock fragments in pyroclastic rocks became weaker when they are oven dried under 105º C. 3.3. Shear test on joints

A total of 16 specimens with a single natural joint associated with different mineral veins or traces were tested at the Nanyang Technological University, (NTU, 2013). In the test, the specimen surfaces were elliptical shape and in dry conditions. A shear strength determination should preferably comprise at least five repeated shear tests at a different but constant normal stress on the same shear surface with each specimen tested. Taking account of the in situ stress condition of JRC, normal stresses of 1, 2, 3, 4 and 5 MPa were applied to each specimen for multiple runs. The shearing direction was along the long axis of its elliptical

ACCEPTED MANUSCRIPT joint surface. The shear load was applied continuously at the selected shearing rate of 0.5 or 1 mm/min. The strength parameters are summarized in Table 3. The cohesion values obtained for these discontinuities represent the behavior of closed joints, which were observed in the rock masses. However, for design purposes and in consideration of the occurrence of some open joints these cohesion values are conservatively reduced to 0.

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3.4. The Geological Strength Index (GSI) of JRC Project The GSI system, which was briefly introduced by Hoek et al. (1992) and developed by Hoek

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(1994) and Hoek et al. (1995), gives a number that, after combination with the intact rock properties, can be applied as a reduction factor to estimate the rock mass strength under

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different geological situations. The quantitative GSI was calculated with the help of four different approaches, all the results are approximately in the same range from 45 to 55 (Fig.

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7) (Kar Winn & Wong, 2019). Comparison was made with the qualitative GSI which were obtained based on assessment on the rock faces using the chart by Hoek et al. (1995). The

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quantitative GSI and qualitative GSI varied within a range of ±10. Values in the range of 40 to 55 are considered for rock mass characterization purposes.

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3.5 Disturbance factor (D) estimation of excavated faces in JRC Project The blast damaged disturbance factor D is determined on the excavated rock faces in the

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field. If possible, undamaged face is to be assessed for the evaluation of disturbance factor to be used. The disturbance factor (D) was determined following the guideline by Hoek, et al. (2002). Both values of D=0 and D=0.5 (Fig. 8) were adopted for JRC project.

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3.5. Rock mass properties

Subsequent Hoek-Brown criterion of rock mass (mb, s and a) can be calculated by applying Eq. (1, 2 & 3) based on the intact rock parameters (mi) in Table 2 with input of GSI values (GSI = 40 and 55 in Fig.7) and factor D in Fig. 8. The rock mass modulus (Erm ) was calculated by using equation Eq. (4) offered by Hoek and Diederichs (2006) with modulus ratio (MR= E/UCS) of 200 for mudstone and 300 for pyroclastic rock respectively. The calculated results of intact and rock mass properties were summarized in Table 4 for wet mudstone rock types (Kar Winn, 2018b). 𝑚 𝑏 = 𝑚 𝑖 𝑒𝑥𝑝 (

𝐺𝑆𝐼−100 28 −14𝐷

)

(1)

ACCEPTED MANUSCRIPT 𝑠 = 𝑒𝑥𝑝 ( 1

𝑎=

2

+

1 6

𝐺𝑆𝐼 −100 9−3𝐷

)

(2)

𝐺𝑆𝐼

(𝑒− 15 − 𝑒 −20/3 ) 1-D/2

Erm = Ei (0.02+ 1+

60+15D-GSI ( ) 11 e

(3)

)

(4)

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where Ei = Intact rock elastic modulus 3.6 Measurement of in-situ stresses

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During construction period of JRC Project, in situ stress of the area was measured inside one tunnel gallery at level 1 which is about 98 m below ground level. HF (Hydraulic Fracturing)

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and HTPF (Hydraulic Test on Pre-existing Fractures) were carried out in 3 NX boreholes

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with core samples taken (one vertical and 2 sub-vertical bore holes). The range of maximum, intermediate and minimum principal stresses are 6.2-7.8 MPa, 4.26.1 MPa and 2.9-3.9 MPa respectively in test depth of 120-160 m from ground. The trend and

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plunge of those principal stresses are N26±2ºE/-5±2º, S62±2ºE/-17±2º and S81±3ºE/-71±2º respectively. It is clearly shown that the trend of the maximum principal stress is generally

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NNE-SSW. The stress ratio of σ H : σh : σv = 2.2 : 1.8 : 1 (at 100 m depth) to 1.9 : 1.7 : 1 at 160 m depth) were determined (Kar Winn & Ng, 2013). A high horizontal stress factor

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around 2 is identified in the area.

4. Rock mass classification systems and basic tunnel and cavern design

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4.1. Q and RMR

Two rock classification systems (Q and RMR) were determined independently during geological mapping on the 394 excavation faces at some designated locations (up to 6% of tunnel progress). The results were plotted in Fig. 9 showing that 95% of RMR values are between 60 and 80, whereas Q values are scattered between 4 and 70. The general correlation equation hence obtained is; RMR = 6.95 ln (Q) + 49.5

(5)

This correlation is comparable with that proposed by Rutledge and Perston (1978). For basic design purposes and taking into account the variability of the rock mass classifications, conservative Class 3 (Fair) model and a value of Q=5 were considered based

ACCEPTED MANUSCRIPT on the support design proposal by Grimstad & Barton (1993). This approach suggests a support consisting of an 80 mm thick shotcrete layer together with a reinforcement of 5 m long bolts spaced 2.2 m in both directions, that is, normal and parallel to the axis of tunnels and caverns. This conservative support and reinforcement was proposed for the project based on a basic previous characterization. 4.2

Tunnel support system

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Eighty (80) mm thick shotcrete (Table 5) was sprayed on the excavated roof (TH only) and side walls (TH, B1 & B2) after mucking out of the rock debris. GFRP (Glass Fibre

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Reinforced Polymer) bolts were installed after shotcrete spray. They were fully grouted bolts

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with 5 m embedded length (Table 6) inside rock mass. In this analysis, reinforced concrete type for shotcrete and swellex-split type for rock bolts are chosen with spacing of 2.2 m.

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4.3. Stability graph

To determine the stability condition of JRC project, about 50 tunnel and cavern roofs at

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different locations were assessed using the method proposed by Potvin (1988), Nickson (1992) and Hutchison and Diederichs (1996). The necessary tunnel dimensions, joint orientation and Q’ data (Q’ = (RQD/Jn ) x (Jr/Ja)) were taken from the respective face

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geological mapping sheets. Subsequent stability parameters A, B and C were determined by applying the graphs Hutchison and Diederichs (1996) and hydraulic radius (HR) was

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calculated using Eq. 6. The modified stability number (N’) was again calculated using Eq. 7. HR =

Area Perimeter

=

xy 2𝑥 + 2𝑦

------------------------------------- (6)

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where x and y are the length and width of the surface to be computed. N′ =

RQD Jn

Jr

x Ja x A x B x C = Q ′ x A x B x C ----------------------------- (7)

It was observed that almost all of the tunnel and cavern roofs are in the transition zone and collapse zone (without support application) when plotted on the stability graph of Hutchison and Diederichs (1996) as presented in Fig. 10. Based on the reinforcement recommendation proposed by Hutchinson and Diederichs (1996), this would require a cable-bolt or bolt spacing in the range of 2 to 2.5 m. This reinforcement density is consistent with the reinforcement proposal based on the Grimstad and Barton’s

ACCEPTED MANUSCRIPT (1993) approach and with the support and reinforcement proposed for this project as presented in the previous section. 5. Structurally controlled instabilities The initial design was based on empirical approaches and traditional rules of thumb. Once this initial design was proposed, it is convenient to check it in detail against different geomechanical issues (potential failure mechanisms) expected to be encountered during the

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excavation stage. As stated in the introduction structurally controlled instabilities including

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the occurrence of wedge failures and bed separation are analyzed in the following section.

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5.1. Wedge analysis

Wedge analysis for the JRC Project has been carried out with the aid of computer program

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UNWEDGE v 4.0 (Rocscience, 2014), a 3D stability analysis code for the wedges occurring in underground excavations in rock containing intersecting structural discontinuities.

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The 5 discontinuity sets presented in Table 1 are used as input in the code together with the opening section of the design. The input rock strength parameters include two cases; the first one includes the laboratory determined values (NTU, 2013) with cohesion 0.2 and 0.3 MPa

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for discontinuities of the bedding and joint planes sets respectively; and for the second one, null cohesion is considered in all cases, referring to existing open discontinuities representing

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the less conservative case. Null tensile strength has been also assigned to all joints. Additionally the stress field measured and presented in Section 3.6 is input so the code can account for its stabilization effect.

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The code computed factors of safety for all possible occurring wedges associated with all possible combinations of joint sets. Only for two combinations of joints unstable wedges are observed namely combination J1-J3-J4 and combination J1-J4-J5. The latter appears to be less stable wedge. For the high cohesion values this combination (J1-J4-J5) produces instability of the wedge in the right-hand shoulder (8) of the cavern (FoS) =0.579. Although cohesion of joints was estimated to be 0.2 MPa, when some block movements or damage of joints associated

with excavation occurs,

this cohesion can be diminished. If we

conservatively consider that the joint cohesion became null, also wedges 5 and 6 will become unstable as depicted in Fig. 11.

ACCEPTED MANUSCRIPT It is relevant to note that the roof wedges may fall from the roof in line with the advance of the cavern face, i.e. just after blasting before any tunnel supports are applied, this type of failure has been observed in couple of occasions during the excavation works. If we input in the code the prescribed support and reinforcement including 80 mm of shotcrete and 5 m long grouted bolts with a tensile capacity of 0.35 MN installed every 2.2 m x 2.2 m, all the wedges became stable including wedge 8 that becomes stable with a FoS over

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2.5. This ensures stability once the cavern is constructed. However during excavation, if some upper local wedges fall, a section of the cavern may present a flat roof as illustrated in

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Fig. 12.

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In this case, the roof is formed by a bedding plane and a rock stratum forms the immediate roof. As briefly introduced above, conventional elastic beam mechanics is not applicable in

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this case where due to rock tensile strength and presence of preexisting joints, mid-span and other cracks may form. The stability analysis of this beam should be based on the Voussoir

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beam theory and it is presented in the following sub-section for the case under study. 5.2. Flat-roofed tunnels and caverns observed in the project Storage cavern (20 m spanned hydrocarbon storage cavern)

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5.2.1

Liquid hydrocarbon storage caverns with dimensions of 340 m length, 20 m width and 27 m

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height were excavated at 135 m below ground level in rock masses of Ayer Chawan Facies, Jurong Formation having an average dip angle of 0 to 10 degrees with the observed average bed thickness of 0.40 m. A flat roof formed by those sub-horizontal bedding planes

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encountered in the cavern was shown in Fig. 12. Two sub-vertical joint sets were observed in the area. The rectangular shaped cavern shoulder which was formed by the intersection of the vertical joint planes with the sub-horizontal beddings was shown in the upper right of the figure. The cavern was excavated in three stages with 9 m height in each stage. 5.2.2

Pump house (16 m spanned gallery)

A tunnel gallery was excavated at a depth of 100 m in rock masses having observed average bed thickness is 0.35 m. The gallery was excavated in two stages. In stage 1, a 10.2 m wide and 12.3 m high operation access gallery was first excavated which was followed by rock support installation. In stage 2, the gallery was widened for housing pumping equipment. The

ACCEPTED MANUSCRIPT final extended width and height were 16 m and 12.3 m respectively as shown in Fig. 13 a & b. 5.2.3 Access tunnel (7.9 m spanned gallery) This access tunnel was excavated at the same depth level and of similar geological condition as in the pump house. However, a smaller tunnel of 7.9 m span and 9.9 m height as shown in

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Fig. 14 a and b was excavated. 5.3. Voussoir analogues stability analysis

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When a flat roof formed by a beam appears, as it is the case for JRC Project when wedge

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instabilities occur, the corresponding beam analysis is necessary. In the case of a continuous beam, it should be first analyzed whether the beam cracks or not, typically due to tension in the upper part of the abutments and in the lower central point of the beam (Fig. 15a). This

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analysis can account for the occurrence of another rock bed lying over the immediate roof. For the case under study considering a 0.4 m thick continuous mudstone beam, and tensile

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strength estimated to be 7 MPa, cracking of the beam will occur for an opening width around 15 m for the standard case and for 14 m if a second beam stratum is resting on the lower one, according to equation presented in the case (Fig. 15a). This means that for the case under

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study, if a continuous beam existed, it will crack for the mentioned cavity spans.

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However this cracking of the beam does not imply instability, because a Voussoir arch tends to form. This implies that a compression arch is able to deviate the stresses associated with the beam weight towards abutments making the structure potentially stable (Fig. 15b).

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This so-called Voussoir arch may form due to cracking of a continuous beam in case only bedding and no other discontinuities exist in the rock mass, but it can also occur due to the previous occurrence of joints striking more or less normal to cavern axis. For the case of JRC Project a set of sub-vertical discontinuities striking subparallel to excavation axis exist (J3) with average spacing around 1.0 m and continuity in the range of 5 m. This discontinuity would tend to produce Voussoir or ‘bricks’ in the roof beams. Voussoir beams have been observed to fail due to various instability mechanisms including 1) beam buckling, with no significant crushing or spalling; 2) failure by crushing or spalling of central or abutment Voussoir; and 3) shear failure at the abutments, which may produce diagonal cracking in weak rock (Fig. 15c) (Diederichs and Kaiser, 1999). Shear failure is more commonly observed in thick beams due to the driving force of the beam weight.

ACCEPTED MANUSCRIPT However, buckling and crushing are more likely to occur in thinner beams with high span/thickness ratios, being buckling more common in hard rocks such as those under scrutiny. If such a flat roof as that presented in Fig. 12 and 15d, representing the potential immediate roof occurring at JRC Project after wedge failures, Voussoir stability analysis is in order. The maximum span of the beam will be 20 m, but due to round corners of the cavity this span will

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tend to be narrower than that. The typical thickness of the beam associated with strata bedding spacing will be 0.4 m. Based on rock mass characterization an average expected

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Young’s elastic modulus is taken as 6 GPa, though a less conservative value of 3 GPa will be

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also considered to assess the influence of this parameter on beam response. The Voussoir beam stability computation approach proposed by Diederichs and Kaiser

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(1999) has been used in this study. The authors refer to the original paper and Alejano et al. (2008) to check the application procedures of this method. This approach is selected because it is found to be more realistic and reliable from a practical engineering scope in relation to

(Yiouta-Mitra & Sofianos, 2018).

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other existing approaches (Sofianos, 1996), though this method has been recently updated

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The Diederichs & Kaiser (1999) model calculates 2 factors of safety, against crushing and shearing. As far as buckling is concerned, these authors preferred to calculate what they

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called a buckling limit (BL), given that buckling is an evolving phenomenon. Even if a theoretical BL of 100% is required for instability, instability processes—which are very difficult to control—tend to commence from a BL of around 35%, which is why the authors

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indicated a BL value of less than 35% as an appropriate design criteria. The authors also pointed out that this BL is usually achieved when deflection attains a value of around 0.1 times the thickness of the roof bed. For the purpose of this study, the authors have resorted to the so-called deflection factor of safety (Alejano et al., 2008), calculated as the relationship between a value of 10% of roof bed thickness and deflection calculated according to the technique described by Diederichs and Kaiser (1999). The methods are suitably applied to 7.9 m wide small gallery but 10 m and above wide galleries are close to the maximum stable limit with respect to buckling instability phenomena (Table 7). It is not possible to apply the method to larger caverns since an equilibrium is not found, so it is not possible to compute the forces and corresponding factors of safety (FoS).

ACCEPTED MANUSCRIPT The obtained results are presented in Table 7. For the standard case (for E = 6 GPa) with one beam in the immediate roof, the structure is stable for a 7.9 m wide gallery, and also for 10 m wide gallery, though in this case the buckling limit (BL) is 28% , close to the 35% that represents the maximum design limit recommended. A 12.65 m wide gallery will represent the maximum theoretically stable gallery, with a buckling limit close to 100%. Remark that the factors of safety against crushing and shear in all these cases are well above one, so these mechanisms are not expected. Also observe that the FoS against crushing decreases for larger

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spans, whereas the FoS against shearing in the abutment increases due to the increasing

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normal pressure exerted by the beam in the abutments.

If we re-analyze the same case, but considering that another bed 0.35 m thick (we chose this

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value because for a bed of the same material, it should be less thick to ensure that its stiffness is low enough so as to rest on the lower 0.40 m thick bed), we obtain similar results. The

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structure is stable for a 7.9 m wide room; but for a 10 m wide room, the buckling limit (BL) is 51%, well over the limit of 35%. This design would therefore not be recommended. In this

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case an 11.1 m wide room will represent the maximum theoretically stable room, with a buckling limit close to 100.

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We have repeated the calculations for the case in which the beam presents a lower value of the elastic modulus equal to 3 GPa, the lower expectable value. We have not repeated for larger values of elastic modulus, because the result will be more conservative. For this new

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case the structure will be also stable for a 7.9 m wide room but with a buckling limit of 21%; now a 10.65 m wide room will represent the maximum theoretically stable room, with a buckling limit close to 100%. In case we include a 0.35 m thick overlying stratum, the 7.9 m

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wide room will be over the design criterion (BL=39%) so it will not be recommended. In the case of 9.35 m wide gallery, buckling limit reached 95% which is close to 100%. As a result of this study, it is clear that for flat roofs with spans in the range 12.65, 11.1, 10.65 and 9.35 m, instability problems will arise according to the conditions encountered. Regarding design according to the Voussoir arch approach in the worst possible conditions (low elastic modulus of 3 GPa and two beds forming the immediate roof) gallery unsupported spans somewhat smaller could only be recommended. It is convenient to note that the Voussoir approach considers fixed abutments. In this case of JRC Project, large horizontal stresses were measured which could potentially contribute to more stable situations (Arzúa et al., 2015). However since a series of parallel caverns are

ACCEPTED MANUSCRIPT excavated, the presence of contiguous caverns would limit this horizontal stress stabilizing effect. The results provided by the application of the Diederichs and Kaiser (1999) approach are thus deemed to be reasonably realistic. All these indicate that some reinforcement is needed to control the potential unstable phenomena for large spanned caverns. We can compute a FoS for the prescribed reinforcement including 5 m long grouted bolts with a tensile capacity of 0.35 MN installed

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every 2.2 m x 2.2 m. To compute that from a conservative point of view, we consider that a beam formed by two mudstone beds with a combined thickness of 0.75 m occurs in the roof.

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We design the reinforcement to hold the weight of this material following the approach provided by Brady & Brown (2006) as illustrated in Fig. 16. According to this approach the

T  ·D·s 2

------------------------------------------------ (8)

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FoS 

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factor of safety of reinforcement can be computed as:

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where T = working load per rock bolt 0.35 MN in our case, γ = unit weight of the rock 27 kN/m3 , D = height of the unstable zone 0.75 m, and s = rock bolt spacing in both the longitudinal and transverse directions 2.2 m. This would yield a FoS over 3.5, which clearly

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indicates that the proposed reinforcement will ensure stability of these potential failure mechanisms.

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6. Discontinuum finite element analysis (RS2 Rocscience Inc., 2012) Intact wet mudstone properties shown in Table 2 and representative values of bedding and

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joint cohesion and friction angle from Table 3 were used in this current analysis as mudstone is the common rock mass in the project. Eighty (80) mm thick shotcrete was sprayed on the excavated roof (TH only) and side walls (TH, B1 & B2) after mucking out the rock debris. GFRP (Glass Fibre Reinforced Polymer) bolts were installed after shotcrete spraying. They were fully grouted bolts with 5 m embedded length inside rock mass. In this analysis, reinforced concrete type for shotcrete and swellex-split type rock bolts spaced at 2.2 m are studied. Gravity loading field stress type with actual ground surface, which is 145 m above the base level of twin caverns, was modelled. Horizontal to vertical stress ratio of maximum (σH / σv ) 2.2 and minimum (σh / σv ) 1.8 were considered in the analysis.

ACCEPTED MANUSCRIPT Maximum vertical displacement of about 8.8 mm (downward settlement, in negative values) was observed at roof centers of both left and right caverns. It is noted that about 90 % of the vertical displacement occurred at the roof during top heading excavation. The displacement is very minimal during the excavation of the remaining 2 benches. The similar vertical displacement is observed in the study by Mandel et al. (2013). Almost no upward vertical movement (heave, in positive values) was observed at the final floor of caverns as shown in Fig. 17a. Remark that this deflection is below the values computed by the Voussoir approach

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for the largest stable cavity spans, something logically attributable to the role played by

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reinforcement.

In terms of horizontal displacement, outer side walls of both caverns, i.e. left wall in left

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cavern and right wall in right cavern, showed around 18 mm (maximum) inward movement. However both inner walls of caverns, i.e. right wall of left cavern and left wall of right

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cavern, showed lesser horizontal movement about 8 mm (Fig 17b). The higher horizontal displacement on outer walls is due to the higher stress movement of rock mass than those on

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inner walls. The distribution of major principal stress (Sigma 1, σ1 ) and stress along X direction (Sigma XX, σxx) are presented in Fig. 17 c and d. The major principal stresses around openings are in compression, and compressive stress levels are about 8 MPa and

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lower than the compressive strength (UCS) of the rock mass. There is very minimal stress interaction between the two adjacent storage caverns due to the large pillar width (40 m)

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between them (> height of cavern = 27 m). The formation of yield points (plastic deformation zone) around each excavated opening are shown in Fig. 17 e. It is observed that the extent of yield zones at the crown is shallower than

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the rock bolt length. It shows that the rock bolts are in stable condition. Vertical displacement was monitored by installing glass prisms on the cavern crown and side walls at different locations along the alignment shown in Fig. 18a. The measured results at cavern crown (points 2, 3 & 4) in a particular section over time are shown in Fig. 18b. The maximum vertical displacement was measured around 5 mm (negative value in Fig. 18b), which was observed to remain generally stable throughout the excavation period. The monitored displacement results should be interpreted with caution, as the measurement accuracy depends on several factors such as the time of installation of the instruments after blasting, geological condition, and damage caused by blasting. In addition, the calculated results are based on a simplified analysis using a simple plasticity-based model. Nevertheless,

ACCEPTED MANUSCRIPT despite these restraints, comparisons between predictions and measurements can provide useful information. The main factor that accounts for such large difference between predicted and measured values is the time and place of monitoring device installed after face excavation. In JRC project, the instruments were installed a few tens of meters behind the excavated faces due to safety reasons. Abutment relaxation

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7.

in

the

behavior

of underground

structures,

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Diederichs (1999) and Diederichs and Kaiser (1999) reported the important role of relaxation highlighting

that

in complex geometry

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underground mines, excavations may affect each other in such a way that relaxation becomes a key parameter in controlling room and stope caving and instability. Due to the excavation

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of parallel tunnels, abutment relaxation (side walls displacement) in the tunnel excavated in stratified medium may affect the roof behavior in a similar way to convergence displacement

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of these walls. For the Voussoir analog approaches, this means an increase both in deflection and buckling tendencies in the roof beds. This relaxation could be more significant in distressed areas, as in the case of cross zones in rooms or faulted zones. A certain degree of

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relaxation may lead to a compressive force reduction of the beam, and consequently, an increase in the deflection necessary to attain equilibrium. In stable cases, the beam will be

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capable of supporting this increase in deflection, but for a beam already close to equilibrium, this relaxation could ultimately cause roof failure. In JRC Project which is aimed for 50 years lifespan usage, the pillar width, rock mass

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medium between two adjacent excavations, is designed to balance the overstressing caused by the combination of rock mass strength and in situ stresses. A good rule of thumb is that the distance between two excavations should be approximately equal to the height of the larger tunnel (Hoek, 2007). The design consultant of JRC Project adopted 40 m of the pillar width between twin storage caverns which is more than the height of caverns (27 m). The stress distributions associated with the various major excavation sections (operation tunnels, access tunnels and storage caverns) and possible stress interactions between excavations were analyzed using computer code Examine2D (Rocscience, Inc. 2010). The code is a boundary element computer program for the elastic stress analyses of underground excavations.

ACCEPTED MANUSCRIPT Fig. 19 shows the major principal stress distribution of excavations for both levels of caverns and tunnels. The major and minor principal stresses around openings are in compression, and compressive stress levels are lower than the uniaxial compressive strength (UCS) of the rock mass. There is very minimal stress interaction between the two storage caverns due to the relatively large pillar width between them (>height of cavern = 27 m). Also, there is minimal stress interaction between the caverns and respective maintenance chambers due to the sufficient pillar width among them. The stress distribution (Figs. 17 c & d) also demonstrates

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that there is minimal stress interaction between adjacent caverns. Therefore abutment

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relaxation is unlikely to occur (convergence displacement of walls) in JRC project. In underground hydropower stations in many other countries, there are numerous intersecting

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chambers where the pillar width between adjacent chambers does not meet the above criterion. Microseismicity monitoring system is used to detect the incipient fracturing process

by adopting effective supporting measures.

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8. Conclusions

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of those pillar rock masses (Zhao et al., 2018) with the aim to restrain the initiation of cracks

The multi-approach analysis performed on JRC Project highlighted a number of key issues

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concerning stability of tunnels and caverns. Different approaches including geomechanical classifications, analytical approaches computing the stability against various failure modes,

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and numerical computational models have been of great help to identify potential problems during excavation and support of these tunnels and caverns. A correct determination of classification, intact rock and rock joint data and actual field conditions is essential for

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obtaining the reliable results by the empirical, analytical and numerical approaches in stability analyses.

The analytical methods applied to structurally controlled instability phenomena (wedge failure and gallery Voussoir beams) have been able to identify failure mechanisms locally observed in the excavation works, so they indicate that the use of support and reinforcement is necessary. The results obtained by these methods suggest that the reinforcement recommended

based

on

geomechanical

classifications

can

be

overestimated.

This

conservatism has already been put forward in the literature, but the authors suggest that some degree of conservatism is convenient when using these natural, complex, heterogeneous, variable and discontinuous rock masses as construction materials, particularly in the field of public works.

ACCEPTED MANUSCRIPT The Voussoir beam stability computation approach proposed by Diederichs and Kaiser (1999) has been applied for the case under study and results tend to make sense in relation to field observations. The authors have resorted to the so-called deflection factor of safety (Alejano et al., 2008), calculated as the relationship between a value of 10% of roof bed thickness and deflection calculated by Diederichs and Kaiser (1999). As a result of this approach, the roof instability problems are observed in the wider tunnel and cavern except smaller one with 7.9 m width. All this indicates that proper tunnel support is required for

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stability concerns.

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The FEM analysis reveals that the 20 m spanned cavern with the required tunnel support applied is stable. Small vertical displacements are predicted. The favorable horizontal stress

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parameters and other geomechanical parameters such as uniaxial compression strength, shear

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strength properties of discontinuities are accounted for in the calculations. As a general conclusion, the authors have observed that designing underground tunnel and cavern based only on geomechanical classification systems may provide stable designs, but a

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number of potential failure mechanisms can be overlooked. The application of wedge and beam analytical techniques helps to better understand the bedded rock mass behavior, which

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could contribute to a more comprehensive understanding of observed in situ phenomena and ultimately to more insightful and safe excavation works. This study clearly demonstrates that the geological structures play an important role in controlling the configuration of the tunnels,

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caverns and galleries in the excavation work. 9. Acknowledgement

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The authors would like to thank JTC Corporation to allow them to publish “Multi-approach stability analyses of large caverns excavated in low-angled bedded sedimentary rock masses in Singapore”. The authors also thank the anonymous reviewers for their time and invaluable input which has greatly contributed to this paper. The second author also acknowledges the support from the University of Hong Kong Start-up fund, the Seed Funding Program for Basic Research for New Staff at the University of Hong Kong, the Hung Hing Ying Physical Sciences Research Fund 2017-18, and the support of the “China University of Geosciences Scholar” Program (2017046). References

ACCEPTED MANUSCRIPT Alejano, L. R., Taboada, J., Bastante, F. G., Rodríguez, P. (2008). Multi-approach backanalysis of a roof bed collapse in a mining room excavated in stratified rock. International Journal of Rock Mechanics and Mining Science, V. 45, pp: 899-913 Arzúa, J., Alejano, L.R., Rodríguez, P. (2015) Back-analyses of failures and support redesign of a magnesite room-and-pillar mine excavated in stratified rock. 13th ISRM International Congress of Rock Mechanics, 2015- MAY. Montreal, Canada.

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Barton, N. R., Lien, R., Lunde, J. (1974). “Engineering classification of rock masses for the design of tunnel support” Rock Mech. 6(4). 189-239

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Beer, G., Meek, J.L. (1982) Design curves for roofs and hanging-walls in bedded rock based

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on voussoir beam and plate solutions. Trans, Instn. Min. Metall. 1982. 1 A: 18-22. Bieniawski, Z.T. (1976). Rock mass classification in rock engineering. Proceedings of the

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symposium on exploration for rock engineering, Johannesburg. pp: 97-106. Brady, B.H.G., Brown, E.T. (1985) Rock mechanics for underground mining. Second edition.

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George Allen & Unwin Pub.

Brady, B.H.G., Brown, E.T. (2006) Rock Mechanics for underground mining: Third edition. Springer.

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Cai, M., Kaiser, P.K., Uno, H., Tasaka, Y., Minami, M. (2004). Estimation of rock mass deformation modulus and strength of jointed rock masses using GSI system, Int. J. of

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Rock Mech. Min. Sci. 41 (1) (2004) pp. 3-49. Defence Science and Technology Agency (DSTA) (2009). “Geology of Singapore” 2 nd Edition.

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Diederichs, M.S., (1999). Instability of hard rock masses: the role of tensile damage and relaxation. Ph. D Thesis. Department of Civil Engineering, University of Waterloo, Waterloo, Ontario.

Diederichs, M. S. & Kaiser, P. K. (1999). Stability of large excavations in laminated hard rock masses: the Voussoir analogue revisited. International Journal of Rock Mechanics and Mining Sciences V 36: pp: 97-117 Evans, W. H. (1941). The strength of undermined strata. Transactions of the Institution of Mining and Metallurgy 50: 475-500

ACCEPTED MANUSCRIPT Grimstad, E. & Barton N. (1993). Updating the Q-system for NMT. In: Proc. Int. Symp. on Sprayed Concrete, Fagernes, Norway 1993, Norwegian Concrete Association, Oslo, 20 p. Hoek, E., Wood, D. and Shah, S. (1992). A modified Hoek-Brown criterion for jointed rock masses. Proc. Rock characterization, symp. Intl. Soc. Rock Mech.: Eurock 1992, (Ed. J.A. Hudson), 209-214. London: Brit. Geol. Soc.

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Hoek, E. (1994). Strength of rock and rock masses, ISRM News J, 2(2) 4-16 Hoek, E., Kaiser, P.K. and Bawden. W.F. (1995). Support of underground excavations in

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hard rock. Rotterdam: Balkema.

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Hoek, E., Carranza-Torres CT., Corkum, B., (2002) “Hoek-Brown Failure Criterion – 2002 edition. Proceedings of the 5th North American Rock Mechanics Symp., Toronto,

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Canada, pp 267-273.

Hoek, E. and Diederichs, M. 2006. Empirical estimates of rock mass modulus. International

Hoek,

E.

(2007),

"Practical

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Journal of Rock Mechanics and Mining Science V 43, pp : 203-215 Rock

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hoeks_corner.

Engineering." www.

rocscience.com/

Hoek, E., Carter, T.G., Diederichs, M.S. (2013). Quantification of index chart, 47th US Rock Mechanics /

education/

the geological strength

Geomechanics Symposium, American Rock

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Mechanics Association, (2013) (ARMA 13-672) Hutchison, D.J., Diederichs, M.S. (1996). Cable bolting in underground mines. Vancouver:

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Bitech Publishers, 416 pp.

Kar Winn and Ng, M. (2013). In situ stress measurement in Singapore, 18th SE Geotechnical and Inaugural AGSSEA Conference, Singapore, May 2013. Kar Winn., Ng, M., Wong, N.Y. (2017). Stability Analysis of underground storage cavern excavation in Singapore. Symposium of the International Society for Rock mechanics (Eurock 2017) Czech Republic. Procedia Engineering 191 (2017) pp: 1040-1045 Kar Winn, Wong, N.Y., Khin Zaw and Thompson, J. (2018a). The Ayer Chawan Facies, Jurong Formation, Singapore: Age and observation of syndepositional pyroclastic sedimentation process with possible peperite formation. Bulletin of the Geological Society of Malaysia, No.66, December 2018, pp. 25-31

ACCEPTED MANUSCRIPT Kar Winn. (2018b). Engineering Geological Properties of Jurong Formation for Underground Cavern Excavation in Singapore, unpublished PhD Thesis (2019), NTU, Singapore. 324 pp Kar Winn., Wong, N.Y. (2019). Quantitative GSI determination of Singapore’s sedimentary rock

mass by applying four different approaches. Geotechnical and Geological

Engineering. V37, pp : 2103-2119

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Mandal, A., Chakravarthy, C.P., Nanda, A., Rath, R., Usmani, A. (2013). Analysis and Design Approach for Large Storage Caverns. International Journal of Geomechanics

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Jan/Feb. pp: 69-75

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Li, D., Wong, L.N.Y., Liu, G., Zhang, X. (2012). Influence of water content and anisotropy on the strength and deformability of low porosity meta-sedimentary rocks under triaxial

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compression. Engineering Geology. Vol. 126. pp: 46-66

Nickson, S.D. (1992). Cable support guidelines for underground hard rock mine operations.

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M.A.Sc. Thesis. Department of Mining and Mineral Processing. University of British Columbia. 343 pp.

NTU. (2013). “Final Report of the 2007-2012 Jurong Rock Cavern Research” by Wong, N.

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Y., Yang, Y., Zhao, Z., Goh, T. C., Low, B. K., Vol. I & II, School of Civil & Environmental Engineering, Nanyang Technological University, Singapore

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Potvin, Y. (1988). Empirical open stope design in Canada. Ph. D. Thesis, Department of Mining and Mineral Processing, University of British Columbia. 343 pp. PWD, Public Works Department (1976). "The geology of the Republic of Singapore," 79 p &

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1 Geological Map of Singapore at scale 1:50,000. Rocscience, Inc. (2010) Examnine 2D, V.4.0, User's manual, Toronto, Canada Rocscience Inc. (2012). RS2 V 8.0, User's manual, Toronto, Canada Rocscience Inc. (2013a). Dips V 6.0, User's manual, Toronto, Canada Rocscience, Inc. (2004) RocData 3.0, User's manual, Toronto, Canada Rocscience, Inc. (2014) Unwedge v 4.0, User's manual, Toronto, Canada Russo, G. (2009). A new rational method for calculating the GSI. Tunneling and Underground Space Technology. Vol. 24, pp: 103-111

ACCEPTED MANUSCRIPT Rutledge, J.C., Perston, R.L. (1978). Experience with engineering classifications of rocks. Proceedings of Intl. Tunnelling Symposium, Tokyo. Sofianos, A. I. (1996). Analysis and design of an underground hard rock Voussoir beam roof. International Journal of Rock Mechanics and Mining Science & Geomechanics V 33, pp : 153-166 Sonmez, H., Ulusay, R. (1999). Modification to the Geological Strength Index (GSI) and

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their applicability to stability of slopes. Intl. Journal of Rock Mechanics and Mining Science. Vol. 36, pp: 743-760

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Yiouta-Mitra, P., Sofianos, A.I. (2018). Μulti-jointed stratified hard rock roof analysis and

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design. International Journal of Rock Mechanics and Mining Sciences, 106, pp: 96-108. Zhao, J., Q. Liu., Lee, K.W., Choa, V., Teh, C.I. (1999). "Underground cavern development formation," Tunneling and Underground Space

Technology, Vol. 14, pp: 449-459.

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in the Jurong sedimentary rock

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Zhao, J.S., Feng, X. T., Jiang, Q., Zhou, Y. Y. (2018). Microseismicity monitoring and failure mechanism analysis of rock masses with weak interlayer zone in underground intersecting chambers: A case study from the Baihetan Hydropower Station, China.

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List of Figures

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Engineering Geology. V 245, pp: 44-60.

Fig. 1. Different shape excavations in stratified medium.

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Fig. 2. a) Location of Jurong Rock Cavern (JRC) Project (www.googlemaps.com) b) Bird eye view of JRC Phase 1 with 2 levels of excavation; Green (AT & OT) & blue (WG) at upper level; purple (Storage caverns) at lower level (Kar Winn, 2018b) Fig. 3 Geology Map of Singapore (DSTA, 2009) Fig. 4 Schematic profile of the tunnel layout and distribution of soil and rock units (Kar Winn et al., 2017) Fig. 5a Contour plot of bedding planes (equal Area, lower hemisphere) Fig. 5b Contour plot of joint planes (equal Area, lower hemisphere) Fig. 5c Contour plot of fault planes (equal Area, lower hemisphere)

ACCEPTED MANUSCRIPT Fig. 6 Mohr-Coulomb criterion and Mohr circles for the strength of dry mudstone (Kar Winn, 2018b) Fig. 7 Comparison of quantitative GSI (by modified after Hoek et al., 2013; Cai et al., 2005; Sonmez & Ulusay, 1999; Russo, 2009 approaches) with qualitative GSI (field assessment using Hoek et al., 1995) : selective examples from 420 excavated rock faces (Kar Winn & Wong, 2019)

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Fig. 8 Estimated disturbance factor (D) of common excavated rock faces at JRC

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Fig. 9 Correlation of RMR and Q from 394 excavation faces of JRC project

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Fig. 10 Condition of tunnel and cavern roofs of JRC project after plotted on stability graph of Potvin (1988), Nickson (1992), Hutchison & Diederichs (1996).

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Fig. 11 Results of wedge stability analysis based in UNWEDGE 4.0 for the less conservative case.

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Fig. 12. Mechanism of wedge failure and formation of a flat roof (20 m Width x 9 m Height at top heading excavation in storage cavern).

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Fig. 13 (a) Pump house – Tunnel section showing horizontal bedding at the roof with right angle cornered shoulders (16 m width  12.3 m height), (b) The roof was secured with rock

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supports

Fig. 14 Access Tunnel section showing horizontal bedding at the roof with right-angled corners at shoulders (7.9 m Width  9.9 m Height), (b) Tunnel section showing horizontal

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bedding at the roof with right-angled corners at shoulders (7.9 m Width  9.9 m Height) Fig. 15 a) mechanics of a continuous beam with two abutments with the values of maximum tensile stress max and factor of safety against cracking for the standard case and for the case a second beam is resting on the lower one. Symbols:max refer to the maximum tensile stress occurring in the upper part of the abutments and in the lower part of the beam center,  refers to the specific weight of the rock, S is the beam span, T the beam or bed thickness and t is the tensile strength of the rock. In case a second upper bed is potentially resting on the first one, this should be checked by comparing the stiffness relations of each bed computed as the ratio of the load q produced by each bed divided by the product of the elastic Young’s Modulus E times the moment of inertia I. b refers to beam width and it is taken as 1 m for the

ACCEPTED MANUSCRIPT presented calculations. When this resting criteria is fulfilled as shown in the Figure, max is computed based on the apparent specific weight. In case beds are not horizontal the dip,  , should be accounted for as indicated in the formulation.

b) compression arch formed in a

cracked beam and pictures showing the effect of Voussoir arches for an artistic pebble arch and a single masonry layer bridge, illustrating how Voussoir arches can be stable when an appropriate compression arc occurs transmitting all load to abutments. c) illustrative sketches

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of potential failure mechanisms of Voussoir arches (Diederichs and Kaiser, 1999; Alejano et al, 2008) and d) Voussoir beam roof in an underground excavation with a numerical and a

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real example of buckling failure in this kind of structures.

Fig. 16 Basic reinforcement design considering all the load of potentially unstable beam is

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supported by reinforcement. T = working load or tensile capacity per rock bolt, D = height of the unstable zone, s = rock bolt spacing, L = length of bolts and B = cavern span. Based on

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Brady & Brown (2006).

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Fig. 17 a) Results of vertical displacement (m) at final stage of excavation and support application b) Results of horizontal displacement (m) c) Major principal stress distribution between adjacent caverns d) Stress (along X direction) distribution between adjacent caverns

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e) Formation of yield zones around each cavern profile Fig. 18 a) Location of monitoring points for displacement measurements on cavern roof and

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walls in Area 1, b) Vertical displacement monitoring results at points 2, 3 and 4 (Kar Winn, et al., 2017)

Fig. 19 Major principal stress distribution of excavations for both two levels with some

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caverns and tunnels (Kar Winn, 2018b)

List of Tables

Table 1 Identified Beddings, Joints and Faults sets in CS 1/1 Table 2 Summary of cohesion and friction angles of different rock types under dry and wet condition with other intact rock properties (σci and mi) (Ayer Chawan Facies, Jurong Formation in JRC Project) (Kar Winn, 2018b) Table 3 Strength parameters of discontinuities

ACCEPTED MANUSCRIPT Table 4 Rock mass properties of wet mudstone, Jurong Rock Cavern Project (Kar Winn, 2018b) Table 5 - Shotcrete Properties Table 6 - GFRP Rock Bolt Properties (Glass Fibre Reinforced Polymer Bolts) Table 7 Factors of safety and stability conditions computed for different wide excavations, with a top mudstone stratum 0.4 m thick and for the case it is overlaid by another 0.35 m

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thick bed.

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Fig. 1. Different excavation shapes in stratified media.

JRC project

a)

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Fig. 2. a) Location of Jurong Rock Cavern (JRC) Project (www.googlemaps.com) b) Bird eye view of JRC Phase 1 with 2 levels of excavation; Green (AT & OT) & blue (WG) at upper level; purple (Storage caverns) at lower level (Kar Winn, 2018b)

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Fig. 3 Geology Map of Singapore (Defense Science and Technology Agency, 2009)

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Fig. 4 Schematic profile of the tunnel layout and distribution of soil and rock units (Kar Winn et al., 2017)

Fig. 5 (a) Contour plot of 81 bedding planes (equal area, lower hemisphere) (b) Contour plot of 242 joint planes (equal area, lower hemisphere) (c) Contour plot of 25 fault planes (equal area, lower hemisphere)

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Fig. 6 Mohr-Coulomb criterion and Mohr circles for the strength of dry mudstone (Kar Winn, 2018b)

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Fig. 7 Comparison of quantitative GSI (by modified after Hoek et al., 2013; Cai et al., 2005; Sonmez & Ulusay, 1999; Russo, 2009 approaches) with qualitative GSI (field assessment using Hoek et al., 1995) : selective examples from 420 excavated rock faces (Kar Winn & Wong, 2019)

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D = 0.5

D=0

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Fig. 8 Estimated disturbance factor (D) of common excavated rock faces at JRC

Fig. 9 Correlation of RMR and Q from 394 excavation faces of JRC project

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Fig. 10 Condition of tunnel and cavern roofs of JRC project after plotted on stability graph of Potvin (1988), Nickson (1992), Hutchison & Diederichs (1996).

Fig. 11 Results of wedge stability analysis obtained by UNWEDGE 4.0 for the less conservative case (Numbers 1 to 8 refer to wedges formed along tunnel profile)

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Fig. 12. Mechanism of wedge failure and formation of a flat roof (20 m width x 9 m height at top heading excavation in storage cavern).

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b) Fig. 13 (a) Pump house – tunnel section showing horizontal bedding at the roof with right angle cornered shoulders (16 m width  12.3 m height), (b) The roof was secured with rock supports

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Fig. 14 a) Access Tunnel section showing horizontal bedding at the roof with rightangled corners at shoulders (7.9 m width  9.9 m height), (b) Tunnel section showing horizontal bedding at the roof with right-angled corners at shoulders (7.9 m width  9.9 m height

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Fig. 15 a) Mechanics of a continuous beam with two abutments with the values of maximum tensile stress max and factor of safety against cracking for the standard case and for the case a second beam is resting on the lower one. Symbols:max refers to the maximum tensile stress occurring in the upper part of the abutments and in the lower part of the beam center,  refers to the specific weight of the rock, S is the beam span, T is the beam or bed thickness and t is the tensile strength of the rock. In case a second upper bed is potentially resting on the first one, this should be checked by comparing the stiffness relations of each bed computed as the ratio of the load q produced by each bed divided by the product of the elastic Young’s Modulus E times the moment of inertia I. b refers to beam width and it is taken as 1 m for the presented calculations. When this resting criterion is fulfilled as shown in the Figure, max is computed based on the apparent specific weight. In case beds are not horizontal, the dip,  , should be accounted for as indicated in the formulation. b) Compression arch formed in a cracked beam and pictures showing the effect of Voussoir arches for an artistic pebble arch and a single masonry layer bridge, illustrating how Voussoir arches can be stable when an appropriate compression arc occurs transmitting all load to abutments. c) Illustrative sketches of potential failure mechanisms of Voussoir arches (Diederichs and Kaiser, 1999; Alejano et al, 2008) and d) Voussoir beam roof in an underground excavation with a numerical and a real example of buckling failure in this kind of structures.

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Fig. 16 Basic reinforcement design considering all the load of potentially unstable beam is supported by reinforcement. T = working load or tensile capacity per rock bolt, D = height of the unstable zone, s = rock bolt spacing, L = length of bolts and B = cavern span. Based on Brady & Brown (2006).

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e Fig. 17 a) Results of vertical displacement (m) at final stage of excavation and support application b) Results of horizontal displacement (m) c) Major principal stress distribution between adjacent caverns d) Stress (along X direction) distribution between adjacent caverns e) Formation of yield zones around each cavern

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Fig. 18 a) Location of monitoring points for displacement measurements on cavern roof and walls at one storage cavern, b) Vertical displacement monitoring results at points 2, 3 and 4 (Kar Winn, et al., 2017)

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Fig. 19 Major principal stress (Sigma 1, MPa) distribution of excavations for both two levels with some caverns and tunnels (Kar Winn, 2018b)

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Table 1 Identified bedding planes, joint planes and fault planes sets in one cavern Sets Bedding planes Joint planes Fault planes Dip / dip direction Set 1 5-25/055 (B1) 85/264 (J1) 80/175 Set 2 79/185 (J2) 80/270 Set 3 85/131 (J3) Set 4 80/220 (J4)

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Table 2 Summary of cohesion and friction angles of different rock types under dry and wet condition with other intact rock properties (σci and mi) (Ayer Chawan Facies, Jurong Formation in JRC Project) (Kar Winn, 2018b)

6 3 7 12

Mean value

Standard deviation

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26 36 29 55

Standard deviation

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Dry mudstone Wet mudstone Dry pyroclastic rock Wet pyroclastic rock

Mean value

54 37 59 45

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Water condition and rock type

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Friction angle (,degree)

Cohesion (c, MPa)

3 3 4 6

Uniaxial compressive strength

HoekBrown constant

(σci , MPa)

(mi )

156 143 207 266

32 7 34 13

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Table 3 Strength parameters of discontinuities

Friction angle ( , degree) 30 30

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Bedding plane Joint planes

Cohesion (c, MPa) 0.3 0.2

Table 4 Rock mass properties of wet mudstone, Jurong Rock Cavern Project (Kar Winn, 2018b) Classification wet mudstone Disturbance factor D=0 D = 0.5 Parameters GSI 40 55 40 55 MR 200

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Table 5 - Shotcrete Properties Liner Type Formulation

11700

2199

5800

Poisson's ratio

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(Glass Fibre Reinforced Polymer Bolts) Swellex / Split Sets In plane spacing 2.2 m

0.188 (MN/m)

Out of plane spacing Bond shear stiffness Residual tensile capacity Allow joints to shear Bolt

2.2 m 100 (MN/m/m) 0 Yes

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Bond strength

4566

Young's Modulus 20000 MPa

0.08 m

Table 6 - GFRP Rock Bolt Properties Bolt Type Tensile 0.35 MN Capacity Tributary area 380 mm2 Bolt modulus 50000 MPa Bolt length 5m

143 7 0.82 0.0025 0.5

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143 7 0.40 0.0003 0.51

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Standard Beam

Thickness

143 7 1.4 0.0067 0.5

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Rock mass properties

143 7 0.82 0.0013 0.51

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σci (MPa) mi mb s a Erm(MPa) Hoek and Diederichs (2006) (Eq. 4)

Intact rock properties

Table 7 Factors of safety and stability conditions computed for different width excavations, with a top mudstone stratum 0.4 m thick and for the case it is overlaid by another 0.35 m thick stratum.

Elastic modulus=6 GPa

Simple 0.4 m thick beam

Elastic modulus=3 GPa

Beam width

7.9 m

10 m 12.65 m

7.9 m

10.65 m

FoS crushing

24.14

13.41

3.89

22.47

4.71

FoS shear (vertical joints)

5.61

7.05

9.44

5.59

8.94

Buckling limit (%)

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28

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21

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0.32

1.60

0.26

Deflection (m)

0.013

0.032

0.126

0.025

0.151

Beam width

7.9 m

10 m

11.1 m

7.9 m

9.35

FoS crushing

13.7

6.79

3.14

11.95

3.79

FoS shear (vertical joints)

5.63

7.06

7.92

5.57

7.54

Buckling limit (%)

16

51

92

39

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FoS deflection

1.86

0.71

0.34

Deflection (m)

0.021

0.051

0.117

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0.93

0.27

0.043

0.144

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0.4 m thick beam with a 0.35 m thick beam resting on it.

FoS deflection

Note: The underlined entries refer to typical unstable cases which are associated with

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buckling limit over 35%.

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Flat-roofed caverns were excavated in sub-horizontally bedded rocks in JRC Project Stability of three flat-roofed caverns was assessed by Voussoir beam theory Stability analysis was also carried out by RS2 FEM program for comparison In situ stress and geomechanical parameters are accounted for in the calculations The abutment relaxation is unlikely to occur in JRC Project

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