Pan, Xiaohua (Nanyang Technological University) | Oliver, Grahame John Henderson (Nanyang Technological University) | Chu, Jian (Nanyang Technological University) | Goh, Kok Hun (Infrastructure Design & Engineering, Land Transport Authority) | Wei, Xiaoqian (Infrastructure Design & Engineering, Land Transport Authority) | Kumarasamy, Jeyatharan (Infrastructure Design & Engineering, Land Transport Authority)
The Sajahat Formation is considered to be the oldest rock unit in Singapore. However, the age of deposition is uncertain. According to the Geological Map of Singapore, the Sajahat Formation has been found on Pulau Tekong, Pulau Sajahat and at Punggol Point. However, the occurrence at Punggol has not been confirmed due to the lack of present day outcrops. As part of a site investigation, two boreholes were drilled at Punggol. Hornfelsed quartzite (very similar to that found on Pulau Sajahat) cut by diorite and granodiorite dykes were logged in the core samples. Zircons from these rocks were radiometrically dated using the Laser Ablation ICPMS U-Pb method. The results of the analysis of the detrital zircons indicate that the quartzite was deposited at or later than 337±3 Ma (Early Carboniferous) and before the intrusion of a diorite dyke at 285±1 Ma (Early Permian). A granodiorite dyke was dated at 260±3 Ma (Late Permian). Therefore, the quartzite at Punggol can be confirmed to be the Sajahat Formation of Carboniferous age and is the oldest dated rock in Singapore. The engineering implication of identifying the types of formations is discussed.
The Sajahat Formation in Singapore is defined as those variably metamorphosed, unfossiliferous, sedimentary rocks comprising quartzite, sandstone, and argillite (Public Works Department 1976, Sharma et al., 1999; Lee and Zhou, 2009; Zhou and Cai, 2011). Previous studies indicate that it is probably the oldest rock unit in Singapore based on outcrops found in Pulau Tekong, Pulau Sajahat and Sajahat Kechil. Lee and Zhou (2009) proposed that the age of deposition of the Sajahat Formation was probably Lower Palaeozoic based on its complex deformation history and multiple intrusion of dykes. However, a Carboniferous to Permian age cannot be ruled out. The Sajahat Formation is very similar to the Mersing Formation in eastern Johor which is assumed to be Carboniferous in age since it is overlain by fossiferous Permian conglomerates with an angular unconformity (Oliver and Gupta, 2017). The deformed Sajahat Formation was considered to predate the undeformed Gombak Norite which has been U-Pb zircon dated by Oliver et al. (2014) at 260±2 Ma (Late Permian). The Sajahat Formation is therefore probably pre-Late Permian in age (Oliver and Gupta, 2017). However, there is no direct evidence of the age of deposition of the Sajahat Formation so far.
Zhaoi, Zhiye (Nanyang Technological University) | Nie, Wen (Nanyang Technological University / Hyundai Engineering & Construction Co.) | Yokota, Yasuhiro (Nanyang Technological University / Kajima Corporation) | Xiao, Fei (Nanyang Technological University) | Jayasinghe, Laddu Bhagya (Nanyang Technological University)
This paper provides an overview of Singapore’s drive in utilising underground space over the last 3 decades, including the past key projects and potential future developments. Two key development works undertaken by the Nanyang Centre for Underground Space, Nanyang Technological University are presented: a) rock bolt modelling in collaboration with Kajima/Japan; b) rock grouting modelling and simulations under the joint NTU/SINTEF TIGHT project. Rock bolt models based on the discontinuous deformation analysis (DDA) are developed to study the interface behaviour, and laboratory tests are used to verify the numerical simulations. New flow models are proposed and 2D and 3D simulations are developed in which available laboratory test data are used for verification study.
1. Singapore’s push in underground development
The main drivers for the use of underground space in Singapore are often the land scarcity, rapidly developing economy, high population density, security requirements and the need for sustainable development. In Singapore, initial and principal use of underground space was in the area of transport systems (e.g. tunnels for road and subway lines) and commercial uses (e.g. basements for shopping and parking) in the densely populated urban areas. Those projects have been carried out either by cut and cover method or by drill and blast method or by the use of tunnel boring machines (TBM). Due to the thick overburden of the Singapore geology, most of these projects were constructed in soft soil. However, in recent times, there is a growing interest in Singapore in placing facilities and services in deep rock formations.
The studies of underground rock cavern developments in Singapore began in the late 1980s (Broms, 1989). Subsequently, a series of feasibility studies for rock cavern development in different geological formations in Singapore was conducted (Broms and Zhao, 1993; Zhao et al. 1994; 1996; Zhao, 1996), which led to the development of the Underground Ammunition Facility (UAF, Fig. 1). The successful completion of UAF demonstrated the significant benefits of using underground space to create more space by reducing the environmental impacts. Consequently, many studies have been carried out to improve the safety of rock cavern construction and to use of rock cavern space for other possible uses (Zhao et al., 2001; 2004). The Jurong Rock Caverns (JRC) project is the second major rock cavern project in Singapore, and the primary purpose of this project was store the petrochemical (Zhao et al., 2004). JRC is located 130 m below the seabed at Banyan Basin on Jurong Island, and it was constructed in the sedimentary rocks of the Jurong Formation (Kar and Ng, 2012). In this project, horizontal directional coring was used for the first time in Singapore (Wong et al., 2012). Fig. 2 illustrates the cavern design layout of the JRC project. The first phase of the project comprises of 5 rock caverns with a typical cross-section of 20 m wide, 27 m high and 340 m long. The JRC phase I has storage of 1.47 million m3 of crude oil and condensate. The construction works were commenced in 2007, and the first two caverns were completed and were officially opened in 2014 (The Straits Times, 2014). Because of the underground land used for the oil storage, approximately 60 hectares of surface land was freed up. The critical challenges of the JRC project were the unpredictable geological conditions and the groundwater seepage. Thus, continual probing and site investigations were carried out before excavation. Extensive umbrella grouting around the shafts had to be carried out to minimise the groundwater seepage (Kim et al. 2012).
Jayasinghe, Laddu Bhagya (Nanyang Technological University) | Zhao, Zhiye (Nanyang Technological University) | Chee, Anthony Goh Teck (Nanyang Technological University) | Zhou, Hongyuan (Beijing University of Technology) | Gui, Yilin (Newcastle University)
This paper presents a series of field test results conducted at a site in the north western part of Singapore to investigate blast-induced ground vibration propagation through mixed geological media which comprised of residual soils and granitic rock. In total, six blast tests were carried out. Tri-axial accelerometers were placed at different standoff distances from the blasting, and the acceleration time histories were recorded at the ground surface, at various depths in the soil mass, as well as in rock mass to monitor the vibrations. Empirical formulae for predicting the peak particle velocity (PPV) attenuation along the ground surface and in soil/rock were derived from the measured data. The ground vibration attenuation across the soil-rock interface was carefully examined and the study shows that the soil-rock interface plays a significant role in changing the ground vibration intensity. Results from this field test have provided valuable data for calibration of numerical models developed to study the ground shock propagation.
Singapore’s rapid development requires extra space for residential, commercial and industrial use. Drilling and blasting is the widely accepted method for large-scale rock breaking activities in civil engineering constructions due to its cost effectiveness, higher efficiency and ability to break hard rock.
However, ground vibrations from blasting are undesirable and it can cause damage to nearby structures. In practice, the damage to nearby structures due to ground vibrations has been controlled by various rules and regulations available. The existing vibration limits are not always applicable, as they depend on the geological conditions of the site and dynamic characteristics of the structure.
Over the years, structural responses and damage to ground vibrations from blasting have been extensively studied and those studies concentrated on establishing allowable ground vibration levels in terms of peak particle velocity (PPV) together with the frequency of the ground vibration to limit the structural damage (DSTA, 2009). However, safe vibration limits given by different researchers differ because those limits were usually obtained based on field observations of low-rise residential buildings. A number of researchers have investigated the problem of ground vibration prediction and have proposed various formulae (Langefors and Kihlstorm, 1978; Wiss, 1982; Dowding, 1985; Zhou et al., 1998). These formulae were obtained based on field observations from various sites. According to Siskind et al. (1980) amplitude, frequencies and durations of the ground vibrations change during the wave propagation due to various factors such as geometric absorption and interaction with various geological media and structural interfaces. Zhou et al. (1998) experimentally studied the ground shock wave propagation through mixed geological media due to detonation of explosive in an underground storage chamber. Kahriman (2004) carried out field tests at a limestone quarry to establish a reliable formula to predict the peak particle velocity.
Grouting has been used frequently as a countermeasure of seepage control for underground construction. Conventional grouting techniques including cement grouting and chemical grouting can be used for seepage control for underground construction in rock when relative large joints are present. However, the high viscosity and large particle size of grouting material makes the grouting materials difficult to permeate into fine cracks. A novel biogrouting technique which employs biocement as the basic grouting material is presented in this paper. Laboratory experiments for use of biogrout to seal the interfaces between 18 pieces of horizontally laid granite slabs were carried out. The results of hydraulic pumping tests show that the seepage rate through the interfaces of the 18 pieces of granite slabs reduced by 1000 folds. A numerical analysis of the biogrouting process were also conducted to simulate the biocementation process and its pattern on the granite sheets. A comparison of the experimental data and the simulation results were made. Factors controlling the biogrouting process are discussed.
Safety and stability are always important issues in underground constructions. However, due to the sophisticated geological conditions, underground construction such as tunnelling or cavern excavation can be risky and expensive. Especially when unforeseen discontinuities such as faults or fractures in sedimentary rock mass are encountered, excessive and uncontrolled seepage may occur and even result in devastated consequences. This has been one of the major factors for the failures of a number of underground constructions.
Conventional cement or chemical grouting have often been employed to cope with the risk of undesired seepage and increase work safety level in underground constructions. Successful history cases have shown their potential of dealing with excessive and uncontrolled seepage. However, shortcomings also have emerged. The cement grout suffers two big disadvantages: firstly the viscosity of the grouting material is high, and secondly the cement grouting cannot permeate into fine cracks due to the size of particle in suspension. In recent years, the availability of ultrafine cement has extended the performance of hydraulic base grout for crack filling. Unfortunately, the high cost of ultrafine cement makes its vast application impracticable. Furthermore, even ultrafine cement may not be fine enough to permeate some fine cracks. Chemical grouting is another alternative to regular cement grouting. However, suspended solids are often added to chemical grouts to modify the solution properties as additives (Karol, 2003). So its application is also restrained by the size of the additives not to mention its high cost and potential negative environmental impact.
We report experimental research on frictional strength and stability properties of shale fractures during slip. Longmaxi shale, Green River shale and Marcellus shale are selected for constant-velocity and velocity-stepping fracture shear experiments. Combing with theoretical analysis, micro controlling mechanism of mineralogy on frictional strength and stability properties of shale fractures is discussed. Results indicate that the fracture friction-stability relationship is largely affected by shale mineralogy. Frictional strength of shale fractures increases when tectosilicate content increases and phyllosilicate content decreases; during velocity-stepping experiments, three kinds of shale show velocity-strengthening behavior, which means an aseismic creep tends to occur; with the increase content of tectosilicate, shale fractures tend to have seismic slip while with the increase of phyllosilicate content, stable aseismic creep happens.
With the advances in horizontal drilling and multi-stage fracturing technologies, shale gas production has a substantial growth not only in North America, but also in other areas around the world (Jia et al., 2018; Wu et al., 2017). However, as the basic stimulation method to enhance shale gas recovery, hydraulic fracturing is implemented for almost every shale gas wells. After hydraulic fracturing operations, large-scale waste water re-injection has been linked to seismic activities (Bao and Eaton, 2016; Ellsworth, 2013; Elsworth et al., 2016), which has raised public concerns and makes hydraulic fracturing prohibited in many areas across the world.
The large-scale injection of waste water generates overpressures and decreases effective normal stresses which leads to reactivation of pre-existing faults and fractures in formations (Ellsworth, 2013), which is shown in Fig 1. In addition, the fracturing operations induced fractures may also be the source for seismic activities. Hence, the stability of pre-existing and induced fractures will decide whether seismic activities happen.
Fig. 1. Mechanism of induced seismicity by large-scale water re-injection. Natural fractures and hydraulic fracturing induced fractures may be re-activated by overpressure caused by fluid injection.
This paper shows a modelling framework for simulation of thermal effect on the fracture behavior of a clay-rich sandstone. The framework was based on the particulate discrete element method (DEM), combined with a coupled thermal-mechanical scheme. Pure mode I and mode II, and mixed-mode (I+II) fracture toughness of the rock was measured under elevated temperatures (up to 600°C) using the ISRM-suggested semi-circular bend (SCB) specimens. The simulation results were validated against analytical predictions.
This study relates to thermal effect on fracture toughness of rock. Fracture toughness is a parameter of geo-materials that describes its ability to resist fracturing. This parameter is fundamentally important to rock and reservoir engineering applications, for example hydraulic fracturing for geothermal energy, oil and gas extractions (especially in the tight and low permeable formations), as well as wellbore stability assessment.
Although fracture toughness is often deemed as an intrinsic property of rock, it still can with factors including geological anisotropy (e.g., Chandler, 2016), temperature and confinement (e.g., Funatsu et al., 2014), experimental setup and loading conditions (e.g., Shang et al., 2018a). Chong and Kuruppu (1987) experimentally investigated the fracture behvaiour of layered oil shale, where semi-circular bend (SCB) specimens (later suggested by ISRM) containing different proportions of organic matter were prepared. It was found that organic-rich shale specimens had a higher fracture toughness than leaner specimens. Chandler et al. (2016) reported fracture toughness measurements on Mancos shale in three principal crack orientations (i.e., arrester, divider and short-transverse) using a short-rod method, and found that fracture toughness of the divider oriented specimens was much higher (increased by a factor of 3.4) than that measured using the short-transverse oriented specimens. Temperature can be another important factor that affects fracture toughness of rock, especially in the deep underground. It is well accepted that fracture toughness of a rock material can increase under elevated temperatures until it reaches an elasto-plastic transition phase (Mahanta, 2016), after which a decrease in fracture toughness can be seen. The transition phase is rock type- and temperature-dependent, which is probably due to the variations in mineral composition leading to various thermal dilations, as well as due to mineral grain interactions at microscale. This paper reports a particulate DEM study on the thermal influence on mixed-mode fracture toughness of a clay-rich sandstone, where a fully coupled thermal-mechanical model was used.
In the study, a series of semi-circular bend (SCB) specimens with the ISRM-suggested dimensions were numerically manufactured using the Particle Flow Code (PFC) to measure the fracture toughness of rock under elevated temperatures (up to 600°C). Fig.1 shows a representative numerical sample used in the study, where main minerals in the Midgley Grit Sandstone (MGS, Shang, 2016) were differentiated by assigning particles with different thermal expansion coefficients (i.e. quartz, 24.3×10−6K−1; feldspar, 8.7×10−6K−1; biotite, 1.0×10−6K−1; clay, 3.6×10−6K−1) (Fei, 1995; Zhao, 2016). Thermal strains can be produced in the sample by accounting for the thermal expansion of the particles as well as the bonds which act as thermal pipes (only apply to parallel bond). Specific heat of each particle and thermal expansion per unit length were 1.0e3 J/kg°C and 0.3°C/Wm, respectively. The DEM samples used in the study were first heated to desired temperatures up to 600°C, followed by three-point bending tests (without cooling phase) as shown in Fig. 1. It is worthwhile mentioning that a group of particles (green particles in Fig. 1) contacting the loading bar and the two supporting bars was generated and these particles were not heated (thus, no thermal expansion) so as to eliminate stress concertation which can be due to the different contact conditions arising from different expansion coefficients of the particles. Sample dimensions are shown in Fig. 1, in which sample radius R (50 mm), span length 2s (55 mm), thickness t (30 mm), and crack length a (25 mm) are constants, and the crack angle β varied from 0° to 46° to allow a complete modes of fracture toughness to be measured (Shang et al. 2018a). Table 1 shows the micro-parameters used in the study which have been calibrated by Shang et al. (2017).
Rathnaweera, Tharaka D. (Monash University /Nanyang Technological University) | Gamage, Ranjith P. (Monash University) | Wei, Wu (Nanyang Technological University) | Perera, Samintha A. (The University of Melbourne) | Haque, Asadul (Monash University) | Wanniarachchi, Ayal M. (Monash University) | Bandara, Adheesha K. (Monash University)
Over the last several decades, many studies have generated a large amount of proppant performance data, but these studies have only focused on proppant conductivity, with no attention to how proppant mechanical properties vary under loading conditions. The impact of mechanical behaviour on proppant performance can only be fully understood by the combined investigation of micro-structural and mechanical changes with increasing loading. Therefore, this study aims to identify such micro-structural behaviour, and in particular the impact on proppant mechanical properties. Proppant samples were tested under one-dimensional compression loading using high-resolution X-ray CT scanning technology. The reconstructed images taken at different load stages were analysed to capture the micro-structural behaviour and finally correlated with the mechanical behaviour of the proppant.
According to the results, there are significant micro-pore voids inside the proppant mass. When the proppant has a higher degree of porosity, there is a considerable reduction of the compressive strength which is not favourable for hydro-fracturing treatment designs. Moreover, it is clear that the brittleness of the proppant decreases with increasing porosity, as its Young’s modulus reduces with increasing pore voids. Therefore, it is important to have high manufacturing standards to achieve effective proppant performance at great depths. The micro-structural behaviour under increasing loading was investigated by performing comprehensive CT image analysis using Drishti software. According to the results, under compressive loading, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages.
Unconventional oil/gas production has recently attracted the research community due to the uncontrollable increasing demand for primary energy sources (Perera et al., 2016; Wu et al., 2017). Since this method provides a good solution to energy scarcity, over the last several decades, the industry has tried to enhance the production rate, mainly focusing on production enhancement techniques which can be effectively used in the energy extraction from sub-surface geological formations. Of the various options, hydraulic fracturing is one of the best ways to enhance oil/gas extraction, as it increases the formation’s permeability, allowing easy movement of the extracted oil/gas towards the production well (Rutledge and Scott, 2003; Orangi et al., 2011; Vengosh et al., 2014; Wanniarachchi et al., 2015). However, this process may be jeopardised due to the high stress levels acting on the formation at great depths (both vertical overburden and confining pressures). One possible consequence is re-closure of the fracture network under downhole stress conditions, which severely affects the post-fracturing production. Such issues can negate the use of proppant as a hydraulic fracture treatment method where proppants injected with the fracturing fluid prop the fractures, withstanding the fracture-closure stress (Wanniarachchi et al., 2015). Although the proppant gives a reliable solution to overcome this issue (propping the fracture network), sufficient closure stress can cause mechanical failure of the proppant, changing the fracture conductivity, causing re-closure of the fracture network, and altering the bulk properties of the proppant pack, which can negatively influence oil/gas extraction. Therefore, it is important to understand the mechanical behaviour of proppants under downhole stress conditions before injecting proppant with the hydro-fracturing fluid.
The effect of cyclic normal load on the direct shear response of planar joints has been investigated using a dynamic shear box device GS-1000. The shear tests were investigated under the influence of different slip velocities, vertical cyclic frequencies, vertical cyclic force amplitudes and constant normal load levels. Laboratory test results show that, the shear force and the dynamic friction coefficient show cyclic behavior under cyclic normal load conditions. A significant time shift between peak shear force and peak normal force with peak shear force lagging is observed, where the time shift is mainly caused by the shear stiffness of the contact surface. The relative time shift decreases with increasing normal load and vertical cyclic force amplitude. The peak value of the friction coefficient is nearly identical with the static friction coefficient, while, the minimum value of the dynamic friction coefficient changes with changing in slip velocity and vertical cyclic force amplitude. Finally, a shear strength criterion is proposed, which can predict the shear strength of planar joints under constant shear velocity and cyclic normal force conditions.
Discontinuities, joints and anisotropy are the main features of rocks and rock masses in geotechnical engineering projects, where the frictional behavior of joints plays a central role in the stability and hazard evaluation, e.g. for surface and underground excavations, slopes, dam foundations or geothermal reservoirs (Hoek and Brown, 1980; Hoek and Bray, 1981; Babanouri et al., 2011, Du et al., 2016). In order to gain a deeper understanding of the shear behavior of joints, direct shear box tests under constant normal load and constant normal stiffness conditions are becoming increasingly popular (e.g. Barton and Choubey, 1977; Lee et al., 2014; Nguyen et al., 2013 and 2014; Dang et al., 2016a). Due to blasting, explosions or earthquake excitation, rock masses suffer dynamic loadings in addition to static loads. Therefore, dynamic effects on rock masses need to be considered and several researchers investigated the rock, rock mass and joint behavior, respectively, under dynamic loading conditions (e.g. Crawford and Curran, 1981; Kana et al., 1996; Lee et al., 2001; Jafari et al., 2003; Bagde and Pertros, 2005; Belem et al., 2007; Guo et al., 2011; Liu et al., 2011, 2012; Konietzky et al., 2012; Cabalar et al., 2013; Nguyen et al., 2014; Zhou et al., 2015; Dang, 2016b).
The Jurong Series of rocks in Singapore primarily comprise weakly metamorphosed sedimentary rocks that have been folded and faulted. The Series occupy a large proportion of the western side of Singapore where many major civil engineering works underground are underway or are planned. Although many geotechnical investigations have been conducted and several projects have been completed published data on the properties of these rocks is sparse. Geotechnical investigations have been carried out on project specific basis and there has not been a collation of material or mass properties in the public domain. For example, interpretative reports have described the rocks qualitatively as water-bearing with highly conductive features. In practice claims have arisen regarding expectations of strength and conductivity and these are difficult to resolve in the absence of an adequate data base. The Series includes diverse rock materials such as conglomerates, sandstone, siltstone, claystone, limestone and tuff amongst others. These rocks have a wide range of properties. The authors have collected basic properties of strength of rock material for different types of rock and for various grades of weathering and in situ conductivity from packer tests with the intention of providing basic data illustrating the broad range of properties of these rocks to which others may add subsequently.
The Jurong Series of rocks (Jurong Rocks) are found extensively over the western side of Singapore. They primarily comprise sedimentary rocks that have been subjected to a low grade of metamorphism and subjected to weathering in situ. The rocks of the Jurong Series are complicated because they comprise several formations with diverse lithology. They are extensively and intensively folded, sheared and faulted. Because of their complexity, determination of engineering properties can be very difficult (Zhao. 2001). In particular packer tests to determine conductivity yield widely ranging values within short distances and strength tests vary widely between adjacent specimens even when taken from the same run of core.
Zhang, Wengang (Chongqing University) | Han, Liang (Chongqing University) | Yang, Changyou (Chongqing University) | Zhou, Xiaowan (Chongqing University) | Wu, Chongzhi (Chongqing University) | Goh, ATC (Nanyang Technological University)
The Bukit Timah Granite (BTG) formation is widely distributed in the central and northern parts of Singapore Island. This paper presents the key mechanical and physical properties of Singapore BTG rocks and residual soils, based on the Factual Geotechnical Reports of Downtown Line stage II sites. The variations of parameters including the index properties, the hydraulics, the strength and stiffness, the compressibility for residual soils, the unconfined compressive strength, the point load strength index, the abrasivity, and slake durability index for rocks are derived from laboratory tests based on samples from different depths of different borelogs. Statistical information including the average mean values, the standard deviations and the coefficient of variations are provided for these parameters. It is hoped that these statistics will provide useful reference and insights for future projects involving in BTG Formation.
The Bukit Timah Granite (BTG) is an acidic igneous rock formed in the lower middle Triassic period. There is considerable hybridization of the rock within the formation and evidence of assimilation (Pitts, 1984). Therefore, there is also a great variation in the mechanical and physical properties of BTG rocks. Through field investigations and laboratory tests, Zhao et al (1994) investigated the influences of the weathering grade and the weathering processes on the mechanical and physical properties of the weathered granitic rocks. Rahardjo et al (2012) compiled the variation of index and engineering properties of BTG residual soils with depth. Based on a large database from the Factual Geotechnical Reports on over 200 boreholes of the Singapore Downtown Line stage II (DTL2), this study presents the key mechanical and physical properties of BTG rocks and residual soils.
2. Properties of BTG rocks
2.1 Unconfined compressive strength (UCS)
In this unconfined compressive strength test, the test specimen, the loading rate as well as the testing environment are shown in Table 1.
As can be seen from Table 1, the specimens in the test are generally standard test specimens. For some of them, due to sampling or storage, the specimen is not standard in geometry, and its tested strength needs to be converted to the value under standard condition (H/D=2). The specific calculation is performed according to equations (1) and (2) (ASTM 1986).