STRENGTH BEHAVIOUR OF JOINTED ROCKS REINFORCED WITH PASSIVE BOLTS

Abstract

Rocks encountered in civil and mining engineering structures are generally intersected by joints. The joints introduce planes of weakness, and sliding of blocks can occur easily along joint planes. Consequently, the jointed rock offers relatively little resistance against failure and strength of rock is considerably reduced. Strength behaviour of jointed rocks (unreinforced) is a complex phenomenon and various studies have been conducted during past to understand the strength behaviour. The studies can be grouped into two major groups. Group I comprises of studies where analysis is conducted in σ, τ space (Patton, 1966; Ladayni and Archambault, 1970; Jaeger, 1971; Barton, 1973). The group II involves those studies where analysis is done in terms of σ3, σ1 space (Hoek and Brown, 1980, 1997, 2002; Ramamurthy, 1993; Ramamurthy and Arora, 1994; Singh et al., 2011; Singh and Singh, 2012). Rock bolts are extensively used to enhance strength of jointed rocks, however assessment of strength enhancement in jointed rocks due to provision of bolts is still a challenging task faced by designers. Based on studies available on reinforced rock, it is understood that the strength behaviour of reinforced rock depends on various factors like the strength of the parent rock (Dight, 1982; McHugh and Signer, 1999; Sakurai, 2010), joint orientation, angle of inclination between joint and bolt and diameter of bolt (Bjurström, 1974; Ludvig, 1983; Grasselli et al., 1999, Grasselli, 2005). Further pre tensioning of the bolt has also been an important factor (Dight, 1982; Ferrero, 1995; Jalalifar and Aziz, 2010). An excellent discussion on the difference in the strength behavior of “individual bolt placed to reinforce single joint” and “interaction between bolts and rock mass” has been presented by Ferrero (1995) and Ferrero et al. (1997). Ferrero et al. (1997) emphasized that the assessment of enhancement in the engineering properties of a rock mass is very difficult since it involves complex failure mechanisms, the interaction between different materials and the characteristics of joints. In addition, the complexities of interactions between a large numbers of blocks add to the difficulties. Though several studies have been conducted during past on rocks reinforced with bolts, these studies focused on strength behaviour of single joint rather than a mass. In field, the rock mass comprises of intact rock blocks separated by discontinuities. The strength behaviour of such a ii blocky mass is substantially different from single joint due to complex interaction of blocks and scale effect. The first major objective of the present study has been to investigate the strength behaviour of a blocky mass under un-reinforced and reinforced (with bolts) cases. The objective was achieved by conducting direct shear tests on large sized jointed block mass (750 mm x 750 mm x 900 mm) specimens. Passive rock bolts were used to investigate strength enhancement due to reinforcement. The outcome of this part is applicable in designing rock bolt reinforcement in situations like rock slopes where analysis is done σ, τ space. The another area where rock bolts are frequently used is underground openings in jointed rocks. The analysis in such conditions is done in σ3, σ1 space, and a failure criterion is used to assess the strength of unreinforced or reinforced rock subjected to given confining pressure. The stress conditions may vary from almost uniaxial loading condition to very high confining pressure range (σ3 As a part of first objective, the direct shear tests were conducted on blocky mass specimens of size 750 mm x 750mm x 900 mm (height). For this purpose a specially designed and fabricated direct shear test apparatus was used. The size of shear box in the apparatus is 750 mm x 750mm x 1000 mm (height). To form the elemental blocky mass, concrete blocks of size 150 mm x150 mm x150 mm were used. About 150 blocks of concrete were required for conducting one test. The uniaxial compressive strength of intact material is about 42 MPa. Formed blocky mass consists of three orthogonal joint sets spaced at 150 mm centre to centre. To ≫ 0). The second part of the present study has focused on assessment of strength of reinforced rock under uniaxial and triaxial loading conditions. It is envisaged that if uniaxial compressive strength of jointed rock (reinforced or unreinforced) is assessed with confidence, the additional effect of confining pressure could be incorporated through a suitable strength criterion (Hoek and Brown, 1980, 2002; Ramamurthy, 2001; Singh and Singh, 2012). Uniaxial and triaxial tests were conducted on intact and jointed specimens of synthetic and natural rocks. Tests were also conducted on reinforced specimens. The results were analysed and an approach for assessing the strength of reinforced rock has been suggested. In the following sections, a brief abstract of the study is presented. iii reinforce the mass, six mm diameter steel bars having tensile strength 550 MPa were used. The bolts were installed perpendicular to the shearing direction, and were grouted with cement mortar. Three different configurations of bolts were used in which numbers of bolts and bolt spacing were varied. Tests were performed for a normal stress range of 0 to 2 MPa. In case of unreinforced blocky mass, sliding of blocks was observed at all the normal stress levels. When bolts were installed, the sliding through the joint plane was restricted. The results indicated increase in shear strength with increase in normal stress for unreinforced as well as reinforced specimens. The shear stress (τ) vs horizontal shear displacement (δH) plots of reinforced blocky mass exhibit two distinct segments. The first part of the plot was relatively flat and second part was steep. The first part indicates the mobilization of shear stress due to the interaction of the blocks, while second part indicates the mobilization of shear stress through the bolts. It is found that for all the normal stress levels, provision of bolts enhances the shear strength of blocky mass. The main reasons for enhancement in shear strength are improved interlocking by bolts, and generation of additional normal stress on joint surfaces due to development of tensile stress in the bolt. Due to improved interlocking produced by the bolts, the stiffness of the mass enhances. The rock mass, therefore, becomes stiffer and stronger. Installation of bolts enhances the cohesion (cj_mass) of blocky mass whereas the friction angle (φ j_mass) remains almost constant (with some scatter). For practical purpose, it may be assumed that the friction angle of series of joints in mass φ j_mass is equal to the friction angle of a single joint φj and effect of reinforcement may be considered through enhanced value of cohesion. The enhanced value of 𝑐𝑗_𝑚𝑎𝑠𝑠 is always less then intact rock cohesion (ci). The enhancement in shear strength depends on amount of reinforcement, spacing between joints, spacing between bolts and imposed normal stress. Increase in amount of reinforcement and reduction in bolt spacing results in increase in the shear strength of blocky mass at all normal stress levels. However, the change in shear strength enhancement per unit change in normal stress decreases with increasing normal stress for each configuration of reinforced mass. Based on the results of direct shear tests the following correlation has been developed to assess the shear strength of a reinforced mass subjected to a given normal stress (σn) iv 𝜏𝑓=𝑐𝑖􁉀0.04 𝑙𝑛􁉀𝐴𝑟𝑁􁉁+ 0.24􁉁+𝑐𝑗+𝜎𝑛 𝑡𝑎𝑛𝜙𝑗 where τf = shear strength of reinforced mass; ci = cohesion of intact rock material; Ar = percent area ratio, N = spacing ratio, cj = cohesion of a single joint and ϕj = friction angle of single joint. The proposed correlation may be used in the field to assess shear strength of mass where analysis is done in τ - σn The designer needs an understanding of triaxial strength behaviour of rock for designing structure in rocks. For assessing triaxial strength of reinforced rock, a suitable strength criterion may be used. The starting point of the strength criterion is the uniaxial compressive strength (UCS). If an accurate assessment of UCS could be made, triaxial strength can also be predicted accurately. Experimental studies have been conducted to get insight into UCS of jointed unreinforced and reinforced rocks. As second objective of the study, uniaxial compression tests were performed on the prismatic specimens of synthetic rocks (size 150 mm x 150 mm x 300 mm (height)). Two different grades of concrete (referred to T2 and T3) were used as model material. The uniaxial compressive strength of cylindrical specimens of the model material was 84 MPa and 127 MPa for T2 and T3 types of synthetic rock respectively. The jointed specimens of synthetic rocks consist of one smooth joint orientated at 0° to 90° with the base of the specimen. For preparing reinforced jointed specimens, two steel bars of diameter 6 mm were installed in the specimens, and were grouted with 2 mm thick cement-mortar. Bolts were installed perpendicular to loading direction and spaced at 50 mm c/c. The results of uniaxial compression tests were obtained in the form of uniaxial compressive strength (σ space like slopes. The correlation may also be used to optimise the number of bolts to reinforce the slopes. ci) and tangent modulus (Et50) and failure modes were recorded. In general, the specimens of unreinforced and reinforced synthetic rocks (both T2 and T3) failed due to splitting. Sliding failure was observed only for unreinforced specimens having θ = 45° and 60°. Due to installation of bolts, the sliding failure mode was altered to splitting. It was found that the strength as well as modulus of jointed rocks were enhanced due to provision of bolts. The enhancement in strength and modulus was mainly due to improved interlocking produced by the bolts. The maximum enhancement was found at orientations where sliding mode of v failure was observed (θ = 45° and 60°) in unreinforced specimens and reinforcement altered the failure mode. The third part of study involves laboratory tests for evaluating failure criteria for reinforced rock. Triaxial compression tests were performed on the specimens of natural jointed rocks (cylindrical specimens of NX size) without and with bolt. The uniaxial compressive strength of intact material was about 87 MPa. The height to diameter ratio of the prepared specimens was about 2. The joint orientation (θ) of specimens was varied from 0° to 90° with respect to base of the specimens. To reinforce the rock, 4 mm diameter bolt was installed perpendicular to loading direction. The bolt was grouted with 1 mm thick cement mortar. The ends of the bolt were tightened by nut washer system. The specimens were tested at confining pressure of 0, 5, 20, and 40 MPa respectively. Load, displacement and failure modes were recorded. It is observed that the unreinforced jointed rocks exhibit splitting, shearing, and sliding or a combination of these failure modes. In case of reinforced specimens, only sliding and shearing or combination of theses failure modes were observed. Sliding failure was observed only between θ = 30° to 80° at σ3 = 0 MPa for unreinforced jointed rocks. Provision of bolt altered the sliding mode of failure into splitting or shearing. The results of triaxial tests were plotted in the form of σ1 vs σ3. For both unreinforced and reinforced jointed rocks, an increase in confining stress (σ3) results in increase in the strength (σ1). The variation of σ1 with σ3 The results obtained from uniaxial compression tests on natural and synthetic rocks were plotted on Deere-Miller (1966) classification chart. It was observed that strength and modulus of intact, unreinforced and reinforced rocks are uniquely correlated with each other and assessment of UCS can be done if modulus is available. The following correlations were obtained is found to be non-linear for all the cases. Results also indicate that the provision of bolt enhances the strength of rocks at all the confining stress levels and reduces the anisotropy ratio. However, increase in confining pressure reduces the bolt contribution toward strength enhancement. The value of strength parameters (c and ϕ) were also altered due to installation of bolt. vi 0.67cjujuciiEEσσ=0.67cjrjrciiEEσσ= where σci and Ei = uniaxial compressive strength and modulus of intact rock; σcju and Eju = uniaxial compressive strength and modulus of unreinforced jointed rock; σjr and Ejr Analysis of triaxial results suggested that for sliding failure in jointed rocks the single plane of weakness theory associated with Barton (1976) criterion could be used with confidence. For non-sliding failure the criterion proposed by Singh and Singh (2012) can be used if effect of JCS and JRC are incorporated. The following criterion is proposed for non-sliding failure of unreinforced rock as well as for reinforced rocks = uniaxial compressive strength and modulus of reinforced jointed rock. In the field, it is relatively easy to get modulus of the mass either through testing or through back analysis. Using the modulus of rock mass and laboratory value of strength and modulus of intact rock, a reasonable estimate can be made on UCS of reinforced rock. 𝜎1=𝜎𝑐𝑗 (𝑜𝑟 𝜎𝑐𝑟 )+􁉀2 𝑠𝑖𝑛(𝜙𝑖0+𝐽𝑅𝐶) 1−𝑠𝑖𝑛(𝜙𝑖0+𝐽𝑅𝐶)+1􁉁𝜎3−1𝐽𝐶𝑆𝑠𝑖𝑛(𝜙𝑖0+𝐽𝑅𝐶) 1−𝑠𝑖𝑛(𝜙𝑖0+𝐽𝑅𝐶) 𝜎32 𝑓𝑜𝑟 𝜎3𝐽𝐶𝑆≤0.3 and 𝜎1=𝜎𝑐𝑗 (𝑜𝑟 𝜎𝑐𝑟 )+􁉀2 𝑠𝑖𝑛(𝜙𝑖0) 1−𝑠𝑖𝑛(𝜙𝑖0)+1􁉁𝜎3−1𝐽𝐶𝑆𝑠𝑖𝑛(𝜙𝑖0) 1−𝑠𝑖𝑛(𝜙𝑖0) 𝜎32 𝑓𝑜𝑟𝜎3𝐽𝐶𝑆>0.3 where σcj and σcr is the uniaxial compressive strength of unreinforced and reinforced jointed rock respectively. The above-proposed criterion can be used for assessing the strength of reinforced rock subjected to a given σ3.