A new approach in determining the load transfer mechanism in fully grouted bolts

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2006 A new approach in determining the load transfer mechanism in fully grouted bolts Hossein Jalalifar University of Wollongong Jalalifar, Hossein, A new approach in determining the load transfer mechanism in fully grouted bolts, PhD thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, This paper is posted at Research Online.

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3 CHAPTER THREE REVIEW OF SHEAR BEHAVIOUR OF BOLTS AND MATERIAL PROPERTIES

4 CHAPTER THREE REVIEW OF SHEAR BEHAVIOUR OF BOLTS AND MATERIAL PROPERTIES 3.1. INTRODUCTION Rock bolts are the main elements of support in modern stabilisation techniques for geotechnical engineering. They generally work as an additional resistance against shear failure along joints and weakness planes. The internal steel bar within the system is the main element resisting axial load under suspension and transverse shear loads caused by beam bending and slip on joints. Axial forces in the bolt consist of a component perpendicular to the shear joint, which contributes frictional strength, and another component parallel to the shear joint plane in the shear direction, which contributes to the dowel effect. When rock bolts are used to support rock slope and underground excavations they are affected by axial and shear loading during movement on the blocks (Figure 3.1). Bolt behaviour under load and how the load is transferred along its length is important. These are discussed in this chapter. This chapter consists of two main parts. The first part summarises studies undertaken by various workers on shear behaviour, and the second part describes the laboratory tests conducted to define the material properties used in the next chapters. These studies were first initiated by Dulascka (1972), she was then followed by Bjurstrom (1974), Haas (1976,1981), Azuar (1977,79), Hibin and Motojim (1981), Egger and Fernands (1983) and Ludvig (1983), Gerard (1983), Dight (1983), 51

5 Bjornfot & Stephansson (1984), Larsson (1984), Schubert (1984), Lorig (1985), Yoshinaka et al. (1987) Spang and Egger, (1990), Stillberg (1991), Holmberg (1991), Egger and Zabuski (1991), Ferrero (1995), Robbert (1995), Pellet and Boulon (1995), Pellet et al. (1995, 1996), Goris et al. (1996), Grasselli et al (1999), Grasselli (2005) and Mahony (2005) worked on the mechanical behaviour of rock bolts. Ground surface Joint Rock Bolt Tunnel axis Figure 3.1. Stability issues in rock mass reinforced by fully grouted bolts All experimental testing of grouted bolts were performed as a single shear test using single shear apparatus, which results in difficulties in the shear joint due to nonequilibrium and non uniform load on the shear joint. None of the works included applying tensile loads on the bolt but several studies applied confining pressure on the moving block. Thus a new method is designed in present research to evaluate bolt bending in a proper manner, which is discussed later. 52

6 3.2. PAST RESEARCH Dulascka (1972) established the following expression to find the shear force carried by a bolt, based on an idealised stress distribution at the point of contact. Her theory was based on the development of a plastic hinge at the point of maximum moment given by; 2 σ c T = 0.2D b σ y[ 1 + ( ) 1] σ sin β y (3.1) where; T σ c = Shear force carried by bolt = Uniaxial compressive strength of rock D b = Bolt diameter σ y = Yield stress of bolt β = Angle between bolt and normal to the joint The crushing strength of the concrete was at least four times greater than the compressive strength. As shown in Figure 3.2 there is no static equilibrium condition in both sides of the shear joint, which limits the system. Bjurstrom (1974) direct shear test on cement grouted bolts in granite blocks was aimed at evaluating the influence of various factors affecting the shear strength of rock joints. The bolts had inclinations between 30 o -90 o with respect to the joint surface. He found that for angle <40 o bolts failed in tension and for angles >40 o the bolts failed in a combination of shear and tension. 53

7 (a) (b) Figure 3.2. Shear test arrangement in (a) and (b) probable load generation (after Dulasck 1972) Bjurstrom provided an analytical solution based on an equilibrium of forces acting on the system and expressed that the total shear strength of a bolt reinforced joint was dependent on the following three parameters: i) Shear resistance due to reinforcement effect: T b = p(cos β + sin βtagϕ) (3.2) where; T b = The reinforcement effect in shear resistance due to bolting p = Axial load corresponding to the yield strength due to shear displacement = Initial angle between bolt and joint direction 54

8 = The friction angle of the joint ii) Shear resistance due to the dowel effect: T d = 0.67d b ( σ yσ c ) (3.3) where; d b = Bolt diameter σ y = Bolt yield strength σ c = Uniaxial compression strength of the rock iii) Shear resistance due to friction of joint: T f = A σ tagϕ (3.4) j n j where; A j = Joint area σ n = Normal stress on joint and ϕ j = Joint friction angle According to Bjurstrom the total contribution from the bolt to the shear strength of the joint, shown in Figure 3.3, is given as: T A σ tagϕ (3.5) t = p(cosβ + sin βtagϕ) D b ( σ yσ c ) + j n j Bjurstrom s estimate of the contribution to increase in strength is acceptable at first glance, however the mode of failure in surrounding materials was neglected, which is a limitation. 55

9 Figure 3.3. Components of shear resistance offered by a bolt (after Bjurstrom, 1974) Hass (1976) carried out a series of single shear tests on chalk and limestone and reported that the block split during shearing. The stresses on both sides of the shear joint were suggested to be different which is not a realistic situation around the shear joint plane (Figure 3.4a). If the loading were truly symmetrical there would be an equal probability of either block splitting. To better distribute the shear load, Hass applied a large bearing plate on the moving block, but it was unsuccessful. Figure 3.4b shows the deformed bar subjected to lateral loading. It reveals a non-uniform situation along the joint plane. It is clearly understood that the single shear test has difficulties in equal load distribution in the shear joint. One method of minimising this problem was to by maintaining high confining pressures in order to reduce the imbalance in the vicinity of the shear joint plane. Non-uniform stress distribution across the shear joint plane was also investigated by numerical analysis (Afridi and et 56

10 al, 2001), thus confirming the existence of a non-equilibrium condition across the shear joint sides (Figure 3.5). Hole diameter Fracture a b Figure 3.4. (a) Block splitting in one side of shear joint (b) non equilibrium situation in vicinity of shear joint Figure 3.5. (a) Finite element mesh and (b) deviatoric of stress distribution across the joint (Afridi and et al. 2001) 57

11 Azuar (1977) found that for bolt installed perpendicular to the joint, the frictional effect is negligible. This finding is not consistent with the confining theories, which attribute part of the increase in strength to a frictional component. Azuar also found; i. The maximum contribution of a rock bolt to the shear resistance of a joint is influence by bolt orientation to the joint surface. It ranges from 60 to 80 % of the ultimate tension load of the bolt is installed perpendicularly, and 90 % for an inclined bolt. ii. The friction characteristics of the joint do not influence the contribution of the bolt. iii. For a given shear displacement, dilatancy increases the resistance of the bolted joint. Hibino and Motojima (1981) reported on shear tests on non grouted 2 mm diameter bolts installed in concrete blocks. They considered bolts placed in 2 mm and 40 mm borehole for fully bonded and point anchored respectively, and reported that: i. For a given shear displacement the shear resistance of fully bonded bolts was significantly higher than point anchored ones. ii. The shear resistance did not increased by bolt inclination. This is in contrast with other investigators. iii. Pre-tension loading the bolt reduced the shear displacement but did not influence shear resistance. This result is not consistent with the laboratory and numerical results obtained by this author and discussed later in the thesis. Hass (1981) reported on the laboratory tests on limestone with artificially cut joints reinforced by different types of bolts and different orientations (0 o, +45 o and -45 o ) to 58

12 the shear plane, as shown in Figure 3.6. He suggested that bolts would act more effectively when they are inclined at an acute angle to the shear surface rather than in the opposite direction, as they tend to elongate as shearing progresses. The total shear strength offered by a bolt was given by the summation of the bolt contribution and frictional strength along the shear surface from stress on the shear plane. Hass could not apply the bolt pre-tension effect because the device designed was incapable. With increased shear displacement the bars started to pull into the rock and consequently bolt resistance was reduced. However for bolts with a bearing plate, the shear resistance increased around 23%. Figure 3.6. Arrangement for bolt shear testing (after Hass, 1981) 59

13 Dight (1982) conducted a theoretical analysis of the grouted bolt performance. Dight assumed that the bolt contribution to the strength of a sheared joint was a resultant of tensile force in the bolt and the dowel effect (Figure 3.7). The angle of dilation was given by the following relationship: Angle of Dilation = 1 δv tag ( ) = i δ Reinforcing bar Grout Figure 3.7. General deformation patterns for a dowel in shear The dowel force was determined by Eq (3.6) t p 2 d t 2 = 1.7σ y puπ (1 ( ) ) (3.6) 4 t y where; 60

14 p u = The bearing capacity of the grout or rock t = Axial bolt load in the position of the plastic moment, t y = Axial load corresponding to the yield strength σ y = Yield stress of the steel, d = Bolt diameter And at the magnitude of t p, the location of plastic hinge was as follows: l pg = 0.58d σ p y u t (1 ( t y ) 2 ) (3.7) Dight did not make any predictions on bolt behaviour in elastic conditions, if tension prevails then the yield strength develops immediately. He considered the Eq (3.8) for a component of axial load in shear and suggested the bolt contribution would be a summation of Eqs (3.6) and (3.8). t = t (sinθ + cosθtag( ϕ i) (3.8) c y b + where; θ = The angle between the normal vector to the joint and the bolt, and ϕb is the basic joint friction angle. Dight reported: i. The normal stress acting on the joint plane does not influence shear resistance which is against the criterion of joint confining effect and results reported by Saeb and Amadei (1992). 61

15 ii. Joints with inclined bolts had stiffer behaviour than those perpendicular ones. The deformed length of the bolt was related to the deformability of the rock. Egger and Fernandez (1983) carried out tests in a high capacity press on samples of bolted concrete blocks, and found: i. The optimum angle of bolt inclination with respect to the joint varied from 30 o to 60 o. However Sharma and Pande (1988) found that the best direction of reinforcement is normal to the major joint direction. ii. iii. Perpendicular bolts appeared to have the lowest shear resistance. Shear displacement at failure was minimal for bolts inclined between 40 o and 50 o. Ludvig (1983) performed tests on swellex bolts, split sets, and two sizes of non grouted bars. The bolts were at 45 o and 90 o to the shear joint. Under shear the tube bolts were generally weaker than the solid bars. He suggested that the swellex bolt has approximately the same shear resistance as a solid 14 mm diameter non - grouted bar. Schubert (1984) proposed an analytical analysis based on the equilibrium of forces acting on the deformed system and conducted shear tests on bolted concrete and limestone blocks. The sketch of the shear device used by Schubert is shown in Figure 3.8. His results lead to the following findings: i. The deformability of the surrounding rock is important for bolt reaction. ii. Bolts embedded in harder rock require smaller displacements for attaining a given resistance than those in softer rock. iii. Soft steels improve the deformability of the bolted system in soft rock. 62

16 Yoshinaka et al. (1987) study on the direct shearing of 16 mm diameter bolt suggested 35 o 55 o angles against the joint plane as most favourable. In addition, a perpendicular bolt showed lowest contribution to shearing compared to those at a low angles (Figure3.9). Moreover, no pre-tension was considered. Figure 3.8. Shear test machine used by Schubert (after Schubert1984) Shear Stress (MPa) Shear displacement (mm) Figure 3.9. Relationship between shear stress and shear displacement (after Yoshinaka 1987) 63

17 Spang and Egger (1990) made an extensive series of shear tests on grouted bolts and used sandstone, concrete, and granite. They found the maximum bolt contribution to the shear strength of the joint was a function of the ultimate strength of the bolt, Tu To = Tu [ σ c sin ( β + i)] σ c ( tagφ ) (3.9) where; T u = Ultimate strength of the bolt σ c = The uniaxial compressive strength of the rock, i d = Inclination between the bolt and the shear surface = Dilation = Dimeter of the bolt = Friction angle of the joint and following Eq (3.10 ) was expressed for the shear deformation of the bolt. u o = d( σ 0.14 c σ 0.28 c But this theory was limited to: i. Steel bolts grouted with cement, 70 )[1 ( ) σ c tagβ ] cos β (3.10) ii. iii. Borehole diameter approximately twice that of the bolt, A uniaxial strength of rock between MPa, iv. Deformation formula is not accepted for bolts perpendicular to the joint ( =90 o ) and, v. Bolt not subjected to pre-tension Egger and Zabuski (1991) carried out a single shear test on small diameter bolts between 2.5 mm to 5 mm. Tests were made without the normal pressure and no pretension across the joint. Figure 3.10 shows the direct shear test apparatus. Bolts 64

18 failed under a combination of shear and axial forces. Only low strength steel was used as the technique was not suitable for high strength steel because the load distributed on the shear joint was not uniform or in equilibrium. Figure Direct shear test device (after Egger and Zabuski 1991) Holmberg (1991) theoretically examined the mechanical behaviour of non-tension grouted rock bolts in elastic and yielding conditions. His analytical model was based on the equilibrium of forces acting on the deformed system. He expressed three stages and an ultimate condition of bolt, grout interaction. These stages are shown in Figure 3.11 and were distinguished as follows: i ii iii iv Bolt and surrounding medium are in an elastic state, Bolt is in elastic and surrounding medium in a yielding state, Bolt and surrounding medium are yielded, Ultimate condition. 65

19 Holmberg s theory disregarded the influence of the grout material. The following conclusions were drawn: a: Elastic condition b: Elastic bolt and yielding subgrade c: Yielding bolt and yielding sub-grade T t p u l y y y = u y T ty d: Ultimate condition Figure Bolt grout behaviour (after Holmberge 1991) 66

20 i. The bolt contribution to the shear resistance of a bolted joint from dowelling and axial load can be determined as a function of deformation for different load conditions, ii. The initial angle of the bolt with respect to the direction of deformation is of minor importance compared to the maximum resistance of the bolted joint, iii. The initial angle has a great influence on the maximum deformation of the bolt, iv. A bolt inclination of 60 o with respect to the direction of deformation reduces the total deformation by four fold compared to a bolt perpendicular to the direction of deformation, v. When a steel bolt crushes into the rock mass and develops a shape similar to a crank handle its ability to resist larger deformation before failure is increased significantly, In a jointed rock mass the shear resistance becomes important where the bolt intersects the joint. When deformation occurs in the rock mass the grouted rock bolt will be subjected to loading which generates axial and lateral forces in the bolt (Figure 3.12). Factors influencing include, bolt and hole diameter, steel quality, bolt elongation, rock and grout strength. The angle between the bolt and the joint is very important for the behaviour of the bolted joint surface, especially in determining the type of failure. If the angle is less than 35 o it seems to be a tension failure, and if the angle is approximately 90 o, it is in shear. Ferrero (1995) proposed a shear strength model for reinforced rock joints based on the numerical and laboratory studies of large shear blocks. He suggested that the overall strength of the reinforced joint could be attributed to a combination of the 67

21 dowel and the incremental axial force due to bar deformation. Figure 3.13 shows the shear test apparatus which tends to suffer from an out of balance load on the shear joint plane. Figure A grouted rock bolt subjected to lateral force Ferrero s analytical model was applicable to bolts installed perpendicular to the joint surface in stratified bedding planes. As shown in Figure 3.14, the proposed analytical model was expressed by F = t cosα Qsinα ( t sinα Qcosα tagϕ (3.11) r r ) where; ϕ = Joint friction angle t r = Load induced in the bolt Q = Force due to dowel effect α = Angle between the joint and the dowel axis and F = Global reinforced joint resistance. 68

22 According to his experimental and modelling evidence, Ferrero suggested failure could possible occur in one of the following ways, depending on the prevalent type of stress: i. Failure due to the combination of the axial and shear force acting at the bolt-joint intersection. ii. Failure due to the axial force following the formation of hinge points. Figure Ferrero s shear test machine Figure Resistance mechanism of a reinforced rock joint (after Ferrero 1995) 69

23 The first yielding mechanism is likely to occur with stiffer and stronger rock at the bolt, joint plane intersection under a combination of shear and normal forces. As shown in Figure 3.15, the bolt is loaded by the axial and frictional forces that develop between the bolt and surrounding grout. The following equations were developed to describe the relationship between the bar tension at the point of maximum moment and bolt, joint intersection respectively. 2 0 x t r = p u D b (3.12) 2 y 0 t 2 2 x0 4y0 1.5 r = pu Db (1 + ) (3.13) 2 2y0 x0 The second failure mechanism occurs when the maximum computed bending moment in A exceeds the maximum yielding moment of the bolt. Usually this kind of failure occurs in weak and less stiff rocks. Figure Forces acting on the failure mechanism (after Ferrero 1995) 70

24 The yielding conditions propagate from the plastic hinges up to the joint intersection, which causes tensile stress to affect the bolt. However, Ferrero stated that pre-tension does not influence maximum resistance of the system. This appeared to be in contrast with both the experimental and numerical studies undertaken in this current thesis, which is discussed later in Chapters 5 and 7. Pellet and Egger (1995) analytical model for the contribution of bolts to the shear strength of a rock joint, took into account the interaction between the axial and the shear forces mobilised in the bolt, and large plastic displacements of the bolt during loading. A description of bolt behaviour must be divided in two sections. The first concerns the elastic range (from the beginning of the loading process) and the second deals with the plastic range (from the yield to the failure of the bolt). The shape of the stressed bolt and the failure envelope for both elastic and plastic deformations are shown in Figure 3.16 and Figure 3.17 respectively. They used the Tresca criterion as a failure criterion for the bolt. a) (a) 71

25 b) Figure Force components and deformation of a bolt, a) in elastic zone, and b) in plastic zone (after Pellet and Eager 1995) Relationship between axial and shear forces in elastic conditions Axial and shear forces at the yield limit Yield limit a) Failure criterion Axial and shear forces at failure b) Figure Evolution of shear and axial forces in a bolt, a) in elastic zone, and b) in plastic zone (after Pellet and Egger, 1995) 72

26 The shear forces at the end of both the elastic limit and plastic region are obtained from Eq 3.14 and Eq 3.15 respectively. Q oe πdbσ el = 0.5 pu Db ( N oe ) 4 (3.14) Q of N 2 πdb of 2 = σ ec 1 16( ) (3.15) 2 8 πdb σ ec where; Q oe = Shear force acting at point O at the yield stress of the bolt N oe = Axial force acting at shear plane at the yield stress of the bolt σ el = Yield stress of the bolt D b = Dimeter of the bolt Q of = Shear force acting at shear plane at failure of the bolt N of = Axial force acting at shear plane at failure of the bolt σ ec = Failure stress of the bolt The displacement of the bolt in elastic and plastic stages were expressed by the following equations: Qoe b U oe = (3.16) Eπ D p sin β b u U of Qoe sin ω op = (3.17) p sin( β ω ) u op Where le 2 2 le 2 2 ω op = arccos[ sin β ± cos β (1 ( ) sin β ) (3.18) l l f f 73

27 where: l e = Distance between bolt extremity (point O) and the location of the maximum bending moment (point A) l f = The length of the part O-A at failure Pellet and Eagers evaluations showed that bolt inclination has a significant influence on maximum joint displacement. The greatest displacement is reached when the bolt is normal to the joint. As the angle between bolt and joint decreased, displacement drops rapidly (Figure 3.18). Figure Joint displacement as a function of angle for different UCS value (after Pellet 1994) Pellet s theory is valid for the inclined bolts less than 90 o and is not properly acceptable for bolts sharply perpendicular to the joints. Robert (1995) reported shear tests on smooth bars and cone bolts by his double shear apparatus. He found that failure only happened in one of the joint intersections. His results showed a non-symmetric situation on both sides of the shear joint, which is 74

28 likely due to the generation of imbalance in three blocks and is contradicted with results from DSS in this research (see experimental results in Chapter 5). Goris et al (1996) carried out a direct shear tests on 69 MPa concrete blocks with in joint surface area of m 2 (Figure 3.19). A mm diameter cable bolt (258 kn yield strength) was placed perpendicularly into a 25.9 mm diameter hole. It was found that yield occurred at 220 kn with 4 mm of displacement, which is higher than the double shear test carried out on the same type of cable bolt. It appears that the single shear test has a higher shear resistance than the double shear test. This is due to an unequal distribution of load on the shear joint and concentration of load through the blocks in front of the bolt, which pushes them together (zone A) resulting in a higher shear resistance which is not an actual bolt contribution. Another limitation of the test set up was the maximum shear displacement available being limited to 46 mm, which prevented the cable from failing. A Figure Shear block test assembly (after Goris and et al. 1996) 75

29 3.3. PRE-TENSION EFFECT IN FULLY GROUTED BOLTS A bolt under tension compresses the rock, which prevents bed separation and frictional forces developing between the layers, but this does not mean that more tension creates better stability (Peng 1992). When a bolt is pre-tension loaded it would influence the shear strength of the joint with forces acting both perpendicular and parallel to the sheared joint by inducing confining pressure. A general rule for determining maximum pre-tension is that it should not exceed 60% of the bolt yield strength or 60% of the anchorage capacity. Nearly all the tests that were conducted by various authors related to bolt behaviour under shear were accomplished without pre-tension loading. However, in field studies and numerical simulations, pre-tension loading was applied and it was unanimously agreed that it increases reinforcement and improves stability, Lang et al. 1979, Maleki 1992, Peng and Guo 1992, Jafari and Vutukuri 1994, 1998, Stankus and Guo 1997, Unrug and Thompson 2002, Zhang and Peng 2002, and Hebblewhite However, numerical studies placed limitations on bolt, grout, rock contact interfaces. In addition no experimental tests were conducted to apply pre-tension in fully encapsulated high strength bolts, especially an evaluation of bolt profile on shear resistance under various levels of pre-tension loading. In this current research whole assumptions and limitations from both laboratory and numerical design were carefully removed. Pre-tension loading was conducted in 0, 5, 10, 20, 50, and 80 kn loads in laboratory and numerical simulations. In the numerical chapter a new design of bolt model and contact interfaces is discussed. As discussed above, there are pros and cons in each method used so far. A brief review of the methods is shown in Table

30 Table 3.1. A brief comparison of the used methods in bolt shear behaviour Author Base of the method Advantages Disadvantages Dulascka (1972) Bjurstrom (1973) Hass (1976) Azuar (1977) Hibino (1981) Hass (1981) Dight (1982) Egger and Fernandz (1983) Ludvige (1983) Schubert (1984) Yashinaka (1987) Spang and Egger (1990) Egger and Zabuski (1991) Development of plastic hinge after max. Moment Equilibrium forces acting on the system Single shear test Single shear test Prediction of shear force by bolt Estimation of shear resistance: due to dowel, reinforcement and friction effect, Test were performed on real rocks Different bolt angles were considered Single shear test Pre-tension was applied Single shear test Real rocks with different bolt angles were considered Theoretical analysis The prediction of dowel effect and hinge point was considered Single shear test Single shear test Equilibrium forces acting on the deformed system Direct shear test Different bolt angles was applied Different bolt angles was applied Real rocks was tested Different bolt angles was considered Single shear test Real rocks was tested, max bolt contribution and displacement was predicted Single shear test Prediction of bolt failure at a combination of axial and shear Non static equilibrium condition in shear joint Mode of failure in surrounding materials was neglected Non-uniform stress distribution along the shear joint Influence of friction effect could not properly considered Pre-tension and bolt s inclination could not considered properly Pre-tension was not applied Neglecting the bolt behaviour in elastic range, poor effect of normal stress on joint Pre-tension was not applied No fully grouted bolt was tested Pre-tension was not considered Pre-tension could not apply Limited in: grout types, annulus thickness, rock strength and pre-tension No joint confinement and bolt pre-tension was considered 77

31 Table 3.1. Continued Author Base of the method Advantages Disadvantages Holmberge (1991) Ferrero (1995) Pellet and Egger (1995) Goris et al. (1996) The equilibrium of forces acting on the deformed bar Single shear test Bolt behaviour was analysed in both elastic and plastic stages The plastic stage of the system was considered Theoretical analysis Both elastic and plastic stages was analysed Single shear test Perpendicular bolts was analysed Grasselli Double shear test Symmetric situation (2005) around the shear joint Mahoni, et Lengthy bolt-groutconcrete anchorage al. (2005) Single shear test Aziz et al Symmetric situation (2005) Double shear test around the shear joint, pretension effect, bolt profile, any grout, bolt & hole diameter The effect of grout was disregarded In-capability of the method to show the effect of pre-tension The effect of grout material was neglected Non-equilibrium load distribution on the shear joint, Max. Displacement was up to 46 mm Bolt pre-tension was not considered - The size of the shear box is small for large bolt diameters and strong steel bolts 3.4. MECHANICAL PROPERTIES OF REINFORCING MATERIALS In this part the strength properties of bolts, resin, and concrete are studied. All the tests were carried out in the laboratory under controlled conditions. Parameters examined include uniaxial compression strength, shear strength, and modulus of deformations. These parameters are pertinent to the overall study of the load transfer mechanism of bolts, resin, and concrete interactions Bolt types Seven different types were tested for tensile strength. Three bolts are the popular types used widely by the Australian mining industry. Figure 3.20 shows the 78

32 photographs for various bolts and Table 3.2 lists their physical specifications. They are similar in diameter core size, but have different profile heights and spacings. Figure 3.21 shows the general profile details of the bolts. Tensile, bending, and shear strength of the steel bolt are the most important mechanical parameters that influence its behaviour when loaded axially and in shear. T1 T2 T3 T4 T5 T6 Figure Different Bolt Types used for axial and shear behaviour tests Rib Width Rib Spacing Rib Height Core Diam. (mm) Outer Diam. (mm) Figure Profiles specification 79

33 Bolt strength tests Three kinds of laboratory tests were carried out on different Types of bolts (Table 3.2). They are: Tensile strength Bending strength Direct shear test Table 3.2. Physical specifications of different bolt types Bolt Bolt Commercial name Rib Spacing (mm) Core diameter (mm) Rib height (mm) T1 AX T2 AXR T3 JX T4 Deformed T5 All Thread T6 N Tensile strength test A 33 cm bolt length, was cut and tested for tensile strength by pull testing. A universal Instron tensile testing machine was used to carry out the tensile test. The tensile test on all re-bar specimens were carried out in accordance with the Australian Standards for tensile tests No AS A typical tensile test arrangement is shown in Figure The test specimen was installed between the two large grips of the testing machine and then loaded in tension. The computer controlled tensile test loaded the specimen at a constant rate until failure. While the test progressed load 80

34 and displacement values were monitored by the computer. The load displacement curves in Figure 3.24 to 3.27 show a typical behaviour of the steel with elastic behaviour in the beginning of the test and small displacement till yielding point. Beyond the yield point the bolt will deform without any further increase in the load until it is strain hardened. Finally the bolt fails when the cross section contracts in the form of a cap and cone known as (necking). Grips Bolt Figure Bolt clamped in Instron Universal Testing Machine As can be seen from the loading profile of the tested bolt (Figure 3.23) the following features were deduced; a) Elastic range b) Yield point c) Elasto-plastic range d) Failure range The yield strength is an important factor in determining tension, which influences its performance. It should be noted that although a roof bolt of high yield strength is desirable, its use in situ should be avoided. When a high strength bolt fails it is most 81

35 likely to shoot out of the hole so quickly it could severely injure anyone in its path (Peng 1986). Accordingly, current bolts used in mines are 320 kn. The value of the yield and ultimate failure loads in all types of bolts are described in Table 3.3. Table 3.3. Bolt tensile strength Bolt Yield Point (kn) Tensile Strength (kn) Yield stress (MPa) Ultimat e stress (MPa) T T T T T T Necking/Yielding/Failure T1 T2 T3 T4 T5 T6 Figure Stretching of the bolts after tensile test 82

36 Tensile Load (kn) T3 T1 T2 T T5 T6 Displacement (mm) Figure Load- deflection curve at tensile test in various bolts Figure Load- deflection curve at tensile test of Bolt Type T5 and T6 Tensile load (kn) Tensile load (kn) Displacement (mm) Displacement (mm) Figure Load- deflection curve at tensile test in cable bolt Figure Load- deflection curve at tensile test of Bolt Type T Three point load bending test For a better understanding of the bending behaviour of the bolts used, several tests were carried out in 3PLBT (three point load bending test). Figure 3.28 shows the three point load test set up. Three types of bolts used for axial and double shearing tests were tested under pure bending by this method. The bending behaviour of Bolt 83

37 Types T1, T2 and T3 is displayed in Figure Bolt Type T1 has the lowest bending strength while Bolt Types T2 and T3 exhibited higher bending loads Load (KN) T2 AXR T3 JAB T1 AX Displacement (mm) Figure Three point load bending test set up Figure Load- displacement behaviour of 3PLBT Direct shear test The direct shear tests were carried out with a guillotine especially designed with replaceable bushes to ensure a proper fit and that the bolt will not bend before being sheared. The shear forces are the resultant of shear stresses distributed over the cross sectional area and act parallel to the cut surface. Figure 3.30 shows the average shear load versus shear displacement for Bolt Type T1 and T3 respectively. Table 3.4 shows the results of direct shear tests two types of bolts. The direct shear test was conducted in an Instron 8033 Servo Controlled 50 tone Compression Testing Machine. 84

38 250 Shear Load (kn) Shear displacement (mm) T1 T3 Figure direct shear test trend in Bolt Types T1 and T3 Table 3.4. Specification of bolts shear test Bolt type Shear load (kn) Shear strength (MPa) Displacement (mm) T T T Average T T T Average Resin grout Epoxy and polyester resins are the most commonly forms of chemicals used in bolt installation in Australian Mines. The most popular type used is the resin combination sausage capsule supplied by Minova Australia (formerly known as Fosrock Mining). Strength tests was carried out on resin, including uniaxial compression tests, double shear tests, and modulus deformation tests. These tests were carried out on slow setting (20 minutes) PB1 Mix and Pour resin. A longer setting time was essential for 85

39 the strength tests. The diameter of the prepared samples was different for different tests carried out. a) Uniaxial Compression Test: A uniaxial compression test is the most common test performed on rock and other samples, in this case resin. The samples prepared were 50 mm diameter and the length to diameter tests was 2.5: 1. The samples were cast in a special plastic mould and tests were accomplished with an Instron machine of 500 kn capacity. A constant displacement rate of 0.25 mm/min was used to load the samples to failure. In reality, tested samples break similar to Figure 3.31 and sometimes the failure cracks are parallel to the axial direction. Figure 3.32 shows the compression test set up and subsequent tests undertaken. Although simple, care must be taken when carrying out the test so that errors are minimised and interpretations are as accurate as possible. The procedure for conducting a UCS test was carried out in accordance with International Rock Mechanics Standards. Samples were polished and cut till the height to diameter ratio of was achieved. Table 3.5 list the details of the samples tested and the UCS values obtained. Seven samples were tested. The average UCS values were 70.8 MPa with SD of +/ The UCS Value obtained was in agreement with the manufacturer s specified strength of 71 MPa. Figure 3.33 shows the relationship between stress and strain in resin. Figure 3.34 displays the load versus displacement. Some of sample was instrumented with strain gauges to monitor, axial and lateral deformation of the sample during loading process. b) Shear Strength: The shear strength tests were undertaken using double shear tests with a 50 tonne Avery machine as shown in Figure The samples were prepared by casting in specially prepared moulds 32 mm diameter, which fitted snug inside the double shear barrel. Four tests were carried out with the average shear 86

40 strength of 16.2 MPa +/- 1.1 standard deviation. The resin was different from the sausage type as it had a setting time of 20 minutes, allowing for a proper preparation of the samples. Resin Sample Fracture Plane Hemispherical Seating Angle of Fracture α Figure Typical fracture plane and fracture angle for compression test samples Table 3.5. Summary of the results obtained from UCS test Sample Length Failure load Ucs (mm) (KN) (MPa) S S S S S S S SD SD = Standard deviation 70.8 ±

41 Strain gauge Figure Compression test set up Axial stress (MPa) UCS=73 MPa, E= MPa Poisson ratio= strain axial lateral Figure Stress strain curve for resin 88

42 Compressive load (kn) Figure Load versus displacement Axial displacement (mm) 32mm diameter samples of resin were cast in PVC tube 100mm long for double shear test. Each sample was placed inside the double shear testing rig and then loaded by the Avery testing machine until they failed at a standard rate of 2.5 kn per minute. The double shear test rig is outlined in Figure There are two shear locations to accurately determine the shear properties of the material being tested. A total of four double shear tests were conducted to accurately determine the peak shear force of the resin and to ensure consistency of both testing methods and results. Sample measurements are shown in Table 3.6. Table 3.6. Double shear test specifications Sample Diameter Sample area Failure load Shear strength (mm) (mm*2) (kn) (MPa) S S S S S.D SD = standard deviation 16.2 ±

43 32 mm Location of Shear Failure a Figure Double shear test set up (a) shear box set up (b) induced loads b Concrete Uniaxial compressive strength Four nominal strength 20, 40, 50 and 100 MPa concretes were used in the double shearing tests. These strengths compare well with the range of rock strength. Some cylindrical samples from each batch were cast to measure the strength of the concrete. It was tested in compression to ensure that the required strength had been obtained. Figure 3.36a & b show the sample during the test and the concrete blocks after taking out from the water tank. The modulus of elasticity was calculated from equation which was expressed from Australia standard AS3600 (1994) and also the typical value of Poisson s ratio specified by AS3600 is E c = 0.043ρ f cm (3. 19) 90

44 A suitable expression, which applies for concrete excess of 50 MPa, has been recommended by ACI Committee 363 (1992): E c = 3320 fcm (3. 20) where; E c = Modulus of elasticity (MPa) 3 ρ = Concrete density ( kg / m ) f cm = Mean value of the concrete compressive strength at the relevant age (MPa) a b Figure Concrete sample: (a) concrete under the test (b) concrete after 30 days Concrete joint surface properties In order to estimate the strengthening effect of bolting one has to know the friction properties of unbolted joints. For this reason a series of direct shear tests was performed on specimens of broken and intact concrete under a variety of normal loads. All the samples were tested in direct shear using a direct shear machine, and some important parameters can obtained such as, peak shear strength, residual shear 91

45 strength, cohesion and angle of internal friction (See Moosavi and Bawden 2003). The specimen was positioned and then the lower half of the sample was potted in the shear box ring with the potting compound. After the compound hardened the appropriate thickness of Plexiglas sheets were placed on top of the lower shear box to form the shear plane. Whereas the specimen being tested had a weakness plane (concrete, concrete interface) it was placed in the shear machine such that the joint plane coincided with the plane of the machine. The friction joint angle can be estimated by performing repeated shear tests under different normal loads. To estimate shear resistance of a joint Barton (1966) developed an empirical model (Brady and Brown 1985). Which can be written as following. JCS τ p = σ ntgjrc log 10 ( ) + ϕb σ n (3.21) Where, τ p = peak shear stress, σ n = normal stress, JRC = joint roughness coefficient, JCS = joint compressive strength, and ϕ b basic friction angle. From the data analysis it was found that the joint surface cohesion in both concrete 20 and 40 MPa was zero and the angle of friction was 31 and 38 degree respectively (Figure 3.37 a and b). As Figure 3.38 shows, once the peak shear strength was overcome, there was considerable loss of shear resistance. From the laboratory results the concrete specifications were found as shown in Table 3.7. Also it was found that the relation between shear stress and normal stress was nearly 0.9 to 1.7 normal stress in 20 and 40 MPa concrete respectively. 92

46 Shear stress (MPa) Normal stress (MPa) Shear stress (MPa) Normal stress (MPa) a b Figure Variation of peak shear stress versus different normal stress in shear joint plane in a: 20 MPa and b: 40 MPa concrete Table 3.7. Concrete joint properties Ucs Strength (MPa) Modulus of Elasticity (MPa) Poisson ratio Friction angle (o) kn 7.5 kn 5 kn Shear load (kn) displacement (mm) Figure concrete Shear load versus shear displacement in joint plane in 40 MPa 93

47 3.5. SUMMARY The following were deduced from the review of bolt reinforcement across the joint planes; Bolt orientation, dowel effect, installation type (full encapsulation versus point anchor), joint surface friction, bolt material type, medium strength are important factors for the bolt effectiveness in joint reinforcement Bolts installed at inclination to the sheared joint plane contribute to a greater resistance to shearing than perpendicular bolts. The effectiveness of bending and location of the hinge points across the joint planes is influenced by the pre-tension loads and subsequent development of axial loads along the bolt. There was no reported citing of any study making reference to bolt surface profile configuration on the load transfer mechanism across the bolt. There are no reported results in the case of diversity of resin thickness and quantitative significance of shear resistance mechanism in different surrounding rock strengths. The effect of pre-tension load on shear behaviour and load transfer mechanism was not subjected to qualitative analysis. All reported shear tests were conducted under single shear test condition, where there will undoubtedly be asymmetric and a non-uniform distribution of load across the joint plane. It is clear that for bolts shearing under symmetrical conditions, the bolt profile configuration and changes in axial loads require further investigation and to achieve these aims, extensive laboratory tests were under taken together with numerical 94

48 simulations and analytical studies which are the subject of research reported in this thesis 95