Comparative Shear Tests of Bolt Rods Under Static and Dynamic Loading

Maintaining the stability of mine workings under the conditions of increased static loading and dynamic impact loading constitutes a significant problem in underground mines. This particularly concerns ultra-deep mines with depths under 1000 m, where excavations are established by means of roof bolting. This can be observed in the cases of working stability loss in the Polish copper mines, occurring as a result of rock bursts (Dubiński & Konopko, 2000; Fuławka et al., 2018) induced by mining activities. Seismic activity analysis showed (Burtan et al., 2017) that the rock bursts were primarily caused by high-energy rock mass tremors with an energy of A_s ≥ 10⁵ J. High-energy tremors occur primarily as a result of the presence of tremor-generating rock formations above the mined deposit, as well as due to the influence of faulting and also the depth and already moved-out area. The maximum mining depth of those mines has been already 1300 m, the copper ore deposits are extracted via various forms of room-and-pillar mining, while the most numerous occurrences of rock bursts have been observed with tremors with energy of A_s = 10⁶–10⁷ J (Gogolewska & Kaźmierczak, 2014; Pytel et al., 2019).

In the case of the utilisation of roof bolting under the conditions of a discontinuous rock mass, that is, fractured (Skrzypkowski et al., 2017) or blocky (Cała et al., 2001), bolt rod shear strength plays a significant role. An example diagram of a gallery working in a blocky rock mass secured against the falling of a block with a weight W via roof bolting is presented in Fig. 1.

Figure 1

A three-dimensional numerical analysis of a rock burst that occurred in the Kiirunavaara Mine (LKAB, Sweden) (Sjöberg et al., 2011) shows that a shear along pre-existing geological formations (with block structures) was the probable cause of the seismic event that resulted in a fatal accident in the mine in 2008. Therefore, a significant problem exists regarding the selection of rock bolt supports for tremor and rock burst conditions that would dissipate the energy of the rocks moving into the working. Thus, as well as static mechanical properties, the dynamic properties of bolts should also be determined.

Major problems connected to maintaining mine working stability also occur in ultra-deep gold mines (extraction at a depth of 4000 m) in the Republic of South Africa (Sengani, 2018). Increasing work safety is possible only via the utilisation of rock mass destressing, correct support design and support element technical parameter testing over the course of numerous tests conducted under laboratory conditions (static and dynamic tests) and underground (static tests).

World-widely, test methodologies and standardisations concerning the mechanical elements and steel from which bolt rods are formed can be found, for example, in countries such as: Australia, Austria, Canada, Germany, Poland, the Republic of South Africa, Sweden, the United Kingdom and the USA. In Europe, for example, the general requirements for steel and other materials are defined in the European standard EN 1537 (2000), which has been translated into many languages (e.g., British standard BS EN 1537:2000). General requirements for mechanical elements and steel have also been defined in the Republic of South Africa: SABS 1408 (2002) and SANS 920 (2005).

In Poland, detailed requirements for bolts under static loading are defined in standard PN-G-15091 (1998), whereas their tests are defined in standard PN-G-15092 (1999). Static strength tests are conducted for bolt rods (tensile, bending, shear and torsional strength; impact testing), washers, lagging, grouting agents and grout cartridges. Bolt rod load capacity is tested under laboratory or underground conditions (in situ) (Niedbalski et al., 2013). Bolt rod shear strength tests are based on a single shear test of only the rod itself (not grouted into a tube or cylinder).

Example standardisations that include test methodologies of bolts under static loading can be found in, for example, German (DIN 21521 Teil 1 1990; DIN 21521 Teil 2 1993), British (BS 7861-1 2007) and American (ASTM F432-13 2013) standards. In the case of the German and British standards, single shear tests of bolt rods are also conducted, but the bolts are grouted into tubes or cylinder segments.

Static double shear tests of bolts and numerical modelling results (Aziz et al., 2003; Li et al., 2016; Mirzaghorbanali et al., 2017), the purpose of which is to inspect the influence of various parameters on the behaviour of the bolt during shear tests, can also be found in literature.

Shear strength tests of bolts under dynamic loading are currently not standardised. Scientific work conducted at the University of Wollongong (Australia) concerning the test methodology of bolts under static and dynamic shear loading (Aziz et al., 2008; Gilbert et al., 2015; Li et al., 2014; Yang et al., 2018; Jalalifar, 2006) can be found in literature. The bolt tests are conducted via double shear testing of bolts or single shear testing. For double shear testing (Craig & Aziz, 2010; Rasekh et al., 2017; Jalalifar et al., 2005), the tests utilise three concrete blocks, whereas the single shear tests (Aziz et al., 2017; Aziz et al., 2018; Li et al., 2019) utilise cylinders filled with concrete, consisting of two sections. The shearing cylinders are fabricated in two sections, each containing 1.8 m of a concrete anchor cylinder, providing a centrally located shearing plane. The double shear tests of bolts under dynamic loading are based on the method of exerting loads by means of a drop hammer (pile-driving hammer) with a mass of 600 kg.

However, the double and single shear tests of bolts installed in concrete or rock blocks do not yield satisfactory results for bolt manufacturers, who expect a method enabling the evaluation of the shear strength of a bolt rod under impact loading and the strength comparison of various materials and types of bolts. The influence of grout, rock or concrete strength during shear tests is significant enough to make the evaluation of the strength of the bolt rod itself difficult (Jalalifar et al., 2005). Another reason for the difficulty in assessing bolt rod shear strength are local instances of spalling (occurring at the point where the guillotine is applied) of the grout, cement or concrete, which result in the formation of complex stress, which is also significantly influenced by bolt rod bending and tension.

The American standard ASTM D7401-08 (2008) concerning dynamic drop tests is one example of a standard that defines the test methodology of a bolt under dynamic loading. The bolt rod is grouted into a thick-walled steel pipe and not into a concrete or rock block. According to this standard, the tensile impact load exerted on the bolt is based on the drop hammer method. Tests of bolts under dynamic loading are conducted by world-renowned laboratories in Australia, Canada, South Africa and Sweden. One of the best-known laboratories in the world is CANMET-MMSL in Canada (CANMET – Mining and Mineral Sciences Laboratories), which has been conducting tests under both static and dynamic loading for many years (Labrie et al., 2008; Plouffe et al., 2008).

Bolt tests under impact loading by means of the drop hammer method are also carried out at the Central Mining Institute in Poland (Nierobisz et al., 2001; Prusek et al., 2016; Pytlik et al., 2016). Initially, the tests were conducted using an impact mass ranging from 4000 kg to 20,000 kg at maximum impact velocities of up to v = 3 m/s. Due to the fact that the PPV (Peak Particle Velocity) values within the near wave field determined on the basis of 120 rock bursts that had occurred in Polish hard coal mines did not exceed v = 1 m/s (Dubiński & Mutke, 1996; Mutke, 2007; Mutke et al., 2016), the bolts were mostly tested using an impact mass of 20,000 kg and an impact velocity of 0.4-1.2 m/s.

Other than bolts, the test facility is also used to conduct impact tests of powered support and single prop support hydraulic legs (Pytlik, 2015a; Pytlik, 2015b; Prusek et al., 2016; Pytlik, 2018), friction props, ropes, chains and mine grids. Tests of shotcrete and membranes are also performed by means of box of rocks tests, using a drop mass of up to 4000 kg.

The current technical capabilities of the Central Mining Institute drop hammer facility for mine support element testing allow the tests of structural elements, that is, hydraulic legs, chains, ropes and cable bolts, with a maximum impact energy E_p ≈500 kJ (m = 20,000 kg, h = 2.5 m) to be conducted. As for bolts tested based on the American standard ASTM D7401-08 (2008), the current capabilities allow the testing of bolts with a maximum length of 3 m by means of an impact mass of 5600 kg dropped from a height of approximately 3 m. This makes it possible to load the bolt with an impact energy E≈165 kJ and impact velocity v≈7.67 m/s.

These test facility parameters allow to simulate impact loads at broad ranges of impact energy and velocity, which are observed in situ. For example, during tests that were conducted in iron ore mines (Shirzadegan et al., 2016a; Shirzadegan et al., 2016b) belonging to LKAB (Sweden), the measured maximum PPV was 7.5 m/s. Rock burst simulations performed under gold mine conditions (Haile & Le Bron, 2001; Milev et al., 2001; Milev 7 Spottiswoode, 2005) in the Republic of South Africa also revealed high PPV values, reaching up to 3.3 m/s. PPV measurement results (at depths of up to approximately 1000 m) in Polish hard coal mines showed that maximum vertical PPV component values reached up to 1 m/s, while the horizontal component values reached up to 1.56 m/s, and incidentally to 1.97 m/s (Kidybiński et al., 2009; Mutke, 2007). The corner frequencies varied between 8.3–31.1 Hz. The corner frequencies are the frequencies that carry most of the seismic energy and momentum. For the 85% of rock bursts, the corner frequency does not exceed 23 Hz (Mutke et al., 2016). In Polish copper mines (for seismic events of energy greater than 10⁵ J monitored in the near-field wave), the velocity may occur in the range of 0.027–0.095 m/s. The highest values of velocity have been measured by frequency of several Hz (Pytel, 2003). The maximum value of the PPV equals to 0.197 m/s measured at a hypocentral distance of 64 m from seismic event of energy A_s = 5.5·10⁵ J, but this value did not cause a rock burst (Nierobisz, 2004). Values of rock vibration velocity generated by seismic waves for Polish copper mines have not been sufficiently recognized and more researches are needed.

The impact velocity v of the free-falling mass in the GIG test facility, is not the same as the PPV on the surface of the mine workings (Pytlik et al., 2016). The methodology assumes that such correlation exists and applies only to the first impulse of the load and not to the wave motion. This is because in addition to the wave motion, there is another rock mass movement, which is a common cause of the destruction of mine workings (Drzewiecki, 2002). This movement is associated with the motion of large volumes of rock mass initiated by the creation or propagation of discontinuities. The direction of this movement is consistent with the area with the most degrees of freedom.

This article presents a methodology for the single shear testing of bolt rods and the results of bolt tests that enable the determination of bolt dynamic shear strength under impact loading exerted by means of a drop hammer. A maximum impact velocity of v = 2 m/s was adopted for the tests, as an implication of Polish experiments conducted during PPV in situ measurements. A comparison was also performed for the work dissipated by the bolt rods under static and impact loading, from which two types, AM18 and AM22, were formed via thermal hardening (heating – stamping – quenching), whereas the other two types, APP and APB, were formed via hot rolling, while the steel was smelted in electric arc furnaces, cast using continuous casting machines and finally subjected to slow cooling. Thus, the APP and APB bolt rods exhibit more uniform structures across their entire cross sections compared to the AM18 and AM22 bolts, which are surface-hardened, while their cores exhibit higher plasticity.

The obtained test results allowed to formulate mathematical relationships for the bolt shearing force under impact loading as a function of time of the duration of the impulse of the force.

The test methodology for single shear testing of bolts under static loading was developed based on a Polish standard (PN-G-15092:1999). The shearing of a bolt rod with an outer diameter D is based on the gradual material integrity deterioration initiated by the excessive pressure of the guillotine, exerted against the rod on both sides of the sheared cross section. The 250 mm-long bolt rod sample is placed in a hole with a maximum diameter of D+2 mm located in the die of the bolt rod shearing device, presented in the form of a diagram in Fig. 2.

Figure 2

Bolt shear testing is based on loading a bolt rod sample with an increasing shearing force until the rod is shorn. The shearing force is exerted on the bolt rod by a punch (guillotine) with a minimum outer diameter of 3×D and a maximum load rate of up to 5 kN/s. Sliding the (cylinder-shaped) punch in the die, which has a very small clearance, up to 136 mm, made possible by the fitting tolerance between the die and the punch, allows the bending stresses in the shorn bolt rod sample to be minimised.

Bolt shear tests under static loading were conducted using a universal testing machine equipped with a strain gauge force sensor (accuracy class 0.5) and a potentiometric displacement sensor (accuracy class 0.1). The measuring sensors were connected to an MGCplus-type measuring amplifier (accuracy class 0.03) coupled with a computer. The measurement data is recorded on the computer with a sampling frequency f_s = 10 Hz. Based on the measurement data, a graph of the static shearing force F_s as a function of bolt rod shear displacement L is generated, and the maximum shearing force F_smax measured during the test is defined.

Bolt rod shear strength t_s (Niezgodziński M.E. & Niezgodziński T., 1996) under static loading is calculated using the following formula: (1)τs=FsmaxAe103,MPa\tau _s = {{F_{smax } } \over {A_e }}10^3 ,{\rm{MPa}} where: F_smax – maximum force during the shear test under static loading, kN; A_e – effective bolt rod cross section area, mm².

In the case of a bolt rod formed from a round plain bar with a diameter D, the bar cross section is calculated using the following formula (2): (2)Ae=A=π⋅D24,mm2A_e = A = {{\pi \cdot D^2 } \over 4},\;{\rm{mm}}^2 whereas, for a rod formed from a ribbed bar, the effective cross section area A_e and effective diameter D_e of the bar are calculated using the following formulas (PN-H-93215:1982): (3)Ae=mrρ⋅lr106=127.4mrlr,mm2A_e = {{m_r } \over {\rho \cdot l_r }}10^6 = 127.4{{m_r } \over {l_r }},\;{\rm{mm}}^2 (4)De=12.74mrlr,mmD_e = 12.74\sqrt {{{m_r } \over {l_r }}} ,{\rm{mm}} where: D – diameter of the rod formed from a round plain bar, mm; D_e – effective diameter of the rod, mm; l_r – length of the rod sample, m; m_r – mass of the rod sample, kg; r – density of steel (r = 7850 kg/m³)

Based on the chart of the course F_s = f(L) (Fig. 3), the bolt rod shear work W_s under static loading is calculated as well.

Figure 3

The objective of the developed single shear testing methodology of bolt rods under impact loading is to conduct relatively simple and repeatable tests, whose results could be used by bolt rod manufacturers, rock bolt support designers and for the purposes of product certification. The methodology also allows to compare the shear impact strengths of various types of bolt rods. The primary goal of the test is to investigate whether the bolt can be safely utilised in underground mines prone to tremors and rock burst hazards.

The tests are conducted in the testing facility presented in Fig. 4. The drop mass (ram) m = 2429 kg used during the tests simulates the impact load exerted on a bolt rod with varying impact velocity v.

Figure 4

The same shearing device is used for bolt rod shear tests under impact loading as for static tests.

The single shear test of a bolt under impact loading is based on the free fall of the mass m = 2429 kg from various heights h directly onto the impact that shears the bolt rod. Each rod is tested until it is completely shorn – via single or multiple impacts of the mass. After the rod is shorn, the ram is intercepted by two buffers (equipped with springs and a liquid elastomer) that secure the test facility from damage.

The bolt impact velocity v during the test is calculated using the following formula (6): (6)v=2gh,m/sv = \sqrt {2gh} ,{\rm{m}}/{\rm{s}} where: h – impact mass height of fall, m; g – gravitational constant, 9.81 m/s².

During the test of the bolt under shear impact loading, the bolt dynamic resistance force F_d is registered with a sampling frequency f_s = 19.2 kHz. The maximum value F_dmax of the bolt dynamic resistance force F_d at a given impact velocity v is also determined during the test.

Bolt rod shear impact strength t_d is calculated using the following formula (7): (7)τd=FdmaxAe103,MPa\tau _d = {{F_{dmax } } \over {A_e }}10^3 ,{\rm{MPa}} where: F_dmax – maximum force during the shear test under impact loading, kN.

Based on the chart of the course F_d = f(t) (Fig. 5), the value of the impulse J of the force (Halliday et al., 2007) is calculated using the following formula (8).

Figure 5

The average force F_a is calculated using the following formula: (9)Fa=Jt1,kNF_a = {J \over {t_1 }},{\rm{kN}} Bolt rod shear work W_d value under dynamic loading is calculated using the following formula: (10)Wd=Fa⋅Lc,kJW_d = F_a \cdot L_c ,{\rm{kJ}} where L_c – displacement until the point of bolt rod shearing under static loading, mm. The L_c value is determined during a bolt rod shear test at static load (Figure 3). Due to the significant difficulties in measuring the displacement located in the test facility axis (due to a lack of space to mount a sensor in the facility axis as well as major sensitivity to impact and vibration of the existing sensors on the market), the L_c value from the static test was adopted for calculations in the formula (10).

Four types of bolts with varied mechanical properties and rods formed from different grades of steel were subjected to testing.

Two types of bolt rods:

AM18 – A500S steel – steel intended for concrete reinforcement,

The other two types of bolt rods:

APP – AP600V steel – steel intended for rock bolt production,

The basic dimensions of the rods and the mechanical properties of the steel used to produce the bolt rods are presented in Table 1. The unit elongation A₅ value is given based on mill certificates (inspection certificates 3.1 in accordance with standard BS EN 10204:2004) of the steel used to produce the bolt rods. Bolt rod ultimate tensile strength R_m under static loading was determined based on tests of samples extracted from 2 m long rods. Then, further samples were extracted from the same rods, which were subjected to single shear tests under static and dynamic loading.

A500S and A500sh steel designations were obtained from the catalogue of the bar manufacturer (ArcelorMittal Kryvyi Rih 2016) and from mill certificates. AP600V and AP770 steel designations were obtained from the catalogue of the bar manufacturer (Minova Arnall, 2018) and from mill certificates.

4.1

AM18 And AM22-Type Grouted Bolt Rods

AM18-type bolt rods are produced as bars with trapezoidal threads along the entire rod. Fig. 6 presents a view of the bolts, as well as the thread profile and cross section diagrams of the rods.

Figure 6

A result compilation of the single shear tests of AM18 and AM22-type bolt rods under static loading is presented in Table 2, whereas the load F_s curves as functions of shear displacement L are presented in Fig. 7.

Table 2

Mechanical properties of AM18 and AM22 bolt rods under static shear loading.

Bolt type	Maximum force Fsmax, kN	Shear strength ts, MPa	Shear displacement Lc, mm	Work Ws, kJ
AM18	131.9 ± 4.7	542.3 ± 19.3	8.6 ± 0.2	0.775 ± 0.061
AM22	217.1 ± 6.7	592.5 ± 18.4	9.7 ± 0.6	1.221 ± 0.131

Figure 7

4.2

APP And APB-Type Grouted Bolt Rods

APP and APB-type bolt rods are made of reinforcing steel bars, whose pictures and cross section diagrams are presented in Fig. 8.

Figure 8

A result compilation of the single shear tests of APP and APB-type bolt rods under static loading is presented in Table 3, whereas the load F_s curves as functions of shear displacement L are presented in Fig. 9.

Table 3

Mechanical properties of APP and APB bolt rods under static shear loading.

Bolt type	Maximum force Fsmax, kN	Shear strength ts, MPa	Shear displacement Lc, mm	Work Ws, kJ
APP	165.2 ± 2.7	541.9 ± 9.0	9.1 ± 0.4	0.971 ± 0.036
APB	235.1 ± 1.4	612.8 ± 3.7	8.5 ± 0.2	1.340 ± 0.053

Figure 9

During the bench testing of AM18, AM22, APP and APB-type bolt rods, a total of 88 single shear tests under impact loading by means of a drop hammer were conducted. This made it possible to determine the mechanical properties of bolt rods under impact loading and present them in the form of tables and graphs. It was also possible to formulate mathematical relationships for the bolt rod shearing force under impact loading as a function of the duration of the impulse of the force, which could become useful in the future for modelling rock bolt support loads and for the correct selection of bolts to the geological and mining conditions found in mine workings.

5.1

AM18 And AM22-Type Grouted Bolt Rods

A result compilation of the single shear tests of AM18 and AM22 bolt rods under impact loading where rod shearing occurred is presented in Table 4.

Table 4

Mechanical properties of AM18 and AM22 bolt rods during single shearing under impact loading.

Bolt type	Maximum force Fdmax, kN	Shear strength td, MPa	Work Wd, kJ
AM18	135.3 ± 4.0	556.0 ± 16.6	0.630 ± 0.032
AM22	225.5 ± 7.0	615.6 ± 19.1	1.203 ± 0.064

Example load F_d curves as functions of time t during AM18-type bolt rod tests where rod shearing occurred are presented in Fig. 10a. During the tests, the minimum impact velocity v of the mass m = 2429 kg that resulted in the shearing of the bolt rod was 0.990 m/s. Example load F_d curves as functions of time t during AM18-type bolt rod tests at impact velocities v = 0.767 m/s and v = 0.886 m/s that did not result in the shearing of the bolt rod are presented in Fig. 10b.

Figure 10

Example load F_d curves as functions of time t during AM22-type bolt rod tests where rod shearing occurred are presented in Fig. 11a. During the tests, the minimum impact velocity v of the mass m = 2429 kg that resulted in the shearing of the bolt rod was 1.253 m/s. Example load F_d curves as functions of time t during AM22-type bolt rod tests at impact velocity v = 1.172 m/s that did not result in the shearing of the bolt rod are presented in Fig. 11b.

Figure 11

5.2

APP And APB-Type Grouted Bolt Rods

A result compilation of the single shear tests of APP and APB bolt rods under impact loading where rod shearing occurred is presented in Table 5.

Table 5

Mechanical properties of APP and APB bolt rods during single shearing under impact loading.

Bolt type	Maximum force Fdmax, kN	Shear strength td, MPa	Work Wd, kJ
APP	174.2 ± 2.5	571.6 ± 8.3	0.945 ± 0.033
APB	252.0 ± 3.6	656.9 ± 9.5	1.269 ± 0.062

Example load F_d curves as functions of time t during APP-type bolt rod tests are presented in Fig. 12a. During the tests, the minimum impact velocity v of the mass m = 2429 kg that resulted in the shearing of the bolt rod was 1.085 m/s. Example load F_d curves as functions of time t during APP-type bolt rod tests at impact velocity v = 0.990 m/s that did not result in the shearing of the bolt rod are presented in Fig. 12b.

Figure 12

Example load F_d curves as functions of time t during APB-type bolt rod tests where rod shearing occurred are presented in Fig. 13a. During the tests, the minimum impact velocity v of the mass m = 2429 kg that resulted in the shearing of the bolt rod was 1.401 m/s. Example load F_d curves as functions of time t during APB-type bolt rod tests at impact velocity v = 1.329 m/s that did not result in the shearing of the bolt rod are presented in Fig. 13b.

Figure 13

The developed method of single shear testing of bolt rods under impact loading allows to obtain repeatable test results concerning maximum bolt rod shearing force, shear stress and shear work values.

It is proposed that the shear work value W_d obtained during single shear tests should become an additional assessment parameter determining whether bolt rods are appropriate for utilisation under roof caving, tremor and rock burst conditions. This parameter can also be used during rock bolt support design as well as for comparisons of various types of bolt rods, which is of significant importance to bolt manufacturers.

Comparative shear tests of four types of bolt rods under static and impact loading showed that APB-type bolt rods made of AP770 steel, which is characterised by the highest strength (R_m = 830.4 MPa at A₅ = 19%), exhibited the greatest shear work W_s = 1.340 ± 0.053 kJ and W_d = 1.269 ± 0.062 kJ. AM22-type bolt rods exhibited a very similar work value (W_s = 1.221 ± 0.131, W_d = 1.203 ± 0.064 kJ). Though the AM22-type bolt rods made of A500sh steel demonstrated lower strength (R_m = 762.3 MPa) than the APB-type bolt, as well as a smaller diameter and cross section; they dissipated the impact energy better thanks to their higher plasticity (A₅ = 23%). This could indicate a method of optimisation for bolts in order to increase their dynamic strength.

The mathematical relationships determined in the form of ‘extreme’ functions describing the actual F_d = f(t) courses of single bolt shearing under impact loading could find application in the load assessment process of bolt rods utilised under roof caving, tremor and rock burst conditions.

Further single shear tests of various types of bolt rods under impact loading are planned in the future.