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
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
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
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
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
The impact velocity
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
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
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×
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
Bolt rod shear strength ts (Niezgodziński M.E. & Niezgodziński T., 1996) under static loading is calculated using the following formula:
In the case of a bolt rod formed from a round plain bar with a diameter
Based on the chart of the course
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)
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
The bolt impact velocity
During the test of the bolt under shear impact loading, the bolt dynamic resistance force
Bolt rod shear impact strength
Based on the chart of the course
The average force
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, AM22 – A500sh steel – steel intended for rock bolt production,
are manufactured via thermal hardening: heating – stamping – quenching.
The other two types of bolt rods:
APP – AP600V steel – steel intended for rock bolt production, APB – AP770 steel – steel intended for rock bolt production,
are manufactured as hot-rolled bars, whereas the steel is smelted in electric arc furnaces and cast using continuous casting machines as 120×120 mm squares. Rods III and IV exhibit more uniform structures across their entire cross sections compared to rods I and II, which are surface-hardened and whose cores retain plasticity.
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
Basic rod dimensions and mechanical properties of steel.
AM18 | A500S | 17.6 | 243.3 | 721.5±16.8 | 18 |
AM22 | A500sh | 21.6 | 366.4 | 762.3±4.3 | 23 |
APP | AP600V | 19.7 | 304.8 | 681.3±2.9 | 24 |
APB | AP770 | 22.1 | 383.6 | 830.4±7.3 | 19 |
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.
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.
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
Mechanical properties of AM18 and AM22 bolt rods under static shear loading.
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 |
APP and APB-type bolt rods are made of reinforcing steel bars, whose pictures and cross section diagrams are presented in Fig. 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
Mechanical properties of APP and APB bolt rods under static shear loading.
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 |
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.
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.
Mechanical properties of AM18 and AM22 bolt rods during single shearing under impact loading.
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
Example load
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.
Mechanical properties of APP and APB bolt rods during single shearing under impact loading.
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
Example load
The results of the single shear tests under impact loading demonstrated that the greatest shear work
A comparison of the mechanical properties of bolt rods during shearing under static and dynamic loading as well as their reference to a static tensile test are presented in Tables 6 and 7.
Comparison of bolt rod mechanical properties under static and dynamic loading (part 1).
AM18 | A500S | 175.5 ± 4.1 | 721.5 ± 16.8 | 131.9 ± 4.7 | 542.3 ± 19.3 | 0.775 ± 0.061 | 0.752 | 135.3 ± 4.0 | 556.0 ± 16.6 | 0.630 ± 0.032 | 0.771 |
AM22 | A500sh | 279.3 ± 1.6 | 762.3 ± 4.3 | 217.1 ± 6.7 | 592.5 ± 18.4 | 1.221 ± 0.131 | 0.777 | 225.5 ± 7.0 | 615.6 ± 19.1 | 1.203 ± 0.064 | 0.808 |
APP | AP600V | 207.7 ± 0.9 | 681.3 ± 2.9 | 165.2 ± 2.7 | 541.9 ± 9.0 | 0.971 ± 0.036 | 0.795 | 174.2 ± 2.5 | 571.6 ± 8.3 | 0.945 ± 0.033 | 0.839 |
APB | AP770 | 318.6 ± 2.8 | 830.4 ± 7.3 | 235.1 ± 1.4 | 612.8 ± 3.7 | 1.340 ± 0.053 | 0.738 | 250.8 ± 1.9 | 656.9 ± 9.5 | 1.269 ± 0.062 | 0.791 |
Comparison of bolt rod mechanical properties under static and dynamic loading (part 2).
AM18 | 102.6 | 102.5 | 81.3 |
AM22 | 103.9 | 103.9 | 98.5 |
APP | 105.4 | 105.5 | 97.3 |
APB | 106.7 | 107.2 | 94.7 |
Under static loading, the calculated maximum stress ratio values
Under dynamic loading, the calculated maximum stress ratio values
Bolt rod tests under tensile impact loading with a maximum load rate of
The
Although the AM18 (A500S steel) and AM22 (A500sh steel) bolt rods are produced using similar technology that involves thermal hardening (with quenching), the A500S steel has lower content of carbon and other main alloying agents such as manganese (Mn) and silicon (Si), which influence the increase in strength and yield stress. Similarly with APP (AP600V steel) and APB (AP770 steel) bolts, which are also produced using the same technology (continuous casting and slow cooling), differences can be found in their chemical compositions. Compared to the AP600V, the AP770 steel has a comparable content of carbon, chromium (Cr) and molybdenum (Mo), and a higher content of manganese, nickel (Ni), silicon and vanadium (V – also improves the impact strength of steel), which influence the increase in strength and yield stress. Austenite forming elements such as Mn, Ni and cobalt (Co) (which does not appear in the chemical compositions of the steel of the tested bolt rods) have a beneficial influence on the structure of steel even in minor amounts. The fine-grained structure of primary austenite usually also serves to improve the mechanical properties and performance of steel. Increasing the concentration of an austenite forming element results in the occurrence of a semi-austenitic structure, consisting of a mixture of austenite and ferrite (Dobrzański, 1998). Fig. 14 presents example views of bolt rods after single shear testing under impact loading.
The
Analysing the tests that resulted in bolt rod shearing confirmed that the impulse
It is thus proposed that an additional assessment criterion of bolts intended for use under dynamic loading be adopted in the form of the shear work value
Due to the fact that most of the shear bolt rod tests’ results published in research studies are performed on rods installed in rock or concrete blocks, it is impossible to directly compare them with the test results presented in this article. These differences are connected with the fact that in the area of the bolt rods’ destruction installed in rock or concrete blocks, there is not only a shear zone, but also a zone of the rod tension and compression (Jalalifar et al., 2005). However, the static shear tests’ results presented in this article are comparable and converge with the results of direct shear tests only of the bolt rod presented by Jalalifar (2006) in his PhD thesis. The results above apply to both the shape of the charts
Thus far, when designing workings in Polish underground mines, the selection of bolts for the given geological and mining conditions was typically influenced by the value of the force and the stress occurring in the bolt rod. Analysis of recent accidents in workings with roof bolting in Polish copper mines indicates that greater attention needs to be paid to the energy dissipated by the bolts during tension and shearing under impact loading. Currently, copper mines are more focused on the impact strength value of the steel used to produce the bolt rods, which is a positive phenomenon serving to improve the work safety of the miners and the stability of the working support.
The characteristics
The mathematical formula of this function is as follows:
Function estimations are performed based on the measurement data originating from specific tests, with assumed amplitude of
Fig. 16 presents example courses of the
Fig. 17 presents example courses of the
The courses of ‘extreme’ functions obtained via computer estimation indicate their correct selection for the registered measurement data, which is confirmed by the high coefficient of determination
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
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 (
The mathematical relationships determined in the form of ‘extreme’ functions describing the actual
Further single shear tests of various types of bolt rods under impact loading are planned in the future.