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![Situation of the tested (no.1-12) and complementary (no.13-18) localities in the map of the regionalgeological division of the Czech Republic 1:500 000 [www.geology.cz].](profile/Ondrej-Holy/publication/328163408/figure/fig1/AS:679765790953472@1539080202285/Situation-of-the-tested-no1-12-and-complementary-no13-18-localities-in-the-map-of.jpg)
Situation of the tested (no.1-12) and complementary (no.13-18) localities in the map of the regionalgeological division of the Czech Republic 1:500 000 [www.geology.cz].
Source publication
The anchor length d b of rock bolts is often determined empirically by the insertion of the bond friction constant τ b at the grout-rock interface. The relationship between force F b by limit bond stress and bond length (or bond area) is their ratio. Within the same location, the anchor length can be overestimated or underestimated by usage τ b = c...
Citations
... In case of a very low rock mass strength, rock bolts anchorage performances are notably decreased [28][29][30]. Additionally, rock masses are disturbed while drilling processes because of low slake durability indexes of the poor rock materials. ...
Effect of the rock material strength on the RMR value and tunnel support designs was investigated within this study including site works, analytical and numerical analyses. It was found that rock material strength effect is quite limited in the RMR method to determine an accurate rock mass class to design tunnel support. Since the limitation, rock mass classes are evaluated to be usually misleading and supports designed in accordance with the RMR value are insufficient for tunnels excavated in rock masses with low strength values of rock materials. Totally, five different tunnels in Turkey have been supported using a new strength adjustment factor calculated in consideration of the in-situ stress and the uniaxial compressive strength values of rock materials. As confirmed by the field applications, analytical and numerical analyses, a newly modified RMR value (RMRus) was suggested to use in tunnel support design works.
... Since the increase in bolt length causes also increases in the stresses induced in the bolt steel material, the steel failure becomes possible instead of the sliding at the friction surface. The increase in the bolt length sometimes has not a positive effect on the load capacity, because the strength of the bolt steel material can make limitations in case of using high bolt lengths [29][30][31][32]. The maximum bolt length was 5 m in this study. ...
Change in the load bearing capacity of the split set type friction rock bolts with variations of bolt lengths was investigated within this study. To determine a relation between the load bearing capacity and bolt length parameters, different friction bolt models with various lengths were analysed with a numerical modelling study. In addition, a series of pull-out tests was carried out to evaluate the load bearing capacities of the split set type friction rock bolts with different lengths. The load bearing capacity of the bolts was found to decreasingly increase with the increase in the bolt length. As an outcome of this study, a relation between the load bearing capacity and rock bolt length parameters is suggested in accordance with the results obtained from both numerical and experimental studies.
... Results (32 MPa) obtained from the simulated study found to be close to these empirical pillar strength formulae. The same rock mass properties are used for simulation of Panel V. Properties of different elements of reinforcement and other parameters reported by Holy (2018) were used in CSM mine were considered during the simulation. Grout stiffness, K g and cohesive strength, C g are determined (FLAC3D, 2012) using equations 7 and 8 respectively. ...
Rhomboid shaped coal pillars (35 m x 30 m to 26 m x16 m) were formed by a modified Room and Pillar method below 850 m depth from surface at the CSM mine in the Czech Republic. The pillars were developed in a shaft protective pillar by driving roadways of 3.5-4.5 m in height and 5.2 m in width within Panel V of Seam No. 30. Development of pillars at such great depth is prone to spalling/fracturing (pillar rib dilation) due to redistribution of the high stress regime. The induced stress driven dilation was measured during partial extraction of the coal seam within the shaft protective pillar using rib extensometers. In order to stabilize the pillar ribs, four rows of rock bolts with 2.4 m length were installed into the pillar from all sides at different heights. The immediate roof was also supported by rock bolts at a 1 m grid pattern. Three-way intersections were made to control the deformation of developed pillars and other underground structures. Further, an attempt was made to understand the rock bolt loading characteristics at different stages of rib dilation using numerical modelling with the available properties of rock mass and reinforcement for the studied site. Elastic and Mohr Coulomb strain-softening constitutive models are considered in FLAC3D to evaluate the performance of the rock bolts. Results obtained on numerical models were found to be in good tune with the rock bolt loading characteristics monitored during the field study. This paper presents a discussion about the impact of rib bolting on pillar safety factor and induced load on rock bolt with respect to the dilation/spalling of pillar ribs at the studied site.
