• Geomechanics and Engineering
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• v.20 no.1
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• pp.29-41
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• 2020

It is extremely important to obtain rock strength parameters for geological engineering. In this paper, the evolution of sandstone cohesion and internal friction angle with plastic shear strain was obtained by simulating the cyclic loading and unloading tests under different confining pressures using Particle Flow Code software. By which and combined with the micro-crack propagation process, the mesoscopic mechanism of parameter evolution was studied. The results show that with the increase of plastic shear strain, the sandstone cohesion decreases first and then tends to be stable, while the internal friction angle increases first, then decreases, and finally maintains unchanged. The evolution of sandstone shear strength parameters is closely related to the whole process of crack formation, propagation and coalescence. When the internal micro-cracks are less and distributed randomly and dispersedly, and the rock shear strength parameters (cohesion, internal friction angle) are considered to have not been fully mobilized. As the directional development of the internal micro-fractures as well as the gradual formation of macroscopic shear plane, the rock cohesion reduces continuously and the internal friction angle is in the rise stage. As the formation of the macroscopic shear plane, both the rock cohesion and internal friction angle continuously decrease to a certain residual level.

• Journal of the Korean GEO-environmental Society
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• v.20 no.5
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• pp.13-21
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• 2019

There are many cuts or natural rock slopes that remain stable for a long time in the natural environment with steep slopes ($65^{\circ}$ to $85^{\circ}$). In terms of design practice, the rock mass consisting of similar rock condition and geological structures is defined as a good continuum rock slope, and during the process of decision making angle of this rock slope, it will be important to establish the geotechnical properties estimating method of the continuum rock on the process of stability analysis in the early stages of design and construction. In this study, the stability analysis of a good continuum rock slope that can be designed as a steep slope proposed a practical method of estimating the shear strength by induced from the Hoek-Brown failure criterion, and in addition, the design applicability was evaluated through the stability analysis of steep rock slope. The existing method of estimating the shear strength was inadequate for practical use in the design, as the equivalent M-C shear strength corresponding to the H-B envelope changes sensitively, even with small variations in confining stress. To compensate for this problem, it was proposed to estimate equivalent M-C shear strength by iso-angle division method. To verify the design applicability of the iso-angle division method, the results of the safety factor and the displacement according to the change in angle of the cut slope constructed at the existing working design site were reviewed. The safety factor is FS=16~59 on the 1:0.5 slope, FS=12~52 on the 1:0.3 slope, most of which show a 10~12 percent reduction. Displacement is 0.126 to 0.975 mm on the 1:0.5 slope, 0.152 to 1.158 mm on the 1:0.3 slope, and represents an increase of 10 to 15%. This is a slightly change in normal proportion and is in good condition in terms of stability. In terms practical the working design, it was confirmed that applying the shear strength estimated by Iso-angle division method derived from the H-B failure criterion as a universal shear strength for a good continuum rock mass slope was also able to produce stable and economic results. The procedure for stability analysis using LEM (Limit Equilibrium Analysis Method) and FEM (Finite Element Analysis Method) will also be practical in the rock slope where is not distributed fault. The study was conducted by selecting the slope of study area as a good rock condition, establishing a verification for which it can be applied universal to a various rock conditions will be a research subject later on.

• Journal of Korean Tunnelling and Underground Space Association
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• v.15 no.4
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• pp.443-453
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• 2013

This study aims to evaluate the mechanical behaviors of unsupported rock pillars in a room-and-pillar underground structure by a series of numerical analyses. In addition, rock pillar strengths estimated by a few empirical equations proposed for underground mines are compared with those from numerical analyses. Based on the results from the numerical analysis, the ratio of pillar strength to rock mass strength increases as the ratio of the width of a pillar to its height becomes bigger. It means that higher ratio of pillar width to its height is much more favorable for stabilizing a room-and-pillar underground structure. Especially, unsupported pillar strengths estimated from numerical analyses are higher than rock mass strength when the ratio of pillar width to height is approximately over 1.5. It is also found that the choice of an empirical equation appropriate for a given geometric condition of a pillar is important for its feasible application to the stability analysis of a pillar in the room-and-pillar method.

• Tunnel and Underground Space
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• v.17 no.2 s.67
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• pp.119-127
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• 2007

Based on the critical plane approach, a methodology far predicting the anisotropic strength ot transversely isotropic rock is Proposed. It is assumed that the rock failure is governed by Hoek-Brown failure criterion. In order to establish an anisotropic failure function, Mohr envelope equivalent to the original Hoek-Brown criterion is used and the strength parameters m, s are expressed as scalar functions of orientation. The conjugate gradient method, which is one of the robust optimization techniques, is applied to the failure function for searching the orientation giving the maximum value of the anisotropic function. While most of the existing anisotropic strength models can be applied only when the stress condition is the same as that of conventional triaxial compression test, the proposed model can be applied to the general 3-dimensional stress conditions. Through the simulation of triaxial compression tests for transversely isotropic rock sample, the validity of the proposed method is investigated by comparing the predicted triaxial strengths and inclinations of failure plane.

• Geomechanics and Engineering
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• v.17 no.1
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• pp.57-67
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• 2019

Water-induced strength reduction is one of the most critical causes for rock deformation and failure. Understanding the effects of water on the strength, toughness and deformability of rocks are of a great importance in rock fracture mechanics and design of structures in rock. However, only a few studies have been conducted to understand the effects of water on fracture properties such as fracture toughness, crack propagation velocity, consumed energy, and microstructural damage. Thus, in this study, we focused on the understanding of how microscale damages induced by water saturation affect mesoscale mechanical and fracture properties compared with oven dried specimens along three notch orientations-divider, arrester, and short transverse. The mechanical properties of calcite-cemented sandstone were examined using standard uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests. In addition, fracture properties such as fracture toughness, consumed energy and crack propagation velocity were examined with cracked chevron notched Brazilian disk (CCNBD) tests. Digital Image Correlation (DIC), a non-contact optical measurement technique, was used for both strain and crack propagation velocity measurements along the bedding plane orientations. Finally, environmental scanning electron microscope (ESEM) was employed to investigate the microstructural damages produced in calcite-cemented sandstone specimens before and after CCNBD tests. As results, both mechanical and fracture properties reduced significantly when specimens were saturated. The effects of water on fracture properties (fracture toughness and consumed energy) were predominant in divider specimens when compared with arrester and short transverse specimens. Whereas crack propagation velocity was faster in short transverse and slower in arrester, and intermediate in divider specimens. Based on ESEM data, water in the calcite-cemented sandstone induced microstructural damages (microcracks and voids) and increased the strength disparity between cement/matrix and rock forming mineral grains, which in turn reduced the crack propagation resistance of the rock, leading to lower both consumed energy and fracture toughness ($K_{IC}$).

• Tunnel and Underground Space
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• v.6 no.2
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• pp.131-143
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• 1996

In-situ survey and laboratory rock test were carried out for rating rock mass around the tunnel that some failures had been occurred in Danyang limestone quarry. For rating rock mass, several methods such as RMR, Q-system, rock strength etc. were applied. The stability analysis on tunnel was evaluated by numerical method FLAC. And The block theory using streographic projection was also applied for stability analysis. The 3-4 major discontinuity sets are distributed in rock mass around tunnel.

• Proceedings of the Korean Geotechical Society Conference
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• 2003.03a
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• pp.431-438
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• 2003

The domestic design method for the shaft resistance of drilled shafts into a bedrock Is based on the empirical method, where the uniaxial compressive strength of rock specimen is utilized for calculation of the shaft resistance. This method has uncertainties in prediction of capacity of drilled shafts and result in uneconomic engineering design. Recently a new improved design method was suggested, which reflects important factors that affect the strength of pile sockets. Socket roughness is one of significant factors influencing the shaft resistance of drilled shaft socketed into rock In this paper roughness information for the shaft resistance design of socket pile was suggested on the basis of statistical analysis of data measured from wall surface In the bore holes of drilled shafts.

• Advances in materials Research
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• v.5 no.3
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• pp.171-179
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• 2016

Fusion and characterization of bisphenol-A diglycidyl ether based thermosetting polymer mortars containing an epoxy resin, Fly ash and Rock sand are presented here for the Experimental study. The specimens have been prepared by means of an innovative process, in mild conditions, of commercial epoxy resin, Fly ash and Rock sand based paste. In this way, thermosetting based hybrid mortars characterized by a different content of normalized Fly ash and Rock sand by a homogeneous dispersion of the resin have been obtained. Once hardened, these new composite materials show improved compressive strength and toughness in respect to both the Fly ash and the Rock sand pastes since the Resin provides a more cohesive microstructure, with a reduced amount of micro cracks. The micro structural characterization allows pointing out the presence of an Interfacial Transition Zone similar to that observed in cement based mortars. A correlation between micro-structural features and mechanical properties of the mortar has also been studied.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 2003.03a
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• pp.173-180
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• 2003

• Proceedings of the Korean Geotechical Society Conference
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• 2003.03a
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• pp.861-866
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• 2003

While coarse geomaterials with abundant fine particles are common, comparatively little information is available to know their engineering behaviour. In this study, the effects of fine particle content of coarse geomaterials on engineering properties, such as shear strength, deformability and permeability were investigated. It was known through large triaxial compression tests that when they are compared with good rock materials, the rock materials with abundant fine particles have different compaction characteristics, low shear strength, low stiffness, and low permeability.