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Failurepatternsofgeomaterialswithblock-in-matrixtexture:Experimentalandnumericalevaluation

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ORIGINAL PAPER

Failure patterns of geomaterials with block-in-matrix texture:experimental and numerical evaluation

Mohammad Afifipour & Parviz Moarefvand

Received: 1 October 2012 /Accepted: 28 February 2013# Saudi Society for Geosciences 2013

Abstract In nature, due to complex geological process,some geomaterials with block-in-matrix texture exist thatcharacterized by a heterogeneous structure including rockblocks embedded in a small-grained matrix. In literature,these materials also referred as bimrock (block-in-matrixrock). Typical examples of these complex materials areconglomerates, breccias, glacial till, coarse-grained alluvi-ums, and mélanges formations. When dealing with thesecomplex media in engineering, it is important to understandthe process by which the geomaterials fail under commonloading conditions. In this paper, artificial bimrock speci-mens were prepared for different percentage of rock blockproportions. The failure mechanism under uniaxial com-pression and indirect tension loading were studied. In orderto compare the failure modes with homogeneous specimens,a numerical simulation of laboratory tests including uniaxialcompression and Brazilian test was conducted on a typicalbimrock and a homogenous specimen. The results showeddifferent features of failure pattern for bimrocks with highproportions of rock blocks in comparison with homoge-neous specimens. According to the experimental results,three main features were observed including a continuoustortuous failure surface, multiple localized shear failure sur-faces, and detachment of rock blocks from the periphery ofspecimens.

Keywords Bimrocks . Failure pattern . Experimental study .

Numerical simulation

Introduction

In nature, there are a great number of geomaterials (rock orsoil) with a texture of stiff rock blocks surrounded byweaker soil-like matrix. In the literature, these heteroge-neous rock mixtures are commonly referred as block-in-matrix rock (bimrock) (Medley 1994), soil–rock mixture(Xu et al. 2008), or stiff rock–soil mixture (Afifipour andMoarefvand 2012). As a general term, these formations canbe characterized as “geomaterials with block-in-matrix tex-ture.” In this paper, the “bimrock” term is used to describesuch formations. The most widespread bimrocks are con-glomerates, breccias in sedimentary rocks, agglomerates andpyroclastics in igneous rocks, tectonic mélanges and flyschin metamorphic rocks and in artificial forms, mine wastedumps and tailing dam materials (Medley 1994; Afifipourand Moarefvand 2012). The most popular bimrocks are insedimentary rock category including coarse-grain alluviumsand colluviums, conglomerates, and breccias. Figure 1 illus-trates some typical samples and outcrops of bimrocks.

Due to a wide distribution of bimrocks in nature, designand construction of engineering structures on and in suchformations are inevitable. When dealing with bimrocks inengineering, it is essential to understand the failure processunder different loading conditions, in order that safe struc-tures can be constructed. Therefore, knowledge of failureprocess and related mechanism is an important requirementin the analysis of a wide range of rock mechanic problemssuch as tunnel design and slope stability and other relatedapplications such as earthquake prediction in these challeng-ing materials.

In literature, many laboratory tests, analytical studies, andnumerical modeling have been carried out to examine thefailure pattern of rocks, as homogeneous materials, undercommon loading conditions. Though the failure mechanismof rocks under compression and indirect tensile loading has

M. Afifipour : P. Moarefvand (*)Department of Mining, Metallurgy and Petroleum Engineering,Amirkabir University of Technology, Hafez 424,Tehran 15875-4413, Irane-mail: parvizz@aut.ac.ir

M. Afifipoure-mail: afifipour@aut.ac.ir

Arab J GeosciDOI 10.1007/s12517-013-0907-4

been studied for decades, the details of failure mechanism ofheterogeneous rocks such as bimrocks are not fully under-stood and still remain the subject of considerable scientificinterest. Moreover, for the gematerials with block-in-matrixtexture of higher rock block proportion like conglomeratesand breccias more researches due to lack of enough knowl-edge are required.

In recent years, many researchers have studied the mechan-ical properties of bimrocks from different points of view.Lindquist (1994) and Sonmez et al. (2006a, b) studied theeffects of important factors on the mechanical behavior ofbimrocks through performing tests on artificial laboratoryspecimens. Their model bimrocks were made of rock blockproportions up to around 70 %. About developing equationsin terms of mechanical properties and effective factors,Sonmez et al. (2009) proposed an empirical approach usinga series of statistical regression analyses on the results oflaboratory strength tests on artificial bimrocks. Moreover,numerical models, as an economically feasible way, are usedto model the fracture process of rock under different loadingconditions. Pan et al. (2008) carried out a series of numericalsimulation on virtual compression tests in order to assess theeffects of important factors on compressive strength of block-in matrix colluviums. Xu et al. (2008), using the technique ofdigital image processing based on finite element method,showed some aspects of shear behavior of soil–rock mixtures.Barbero et al. (2008) conducted 3D finite element simulationof compression tests on bimrocks in order to understandproperly the mechanical behavior of bimrocks with respectto different block volumetric proportions.

Despite the aforementioned researches on bimrocks’ me-chanical properties, there are few relevant studies in theliterature considering specifically failure patterns ofbimrocks. Lindquist (1994), through conducting triaxialtests on artificial bimrocks, addressed the dominant failurepattern of samples. The results showed that failure surfacesof bimrocks generally pass around blocks. Medley (2004)from a study of failed physical model mélanges summarizedthe characteristics of failure surfaces and their dependenceon volumetric block proportions (up to around 70 %) andproposed a preliminary guideline for estimating the thick-ness of potential failure zones in bimrocks. Kahraman andco-workers carried out a series of uniaxial laboratory testson a special fault breccia with clasts weaker than the matrix(i.e., different from common bimrocks with stiffer blocksembedded in a weaker matrix). Their experimental resultsshowed that three failure patterns, including failure planesthrough blocks, matrix, and matrix–block interfaces, wereobserved in the samples having different volumetric blockproportions (Kahraman and Alber 2006; Kahraman et al.2008). Delenne et al. (2009) investigated the strength andfailure properties of a model cemented granular materialsunder simple compressive deformation. They suggested sev-eral regimes of crack propagation depending on the matrixvolume fraction and effective particle-matrix adhesion.

The former studies revealed that, rock block proportionand strength contrast of constituents in bimrocks are themajor factors for the estimation of their overall mechanicalproperties and failure mechanism. Although recent re-searches were successful in providing an understanding of

Fig. 1 Some samples and outcrops of typical bimrocks

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mechanical properties of bimrocks, more researches areneeded in order to understand the dominant failure patternsof bimrocks, especially with high content of rock blocks, incompression and tension loading. In the recent researchesconcerning bimrocks, the artificial bimrocks were fabricatedwith respect to the rock block proportion up to around 70 %.

To properly interpret the failure mechanism and patternsof resultant failure surfaces in different loading conditions,in this paper model bimrocks with rock block proportionsmore than 70 % were fabricated. In more details, modelbimrocks were fabricated for three high content of rockblocks including around 70, 80, and 90 %. Uniaxial com-pression test on two sizes of cylindrical specimens andBrazilian test on one size of disk specimen were conductedto consider the dominant failure pattern of model bimrocksin common loading conditions. In order to compare quali-tatively failure mechanism of model bimrocks with homo-geneous samples, two-dimensional (2D) numericalsimulations of mentioned laboratory tests were performed.

In the following, we first introduce the experimentalprocedure including sample preparation and tests protocol.After that, the experimental results will be analyzed anddiscussed in detail. The numerical models are briefly intro-duced and followed by the presentation of numerical resultsof two typical specimens in two loading conditions. Finally,we conclude with a discussion and remarks about potentialperspective of this work.

Sample preparation and experimental tests

The main objective of these experiments is to investigate thefailure mechanism and corresponding failure surfaces ofmodel bimrocks. In this research, two cylindrical specimensof sizes 150×300 and 100×200-mm were fabricated inorder to consider one of the aspects of scale effect, namelysize effect, in failure mechanism of bimrocks. In addition, toobserve the failure in tension, one size of disk specimen wasprepared for Brazilian test. The prepared test specimen wasstandard Brazilian disk with a diameter of 100 mm andthickness of 50 mm (a diameter, thickness ratio of 0.5).

Artificial bimrocks were prepared by mixing of rockparticles and a cementing agent; in this research, Portlandcement was used. For rock particles, river aggregates werechosen. The particles have a density of 2.53 gr/cm3 withsmooth surface and a good variability in spherity and almostnormally distributed sizes. The compressive strength of theparticles is around 45 Mpa according to the correlation bypoint load test, so that they can break under moderatecompressive loads.

To conduct the tests for different shape and size of molds,the particle sizes must be scaled down. For this purpose, twomajor points should be considered. Firstly, maximum particle

size according to minimum size of test mold should be deter-mined. Secondly, a proper gradation modeling techniqueshould be considered. In a laboratory specimen, the maximumparticle size (d) is determined according to the minimumdimension of specimen (D). For materials with broad gradationD/d=4 is proposed (Marsal 1973; ISRM 1983). The specimengradation can be selected by one of the four modeling tech-niques namely, scalping, parallel gradation, quadratic grain-size distribution, and replacement technique (Iannacchioneand Vallejo 2000; Asadzadeh and Soroush 2009).

In the current research, in order to have uniform particle sizedistribution for all specimens, parallel gradation method wasemployed for modeling the gradation curve. Consequently, therock particle sizes range approximately from 9 to 19 mm for150×300-mm cylindrical specimens, 7 to 12 for 100×200-mmcylindrical specimens and 5 to 10 mm for disk cylinder of size100×50-mm. The rock block sizes and size distribution curveused for each samples were depicted in Fig. 2.

For the matrix, Portland cement and water were mixed inthe ratio of 0.3(water/cement) to prepare mortar, which canbe filled easily into the mold while embedding the rockblocks. Three artificial bimrock compositions were preparedwith mixture ratio by weight of rock blocks range around70, 80, and 90 %. The rock particles were mixed withdesired amount of cement and water in a box at ambienttemperature and carefully stirred manually until a homoge-neous mixture was obtained. Then, the mixture was placedin the molds whose internal wall was covered by a greasyfilm, which had a weak adhesion with the cementing paste.The mixture was compacted layer by layer in order to obtaina dense packing. According to ASTM C192 (2007), a com-paction bar was provided for compacting the samples. Eachmould was filled in three approximately equal layers, andeach layer with 25 strokes was compacted. The specimenswere unmolded after 24 h. For curing the specimens, theywere kept for 28 days in a wet environment at a temperatureof around 25 °C and relative humidity of 95 %. Differentsets of cylindrical and disk specimens were fabricated, asshown in Fig. 3.

The compression test was performed according to ASTMC42(1995) which has several requirements regarding the shapeof the test cylinders used for the compression test. One of themain requirements is that, the ends of the specimens arerequired to be within 0.5° perpendicular to the axis, and mustbe plane. Because of the high proportion of rock blocks in themodel bimrocks, none of them met this requirement. There-fore, capping the ends of the specimens is required. Thecylinder specimens were capped at both ends with a gypsumcapping compound following ASTM C617 (1995).

The servo-hydraulic testing machine and indirect tensionloading frame used to perform uniaxial compression andBrazilian test are shown in Fig. 4. All the compression testswere carried out under displacement control at a rate of 0.

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05 mm/s. In each test, the loading was continued until fullfailure of the specimen occurs. In the following section, wepresent and discus our main experimental results by focus-ing on the governing mechanism and failure pattern undercompression and tension loading as function of rock blockproportions.

Failure pattern under compression

The most common and applicable direct strength test ofrocks is the uniaxial compression test. The failure mode of

the typical brittle and approximately homogeneous rocks inthe uniaxial compression testing is mainly axial splitting,that is, crack propagation from top to bottom and parallel tosub-parallel to the loading axis (Fairhurst and Cook 1996).To investigate the failure pattern of model bimrocks undercompression loading, a series of uniaxial compression testshave been carried out in this study. The failure modes of100×200 and 150×300-mm cylindrical model bimrockswere shown in Figs. 5 and 6, respectively.

According to the Figures, as a general term, instead of asharp splitting failure surface like in homogeneous brittlerocks, the specimens experienced one or more tortuous failuresurfaces. Furthermore, increasing rock block proportionschanged the general configuration of resultant failure surfacesin the specimens. During testing, crack propagation started atthe interfaces of blocks and matrix, and the cracks grewthrough the matrix. Coarse rock blocks arrested crack growth,producing meandering and branching of cracks. The averageuniaxial compressive strength of specimens was around 12, 6,and 2 Mpa for the specimens of around 70, 80, and 90 % ofblock proportions, respectively. Further experimental work isunderway in order to check the effects of rock block propor-tions and cement types on mechanical properties of bimrocks.

Figure 5 corresponds to the failure modes of 100×200-mmcylindrical model bimrocks for different rock block propor-tions. For the specimen with around 70 % of rock blockproportion, a continuous tortuous failure surface was observed(Fig. 5a). In this specimen rather than the other ones, theindividual resulting continuous failure surface was more sim-ilar to the general axial splitting but with tortuous configura-tion. For the specimens with higher block proportions (morethan 70 %) instead of one unique tortuous failure surface, weobserved multiple failure surfaces especially some shear sur-faces in the circumference of specimens (Fig. 5b, c).

Fig. 3 Fabricated model bimrocks with three different rock blockproportions in different sizes and shapes, a cylindrical specimens forcompression testing in two sizes, b disk specimens for Brizilian test inone size

Fig. 2 Rock particles’ sizedistribution prepared forartificial bimrocks withcorresponding sieve sizes

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In 80 % block proportion specimen, multiple tortuousfailure surfaces were occurred. The relative positions of suchfailure surfaces were dependent on the positions of coarserrock block in the specimens. In addition, occurrence of shearfailure surfaces observed in the periphery of the specimencould be the result of lower confinements of these parts withrespect to the inner parts of specimens (Fig. 5b). The specimenwith 90 % of block proportion experienced a combination ofmultiple shear failure surfaces and localized detachment ofrock blocks in the periphery of specimen (Fig. 5c). The laterfeature, detachment of rock blocks in the periphery of speci-men, occurred in 90% and in some of 80 % block proportionsspecimens. This form of failure was due to the high void spaceand low grain-to-grain contact adhesion in rock blocks due tohigh rock block proportion in the mentioned specimens.

It was also evident during testing that the specimens withhigher block proportion (around 80 % and more) underwentmore lateral expansion (more dilation). In these specimens,the dominant failure modes were multiple localized shearsurfaces and in some specimens, rock blocks detachmentalong the circumference of specimens.

Figure 6 illustrated the failure modes of 150×300-mmcylindrical model bimrocks. According to the Figure, the

failure mode approximately was similar to the 100×200-mm specimens for different percentage of rock blockproportions.

Increasing rock block proportion in the specimenschanged a continuous tortuous failure surface (Fig. 6a) tomultiple shear failure surfaces and detachment of rock par-ticles along the periphery of specimens (Fig. 6b, c). Com-parison of Figs. 5 and 6 showed that dominant failurepatterns for different size of cylindrical specimens, consid-ering specimens with equivalent percentage of rock blockproportion in each size, were approximately similar to eachother.

The phenomenon of scale effect in the rock fractureprocess in experiments has greatly concerned many re-searchers in the last decades. Most researchers believed thatscale effects do exist in the failure process of rocks(Bieniawski 1968; Jirasek et al. 2004), although some havefound that there is no scale effect of rocks under uniaxialcompression (Hodgson and Cook 1970; Pan et al. 2009). Incurrent study, three main modes of failure were observed forthe cylindrical specimens under uniaxial compression test-ing including continuous tortuous failure surfaces, multiplelocalized shear failure surfaces, and detachment of rock

Fig. 4 Test equipmentcontaining model bimrocks, aServo hydraulic 8502 Instroninstrument for compressiontesting, b Brazilian testapparatus

Fig. 5 Failure modes of 100×200-mm cylindrical specimenswith different rock blockproportions by weight,including around a 70, b 80,and c 90 % of rock blockproportion

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blocks along the circumference of specimens for differentpercent of block proportions. For two sizes of cylindricalspecimens, approximately similar failure patterns were ob-served with respect to the equivalent percentage of rockblock proportions in each cylindrical specimen size.

Failure pattern under indirect tension

The Brazilian test is a common means of indirectly evalu-ating the tensile strength of materials. The test involvesplacing a cylindrical disk diametrically between the platensof a compression testing machine. According to the testconfiguration, a line load is applied to the diameter of thesample. The indirect tensile strength of a cylindrical sampleunder Pmax load with radius R and thickness t is obtainedusing the following relation (ISRM 1978, 2008):

σt ¼ 2P

pRt¼ 0:637

P

Rtð1Þ

In a homogenous material, Brazilian test leads to developa tensile crack propagating from the center of the disk

perpendicular to loading platens. A series of Brazilian disktests was carried out on model bimrocks including threedifferent proportions of rock blocks. The resulting failurepatterns of model bimrocks were depicted in Fig. 7.According to the Figure, increasing the block proportion inthe specimens changed “the central splitting fracture,” usualin homogeneous specimens, to a tortuous failure surface andlocalized shear failure surfaces adjacent to the loadingpoints. During testing, cracks were initiated mostly alongthe weaker rock particle–matrix interface that were adjacentto the coarser rock particle, and then propagate within thematrix itself following a path to reach to the next particleinterface. The average tensile strength of model bimrockswas around 1.4, 0.5, and 0.15 Mpa for the specimens ofaround 70, 80, and 90 % of block proportions, respectively.

In the specimen with 70 % of block proportion, only acontinuous tortuous failure surface was observed (Fig. 7a).This specimen, beside a tortuous failure in matrix, experi-enced rock particles breakage in the line of failure. In suchgranular medium, the interaction forces are transferredthrough the contact between particles. Whenever the bond

Fig. 6 Failure modes of 150×300-mm cylindrical specimenswith different rock blockproportions by weight,including a 70, b 80, and c90 %

Fig. 7 Failure modes of disk specimens with different rock block proportions by weight, including a 70, b 80, and c 90 %

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strength between the particles is higher than the strength ofone particle, breakage may occur under loading conditions(Buyukozturk and Hearing 1998; Delenne et al. 2009). Inthis specimen, due to higher matrix content with regard tothe other ones, particle breakage was observed. In otherspecimens, due to lower bond strength between particles,breakage did not occur.

For the 80 % block proportion specimen, the tortuosity offailure surface was increased and localized shear failure anddetachment of rock particles adjacent to one of loadingpoints was formed. Occurrence of tortuosity in failure sur-face of specimens could be related to the low contactstrength of bimrocks and non-uniform distribution of load-ing due to the relative position of rock blocks and high voidspace between them.

The specimen with the highest rock block proportion(approximately 90 %) showed both the tortuous failuresurface and localized shear failure and detachment of rockparticles adjacent to both points of loading. Due to the lowercontent of matrix in the specimen, detachment of rockparticles increased considerably and the failure surface di-vided into multiple branches.

In summary, for the model bimrocks with three differentrock block proportions under indirect tension loading, par-ticular failure patterns were observed. Variety of rock blockproportions in the specimens and coarser rock particle rela-tive positions controlled the tortuosity of failure surface.Occurrences of shear failure adjacent to the loading pointsand detachment of rock particles for higher block propor-tions specimens were dominated. The experiments provideuseful knowledge that highlights convincingly the respec-tive roles of rock block proportion in the failure mechanismof bimrocks with high contents of rock blocks. Furtherexperimental work is underway in order to identify therelationship between failure mode and correspondingstrength properties.

Numerical modeling

In this section, to compare the main differences of the failurepattern between homogeneous and heterogeneous rock(such as bimrocks), numerical simulation was utilized. Itshould be noted that the goal of these 2D numerical simu-lations is not to fit experimental data. We are mainly inter-ested in the influence of heterogeneity caused by rock block

proportion on failure pattern and the differences of failurepattern between typical homogeneous and heterogeneousspecimens.

Numerical models and materials parameters

The finite element code Plaxis was used for this purpose.Finite element is a capable method in simulation of hetero-geneous materials with complex nonlinear behavior. In thefollowing sections, the simulation of uniaxial compressionand Brazilian tests were described, respectively. For eachtest, a set of two specimens was simulated. The heteroge-neous model contains random distribution of stiff rockblocks embedded in a weaker soil–like matrix. In otherwords, the numerical model of a typical bimrock was sim-ulated. The homogenous specimen has been simulated usingthe material properties equal to the average properties ofweaker matrix and the interior stiff rock blocks. Both mate-rials were described according to Mohr-Coulomb elasticperfectly plastic constitutive model. The same values ofthe mechanical parameters used by Xu et al. (2008) havebeen used for the current numerical calculation, and weresummarized in Table 1. The bottom of the specimens hasbeen considered as fixed, while the upper boundary in eachtest was constrained to move according to each test loadingcondition.

Fifteen-node triangular element was used to mesh themodels. A fine mesh with a size range of 3–8 mm has beenused for each specimen. With respect to the finite elementbased software, all numerical tests at a certain value of theimposed displacement or load did not converge anymore, andthe solution procedure stopped. The state of the specimen atthe last converged step has been considered as representativeof ultimate deformation conditions and failed specimen. Thecorresponding failure surfaces at each test have been consid-ered in the subsequent comparison analysis as the governingfailure mechanism. In the following sections, simulation ofeach test has been described respectively.

Uniaxial compression test

To observe the full configuration of failure mechanics, uni-axial compression test has been modeled in 2D plain strainconditions rather than axisymmetric condition. Of course,axisymmetric condition is more convenient to incorporatewith the real 3D condition of testing, but in this paper, the

Table 1 Physical andmechanical parameters of thecomponents of model bimrocksand homogenous specimen (Xuet al. 2008)

Materials and properties ρ (kN/m3) E (MPa) υ− C [MPa] φ deg.

Rock blocks 24 1,040 0.2 1.2 39

Matrix (cemented soil) 19 60 0.3 0.1 26

Homogenous specimen 22 560 0.25 0.65 33

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concentration was on the ultimate failure mechanism oftests. The specimen had a width (diameter) of 150 mm anda height to diameter ratio of about 2. The material propertiesof constituents have been assigned as listed in Table 1.

In this model for simulation of uniaxial compressionloading, a constant vertical displacement (up to 15 mmequivalent to an axial strain of 5 %) was applied directlyon the specimen without any platens to prohibit stress

Fig. 8 Dominat failure patternof a homogeneous specimen,b model bimrock specimenunder uniaxial compressionincluding shear strain shadedcontours and principledirection of strain

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concentration. A mesh size of approximately 8 mm wasassumed for the specimen. The governing failure surfaceswith regard to the distributed shear strain (shading andprinciple direction mode) in two numerical specimens underuniaxial compression were shown in Fig. 8. According tothe Figure, the well-known “hour-glass” failure mode oc-curred in the homogenous specimen (Fig. 8a). The resultsobtained were in general agreement with experimentallyobserved phenomenon (Tang et al. 2000).

However, for the heterogeneous model bimrock speci-men, a different response and failure mechanism was devel-oped. In the heterogeneous model bimrock specimen, thefailure planes were forced to negotiate around blocks tortu-ously owing to the different physical and mechanical

properties of specimen constituents (stiffer rock blocks andweaker soil-like matrix; Fig. 8b). Micro-cracks aroundblocks coalesced and created continuous tortuous failuresurfaces. Accordingly, multiple failure surfaces were devel-oped rather than a sharp and individual shear failure surface.Existence of rock blocks and their relative positions alteredsignificantly the stress distribution in the specimen(Afifipour and Moarefvand 2012).

Brazilian tests

In the numerical simulation of the Brazilian test in thispaper, the specimen had a radius of 100 mm. The materialproperties assigned to rock blocks and matrix were listed in

Fig. 9 Dominant failure pattern of a homogeneous and b heterogeneous bimrock specimens under indirect tension loading in Brazilian test

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Table 1. For this model, a mesh size of 3 mm was assumed.In order to model numerically such a test, the bottom pointof the circular specimen was fixed with respect to translationin both directions. A stiff platen was considered on top ofspecimen to use as loading agent. A prescribed load wasapplied on the mentioned loading platen. The platen wasallowed to move only in vertical direction.

The results of two simulations, on homogenous andtypical bimrock specimens, in terms of failure mechanismwere shown in Fig. 9. According to this Figure, in homoge-neous specimen, a straight failure surface in tension wasdeveloped. In bimrock specimen, the tensile cracks andrelated failure surface has a non-straight path that is becauseof existence of stiff rock blocks and developed aroundblocks in multiple directions.

Discussion

As a general rule for different loading conditions inbimrocks, the failure surfaces configuration depend on rockblock proportion, the property contrast of rock blocks tomatrix, and rock blocks’ geometrical properties (Lindquist1994; Medley 1994; Ke 1995; Sonmez et al. 2009; Coli etal. 2012; Afifipour and Moarefvand 2012). According tothese effective factors, three special features can be consid-ered for failure paths. The schematic descriptions of threepossible failure modes in bimrock specimen are depicted inFig. 10 (from Xu et al. 2008).

Mode one Deviation of failure path around rock blocks.This kind of failure occurs where the matrix is weaker andsofter than interior rock blocks and where the blocks haveenough relative space from each other (i.e., floating condi-tion). Typical examples of this condition are matrix-

supported conglomerates, non-cemented coarse-grain allu-viums and colluviums, mélanges (Medley 1994; Lindquist1994), and andesite-tuff agglomerate (Sonmez et al. 2004).

Mode two Branching or widening the failure surface. Thiscondition may be observed where the embedded rock blocksare stiffer than matrix, but the relative position of rockblocks is small and rock block is very close to each other(i.e., grain-to-grain contact). A typical example of this con-dition is clast-supported conglomerate (a conglomerate withhigh proportion of rock blocks) and the model bimrocks ofthis research.

Mode three Passing through rock blocks and matrix. Thiscase corresponds to the conditions where the property con-trast of rock blocks to matrix is very low (i.e., no significantdifferences between geomechanical properties of bimrockconstituents). Typical examples of this condition are calcite-cemented alluviums and colluviums, calcite-cemented con-glomerates, and fault breccias (Kahraman and Alber 2008).

As discussed above, dominant failure mode of the modelbimrocks of this research can be attributed to “mode two”from the aforementioned category. In more detail, accordingto the results, for the specimens with high rock block pro-portions (above 80 %), due to the porous texture of speci-mens, localized shear failure and rock block detachmentwere observed. Dominant failure mode in porous soft rockis associated in most cases with the phenomenon of locali-zation (Besuelle et al. 2000).

In the model bimrocks with lower rock block proportions(around 70 %) “mode one” were also observed for some ofthe specimens. This mode of failure was also reported forsimilar natural bimrocks. Sonmez et al. (2004, 2006) ob-served a similar failure mode (propagation around blocks)for some of Ankara Agglomerate samples. Kahraman andAlber (2006) also observed a similar result for Misis faultbreccias (Turkey) when the strength of matrix was weakwith respect to rock blocks. They also observed “modethree” (i.e., passing through blocks) for the samples inwhich the strength of matrix was high with respect to theblocks.

However, natural bimrocks due to inherent heteroge-neous texture may have a mix mode of failure from theaforementioned category. In addition, the failure mode inbimrocks could be affected by other parameters such as rockblocks shape, orientation and size distribution, relative po-sitions of larger blocks, and strength contrast of rock blocksto matrix. In this paper, we focus on the effect of the mainparameter, rock block proportion as reported by severalresearchers, especially for high values. Thus, other men-tioned parameters were considered similar in the fabricationof specimens. Aforementioned modes of failure of bimrocks

Fig. 10 Three possible failure propagation models of bimrocks (fromXu et al. 2008)

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could be further subdivided considering more effectiveparameters.

Conclusion

In this paper, physical model bimrocks made up of rockblocks embedded in a small-grain matrix were tested inuniaxial compression and indirect tension to study the effectof high block proportions on failure mechanism and resul-tant failure surfaces configuration. The results of these testsshowed that rock block proportion plays an important role inthe failure modes of model bimrock specimens and controlsthe configuration of fracture surfaces in common loadingconditions. Coarser rock blocks control the orientation anddegree of tortuosity of resultant failure in bimrock undercompression and indirect tension loading.

Three main modes of failure were observed for the cy-lindrical specimens under uniaxial compression testing in-cluding continuous tortuous failure surfaces, multiplelocalized shear failure surfaces, and detachment of rockblocks along the circumference of specimens for differentpercent of block proportions. Under indirect tension load-ing, model bimrocks experienced particular failure patternsincluding shear failure adjacent to the loading points anddetachment of rock particles for higher block propor-tions specimens and tortuous splitting surface. The nu-merical approach proves to be a useful tool to improvethe knowledge of failure mechanics and its dominantpattern in mentioned media, but it needs to be extendedto three dimensions for a direct comparison with theexperiments.

The current experimental study and numerical modelingprovide useful knowledge that highlights convincingly therespective roles of rock block proportion in the failuremechanism of bimrocks with high contents of rock blocks.Further experimental work is underway in order to identifythe relationship between failure mode and correspondingstrength properties of bimrocks.

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