Dov Bahat

Mechanism of exfoliation joint formation in granitic rocks, Yosemite National Park

Abstract Fractographic techniques reveal mechanical aspects of exfoliation in granitic rocks at Yosemite National Park, and electronic surveying provides information on their attitudes and dimensions. In the middle elevations of the cliff of El Capitan, exfoliation consists of early fractures a few meters to tens of meters in size which are fan-shaped. Fans at upper elevations point upwards, whereas fans at lower elevations point downwards parallel to the cliff. The fans interact with each other and merge into composite joints hundreds of meters in size, normal to the minimum compressive principal stress. This polarity indicates exfoliation by longitudinal splitting and buckling. The palaeostress causing the exfoliation on the southwestern side of Half Dome is estimated to range between 0.01 MPa and 0.94 MPa. Differences in fracture paleostresses of joints may be used in comparing their relative propagation velocities. It is suggested that the large exfoliation on the southwestern side of Half Dome had undergone a prolonged sub-critical growth before attaining rapid fracture velocities under post-critical conditions.

Fracture area calculation from electromagnetic radiation and its use in chalk failure analysis

Fracture area calculation from electromagnetic radiation and its use in chalk failure analysis A. Rabinovitch*, V. Frid, D. Bahat, J. Goldbaum The Deichmann Rock Mechanics Laboratory of the Negev, Israel Physics Department, Ben Gurion University of the Negev, PO Box 653, 84105, Beer-Sheva, Israel Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, PO Box 653, 84105, Beer-Sheva, Israel

Theoretical considerations on mechanical parameters of joint surfaces based on studies on ceramics

Morphological features of brittle fracture in glass, ceramics and rocks include fracture origin, mirror plane, mist, hackle, radial striations and concentric undulations. The plume structure which characterizes many joint surfaces is analogous to striations or ‘grainy steps’ developed on fracture surfaces of ceramics. Joint surfaces with plumose structures result essentially from tensile failure, but may also be affected by shear stresses. The ellipticity of the plumose structure commonly observed reflects a region of secondary tensional strain parallel to bedding developed due to anisotropic elastic properties of the rock as a result of formational loading. Striations and hackles may have similar orientations. A distinction between them is important since hackles on a joint surface indicate a rapid fracture whereas striations can result from a slow fracture. Intense undulations without hackling may indicate a slow fracture. A smooth mirror plane on a joint surface indicates that while the fracture had been formed, stresses had reached a certain maximum and then subsided. On the other hand, a joint surface that shows a mirror plane with hackle around its boundaries indicates that no decrease of stresses occurred during fracture, and fracture was possibly produced under a constant stress. Since A H is a material property, the fracture stress σ f responsible for a rapid fracture may be calculated following a determination of the mirror constant A H of the rock by σ f r H ½ = A H

Parametrization of electromagnetic radiation pulses obtained by triaxial fracture of granite samples

We have accurately measured and parametrized individual pulses of electromagnetic radiation (EMR) obtained during a fracture experiment. Analysis of the parameters shows that they follow a log-normal distribution. Results indicate no dependence between fracture lengths and widths.

Experimental and theoretical investigations of electromagnetic radiation induced by rock fracture

Frid, V., Bahat, D., Goldbaum, J., Rabinovitch, A. 2000. Experimental and theoretical investigations of electromagnetic radiation induced by rock fracture. Isr. J. Earth Sci. 49: 9‐19. There is a general agreement in the literature that the technique of measuring electromagnetic radiation (EMR) emitted from cracked rock is a good candidate for forecasting of earthquakes. Our immediate objective in pursuing this goal is to correlate EMR with crack dimensions in micro-scales (mm–cm), coupling it with the understanding of atomic-scale phenomena for coherently understanding the EMR process. We review some of the results obtained in this laboratory. They include the isolation, both experimentally and theoretically, of an individual EMR pulse. Individual EMR pulse parameters are correlated with crack dimensions: the time from pulse origin up to its maximum is proportional to the crack length, and the frequency of the EMR pulse relates to the crack width. Individual EMR pulses are classified both according to their length and according to their location on the stress–strain curve. We find that the key elastic parameter for EMR characterization during triaxial compression is the Poisson ratio: the lower the Poisson ratio, the higher the EMR activity. Amplitudes of EMR and their changes with loading are shown to be independent of crack mode (tensile vs. shear), they are only dependent on the entire crack area. In order to experimentally overcome load limitations we introduce a new sample shape, the truncated cone, that fails more readily than standard cylindrical samples.

Jointing and fracture interactions in middle eocene chalks near beer sheva, Israel

Abstract This study presents some observations on jointing and fracture interaction in synclinal Middle Eocene chalks (Horsha formation) near Beer Sheva, Israel. Three joint sets occur in an outcrop; set 344° (N16°W) developed first, and was followed by sets 028° and 062°. Each set reflects a different fracture mechanism. Joint spacing and fracture length were investigated in set 028° but there did not appear to be any correlation between the two parameters. The mean spacings in two layers 30-cm- and 60-cm-thick range closely between 23 ± 16 cm and 26 ± 19 cm, suggesting independence of joint spacing from bed thickness in set 028°. Log-probability plots of joint lengths from these two layers suggest an approximate lognormal distribution. The mean joint lengths are 82.2 cm ± 56.8 cm and 64.1 cm ± 44.0 cm in the thicker and thinner beds, respectively. Statistically, the length populations in the two beds are considered to be significantly distinguishable, implying that the mean fracture lengths show some dependence on layer thickness. Joint sets that reflect different fracture mechanisms show differences in intensity and in style of fracture termination. Terminations between parallel joints of the same set are differentiated from interactions between joints of different sets at acute angles. In joints of set 028°, various crack terminations are classified into seven styles. There is on the average a visible interaction every 120 cm along joints of set 028°. Interactions between sets 0.28° and 344° induce secondary fractures, which are identifiable as three style groups. The most frequently induced secondary fractures are either subparallel or subperpendicular to the two sets. The sum length of the secondary cracks in an interaction zone does not seem to be akin to the lengths of the parent joints. Especially puzzling interactions between sets 028° and 344° are triple junctions and complex triple junctions defined by these fractures. Interactions between sets 028° and 0.62° result in rhomboid structures at acute angles ranging between 18° and 51°. Evidently, these structures are the product of extension, and no shear offsets are visible along the fractures. Characteristically, some of these rhomboids are being subdivided into smaller structures by the same sets.

Similarity and dissimilarity of electromagnetic radiation from carbonate rocks under compression, drilling and blasting

1. IntroductionNumerous investigations have examined differentaspectsofelectromagneticradiation(EMR)emittedbyfracture [1–6]. For example, it was noted that anincreaseofYoungmodulus,strength,andloadingrateenhancestheEMRamplitude[3,7–9].IndividualEMRpulseswerecarefullyinvestigatedunderuniaxialandtriaxialstiffcompression[8–11].AnexampleofanEMRpulsefromchalkcompressionisshowninFig.1.Itisknownnowthatapropagatingcrackconsistsofatomicbondseverage,whichexcitesatomic(orionic)oscilla-tions along the crack surfaces. These oscillations(Rayleigh-typewaves)induceelectromagneticradiation.Hence,eachcrackconstitutesasourceofanindividualEMRpulse.TheEMRamplitudeisafunctionofthecrackarea[12,13];thetimefromthepulseorigintothemaximumofitsenvelope,isproportionaltothecracklength;thefrequencyoftheEMRpulseisrelatedtothecrackwidth[12,13].AnanalysisoftheEMRpulsesemanating during uniaxial compression [14] showedthatindividualshortpulses(ofdurationof0.5–6ms,Table1)arecorrelatedwiththestageofindividualmicro-cracksformation;multi-pulsestringsarecorre-lated with the crack coalescence stage;andlengthypulses(ofdurationof30–400ms,Table1)arecorrelatedwithrockfailure.All these investigations were carried out in thelaboratory.Large-scaleEMRstudiespriortorockburstsinminesandtoearthquakes(EQ)arealsoregistered.Thus,Khatiashvilli[15]carriedoutaninvestigationofEMRintheTkibullideepshaft(Georgia)priortoanEQofa5.4magnitude.Theregistrationpoint(attheshaft position) was located 250km from the EQepicenter.PriortotheEQitself,anincreaseofintensityofthelowerpartofthespectrum(1–100kHz)andacorrespondingdecreaseofintensityofhigherfrequen-cies(100–1000kHz)wereobserved.NesbittandAustin[16]registeredEMRinagoldmine(2.5kmdepth).Frid[17,18]observedEMRanomaliesbeforerockburstsandgasoutbursts.Itisclaimed[19]thatanabnormallyhigh-EMRleveloccurshoursorevendaysbeforeanEQ,afterwhichEMRdecreases.Rikitake[20],analyzing60EQeventsmeasuredinJapan,alsoshowedthatEMRisa‘‘short-term’’precursor,withanestimatedmeantimepriortoan EQ of B6h. It was assumed [21–25] that theanomaliesofEMRpriortoanEQwereduetoadeformationoftheEarthsurface,whichresultedintheformation of micro-fractures and in friction of thenearbyrockblocks.Parrotetal.[26],afteradetailedconsiderationofalargenumberofEMR–EQinvestigations,remarkedthatalthoughtheexistenceofEMRinrelationtoseismicand/orvolcanicactivitieswereclear,EMRselectionoutofahostofartificialsignalsremainedasignificantproblem. Nevertheless, investigations of EMR as aprecursortolarge-scalefailurecontinue.In this work, we present the results of ourEMR investigations on carbonate rock fractureboth in the large scale (blasting in an openquarry) and in the micro-scale (drilling in the lab)and show the similarities and dissimilarities oftheseEMRresultstoourpreviousEMRlaboratorystudies during regular compression tests in thelaboratory.

Exploration via electromagnetic radiation and fractographic methods of fracture properties induced by compression in glass-ceramic

The fracture properties of glass ceramic induced by compression were investigated by combined electromagnetic radiation (EMR) and fractographic methods. The study of a transparent sample enabled us to elucidate the sequence of crack nucleation, growth and interaction, and the ultimate longitudinal splitting under incremental increase of uniaxial stress in five stages. The fracture process was accompanied by some 18 EMR pulses. The short EMR pulses (of a duration of 0. 8–1.5 μs) occur under low stresses (0.36–1.7 MPa) in association with microcracking at the sample outer surface. Medium pulses (durations of 15–25 μs) are associated with stresses of up to 65 MPa and are correlated with crack limited growth outside the specimen. A lengthy pulse (duration of more than 40 μs) occurs under greater stresses (112 MPa) and correlates with the longitudinal splitting at failure. A return to the 17–20 μs range occurs for the post-failure cracking during stress relaxation.

SINGLE-LAYER BURIAL JOINTS VS SINGLE-LAYER UPLIFT JOINTS IN EOCENE CHALK FROM THE BEER SHEVA SYNCLINE IN ISRAEL

Abstract The single-layer (s.l.) joints that occur in the Lower Eocene chalks near Beer Sheva, Israel, developed during the burial history of the rock, whereas the s.l. joints in adjacent Middle Eocene chalks developed during the uplift stage. Characteristically, s.l. burial joints occur in orthogonal cross-fold and strike sets, and as conjugate sets. They precede normal faults and multi-layer joints, and they do not exhibit strike rotation, en echelon segmentation or fracture interaction with each other. These joints are generally closed, and during subsidence older beds fracture first. On the other hand, s.l. uplift joints do not occur in orthogonal or conjugate sets. They are post strike-slip faulting, contemporaneous with multi-layer joints, and exhibit strike rotation, en echelon segmentation and often interact with each other. They are occasionally opened up to several millimetres, and during uplift younger beds fracture first.

Comparison of electromagnetic radiation and acoustic emission in granite fracturing

The electromagnetic radiations and acoustic emision from two granite samples under compressive loading are studied in experiences. The two samples exhibit respectively plastic and brittle character. Differences in observed signals for these two samples are considered to be related to their different compositions.