Question

Discuss the method of obtaining joint samples of rock. Draw the necessary sketch.

Answer 1

The joint roughness coefficient (JRC) of rock joints has the characteristic of scale effect. JRC measured on small-size exposed rock joints should be evaluated by JRC scale effect in order to obtain the JRC of actual-scale rock joints, since field rock joints are hardly fully exposed or well saved. Based on the validity analysis of JRC scale effect, concepts of rate of JRC scale effect and effective length of JRC scaleu effect were proposed. Then, a graphic method for determination of the effective length of JRC scale effect was established. Study results show that the JRC of actual-scale rock joints can be obtained through a fractal model of JRC scale effect according to the statistically measured results of the JRC of small-size partial exposed rock joints and by the selection of fractal dimension of JRC scale effect and the determination of effective length of JRC scale effect.

roughness coecient (JRC) is a vital parameter of theJRC-JCS model for estimation of shear strength of rock joints[–].eJRCwilldecreasewithanincreaseinsamplinglengthduetoJRCscaleeect[–].Fieldinvestigationsshowthatitisusuallyhardtomeasurethe JRC of actual-scale rock joints directly because theeld rock joints are hardly fully exposed or well saved.e estimation of the JRC of actual-scale rock joints bystatistically measured results of the small-scale JRC of partialexposed rock joints is critical to the reliability of the shearstrength of rock joints empirically estimated by the JRC-JCSmodel.Based on the analysis on quantities of rock joints throughjoint model test, Barton and Bandis [] proposed a modiedcurveofJRCscaleeect(seeFigure)andamodiedformuladened asJRC≈JRC00−0.02JRC0,()where JRCis the JRC value of rock joints with samplinglength and JRC0is the JRC value of standard-size joints(the sampling length is 0, i.e.,  cm).Du et al. [] have statistically measured the JRC valuesof  prole curves on SSE group joints from Xiaolangdibytheuseoftheprolographandroughnessrulerandgotthe regularity of the decrease characteristic of JRC with anincrease in sampling length (see Figure ).In this paper, a new method is introduced to studythe estimation of the JRC of actual-scale rock joints bystatistically measured results of the small-scale JRC of partialexposed rock joints through the analysis of JRC scale eectof rock joints. A fractal model of JRC scale eect wasestablished. e physical meaning of the fractal dimensionof JRC scale eect was dened. A method to determine theeective length of JRC scale eect was proposed based on thestudy of the eectiveness of JRC scale eect.2. Fractal Dimension of JRC Scale EffectAccording to () and the regularity of JRC scale eect inFigures  and , the fractal expression of JRC scale eect canbe obtained:JRC=JRC00− ,()where is the fractal dimension of JRC scale eect, whichdenes the velocity rate of JRCdecreases with an increase insampling length

Ln/L0JRCn/JRC0LnL0JRCnJRC050.91.00.30.40.50.60.70.824681010⩾15Appro z. JR C0F : Scale eect related to JRC0[].Statistically measured results show that the fractal dimen-sion of JRC scale eect is comparatively stable. ,calculatedfrom the fractal model of JRC scale eect according to thestatistically measured results of JRCof random samplinglength along the same direction of rock joints in the samewall rock, distributes over a stable interval (see Table ).From Table , it can be seen that the fractal dimension alongthe tonalite joint (1−1) trend direction ranges from .to ., with mean value of .; the range of fractaldimension along ∘direction is .–., and meanvalue is .; the range of fractal dimension along thestrike direction is .–., and mean value is .;0.40.50.60.70.80.91012345678910111234Ln/L0JRCn/JRC0F : JRC scale eect of SSE group joint in Xiaolangdi.(1) Calcareous packsand along joint strike direction (∘), (2)calcareous packsand along joint trend direction (∘), (3)silty clayrock along joint strike direction (∘), and (4) silty clay rock alongjoint trend direction (∘).the range of fractal dimension along ∘direction is .–., and mean value is .. is means that the fractaldimension of JRC scale eect (30)convertedbytheJRCmeasurement results of small-size joint is the same as thefractal dimension of JRC scale eect (100)convertedbythe JRC measurement results of large-size joint. 100 and