Fragmentation characteristics analysis of sandstone fragments based on impact rockburst test

Dongqio Liu,Dejin Li,Fei Zho,b,Chengcho Wng,b

aState Key Laboratory for Geomechanics and Deep Underground Engineering,China University of Mining and Technology,Beijing 100083,China

bSchool of Mechanics and Civil Engineering,China University of Mining and Technology,Beijing 100083,China

Fragmentation characteristics analysis of sandstone fragments based on impact rockburst test

Dongqiao Liua,b,*,Dejian Lia,Fei Zhaoa,b,Chengchao Wanga,b

aState Key Laboratory for Geomechanics and Deep Underground Engineering,China University of Mining and Technology,Beijing 100083,China

bSchool of Mechanics and Civil Engineering,China University of Mining and Technology,Beijing 100083,China

A R T I C L E I N F O

Article history:

Received 2 April 2014

Received in revised form

10 April 2014

Accepted 23 April 2014

Available online 9 May 2014

Impact rockburst test

Impact rockburst test on sandstone samples with a central hole is carried out under true triaxial static loads and vertical dynamic load conditions,and rock fragments after the test are collected.The fragments of sandstone generated from strain rockburst test and uniaxial compression test are also collected.The fragments are weighed and the length,width and thickness of each piece of fragments are measured respectively.The fragment quantities with coarse,medium,f i ne and micro grains in different size ranges, mass and particles distributions are also analyzed.Then,the fractal dimension of fragments is calculated by the methods of size-frequency,mass-frequency and length-to-thickness ratio-frequency.It is found that the crushing degree of impact rockburst fragments is higher,accompanied with blocky characteristics observably.The mass percentage of small grains,including f i ne and micro grains,in impact rockburst test is higher than those in strain rockburst test and uniaxial compression test.Energy dissipation from rockburst tests is more than that from uniaxial compression test,as the quantity of micro grains generated does.

?2014 Institute of Rock and Soil Mechanics,Chinese Academy of Sciences.Production and hosting by Elsevier B.V.All rights reserved.

1.Introduction

Under deep mining conditions,high in situ stresses,high water pressure,and high temperature combined with disturbance of mining engineering can lead to more and more engineering disasters(He et al.,2005;He and Qian,2010).Among them,rockburst,a sudden violent ejection of rock fragments and/or blocks from surrounding rocks,may bring greater threats to underground openings,equipments,and the safety of mining workers.

From the mechanical state of rockburst occurrence,rockburst can be basically divided into strain rockburst and impact rockburst. Strain rockburst is caused by excavation of deep rock mass under static action,and impact rockburst is triggered by combined static and dynamic actions of deep rock mass after excavation(He et al., 2012).

Understanding rockburst mechanism is a long-term goal for many scholars in the world.Many theories such as energy theory (Cook et al.,1966),stiffness theory(Cook,1965),fracture and damage theory(Pan and Xu,1999),dynamic disturbance theory (Wang and Huang,1998),and catastrophic theory(Tang,1993), were used to investigate the characteristics of rockburst.Furthermore,for the fragments generated in rockbursts,some scholars proposed that the fragmentation characteristics and the broken extent offragments can re f l ect the mechanismofrockburst.He etal. (2009)classi f i ed fragments of strain rockburst into four categories, i.e.coarse,medium,f i ne and micro grains,and corresponding methods were also proposed.Li et al.(2009)suggested that the crushing degree of granite fragments of strain rockburst is higher with obvious slab characteristics.Miao(2009)found that the fractal dimension of strain rockburst fragments is larger than that of uniaxialand true triaxial compression.Nie(2011)found that sandstone fragments of strain rockburst are mostly in rectangle shape.However,papers on impact rockburst have been rarely reported,especially the ones on its fragments.It is noted that analyzing the fragmentation characteristics of impact rockburst will contribute to understanding the energy dissipation characteristics.

This paper analyzes the fragmentation characteristics of the sandstone fragments from impact rockburst test in State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology(Beijing).Besides,the comparison test is also made on the sandstone fragments from strain rockburst test and uniaxial compression test.

Fig.1.The experimental system of impact rockburst(ESIR).

2.Impact rockburst test

The impact rockburst test system can independently provide static loads in three directions,one dynamic load in one direction, or multiple loads in multiple directions at the same time.So the impact rockbursts induced by dynamic disturbance,such as blasting,roof collapse and fault slip,can be simulated in the laboratory. The test system is shown in Fig.1,and the test model is shown in Fig.2.

The system consists of main stand,servo-controller,hydraulic power and image acquisition system,as shown in Fig.1a.It has 16 basic waveform signals(see Table 1),including ramp wave,sine wave,triangle wave,square wave and so on.The amplitude range of these disturbance waves is 0-1 mm in displacement control way and the frequency range is 0-1 Hz.

The specimen dimensions are 110 mm×110 mm×110 mm and the diameter of the hole is 50 mm.The loading process is illustrated as follows:f i rstly,the static stresses were applied on the specimen to simulate the in situ stresses state as shown in Fig.1b;secondly, the disturbance wave was loaded in one,two or three directions and the burst phenomenon could be observed.In this paper,the load with the disturbance wave was only applied inσ1-direction,as shown in Fig.3.The burst phenomenon observed is shown in Fig.4 with a lot of grains or fragments ejecting out from the free face.

3.Fragmentation characteristics

3.1.Classif ication and mass distribution of fragments

Firstly,the fragments of sandstone from impact rockburst test were screened by sieves with diameters of 0.075 mm,0.25 mm, 0.5 mm,1 mm,2 mm,5 mm and 10 mm,respectively.Secondly,the mass of fragments passing through different sieves was measured.And then the length,width and thickness of fragments passing through sieves with diameters more than 5 mm were measured by a vernier caliper.In order to unify the measuring standard,the largest value was chosen as the length,and the least value as the thickness.

Table 1Different impact waveforms for simulating rockburst induced by blasting,roof collapse or fault slip.

Fig.2.Impact rockburst test model by dynamic cyclic loading.

According to the fragments classi f i cation of rockburst test proposed by He et al.(2009),the fragments after impact rockburst test can be divided into four groups:coarse grain(＞30 mm),medium grain(5-30 mm),f i ne grain(0.075-5 mm),and micro grain (＜0.075 mm).Fragments classi f i cation photo is shown in Fig.5.

The results of fragments classi f i cation show that 50.02%of total fragments mass(2.57 g)is of medium grain,47.51%is in f i ne grain (2.44 g),and 2.47%of micro grain(0.13 g).Note that there are no fragments with coarse grains.

In order to better understand fragments characteristics,a comparison of sandstone fragments from impact rockburst test, strain rockburst test and uniaxial compression test was made.In strain rockburst test,one surface of the specimen was unloaded suddenly from a true triaxial stress state.That could simulate the strain rockburst induced by excavation.The conventional uniaxial compression test was conducted to obtain rock strength.The mass of different grain groups in 3 tests was counted up and its percentage was calculated.The results are shown in Table 2 and the distribution of fragments mass is shown in Fig.6.

From Table 2 and Fig.6,it can be found that the mass of fragments from impact rockburst test is only 5.14 g which is 5.10%and 2.29%of the mass from strain rockburst test and uniaxial compression test,respectively.And the mass percentage of fragments with small grains including f i ne and micro grains is 49.98%, 7.25%and 2.36%for impact rockburst test,strain rockburst test and uniaxial compression test,respectively.

Fig.3.Loading path of impact rockburst test.

Fig.4.Phenomenon of sandstone impact rockburst.

Table 2Mass and percentage of different grain groups.

3.2.Size characteristics of fragments

To study the size characteristics of sandstone fragments,the length,width and thickness of fragments passing through sieves with diameter greater than 5 mm were measured.And then the length-to-width ratio,length-to-thickness ratio and width-tothickness ratio were calculated.The ratios distributions are presented in Fig.7,and their ranges and average values are shown in Table 3.

According to the size classi f i cation of fragments proposed by Li et al.(2009),fragments from impact rockburst test can be divided into 4 groups:blocky fragment whose length-to-width ratio is less than 3,platy fragment whose length-to-width ratio is 3-6,lamellate fragment whose length-to-width ratio is 6-9,and lamellar fragment whose length-to-width ratio is greater than 9.From Fig.7, it can be found that all the fragments obtained from impact rockburst test are blocky and platy ones,and the representative value of length:width:thickness is 3.6:1.8:1.0.The lamellate characteristics of fragments from strain rockburst test are obvious,and therepresentative value of length:width:thickness is 6.6:3.9:1.0. However,the fragments from uniaxial compression test are mostly blocky and platy ones whose mass percentage is 82.35%,and the representative value of length:width:thickness is 4.6:2.1:1.0.

Fig.5.Classi f i cation of fragments from impact rockburst test.

Fig.6.Distribution of fragments mass.

Table 3Size distribution of fragments.

Fig.7.Size ratios distributions of fragments.

Fig.8.Logarithmic relationships betweenLeqmax/LeqandNs/Ns0.

4.Fractal dimension of fragments

Any complex morphology,such as fracture surface of materials, could be quantitatively described by fractal theory.In this paper,we selected the size-frequency,mass-frequency and length-tothickness ratio-frequency to analyze the fractal characteristics of fragments.

4.1.Size-frequency

For the measurable medium and coarse grains,the fragments were assumed as cuboid and their volume can be calculated using the measured length,width and thickness.Then the fragments were converted into cubes with the equivalent side length.For the fi ne and micro grains which are dif fi cult to be measured,we screened them and assumed the sieving diameter as the equivalent side length.Then,we counted up the grains’number by sampling statistical methods and therefore the fractal dimension can be calculated according to the following formula:

whereNsis the number of fragments whose equivalent side length is greater thanLeq,Ns0is the number of fragments whose equivalent side length isLeqmax,andDis the fractal dimension.The logarithmic relationship betweenLeqmax/LeqandNs/Ns0is shown in Fig.8.The absolute value of slope of the linear f i tting curve in Fig.8 is the fractal dimension(Shan and Li,2003).

Fig.9.Logarithmic relationships betweenMmax/MandNm/Nm0.

Fig.10.Logarithmic relationships betweenrmax/randNr/Nr0.

Table 4Fractal dimension of fragments.

4.2.Mass-frequency

By the statistical method,the relationship betweenM/MmaxandNm/Nm0is presented as follows:

whereNmis the number of fragments whose mass is equal to or greater thanM,Nm0is the number of fragments whose mass isMmax,andbis the distribution exponent of mass-frequency.The logarithmic relationship betweenM/MmaxandNm/Nm0is shown in Fig.9 and the absolute value of slope of the linear f i tting curve is the distribution exponent of mass-frequency.

Note that the correlation between the massMand the sizeLof fragments isM∝L3.Therefore,the relationship between the distribution exponent of mass-frequency and the fractal dimension isD=3b.

4.3.Length-to-thickness ratio-frequency

Length-to-thickness ratio of fragments can be calculated by the length,width and thickness measured.The relationship betweenr/rmaxandNr/Nr0is presented as follows:

whereris the length-to-thickness ratio,rmaxis the maximum one,Nris the number of fragments whose ratio is equal to or greater thanrandNr0 is the number of fragments whose ratio isrmax.The logarithmic relationship betweenrmax/randNr/Nr0is shown in Fig.10 and the absolute value of slope of the linear f i tting curve is the fractal dimension.

4.4.Calculation results

The fractal dimension of fragments from impact rockburst test, strain rockburst test and uniaxial compression test were calculated by the above-mentioned methods,and are shown in Table 4.

Table 4 shows that the fractal dimension of fragments obtained from strain rockburst test is the largest whereas that from uniaxial compression test is the least.The larger the fractal dimension is,the greater the consumed energy is.So the energy consumption for rock failure inthe strainrockbursttestis the highest,while itis the leastin the uniaxial compression test,suggesting that the rockburst occurrence consumes more energy.During strain rockburst test,the high true triaxial stress state makes elastic energy accumulated in rock specimen,which is high enough for rock failure under uniaxial compression before unloading.Once one surface of the specimen is unloaded suddenly,the energy is released from the surface very quickly,the crushing degree of fragments is extremely high and the specimen is out-of-shape.In impact rockburst test,the true triaxial stress state also leads to accumulation of elastic energy in rock specimen,while it is less than that in strain rockbursttest.Due to the dynamic stress,large energy is released and accompanied with lots of grains ejection from the hole surface where high stress concentration occurred,resulting in high level crushing of fragments.

5.Conclusions

In this paper,the size and mass of fragments from impact rockburst test,strain rockburst test and uniaxial compression test were measured.Based on these results,the fractal dimension of fragments was calculated.The fragmentation characteristics can be summarized as follows:

(1)From the results of size characteristic analysis,it can be concluded that blocky characteristics of fragments from impact rockburst test is more obvious than that from strain rockburst test and uniaxial compression test.

(2)The distribution of fragment mass shows that the mass proportion of small grains including f i ne and micro grains from impact rockburst test is greater than that from strain rockburst test and uniaxial compression test.

(3)From the fractal dimension results,it can be obtained that the energy consumption at rock failure of impact rockburst test is between those of strain rockburst test and uniaxial compression test.

Con f l ict of interest

The authors wish to con f i rm that there are no known con f l icts of interest associated with this publication and there has been no signi f i cant f i nancial support for this work that could have in f l uenced its outcome.

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*Corresponding author.Tel.:+86 10 51734047.

E-mail address:liudongqiao@yeah.net(D.Liu).

Peer review under responsibility of Institute of Rock and Soil Mechanics,Chinese Academy of Sciences.

Fragments

Fragmentation characteristics

Fractal dimension