Rockburst induced ground motion—a comparative study
Zbigniew Zembaty *
standby什么意思Faculty of Civil Engineering,Technical University of Opole,ul.Mikolajczyka 5,45-233Opole,Poland
Accepted 9October 2003
Abstract
While rockbursts from underground copper mining in Western Poland normally produce surface peak ground accelerations (PGA)and velocities of 0.05–0.1g and 1–3cm/s,occasionally the peak motions may exceed 0.15g and 10cm/s,respectively.The larger motions are of considerable concern and an investigation has been undertaken to define the nature of the larger induced ground motions.This paper compares the rockburst motions with low intensity earthquakes.Various strong motion parameters such as PGA,peak ground velocity (PGV)and displacements as well as strong motion duration,Arias intensity,Fourier and respon spectra are compared with tho from earthquakes.It is concluded that although short duration is the most obvious parameter that differentiates rockbursts from earthquakes,in fact their high dominant frequencies,which result in high PGA/PGV ratios differenti
ate them the most.Two types of rockburst-induced ground motions are indicated in this paper:typical—with 3–6months return period and characteristic,high frequency content—as well as rare events similar to shallow,low intensity earthquakes.q 2003Elvier Ltd.All rights rerved.
Keywords:Mines;Rockbursts;Ground motion;Respon spectra
1.Introduction
Nowadays structural vibrations induced by ground motion can be caud not only by earthquakes but also by the activity of man.On the one hand,traffic vibrations and industrial explosions may be regulated to control the potential for cosmetic cracking (Dowding [3]).On the other hand,rockbursts and rervoir-induced ground quakes are random events with respect to their time,magnitude and place although this randomness is not as obvious as for earthquakes.The event from March 13th 1989in Germany near Merker ðM L ¼5:4Þ;which caud injuries to three people and substantial damage to buildings proved that the problems should be treated riously by civil engineers.Rockbursts belong to a broader category of ground failures caud by human activities.The ground failures together with typical chemical explosions are subject of detailed studies to differentiate them from underground nuclear explosions.The rearch in this field is spons故事书的英文
ored by various governmental and international organizations for the benefits of proper implementation of the Nuclear Test Ban Treaty [1].This rearch is concentrated at teleismic (.2000km)or regional (.1000km)epicentral distances whereas the surface
effects of the mine-induced ground quakes can usually be obrved only at near field distances of less than 10–20km around mines.
consolidationWith respect to surface intensity,rockbursts should be classified right after underground nuclear explosions,but before surface mine explosions and construction blasts.The latter two phenomena were studied extensively by Duvall and Fogelson [17],Siskind et al.[2]as well as by Dowding [3].It was concluded that surface particle velocity was the parameter directly correlated with the amount of damage obrved for buildings located in the vicinity of blast activity (coal surface mines,quarries).It was also noted that the spectral content of blast-induced ground vibrations was shifted towards higher frequencies as compared to earth-quakes or nuclear explosions.
Rockbursts occur when accumulated stress fracture intact rock,usually ahead of an advancing mine face (Fig.1).Johnston [10]gives more preci definition describing six various mechanisms of rockbursts.She also divided rock-bursts into two general categories:Type I—directly correlated with
mining activity with low to medium magnitude and type II—only looly correlated with mining activity and with focus in wider area around the mine,but with potentially higher magnitude.From civil engineering point of view,particular attention should be paid to the events of type II which occur randomly,with return period of a few months to a few years.
0267-7261/$-e front matter q 2003Elvier Ltd.All rights rerved.
doi:10.1016/j.soildyn.2003.10.001
Soil Dynamics and Earthquake Engineering 24(2004)11–23
/locate/soildyn
*Fax:þ48-77565-84.
E-mail address:zet@po.opole.pl (Z.Zembaty).
Mine-induced rockbursts can occur in many countries. When mines are located in the vicinity of cities,rockbursts may affect life of ordinary people and at the extreme may cau some damages to buildings or equipment.Rockbursts are usually carefully monitored by special geophysical rvices of mines,which are primarily interested in the safety of the crews working below the surface of the earth, so networks of measuring devices are installed below the ground and at lected places on the surface.The problems are extensively studied from ismological point of view[4,5].From the results of ismological rearch, one may e that almost every mine has its own specifics, generating particular type of ground motion,depending on the type of ore,technology and local geology.The local differences are usually much more pronounced compared to regional differences of earthquakes.In spite of substantial ismological rearch,the studies often do not meet direct civil engineering needs as they are concentrated on physics of the events and on statisti
cal models of their occurrences rather than on spectral and peak ground parameters[4].
The purpo of this paper is to prent results of an analysis of recorded ground motions during rockbursts taking place in the vicinity of a copper mine from Western Poland.Classic ismic strong motion parameters like peak ground accelerations(PGA),velocity(PGV),displacements (PGD),strong motion duration as well as Fourier and respon spectra of the mine tremors are calculated and compared with the same parameters from earthquakes. The main aim of this study is to better asss the intensities of mine ismic events and investigate the differences and similarities of rockbursts and earthquakes.Such comparison is thefirst step to adapt methods of ismic engineering to mitigate rockbursts effects on buildings.
2.Ground motion parameters
For the purpo of this study,the ground motion parameters can be divided into four categories
†descriptive intensity parameters(local Mercalli intensi-ties),
广元在线
†energy measures of ground motion records like Arias[6] intensity,
†measures of peak ground motion(acceleration—PGA, velocity—PGV and displacements—PGD),
†spectral parameters(Fourier and respon spectra),
†measures of strong motion duration.
The local Mercalli intensities applied here are Modified Mercalli(MM),Medvedev,Sponhauer Karnik(MSK-64) and Mercalli,Cancani,Sieberg(MCS).Thefirst two of them(MM and MSK-64)can be treated as almost
identical
for civil engineering purpos[7].The third(MCS)differs more,though not very substantially[8].
The Arias intensity,as applied in this study,is a direct measure of energy transmitted through the ground surface
I A¼ðt k
a2ðtÞd tð1Þ
nate ruesswhere aðtÞis the acceleration record with total duration t k:Using this definition and the notion of so called Husid plot
HðtÞ¼ðt
a2ðtÞd t
ðt k
a2ðtÞd t
ð2Þ
the definition of strong motion duration t d as the time for the Husid plot to stay between5and95%can be formulated.This definition of strong motion duration has been propod by Trifunac and Brady[9].
To measure variations in spectral content of the accelerograms,one may need not only to e plots of Fourier and respon spectra,but also to formulate some quantitative parameters.The simplest parameter of this kind can be formulated as mean,or central frequency calculated here in Hz as follows
f centr¼
1
2p
ð1
v SðvÞd v
ð1
SðvÞd v
ð3Þ
where SðvÞis the power spectral density(Fourier spectrum) of the analyzed record.The integral in the numerator is a spectral moment.So the result of Eq.(3)can be interpreted as value of abscissa of center of gravity of thefigure described by the plot of spectral density SðvÞ:
Except for descriptive Mercalli intensities,all the other ground motion parameters are parately defined for each of three Cartesian axes x;y;z:To apply the parameters independently of the instrument orientation,they should be reformulated parately for one horizontal and the vertical direction.The latter one remains the same as there is only one vertical axis,whereas from two horizontal records along x and y axes,one measure should be derived.From civil engineering point of view,the role of horizontal and vertical excitations is different,as buildings differ with respect to their horizontal and vertical properties.In particular, vertical building stiffness is usually much grater than the horizontal one.
全国英语四级成绩查询The peak ground motion parameters(PGA,PGV and PGD)are formulated as vertical and horizontal,with the latter ones being just surface maxim:max
ffiffiffiffiffiffiffiffiffi
x2þy2
p
:
Fig.2.Time history measurements of a rockburst from February2nd2001(station‘3Maja’,g03),horizontal x;y and vertical z:
Z.Zembaty/Soil Dynamics and Earthquake Engineering24(2004)11–2313
The horizontal and vertical Arias intensities can be formulated as follows:
I hor A¼1
2
ðt k
½a2xðtÞþa2yðtÞ d tð4aÞ
I ver A¼ðt k
goldilocksa2zðtÞd tð4bÞ
The horizontal and vertical strong motion durations are formulated from appropriately modified Husid plots
H horðtÞ¼ðt
½a2xðtÞþa2yðtÞ d t
ðt k
hbsc½a2xðtÞþa2yðtÞ d t
ð5aÞ
H verðtÞ¼ðt
a2zðtÞd t
ðt k
a2zðtÞd t
ð5bÞ
Analogously,the horizontal and vertical central frequencies
may be defined as follows
f hor
tinycentr
ðvÞ¼
1
2p
ð1
½v S xðvÞþv S yðvÞ d v
ð1
½S xðvÞþS yðvÞ d v
ð6aÞ
f ver centrðvÞ¼
1
2p
ð1
v S zðvÞd v
ð1
S zðvÞd v
ð6bÞ
Both horizontal Arias intensity and horizontal central
frequency can be viewed upon as averaged values of the
quantities derived from respective values measured on x and
y
服装样衣
axes.
Fig.3.Fourier spectra of a rockburst from February2nd2001(station‘3Maja’,g03),horizontal x;y and vertical z:
Z.Zembaty/Soil Dynamics and Earthquake Engineering24(2004)11–23
14
3.Comparative study
The copper mine‘Rudna’is situated clo to a small town Polkowice(about20,000inhabitants).It is a part of a copper ‘LGOM’basin consisting of also a few other,smaller mines. Typical buildings of the Polkowice are small,residential masonry buildings and prefabricated reinforced concrete residential buildings of6and11stories with natural periods of0.5–0.9s.The mine operates at depths of600–1000m practically below the whole town and neighboring villages.For this reason,the surface network of measur-ing devices is located within the radius of about6–8km inside and around the city.The instruments are installed at the foundations of some buildings as well as on the ground.Most of the instruments measure three components of accelera-tions.Some devices measure also velocity.
The measure-ments are triggered by events with PGA exceeding10cm/s2. Stronger mine tremors occur with return period longer than 3–6months causing usually only minor damage,mostly to non-structural elements of the buildings.The local magni-tudeðM LÞof the strongest rockbursts reach4–4.5for Lubin–Polkowice region.It is rather high value as compared with other mining regions of the world(Table2in Ref.[10]).The largest rockbursts in the region belong to type hey are strong and occur randomly.The geologic conditions of the mine and its neighborhood favor accumulation and sudden releas of energy in the calcium–dolomite and anhydrite rocks overlying the exploited deposit[11]and do not cau greater static ground deformations met in many coal mine regions.On the other hand,the surface layers of soil are not particularly susceptible to amplification phenomena as the shear wave velocity in the upper soil layer equals about400–600m/s[12].For each detected rockburst,the geophysical rvices calculate approximate energy relea and location of epicenter.Typical energies of strongest rockbursts equal for this mine106–109J.It should be noted,however,that the greatest surface PGA or PGV)are not directly correlated with the maximal energies.
The main purpo of the reported rearch was to asss the intensity and destructive capacities of the rockbursts as compared to earthquakes.The investigations were motivated by unusually high pe
ak ground accelera-tions as for rockbursts,often exceeding0.1g as well as controversies regarding the asssments of local intensi-ties of the measured events.The problems are important for the mine authorities when responding to numerous claims of local residents for damages to buildings or equipment.
Fig.2prents typical time history record of surface ground motion caud by a rockburst.As it can easily be en,the characteristic feature of the mine tremor is its very short duration.But this is not the only difference.Obrving the velocities and displacements reveals their small values as compared to earthquakes with similar values of PGA. Another difference can be en in Fig.3,which shows respective Fourier spectra of the accelerations from Fig.2.It can be en from thisfigure that the spectral content display clear shift of the dominating parts of the plots into higher frequencies as compared with the earthquakes.Fig.4 prents respective respon spectra which show in
turn Fig.4.Respon spectra of a rockburst from February2nd2001(station‘3 Maja’,g03),horizontal x;y and vertical z componentsðj¼0:05Þ:
Z.Zembaty/Soil Dynamics and Earthquake Engineering24(2004)11–2315
