Blasting vibration criterion for a rock-anchored beam
in an underground powerhouse
Ming Chen a ,Wenbo Lu
a,*
,Changping Yi
b
a
State Key Laboratory of Water Resources and Hydropower Engineering Science,Wuhan University,Wuhan 430072,China
b
College of Resource and Engineering,Wuhan University of Technology,Wuhan 430070,China
Received 21September 2005;received in revised form 12January 2006
Available online 5June 2006
Abstract
The design approach for a rock-anchored beam (RAB)in an underground powerhouse is brie?y introduced,and the dynamic response characteristics of an RAB under blasting vibration are presented.It is found that the key to ensure the safety of RAB under blasting vibration is to prevent the split of the bonding interface between the RAB and a vertical rock wall,and to control the horizontal peak particle velocity (PPV).On the basis of the practices at Longtan hydropower station and other projects,and taking into account the comprehensive factors such as the structure characteristics of RAB,the age of concrete and the strength of bonding interface,the safe PPV for an RAB in a large underground powerhouse are thoroughly studied.It is concluded that a PPV of 7cm/s for an RAB with an age older than 28d would be of some safety.The safe PPV for young concrete proposed by GB6722-2003could be adopted as those for an RAB with an age no older than 28d,i.e.,a safe PPV of 3–7cm/s for an RAB at an age range from 3d to 7d,and 2–3cm/s for an RAB at age range from 0d to 3d.
ó2006Elsevier Ltd.All rights reserved.
Keywords:Underground powerhouse;Rock-anchored beam;Blasting vibration;Peak particle velocity
1.Introduction
Because of its advantages in the arrangement of pivotal buildings,the construction and the function of a powersta-tion,an underground powerhouse is widely used in modern hydroelectric engineering.And with the accretion of the scale of a hydropower station,more and more large under-ground powerhouses appear.In order to shorten the plant span and to speed up the construction,a rock-anchored beam (RAB)is widely employed as the structure to arrange tracks for travelling cranes in an underground powerhouse.The RAB is anchored on the rock wall through anchor bolt,all loads of the crane being transferred to the rock body through the anchor bolt and the frictional force of bonding interface between the beam and rock wall.Fig.1is a sketch map of an RAB of Longtan hydropower station.
When an RAB is used in an underground powerhouse,the traditional concrete pillars or walls are no longer required,and it can make full use of the loading capacity of sur-rounding rock mass,which not only reduces the under-ground powerhouse span by 2–4m,but is also helpful for the stability of rock wall,and to reduce the rock excava-tion.Furthermore,the crane could be installed in advance to facilitate the placing work of factory concrete and the installation of turbo-generators,thus speeding up the con-struction progress,and saving the investment.
The RAB was ?rst adopted in Norway in Skjomen and Kvilldal powerstation at the end of 1970s (Lien et al.,1990;Johansen,1984).Now it is a pivotal building and is widely applied in large underground powerhouses (Lien et al.,1990;Johansen,1984;Stokkebo and Tondevold,1978;Gill and Lawrenson,1986).Tens of underground hydropower stations with RABs have already come into operation for several years in Norway.It has been proved that an RAB is a kind of economical and safe structure.In 1986,Chinese
0886-7798/$-see front matter ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.tust.2006.03.005
*
Corresponding author.Tel./fax:+8602768773652.E-mail address:wblu@https://www.doczj.com/doc/82378521.html, (W.Lu).
https://www.doczj.com/doc/82378521.html,/locate/tust
Tunnelling and Underground Space Technology 22(2007)
69–79
Tunnelling and
Underground Space Technology
incorporating Trenchless Technology Research
engineers accepted Norway experts’suggestion,and successfully used RABs for the?rst time in the Lubuge underground hydropower station.In addition to Lubuge, RABs were applied in Guangzhou Pumped storage power-station,Daguangba,Dongfeng,Taipingyi,Tianhuangping, Jiangya,Dachaoshan,Xiaolangdi and other large scale underground hydropower stations(Wang and Solyma′r, 1997;Zhao and Zeng,2002).All of them perform safely and economically.Several large scale projects are under con-struction such as Three gorges project,Longtan and Pubu-gou.RAB will be employed as the runway for traveling crane.
During the excavation of lower layers of rock mass of the underground powerhouse,there is a potential danger of the RAB being damaged by the repeated vibrations induced by blasting,especially,after placing the concrete of RAB,though lots of experiences are accumulated during the construction of RABs.There are few reports in the lit-erature about the mechanics and numerical simulations of blasting vibration induced damage on RABs.It is essential to study the dynamic response through simulation,experi-mentation and analysis of the monitored peak particle velocity(PPV)during construction so as to determine the safety criterion for RABs under blasting vibration,and to propose a reasonable construction method and detailed blasting parameters.
2.Design approach for an RAB
The design approach of an RAB is to suppose the sec-tion of the beam and the obliquity of the anchor bolt?rst and then to calculate the internal force of the anchor bolt by rigid body limit equilibrium method.It is also necessary to check the RAB’s stability against sliding along a rock platform and the compression strength of a rock platform to see if it meets the challenge,if not,then adjust the sec-tions,the obliquity of the anchor bolt and rock platform (Liang,1999;Yu,1998;Hong,1996),and calculate again. During the design,di?erent beam section,obliquity of the anchor bolt and rock platform have been calculated, and the last step is to choose the best one among the di?er-ent projects.
During the design of the RAB,some hypotheses were considered(Liang,1999
):The concrete RABs were a rigid
body;No force exists in the interface between the wall and the RAB,and the beam turns around with the point O(shown in Fig.1),which is the intersection of the rock platform and the compressive anchor
bolt;The top line tensile anchor bolt just bears the tensile and the force was scaled with the arm of force of bolt to the intersection point O.If compressive anchor bolt does not bear the force,then just?x the beam.
In the design process of a RAB,dynamic loadings are not taken into account.The main loadings to RABs are ring pressure of crane P1,the deadweight of the beam G,hori-zontal braking force H,the tensile force of the anchor bolt and the friction force between the beam and the rock wall.
2.1.The design of an anchor bolt
The anchor bolt section is designed according to its force.When the RAB is at work,according to the rigid body limit equilibrium method,the RAB must be kept in stability,and must satisfy the equation:
F1L F
1
tF2L F
2
?
X
Me1TX
M?P1L P1tHL HtGL Ge2
T70M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–79
According to
hypothesis it has:
F1=F2?L F
1=L F
2
e3T
where F1,F2are the forces of the tensile anchor bolt;M is
the moment of forces;L F
1,L F
2
,L P
1
,L H,L G are the arms of
each force to the point O.
But in the process of an underground powerhouse exca-vation,the crane does not play a role.So the main loadings to RABs are blasting vibration,the anchor bolt force,the support force of the rock platform and the deadweight of the beam.
2.2.Safety coe?cient of an anchor bolt
The design anchor bolt force dividing the practical force of an anchor bolt is the safety coe?cient
K?A gáR g=Fe4THere F is the force of the anchor bolt,A g is the section area of the anchor bolt,and R g is the tensile strength of the anchor bolt.The minimum allowable values of K are2.0 and 2.5for usual and unusual loading combinations, respectively.
2.3.The obliquity of the rock platform
The RAB’s stability is controlled by the friction coe?-cient g of the beam and the rock platform interface,it requires g<1
g?S=N?tg ue5Twhere S is the tangential component of the resultant coun-terforce R to the rock platform and N is the normal com-ponent of the resultant counterforce R to the rock platform
u?90 àaàcosà1PàF sin b
R
e6T
where P is the resultant force of all external force;a is the obliquity of the rock platform;F is the resultant force of the top and down line tensile anchor bolt force.The forces are shown in Fig.2F?
P coseatuàcT
siseatbtuT
e7T
As rigid body limit equilibrium method is not expressed the main stress of the anchor bolt coming from the surround-ing rock stress is released after excavation,and it does not roundly take the bonding force and the frictional force lying in the interface between the RAB and the rock wall into account;therefore,the practical stress in the anchor bolt is much less than the results obtained from the calcu-lation of rigid body limit equilibrium method.So some designers suggest applying?nite element method and imi-tative models testing to design the RAB.On the one hand, all factors,such as the condition of surrounding rock,the geological structure,the initial ground stress and the cham-ber span can be taken into account,on the other hand,it can make certain the ring pressure per meter on the RAB and the lateral sti?ness which is beyond the capacity of ri-gid body limit equilibrium method(Zhang and Yu,2004).
3.The in?uence of blasting vibration on RABs
The e?ect of blasting vibration on RABs may result in reducing the anchor strength of an anchor bolt to an RAB,producing new cracks on RAB concrete,and decreasing the bonding strength of the interface between the RAB and rock wall.All these factors will endanger the bearing condition of an RAB.Especially,when the bonding interface is cracked,the anchor bolt will be exposed to the atmosphere.If so,the steel bar will easily be carbonized and rusted,and the anchor bolt strength will be reduced quickly,which will endanger the function of an RAB.
The anchor bolt stress is closely correlated with the state of the surrounding rock.According to Zhang and Yu’s research(2004),when the crane is not in function,the stress on the tensile anchor bolt of an RAB mainly comes from the surrounding rock stress released after rock excavation, and the stress has relation with geological structure of the surrounding rock,initial ground stress?eld,the scale of chamber,the deadweight of an RAB,and the rock excava-tion and support method.The research of the Yangtze River Scienti?c Research Institute also indicated(Zhang et al.,2003)that the variety of anchor bolt stress is mainly con?rmed by the unloading wave of surrounding rock con-straints released,the stress distribution after the release of initial stress,and the function of blasting vibration.Among them,the latter’s e?ect is the least.If only considering the e?ect of blasting vibration to the surrounding rock,when the PPV on the surface of the surrounding rock is less than 16cm/s,blasting vibration will not cause a great distur-bance to the surrounding rock;as a result,the anchor bolt stress varies a little.As to the design rule in China,design-ing safety factor K of an anchor bolt is more than2.0, checking safety factor K is more than2.5.So anchor bolt stress varied in a certain range would not negatively a?ect the anchor bolt strength to an RAB.If the intensity of
the Fig.2.Force sketch map of RAB.
M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–7971
blasting vibration is too much to make the surrounding rock layers move,then inhomogeneous deformations occur,which may result in RAB fracture.
The concrete and rock cementatory body may be con-sidered as laminated compound material.The bonding of concrete and rock mainly depends on van der Waals force and mechanical interlocking force.Bonding interface is composition face,which contains many detrimental ele-ments in bonding.Take the rock mass as a large coarse aggregate,an interfacial transition zone appears between the concrete and the rock;it is just as the interfacial tran-sition zone in concrete between the matrix and the coarse aggregate.In the zone,all the e?ects–the higher ratio of water–cement,the more pores and cracks,the weaker van der Waals force,the bigger crystal style(Kumar Mehta, 1986;Liao et al.,2004),the volume contraction of concrete, the shrinkage crack in bonding interface produced by the restriction of bedrock,the concrete thermal stress,etc.–make the bonding interface strength lower than the con-crete itself.The tensile strength test and the shear strength test of evenness bonding interface of concrete and bedrock also indicated that cracks spread along bonding interface. Therefore,the bonding interface is the weakest place in cementation body strength.
As the bonding interface’s strength is weak,the failure of an RAB will?rstly occur at the bonding interface,but if under blasting vibration,an RAB is unlikely to bear ?erce shock waves.The compressive failure is impossible to happen because concrete and bonding interface have stronger compressive strength;moreover,shear strength is twice more than tensile strength(Gao et al.,1996;Lo et al.,1991),and an RAB is sustained by the inclining rock platform.Therefore,the possibility of shear failure in a bonding interface is less.But according to the location, the structure and bearing characteristic of an RAB,and the lower tensile strength of the bonding interface,tensile failure will take place along the interface in horizontal ori-entation because of the function of stress concentration under blasting vibration.Therefore,it can be concluded that the safety measure of an RAB is to control the inten-sity of blasting vibration,and to prevent the bonding inter-face from tensile failure.
4.Shock absorption measures in underground powerhouse excavation
As the span and the height of an underground power-house is bigger,the construction of an underground power-house usually adopts a multi-layer step in blasting excavation.In order to improve the construction e?ciency, the large diameter deep-hole bench blasting is applied.As the vibration of deep-hole bench blasting is great,it may have great negative e?ects on the top arch of the power-house,and the rock wall and the RAB.To solve this prob-lem,the protective rock layer and the presplit crack should be taken into consideration,and the weight of explosive charge is to be controlled.Rules in hydraulic structure construction speci?cation of underground engineering exca-vation SDJ212-83demand that in the extraordinary big sec-tion chamber excavation,if deep-hole bench blasting is used,it must satisfy the following
claims:Perimeter blasting is used
?rstly;Using detonator and segmented
Control the maximal weight of explosive charge in with the aseismatic demand of surrounding rock and the structure.
Modern large underground powerhouse excavation usu-ally applies one of the following two schemes:
(1)Perimeter blasting is carried out?rstly,cutting blast-
ing is used in the middle of the powerhouse,and a line of dense hole is added near to the rock wall.
Finally,short-hole blasting is done to excavate the remainder.
(2)In the middle part of the powerhouse,cutting blasting
is carried out?rstly,reserving a protective layer which is close to the rock wall.Then smooth blasting is done to excavate the protective layer.
At present,a majority of large underground powerhouse excavations are supposed to apply the second scheme while in the excavation of the third layer of the underground powerhouse,usually perimeter blasting is carried out?rstly to reduce the negative e?ect of blasting vibration to the RAB.
5.Practice observation on RAB under blasting vibration
From December2002to January2003,two concrete blocks with one anchor bolt were made on both upstream and downstream rock wall in the underground powerhouse to simulate the bonding condition.Joint gauge and strain gauge were mounted in the interface to monitor the inter-face’s strain and opening under blasting vibration.Yangtze River Scienti?c Research Institute and techno-o?ce of 1478a?liated company studied the e?ect of blasting vibra-tion of cutting blasting excavation in layer III to the bond-ing interface of RAB and rock wall through dynamic and static observation.Data from observation instrument var-ied little,it is proved that the limiting blasting vibration has little disadvantage to the bonding interface.
After the concrete of RAB was cast,observation points with joint gauge and strain gauge were mounted in the up and downstream RAB.The PPV exceeded the controlling standard about28times when excavation was carried out at the access tunnel to transformer chamber,bus-bar cham-ber,and layers below layer III.The maximal PPV is 15.33cm/s vertically,and14.99cm/s horizontally,of which the PPV bigger than10cm/s occurred18times,accounting for about64.3%in all.The stress of anchor bolt in RAB exceeded the measure capacity by5times.The measure capacity is350MPa or400MPa,and the change rate of 16observation points was obviously bigger than others over5MPa/d.The data from joint gauge that is above 1mm have6points,among which,the charge rate of4
72M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–79
points is more than0.1mm/d.Although the PPV exceeded the safe PPV much more,and many a times,the function of the RAB and the engineering construction is in good condition.
In the process of constructing the underground power-houses of Lubuge,Daguangba hydropower station,etc., the PPV from excavation of under layers of middle part in powerhouse and the bus-bar chamber reached16–32cm/s on RAB,but no abnormality was found during the operation of the RAB.
6.Numerical simulation of RAB under blasting vibration
In this paper,it has been noted that dynamic response numerical simulation parameters of RAB at28d of age come from Longtan underground hydropower station.
6.1.Project introduction
Longtan project is located on the Red river in Tian’e
country,Guangxi province,the People’s Republic of China. It is a key project of Red river cascade development with its great contribution in hydroelectric generation,?ood con-trol,irrigation and navigation.The project involves the con-struction of RCC gravity dam,water diversion and power generation system in the left bank,and navigation structure in the right bank.Its installed capacity is6300MW,with nine turbine generators.The underground powerhouse is made up of main and vice-equipment installation,which is398.19-m-long,30.17-m-wide by77.13-m-high,with a bigger span,longer chamber,and a higher intensity of exca-vation and support.The total excavation volume of the cav-ern is about636,000m3,and30,600piece anchor bolt is used,and the number of tunnels across the powerhouse is more than40.So its construction was disturbed by other working face seriously.Drilling and blasting method was used in rock excavation in underground powerhouse.Con-trolling the blasting vibration during construction is one of the key techniques in the project.
The main underground powerhouse is divided into nine layers to excavate,then each layer is further divided into several blocks.Sketch map of excavation procedure of underground powerhouse is shown in Fig.3.
The RAB locates in layer II in the underground power-house.Its designing ring pressure is1400kN of each wheel, which is a world record.Section view of RAB of Longtan hydropower station is shown in Fig.1(Zhang and Yu, 2004).Because concrete casting and anchor bolt construc-tion begin as soon as layer II of powerhouse is excavated, the RAB will be a?ected by the blasting vibration of blast-ing excavation at layer III and other lower layers.
6.2.Dynamic response numerical simulation of RAB
6.2.1.Parameters
In the dynamic response calculation of RAB under blasting vibration,the e?ects of the varieties of excavation methods and the parameters to the rock wall and the RAB were simulated roundly.Exploding process simulating function of LS-DYNA was used,just according to the install charge and dynamite parameter,then the detonation loading was ascertained through the interaction simulation of detonation products and blasting hole wall.
The parameters of blasting hole are depth ranging from 5to10m,diameter ranging from42to76mm,and a stem length of about2m.In order to simulate the excavation process,all rock layers excavation simulation use two-dimensional model,which neglect the function of the anchor bolt.To explore the e?ect of the anchor bolt to RAB’s bear condition,a three-dimensional model at layer IV excavation numerical simulation including anchor bolt is applied,then the results are compared.Result from three-dimensional model are described in detail in this paper.Model and element are shown in Figs.4and5
.
Fig.4.Sketch map of three-dimensional model and elements.
M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–7973
The calculation uses ideal elasto-plastic kinematic mate-rial model for rock and concrete,strain rate accounted by the Cowper–Symonds model which scales the yield stress by the strain rate dependent factor is shown below(Cowper and Symonds,1957):r y?1te_e=CT1P
h i
er0tb E p e eff
p
Te8Twhere r y is yield stress considering strain rate,r0is the ini-
tial yield stress,_e is the strain rate,C and P are the Cow-
per–Symonds strain rate parameters,e eff
p
is the e?ective plastic strain,b is hardening parameter,and E p is the plas-tic hardening modulus which is given by
E p?
E tan E
EàE tan
e9Twhere E is elastic modulus and E tan is tangent modulus;the material parameters are shown in Table1.
The JWL equation of state which is usually used for det-onation of high explosives de?nes the pressure as:
P?Ae1à
x
R1V
TeàR1VtB1à
x
R2V
eàR2Vt
x E0
V
e10T
where V is speci?c volume,A,B,x,R1,R2,E0are material parameters;ammonium nitrate explosive parameters are listed in Table2.
6.2.2.Numerical simulation results
Taking the horizontal and vertical PPV,horizontal ten-sile stress and the maximal shear stress from the numerical simulation results of RAB listed in Table3,studying the maximal compressive stress,and comparing the
horizontal Fig.5.Nodes and elements position of RAB middle section.
Table1
Material mechanical parameters
E/GPa E tan/GPa q/kg mà3m r0/MPa e y b C/sà1P Rock80027000.21000.031403 Concrete25024000.167300.0021403 Anchor bolt210/78000.3/0.3///
Table2
Material parameter of ammonium nitrate explosive for JWL equation
Density/kg mà3A B R1R2x E0Detonation velocity/m sà1 1000 2.14E+11 1.82E+09 4.20.90.3 4.192E+093600
Table3
Results of dynamic response of RAB under blasting vibration
Excavation type Horizontal
PPV/cm sà1Vertical
PPV/cm sà1
Maximal shear
stress/MPa
Maximal horizontal
tensile stress/MPa
Permitting tensile
stress/MPa
Safe
PPV/cm sà1
Layer III1enlarging2017 1.00.9 1.022.2 Layer III2draw–cut1070.450.5 1.020.0 Layer III2enlarging11.8150.560.87 1.013.6 Layer IV draw–cut 2.4 4.60.320.32 1.07.5 Layer IV enlarging 4.28.8 1.00.6 1.07.0 Layer IV enlarging a 3.2 5.390.250.262 1.012.2 Layer V draw–cut 1.8 4.00.140.13 1.013.8 Layer V enlarging 2.28.00.40.29 1.07.3 Layer VI draw–cut 3.4 5.20.180.27 1.012.6 Layer VI enlarging 3.3 6.10.220.38 1.08.7 Layer VII enlarging 3.5 2.80.150.4 1.08.7 a Remarks:results in this column are from three-dimensional,others are two-dimensional results.
74M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–79
and the vertical tensile stress on RAB,it is easy to conclude that there is a small compressive stress on the RAB,and the horizontal and the vertical tensile stress are comparative which varied between 0.1and 0.9MPa.Results from two-dimension indicated that the maximal horizontal ten-sile stress appears at the bonding interface top,and vertical tensile stress appears at the top or the bottom of the bond-ing interface,the maximal tensile stress is shown in Figs.6and 7.In the three-dimension calculation results,tensile stress at the bonding interface top decreases due to the e?ect of anchor bolt.The maximal tensile stress of RAB appears at the middle of top surface,as is shown in Fig.8,which indicates that forces in bolt can reduce the stress of interface,thus guaranteeing the safety of RAB.
The maximal shear stress is not much di?erent from hori-zontal tensile stress,it is even smaller than the horizontal tensile stress.Tensile strain on each direction changes from 2to 6microstrain,and the maximal main strain is 15microstrain,much less than the concrete limit tensile strain which ranges from 80to 150microstrain.The tensile stress of anchor bolt is 2.5MPa,and the maximal PPV on rock-anchored edge of top surface is about 1.5–3.0times the PPV at the RAB inside of the top surface.The classical time history of velocity and stress is shown in Figs.9and 10.
As the damage of RAB is determined by horizontal ten-sile stress under blasting vibration,and the horizontal ten-sile stress on RAB is ranged from 0.1to 0.9MPa,the bonding interface tensile strength is more than 60%of con-crete tensile strength and the test value is over 1.0MPa (Gao et al.,1996;Lo et al.,1991).A conclusion can
be
Fig.6.Horizontal stress contour of RAB when bonding interface at its maximal tensile
stress.
Fig.7.Vertical stress contour of RAB when bonding interface at its maximal tensile
stress.
Fig.8.Maximal vertical tensile stress contour on
RAB.
M.Chen et al./Tunnelling and Underground Space Technology 22(2007)69–7975
drawn that the bonding interface is safe under a certainty intensity blasting vibration because the tensile strength of common concrete is more than 1.7MPa.That is to say,the RAB is safe.
The average accelerated velocity on RAB ranges from 200to 400m/s 2,and the basic frequency is more than 500Hz,and the frequency and accelerated velocity is sev-eral magnitudes more than that of natural earthquake.For example,the accelerated velocity of nodal 41214is 211m/s 2,but the basic frequency is 1126Hz.The curve of accelerated velocity and the power spectrum are shown in Figs.10and 11.The practice observation of basic fre-quency usually ranges from 200Hz to 500Hz.Supposing RAB bears most disadvantage of loading,the bonding interface is destroyed under blasting vibration,and the stress on bonding interface releases at a moment,then all the mass force of RAB is borne by anchor bolt.According to the design law and the hypothesis,the results from numerical simulation should be made use of.Due to the rigid body limit equilibrium method,and Eq.(1),we can
know that the anchor bolt stress will be much bigger than its design strength,and the RAB will be thrown out from the rock platform.In fact,the tensile stress on RAB and bonding interface is less than the tensile strength of bond-ing interface,which indicates that the accelerated velocity has little correlation with the stress in underground engi-neering.The accelerated velocity cannot re?ect the fracture state of RAB.So accelerated velocity may not be an exact safe criterion to underground engineering.6.3.Safe PPV for RAB
From the above analysis,the main fracture form of RAB is the breakage of bonding interface of RAB and rock wall.Because the bonding interface tensile failure occurs horizontally,and the time history curves of PPV well cor-respond to the maximum horizontal stress,the safe PPV is controlled by horizontal velocity.For RAB concrete older than 28d,according to the calculation results,the tensile strength of bonding interface and the limiting stress rule of concrete,and making full use of the correlation among horizontal tensile stress,velocity and the bonding interface tensile strength,safe PPV for RAB can be calcu-lated by Eq.(11),its results are listed in Table 3V s ?
?r l
v e11T
where [r l ]is tensile strength of bonding interface,r is the maximal horizontal tensile stress,v is the horizontal PPV.Without allowing the stress on bonding interface attain the maximum tensile stress at the same time into account,the calculate tensile strength is a lower static strength,only 1.0MPa,and the increase of strength under dynamic load-ing and not involving the increase of strength with concrete age,so the calculation results have some safe storage.The blasting excavation below the RAB in Longtan underground powerhouse was not permitted when concrete strength less than its design strength at 28d,and the safe PPV is 7cm/s on the RAB when concrete is older than 28d.According to Table 3,di?erent control standards are proposed as the di?erent response of RAB in di?erent elevation.PPV of 7cm/s is reasonable in the blasting exca-vation in layer IV to layer VII,while it is more strict in layer III,whose safe PPV can reach to 13.6cm/s.Results from three-dimensional model indicate that the safe PPV of 7cm/s to RAB at the age of 28d has safe storage too.7.The feasibility of blasting excavation when RAB is of early age
In the excavation process of large underground power-house,the blasting excavation is forbidden before the con-crete strength of RAB attains its 28d design strength according to the design requirement.On one hand,it requires the concrete strength to attain 28d design strength,on the other hand,stick to the PPV of 7cm/s as the safe criterion.If so it will seriously a?ect the
time
76M.Chen et al./Tunnelling and Underground Space Technology 22(2007)69–79
limit of project in waiting for the last placed concrete strength attain28d design strength if no special measures are taken.In order to speed up the construction and to save the project investment,it is necessary to study the feasibil-ity of blasting excavation in underground powerhouse when the concrete age of RAB is less than28d(see Fig.12).
There is much construction experience of blasting exca-vation at the rock layers lower than RAB or neighboring chamber when the age of the concrete less than28d in underground powerhouse.For example,in the construc-tion of RAB in Lengzhuguan Powerstation’s underground powerhouse,the design concrete C25was taken place by C30with rapid hardening additives.When concrete reaches the age of10d,layer III bigins its excavation,and the result showed that blasting vibration did not a?ect the RAB(Kong,2001).In the construction of Daguangba underground powerhouse,diversion tunnel excavation was carried out when the concrete of RAB is at the age of3d and the PPV on RAB is3.43cm/s.The observation of static state showed that the interface between concrete and rock wall had not been destroyed(Wu and Wang, 2004).The examples showed that blasting excavation in underground powerhouse is feasible when RAB concrete age is less than28d.
In order to study the dynamic response character of early age RAB concrete under blasting vibration,the three-dimensional compute model mentioned above should be used,and concrete numerical simulation parameters should be changed.Adopting7d concrete parameters to simulate the dynamic response of RAB,the dynamic elastic modulus is25GPa,compressive strength is20MPa,and limit compressive strain is0.001microstrain.The numera-tion results show that the early age RAB have the same response characters with RAB at the age of28d except for the value.The compared curves of peak particle veloc-ity are shown in Fig.13.
From the comparative curve above,it is concluded that the PPV of RAB at the age of7d is more than28d’s,but 7d old concrete’s element stress is less than28d’s,mainly for concrete elastic modulus is smaller,its compressive strength is weaker,and the restriction of rock wall to RAB is smaller too.
According to concrete mechanics,concrete strength increases with its age,and its aseismatic capacity enhances with the increasing of the strength(Chen,2003).The safe vibration velocity of RAB is mainly con?rmed by concrete strength,as long as the concrete has some strength which can resist a certain blasting vibration,so it is unnecessary to restrict the blasting excavation carry out no other than the RAB concrete age of28d.The safe PPV for RAB in di?erent ages may be adopted according to the strength of di?erent age concrete.
From the growing rules of concrete strength,the strength of concrete at the age of7d can reach to50–70%of28d’s.According to the response characters of RAB under blasting vibration,the vibration velocity is large but the stress of interface is small for7d age
RAB,
M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–7977
so the safe PPV of7d concrete should be more than50%of 28d’s.This value is almost identical with the value pro-posed by blasting safety regulation.In order to speed up the construction and save limiting time,di?erent safety cri-terions can be proposed to RAB according to the age and strength of concrete.It is unnecessary to restrict the blast-ing excavation carry out no other than the RAB concrete age of28d.The safe PPV for young concrete proposed by GB6722-2003could be adopted as those for RAB with an age no older than28d,i.e.,a safe PPV of3–7cm/s for RAB at age range from3d to7d,and2–3cm/s for RAB at age range from0d to3d.
8.Blasting vibration criterion
During the blasting excavation of underground power-house,PPV is currently adopted as the safety criterion for RAB.For most of the built projects,the safe PPV for RAB is employed according to the safe PPV for early age concrete.For example,the safe PPV is not more than 7cm/s for RAB older than28d in Tianhuangping Power-Station.The safe PPV is7cm/s in Lengzhuguan and Longtan PowerStation et al.,too(Kong,2001;Liu and Zhou,2000;He and Xie,2003).The safe PPV of7cm/s is proved by project practice.But the PPV on RAB is over 16cm/s which was monitored by Yangtze River Scienti?c Research Institute in the underground powerhouse of Dongfeng PowerStation.There is no abnormal phenome-non by macro-investigation in RAB and it is also proved to be safe by static state observation(Zhang et al.,2003; Ji,1995).In the construction of Daguangba PowerStation underground powerhouse,the maximal vibration velocity is11.56cm/s at the top of RAB near to rock wall and the maximal vibration velocity is18cm/s at the bottom of RAB,but no damage was found on the RAB.So the safe PPV of7cm/s has the feasibility to increase in project construction.
Through the discussion of the blasting excavation feasi-bility for RAB at the age of7d,the monitoring data from practice,and the numerical simulation data in Table3,a conclusion can be drawn that safety criterion of PPV for RAB in di?erent ages can adopt the criterion for early age concrete proposed by GB6722-2003;the safe PPV for early age concrete is shown in Table4,di?erent ages with di?erent safety criterions.
RAB is a pivotal structure in underground power-house,so its safety is the precondition of normal running of powerstation.The PPV for RAB should be strictly con-trolled to avoid the negative e?ect caused by blasting vibration.9.Conclusions
Through theoretical analysis and numerical simulation, the following conclusions could be included:
a.Bonding interface between RAB and vertical rock
wall is a weak face of the whole RAB structure.
The possible failure is the tensile fracturing of the bonding interface.The key to guarantee the safety of RAB is to prevent the cracking of the bonding interface.
b.The safe PPV for freshly placed concrete proposed by
GB6722-2003under di?erent ages could be adopted as that for RAB concrete,and it is not a prerequisite to limit blasting work for the excavation of lower lay-ers if the age of RAB concrete is below28d.
c.The results of numerical simulation of RAB response
under blasting vibration indicate that the maximum horizontal tensile stress of the bonding interface between RAB and vertical rock wall appears at the top of the interface,the maximum vertical tensile stress appears at the top or the bottom of the interface.
Bonding interface could be prevented from damaging under blasting vibration under proposed safe PPV.
Acknowledgements
The research work reported was carried out at Whuhan University,the author is grateful to the National Natural Science Fund Council for its?nancial support(No. 50239070)and acknowledges the Support provided by the Excellent Young Teachers Program of MOE,P.R.C.(2081). References
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Table4
Safe PPV for early age concrete
Age(d)0–33–77–28
Safe PPV(cm sà1)2–33–77–12
78M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–79
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M.Chen et al./Tunnelling and Underground Space Technology22(2007)69–7979
(一) 假定你是红星中学初三学生李华。你的美国朋友Jim在给你的邮件中提到他对中国新近出现的一种共享单车“mobike”很感兴趣，并请你做个简要介绍。请你给Jim回信，内容包括： 1. 这种单车的使用方法(如：APP查看车辆、扫码开锁等); 2. 这种单车的优势; 3. 你对这种单车的看法。 注意：1. 词数不少于80; 2. 开头和结尾已给出，不计入总词数。 提示词：智能手机smartphone, 二维码the QR code 参考范文 Dear Jim, I’m writing to tell you more about the new form of sharing bike mobike mentioned in your latest letter. It’s very convenient to use if you have a smartphone. What you do is find a nearest mobikethrough the APP, scan the QR code on the bike, and enjoy your trip. Compared to other forms of sharing bike, the greatest advantage of mobike is that you can easily find one and never worry about where to park it. It is becoming a new trend as a means of transportation, which relieves the traffic pressure and does good to the environment as well. Hope to ride a mobike with you in China. Yours, Li Hua (二) 最近很多大城市都投放了共享单车(shared bikes)，比如摩拜单车(Mobike)、Ofo共享单车等。由于它们方便停放，骑车也能起到锻炼身体的作用，作为代步工具很受大家欢迎。但是，各地也出现了很多毁车现象，比如刮掉车上的二维码(QR code)、上私锁等。 你对这种现象怎么看?你对共享单车公司有什么建议吗?写一篇符合逻辑的英语短文，80词左右。 参考词汇：bike-sharing companies 共享单车公司，Mobike 和Ofo 是两家共享单车公司，convenience 方便，register登记 参考范文 The shared bikes like Mobike and Ofo bring great convenience to people. You needn’t lock them by simply using your smart phone. They can take you where the subway and bus don’t go. And they can be left anywhere in public for the next user. However, bad things happen. Some people damage the QR code on the bike, or use their own lock, which causes trouble to other users. In my opinion, it’s difficult to turn these people’s ideas in a short time. Therefore, bike-sharing companies like Mobike and Ofo need to do something. For example, those who damage the bike should pay for their actions. Also, because people use their real name toregister as a user, it’s a good way to connect to one’s personal credit. In the end, what I want to say is to take good care of public services. (三) 共享单车(bicycle sharing)已成为时下最热的话题之一，请你就这一话题写一篇短文。内容须包括三方面：1. 共享单车蓬勃发展，成为社会热潮;2. 共享单车带来便利，但也存在问题;3. 我对解决问题的建议。 参考范文 Bicycle Sharing With the development of technology, bicycle sharing comes into people's lives. It becomes more and more popular and much news reported it. At the same time, we should see that there are some problems caused by bicycle sharing. On one side, bicycle sharing makes it very convenient of people traveling. You can find a bicycle anywhere at any time when you want to go out for a cycling, and the price of one trip is very low. It can save time for people. On the other side, its management is not perfect. Even kids can open the lock and ride the bicycle, there is no doubt that such behavior is very dangerous.
介绍北京的英语作文(2) AsBeijinghasbeenconfirmedhomecityofOlympics2008,the spiritofgreenOlympics,scientificOlympicsandhumanizedOlymp icwillsurelybringmoreandmorechangestoBeijing,promotethed evelopmentofsportsandOlympicsinChinaaswellasintheworld,a ndstrengthenthefriendlycommunicationsbetweenChineseandf oreignpeople. 篇六：Beijing BeijingisthecapitalofPeoplesRepublicofChinaandthenation scentreforpolitics,economyandculture.Itenjoysalongandrichhis tory.Therearenumerousheritagesitesandwonderfulexamplesof ancientarchitecture,suchastheworld-famousGreatWall,theTem pleofHeavenandtheForbiddenCity. Besidessightseeingplaces,therearemanydeliciousfoodsuch asPekingducksandBeijingsnacks.Beijingisreallyagoodplacetotr avel. 篇七：Beijing AsthecaptainofChina，Beijinghasbeenthemostpopularcityofchina。SomoreandmorepeoplewanttovisitBeijing.
AN EXCITING JOB I have the greatest job in the world. I travel to unusual places and work alongside people from all over the world. Sometimes working outdoors, sometimes in an office, sometimes using scientific equipment and sometimes meeting local people and tourists, I am never bored. Although my job is occasionally dangerous, I don't mind because danger excites me and makes me feel alive. However, the most important thing about my job is that I help protect ordinary people from one of the most powerful forces on earth - the volcano. I was appointed as a volcanologist working for the Hawaiian V olcano Observatory (HVO) twenty years ago. My job is collecting information for a database about Mount Kilauea, which is one of the most active volcanoes in Hawaii. Having collected and evaluated the information, I help other scientists to predict where lava from the volcano will flow next and how fast. Our work has saved many lives because people in the path of the lava can be warned to leave their houses. Unfortunately, we cannot move their homes out of the way, and many houses have been covered with lava or burned to the ground. When boiling rock erupts from a volcano and crashes back to earth, it causes less damage than you might imagine. This is because no one lives near the top of Mount Kilauea, where the rocks fall. The lava that flows slowly like a wave down the mountain causes far more damage because it
[标签:标题] 篇一：关于汽车的英语作文 好的 Nowadays, with the rapid improvement of people’s living standards, cars have become an indispensable part of people's lives,so that more and more people have a car of their own, especially in cities. It brings some benefits for us but also causes many problems at the same time. For one thing,there’s no doubt that cars provide much convenience for people to go where they want to quickly and easily. Especially on weekday,driving a car can save a lot of time for us to go to work.When some places are too far away from our home, driving our own car is also convenient, we can go wherever we want. However,for another, too many cars will lead to the pressure of public transport, a series of problems will appear.First of all,it will bring about more air pollution,a large amount of polluted air given off by cars do great harm to our health.What’s more, as the existing roads are not so wide for the increasing number of cars,undoubtedly,traffic jams will become more and more serious. Last but not least, cars also place burden on the public facilities in providing more parking lots. As far as I am concerned,everything has its advantages and disadvantages. It’s high time that effective action must be token to limit the ever growing number of cars, the government should take measures to control the air pollution from the cars. Some roads should be widened and more new roads should be constructed. Only in this way，will people benefit from the popularity of cars. 坏的 Nowadays, with the rapid improvement of people's living standards, cars have become an indispensable part of people's lives,so that more and more people have a car of their own, especially in cities.It brings some benefit for us but also causes many problems at the same time. For one thing,it's no doubt that that cars provide much convenience for people to go where they want to quickly and easily. Especially on weekday,driving a car can save a lot of time for us to go to work.When some places are too far away from our home, driving our own car is also convenient, we can go wherever we want. However,for another, too many cars will lead to the pressure of public transport, a series of problems will appear.First of all,it will bring about more air pollution,a large amount of polluted air given off by cars do great harm to our health .What's more, as the existing roads are not so wide for the increasing number of cars,undoubtedly,traffic jams will become more and more serious. Last but not least, cars also place burden on the public facilities in providing more parking lots. As far as I am concerned,everything has its advantages and disadvantages. It's high time that effective action must be token to limit the ever growing number of cars, the government should take measures to control the air pollution from the cars. Some roads should be widened and more new roads should be constructed. Only in this way,will people benefit from the popularity of cars. 篇二：雅思作文高分范文：私家车的利与弊 智课网IELTS备考资料 雅思作文高分范文：私家车的利与弊
介绍北京的英语作文1篇 篇一MyFamily Ilovemyfamily,becauseIhaveahappyfamily. MyfatherisanEnglishteacher.HisnameisJacky.Heisthirty-eight.Helikesplay ingbasketball.What’smymotherjop?Issheateacher?Yes,you’reright!Mymotherisverykindandnice,sheisthirty-seven.Mymotherisalways laboriouswork.Ilovemyparents! OnStaurdayandSunday,Ioftengotothelibraryandplaythepiano,Myfathergot oplaybasketball.Sometimes,wewatchTVandlistentomusicathome. Ilovemyfamily.BecauseI’mveryhappytolivewithmyparentstogether! 篇二MyFamily MyFamily Everyonehasafamily.Weliveinitandfeelverywarm.Therearethreepersonsin myfamily,mymother,fatherandI.Welivetogetherveryhappilyandtherearema nyinterestingstoriesaboutmyfamily. Myfatherisahard-workingman.Heworksasadoctor.Healwaystrieshisbesttoh elpevery,patientandmakepatientscomfortable.Butsonetimesheworkssohard thathecan”trememberthedate.
汽车的重要性《英语作文》 The automobile has become one of the most important means/ways of transportation in the world since it was invented. The automobile has completely changed the lifestyles of almost all the people in the world. In the past, animals like horses and camels were used for traveling and transporting goods. Automobiles are more comfortable and faster. Automobiles have also made it possible for us to transport large quantities of goods and people at the same time. Besides, the invention of the automobile has provided jobs for millions of people all over the world. 翻译： 汽车已经成为世界上最重要的交通工具之一，因为它是发明的。汽车已经完全改变了世界上几乎所有的人的生活方式。 在过去，像马和骆驼的动物被用来运送货物。汽车更舒适，更快速。汽车也使我们能够在同一时间运送大量货物和人。 此外，汽车的发明为全世界上百万的人提供了工作。
三一文库（https://www.doczj.com/doc/82378521.html,） 〔我想去北京英语作文〕 我想去北京的英语作文如何写？那么，下面是小编给大家整理收集的我想去北京英语作文，供大家阅读参考。 我想去北京英语作文1 I’d like to go to a beautiful place. I think it would be Beijing. Beijing is not only our capital city, but also a famous city with long history and wonderful culture. Beijing is also China’s political and cultural center. There’re many old places of great interest, such as the Great Wall, the Summer Palace, the Forbidden City, the Temple of Heaven, and Tiananmen Square. Once you see Tiananmen Square, you will think of Beijing. It has been the symbol of Beijing since 1949. 我想去北京英语作文2 I went to Beijing more than eight times. Beijing is the capital of China. It’s a big city. I am very familiar with Beijing. It takes an hour and forty minutes from Nantong to Beijing by plane. There are many tall buildings in Beijing. It’s a modern city. My family visited the Great Wall, the Summer
汽车英文演讲稿 篇一：汽车英语演讲稿 Good morning everyone ,today, the topic of my lecture（演讲） is “Do you love automobiles（'tmbilz）”. before I start my speech ,I want to ask a question,did there anybody present had seen the movie”the Fast and the Furious['fjrs]”.There are many cars of different styles in the movie,for example ,when you see the movie,you can find roadsters（跑车） just like Porsche（保时捷） GT3,sports utility (通用的）vehicles （车）SUV such as Volkswagen ['f:lks,va:gn]Touareg ['twɑ:reɡ](大众途锐）,even armored cars ,all of them are so powerful and beautiful. When I was a child,I dreamed I can have a car like FORD GT 40,this racing bike（跑车） is amazing and fantastic,but as time goes on ,my hobby is changing,now my favorite car is Jeep Rubicon,this is a car I really want ,it’s dynamic performance（性能） and off-road （越野） performance is top-ranking（一流的） ,which other ordinary SUV is unable to compare; the shape and color of this car is magnificent (华丽的） [mg'nfs()nt],and it’s gear-shift system（齿轮转
小学一年级介绍北京的英语作文 Beijing is an ancient city with a long history. Back in 3000 years ago in Zhou dynasty, Beijing, which was called Ji at the moment, had been named capital of Yan. Thereafter, Liao, Jin, Yuan, Ming and Qing dynasty all made Beijing their capital. Therefore, Beijing was famous for "Capital of a thousand years". The long history leaves Beijing precious cultural treasure. Winding for several kilometers in Beijing area, the Great Wall is the only man-made structure that could been seen in the space. The Summer Palace is a classic composition of ancient royal gardens, and the Forbidden City is the largest royal palaces in the world. Tiantan is where the emperor used to fete their ancestors, and also the soul of Chinese ancient constructions. The four sites above has been confirmed world cultural heritage by UNESCO. However, the best representatives for Beijing are the vanishing Hutongs and square courtyards. Through hundreds of years, they have become symbol of Beijing's life. Tian'anmen square being still brilliant today with cloverleaf junctions and skyscrapers everywhere, the old-timey scene and modern culture are combined to present a brand new visage of Beijing. As Beijing has been confirmed home city of Olympics 2008, the spirit of "green Olympics, scientific Olympics and humanized Olympic" will surely bring more and more changes to Beijing, promote the development of sports and Olympics in China as well as in the world, and strengthen the friendly communications between Chinese and foreign people. 北京是一个有着悠久历史的古城。 早在 3000 年前的周朝,北京,这叫霁,被命 名为首都燕。此后,辽、金、元、明、清都是北京首都。因此,北京著名的一千年 的“资本”。 悠久的历史使北京宝贵的文化瑰宝。绕组在北京地区几公里,长城是唯一的 人造结构,可以在空间。 颐和园是古代皇家园林的经典组合,和故宫是世界上最大 的皇家宫殿。 天坛是皇帝用来祭祀他们的祖先的地方,也是中国古代建筑的灵魂。 上面的四个网站已经确认被联合国教科文组织世界文化遗产。然而,北京最好的 代表是消失的胡同和广场庭院。数百年来,他们已经成为北京的生活的象征。天 安门广场到处都在今天依然灿烂的蝶式路口和摩天大楼的,古色古香的场景和现 代文化相结合,提出一个全新的北京的面貌。 随着北京 2008 年奥运会已被证实的家乡,精神的“绿色奥运、科技奥
1/5
英语作文：关于共享单车的4篇范文 共享单车确实是今年最热的话题之一，很多小伙伴们留言问共享单车要怎么写英语作文，这一份写作指南想必大家都很需要。 词汇篇 bicycle-sharing /sharing bike /shared bikes 共享单车 bicycle /bike /cycle 自行车 ride a bicycle /take a ride /cycling /cycle /bicycle /bike 骑自行车 park 停车 Bluegogo 小蓝单车 Ofo bicycle 小黄车 Mobike 摩拜单车 APP 应用程序(全称为application) scan the QR code
扫描二维码 unlock the smart lock 解开智能锁 timer 计时器 user 用户 smartphone 智能手机 keep fit 健身 environmental protection/ environment friendly/ help the environment 环保的 In full swing 如火如荼 brightly coloured 色彩鲜艳 The combination of technology, creativity and market forces 科技、创意、市场力量的结合 has changed the image of the city 改变了城市形象
curb carbon dioxide emissions 减少二氧化碳排放 reduce road congestion 减少道路拥堵的情况 random parking 随机停车 illegal parking 乱停乱放 bicycle damage 车辆损坏 traffic violation 违章 social mind 公德心 quality 素质 范文篇 (一) 假定你是红星中学初三学生李华。你的美国朋友Jim在给你的邮件中提到他对中国新近出现的一种共享单车“mobike”很感兴趣，并请你做个简要介绍。请你给Jim回信，内容包括： 1. 这种单车的使用方法(如：APP查看车辆、扫码开锁等);
暑假去北京的英语作文 北京是我国的首都，有许多历史文化建筑，那么关于暑假去北京的英语作文怎么写呢?以下是为大家整理的暑假去北京游玩的英语作文，欢迎大家阅读。 暑假去北京游玩的英语作文篇一This summer holiday I visited to Beijing with my aunt. We visited to many famous places such as Tian'ann Men Square the Imperial Palace the Summer Palace the Great Wall and so on. Before that I just saw them on TV but this summer holiday they were all in front of my eyes. I was so exciting. Among them I like theSummerPalacethe most because it's so amazing and the scenery is very beautiful. BesidesBeijingis a modern city. There are many tall buildings. I like this city very much and I hope I can visit there again. 翻译： 这个暑假，我和叔叔到北京旅游。我们参观了很多地方，比如天安门广场，故宫，颐和园和长城等等。 在那之前我只是在电视上看见过它们，但是这个暑假我亲眼看到它们就在我的前面，我很兴奋。
八年级下学期全部长篇课文 Unit 1 3a P6 In ten years , I think I'll be a reporter . I'll live in Shanghai, because I went to Shanfhai last year and fell in love with it. I think it's really a beautiful city . As a reporter, I think I will meet lots of interesting people. I think I'll live in an apartment with my best friends, because I don' like living alone. I'll have pets. I can't have an pets now because my mother hates them, and our apartment is too small . So in ten yers I'll have mny different pets. I might even keep a pet parrot!I'll probably go skating and swimming every day. During the week I'll look smart, and probably will wear a suit. On the weekend , I'll be able to dress more casully. I think I'll go to Hong Kong vacation , and one day I might even visit Australia. P8 Do you think you will have your own robot In some science fiction movies, people in the future have their own robots. These robots are just like humans. They help with the housework and do most unpleasant jobs. Some scientists believe that there will be such robots in the future. However, they agree it may take hundreds of years. Scientist ae now trying to make robots look like people and do the same things as us. Janpanese companies have already made robts walk and dance. This kond of roots will also be fun to watch. But robot scientist James White disagrees. He thinks that it will be difficult fo a robot to do the same rhings as a person. For example, it's easy for a child to wake up and know where he or she is. Mr White thinks that robots won't be able to do this. But other scientists disagree. They think thast robots will be able t walk to people in 25 to 50tars. Robots scientists are not just trying to make robots look like people . For example, there are already robots working in factories . These robots look more like huge arms. They do simple jobs over and over again. People would not like to do such as jobs and would get bored. But robots will never bored. In the futhre, there will be more robots everwhere, and humans will have less work to do. New robots will have different shapes. Some will look like humans, and others might look like snakes. After an earthquake, a snake robot could help look for people under buildings. That may not seem possibe now, but computers, space rockets and even electric toothbrushes seemed
描述北京的英语作文 北京是我国的首都，也是个历史悠久、经济繁荣及科技发达的城市，但是北京也有人口拥堵雾霾严重等种种问题，你对北京是怎么看的呢?下面是橙子为大家整理的描述北京的英语作文，希望对大家有帮助。 北京 I’d like to go to asdfs beasdfsutiful plasdfsce. I think it would be Beijing. Beijing is not only our casdfspitasdfsl city, but asdfslso asdfs fasdfsmous city with long history asdfsnd wonderful culture. Beijing is asdfslso Chinasdfs’s politicasdfsl asdfsnd culturasdfsl center. There’re masdfsny old plasdfsces of greasdfst interest, such asdfss the Greasdfst Wasdfsll, the Summer Pasdfslasdfsce, the Forbidden City, the Temple of Heasdfsven, asdfsnd Tiasdfsnasdfsnmen Squasdfsre. Once you see Tiasdfsnasdfsnmen Squasdfsre, you will think of Beijing. It hasdfss been the symbol of Beijing since 1949. 我想去一个美丽的地方。我觉得那应该是北京。北京不仅仅是我们的首都，也是有着悠久的历史和精彩地文化的城市。北京也是中国的政治和文化中心。有很多古迹，比如，长城，颐和园，紫禁城，天坛，天安门广场。只要你看到天安门广场，你就会想起北京。自从1949年起它就是北京的象征了。 北京