The problem of human response to blast induced vibrations in tunnel construction and mitigation of vibration e?ects using cautious blasting in half-face blasting rounds
Cengiz Kuzu *,Erim Guclu
Istanbul Technical University,Mining Engineering Department,34469Maslak,Istanbul,Turkey Received 24October 2007;received in revised form 19February 2008;accepted 20February 2008
Abstract
Blast induced ground vibrations generated by explosives in tunnel construction may cause structural damages in or close to urban areas.Therefore,the aim in blasting must be to suppress the vibration e?ects and mitigate the possible hazard on structures.But the psychological character of human response to vibrations involves highly subjective attitudes about what kind of environment is ‘‘accept-able ”even if no structural damage is occurred.Therefore,we utilize the method of cautious blasting for half-faces that is environmentally friendly,and easy to utilize for tunnel construction.Small charges in this method are detonated sequentially to produce minimum side e?ects.This method is tested in a tunnel construction in Istanbul with numerous experimental shots.In these experiments,the duration and also quantity of explosives are carefully controlled.Regarding human response,better results are obtained with short durations (about 300ms)compared to long durations (9000or 480ms).The quantity of maximum co-operating explosive charge decreases from 3.088to 1.744kg while the vibration levels de?ned as peak particle velocity (PPV)become more tolerable around 300ms shot durations.ó2008Elsevier Ltd.All rights reserved.
Keywords:Blast induced vibrations;Ground vibrations;Tunnel blasting
1.Introduction
Tunnel constructions with blasting in urban areas create unpleasant ground vibrations and also in?ict structural damage if the magnitude of the explosion is not kept at minimum.In tunnel blasting,considerably higher speci?c charges than in bench blasting are required because of the high degree of constriction or ?xation (Persson et al.,1996).On the other hand,it is necessary to guarantee lower vibration levels which is the best indicated with peak parti-cle velocity (PPV).The frequency contents of PPV values are also important.Damage potentials in the low frequency range (<40Hz)are considerably higher than those in the high frequency range (>40Hz),especially due to a possible
resonance e?ect in structures (Singh et al.,1997;Kuz-menko et al.,1993).Performance evaluations of blasting rounds point out that environmental performance is as important as technical performances and human reactions to blasting can be a limiting factor.
It is a well-known fact that human beings notice and react to blast-induced vibrations at lower levels than the damage thresholds (Anonym,1987;Hustrulid,1999).Sis-kind et al.(1980)reported that particle velocities of 0.5in/s from typical blasting (1s vibration)had to be tolerable to about 95%of the people perceiving it as ‘‘distinctly per-ceptible ”.They pointed out also to the importance of the blast duration in forming of human response.Generally,as blast duration increases,the potential for adverse human response increases.The most of the studies on human tol-erance to vibrations have been of steady-state sources or those of relatively longer duration than typical mine-blast-ing operations.Siskind et al.(1980)summarized the results
0886-7798/$-see front matter ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.tust.2008.02.001
*
Corresponding author.Tel.:+902122856341;fax:+902122856131.E-mail address:kuzu@https://www.doczj.com/doc/885735478.html,.tr (C.Kuzu).
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Tunnelling and Underground Space Technology xxx (2008)
xxx–xxx
Tunnelling and
Underground Space Technology
incorporating Trenchless Technology Research
of three experimental studies which belonged to one tran-sient vibration from Wiss and Parmelee(1974)and another two steady-state vibrations from Goldmann(1948)and Reiher and Meister(1946).Raina et al.(2004)gave a help-ful reference list on this topic.
Several researchers recognized that the duration of vibration was critical to its undesirability(Wiss and Parme-lee,1974;Murray,1979;Dowding,2000);the most of the evidents could be tolerated if the event was short.The vibration limits required for reasonable comfort from a long-term vibration source are certainly more restrictive than for sources of short duration and infrequent occur-rence.Therefore,a good planned shot duration(generally less than2s)has a high priority in blasting practice to pro-vide human comfort.
If ground vibration problems are likely to arise,an esti-mate of their e?ects should be considered in the planning of the work.The risk of damage as a function of the size of the charge and of the distance between the charge and structures has to be determined(Langefors and Kihlstro¨m, 1978).A solution to this problem could be‘‘cautious blast-ing”techniques and its elements such as the modi?cation of the blasting patterns,the maximum charge per delay,delay period,?ring sequence and the geometry of the rock vol-umes where the explosives are placed(Lizotte,1996).Also, in cautious blasting,PPV is the key for the environmental performances and it is strongly dependent upon the maxi-mum charge per delay in the near?eld and the total charge in the very far-?eld(Blair,1990).Especially,the duration of the seismic waves is directly proportional to the charge weight in a blast and inversely proportional to the distance (Zhang,2000).But generally,proper delay time is very important to reduce the amplitude and optimize the dura-tion of ground vibrations(Venkatesh,2005).
On the other hand;to divide the necessary total charges using of delay techniques does not give satisfactory results in all cases.Therefore,it is necessary to use an optimum number of individual delays and reach an optimal duration of shots.The duration of shot becomes longer if the total charge is divided in big number of delays,i.e.in very small amount of explosives.This kind of long duration shots, contrary to the short duration shots,are not tolerated by people although very low vibration levels are obtained.
This paper utilizes this attitude of people to the duration of vibrations and describes the mitigation of vibration e?ects using cautious blasting in half-face blasting rounds instead of full face blasting rounds in a case of tunnel dri-vage with blasting near a populated area.Based on the fact that the quantity of consumed explosives in half-face rounds makes possible to get short duration times with small enough PPV values,new blasting practice are exper-imented.Thus,the tunnel face is divided in two parts; upper and lower parts.The blasting work is organized by stages.The lower part is shot in the?rst stage and the upper part is shot after20min in the second stage to bal-ance the people’s annoyance.
2.Site description
Fig.1shows the Ayazaga Tunnel where this study is performed to test di?erent blasts and people’s responses to vibration e?ects.Ayazaga Tunnel is a part of a big muni-cipal water supply tunnel project designed to divert water to Istanbul from the neighboring city of Sakarya.The Aya-zaga Tunnel is about2582m long with a cross-sectional area of22m2.The construction site is highly populated and only a small portion of the tunnel passes beneath a for-est.Especially in a construction section of326m in
length
Fig.1.The location of Ayazaga Tunnel.
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and57–22m in depth beneath a populated area,a special care is required to attain a minimum possible silence.Here, the3–6storey residential structures with a reinforced con-crete quality were built on the relatively competent rock formations.The geological structures of this construction section are shown through the drill cores(BH8and BH9) in Fig.2.The data obtained from drill cores show the exis-tence of ground water at6–8m below the surface.The structures were built above this level directly on the rock body after removing the thin weak layers if
exists.
The rock mass,in which the tunnel is driven,consists of alternated sandstone,mudstone,siltstone and occasional shale bands.The rock quality designation(RQD)and the uniaxial compressive strength(UCS)values of these rock layers are given in Fig.2.One of the rock mass classi?ca-tion systems,Barton’s Q-system,is used frequently in tun-nels(Barton et al.,1974;Harrison and HudsonFREig, 2000;Chakraborty,1998)to assess the support require-ments,the stand-up time,and maximum advance length per blasting round.A new Q value after each shot is obtained directly from the tunnel face based on the new geological conditions of the new working area;this is to control the advancement lengths per blasting round. Table1illustrates the advance rate per blasting based on the estimated Q ranges.Determination of the advance rate and the Q values ensures safe working conditions as the tunnel face advances.A typical example of Q-system rating applied in this study is presented in Table2.Very low Q values are required to support the advance per blasting round as short as 1.5m to avoid roof collapses.The obtained Q values vary in the ranges of less than0.04 (rarely3.2%),0.04–0.1(mostly91.5%)and0.1–0.4(rarely 4.1%)which represent the areas of the Q system from ‘‘extremely poor”to‘‘very poor”,whereas those in the range of0.1–0.4have advancement of more than1.5m. But in all shots and for all Q ranges,small advance rates of about1.5m with angle hole cuts,are used to control roof collapses and mitigate the human annoyance.
3.Set-up procedures and cautious blasting experiments
This work is performed in two stages.In the?rst stage of the work,the blast round is planned as a full face blast round with the aim of?nding an answer to the vibration problem.The vibration e?ect of explosions is mitigated by selecting proper shot durations which also simulta-neously de?ne the amount of co-operating charge weights. The cautious blasting techniques are used in this study as indicated by Lizotte(1996),and the?ring sequence of the shot is modi?ed by dividing the explosives into di?erent amounts to enable the blast within the drill holes for a given time sequence.Towards determination of a proper shot duration,cautious blasting experiments are carried out in which the maximum charge weights are kept in con-servative ranges to provide mitigation of vibration e?ects. Table3illustrates the measured representative PPV values and the corresponding responses to these shots.The dura-tions of the shots are varied between480and9000ms (Table3).In these shots,shock tube initiation system (non-electric initiation)is especially used to achieve long shot durations such as9000ms shot.Details of these exper-imental shots and the respective quantities are illustrated in Fig.3a and b;and Table4.
In the second stage of the work,the vibration e?ect of explosions is mitigated by selecting new blast organizations in the form of half faces in which also the shot durations simultaneously de?ne the amount of co-operating charge weights.The blasting rounds are divided in two partial rounds consisting of upper and lower face portions.The lower face portion is blasted at?rst.Thereafter in20min, the upper face portion is blasted separately to enable a good placement of muckpile for loading,transport and roof safety.The time interval of20min between two shots is nec-essary to balance the people’s annoyance,too.The same technical delay approach is applied into these new blasting rounds.The numerous trial blasts are carried out in which the maximum charge weights are kept in conservative ranges to provide a mitigation of vibration e?ects.Table3illus-trates the measured PPV values and the blast design of these shots.The durations of these shots are realized about the half of the shortest duration of480ms shot-duration of the?rst stage(Table3).In these shots,the milliseconds electric caps (30ms)are used as initiation system due to short duration of the shots.Details of these experimental shots and the respec-tive quantities are illustrated in Fig.3c and Table4.
Special care is taken in central area of lower faces, directly beneath the upper face,in forming of the cut con-?guration.A minimal v-cut con?guration is chosen to avoid any kind of overbreak upwards to the upper face holes and any kind of disturbance of the loaded holes in upper face which could harm to the shot organization of upper face shot accordingly to the Q-system evaluations explained in Section 2.Therefore,total charge(17kg)
Table1
Q values and advance rates
Q ranges Description Recommended
Advance Rates(m) >1.0Poor and upwards63.0
0.4–1.0Very poor63.0
0.1–0.4Very poor63.0
0.04–0.1Extremely poor61.5
<0.04Extremely poor and Downwards61.5
Table2
Q-system parameters and calculation of Q value
Parameter Rating Comment
Rock quality
designation
(RQD)
30Poor
Joint set number
(J n)
12Three joint sets plus random
Joint roughness
number(J r)
1Smooth,planar
Joint alteration number(J a)6Strongly over-consolidated,nonsoftening clay mineral?llings(continuous<5mm in
thickness)
Joint water
reduction
number(J w)1Dry excavations or minor in?ow,e.g.5l/ min locally
Stress reduction factor(SRF)7.5Multiple shear zones in competent rock
(clay-free),loose surrounding rock
(any depth)
Q=(RQD/J n)?(J r/J a)?(J w/SRF)=(30/12)?(1/16)?(1/7.5)=0.055.
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and speci?c drilling(2.290m/m3)values of upper face shots are bigger than lower face values(15.69kg and1.960m/ m3;Table4).By doing so,good performances are obtained (percentage pull0.88to0.95;Table4)and no mis?res are observed.
Each shot carried out at about13:30PM is monitored to study the vibration e?ects in the residential areas using INSTANTEL three axial(transversal,vertical and longitu-dinal)seismographs.The instrument records PPV values ranging from0.005in/s(0.127mm/s)up to10in/s (254mm/s)at a frequency range of2–300Hz.The resolu-tion,sampling rate and accuracy are0.005–0.000625in/s (0.127–0.0159mm/s),1024samples/s and3%at15Hz, respectively.
The performance values such as‘‘speci?c drilling”,‘‘spe-ci?c charge”and‘‘percentage pull”of two kind of blasting works are listed in Table4and no signi?cant di?erences between these performance values are observed.
4.Results and discussions
Although the shots presented in this study along with those that are not presented are under the threshold limits of structural damage;these shots are noticeable in di?erent levels according to the subjective evaluation and tolerance level of persons.
For the evaluation of the human responses to these experimental shots,an evaluation group consisted of two
Table3
Summary of experimental blast vibration records
Shots No.Distance(m)Measured PPV-values People response
PPV(mm/s)Frequency(Hz)Duration of shots(ms)Figure Structural damage Response description Figure Full face shots
1-?s57.2516.185to>100480Fig.4b No damage Barely noticeable Fig.5 2-?s57.2511.472to881500–Distinctly perceptible Fig.5 3-?s57.259.365to833000–Distinctly perceptible Fig.5 4-?s57.257.2968to776000–Strongly perceptible Fig.5 5-?s57.25 4.5464to739000Fig.4a Severe Fig.5 Half face shots
1-ufs34.1711.739to>100330–No damage Not noticeable Fig.5 1-lfs19.037to73330–Not noticeable Fig.5 2-ufs35.6614.764to>100330–Not noticeable Fig.5 2-lfs16.847to>100330–Not noticeable Fig.5 3-ufs44.6117.139to64330–Not noticeable Fig.5 3-lfs25.843to>100330–Barely noticeable Fig.5 4-ufs39.2417.830to85330–Not noticeable Fig.5 4-lfs26.7>100390–Barely noticeable Fig.5 5-ufs39.6114.237to>100270–Not noticeable Fig.5 5-lfs17.0>100330–Not noticeable Fig.5 6-ufs25.6611.134to51330Fig.4d Not noticeable Fig.5 6-lfs23.151to>100330Fig.4c Barely noticeable Fig.5 7-ufs26.3513.140to87330–Not noticeable Fig.5 7-lfs24.555to>100330–Barely noticeable Fig.5 8-ufs22.1514.159to>100330–Not noticeable Fig.5 8-lfs23.855to74330–Barely noticeable Fig.5?s:full face shot,ufs:upper face shot,lfs:lower face
shot.
Fig.3.Firing sequences of the shortest(a)and the longest(b)duration shots of full face shots and half face shots(c).
C.Kuzu,E.Guclu/Tunnelling and Underground Space Technology xxx(2008)xxx–xxx5
researchers and tunnel geologist is organized.This group discusses with people who are known as sensitive or the most problematical from former shots.A classi?cation for di?erent levels of vibrations is described by this‘‘eval-uation group”as‘‘severe;strongly perceptible,distinctly perceptible,barely noticeable and not noticeable”(Table 3).
The results of the?rst stage(full face shots)reveal that the PPV values of shots with the longest durations(espe-cially the shot with9000ms duration)are small enough for a person to tolerate.But,the reaction of people to ground vibrations generated after the long-duration-shot is enormous.This is best exempli?ed with the shot of 9000ms duration rated as‘‘severe”,as shown in Table3. The reason for this high human annoyance level is attrib-uted to long duration of the test shot and the blast vibra-tions.The seismic records and their analyses based on DIN4150standard(Anonym,1999)are presented in Fig.4.Among the other standards,DIN4150is the most restrictive one,which actually intends to minimize percep-tions and complaints(Siskind,2000).In these analyses,the threshold level(L2)for‘‘good condition buildings”are used according to the structural conditions observed in the region.On the other hand,PPV values of the shot with the shortest duration(especially the shot with480ms dura-tion)also from full face shots are higher than those of the long-duration-shot.But,the people’s reaction to ground vibrations is tolerable and this shot provides a reasonable comfort.The results of this stage can be summarized as follows:
The shots with short delay period have the longest dura-tion and the most unfavorable vibration e?ects due to the long duration of vibrations as it is seen from the seis-
mic monitoring data in Fig.4a.This shot is organized such that the quantity of individually detonating explo-sives is minimized while the duration of the shot is max-imized.People responds to the vibration duration of approximately10s as to be severe,although low PPV amplitude of4.54mm/s is de?nitely below the threshold level of any structural damage.
The shots with long delay period have the most favor-able vibration e?ects,as shown in Fig.4b.This shot is organized such that the amount of individually detonat-ing explosives increases and the duration of shot decreases.The duration of shot vibrations is reduced down to about0.5s(480ms)without any people annoy-ance and any other unwanted reactions although the PPV value is increased to16.1mm/s that is about3.5 times greater than the PPV value of the shot with short delay periods.On the other hand,the PPV value of
16.1mm/s still remains below the threshold value of
structural damage.
Decreasing the shot duration from9000to480ms (approximately19times)mitigates people annoyance while increasing the co-operating charges from3.088 up to9.264kg;although this causes an increase in the PPV values,they are still below the threshold level for creating structural damage.The tolerance of people,in this case is excellent,with a rate of‘‘barely noticeable”
as illustrated in Table3,despite the PPV value of about 3times greater than the shots with longer duration.
The second stage of the work involves further mitigation of the vibration e?ects.The shot durations are thus short-ened to investigate the people responses due to vibration level and vibration durations.The results of this stage can be summarized as follows:
Table4
Summary of experimental blast properties
Properties Full face shots Half face shots
Shot with the shortest duration(480ms)Shot with the longest
duration(9000ms)
Lower face shots Upper face shots
Hole length(m) 1.7 2.3 1.7 1.7
Hole diameter(mm)43434343 Number of holes44482518 Explosive Emulsion(cartridged)Emulsion(cartridged)
Size of explosive(mm)34?40034?40034?40034?400 Weight of explosive(kg)0.3860.3860.4360.436 Charge weight per hole(kg)
Cut holes3?0.3862?0.3862?0.4361?0.436 Floor holes3?0.3863?0.3862?0.436–
Holes next to?oor holes3?0.3862?0.386––
Wall and roof holes2?0.3862?0.3862?0.4362?0.436 Stoping holes2?0.3862?0.3862?0.4361?0.436 Max.co-operating charge(kg)9.264 3.088 1.744 1.744 Speci?c charge(kg/m3)0.9700.9150.9320.820 Speci?c drilling(m/m3) 2.026 2.026 1.960 2.290 Total charge(kg)36.28446.3215.6917 Percentage pull0.88–0.900.90–0.920.88–0.920.95
6 C.Kuzu,E.Guclu/Tunnelling and Underground Space Technology xxx(2008)xxx–xxx
Fig.4.Seismic records of experimental shots (a,b;for full face,c,d;for half face shots).
C.Kuzu,E.Guclu /Tunnelling and Underground Space Technology xxx (2008)xxx–xxx
7
The duration of shots is varied between 270to 330ms (Table 3).The PPV values are below the threshold level for creating structural damage and they are still as small enough as PPV values of 480ms full face shot (Fig.4c and d).Decreasing the shot duration from 270to 330ms (approximately 27–33times)mitigates people annoyance while decreasing the co-operating charges to 1.744kg.
There are sometimes malfunctions in blasting process caused by detonators or spacing of the blast holes,i.e.bad hole alignment and deviation in strongly jointed rock masses.These bring up the PPV magnitudes as seen from the same vibration records.Otherwise,the PPV magnitudes are as favorable as the shot organized in the ?rst stage (480ms full face shot).
In lower portion shots relatively high PPV magnitudes are encountered due to the con?nement of this tunnel part (Table 3).But the people responses are more bene-?cial compared to the ?rst stage shots.Especially,the majority of upper part shots have no negativenesses regarding people reactions and are rated as ‘‘not notice-able ”.A common presentation which summarizes ‘‘the accompanying e?ect of PPV and shot duration on the level of people annoyance ”is given in Fig.5.
5.Conclusions
An application is tried and people’s annoyance,i.e.peo-ple responses to the tunnel vibrations,are investigated based on arranging technological conditions in favor of minimizing the PPV values of blast induced vibrations without any performance loss.The experimental shots in the tunnel and the blast induced vibration measurements are carried out in an urbanized area mainly in 3–6storey residential structures located about 60–22m over the tun-nel axis.The approach used in this study is based on divid-
ing the total required explosives into small amounts that are placed in di?erent holes and detonating sequentially at speci?c time lags.This approach eliminates the struc-tural damage,but in return,increases the duration of shots and people annoyance.Therefore,an optimization of the blast duration with an appropriate time sequencing is practiced based on PPV monitoring at di?erent experimen-tal blasts of full face and half face blasting rounds.The results demonstrates that,in their site speci?c conditions,the shots with shorter durations are most tolerable shots against long duration shots especially with respect to the people annoyance.Acknowledgements
The author acknowledges the support of the Alke-Atac Musterek Ortakligi Company and would like to thank the ?eld supervisor and workers for the e?orts to complete the experiments in this work.References
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Fig.5.The accompanying e?ect of PPV and shot duration on the level of people annoyance.
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Cengiz Kuzu obtained his MSc from Istanbul Technical University in 1992and PhD from Berlin Technical University in1995.He worked as mining engineer in Turkish Hardcoal Enterprises.He is currently working as an associate professor of mining engineering at Mining Engineering Department(ITU).His main research areas are blasting optimization and environmental e?ects of mining.
Erim Guclu Holds the degrees of BSc at the year of2004in mining engineering from Istanbul Technical University.He is working as a research assistant at Mining Engineering Department(ITU)where he is currently a MSc student.His main research areas are underground blasting and its environmental e?ects.
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