IMPACT ASSESSMENT OF BLASTING ON VULNERABLE LANDSLIDE AREA. Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak

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1 50 th IMPACT ASSESSMENT OF BLASTING ON VULNERABLE LANDSLIDE AREA Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak ABSTRACT Ground vibrations caused by blasting operations is defined as the velocity of particles within the ground resulting from vibratory motion. The intensity of ground vibration is measured in units of peak particle velocity, generally, millimetres per second (mm/s). Shock waves (energy from the detonation) radiate outwardly during any explosion and the material adjacent to the source gets crushed. A part of the energy is used in fracturing and displacement of ground (approximately 20-30%) while the remaining part of the energy dissipates in the form of ground and air vibrations (concussion). Under typical conditions, blasting vibrations intensity diminishes with distance, at a rate of about one third of its previous value each time the distance from the vibration source is doubled. Hence, extent of damage to the adjacent structures depends on the distance from the blast source and the intensity of the explosion. At dam site of Punatsangchhu-I H.E. Project, Bhutan, a landslide occurred on the right bank slope during excavations by blasting at dam site. Accordingly, apart from the strengthening measures of the vulnerable slide mass, blast vibration studies were also carried out to optimize the blast design. This paper presents the impact assessment on vulnerable right bank slope mass during blasting carried out at dam site on the left bank of Punatsangchhu-I hydroelectric project in Bhutan. Vibration limits recommended by various codes and guidelines have also been discussed. Considering peak particle velocity (PPV) of 5 mm/sec as the threshold values, vibrations have exceeded at three locations. In the present study, distances of the monitoring points from the blast location through ground were approximated from the cross sections drawn at upstream and downstream of dam axis. Blast vibrations were monitored at different locations/distances and using variable quantity of explosives. In the event of data containing variable distance, direct correlation between PPV and quantity of explosives is not feasible. In order to develop the correlation between PPV, distance and quantity of explosives, concept of scaled distance was utilized. Correlation co-efficient for scaled distance was derived as d/w 1/2.2 (m/kg 1/2.2 ) through optimization (d = distance of monitoring point from blast location approximated along the ground/rock line in m and w = total quantity of explosive used, kg)

2 Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak PPV (mm/sec) y = x Scaled Distance (m/kg 0.45 ) Peak Particle Velocity (PPV) v/s scaled distance Further, it is assumed that total quantity of explosives works as single source of explosion. Additionally, the distances are approximated based on the desktop studies. The plot between PPV (m/sec) versus scaled distance (m/kg 0.45 ) was drawn and the regression curve was fitted as shown in Figure above. The best fit correlation between PPV and scaled distance was obtained as given in the following equation : PPV = x (d/w 0.45 ) Correlation Co-efficient (R) = 0.55 Keywords: Ground Vibrations, Peak Particle Velocity, Air Overpressures, Scaled Distance Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak; haridev65@gmail.com

3 50 th IMPACT ASSESSMENT OF BLASTING ON VULNERABLE LANDSLIDE AREA Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak Central Soil and Materials Research Station, New Delhi ABSTRACT: Ground vibrations caused by blasting operations is defined as the velocity of particles within the ground resulting from vibratory motion. The intensity of ground vibration is measured in units of peak particle velocity, generally, millimetres per second (mm/s). Shock waves (energy from the detonation) radiate outwardly during any explosion and the material adjacent to the source gets crushed. A part of the energy is used in fracturing and displacement of ground (approximately 20-30%) while the remaining part of the energy dissipates in the form of ground and air vibrations (concussion). Under typical conditions, blasting vibration intensity diminishes with distance, at a rate of about one third of its previous value each time the distance from the vibration source is doubled. Hence, extent of damage to the adjacent structures depends on distance and intensity of the explosion. At dam site of Punatsangchhu-I H.E. Project, Bhutan, a landslide occurred on the right bank slope during excavations by blasting at dam site. Accordingly, apart from the strengthening measures of the vulnerable slide mass, blast vibration studies were also carried out to optimize the blast design. This paper presents the impact assessment on vulnerable right bank slope mass during blasting carried out at dam site on the left bank of Punatsangchhu-I hydroelectric project in Bhutan. Vibration limits recommended by various codes and guidelines have also been discussed. INTRODUCTION Three types of waves viz. compressive, shear and surface are generated through excitation. Three perpendicular components of motion namely longitudinal, vertical and transverse must be measured to describe the motion completely. The longitudinal component, L is usually oriented along a horizontal radius to the explosion followed by other two perpendicular components i.e. vertical, V and transverse, T to the radial direction. The three main waves can be divided into two types; one is body wave which propagates through the body of the rock and soil and second is surface wave, which is transmitted along a surface (usually the upper ground surface). The most important surface wave is the Rayleigh, denoted by R. Body waves can be further subdivided into two categories compressional or tension or sound-like waves denoted as P-wave and distortional or shear waves denoted as S-wave. Excitations produce predominantly body waves at small distances. These body waves propagate outward until they intersect a boundary such as another rock layer, soil or the ground surface. At this intersection, surface waves are produced. Rayleigh surface waves become important due to large transmission distances. At small distances all three wave types will arrive together and complicate wave type identification, whereas, at large distances, more slowly moving shear and surface waves begin to separate from the compressional wave and allow identification. The pattern of motion depends upon the nature of transmitting media (soil or rock). Due to motion of waves resulting from explosion, structures built on or in soil (or rock) will be deformed differently. The longitudinal (compressional) wave produces particle motions in the same direction as it is propagating, whereas, the shear wave produces motions perpendicular to its direction of propagation i.e. either horizontal or vertical. The Rayleigh wave produces motion both in the

4 Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak vertical direction and parallel to its direction of propagation. Vibration monitoring includes ground vibration (transverse, vertical and longitudinal ground vibrations) and air overpressure. Transverse ground vibrations agitate particles in a side to side motion. Vertical ground vibration agitates particles in an up and down motion. Longitudinal ground vibrations agitate particles in a forward and backward motion progressing outward from the event point. Events also afford air pressure by creating air blast. By measuring air pressures we can determine the effect of air blast on structures. The Peak Vector Sum (PVS) of particle under vibration is defined as: PVS = (T 2 +V 2 +L 2 ) 1/2 (1) Where, PPV = Peak Particle Velocity T = Peak Particle Velocity (PPV) measured by transverse geophone V = PPV measured by vertical geophone L = PPV measured by longitudinal geophone Peak Vector Sum is calculated for each point of the sampled waveforms and gives the largest value. This PVS is not necessarily the peak particle velocity for an individual wave form. Wave propagation phenomena were first investigated by Morris [1] and his principles have been refined ever since to attempt to determine peak particle velocity (PPV). Maximum allowable limit of peak particle velocity (PPV) within a frequency range varies worldwide. For example, in Japan the permitted vibration amplitude has to be between 0.5 and 1.0 mm/s in residential areas, whereas, in New Zealand anything below 5 mm/s is acceptable. The present paper presents the impact of blasting operations carried out along the left bank at the dam pit on the rock mass vulnerable to slide on right bank of Punatsangchhu-I H.E. Project, Bhutan. Blast vibration monitoring was carried out at different locations along the slide prone mass. THE PROJECT Punatsangchhu-I hydroelectric project comprises of a 130 m high roller compacted concrete gravity dam across river Punatsangchhu. The extent of excavation is of the order of more than 70 m below rived bed level. Excavations were being carried at the dam site for removal of river borne materials and loose rock mass for laying the foundation of the concrete gravity dam on the firm bed rock. GEOLOGY OF DAM AREA Left Bank - The left bank along the river line is occupied by hard, fresh and blocky quartzofeldspathic gneiss and its variants are exposed to the ridge top. The strike of the foliation varies from N20 0 E - S20 0 W to N40 0 E- S40 0 W dipping at SE into the hill. The rocks on the left abutment are traversed by four prominent sets of joints, major vertical fractures cutting across the hill and minor shears. The valley ward dipping joint (J2) is very prominent, steep and controls the configuration of the left bank abutment. The vertical fractures are developed along the joints dipping NNE and NW upstream dipping. Right Bank - At right bank, the exposures are restricted. On the right bank of dam, the ongoing excavation for stripping has revealed very limited exposure of rock above cable car bench (EL.1260 m) from U/s 50 m to D/s 100 m of dam axis. The area in the upstream and downstream of this rock ledge comprises thick overburden/hill wash debris. This available rock ledge exposes jointed and blocky quartzo feldspathic gneiss, which is fresh in the middle portion and slightly to moderately weathered (W2-W3) in the upstream and downstream extremities. The general strike of the foliation varies from N10 0 E S10 0 W to N40 0 E- S40 0 W dipping at SE. However, the rocks on the right bank exhibit tight S- shape folding, which has disturbed the normal foliation thus showing rolling dips at many places. The exposed rocks on the right abutment are traversed by four prominent sets of joints and minor shears. Wide topographic depressions filled with thick overburden/slide debris are noticed in the Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak; haridev65@gmail.com

5 N 80 E S 80 W PDH ROAD ROAD SINKING LINE Grouting Gallery SZ DHM-7 DHM Rock mass DHM - 22 DHM yet to expose N N N E E Fol= ( 7 )/N th downstream and upstream portions of the rocky outcrop. The geological section of dam axis is shown in Fig. 1. LEFT BANK CABLE CAR BENCH -0 J.N70 E-S70 W/70 NW(25 ) NATURAL SURFACE PROFILE FOL.N20E-S20 W/20 SE(17 ) J.N50W-S50 E/75 NE(70 ) Design profile 1 2 DH-20(P) DH-26(P) RIVER EDGE (RIGHT BANK) DH-25 DH Fig. 1: Geological section of dam axis THE PROBLEM EXCAVATED PROFILE BEFORE SLIDE PDH- 35(CAN NOT BE PROJECTED) DH-30(CAN 10 NOT BE 11 PROJECTED) EXCAVATED PROFILE AFTER SLIDE DHM-15 NATIONAL HIGHWAY J = (70 )/N PDH-33 (PROJECTED) PDH-40 (CANNOT BE DHM-8 PROJECTED WRT SHEAR ZONE) FAULT / SHEAR ZONE (40 (16 )/N330 ) 40 (16 )/N330 C B A J.N25 W-S25 E/75 NE(72 ) REDUCED DISTANCE (m) SECTION 0-0' DHM-11(PROJECT ED) GABBION WALL On completion of excavation and dressing of slopes for right abutment blocks No. 14 and 13 up to EL 1110 from the river bed level of 1151 m, benching down of block no. 12 was under progress when movement and subsidence of right abutment from 150 m u/s to 140 m d/s of dam axis between EL 1110 m and 1400 m occurred. The probable reason of slide was attributed to shear zones. During benching down, removal of toe material on the right bank in block nos. 12 and 13 (Fig. 1) triggered the movement of geological mass along the shear zone. Due to the geological situation the right abutment of the dam is prone to sliding along various shear planes [2]. Figure 2 shows the extent of slide on the right bank. Excavation in the river bed was immediately stopped. Investigation and instrumentation in the affected right bank slope was started to assess the damages and to suggest the remedial measures. Inclinometers were installed at critical locations in consultation with the designers in the slide area to monitor the movement of ground. Optical targets at critical locations were also installed to monitor the surficial movement. RETAINING WALL (31-36 )/N50 SINKING ZONE (COLONY ROAD) SINKING ZONE (COLONY ROAD) SINKING ZONE (COLONY ROAD) DHM-1 (CANNOT BE PROJECTED WRT SHEAR ZONE) ROCK LINE RIGHT BANK -0' Fig. 2: Extent of slide and imposed dam section Cement grouting was started as immediate rehabilitation measure to strengthen the damages/displaced mass. Right bank slope was unloaded by removing some overburden mass and creating more benches. Cement grouting, micro piling and cable anchoring was initiated at various levels as the long term stabilizing measure. Reinforced concrete piles of 2.0 m diameter are also being provided at different levels for strengthening of vulnerable slope. Excavation is still required for laying the foundation of concrete gravity dam which will be taken up after completion of strengthening measures. In view of the nature of slide mass, excavation using blasting may further trigger the landslide. Hence, it was decided to complete the restoration measures. After sizeable rehabilitation measures, controlled blasting was allowed along the left bank abutment and simultaneously, intensity of blast and ground vibrations were monitored by CSMRS for a period of one month to assess the impact of blasting. BLAST VIBRATION STUDIES After stabilization of right bank slope and completion of restoration measures, controlled blasting was permitted along the left bank and

6 Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak monitoring of vibrations was suggested to be carried on the right bank slope to study the impact of blast on the vulnerable slide mass. Typical locations such as right bank toe in dam pit area, grouting gallery, national highway level, cable car bench and colony area were chosen as monitoring points. Real time monitoring of ground and air vibrations were monitored. Peak particle velocity and air overpressures resulting from blast vibrations were recorded at various locations. The data pertaining to blast such as total quantity of explosives and blast holes were also obtained from the agency responsible for construction of dam. A correlation between the quantity of explosives, distance and peak particle velocity was arrived at through regression analysis. Blast vibration monitoring was carried out at 18 locations on the right bank of dam site focusing on the vulnerable slide mass. A total of 49 blast vibration data was recorded. Blast locations and ground motion monitoring points have been depicted in Fig. 3. Installation of instrument and monitoring of vibrations is shown in Figs. 4a and 4b. Circular No: 7 of 1997). Typical blasting limits for various types of buildings in different countries have been described by Roy [10]. Fig. 3: Blast locations and ground vibration monitoring points SAFETY CRITERIA Ground Vibration: A small part of the blast energy is utilised for breakage and displacement of the rock mass, the rest of the energy accounts for ground vibrations, air blasts, noises, back breaks, flyrocks, dusts etc. [3-6]. The structural damages produced by ground vibration are commonly correlated with the peak particle velocity and safety criteria are suggested accordingly. However, the mechanism of damage cannot be explained only in terms of the peak particle velocity. Persson [7] developed the damage criteria for Swedish hard rock. Li and Huang [8] discussed damage criteria for rock tunnels with slight, medium and serious damage conditions. Director General of Mines Safety [9] has laid out permissible levels of vibration at the foundation level structures when carrying out blasting operations in mining areas (DGMS (Tech) (S&T) a) National Highway Location b) Grouting Gallery Location Fig. 4: Blast Vibration Monitoring in Progress at Right bank of Dam site area. Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak; haridev65@gmail.com

7 50 th Maximum allowable peak particle velocity for the RCC frame structures, brick/plastered houses and mud houses as per DGMS [9] and Bureau of Indian Standards (IS 14881:2001) [11] criteria are shown in Fig. 5. of 19 mm/s (Table 1). These limits are presented below as function of instrument s frequency weighing scales as different sound weighing scales are employed by different monitoring instruments. Most cases of broken glass are reported to have been observed at air over pressures of db (measured with a linear transducer). Table 1: Table showing BIS safety criteria for air overpressure Lower Frequency Limit of Maximum measuring System (Hz 3 db) Levels (db) 0.1 or lower flat response 134 peak 2 or lower flat response 133 peak 6 or lower flat response 7 peak RESULTS AND DISCUSSIONS a) DGMS Safety Criteria b) BIS Safety Criteria Fig. 5: DGMS and BIS safety criteria for ground vibrations Air Vibration (IS 14881: 2001): Limits are based upon wall response necessary to produce strains equivalent to those produced by surface coal mining induced ground motions with peak particle velocity The vibration recording instrument was set to record the vibration in the normal range of geotrigger level starting 0.50 mm/sec to 254 mm/sec and Mic trigger level ranging from 100 to 148 db (L). All the records were individually viewed and interpreted. The recorded PPV and air blast (air overpressure) was compared with the general guidelines set by the DGMS and BIS. Permissible limits of PPV being followed worldwide were also referred. The guidelines for permissible limits of PPV and air blast are generally for the different kinds of buildings including monumental structures. However, Environmental guidelines [12] by Department of Natural Resources and Environment Minerals and Petroleum, Victoria suggests limits for PPV and airblast (Air over pressure) as 5 mm/sec and 115 db, respectively for sensitive sites (new sites). In the present study, distances of the monitoring points from the blast location through ground were approximated from the cross sections drawn at upstream and downstream of dam axis. The distances, quantity of explosives and the corresponding PPV pertaining to various blasts are given in given Table 2.

8 Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak Blast Location along Left Bank Dam, EL 1097 m Dam, EL 1095 m Dam, EL 1097 m EL 1095 m EL 1097 m Power Intake, EL 1120m Power Intake, EL 1120 m Dam, EL 1096 m. EL 1095 m Left Bank, EL 1096m Left Bank dam EL 1105 m EL 1105 m EL 1105 m Table 2: Blast vibrations data Monitoring Point on Right Quantity of PPV Air Over Bank Explosives, mm/se Pressure Location Approximated Distance, m kg c NH, EL m Dam Pit, > 148 db EL 1115 m Grouting > 148 db Gallery, EL 1168 m Bjimthong Colony EL 1330 m Cable car bench, EL 1260 m Grouting Gallery, EL 1168 m Dam Pit, EL 1115 m Bjimthong Colony EL 1330 m Cable car bench, EL 1260 m Right bank, NH, EL 1217 m NH, EL 1221 m NH, EL 1227 m NH () EL 1217 m PHEP-I Colony, EL 1375 m Power Intake NH, EL 1226 m Below NH, EL 1212 m EL 1120 m, 1100 m, 1205 m & 1216 m Power Intake EL 1115 m EL 1205 m and 1119 to 1123 m Near Plunge Pool, EL 1135 m Cable car bench, EL 1265 m > 148 db Blast vibrations were monitored at different locations/distances and using variable quantity of explosives. In the event of data containing variable distance, direct correlation between PPV and quantity of explosives is not feasible. In order to develop the correlation between PPV, distance and quantity of explosives, concept of scaled distance was utilized. Correlation co-efficient for scaled distance was derived as d/w 1/2.2 (m/kg 1/2.2 ) through optimization (Where, d = distance of monitoring point from blast location approximated along the ground/rock line in m and w = total quantity of explosive used in kg). Hence, a plot between PPV and scaled distance was drawn as shown in Fig. 6. PPV (mm/sec) y = x Scaled Distance (m/kg 0.45 ) Fig. 6: PPV v/s scaled distance Further, it is assumed that total quantity of explosives works as single source of explosion. Additionally, the distances are approximated based on the desktop studies. The plot between PPV (m/sec) versus scaled distance (m/kg 0.45 ) was drawn and the regression curve was fitted as shown in Figure 6. The best fit correlation between PPV and scaled distance was obtained as given in equation 2 (with correlation coefficient of 0.55): PPV = x (d/w 0.45 ) (2) Air overpressure has generally exceeded the permissible values as per IS 14881: At occasions, the instrument has indicated out of range values which means that the intensity air blast was beyond 148 db. Hari Dev, Birendra Pratap, Rajbal Singh and Shashank Pathak; haridev65@gmail.com

9 50 th CONCLUSIONS Safe PPV is project specific and it depends upon the topography, geology, blasting practices etc. Maximum PPV of 24.5 mm/sec was recorded at dam pit (right bank). The quantity of explosives corresponding to the maximum PPV was of the order of 1383 kg (969 blast holes). The monitoring point was very near to the blast location approximately 112 m. Considering peak particle velocity (PPV) of 5 mm/sec as the threshold values, vibrations have exceeded at three locations. Air overpressure has exceeded the permissible limits in most of the blasts with some of the data exceeding 148 db also. Best fit correlation between PPV and scaled distance was obtained as PPV=26.032x(d/w 0.45 ) with correlation co-efficient (R) as Due to approximation in distances and limited data, present attenuation law is of limited utility for preliminary safe blast design. Proposed correlation may be further validated with more data. Ground vibrations must be monitored during blasting at right bank for necessary modifications in the blast design, charge, pattern and delay etc. in order to arrive at safe blast design. ACKNOWLEDGEMENTS: The blast vibration monitoring work was possible due to cooperation and active participation of WAPCOS Ltd. REFERENCES [1] MORRIS, G., Vibrations due to blasting and their effects on building structures. The Engineer, 394/95; [2] Peter Jewitt (2014), Interim Report (PS02), Geological Aspects related to the Sliding on the Right Abutment, 30 January. [3] Bajpayee, T.S., Rehak, T.R., Mowrey, G.L., and Ingram, D.K. (2004) Blasting injuries in surface mining with emphasis on flyrock and blast area security, J Safety Res, Vol. 35, pp [4] Hagan, T.N. (1973) Rock breakage by explosives, In: Proceedings of the national symposium on rock fragmentation, Adelaide, pp [5] Singh, D.P., Singh, T.N. and Goyal, M. (1994) Ground vibration due to blasting and its effect, In: Pradhan, G.K., Hota, J..K, editors, ENVIROMIN, Bhubaneshwar, India, pp [6] Wiss, J.F. and Linehan, P.W. (1978) Control of vibration and air noise from surface coal mines III. Report no. OFR 103 (3) 79, Bureau of Mines, US, pp [7] Persson, P.A.: The relationship between strain energy, rock damage, Fragmentation, and throw in rock blasting. International Journal of Blasting and Fragmentation 1 (1997), pp [8] Li, Z. And Huang, H.: The calculation of stability of tunnels under the effects of seismic wave of explosions, In : Proceeding of 26th Department of Defence Explosives Safety Seminar, Department of Defence Explosives Safety Board, [9] Director General of Mines Safety (DGMS), permissible levels of vibration at the foundation level structures when carrying out blasting operations in mining areas (DGMS (Tech) (S&T) Circular No: 7 of 1997). [10] Roy Pijush Pal, Rock blasting (2005): effects and operations, A.A. Balkema, Rotterdam, [11] IS 14881: 2001, Method for Blast Vibration Monitoring Guidelines. [12] Environmental Guidelines (2001), Ground Vibration and Airblast Limits for Blasting in Mines and Quarries, Department of Natural Resources and Environment, Mineral & Petroleum Victoria, 2001.