Article_Impact of Blast Induced Vibrations on Buildings

Reporton impacts of blasting activities Nakatooke quarry. An Assessment of the baseline survey. in the vicinity of ......................................................... 3 2................ 12 6....................................................................................................................... 5 3........................................... SAFE OF LEVELS OF AND AIR BLAST ........... 12 7.................................................................................. EARTH AS A BUILDING MATERIAL: ...............1 Vibration Readings 01 ...... 5 3......................1................................2 Vibration Readings 02 ............................................................... APPENDIX-1 :RECORDS OF VIBRATION MONITORING..1 Existing Recommendations ......... 4 3............................ 8 4..............................1..................................... IMPLEMENTATION OF LIMITS .................... 16 ............................................................. RECOMMENDATIONS................... 14 9................................................................................................................. 13 6..........................1.........................................1....... THE GROUND VIBRATION 3......... 7 GROUND VIBRATIONS AND AIR BLAST............ CONCLUSIONS ................................................................... 13 8........................................................................1..................................... REFERENCES ............................ THE NATURE OF THE BUILDING MATERIAL ................1........................... THE BLAST AND ITS INTERACTION WITH STRUCTURES ............................ 11 5................ 3 1.......................... APPENDICES ....1......................................................................................................................................................................................................................................................................... 3 1............................4................. WHY THE ASSESSMENT? ..................................2 Recommendations for Isimba quarry site....... 14 9............................................................... .............. 13 9.........................2......................................1......................................... 14 9.................................................................3......... INTRODUCTION ........ 12 7.......................2.............. ABSTRACT ........................... ACTIONS TAKEN ................ MEASUREMENT 3........................................... FINDINGS -VIBRATION MONITORING RESULTS .................. 11 6.............. 6 DAMAGE POTENTIAL ...............................................................................................................1..... .................................... which was carried out within the framework of conditions that take into account standardized structures materials such as burnt clay bricks built with the or concrete blocks. the most common structures within rural communities are mud and wattle constructions. This was attributed to the differences in the amounts . The aim of this article is to assess the impact of blasting activities on the structural Uganda. In Uganda. The structures assessed are located within a 2 km radius of the Nakatooke quarry located in Kayunga district.2. forms. sand and occasionally. the need for an assessment of the performance of mud and wattle structures under the cited conditions cannot be understated since blasting activities can have a significant vibration output that has been known to damage structures. integrity The report of also mud and covers wattle structures recommendations vernacular to operation of for machinery.1.1. Introduction 1. Owing to the fact that loam is not a standardized construction material. During the initial survey it was noted that the structures varied in terms of the characteristics. conventional However. earth is referred to as loam. The aggregates were excavated by blasting. Earth as a building material: In scientific terms. 1. It is a mixture of clay. the need for an analysis of its performance has been brought about by the opening up of quarries for large scale industrial constructions. In light of this. larger aggregates. these compressed being unbaked handmade bricks (soil blocks) or compacted within formwork (rammed earth). information on its performance as a construction material has been scarce. In order to assess the performance of the subject structures. Abstract The purpose of this report is to assess the impact of the Construction and blasting activities at Nakatooke quarry on the structures of the residents in the vicinity of Isimba HPP and to make recommendations aimed at mitigating or limiting the possibility of damages due to operation of heavy machinery or blasting activities. However. the performance of mud and wattle structures does not seem to have been factored into compliances the required framework for the of considerations attainment of that permits govern for the blasting activities. It is used all over unbaked bricks the world (mud in several bricks/adobes). a baseline survey was carried out on the structures within a 2 km radius of the blasting epicenter so as to assess their initial conditions. silt (very fine sand). 2. the use of mud and wattle. shifted the and blasting this does the local activities. getting the local communities to temporarily moved further from the epicenter of the blasting activities has not been easy to accomplish. the homes of the locals are mostly mud and wattle and as such require an independent assessment of the impacts of the blasting activities. Despite incentives by the EPC Contractor. These blasting activities carried out at the quarry are approved and a license for the activity has been obtained by the Contractor. With Projects of this scale and magnitude. Why the assessment? The need for the assessment stemmed from the fact that the areas in the vicinity of communities the for quarry whom were the previously populated by only option in terms of several building rural material was and still is. These Project Affected People were compensated and the land was handed over to the Contractor for commencement of works on the Construction of Isimba Hydro Power Plant. A simple disturbance allowance does little or nothing damaged by the blast waves.and types of workmanship clay as used well as for the the construction. This is because mud and wattle is the most abundantly available building material. cannot impacts be of However. there are stringent measures put in place to protect the interests of the local communities. The number of households was just too numerous and the locals will still have to return to their homes after the blasting activities are completed. the adobes. Whereas these may not necessarily damage the concrete block buildings of the Contractor and the Client. soil it blocks or rammed earth. be content. where the clients offices are situated. requiring little or no technology for processing. However the communities adjacent to the Project boundaries are still impacted by the blast waves that occur due to the detonation of between 500 and as much as 3500 Kg of ammonium Nitrate emulsion explosives. This lead to a significant population 675 within the proximity of the dam area alone. The blasting activities have been noted to propagate the effects of the blast wave as far as 5 km away. the permit conditions only oblige the permit holder to clear a 300m radius within the vicinity of the blast epicenter. However. form of the silt construction. to restore or maintain a structure . Community sensitization has been conducted to inform the locals of the potential impact of the blasting activities and of the notice and warning system that not change communities warns the of fact will imminent that have to blasting structures bear the activities. The nature of the building material It has been observed that the impacts of the blasting activities are not so easily ascertained mainly due to the varying degrees of workmanship during the construction of the mud huts.3. However. The blast and its interaction with structures 3. Given the potential for damage to property and the substantial nuisance caused to the local population it is vital to have the impacts quantified so as to determine limits and mitigation measures where applicable and possible. The damage to property can be caused directly by ground wave movements or indirectly via potentially unstable soil or rock conditions in the vicinity of the quarry site (e. it will be composed of differing amounts characteristics. and the application may also and to alter know these. there have been claims that some brick houses have developed cracks as a result of the blasting at the quarry. The linear and to achieve workability). Shrinkage can be minimised by reducing the clay and the water content. shrinkage it was of said the that soil the as well as the pre-existing poor water damage was worsened by the impacts of the blasting activities. and by using additives. to the However. from a site specific characteristics additives.4% and 2% with drier mixtures (used for rammed earth. by and types may correct to judge applying of clay. preparation differ. Loam is not a standardised building material Depending on the site where the loam is dug out. 3. 2. the when specific composition of the loam involved. soil liquefaction. In view of the claims the EPC Contractor has been requested to conduct vibration monitoring at varying distances from the epicenter of the blasting activities. Loam is not water resistant Loam must be protected against rain and frost especially in its wet state. the order necessary. some of the structures had evident pre-existing damage most likely due to poor workmanship and possibly due resistance. shrinkage ratio is usually 12% with wet mixtures (such as those used for mortar and mud bricks). Its site. compressed soil blocks). In addition.1. differ mix its or silt.g. one and needs to aggregates. The above stated have made it difficult to determine the actual impact of the blasting activities. and between 0. by optimising the grain size distribution. slope . of In therefore. Loam mixtures shrink when drying Due to evaporation of the water used to prepare the mixture (moisture is required to activate its binding strength shrinkage cracks between 3% and will occur. The main inherent difficulties with regards to working with adobe are: 1. records of vibration monitoring will be used to assess the impact and determine measures of mitigation if need be. Consequently. mechanisms of breakage fragment the rock mass (crushing. This is caused by the sudden acceleration of rock mass by the detonation gas pressure exerted on the hole wall. the strain waves are attenuated over distance since a fixed amount of energy gets spread over a larger mass of material with increase in distance. and K and B the site constants. The rest of the energy is propagated through the strain waves of lower intensity. 3.failure). R the distance between blast face to vibration monitoring point (m). radial cracking etc) and this impact is localized to the fragmentation zone. The following are some of the vibration predictor equations: where v is the peak particles velocity (mm/s). However. These waves in the elastic zone display visco-elastic behaviour and as such. The Ground Vibration and Air Blast The detonation of an explosive charge in a blast hole results in intense dynamic stresses around the blast hole. The Strain waves propagate through the medium as elastic waves. larger amounts of explosives can still result in the propagation of ground vibrations large enough to cause damage to structures by causing dynamic stresses that exceed the material strength. . unable to cause permanent deformation to the rock mass. which can be determined by multiple regression analysis. In view of the above. Air blast is not considered to be a significant factor in causing damage to structures but is a significant nuisance to the local communities in the vicinity of the quarry. a wave motion is set up in the ground as the strain waves Different are transmitted through the surrounding rock mass. oscillating the particles through which they travel. Q MAX the maximum charge per delay (kg).2. 5 50 2 Set initial Limit of 125 mm/s (5 in. Levels of ground vibrations are also determined by measurement of displacement or acceleration of a particle at the site. So as not to overlook the impact of human exposure to blast induced vibrations.3./s). underlying ground conditions and the building construction as well as the state of repair of the structure. Guidance on the levels of vibrations above which buildings could be damaged is mainly derived from BS 7385*. Measurement of Damage Potential Particle velocity is generally adopted worldwide as the best criterion for relating ground vibrations to building damage. displacement and acceleration: Eventually however. per sec) Maximum at the Crusher .(Reference 5). however. Typical damage that can be expected in relation to the threshold value of the peak particle velocity experienced in the ground waves from the blasts are indicated in table 1. some of which include the type of foundation. from which it is evident that the onset of plaster cracking in a house occurs at a threshold peak velocity of 50mm/s (2in. criteria other than the vibration levels may need to be considered. reference is made to BS 6472-2:2008. for detailed engineering analysis. the performance of a structure will depend on a multitude of factors.3. This criteria is universally accepted in North America. The following equations show the relationship between velocity. table 1: Type of Structure Type of Damage Rigidly Mounted Trip Out Mercury Switches Houses Plaster Cracking Peak Particle Velocity Threshold at Which Damage Starts mm s kv per sec 1225 0. Such as Possible Cracking of Cement Block 1500 60 Safe levels of Ground Vibrations and Air Blast. This report's recommendations for reducing ground vibrations and air blast levels are aimed at minimizing distress to people as well as avoiding damage to buildings. particle velocity is used as a parameter for damage assessment.4. These are shown in Table 2. The current acceptable levels of ground vibration from blasting are recommended as in Australian Standard AS 2187-983 (Reference 1). Loose Ground Mechanical Equipment Pumps Compressors Prefabricated Metal Building on Concrete Pads Shafts Misaligned Cracked Pads Building Twisted and Distorted 3. the limits recommended construction projects are quite conservative. vibratory rollers and traffic  Air blast. there are multiple sources of these vibrations and therefore the limits recommended depend on the vibration source as defined below:  Blasting  Pile Drivers. As mentioned in Section 3. Table 2 – acceptable levels of ground vibration from blasting Notes: .3.Cracks in Blocks Concrete Block as in a New House Cased Drill Holes Horizontal Offset Retaining Walls. It should be noted that at a Construction site such as Isimba HPP. 200 8 375 15 1000 40 Beyond 250 rwn/» (10 In per sec) Mafor Damage Starts. Since humans respond to levels of ground vibrations and air blast considerably lower than those necessary to induce structure damage. . a value of peak particle velocity other than that recommended in the table may be used. The actual results attached herein as appendix . A peak particle velocity (VRmax) limit of 5 mm/sec is therefore recommended. where substantiated by careful investigation. Ground vibrations caused by these sources are of a continuous nature usually lasting for extended time periods. It is shown in Table 4. Because of this. it is proposed that vibration limits should be set at lower levels than from blasting. In particular. the vibration readings are 13 mm/s. the vibrations can easily be brought down to well within more tolerable limits. With adjustments to the quantities of charge used. The peak particle velocities in the table have been selected taking no consideration of human discomfort and the effect on sensitive equipment within the building. Blasting: The control of blasting procedures to limit ground vibration levels to those outlines in Table 1 should automatically limit air blast overpressures to safe levels with respect to building damage. The proposed maximum levels are shown in Table 3 below. the limits recommended for buildings types 2 and 3 may cause complaints. In a specific instance. Vibratory Rollers and Traffic: The Construction Site has heavy machinery many of which could serve as a source of Vibrations.1 indicate for instance that at a distance of 400m. which is close to the range indicated in table 2.1. Tynan (Reference 2) contains a handy user guide applicable to vibrating rollers which approximates the recommended limit. it should be noted that the charge used was over 2000 kg. However. 2. Table 3 -maximum air blast levels Pile drivers. 1 Response of the Human body to mechanical vibration (Goldman.4 . Table 4b:Air Blast: Fig.Table 4a: user guide applicable to vibrating rollers * Values in brackets are those suggested to keep claims and complaints to an acceptably low level. 3. these values would possibly be needed to be increased still further. 1948) . For complaints to be stopp ed completely in residential areas. The derived data is attached as Appendix-1.2 Human and Structural response to sound Pressure levels. transverse and vertical components to the velocities measured. in some instances. Claims have also been made by owners of houses made of burnt brick. The blast records analyzed are taken from data of 5 blasts.4 . The shrinkage which occurs as the adobe dries. causes crack which are easily mistaken for cracks caused by vibrations due to blasting. but rather.Fig. Conclusions The nature of the building material used by the majority of the locals has certain inherent characteristics which make identification of actual impact of the vibration caused by blasting somewhat problematic. The blasting analysis software provides features for graphical output of the wave forms in each of the 3 axes and a comparison of the measured peak particle velocities and frequency content. The 3 axes correspond to the radial. That is not to say that cracks due to blasting do not occur. it implies that the material is not as strong as the Concrete or burnt brick buildings that are within the same range from the blast epicenter. Findings -Vibration monitoring results The vibration monitoring is conducted using a seismograph with analysis software. 3. visual investigations of these cracks seemed to indicate that the damage was a pre-existing condition since the surfaces of the cracks showed signs of aging. 5. 4. The seismograph consists of a 3-axis velocity transducer and a data acquisition storage device. however. These were compared with cracks on buildings were it was evident that the cracks . 5 mm/s and 1% at 2 mm/s. It may turn out that the limits recommended by this report may not be consistent with National guidelines on blasting activities.5 mm/s (0. that the determining factor in setting up a limit for the vibrations is the human factor and its response to the vibrations. In comparison. The types of structures and the quality of workmanship was also analyzed during the baseline survey. It is therefore only fair that the limits should be set based on these limits as is common practice internationally. monitoring has been conducted at various distances from the blast epicentre to establish magnitude of ground vibrations propagated to given distances. Ground vibration and air blast levels are generally measured at the nearest sensitive site. It can be concluded from the findings and from comparisons with international practices./s) have been known to reduce the number of complaints by a factor of three compared to 50mm/s (2 in. 7. The local communities also stand to gain from transfer of skills that is going on as the local workforce interacts with foreign Contractor. could serve as a template for the further development of the region as the locals lean to build better. This transference of skills most of which occurred during the initial stages of the Project as the EPC Contractor Constructed the Camps. The EPC Contractor is guided by the conditions pertaining to the permits obtained for the blasting activities. Actions taken The EPC Contractor has put in place programs to sensitize the local residents of the possible impact of the blasting activities. However. 6./s). 10% at 12.1. Recommendations 7. in the interests of minimising potential negative impacts on the local communities. Implementation of Limits Different limits may apply depending upon whether there are national guidelines in use prior to the introduction of this Report. Canada calls for a maximum peak particle velocity of 10 mm/s. This has been done in accordance with the Explosives' management plan that was submitted by the EPC Contractor. The Effects of vibrations become intolerable to humans at a levels appreciably lower than the levels at which structural damage occurs. It should also be noted as an example that limits used in the US of peak particle velocity of 12.had only recently been formed. Some of these were attributed to poor workmanship.5 in. This is even more pertinent in view of the fact that some of the structures/Buildings within the 2 km radius of the Blast epicenter had showed signs of cracks. The current blasting code for Ontario. which is just the perceptible range. In such cases the applicable limits are those set down in the Licence or Authority. the United States Bureau of Mining (USBM) recorded complaints on one construction site as high as 30% at 50 mm/s. . 3. authorities these being standards may be varied. Second Edition.  In view of the above. Note: In situations where the location or the nature of the operations mean that this is not subject to the relevant achievable. 2.6. BS 6472-2:2008: Guide to evaluation of human exposure to vibrations in buildings. Part 2: Guide to damage levels from ground borne vibration. Australian Road Research Board Special Report. (1973). BS 7385-2:1993: Evaluation and measurement for vibration in buildings. Explosives Code AS2187-983 Part.1 Existing Recommendations In international practice. metallurgy and Exploration (US). Use of Explosives. Ground Vibrations. References 1. 6. .1. Work Authorities or Licence Conditions set limits for air blast and ground vibration measured at sensitive sites and these are set as follows:  Ground vibration at sensitive sites should be below 10mm/s (ppv*) at all times. 8. edited by Bruce A. 4. the EPC Contractor is advised to limit the quantities of explosives used so as to ensure that the ground vibrations and airblast are limited to the ranges indicated above.  Airblast at sensitive sites should be below 115dB (Lin Peak) for 95% of all blasts. Standards Association of Australia (SAA).2 Recommendations for Isimba quarry site The levels for vibrations and Airblast for the communities in the vicinity of Isimba HPP's quarry are recommended as follows:  Ground vibration at sensitive sites should be below 5 mm/s (ppv) for 95% of all blasts. Surface Mining. Tynan A.E. Society for mining. and  Airblast at sensitive sites should be below 120dB (Lin Peak*) at all times.1. Part 2: Blast induced vibrations 5. satisfied that all effected people have given informed consent). Kennedy. Velocity Graph 139 2034 .9. 9.1. Appendix-1 :Records of Vibration Monitoring.1 Vibration Readings 01 Distance from Epicenter 400m Number of Holes Total Charge (Kg) i.1. Appendices 9. ii. Results: Maximum Velocity Frequency 0.13 cm/s 22.3 Hz . 2 Vibration Readings 02 Distance from Epicenter Number of Holes 310 m 148 Total Charge 3456 (Kg) .1.9. i.2 Hz .4311 cm/s 22. Velocity Graph ii. Results: Maximum Velocity Frequency 0.