Establishing Ground Vibration Threshold Level for Open Pit Mining Environment - A Case Study*

S. K. Yomekpe-Agbeno and M. Affam

Yomekpe-Agbeno, S. K. and Affam, M. (2008). “Establishing Ground Vibration Threshold Level for Open Pit Mining Environment - A Case Study”, Ghana Mining Journal, pp. 19 - 24.

Abstract

To assess the impact of blasting activities on building structures in open pit mining districts in Ghana a study was carried out in Prestea, a township very close to the perimeter of the Plant North pit of the Bogoso Gold Ltd (now Golden Star Resources Bogoso/Prestea Mines), in the Western Region of Ghana. It was observed during the study that the quality of most 'residential building structures within the Prestea township and its environs were generally sub-standard. Most of the houses were in a deplorable state of disrepair with their con- ditions already triggering failure. Again, it was noted that no records were available to ascertain the structural state of the buildings prior to the commencement of blasting activities at the pit, necessitating the need to carry out a baseline study of buildings in nearby Himan townships for comparative analysis. It was also observed that even though management had selected 12 mm/s as its threshold for its operations in the pits only 6% of the over 542 blasts monitored over a two and half year period had ground vibration values in excess of 1.5 mm/s, with a maximum recorded value of 8 mm/s.

It is recommended that management should not adopt ground vibration levels in excess of 2 mm/s for their pit operations because of the poor infrastructure within the surrounding communities. Even though this level can increase drilling and blasting costs considerably, it is considered a better option than expensive lawsuits in the likely event of any further damages that may be caused to building structures in the township.

1 Introduction These concessions are located in the Wassa West District of the Western Region of Ghana. An assessment of buildings in some mining The Bogoso/Prestea Mines employ open pit min- districts in Ghana where blasting activities ing operations, one of the main pits is known as simulate strong ground vibrations require Plant North Pit (PNP), at Prestea. In addition to thorough appraisal. Damage caused to the the surface operations there is also the Prestea building structures in these areas cannot be at- underground mine (New Century Mine) which is tributed to a single cause alone. There are sev- currently under care and maintenance. At the eral causative agents such as: poor building time of this study, the only operating open materials quality, poor, foundation problems, pits of the Golden Star Resources (GSR) were differential settlement, ground vibration, age- the Plant North Pit at the Prestea Township and ing and building maintenance culture. The the Ablifa pit at Kwame Niampa village. The ground vibrations from the extensive open pit Prestea Plant North pit is located about 300m to blasting activities can however, worsen the the east of and parallel to the general layout of the already precarious conditions of the buildings Prestea Township. or they can act as catalysts to worsen the already deplorable state of the structures. To assess the 1.1 Conflicts between Residents of Prestea impact of blasting activities on building struc- Township and the Mine Management tures in open pit mining districts in Ghana a study was carried out in Prestea, a township very close Since blasting activities started at the Plant North to the perimeter of the Plant North pit of the Bo- Pit in January 2003), the inhabitants of Prestea goso Gold Ltd., (now Golden Star Resources Township living close to this pit have com- Bogoso/Prestea Mine), in the Western Region of plained of the development of cracks and gen- Ghana. eral deterioration in their buildings, damages to electrical and electronic appliances and gen- Golden Star Resources (Bogoso/Prestea) Ltd. eral nuisance by way of fright and noise. The owns and operates a combined mining lease 2 people have attributed this to the adverse impacts covering an area of 224 km at Bogoso and of blasting at the nearby Plant North Pit. Prestea, known as Bogoso/Prestea Mines.

* Manuscript received February 21, 2007 Revised version accepted August 11, 2008

19 GMJ Vol. 10, December 2008 It was the initial view of the mine management with the holes at the edges of the benches being the effects of the blasting operations in the pit inclined at 60o to 65o Before the disturbances of were not causing what the people complained September 2005 an average of up to 200 holes about as they had been monitoring their blasts were drilled for each blast per paddock or sec- since they began blasting in the pit. The re- tion. This number was however, reduced to 100 sults of the blast monitoring showed that the holes per blast per section with up to 3 sections levels of ground vibrations from these blasts being blasted per blast or shot. This reduction generally fell below the United States Bureau was necessitated by the need to reduce ground of Mines (USBM) threshold limits of 12 mm/s vibration levels to meet the environmental re- adopted for their operations. Management there- quirements at the Prestea township. Bench fore took no positive steps to address the com- heights have been designed for 8.0 m with sub- plaints of the inhabitants. drilling of 0.5 m for all material types in the pit. Operational bench heights however, varied After receiving no positive response from the between 4 m and 12 m, depending on prevailing mine management on their complaints the in- site conditions. habitants resorted to a series of demonstrations against the Company (Anon, 2003). The impasse The method for charging of blast holes was col- culminated in the temporary suspension of blasting umn loading, using 150 g cartridges of pentol- operations at the Prestea pit by the Environ- lite explosives [called ‘booster’ on the mine] mental Protection Agency (EPA) in October with 500 ms in-hole Unidet NONEL detonators 2005. This action was deemed necessary to as primers. Between 25 kg and 60 kg of bulk enable the mine management address concerns emulsion explosive was then pumped into the raised by the people and to find an amicable holes and the remainder of the blast hole settlement between the two parties. (about 2.2 m in length) filled with either drill

During this period of suspension of blasting cuttings or 12 mm quarry chipping as stemming activities the management sought for profes- material. Table 1 shows blast design parameters sional opinion on the matter. This paper pre- for the pit. 25 ms surface delay detonators were sents the findings of such a study. The work used between blast holes in a row and 67 ms included an assessment of the blasting activities between rows of holes. at the pit to determine the ground vibration levels generated using various quantities of explosives, Table 1 Blast Design Parameters Used at the an assessment of the conditions of the building Prestea Plant North Pit structures in the various communities of the Type of Material Oxide Transition Sulphide Prestea township and the determination of a Ore Ore Ore threshold value of ground vibration with the Density Material 1.90 2.10 2.65 corresponding quantity of explosives to be em- (t/m3) ployed for carrying out the blasting activities that would produce minimal adverse environ- Drilling Pattern 4.5"5.0"8.0 3.5*4.0*8.0 3.0*4.0*8.0 mental impact on the Prestea township and its environs. Bottom Charge (g) 150 150 150

2 Drilling and Blasting Activities at Column Charge (kg) 25 30-40 40-60 Total charge /hole the Prestea Plant North Pit (kg) Specific Charge 25.15 30.15-40.15 40.15-60.15

Drilling operations are undertaken by the mine's Powder Factor 0.20 0.30 0.54 own personnel while African Explosives Limited (kg/m3) (AEL) supplies the required explosives and car- ries out the charging, blasting and blast monitor- ing operations on contract for the Golden 3 Structural Defects Star Resources (GSR) Bogoso/Prestea Ltd. The structural defects on the dwelling houses var- Tamrock drill rigs, using 89 mm and 102 mm ied considerably with the type of house and mode diameter drill bits, were employed for all drill- of construction. The houses are categorized into ing operations from the onset of blasting opera- four principal units namely, wattle and daub tions in January 2003. These were however, houses, mud buildings, landcrete, and sandcrete replaced with the Atlas Copco L7 drill rigs, houses. The foundation footings for most of using 115 mm diameter bits, in April 2005 and these structures are often either very shallow or which were in turn eventually replaced with a absent with poor quality building materials. smaller diameter bit of 105 mm in October 2005. The buildings are thus vulnerable to various defects that include associated foundation Blast holes were generally, drilled vertically

20 GMJ Vol. 10, December 2008 problems, poor building material quality, bad Prestea township the prevalent source of vibrations building maintenance culture, differential settle- is attributed to the activities in the nearby open ment and ground vibrations. pit mine. Although investigations by building and road research establishments show that the Associated foundation problems within the set- risk of damage to normal buildings by vibration is tlement resulted from factors attributed to poor extremely small (Seeley, 1985), blast vibrations foundation footings, shallow foundation from the pit may have compounded the already depths, and seasonal weather changes, removal precarious situation at the township (Fig. 2). of vegetation cover, intensive surface run-offs, earth flow and clayey soil conditions. Most of these buildings have no drains around them and as there is little or no vegetation cover the foun- dations have been severely eroded resulting in the virtual suspension of many of structures.

Maintenance work on buildings has to be predictable (i.e. regular periodic work as and when necessary) or avoidable (i.e. required to rec- tify failure). Failure to engage in early repair work could result in eminent structural collapse (Fig. 1). It was noted most of the buildings within the settlement had not been given any major repairs or maintenance for a very long time while some of the Fig. 2 Serious Multiple Crack Damage in structures had been left at the mercy of the weather. Building Attributed to Blasting

4 Results and Discussions

This section discusses the procedure for estimating ground vibration threshold levels for the Plant North Pit at Prestea, using pertinent literature and the results of over 542 blast monitored activities within a period of two and half years.

4.1 Predicting Ground Vibration Levels Us- ing Plant North Pit Blast Design Parame- ters

Ground vibrations due to blasting are generated Fig. 1 A Landcrete House with Distressed Foun- when some of the released explosive energy dation during detonation generates seismic waves within the rock mass. The vibration level may All elements of buildings deteriorate at a greater or be measured in such parameters as peak particle lesser rate depending on material quality. This is velocity (ppv), in millimeters per second, dis- 2 one of the critical causes of early deterioration placement in mm, acceleration in mm/s , etc. of structures. Also the quality of materials used, Generally, the most used parameter for assess- mode of construction and environmental conditions ing damage to buildings is the peak particle of the buildings made them suffer severely from the velocity (mm/s). For a given site or location a harsh weather conditions. Accordingly they responded correlation exercise could be carried out to de- quickly to the fluctuating effects of the high rainfall termine the most suitable relationship for estimat- pattern and the scorching sun prevalent in the area, ing the peak particle velocity generated. thus inducing differential settlement. According to The environmental acceptability of the blast de- Seeley (1985), buildings may suffer differential sign parameters in Table 1 was established by settlement for a variety of reasons including inade- using design parameters generated by the com- quate foundation, low bearing or shrinkable clay pany. The results obtained were then compared soil, presence of trees close to the footing and the with a National Standard (Threshold) and other effects of mining activities close-by. values from actual monitoring activities. The

Vibrations in buildings could be generated from impacts of these blasts on the nearby buildings at machinery, transport and, to a lesser extent, noise the Prestea township and its environs were then within the settlement. It may become objectionable assessed by studying the physical conditions of to people and interfere with their work. In the the buildings. According to Oloffson (1990)

21 GMJ Vol. 10, December 2008 ground vibration levels (V) may be estimated bration levels have been estimated for the using the following relationship: various blast design parameters and are sum- V = K {Q /R 3/2}a marized in Table 3. From Table 3 it can be ob- served that the established blast design parame- Where ters generated ground vibration levels of be- a = 0.6 determined for the Plant North Pit of tween 2.4 mm/s and 4.1 mm/s on structures Prestea. within an average distance of 300 m from the V = ground vibration, (mm/s), blast face, provided that laid down procedures Q = maximum weight or cooperating charge, are strictly applied at the pit. This implies that (kg), instantaneous charge structures beyond 300 m would be subjected to R = distance of blast from the monitoring point or structure, (m) and, far lower vibration levels. K= Rock transmission factor (RTF) for the area. It is a parameter that characterizes the Table 3 Predicted Ground Vibration Levels for homogeneity of the rocks in the blast area and the various Blast its environs. Type of Material Oxide Transition Sulphde For predicting ground vibration levels from the Ore Ore Ore Blast hole dia 115 and 115 and 115 and various blast design parameters at Plant North Pit (mm) 105 105 105 at Prestea the average value of "K" in equation (1) was found to be 59, when monitored results Bench height (m) 8 8 8 of blasting activities together with their corre- sponding co-operating charges and distances of the Distance from face 300 300 300 (m) blast faces from the monitoring points were used 25.15 30.15- 20.15- (Table 2) the Rock Transmission Factor (RTF) Co-operating 40.15 60.15 value obtained compares favorably with the charge (kg) value obtained previously for the erstwhile Satellite Goldfields Ltd (now Golden Star Re- Predicted ground 2.4 2.69-3.19 3.19-4.10 vibration Levels sources, Wassa Mine), (Anon 2002). The (mm/s) Golden Star Resources (Bogoso/Prestea mine) and Golden Star Resources (Wassa Mine) are both 4.2 Assessing Impact of Blasting Activities located on the Prestea-Ashanti Gold Belt with from the Plant North Pit on Prestea similar lithological units. Township and its Environs

Table 2 Rock Transmission Factor (K) Deter- Records on the mine show that most of the mined for Prestea Plant North Pit Av- blasts carried out at the pit between January erage Rock Transmission Factor =59 2003 and September 2005 were monitored for ground vibrations and air blasts. A total of 542 Vibration Co- Monitoring Rock blasts were recorded and out of this only 20 Levels operating Distance Transmiss- (mm/s) Charge from face ion Factor blasts (representing about 6% of total blasts) (Kg) (m) (K) had ground vibration levels above 1.5 mm/s 2.54 70 400 43.61 while 9 blasts had air blast values in excess of 2.03 60 300 29.51 120 dB(L) which is the recommended Environ- 1.78 70 300 23.59 7.37 77 400 119.52 mental Protection Agency (EPA) of Ghana levels. 1.65 60 440 33.86 The records also show that attention was not paid 1.78 24 410 59.4 to Frequencies (at vibration levels were recorded) 2.27 50 370 44.47 and the weather conditions at the times of the 1.65 56 400 32.39 monitoring exercises. Frequencies and weather 3.68 35 310 76.14 conditions are important parameters when as- 2.03 35 300 40.78 3.43 56 440 73.36 sessing the damage potential of a particular 3.68 40 300 68.24 blast in terms of ground vibration and air 3.17 40 270 53.46 blast. According to Konya and Walters (1990), 6.35 40 200 81.75 frequency is an important factor in assessing the 2.29 32 360 57.2 damage potential of vibrations as structural 2.48 35 300 49.82 4.83 22 320 135.87 resonance lies in the low frequency range typi- 2.54 45 360 51.71 cally of 5 to 20 Hz and blast vibration in this 2.73 24 200 47.75 frequency range can cause a resonance response in structures which produces increased displace- ment and strain, giving serious problems in the On hand with the Rock Transmission Factor structures. Also air blast levels rise with in- (RTF) in Table 2 and equation (1) ground vi- creased overcast skies with a corresponding in-

22 GMJ Vol. 10, December 2008 creased damage potential. and blasting operations at the Prestea North Pit.

4.3 Assessing Damage Conditions of Building 4.4 Establishing Ground Vibration Threshold Structures in the Prestea Township Level for Prestea Plant North Pit

At the time of this study, Mine Management With the information on the state of the buildings had not carried out any assessment of the con- in the Prestea area, monitored ground vibration ditions of the houses in the blast area before levels, blast design parameters and the corre- starting blasting and other mining activities. sponding vibrations they generated, the study set Some base line information was needed to help in out to establish appropriate ground vibration the blast damage assessment in the study. Some thresholds for the mine to adopt. base line study was carried out at Himan, a For any blast to be considered safe and environ- nearby village with similar building structures mentally acceptable, the vibration levels gener- and which had not been subjected to any im- ated should not exceed a threshold or maximum pacts of blasting from the Prestea pit, to provide established level for the particular condition and the necessary data for a comparative damage location or country. A threshold level thus estab- assessment of the Prestea buildings. lished could be adopted for use under other con- Assessment of damage caused by ground vibra- ditions provided appropriate corrections or ad- tion due to blasting at the Prestea township and justments are made, taking into consideration its environs is a complex process. The magni- prevailing conditions in this new location. Ac- tude of structural damage caused to building cording to Persson et al, (1990) the correction or structures resulting from blasting activities ap- adjustment to this threshold value, V0, may be peared to be disproportionately high, consider- obtained by applying such correction factors as ing that over a two and half year period only construction quality factor, distance factor, pro- about 4% of total blasts monitored had ground ject time factor, building factor and construction vibration levels higher than the 2 mm/s estab- material factor, using equation (2) below: lished threshold. Experts on the subject of blast V = V x Fk x Fd x Ft (2) damage due to ground vibrations generally agree that most structures have cracks of some kind, where V = Adjusted or corrected peak particle from other sources than blast vibrations. These velocity (ppv), mm/s are usually due to causes ranging from poor con- Fk = Construction Quality Factor = Fb x F,,, struction to normal environmental stresses such Fd = Distance Factor, (varies as to type of as thermal stresses, humidity, wind etc. It is un- material on which the building is con usual to find a house without a crack in it. How- structed and ranges, for example, be ever, when any blasting is undertaken in the tween 0.22 for rock; 0.35 for moraine area environmentally induced cracks are and 0.50 for clay materials for distances of quickly attributed to it. Most of the cases in the over 350 m). Prestea township fall into this category. Ft = Project Time Factor (4.0, for stationary projects such as mines and quarries for It is a fact that vibration damage due to which duration is one year or less and blasting can and does occur but blasting op- 0.75 for duration of 5 years or more). erations nowadays are usually so engineered Fb = Building Factor (=1.20 for industrial and and controlled that genuine cases of blasting office buildings, 1.0 for standard living damage are generally very small and in the mi- buildings and 0.5 for buildings in dam nority. Konya and Walters (1990), point out ageable conditions and certain ruins). that there will always be structures at the Fm = Construction Material Factor (1.20 for point of failure due to normal environmental reinforced concrete, steel and wood; 1.0 stresses waiting for the proverbial "straw that for not-reinforced concrete, brick or broke the camel's back" and this straw may be clinker, 0.75 for autoclave porous con the vibration caused by nearby blasting activi- crete and 0.65 for mexi-brick/artificial ties, even though in reality it may not be the limestone brick). actual cause. In the absence of any local or national ground vi- The above points notwithstanding, it cannot be bration thresholds, the authors adopted the Ger- ruled out that some damage must have been man Standards and made suitable adjustments caused to some of the houses at the Prestea town- to suit the local conditions at Prestea and its ship as a result of blast vibrations. It is difficult to environs. According to German Standards the point out such houses without thorough investiga- maximum allowable level of ground vibration on tion. However, the assessment process would a standard residential building should not exceed have been easier if a baseline study had been 8 mm/s (Persson et. Al. 1990). Considering the sub carried out prior to the commencement of mining standard quality of buildings in the Prestea

23 GMJ Vol. 10, December 2008 township and its environs, authors held the view Community interaction and education is crucial that the German Standard of 8 mm/s was too high for the avoidance of misunderstandings and con- for direct application and therefore not accept- frontation between mine management and the able. Appropriate adjustments were therefore inhabitants in the adjoining settlements. The made to this value using equation (2) together communities must be made aware that in min- with the appropriate correction factors to obtain ing there is generally a trade-off between bene- the value of 2 mm/s. This implies that the fits accruing from the operations and adverse maximum ground vibration due to blasting ac- losses to the affected communities. tivities at the pit that should be allowed on build- ing structures at the Prestea township and its envi- References rons should not exceed 2 mm/s. Viewed against Anon. (2002), “Proposed Mining at Deadman's the results in Table 3 and confirmed by the re- Hill and the Effect on Kubekro”, sults of monitoring where 4% of total blasts Supplementary Report for Satellite Gold- monitored had values in excess of the 2 mm/s, it field Ltd, Unpublished Report, 37 pp. can be seen that the blast design parameters being Anon. (2003), "Report on Perceived Blasting Im used at the pit at the time of this study were pact on Prestea Township and Its rather on the high side and could easily have Environs", Unpublished Report for Bogoso caused some damage to buildings and structures. Gold, 8 pp. The adjusted blast design parameters are summa- Oloffson S.O. (1990), “Applied Explosives rized in Table 4. Technology for Construction and Mining”, Table 4 Recommended Revised Blast Design Applex Pub, Arla, Sweden, pp. 202-207. Parameters Persson, P. A., Holmberg R. and Jaimin L. (1990), “Rock Blasting and Explosives Type of Material Oxide Transition Sulphide Engineering”, RC Pres Inc, Boca Raton, Flor Ore Ore Ore ida, pp. 346-355. Bench height (m) 4 4 4 Konya, C. J. and Walters J. W. (1990), Surface Burden & Spacing (m) 3 x 3 3 x 3 3 x 3 blast design, Prentice Hall Publishers, Engle Wood Cliffs, N.J., pp. 152-249. Sub drill (m) 0.5 0.5 0.5 Seeley, (1985), Building Maintenance, Macmillan Publishers, pp. 23-89. Bottom charge (kg) 0.15 0.15 0.15

Column charge (kg) 14 14 14 Authors S. K. Yomekpe-Agbeno holds Degrees of Co-operating charge 14.15 14.15 14.15 BSc (Mining Engineering) from Heriot- (kg) Watt University, Edinburgh, Scotland and MPhil from the Kwame Nkrumah Specific charge (kg/m3 0.39 0.39 0.39 University of Science and Technology, Kumasi, Ghana. He worked at Ashanti No. of Holes/Blast 100 100 100 Goldfields (Ghana) Ltd. at Obuasi, first as a Technical Mining Assistant, then The maximum vibration level expected from this as a Shift Boss, at Prestea Goldfields Ltd as Mine Captain; at Taylor Woodrow International/ blast design has been estimated as 1.7 mm/s. Fur- Taysec Construction Ltd as Quarry Manager and at ther upward adjustments could be made to the Nsemmere Quarry Ltd in Sunyani as General Manager/ explosive consumptions for improved fragmen- Chief Executive Officer, all in Ghana. He joined the tation, if so desired, so long as the vibration levels University of Mines and Technology, Tarkwa, in 1987, where he lectures in Underground and Surface Mining do not exceed the recommended 2 mm/s. Technologies; Minerals and Mining Law. He is a Char- tered Engineer (CEng), a member of the Ghana Institu- 5 Conclusions tion of Geoscientists and a member of a team of experts that provides consultancy services to the mining and It is imperative that in monitoring blast vibra- allied industries. tions, frequencies at which the vibrations oc- curred must be indicated. Also records of weather M. Affam obtained his BSc. (Hons) degree in Geological Engineering and MSc. degree in Mineral Exploration from conditions at the time of monitoring should be Kwame Nkrumah University of Science and Technology, recorded. Kumasi, in 1996 and 2002 respectively. He has since graduation worked for a number of mining and exploration Initial baseline studies are a good basis for com- companies in Ghana for almost eight years. He joined the parative assessment of damage to building struc- University of Mines and Technology, Tarkwa in 2003 as a tures due to blasting operations in mining dis- lecturer in geotechnical engineering. His research areas include exploration geochemistry; soil and rock mechan-ics. tricts. Based on the quality and standard of most He is a member of a team of experts that provide geotechni- of the buildings in the Prestea township and its cal consultancy services to mining and allied companies environs, maximum ground vibration level of 2 in Ghana. He is a member of GhIG. mm/s has been recommended.

24 GMJ Vol. 10, December 2008