Sea to Sky Geotechnique 2006

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1 A BRIEF CASE HISTORY OF THE APRIL 2006 INFLOW EVENT AT THE ANTAMINA TAILINGS IMPOUNDMENT FACILITY Terry Eldridge, P.Eng., Golder Associates Ltd., Burnaby, British Columbia, Canada Henri Letient, P.Eng., Compañia Minera Antamina, Huaraz, Perú ABSTRACT The Antamina Tailings Impoundment Facility in Peru experienced an inflow of tailings and water into the decant system at the end of April The flow increased over a period of about 5 days, then decreased as the system naturally plugged itself. The cause of the inflow and the method of natural sealing are presented. The environmental water quality monitoring system that allowed deteriorating conditions to be identified early in the event is discussed. As well, how the company s publicly stated values facilitated decision making under uncertain conditions is presented. The tailings dam was not involved in the inflow event, nor was its integrity jeopardized in any way. RÉSUMÉ Le bassin de retenue des résidus miniers de la mine Antamina au Pérou a subi un apport de résidus miniers et d'eau dans le système de décantation à la fin avril L apport a augmenté durant une période d'environ 5 jours et a diminué lorsque le système s'est, par-lui même, naturellement bouché. La cause de l'afflux et la méthode de scellage sont présentées. Le système contrôlant la qualité environnementale de l'eau, qui a permis l identification des conditions de détérioration au tout début de l'événement, est discuté dans cet article. De plus, la façon dont la compagnie a affirmé publiquement ses valeurs, facilitant ainsi une prise de décision sous d'incertaines conditions, est présentée. Le barrage de résidus miniers n'a pas été impliqué dans cet événement et son intégrité n a donc pas été en jeu de quelque façon. 1. INTRODUCTION The Antamina Mine is located in the district of San Marcos, province of Huari, in the Ancash region of Peru. It lies between 4,200 and 4,700 meters above sea level at latitude 9º32 S longitude 77º03 W. Access is via a paved road, about 7 hours from Lima or 3 hours from Huaraz. Antamina is a polymetallic orebody with an estimated 559 million tons of reserves. The mine is operated as an open pit truck/shovel operation with production beginning in The mine will have an operating life of more than 20 years at a production rate of about 85,000 tpd. 2. DESCRIPTION OF DECANT SYSTEM The decant system is part of the overall water management system at the Antamina mine site. The system is designed to convey natural, surface water runoff from the area upstream of the tailings impoundment facility to the Ayash River downstream of the tailings dam and to allow excess water in the tailings pond that is accumulated during large flood events to be discharged safely downstream of the tailings dam. The layout of the tailings impoundment facility is shown on Figure 1. The Stage 1 decant system is located about 500 m upstream of the tailings dam. The main components are an inclined, reinforced concrete sidehill structure with two channels, each 1.5 m wide by 1.4 m deep, draining to a 3.5 m diameter, reinforced concrete drop shaft, which is in turn connected to the decant tunnel. The opening, or orifice, at the lower end of the channels that connects to the drop shaft was designed to limit the maximum flow into the decant tunnel to 25 m 3 /s. The invert of the channels within the sidehill structure extends from a working platform elevation at 4050 m to the connection to the drop shaft at elevation m. The invert of the decant tunnel is at elevation 3973 m at its connection with the drop shaft. A plunge pool is located at the bottom of the drop shaft to dissipate the energy of the flowing water before it enters the tunnel. A concrete-encased, 1.2 m diameter, corrugated steel pipe installed parallel to and on the north side of the twin intake channels provides air to the tunnel during periods of high water flow. These structures are all founded on bedrock. As the level of the water and tailings rises in the impoundment, the inlet channels are progressively closed by installing stop logs. The stop logs consist of a steel I-beam section (IBP360), supported on lips on the inside of each channel wall, and covered with a 300 mm thick layer of concrete. A steel plate covers the top of the channels, which acted as a form to contain the concrete when it was placed. The steel plate, concrete and I-beam act as a composite structural element. The configuration of the Stage 1 decant system is shown on the isometric drawing in Figure 2 and on Figure 3. The Stage 1 decant was designed to operate between elevations 3988 and 4050 m. The Stage 1 decant system was to be sealed by constructing a plug in the tunnel section during the dry season after the Stage 2 decant 1018

2 Figure 1. Antamina Tailings Impoundment Facility Satellite Image Sidehill Structure with Two Channels Stop Log Closure of Channels Sidehill Structure with Two Channels Ventilation Conduit Decant Tunnel Drop Shaft Ventilation Conduit Drop Shaft Plunge Pool Figure 2. Isometric of Stage 1 Decant Figure 3. Stage 1 Decant System 1019

3 began operating. Construction of this plug was planned to start in June In preparation, the top of the two channels were plugged during the wet season of 2005 to prevent water from the rising pond entering the tunnel during the plug construction. At the end of April 2006, the tailings pond was at elevation m and the surface of the tailings near the Stage 1 decant structure was at elevation 4029 m. 3. INFLOW EVENT On Sunday, April 23, 2006, rising dissolved copper levels were noted in the water coming out of the decant tunnel at the Antamina mine site. On April 24 an investigation began, and inspections of the tunnel and Stage 1 decant drop shaft area were carried out on April 25 and on April 26. Inspections of the sidehill channels could not be carried out on those dates because the upper ends of these channels had been sealed in preparation for permanent plugging of the Stage 1 decant tunnel, planned for June The sidehill channels were sealed by placing a concrete plug in the lower part of the channels, above the entry to the drop shaft and plunge pool. An initial section of the plug was poured on May 9, 2006, and a final section on May 11, In total, 119 m 3 of concrete were placed in the sidehill channels to create a 33 m long plug. The emergency was declared over on May 13, 2006, once testing had confirmed the desired strength of the concrete plug had been achieved. 4. MECHANISM FOR INFLOW The inspections of the decant system revealed a number of water inflows into the sidehill intake channels. In one area of the north channel and two areas in the south channel, point source inflows were observed. As shown in Figure 4, these were in the lower part of the walls near the floor and are most likely associated with small zones of segregated concrete. The seepage flow coming out of the Stage 1 decant system increased during this period, with flow estimated to be in the range of 50 to 100 l/s on April 25 and up to 200 l/s on April 26. Solids were observed on the floor of the tunnel near the Stage 1 decant drop shaft on April 25. Testing confirmed that the increased flow was tailings pond water and that the solids were tailings. A Level 1 Alert was raised on April 26, 2006 at 1:40 p.m., activating Antamina s Crisis Management Team (CMT). The alarm level was raised to Level 3 Crisis that evening due to the increase in flow and the observed tailings solids near the drop shaft. Not knowing the location of the leak or how it would evolve, and not able to inspect the sidehill structure, it was decided to evacuate the village of Ayash and shut down the concentrator as precautionary measures. On Thursday, April 27, 2006, tailings were observed in the energy dissipater pool at the outlet of the decant tunnel. By Friday, April 28 the seepage from the Stage 1 decant was decreasing and by Saturday, April 29, 2006, no more solids were observed coming out of the tunnel. The plugs near the top of the sidehill channels were removed by April 29, and inspection of the channels began. On Sunday, April 30, flow into the Stage I decant was estimated to be less than 20 l/s, with no solids. Over the next two days, the flow decreased to approximately 10 l/s. Following inspection of the components of the Stage 1 decant system, it was determined that the system was structurally sound and that the flows came from small cracks in the concrete sidehill channels. The people of Ayash returned to their homes on Saturday, April 29, 2006 and the concentrator was restarted that evening. The level of the event was reduced to Level 2 Emergency on May 1, Figure 4. Water Inflow to North Channel In the south channel at a point 118 m from the upper end of the channel, at about elevation 4008 m, four near vertical cracks were found, as shown on Figure 5. The cracks were about 350 to 400 mm long, a few millimetres wide and extended upward from the floor level. These cracks occurred at the location where the thickness of the concrete wall and floor was reduced from 800 mm to 650 mm, as shown on Figure 6. It is considered that the wall and floor thickness change created a stiffness contrast and stress concentration, that when combined with the hydraulic pressure on the protruding part of the wall, caused the cracking as the load from the water and tailings on the structure increased. The widths of the cracks were controlled by the heavy steel reinforcement in the wall. It is not known if the cracking at this location occurred at one time or a number of times. 1020

4 Construction Joint between Wall Sections flow. The silt is held in place by the Zone 2B material that supports the concrete face. Cracks Figure 5. Cracks in South Channel Thickness Change from 900 mm to 800 mm Thickness Change from 800 mm to 650 mm Figure 6. Wall Thickness Change in Area of Cracking 5. NATURAL PLUGGING OF CRACKS The decant structure was constructed in an excavation to rock. Granular backfill was placed and compacted along the outside of the walls. At the time of the inflow, approximately 18 m of tailings covered this area. The mechanism for plugging of the crack is considered to be water carrying sand or silt sized material from the backfill and tailings into the crack where it was held, while the finer fraction of the material passed into the channel. A filter developed in the crack, holding finer materials, the hydraulic conductivity decreased and the flow decreased. A flow estimate using the Darcy equation shows that flow would be reduced several orders of magnitude with sand sized material plugging the crack. This mechanism is used to seal cracks in the faces of concrete-faced rockfill dams. When leakage occurs, silt is dumped down the face and drawn into the crack by the 1021

5 6. WATER QUALITY MONITORING The occurrence of these leaks was detected early on by Antamina s water quality monitoring system. Water quality is monitored daily at various locations throughout the tailings facility. Field parameters, such as ph, conductivity, turbidity, and flow, are recorded and any sudden changes are immediately notified to a supervisor. In this event, no obvious changes to field parameters were detected. Water samples are collected daily and sent to an on-site laboratory for analyses of key parameters: ph, TSS, cyanide, ammonia, total copper, zinc and iron, and dissolved copper, zinc and iron. These daily samples are for internal compliance monitoring only. Weekly, official compliance samples are sent to an external certified laboratory for a full range of analyses. These weekly samples are what represent the official compliance registry of Antamina. However, it is the daily internal samples that permit early detection of any upset in the system, such as occurred in this instance. Reviewing the results of daily samples, the presence of low levels of copper (0.03 mg/l) was observed in the water flowing out of the decant tunnel on April 23. It appeared that the presence of copper had been increasing since April 19. Prior to that date, dissolved copper was always below detection limits (<0.01 mg/l), as shown in the Table 1. The detection of copper in the discharge from the decant tunnel triggered the investigation into the origin of this copper well in advance of the peak flows that occurred between April 26 and 27. This early detection allowed Antamina to implement its Emergency Response Plan in a timely fashion. Water quality at the receiving water compliance point in the Ayash River downstream of the tailings facility remained within compliance limits at all time and no environmental impacts were detected as a result of this event. Table 1. Water Quality at Decant Tunnel Outlet Date Dissolved Copper Total Copper (mg/l) (mg/l) April 12, 2006 < April 13, 2006 < April 14, 2006 < April 15, 2006 < April 16, 2006 <0.01 <0.01 April 17, April 18, 2006 < April 19, April 20, April 21, April 22, April 23,

6 7. DECISIONS On April 26, the decision was made to shut-down the concentrator, and evacuate the people in the village of Ayash, located several kilometres downstream of the tailings impoundment facility. These were very difficult decisions for the senior management to make. Information was not available on specifically where the leak was developing and it was impossible to know whether a structural failure of a component of the decant system was occurring or whether a crack had occurred and the flow would be limited. At the time of the decision, it was known that flow into the decant tunnel was increasing, tailings were reaching the tunnel, and that evacuation of the communities downstream of the tailings facility would be very difficult if a major breach developed because of the very narrow canyon immediately downstream of the tunnel outlet portal that must be traversed to reach the village. what was occurring inside the decant structure, it is quite possible that different measures would have been taken. Evacuation of the village, investigation of the inflow and sealing of the system was facilitated by the mine s Emergency Response Plan. Future designs of similar decant structures should incorporate methods to inspect all the components of the system at all times until the system is permanently decommissioned. These decisions will be reviewed and second guessed because the inflow event turned out to be minor. However, being unable to inspect the sidehill channels until the plugs were removed, Compañia Minera Antamina made the decision that was consistent with the corporate values and sense of social responsibility. That is, it assumed the worst case in order to protect the people in the downstream community. Once the decision to evacuate the village was made, it was rapidly implemented by the Crisis Management Team following the mine s Emergency Response Plan. When inspection of the system revealed that the decant structures were sound and that the risk to the downstream community had not changed from pre-inflow conditions, people returned to the village and the concentrator was restarted. 8. CONCLUSION Cracks in the wall of the decant system resulted in a water and tailings inflow to the Stage 1 decant system at the Antamina mine. The flow reached a maximum of 200 l/s before naturally decreasing. Approximately 21 m 2 of tailings reached the outlet portal of the tunnel. Water quality at the receiving water compliance point in the Ayash River downstream of the tailings facility remained within compliance limits at all time and no environmental impacts were detected as a result of this event. The environmental water quality monitoring program provided early identification of deteriorating conditions and allowed investigations to begin before any tailings reached the tunnel. However, the inability to quickly inspect the structure itself forced Antamina to assume the worse case scenario. As a precautionary measure, the people living in the village of Ayash downstream of the outlet portal of the tunnel were evacuated to the mine site until it was determined that a breach of the decant system was not developing. The decision was consistent with the values and sense of social responsibility held by the company. If information had been available to assess 1023