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Air Gap Monitoring Project Oktober 14, 2006

Posted by hseclubindonesia in Environmental.

Possibility of An Air Blast occurrence was considered at Freeport’s Block Cave Mines. It was considered important to introduce and conduct Air Gap Monitoring Project as a safety precaution. The aim of this project was to provide means of measuring the air gap between the cave back and the top of existing muck pile through open drill holes. The project was successful to determine air gap distance, dilation zone, and swell factor of muck pile inside the cave. Risk minimization program was arranged to manage the air blast risks. Draw rate reduction, geotechnical monitoring, procedures development, and areas isolation were the actions to anticipate and minimize the risks. This paper elaborates air gap measurements and following actions to manage the risks and the impacts of air blast.

The Deep Ore Zone Mine (DOZ) is the main underground block cave mine at PT Freeport Indonesia (PTFI). It is located about 300 meters below the Intermediate Ore Zone (IOZ) mine and about 1200 meters below the surface (fig.1). The DOZ Mine currently produce 35 kilo tons of copper and gold ore per day. Since the first production at year 2000 till the end of April 2003, the mine has already produced 13 million tons ore grading 1.73 percent copper and 0.89 ppm gold.

Block Caving Method is the lowest cost underground mining method but it has relatively high risks and a high-up front capital and development cost (Heslop, 2000). Besides that, it has some operating risks which can be divided into:
a. Operational Hazards
Rock bursts, air blast, mud rushes and water and slurry inrushes.
b. Design Risks
Those risks that have an economic impact and are the result of incorrect assessments of the ground conditions or effect of stress, etc based largely on geotechnical data collected years before.
c. Draw Risks
Those risks that have an impact on the current and future ore and grades that will be recovered and are the result of incorrect assessment of the issues that influence draw.
d. Automated Equipment Risks
Risk arising from an over-reliance or advanced technology to achieve critical levels of performance from LHD and Drills.

One of operational hazards is an air blast. The Air Blast has become an issue in Freeport’s Block Cave Mines following the incident which happened at North Parkes Mine (Australia) in November 1999. Cave back suddenly collapsed and created air blast. Learning from that incident, it was considered important to introduce and conduct An Air Gap Monitoring Project in the Deep Ore Zone Mine as a safety precaution.

Air Gap Monitoring
Air Blast is a disturbance in underground workings accompanied by a strong rush of air. The rush of air, at times explosive in force, is caused by the ejection of air from large underground openings, the sudden fall of large masses of rock, the collapse of pillars, slippage along a fault, or a strong current of air pushed outward from the source of an explosion (American Geological
Institute, 1996).

Air Gap Project
The aim of this project was to provide means of measuring the air gap distance between the cave back and the top of existing muck pile. The project included:
a. Development of 60 meters excavation/drift to allow vertical or sub-vertical holes to the DOZ Cave Back.
b. Drill four holes (totally 400 meters in length) for Time Domain Reflectometry (TDR) Installation. This monitoring is adopted from telecommunication technology. This device is used for identifying the location of a fault in a cable. When the cave or crack intersects the cable, the cable will form an open circuit and the reading unit will show the distance from collar to the intersected location.

c. Drilling at the air gap drift to get open holes to the cave for allowing direct measurement of Air Gap (fig.2). During drilling activities, monitoring both visual and TDR monitoring were conducted every hour.

The hardest part of this project was to get a directly open hole or a breakthrough hole into the cave. In the immediate vicinity of the cave, the rock mass fractures forming, so called a dilation zone. The drill holes usually get stuck while enter the dilation zone. Besides the water can not flowing well, the rock or ground around the dilation zone is unstable and ready to collapse as cave material. After struggling with collapsed holes, the project was successful to measure the air gap distance.

Project Results
The project was successful to get breakthrough hole and measure the air gap distance as well.
The results of this project are:
a. Air Gap Distance
The measurement showed that the distance between the cave back to the top of the DOZ muck pile was approximately 55 meters. In the other word, there is 240 (fig.2) meters of muck between air gap and extraction level.
b. Dilation Zone
From several holes which drill into the cave indicated that dilation or crack zone around the cave is 20-30 meters.
c. Swell Factor
This measurement allowed to calculate swell factor of muck pile to be 24%.

The Swell Factor calculation from the Air Gap Measurement (Rachmad, 2002):
Assumption : Density of the rock 2.7 ton/m3
Material pulled vertically and independently along the column
Data : Air Gap Distance 50 meters

Height Cave from Extraction 288 meters
Height of Muck Pile 238 meters
Tons Pulled (April 15th,02) 75,361 tons
Area of Draw Point 287 m2
Calculation :
Tonnage Cave = 288 m x 287m2 x 2.7 ton/m3 = 223,171 ton
Tonnage Cave = Tonnage remaining in muck pile + Tonnage Drawn
Tonnage remaining in muck pile = Tonnage Cave – Tonnage Drawn = 223,171 – 75,361
= 147,810 tons

Volume remaining in muck pile = 147,810 ton/(2.7ton/m3)
= 54,744 m3
Volume remaining x % Swell)/Area of Base= Height of Muck Pile
% Swell = (238 m x 287m2)/54,744 m3 = 124%

Swell Factor = % Swell-1 = 124%-100% = 24%

Risk Minimisation
The project showed that the muck pile height is about 240-250 meters from the DOZ Extraction Level (Widijanto, 2002). For comparison, North Parkes Mine adopted minimum of 60 meters of caved material above extraction level (North Parkes, 1999). Based on that result the risk of air blast happen at the DOZ Mine could be neglected because the muck pile can absorb the energy or pressure. For the Intermediate Ore Zone (IOZ) Mine which intersected with the DOZ Air Gap Volume (volume between cave shape and muck shape – fig.2), the risk of that is managed by several actions and monitoring (Widijanto, Rachmad, & Nicholas). Some actions following this project:

1. Draw Rate
The concept of a draw rate implies that a rate of the cave collapse is equal or faster than the draw rate (production rate). The concept requires that collapsed cave has always greater volume than pulled material from the draw points. It means the air gap between the cave back and the top of muck is maintained at a current situation or getting smaller. Further more, the risk of an air blast could be minimized. Draw rate reduction at panels with a significant cave height minimizes the air gap distance.

2. Monitorings
Regular TDR monitoring around the cave can give early warning if there is anomaly of cave progress. Besides that, installing a television camera helps to identify the actual condition at the edge of air gap drift which the nearest point of the existing drift to the cave.

3. Procedures
Procedure was developed to ensure safety for all activities, especially for the area which was close to the DOZ Cave propagation. Distance between the closest excavation or drift to the DOZ cave was used to determine what actions should be taken. Besides that, the procedure stipulated the critical area should be inspected regularly to ensure safety of people who works on the same level or elevation.

4. Isolations
The areas which were considered under potential risk from airblast were identified and isolated. The isolation prevented people to enter or carrying out their activities at that area, except those who had permission from the area owner and underground geotechnical section. The other purpose of isolation by building bulkheads is to direct air pressure to the surface -not flowing to the underground working areas-. Concrete Bulkheads at the north and south of East Exhaust Drift which had the closest distance to the DOZ Cave, Pneumatic Doors at Undercut Level, and Shotcreted Bulkheads were built to isolate some areas.

1. Heslop T. G., 2000.Block Caving – Controllable Risks and Fatal Flaws. In Proceedings of Mass Min 2000 Conference, Brisbane, 29 Oct-2 Nov 2000, 437-454.
2. American Geological Institute, 1996. Dictionary of Mining, Mineral, and Related Terms. American Geological Institute in cooperation with The Society for Mining, Metallurgy, and Exploration Inc., Second Edition.
3. North Parkes, 2000. Findings and Recommendations of Airblast Incident. In Internal Report. North Parkes
4. Widijanto E., 2002. Airgap Monitoring Result. In Internal Report. PT Freeport Indonesia.
5. Rachmad L., 2002. Airgap Calculation. In Internal Report. PT Freeport Indonesia.
6. Widijanto E., Rachmad L., Nicholas D., 2002. Estimate When and Where DOZ Cave will Intersect IOZ/GBT and The Surface. In Internal Report. PT Freeport Indonesia.

Eman Widijanto, Superintendant Geotechnical Underground, Geotechnical Underground Department, PT Freeport Indonesia.

Publication by.
HSE Club Indonesia Journal 1st Edition, 2006.


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