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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.



Posted by hseclubindonesia in Safety.


The world primary energy consumption is getting high from year to year.  In this year (2005), the world primary energy consumption is predicted around 11,063 million TOE, in which around 21,8% of them (2,409 million TOE) is supplied by coal.  In the next 2010, the world consumption is predicted will jump to 12,413 million TOE, in which 21,3% of them (2.638 juta TOE) will be supplied by coal (EIA, International Energy Outlook, 2002). Still based on the information from EIA, in the year of 2002, especially for Asia region, the coal consumption of

Republic will increase from year to year, including for their preparation as the Olympic host in the next 2008.  The detail data about the world primary energy consumption is attached on Table-1. The primary energy consumption in
Asia region, as a big population region, is getting high from year to year.
China, as the biggest population country in this region, is predicted will use coal as it’s main energy.  In 1999,
China consummated around 1,595.5 million TOE.  In the next 2010 and 2020, China as the biggest country in
Asia region will consummate around 2,757.2 and 4,193.1 million TOE of the world primary energy.  Around 60% of them will be supplied by coal energy which is produced by the country and by another countries in the region, including

Mainly, the coal can be mined through two different ways of mining; the Surface Coal Mining (SCM) and the Underground Coal Mining (UCM).  Currently, the SCM that has been implemented for hundred of years, is getting un-popular.  It is in relation to the improvement of people concern to the environmental protection issues.  The UCM is claimed as the best method of mining in relation to the environmental protection issues.  But unfortunately, this mining method has a very high risk to employees’ accident.   

UCM methods have been introduced many years ago in
Indonesia but this method of mining is not as popular as Surface Coal Mining (SCM).
PT. Arutmin Indonesia, as one of the three biggest coal mining company in
Indonesia, has four surface coal mining sites, i.e. Satui, Asam-Asam, Senakin and Batulicin Coal Mine Sites.
  UCM has been newly introduced at Satui Coal Mine site – in
South Kalimantan and is the only one operating in the region. 

SCM methods are developed by digging the overburden materials and taking out the coal from the earth.  These type of mining are limited by an economical “Stripping Ratio”, such as 5:1.  These mining methods can potentially have serious impacts to the environment surrounding the mining area.

Compared to the SCM Method, UCM Methods have minimum impact to the environment.  But unfortunately, this mining method has a higher risk or impact for people who working at the UCM.  Until now, a lot of people are still killed in the UCM area.  For example, on 26 April 1942, 1,527 employees are killed in Honkeikou –

In this paper, I would like to show some risks that could be met by the UCM employees.  The accident risks could be; mine blast, mine fire, hazardous/toxic gas circulation, stone/coal dust circulation, mine flood, and mine fall/collapse accidents.  From the all kinds of the UCM accidents, the UCM blast accident is the worst one that worried by the UCM employees at present, because the accident frequency rate is still high.  Thousands of the UCM employees have been killed by the UCM blast accident.  When the UCM blast accident happened, it is usually followed-up by another type of UCM accidents, such as mine fire, hazardous/toxic gas circulation and/or mine fall/collapse.


Effort to prevent UCM blast accident will be conducted effectively if we know the characteristics of the UCM blast accident.  We should know why the UCM blast accident is happened.  Managing preventive efforts are always more better than conducting curative efforts.  In this paper, we will see why the UCM blast accident is happened and the ways to prevent this accident happened in our UCM areas.  Some materials that I present in this paper are from Ikeshima –
Center –
Japan.  I learned the UCM safety in the training center for about six months. 

2.  CHARACTERISTICS OF UNDERGROUND COAL MINING Compared to the SCM methods, the UCM methods have some characteristics or limitation as follows: 1.   As their name, the UCM methods are developed in the underground area.  In some cases, these type of mining methods can be developed for thousands of meters into the earth. 2.   They have a limitation in free air circulation, especially for oxygen gas circulation required for employees’ breathing who are working in the underground panel.3.   They have a limitation in lighting, causing almost all tunnels and work panels are extremely dark.  This condition can impact the employees’ sight who are working there. 4.   Limitation in working area for employees’ and equipment movement.  This condition has potential problem for increasing of employees’ hit, pinched and scratch accidents. 5.   The UCM methods are affected by the earth warming effect.  More deeper more warmer.  This effect is strongly felt by the employees when the underground ventilation system is not working properly. 6.   They are also affected by the earth’ tension effect.  That’s why the underground tunnels should be maintained regularly to minimize the impact of the effect to the tunnels.  Otherwise the tunnels will be narrower. 7.   These type of mining methods also have a risk of roof falling.  If the tunnel and/or the working panel collapse, it could kill employees working in the area.  This accident has potential to stop air circulation to other working panels.  We can imagine when people working without fresh air (oxygen).8.   The UCM methods also have a high risk of water flooding.  Underground water from around can enter any empty spaces of the underground coal mining area.9.   Some types of hazardous gases can also enter and be able trapped in the UCM spaces. The hazardous gases will decrease the oxygen availability and will impact to the employees’ health and/or safety.10.  Last but not least, the UCM methods have a high risk on the circulation of coal and/or stone dust.  3.  ACCIDENT RISKS IN UNDERGROUND COAL MININGSome characteristics of the UCM have potential risk to cause accidents and/or health impacts to their employees.  Some kinds of underground accident risks that can occur at an UCM are:                  a.  Mine blast accident risk,              b.  Mine fire accident risk,            c.  Hazardous gas circulation risk;             d.  Coal and/or stone dust circulation risk;             e.  Mine flood accident risk;             f.   Mine fall / collapse accident risk; and

            g.  Other general accident risks, such as pinched, scratched, hit, etc.

This paper will only discuss about the underground mine blast accident risk.  Until now, it is the highest accident risk at the UCM.  This risk has great concern and challenges to be managed properly to ensure a safer UCM area.   4.  UNDERGROUND COAL MINE BLAST ACCIDENTMethane gas and/or coal dust in underground coal mining has potential risk causing blasting accident at the underground coal mining. Methane gas has some characteristics, such as:a.       Has no color, b.      Has no taste, c.       Has no smell, and d.      Toxic. This gas has type weight = around 0.56.  It means the weight is lighter than air.  That is why the gas is usually trapped on the ceiling of  the UCM, especially at the curve areas.  This gas has possibility to blast at concentration of 5 15.  At concentration of 9.5%, this gas is easily to blast. Coal dust, which easily to be found at coal transportation facility areas in panel and tunnel areas, has specific characteristic; i.e. easily to fire and blast. The coal dust which produced at work panel areas, at coal seam tunnels and at UCM transportation facilities, has been proofed as a factor causing UCM blasting accident that have killed a lot of the UCM employees around the world. On the next table, we can see how many employees had been killed by the UCM blast accident around the world, especially those which have killed more than 300 employees in each accident. Table 1.  Fatality in Underground Coal Mining around the World (more than 300 victims) 

British Ox  December 12, 1886 361
French Coullier March 10, 1906 1,099
Japan Takashima March 28, 1906 307
Japan Toyokoku July 20, 1907 365
British Monoger December 6, 1907 361
Germany Lad Bold November 12, 1908 360
British Pretoria December 21, 1910 344
British Senghenice December 18, 1913 439
Japan Wakanabe November 28, 1914 423
Japan Usui December 1, 1914 422
Japan Houjou December 25, 1914 687
China Bujun Daisan Kou January 11, 1917 917
Japan Onoura December 21, 1917 369
China Kai October 14, 1920 431
China Sei May 22, 1940 341
China Honkeikou April 26, 1942 1,527
China Seian Taishin Dai Ikkou October 11, 1942 310
Germany Luizendal N/A 300
Japan Mike November 9, 1963 453

Source:   “Underground Mining & Safety Technology” training handbook released by

Ikeshima –
Center –


As previously concerned; by knowing the characteristics and the accident risks of underground coal mining, we can use this as a challenge to improve underground safety performance by reducing any kinds of accidents in these areas, including UCM blast accidents. The “Fire Triangle” concept is typically used to prevent UCM blast accidents. Based on the above concept, we can prevent UCM blast accidents by:a.  Monitoring and handling of fire flame / ignition; b.  Monitoring and handling of explosive / flammable materials (such as methane gas, coal dust, etc); andc.  Managing oxygen gas. 5.1.    Monitoring & handling fire flame / ignition 

There are some kinds of fire flame/ignition sources in underground coal mine areas, such as:a.    Blasting activities,              b.      Electrical equipment / installation,   c.       Safety / head lamps,           d.      Cigarettes / matches,e.       Mine fire / self combustion, and f.        Other sources.                      Based on the flame / ignition sources, we can organize some activities to prevent the UCM blast accidents, as follows: a.    Blasting activities                           – Implement good UCM blasting permit                        g.       Electrical equipment / installation    – Use flame-proof electrical equipment             h.       Safety / head lamps                                    – Conduct pre-start checking prior to use the lampsi.         Cigarettes / matches                          – Organize personal check to employees and/or guests who want to enter the underground coal minej.        Mine fire / self combustion              – Install permanent CO gas detector– Install automatic fire fighting equipmentk.      Other sources                                 – Install “No Flame / Ignition” sign– Organize Safe Behavior Observation– Install “Centralized Monitoring System”– Install “Water Pockets” in some designated tunnels.  

5.2.  Monitoring & handling of explosive materialsExplosive materials in UCM could be:a.  Methane gas (CH4)b.  Coal dustIn relation to monitoring and handling of the explosive materials, some efforts to prevent UCM blast accidents are as follows:             a.  Install methane gas detector             b.  Drain the trapped methane gas             c.  Spraying road header or drum cutter (equipment) with water            d.  Spraying of UCM tunnels with limestone dust            e.  Concreting of UCM tunnelsf.        Regular maintenance / cleaning areas under conveyors and other hazardous areasg.       Install CO gas detector and monitoring system.  

5.3.  Managing oxygen gasSome efforts to prevent the UCM blast accidents could be conducted by managing the oxygen gas circulation in the underground coal mine, such as follows: a.   Increase the oxygen gas supply to where the methane gas is trapped to decrease the methane gas concentration in that area under it’s explosion point.  The explosion point of methane gas is between 5 to 15%.  Install “wind doors” and/or “wind bridges” to increase or decrease the oxygen supply to designated areas. b.   Stop the oxygen gas supply to where the explosion has happened, by blocking the blasted / fired areas with concrete blocks.  As explained in the “Fire Triangle Concept”, stopping the oxygen gas supply will stop the coal fire / blast. 


Based on the previous explanations, there are some conclusions that can be drawn:1.  The world energy consumption for coal is getting higher from year to year.2.  Coal can be mined through surface or underground coal mining methods.3.  The underground coal mining method has some limitations or characteristics. 4.  One of the characteristics is the high accident risk of UCM blast.5.  UCM blast accident has occurred hundreds of years ago, but until now the accident risk is still high. 6.  The principle concept to prevent UCM blast accident is using the “Fire Triangle” concept. 7.  Three items of concern with on the “Fire Triangle” concept are; Flame/Ignition, fuel and oxygen. 8.  Managing the three items properly will prevent the UCM blast accidents.  This type of accident has made thousands of the UCM employees dye.   Personal data of the Presenter:Ir. Eddy Suprianto, MAppSc was born in Banjarbaru on 27 September 1983.  He is working at PT. Arutmin
Indonesia as Safety, Health & Environmental (SHE) Superintendent.  Passed his post-graduate study at
University of
New South Wales –
Australia in Safety Science Department.  Attended the “Underground Mining & Safety Technology” training in Ikeshima –
Center –
Japan for about six months.
  Address:  Jl. Dahlina Raya No. 85 – Intansari – Banjarbaru –
Kalimantan Selatan.  Mobile Phone: 0811-501096.