[hr]Specialist crash investigator and mine site driving trainer, George Foessel examines the reasons why the condition of road surfaces and driver training are so important to mine site safety.[hr]
The haul road by nature is designed to carry extremely heavy loads from one area of an operational site to another. The design of the haul road is based on a set of specifications set for the equipment using the road. For example the width must be adequate to carry approaching and departing haul trucks, road trains, light vehicles and cranes. The road surface must also be adequately designed to withstand the weight of the equipment using the road.
Due to the heavy loads and light and heavy vehicle interaction, a lot of consideration and planning must be made to ensure that the road surface has the correct co-efficient of friction. Therefore the roads are designed with a purpose in mind.
The road must have adequate width, ramp angles, berms and visibility. The important thing the every road needs is traction or friction. Here we briefly look at the co-efficient of friction.
WHAT IS CO-EFFICIENT OF FRICTION?
Co-efficient of friction is a measure of the amount of resistance that a surface exerts on or substances moving over it, equal to the ratio between the maximal frictional force that the surface exerts and the force pushing the object toward the surface.
Effectively, co-efficient of friction is grip or traction. In the simple form, it’s the difference between walking on a high traction surface like bitumen, compared to wet smooth tiles. The slipperier the surface the lower the friction.
One of the common denominators in the vehicle incidents I have investigated is a loss of control of the vehicle by the driver. It is rarely one factor alone that constitutes a vehicle or machinery incident.
For example, an incident involving a driver traveling along a wet haul road who loses control of the vehicle due to a lack of traction, cannot be blamed on the road surface alone. The driver of the vehicle generally shares some of the responsibility as may the organisation’s processes. Examples of this are where drivers have not identified a change in road conditions and were traveling at a speed which was too quick for the changed condition. This is quite common where the driver goes from a dry to wet surface.
It can be exceptionally difficult to foresee a change of environment on a field road compared to a public road. Out on a public road we generally work on the fact that we can see the rain, and if you are sharp, you can smell the rain and therefore know what to expect. In the workplace it is different; you have no real way of knowing where the condition might change.
A wet road equals reduced traction which equals less control.
We measure the co-efficient of friction (traction) to determine how the driver should have adjusted his speed, or if in fact there was anything he / she could have done to control the vehicle.
As a guide we generally conduct a braking exercise using a variety of equipment to measure friction. We generally do three consecutive tests on a surface and then add and average the runs. This gives us a very accurate reading of the level of traction/ friction available on that surface.
I normally measure friction based on g’s of deceleration. A high reading is a good grip, the lower the reading the less grip. For example a good dry bitumen road can achieve a co- efficient of 0 .97g or more.
We normally measure the co-efficient of friction in a straight line, and use the same technique on corners. Wet that same good bitumen road surface down and it may drop to between 0.7 and 0.54.
As the surface deteriorates the g’s of deceleration become less. I have seen readings as low as 0.2 which is equivalent to icy conditions.
The important thing to understand with friction or traction is that it is what is needed to keep the vehicle on its intended path of travel. The other factor is that the vehicle’s stopping distance will increase dramatically as traction decreases.
In a previous article I covered the effects of a low friction surface on ABS equipped vehicles. To conduct a friction test that is valid / accurate / and accepted requires training, and in some instances, specialised equipment.
So remember as the surface deteriorates or becomes slipperier, the longer the stopping distance.
“…it just is not good enough to provide staff with low quality training when they are working in a high risk environment.”
I know of one site with a 10 degree ramp that took a light vehicle 256 metres to stop. Why you may ask? There was “no traction”.
There are a broad range of variables when looking at a loss of control situation on a haul or field road.
It is surprising how many vehicles have defects that have contributed to the incident. This is happening even though there is a vehicle inspection book that has been signed at least once per day to indicate the vehicle has been inspected by a driver and is safe to use.
I would estimate that half of the vehicles involved in incidents have some form of defect as a contributing factor. There are two reasons that the inspection process fails:
- The operators or drivers tend to use the inspection as a process rather than a legal obligation to ensure the vehicle is in a safe operating condition.
- Very few drivers have had formal instruction on how to conduct a thorough and accurate pre-drive inspection. An example of this relates to tyre tread depth. This is a question we ask on each and every course we conduct, yet very, very few people know the legal minimum tread depth of a tyre. Nor do they understand the variation in tread patterns and how it may affect the vehicle’s handling or directional stability under brakes.
- Only one head lamp operational; driver did not see change in conditions. Dry/wet.
- No lighting of instruments; driver did not know what speed he was traveling
- Rear tyres no tread; front tyres new
- High traction mud tyre one side of car; standard road tyre on other
- Cracked and dirty windscreen
- Activated high beam and the lights dropped out
- Draw bar broke on trailer causing loss of control
And the list goes on…
“I would estimate that half of the vehicles involved in incidents have some form of defect as a contributing factor.”
Look at driver competency – has the employee been trained to tow a trailer; correctly conduct a pre-drive inspection; or drive the vehicle with a full understanding of its safety features and when and where to use them?
Lastly understand and identify road conditions.
Having spoken to persons in various industries, some training providers are delivering far from acceptable courses. We see the evidence of these programs through Credit Transfer or RPL applications. We recently had an applicant apply who completed a 40 minute driving course to be permitted to drive on site. This is normally an 8 hour course, yet somehow has been compressed into 40 minutes. Driver competency extends beyond the piece of paper.
Seriously it just is not good enough to provide staff with low quality training when they are working in a high risk environment.
One of the most serious incidents I have attended involved a vehicle roll over. The driver had never been shown how to conduct a pre-drive inspection and the vehicle had 27 faults. One of the major faults was that there were FOUR different tyres on the car.
The next major factor is that drivers need specific training on how to control a skidding car. The lesson here is that there is a big variation in training strategies for vehicles with Dynamic Stability Control versus conventional.
The purpose of the water truck is to wet down a road surface to minimise dust. Dust is minimised to maximise visibility and therefore improve site safety. As for the watering of the ramps and haul roads, the instructions and guidelines are specific and detailed.
Following is an example of a mine site procedure for haul road watering. This was provided to demonstrate the levels of complexity involved in getting the watering process correct.
Alternative Strip Spraying Technique
a) Spray Outside Lines
Spray the outside lines and include side rills. The dry line left in the centre is to be wide enough to allow passing trucks to pass safely with their outside duals steer tyres on dry pavement.
b) Spray Centre Lines
When outside lines are dry, then spray the inside lines by using the appropriate spray nozzle, leaving sufficient room for truck to pass safely with their inside duals and steer tyres on dry pavement.
c) Watering Ramps and Corners
When watering corners or down ramps the following measures shall apply to reduce the risk of loss of traction, loss of steering and reduced break effectiveness.
Strip spray technique shall apply to ramps and corners as for other sections of haul roads. Commence spraying any road a minimum 50 m prior to the crest or toe of a ramp or 50 m prior to the start of a corner.
Extra caution is to be taken when watering down ramps loaded because the load of water does move within the tank which can cause uncontrolled movements.
At intersections, the following process described in Figure 4 – Spraying Technique at Intersections should be carried out, unless high dust levels require the entire intersection to be watered.
Spot Spraying of Ramps and Roads
Spot spraying is not permitted for watering ramps or roads. The reason for this rule is that spot spraying leaves no continuous dry line. Two trucks are not able to pass safely in this situation. It is a hazard particularly as trucks lose traction up and down ramps in spotted areas.
Multiple light sprays are better than one heavy spray. In situations where there are limited water carts in operation, it is important not to apply extra heavy watering in one pass as this may create a major hazard.
Over-watering has the effect of reducing friction between the road and vehicle tyres and increasing the possibility of loss of steering, loss of traction and effectiveness of vehicle brakes.
All of a sudden the job of a water cart operator has actually become quite complex, and requires a degree of training, experience and understanding of the effects of the water on the varying road surfaces.
The Mines Inspectorate have identified wet roads as a real hazard and have distributed a number of safety bulletins.
MINES INSPECTORATE SAFETY BULLETINS
The Mines Inspectorate has released a number of Safety Bulletins regarding the subject of watering of roads.
We can be thankful the collision rate is low, as this is where the risk increases dramatically due to the varying size, weight and shape of the vehicles. For example a haul truck versus a light vehicle or medium truck, will generally end up with a tragic outcome.
As one can imagine, trying to control a massive piece of machinery as it begins to lose traction requires a strong heart and an extremely high skill set. More often than not it is extremely difficult to control large machinery in these circumstances.
A road surface is something we generally take for granted. But we need to recognise that staff are driving in challenging environments with varying levels of experience, be it dust, mud or slippery conditions.
Understand the risks on your work site and ensure you have strong procedures to back up your methods, and there will be a safer workplace for all concerned.
Traction or grip is the key to safety
For many years George Foessel was a Sergeant at the Queensland Police Driver Education unit and was qualified to deliver a broad range of high end Police driving courses. Aside from his Police qualifications he has trained with Police around the world including Germany, United States, Japan, UK and South Africa.
George is now the CEO of Motor School (Four Wheel Drive Training Pty Ltd). He formed the company in 1994 to provide specialist training service to the resources sector. The service extends beyond basic training to include specialised crash investigation and vehicle testing services.