Open Cut » Overburden Removal
The aim of this project was to develop a method for obtaining quantitative data to characterise dragline bucket filling behaviour. This has been achieved and data has been collected for four bucket filling scenarios:
1. Conventional BE bucket (Howick mine)
2. Esco Mark IV (Newlands Mine - Dragline 1)
3. Esco Mark IV (Newlands Mine - Dragline 2)
4. BE HPS III (Ravensworth Mine - Dragline 302)
The original methodology of the project had to be altered as problems arose and practical limitations to image acquisition using automatic digital photogrammetry methods were identified.
Nevertheless the project did show that terrestrial photogrammetrical techniques can be used as a cost effective means of quantifying bucket filling behaviour.
It also successfully quantified dragline bucket filling behaviour that was previously unknown, i.e.:
- Payload density distribution can exist throughout a bucket due to overburden and/or bucket characteristics
- Dragline buckets may carry payload volumes significantly above their rated capacity.
- Importantly the project also documented that significant over drag was common
In all cases the volume did not appreciably increase after 10 seconds of drag. This highlights the importance of minimising overdrag when it is possible to disengage the bucket.
An unexpected spinoff from the project may be a way of estimating minimum fill times.
Using this information to provide visual or audible warnings for operators, could significantly reduce the occurrence of costly overdrag.
The fill rates for the different buckets were essentially linear. The drag velocities for the different machines were not necessarily identical however. Trend lines for the fill rates were as follows:
- Howick 8.75 m3/sec (time 0 to 10 secs)
- Newlands Dragline 1 5.1 m3/sec (time 0 to 10 secs)
- Newlands Dragline 2 4.6 m3/sec (time 0 to 12 secs)
The average payload densities in the zones were as follows:
- Zone 1: Density = 2.23 t/m3
- Zone 2: Density = 2.09 t/m3
- Zone 3: Density = 1.79 t/m3
These results show that it is possible to have a density distribution through the payload in a dragline bucket.
This may be due to overburden characteristics or bucket type.
The implication is that the carry angle of a bucket may be critical to performance since it is essential that a dragline should move material as efficiently as possible, ie., the bucket payload density should be as high as possible so as to move as many tonnes of overburden in the smallest possible space.
If this is achieved then the required bucket basket size is reduced (and subsequent bucket weight).
Conclusions & recommendations
This project has demonstrated a cost effective technique for gaining quantitative data to characterise dragline bucket filling.
Four scenarios have been investigated. Each yielded data that indicate a variety of bucket filling behaviours exist depending upon bucket type, overburden characteristics and operating geometry.
It appears that the payload density in a dragline bucket varies significantly between situations and that a definite density distribution through a bucket can exist depending upon overburden characteristics and bucket type.
The use of digital devices for image acquisition may not be currently possible due to the contradiction of maximising image resolution versus covering a realistic field of view.
Real time reduction of digital images to digital terrain models requires more research due to the inherent nature of dragline operational geometry and conditions.
It is possible however that low resolution application for DTMs of overburden topography (eg. Indicating that the rear of a bucket is full) could be developed from existing hardware and software technology.
With additional work on different dragline buckets and overburden types and a larger statistical universe of results the observation of variation in density distribution throughout a dragline bucket could be confirmed or denied.
Perhaps more importantly, it may be possible to develop a technique for quantifying fill times for particular buckets and overburdens thus helping to control the costly problem of overdrag.