MAGNETIC DEVICES

Report on Field Trials

1. Trial Purpose:

Test the effectiveness of a magnet attachment to a backhoe bucket excavator for collecting metal fragments from hard ground (see illustrations and photographs, and earlier conceptual study).


The magnetic device and its proposed use Design of the magnet device

2. Procedure

The shovel was scraped over the ground so that its teeth break up the surface of the ground. Magnets mounted on the back of the shovel pick up and collect the exposed steel/iron fragments. Afterwards, conduct a metal count of what the magnet has collected, what is still left in the ground, and from that determine the results.

3. Summary

The trial was successfully carried out on May 3rd 1998 at the Bindoon Army Demolition Range approximately 120 km North East of Perth in warm and dry conditions (approx. 32°C).

The test results demonstrated that the excavator would easily break up the surface layer of hard ground with embedded explosion fragments (mainly steel), and the magnet removed nearly all (>95% estimate) of the metal fragments contained in the broken layer of earth. This was a much greater proportion than we expected.

4. Results

Refer to photographs and diagram of magnet mounting

Photographs 1 and 2 show the bucket with the magnet assembly mounted under the back of the bucket.

The bucket and magnet assmbly.
Photo 1		 Close-up of bucket digging into ground.
Photo 2

Magnet assembly on bucket during trial.

Ground is broken to a depth of approximately 75mm.

Before breaking it, the ground was very hard. A chisel was driven in with a hammer - but could only go in about 20mm.

 
Material collected ny the magnet.
Photo 3
Photos 3, 4, 5, and 6 show the material collected on the magnets. Material collected by the magnet.
Photo 4
Material collected ny the magnet.
Photo 5
Material collected ny the magnet.
Photo 6

The magnet assembly consisted of 8 large ferrite permanent magnets encased within a 6mm stainless steel box. The thickness was considered sufficient to protect the magnets without significantly weakening the field gradient. The stainless steel allows the accumulated fragments to be removed easily by hand, using gloves (Photograph 7)

Note that in practice an electro-magnet would be more practical – the magnet could be switched off to release the accumulated fragments.

A shield plate is mounted between the bucket teeth and the magnet, again to improve protection against accidental explosions.

The red strap (shown in the photographs) was used to adjust the angle of the magnet assembly to the bucket.

After one or two passes, the magnet was covered in small and large pieces of metal, and some small laterite stones (a proportion of the stones in the ground are magnetic). (Photographs 3,4,5,6)

Removing collected material from the magnet.
Photo 7 - Removing material from the magnet.
The material collected by the magnet from a small area.
Photo 8 - Approx. 5kg of fragments from about 5sq. metres of ground.

It was difficult for the machine operator to keep the bucket at the right angle to the ground, and some metal pieces were scraped off the magnet towards the inner end of each stroke.

Photograph 8 shows material recovered from about 5 square metres of ground (required about 10 minutes work by excavator).

For this reason, the verification area (1 square metre) was marked out near the outer edge of the working area. This was then carefully inspected by hand to locate any metal fragments missed by the magnet (photograph 9). This was time-consuming and rewarding because very few pieces of metal were found (Photograph 10).

Because the ground was deeply contaminated with metal fragments as a result of several years of tests with large ordnance explosions and demolition tests, it was not feasible to use a metal detector to verify clearance. Many metal fragments are buried in the ground to a depth of at least two metres.


Checking the ground to look for fragments missed by the magnet.
Photo 9 - Verification check on the ground marked by rocks at the corners.
Quantity of metal fragments missed by the magnet.
Photo 10 - Largest quantity collected on verification check. 2 pieces are magnetic - the rest are non-magnetic.

5. Design Implications

Some lessons can be learned for designing a practical device for recovering metal fragments before manual demining.

  1. The magnet needs to be allowed to pivot with a skid to keep the bottom of the magnet at the right angle and height from the ground.
  2. The operator needs a reasonably clear view of the bucket teeth and magnet.
  3. The magnet needs to be retractable so that the bucket can be used for other tasks without having to remove the magnet.
  4. The bucket teeth should be narrower than the ones we used to reduce the amount of material pushed forward by the teeth.


 

6. Appendix - Trial Reference Information

1. Test Site

Bindoon Demolition Testing Range

2. Equipment Used

  1. Army Equipment:
    1. Hydraulic tracked excavator.
    2. Excavator bucket fitted with magnet.
  2. UWA Equipment used:
    1. Magnet attachment
    2. Metal Detector.
    3. Utility Vehicle.
    4. Passenger Car.
    5. Plastic Buckets (for sample collection), plastic bags, sticky labels, bag ties
    6. Prodders/digging implements (including manual excavator & star pickets)
    7. Rope, tape, posts for area marking
    8. Tarpaulin/mat
    9. Video camera & tape
    10. Camera & film
    11. Hats & sunscreen, insect repellent
    12. Pen & paper, recording forms
    13. Lunch & Refreshments

3. Personnel

  • Team leader (James Trevelyan, UWA)
  • Backhoe Operator (Army, 13th Fld Sqdn) (Sgt. Gary Mason)
  • Camera operator (still & video) (UWA, James Trevelyan)
  • Technician (UWA, Robert Heath)
  • 2 Assistants (UWA, Brian McLean, Dino Busuladzic)


 

4. Trial Schedule

The trial ran to the planned schedule as set out below.

  • 0700 Trials party meet at University, load equipment. (Equipment packing completed previous day) (45 mins)
  • 0745 Depart for Bindoon Military Base (refreshments in Bindoon). (2hr 15 mins)
  • 1000 Arrive at Range HQ, confirm with Range Officer, proceed to demolition range. (30min)
  • 1100 Mark test location 1, unload and set up equipment
  • 1130 Run test 1 using excavator (10 mins), followed by ground sampling (50 mins)
  • 1230 Break for lunch. (30mins)
  • 1300 Mark test location 2
  • 1330 Execute test 2, take samples (1 hr)
  • 1430 Execute test 3 (embankment, 1 hr)
  • 1530 Pack equipment, load equipment and samples (30mins)
  • 1600 Depart demolition range via Range HQ (15 mins)
  • 1630 Depart range for UWA. (Refreshment stop at Bindoon)
  • 1845 Arrive U.W.A, unload equipment


 
The project team and the backhoe.
The Trial Team:
Dino Busuladzic, Rob Heath, James Trevelyan, Gary Mason, Brian McLean.

Copyright reserved © 1998 UWA Demining Project
Author: James Trevelyan

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All graphics by Demining Research Team. Aug 1998.
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