Summary

Jacques DEMICHELIS found out that heavy chains dropped on the ground deliver high energy and high pressures, which in turn can trigger the antipersonnel landmines (APL). He joined ARTID (Association for Research of Techniques for Innovative Demining), who began to study and develop this idea. This gave rise to the DEMICHAIN project.

A web of heavy (about 10 kg/m) chains is mounted in a frame ; the weight per square meter ranges from 150 to 250 kg. DEMICHAIN is fixed either at a crane equipped with a free fall winch or at a grip on an hydraulic arm and lifted 4 to 5 meters over the ground to be cleared.

A free fall of the device delivers a mechanical impulse in the ground. The kinetic energy (» 10 000 J/m²) is coupled to the ground for a short duration, generating high pressures. The vegetation is crushed by the chains.

A direct hit on the triggering device of an APL lying on the ground, or of a trip wire, produces the detonation of the APL. If the latter is buried in the soil, the links of the chains penetrate into the soil and release a small part of the kinetic energy into compaction of the soil. Most of the energy propagates in-depth in the form of a "pressure wave", which triggers the landmines. The chains are likely to resist the blast of explosions.

In order to evaluate the efficiency of the DEMICHAIN concept, tests on real mines must be carried out (and this will be done as soon as possible).

However, before performing these final tests, a knowledge of the forces generated by the free fall of a DEMICHAIN device will help to correctly dimension the parameters of the mount. First tests have been made using small lead cones, which are crushed when a force is exerted on them. The crushed surface measures the mechanical impulse.

Force probes are under development, in which the temporal variation of the exerted force will be recorded. In other more simple devices, a micro-switch will close if the exerted force is sufficient to trigger an APL.

The first tests were carried out in the CORBIERES hills, end of February 2002.

Force probes consisting in small lead cones were placed on the ground for several tests. The crushing diameters of the cones exhibit a broad dispersion, resulting from an uneven distribution of the vertical chains on the ground.

The mean value of the measured diameters of the crushed cones is equivalent to the diameter generated by a 20 kg weight, which is larger than the force needed to set off most of the APL.

The device works well on force probes located on the bottom of a ditch.

The resistance of the device to explosions has been checked using up to 400 g of explosives. Except for a deformation of a part of the metallic frame, the chains behaved well.

Several tests have been made on bush. The chains crush the vegetation, the links reach the ground. It is possible that a part of the kinetic energy is absorbed by the vegetation during a first fall. With a second fall on the crushed vegetation, the ground will receive the full energy of the system and the APL will then be triggered.

A small device (50 x 50 cm²) has been set up under a shed. The 40 kg of chains fall on a 1 m² size, 40 cm deep soil bin. The purpose is to carry out parametric studies on the variation of the forces.

First tests have been performed on probes introduced in different depths under the surface. They show that the mechanical impulses do not decrease significantly in-depth. It can also be seen that the measures are rather dispersed. Careful investigations are needed to be sure what operating conditions will give a good efficiency.

First of all, the exerted forces are vertical and do not pulverize the upper layer of soil. DEMICHAIN can be employed on roads or trails and on the verges.

This system can be used on very uneven grounds. If there are holes, ditches, humps, small rocks, barbed wires, slopes, the system can be easily used or adapted (using vertical chains for very uneven grounds).

A free fall of the system crushes the vegetation, provided the bush is not too dense. It would be necessary to make several free falls on the same surface, the first ones crushing the bush and the last ones triggering the APL.

It must be carefully checked if all types of active landmines are triggered by this system. In certain cases, DEMICHAIN can be adapted in order to deliver short duration impulses, or longer impulses (which may be required for triggering mines like the VS50, which has a pneumatic delay to avoid being set off by an explosion).

The set off of an antitank landmine triggered by a magnetic mass is likely to occur during the free fall of the chains, which are a large metallic mass. A careful investigation of the behavior of DEMICHAIN falling on antitank landmines must be performed, because DEMICHAIN must trigger all antitank landmines and also survive the setting off of an antitank landmine.

A carrier needs only to be able to lift and then to release DEMICHAIN in a free fall and to be lightly armored (as is usual for demining tasks). This carrier can be any type of vehicle in use in civil engineering.

The carrier and DEMICHAIN can be coupled by a cable equipped with a free fall winch. In case of a strong effect on DEMICHAIN (explosion of a large landmine), this does not affect the carrier, provided that the distance between the chains and the carrier is large enough.

It is also possible to use an hydraulic arm equipped with a grip. The DEMICHAIN device is then released in a free fall without any coupling between it and the carrier as it hits the ground.

If carriers are available in the vicinity of the demining site, the DEMICHAIN tool can be adapted to these devices. On the contrary, if the carrier must be purchased for demining purposes, a good selection can be made to manage following uses, when the demining program ends: sustainable development.

In order to manage with security reasons, it would be interesting to adapt a remote controlled machine.

DEMICHAIN consists in a web of heavy chains (about 10 kg/m) attached on a frame ; the weight per unit surface ranges from 150 to 250 kg/m². The dimension of the frame and the arrangement of the chains can be adjusted to the requirements of the work to be done as well as to the available carrier. It is very flexible.

Chains are cheap. Mounting them in a frame can be carried out on every demining site. Maintenance is simple, it constitutes a very homely tool.

No patent is registered on this device.

We are looking for an opportunity to carry out series of tests of the DEMICHAIN system on real APL. A system with a realistic size (6 m²) will be built in the next future.

There are two places where tests can be performed :

In that last case, three types of tests can be realized :

1. Tests on APL which have been neutralized, then placed on a test area and rearmed for performing the test

2. Tests during an actual manual demining process : at the end of a day, several APL have been localized. Instead of destroying them by explosion, DEMICHAIN is brought on the place and tested on these APL.

3. Tests before a manual demining process takes place: a mechanical demining using DEMICHAIN is performed on a live area, which is then cleared using manual procedures.

The best way would be to perform successively these three phases, but the first one is possible only if the APL can be neutralized and transported on a test area where they are armed at new. If the APL cannot be neutralized and must be destroyed on place, then the phases 2 and 3 can be used. However, getting enough experience can need a lot of time.

As compared to other mechanical demining systems, DEMICHAIN is a "pulsed" system, which works on a given surface in one fall (or a limited number of falls). However, each fall must be prepared by a lift sequence, and the whole process is discontinuous. Therefore, the system must be operated in a quasi-continuous way, linking a succession of lifts and falls, the carrier moving continuously. The speed of this carrier can be adjusted so that every point of the surface will be affected by several falls, increasing the efficiency.

The main difference of DEMICHAIN principle with the flails consists in the area of action of the mass.

For the flails, the hammers work along a straight line which moves continuously. The resulting mechanical impulse in the ground varies inversely proportional to the depth. In comparison, DEMICHAIN falls over a whole surface, yielding a flat pressure wave which decreases in-depth only by losses, like compaction.

In order to keep a continuous circular motion of the flails, the angle of attack of the hammer with the ground must be shallow and the vertical component of the exerted force (which triggers the landmines) is small as compared to the horizontal component, which produces the unwanted effect of pulverizing the soil. This pulverization effect alters the upper layer of the ground and may greatly affect its agricultural capacity.

On the contrary, DEMICHAIN exerts only vertical forces, which can produce a certain compaction of the soil (if this is loose), but do not alter the structure of the ground. It simulates the forces generated by a person, which the landmines are built to be triggered by.

The sensitive point will be to determine which type of chains, which arrangement of the chains, which operation mode will guarantee that all the surface to be demined will receive the appropriate hits for triggering all active mines?

In other words: how to reach and to guarantee the overall efficiency of the system (the famous >99,6%) ?

With which other techniques can DEMICHAIN be associated ?