Flail Technology in Demining

by Ashish Juneja [ Indian Institute of Technology Bombay ] - view pdf

Figure 1. Typical relationship between speed and cutting depth of a light to medium demining machine in two different soil conditions.Figure courtesy of author.
Figure 1. Typical relationship between speed and cutting depth of a light to medium demining machine in two different soil conditions.
Figure courtesy of author.
Figure 2. The Pearson mine roller. Photo courtesy of Pearson Engineering.
Figure 2. The Pearson mine roller.
Photo courtesy of Pearson Engineering.

With the use of rollers, tillers and chain flails, the focus of minefield clearance has shifted since the early 1980’s from military to humanitarian demining. These machines can clear 200–300 mm of soil depending on the speed of the vehicle and its configuration, the soil type and the terrain. As seen in Figure 1, a 200 mm depth can be cleared if the vehicle operates at 0.5 kph. Unfortunately, heavy machines are difficult to operate at these slow speeds unless large amounts of power are available to run and rotate the flails.1 Moreover, recent literature cites the use of modern technology in demining (e.g., infrared imaging, ground penetration radar, thermal neutron activation and X-ray tomography). Mechanical machines, however, are still considered the safest tool for clearing minefields.

Demining Machines

Demining machines clear minefields by activating or outright destroying landmines. These are all-terrain vehicles, transportable and (partially) resistant to mine blasts.2 A large, steel-wheeled roller is a simple demining machine and uses the static load of its wheels to activate mines. Occasionally, a cam or a spring triggered impacting tool is added to the roller to hammer the ground. However, these machines are not effective for all ground conditions.3 Figure 2 shows a photograph of the Pearson mine roller.

Photo courtesy of Rheinmetall.
Figure 3. The Keiler mine flail.
Photo courtesy of Rheinmetall.
Figure 2. The Pearson mine roller. Photo courtesy of Pearson Engineering.
Figure 4. The MineWolf tiller.
Photo courtesy of MineWolf.

Flails and tillers are the most common mechanical demining machines. With flails, attached chains with hammers are rotated using a horizontal shaft, and the hammers impact or dig through the ground. While a large amount of dust and debris can be generated during flailing, this action results in detonation or fragmentation of near-surface mines. The rotation of the shaft is adjusted to optimize the digging depth. Results can improve if the chain-links are replaced by single or multi-lever links. Figure 3 shows the Keiler flail machine.

Tillers function similar to the flail systems. They use a rotating drum fitted with hardened chisels or teeth on its circumference to help dig or bite through the ground. The length of the chisel controls the clearance depth. A flexible knee joint helps increase the degree of freedom of the chisels and absorb most shocks. However, by doing so, the productivity of the tillers is reduced. Tillers use large, powerful engines and tend to be heavy and difficult to maneuver. Tillers can be coupled with flails to destroy mines more reliably. They have been used in Bosnia and Herzegovina and Croatia. Figure 4 shows the MineWolf tiller machine.

Classification

Depending on their weight, demining machines are classified as light, medium or heavy.4 Light machines weigh less than 5 tons, are remotely operated and can clear 100 mm thick soil. The MV-4 and Bozena (Series 1–4) are examples of light demining machines. Medium demining machines weigh up to 20 tons and are either remotely controlled or directly operated from its cabin. These machines can clear 200 mm thick soil layers. The RM-KA-01 and -02, Samson-300 and Hydrema-910 MCV are examples of medium demining machines. Heavy demining machines weigh more than 20 tons, use construction or military equipment as their under structure and are operated directly from the cabin. Some heavy machines can be equipped with double flails layers. Heavy machines can achieve a digging depth of over 200 mm. The Rhino-2, Zeus-1, Oracle and Scanjack-3500 are examples of heavy demining machines. Table 1 summarizes the significant differences between the light, medium and heavy demining machines. Tires of many light and medium machines are filled with foam or water to help absorb shock and to protect the vehicle in case of detonation. Heavy vehicles have an armor shield and a double floorboard to protect the vehicle and its operator. Table 2 summarizes the demining equipment. As the table shows, not all commercially available machines are suitable for every anticipated minefield condition.

Parameters
Light
Medium
Heavy
Mine clearing capability
AP and AT
AP and AT
AP and AT
Clearing/neutralizing
mechanism specification
31 to 108 chains
48 to 72 chains
1 to 2 flail systems
Clearance width (mm)
100 to 2,100
2,000 to 3,500
2,700 to 4,000
Clearance width (mm)
50 to 100
100 to 200
up to 300
Clearance rate (m2/h)
3,700
highly variable
140 to 8,000
Mode of motion
Both track and wheel
Track most often
Track and wheel
Mass of machine (kg)
2,500 to 7,000
7,800 to 18,000
32,000 tp 35,500
Mass of flail unit (kg)
unknown
1,500 to 4,300
8,300 tp 19,900
Fuel requirement (l/h)
7 to 35
9 to 60
17 to 80
Maneuverability
Very maneuverable
Fairly good, limited with increased size
Limited to heavy and bulky
Transportability
By trailer/air or self-propelled
By trailer of self-propelled
Need low-bed trailer
Control
Remote
Operator controlled and remote
Operator controlled
Table 1. Light, medium and heavy demining machines.4
All tables courtesy of author.

Flail System Design

The magnitude of the impact force and the power required by the flails depends upon:

  Dok-Ing
MV-4
Bozena
04
Minemill MC-2004 Bozena
05
RM-KA-02 Aardvark Mark-4 Hydrema 910 MCV-02 Dok-Ing MV-10 Mine-Wolf Mine-Wolf Rhino-02
Engine make Perkins
1106C-E60TA
Deutz BF 4L913 Iveco turbo engine Tatra T3A-928-30 Perkins 1306-9T New Holland TM165-T - - Deutz engine Perkins 3012-26TA3 -
Power kW 129 98 190 170 168 121 272 - 270 663 656
Weight (tonnes) 5.6 6 9.6 11.4 12.5 15.2 18 18 21.8 33.5 58
Clearing speed kmph 0.5 to 2 - - - 0.3 to 0.9 0.2 to 1.1 1.4 3 0.8 to 1.5 2 1.3
Clearing width (mm) 1725 2225 2000 2810 2000 3500 3500 - 2800 3400 3000
Clearing depth
flail (mm)
200 250 250 300 300 250 200 350 150 300 250
Chain length (m) 410 - - - 450 - 1100 - 1210 1000 -
Number of flails - - 67 - 36 - 72 - - 82 -
Clearing depth tiller (mm) - - - - - - - 390 350 400 300
Rotating speed typical (rpm) 900 400 500 500 600 300 440 - - - 400
Performance
m2/hour
500 tp 2000 520 to 2500 1400 to 2000 1050 to 4900 500 to 2000 - - - 2800 5000 750 to 2500
Fuel consumption (l/hour) 15 to 25 - - - 25 to 40 23 - 30 to 70 42 40 to 80 60 to 110
Table 2. Summary of the demining equipment. Note that “-” indicated data is unavailable or inapplicable.
Figure 5. Types of hammers used in chain flails.Figure courtesy of author.
Figure 5. Types of hammers used in chain flails.
Figure courtesy of author.

At slow speeds, the footprints of the hammers overlap one another as they strike and cut through the soil. However, skip zones will occur if the vehicle moves at a fast speed. At high vehicle speeds, the chains are no longer straight and tend to drag or wobble along the ground. The volume of the soil cut depends on the shape and size of the digging tool. Hammers with sharp geometry increase the penetration and movement through the soil. They also produce erratic impacts. On the other hand, smooth and spherical hammers produce consistent impact.

The energy of the chain flail is calculated using the length and mass of the rotating assembly, and its angular velocity. The repeated impact transfers this energy to the ground. When the hammers impact, the vertical stress distribution in the ground is estimated using Boussinesq’s Equation, which is written as (Eq. 1)Equation 1
where σz is the vertical stress at depth z and radial distance r, and Q is the load at impact. Variable σz is calculated by assuming that the soil is elastic, homogeneous and isotropic. In reality though, most soils are non-elastic, heterogeneous and contain stones or plant roots, all of which can cause the hammers to wobble or meander over the ground. The problem is further complicated by the fact that the chain flails not only hit the ground but also cut through it. The energy demand to cut or shear through the ground can be very high. The effect of all these stresses cannot readily be incorporated within the framework of Equation 1. However, the results do permit a semi-empirical relation to be fitted to account for the state and consistency of the soil. The Equation is modified as (Eq. 2) where k is a soil dependent constant. Table 3 shows the value of k for different soils.

Soil condition k
Elastic 3
Overconsolidated/hard soil 4
Slightly overconsolidated/dry soil 5
Normally consolidated/soft soil 6
Table 3. The relation between soil condition and k (soil dependent constant).

Some researchers relate the soil condition to the coefficient of restitution, CR defined (Eq. 3) Equation 3as where v is the velocity of thehammer after the impact and u is the velocity of the hammer before the impact.5 Variable CR of zero implies that the entire energy is transferred to the ground. CR is greater than one if the hammer hits and explodes a landmine, to release large energy. In the usual case, CR varies between 0.3 and 0.7, because part of the energy is utilized to cut through the soil.

Deming machines are subjected to wear and tear, and harsh environments, which limits the use of complex and expensive equipment in most demining applications. Speed of the demining machine affects the quality of the demining operation. Although a slow machine increases productivity cost, its chain flails will not miss any areas, and the entire ground is cut during the impact process. The load at impact should detonate or fragment any mine within its zone of influence.

Large amounts of power are required to rotate the flails and propel the vehicle. This requirement increases the weight and size of the power-generating equipment. Heavy demining machines require powerful engines to overcome the topography and the soil conditions of any minefield.

Soil
Proportion %
Liquid Limit (%)
Plastic Limit (%)
Dry unit weight (kN/m3)
Water content (%)
Shear strengh parameters
Sand
Silt
Clay

c
(kN/m2)

φ
(degree)

Fine sand
100
-
-
25
-
15
9
5
31
Clayey silt
19
67
14
30
23
18
12
40
28
Table 4. Soil properties for fine sand and clayey silt.
Figure 6. Experiment setup.Figure courtesy of author.
Figure 6. Experiment setup.
Figure courtesy of author.
Figure 7. Hammers used in test.Figure courtesy of author.Figure courtesy of author.
Figure 7. Hammers used in test.
Figure courtesy of author.

Experiment

With demining equipment, the energy available to cut through the ground depends upon the soil characteristics and the configuration of the rotating chain flails. It may not be difficult to characterize the soils using the principles of mechanics if the gradation, packing and stress history of the geomaterial are known. On the contrary, the flail configuration’s effect on the cutting resistance is often hard to perceive because of the complex interaction of the shape, size and rotational speed of the hammers, in addition to its impact angle and penetration depth. Although some of these complexities can be reduced, they cannot be eliminated completely. The objective of this study is to estimate the energy utilized by the chain flails to cut through the soil. Laboratory tests were conducted to investigate the effect of shape and rotational speed of the hammer on the energy transferred to the ground.

Soil beds were prepared in the laboratory using fine sand and clayey silt. Table 4 shows the properties of the two soils used in these tests. As can be seen, both the soils belong to Class I of CEN (European Committee for Standardization) Workshop Agreement (CWA) 15044 classification. In total, 34 test beds were prepared by compacting the soil in a 1270-mm-long, 445-mm-wide, 750-mm-tall, steel container. The samples were compacted in layers at their maximum dry density using a 2.5 kg rammer falling from a height of 300 mm until the container was completely filled.

After sample preparation, the container was carefully placed beneath the flail assembly, fixed to a pedestal. The flailing assembly consisted of three 450-mm-long chains attached at 120 degrees to each other. The chains were rotated by a 415 V, 50 Hz and 20 HP induction motor, which was air-cooled during the tests. The setup was connected to a variable frequency drive to help maintain the motor speed during the flailing operation. The entire flail and motor assembly weighed about 180 kg when placed over the pedestal. Figure 6 shows a photograph of the experimental setup. Also shown in the photograph is the safety cage to enclose the flails during rotation.

Figure 8. Soil container after test using Mushroom-I hammers in clayey silt.Figure courtesy of author.
Figure 8. Soil container after test using Mushroom-I hammers in clayey silt.
Figure courtesy of author.

Hammers were attached to the free end of the chains. Figure 7 shows the four different hammers used in the tests. Each hammer weighed about 1 kg and was about 60 to 80 mm in diameter. It was perceived that the above spherical, cylindrical and mushroom shaped hammers would result in different soils resistance. During the test, the soil container was raised by 4 to 7 mm/s using a pallet truck to cut the soil using the three flails rotating at 150 to 550 rpm. In some tests, the pallet truck was replaced by a forklift to lift the soil container. The torque required by the motor to cut through the soil was recorded using an automated data logger. The test was stopped when over 250 mm thick soil was cut. Figure 8 shows a photograph of the soil container after the test.

Results and Analysis

Figures 9 through 12, and Figures 13 through 16 show the torque versus the depth of cut in fine sand and clayey silt, respectively for the different rotor speeds. As expected, the measured torque in clayey silt was more than that in fine sand tests. The torque increased with the increase in the size of the cut to reach a peak toward the end of the tests. The figures show that Spherical hammers required the largest torque and hence offered the greatest resistance to shear the soil, followed by Mushroom-II hammers. These findings also indicate that T and Mushroom-I hammers are more efficient to cut through the soil without causing a significant spike in the power demand.

Figures 13–16. Torque versus depth of cut in clayey silt using: (a) Spherical hammers, (b) T hammers, (c) Mushroom-I hammers and 
(d) Mushroom-II hammers.Figures courtesy of author.
(click here to enlarge figures)
Figures 9–12. Torque versus depth of cut in fine sand using: (a) Spherical hammers, (b) T hammers, (c) Mushroom-I hammers and
(d) Mushroom-II hammers.
Figures courtesy of author.
Figures 13–16. Torque versus depth of cut in clayey silt using: (a) Spherical hammers, (b) T hammers, (c) Mushroom-I hammers and 
(d) Mushroom-II hammers.Figures courtesy of author.
(click here to enlarge figures)
Figures 13–16. Torque versus depth of cut in clayey silt using: (a) Spherical hammers, (b) T hammers, (c) Mushroom-I hammers and (d) Mushroom-II hammers.
Figure courtesy of author.
Figure 17. Surface area of the soil bed cut by the flails. Figure 18. Cutting res
istance of the soils. Figure 19. Torque calculated in fine sand. Figure 20. Torque 
calculated in clayey silt.
(click to enlarge figures)
Figure 17. Surface area of the soil bed cut by the flails. Figure 18. Cutting resistance of the soils. Figure 19. Torque calculated in fine sand. Figure 20. Torque calculated in clayey silt.
Figure courtesy of author.
Figure 21. Ratio of measured and calculated torque versus rotor speed.
Figure 22. Comparison of the measured torque to the modified calculated torque.
(click here to enlarge figures)
Figure 21. Ratio of measured and calculated torque versus rotor speed.
Figure 22. Comparison of the measured torque to the modified calculated torque.
Figure courtesy of author.

Figure 17 shows the surface area of the soil bed cut by the flails. In the figure, the area of the circular segment, A1 is given by (Eq. 4) Equation 4
where R is the radius of the chain-flail (equal to 450 mm) and x is the depth of the cut after each revolution. Variable x equals the relative motion between the flails and the soil bed. Because there are two sides of the circular segment, the total surface area of the circular segments is equal to 2A1.

Figure 17 also shows the surface area produced by the hammer’s width. This area is generated at the base of the arc and is given by (Eq. 5) Equation 5
where A2 is the area produced by the hammer along the arch and d is the average diameter of the hammer. Since the overburden is small and insignificant, the effect of the vertical stress component on the shear resistance can be ignored. The cutting resistance or the force to shear through the soil can therefore be written as (Eq. 6) Equation 5where c is the cohesion component of the shear strength (see Table 4).

Figure 18 compares the cutting resistance deduced from Equation 6 with Mikulic’s equations for loose/soft and compact/stiff deposits.6 In Figure 18, the depth of the cut, x, is normalized by the length of the chord AB (see Figure 17). As Figure 18 shows, Mikulic’s equation tends to slightly overpredict the results of both the soils from about mid depth.6 Some difference between the two predictions also exists at shallow depths in clayey silt. But then, the difference is less than 10% and is considered to be within the acceptable range.

Since the effect of F in Equation 6 is to cut and remove the soil up to the surface, the work done or the energy, E1 is equal to (Eq. 7) Equation 7where H is equal to the depth of the cut (see Figure 17). E1 will be the maximum (E1max) when H is equal to 250 mm, and A1 and A2 are measured in the last cycle. In addition, the energy to lift the 3 hammers in one revolution, E2 is written as (Eq. 8) .

Therefore the maximum torque, T required to cut through the soil bed is equal to (Eq. 9) Equation 9.

Ito and Fujimoto suggested that T in Equation 9 be increased by 15% because of the impact loading.7 Watanabe and Kusakabe (2013) observed that the increase was not uniform but varied from 0% to 10% under high frequency loads in different soils.8 Because of the above uncertainties, the torque was varied from 1T to 1.15T in the above Equation. Figures 19 and 20 show the torque calculated using Equation 9 for rotor speed of 150 to 600 rpm. The effect of the impact loading on the calculated torque is shown as a thick blue curve in the two figures. Figures 19 and 20 also show the maximum measured torque deduced from Figures 9 through 16 for Spherical, T, Mushroom-I and Mushroom-II hammers. As can be seen, the calculated torque significantly underestimates the measured torque, the difference being more in fine sand tests. Also noteworthy is the scatter in the measured data. While some difference between the measured and calculated torques can be attributed to the effect of the chain and its weight, it still cannot explain the significant spread in the data. Because of low confining pressures, it is also unlikely that the soil particles would have crushed under the impact thereby increasing the measurement.

The difference can be resolved by further adjusting the calculated torque for high frequency loads. However, as Figure 21 suggests, the ratio of the measured and calculated torque is somewhat unaffected by the rotor speed. This imposes a limit on the adjustment, which would otherwise unrealistically model the soil yielding. Unfortunately, this procedure will not help tighten the observed data. One possible reason to explain the discrepancy is that when the soil flows past the hammer during impact, it has a tendency to dilate or expand, resulting in an increased penetration resistance. Dilation can be significantly high in fine sands. The buildup of pressure bulb ahead of the hammer and the tendency of the soil to dilate, depends upon the shape of the hammer. Spherical shapes mobilize significantly high resistance loads compared to shapes that have sharp edges.9 These effects cannot readily be incorporated in the simple framework but do permit a semi-empirical fit to the measured torque. Equation 9 can now be modified as (Eq. 10) Equation 10 where α is a fitted parameter. Table 5 shows the value of α for the four hammers in fine sand and clayey silt.

Soil Hammer α
Clayey site Spherical 1.6
  T 1.2
  Mushroom I 1.3
  Mushroom II 1.4
Fine sand Spherical 2.9
  T 2.1
  Mushroom I 1.9
  Mushroom II 2.6
Table 5. Parameter α in clayey silt and fine sand.

Figure 22 compares the measured and the modified calculated torques using Equation 10. As can be seen, the measured torque is now reasonably well predicted, indicating that the data can be accommodated within a highly deterministic framework.

Conclusion

The discussion presented data relating to the effect of the soil type and the shape of the hammer on the energy transferred to the ground. The results showed that the torque required to flail through clayey silt was about twice to that required in fine sand. Spherical shaped hammers utilized the maximum torque in both the soils. The least resistance was observed with Cylindrical and Mushroom-I shapes. Torque calculated using Mohr Coulomb shear failure criterion underestimated the measured torque. This is partly attributed to the shape and size of the hammer, which cannot readily be incorporated within the above framework. However, the results do permit a semi-empirical relation to be fitted to the data. c

The author wishes to thank Dr. B.A. Mir and Dr. Raghunandan M.E. for their help performing the laboratory tests during their teaching assistantship at IIT Bombay. This work was supported by R&DE, Defense Research and Development Organization, Pune, vide project code RDE/91335/CMF/CE. The contents of this paper are solely the responsibility of the author and do not necessarily represent the official views of R&DE, Pune.

 

Biography

Ashish Juneja Ashish Juneja, Ph.D. is an associate professor at the Indian Institute of Technology (IIT) Bombay. He holds a doctorate in geotechnical engineering from the National University of Singapore (2002). He holds a master’s degree in soil mechanics and foundation engineering from IIT Delhi (1996) and a bachelor’s degree in civil engineering from the University of Roorkee 1993 (now IIT Roorkee). Dr. Juneja worked intermittently in the industry for over six years in India and the United Kingdom before returning to academia in 2005. His research interests are in dynamic soil behavior, numerical and physical modelling of underground structures and ground improvement works.

Contact Information

Ashish Juneja, Ph.D.
Associate Professor
Department of Civil Engineering Indian Institute of Technology Bombay
Powai, Mumbai 400 076 Maharashtra / India
Tel: +91-22-25767327
Fax: +91-22-25767302
Email: ajuneja@iitb.ac.in
Web: http://www.civil.iitb.ac.in/~ajuneja/

Endnotes

  1. Leach, C.A. “Evaluation of Aardvark Mk IV Flail.” Defence Evaluation and Research Agency report for Department for International Development (December 2000) 77. Accessed 18 May 2016. http://bit.ly/1NypXD0.
  2. Cvetkovic, Milenko. “Minemill MC2004 demining machine: a machine for mechanical ground penetration for demining an destruction of AP landmine and unexploded ordnance.” Paper presented at the International Symposium for Use and Testing of Demining Machines, Sibenik, Croatia. 25–28 April 2005. 53–54. Accessed 18 May 2016. http://bit.ly/1TjWfhY.
  3. Burton, Thomas. “Development of a mechanical means for AP landmine neutralization.” Master of Science thesis, Department of Agriculture & Bioresource Engineering, University of Saskatchewan, Saskatoon, Canada, 19 May 2006. Accessed 18 May 2016. http://bit.ly/23XfTFt.
  4. Bach, Havard. “A study of mechanical application in demining” Paper for Geneva International Centre for Humanitarian Demining, Geneva, Switzerland. May 2004. Accessed 18 May 2016. http://bit.ly/1TZu1v9.
  5. Shankhla, V.S. “Unravelling flail-buried mine interaction in mine neutralization.” Technical report for Defence Research Establishment, Suffield, Connecticut. December 2000. Accessed 18 May 2016. http://bit.ly/206mj4m.
  6. Mikulic, Dinko. Design of Demining Machines. London: Springer-Verlag, 2013.
  7. Ito, T. and Fujimoto, K. “Strain rate effects on stress–strain relationships of sand.” Journal of Theoretical and Applied Mechanics 30, no. 1 (1981), 17–24.
  8. Watanabe, Koji and Osamu Kusakabe. “Reappraisal of loading rate effects on sand behavior in view of seismic design for pile foundation.” Soils and Foundations 53, no. 2 (March 2013), 215–231. Accessed 18 May 2016. http://bit.ly/1srUqso.
  9. Kelleher, P.J. and M.F. Randolph. “Seabed geotechnical characterization with a ball penetrometer deployed from the portable remotely operated drill.” In Frontiers in Offshore Geotechnics: Proceedings of the International Symposium, ed. Susan Gourvenec and Mark Cassidy 365–370. London: Taylor & Francis Group, 2005. Accessed 18 May 2016. http://bit.ly/1OIWOFx.