Suitablity of Soils for Vehicle movement

Discussion in 'Members Corner' started by DivineHeretic, Nov 22, 2013.

  1. DivineHeretic

    DivineHeretic Senior Member Senior Member

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    During my nearly one year in DFI, the following questions have risen countless times

    1. Is this region suitable for armored warfare?
    2. Is total weight the defining factor for determination of soil suitability or is it the contact pressure?
    3. Will Arjun Tank be able to move over this surface or will it sink?

    This time, I shall attempt to provide a rational understanding of when and where soils fail under wheel and track load.

    But before I begin, These basic points must be kept in mind.
    • The maximum axle load for a vehicle as defined by IRC is 8170kg.
    • The maximum wheel load as defined by IRC is 4085kg.
    • The contact area is assumed elliptical.

    However, for larger vehicles, we find that they usually employ dual wheel, tandem wheel even dual tandem wheel configuration to carry extra load without violating the IRC regulations. This load must be converted to a standard load so as to be able to compare and calculate for the multiple types of vehicles. This is done by ESWL.

    Equivalent single wheel load (ESWL) is the single wheel load having the same contact pressure, which produces same value of maximum stress, deflection, tensile stress or contact pressure at the desired depth as that caused by the multiple wheel arrangement.

    [​IMG]

    Where P is the wheel load, is the center to center distance between the two wheels, is the clear distance between two wheels, and is the desired depth.

    [​IMG]

    This transformed or equivalent wheel load is taken for all calculations in road design and also for soil suitability tests. Any loads and contact pressures calculated hence will refer to this load only.

    Now, Let's begin the main story.

    Stress on soil at a depth Z

    The following are the causes of stress in soil at any depth

    1. Stress due to the load of soil above the point

    2. Extra load, either point or uniformly spread load applied on the top surface of the soil.

    For vehicle movement, we are interested in the upper layers of soil only, up to a depth of around 2 m from the surface of the soil. As such, the weight of the soil is not a significant factor in soil failure.

    Soil stress due to wheel load, or due to any other object is given by,

    [​IMG]

    Please note that this pressure, known as contact pressure or ground pressure, is very important in determining whether the soil can or cannot bear the given load.

    The above equation would seem to imply that the stress at any particular depth is dependent only on the ground pressure of the vehicle. However, when the graph is plotted for stress v/s depth, we find an interesting link between stress and pressure and overall load.

    I tried to find the graph online but it seems the graph is only available in the book Highway Engineering by CEG JUSTO & S.K Sharma. Anyway, I’ll provide the details of the graph.

    The tests were carried out using a 18 ton load and a 36 ton load with ground pressures (contact pressure) of 7 kg/cm^2 and 14 kg/cm^2. Each load was tested with both the pressures and the final result tabulated against depth of soil.

    The following are the trends discovered from the graph

    (This is the overall summary of this post)

    1.For any load at any pressure, for the initial depths, up to 10-15 cm, the contact pressure was the dominant factor in determining soil stress at that depth. I.e. the test with higher ground pressure had greater soil stress.

    2.At depth greater than 15 cm, however, the load became the dominant factor.

    3.When the pressure was kept constant and the load varied, we find the higher load has higher stress at any depth.
    This means the 36 ton load caused higher stress than the 18 ton load despite both having same stress.

    4.When the load was kept constant and the pressure varied, higher the ground pressure, higher was the soil stress at all depths. This is as per our expectations.

    5.The soil stress due to vehicle load and pressure becomes negligible after around 2 m depth.


    These trends show, very clearly, that the overall load is just as important, if not more important, in determining the soil stress at a depth.

    Particularly in case of tanks and other tracked vehicles, where 10-15 cm difference in soil elevation will be inconsequential to their mobility, it is seen that the weight of the tank will have a significant impact on the suitability of the soil to its movement.

    The failure caused by this is akin to walking on wet swampy soil, where we find our feet suddenly sink deeper and being unable to move. This form of failure, or failure by shear, is the biggest impediment to tank or vehicle movement.

    However, there is another failure that must be kept in mind, particularly for stationary vehicles. This failure is caused by consolidation of soil.

    Please note that this is just a simplistic overview of soil strength & not its failure criteria. I'll provide the failure criteria in the next post here. Also there exist several inexpensive and fast methods of quickly building up soil strength by a reasonable degree should such a need arise. If the members wish, I can provide some of the ideas
     
    Last edited: Nov 22, 2013
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  3. DivineHeretic

    DivineHeretic Senior Member Senior Member

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  4. Damian

    Damian Defence Professionals Defence Professionals

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    Actually modern tanks can be deployed in vast types of terrains, climate conditions etc. They are very universal in that matter. Of course there are obstacles that can stop tanks, but this is more a problem of proper training for tank commander and drivers to recognize such obstacles and avoid them.
     
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  5. DivineHeretic

    DivineHeretic Senior Member Senior Member

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    True that. But you can only push the laws of soil mechanics so much. You are limited by the soil on which your tracks roll. If the soil fails, you tank stops, no matter how universal they may be.
    This is why Indian army has not moved armored units to the North east of India, where the soil simply cannot support the load of a 45-50 ton tank.
     
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  6. Damian

    Damian Defence Professionals Defence Professionals

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    I don't know from where it comes, NATO countries use heavier 55-60+ tons tanks in vast types of environement and we do not have any significant problems. Sure sometimes one tank stuck somewhere, but this is also good opportunity for soldiers to train recovery operations.
     
  7. W.G.Ewald

    W.G.Ewald Defence Professionals/ DFI member of 2 Defence Professionals

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    The longest day I ever had at Fort Bragg happened when our M60 got stuck and the driver ended up breaking the drive axle and we got towed to the MATES by an M88.

    Here's a video of another recovery. (Excuse the language.)

     
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  8. W.G.Ewald

    W.G.Ewald Defence Professionals/ DFI member of 2 Defence Professionals

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  9. Damian

    Damian Defence Professionals Defence Professionals

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    I wonder if M88A2 is also considered to receive that new 1630HP diesel engine that was integrated with M1A2 recently, it would be nice to replace that currently used 1050HP diesel.
     
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