Ground Pressure / Compaction

[Soundguy]

For the generic tractor compaction 'argument' I think 'average' PSI should be fine..

soundguy

 

[John_Mc]

Found a good research article on the subject at this link the source is a professor at Penn State's College of Agricultural Sciences.

The shocker to me was the statement that "With most farm tires, surface contact pressure is about 1 to 2 psi higher than tire pressure due to stiffness in the tire." However, it did go on to say "The best way to determine contact pressure is to calculate the load in pounds per wheel and divide it by the area of the tire that touches the soil (in square inches). This will give you the average contact pressure under that tire in psi."

Another interesting wrinkle: compaction of Topsoil (0-12" depth) is dependent on "contact pressure" (basically weight of equipment divided by the footprint). Compaction in the upper subsoil (12-20" depth) is a combination of contact pressure and Axle Load. Below 20", soil compaction is entirely a function of axle load.

Axle load is the total load supported by one axle, usually expressed in tons or pounds. Farm equipment with high axle loads will cause compaction in the topsoil and subsoil, whereas low axle loads will cause
compaction in the topsoil and the upper part of the subsoil only

topsoil compaction will genreally go away in about 5 years. Upper subsoil in about 10 years. Lower subsoil compaction is essentially permanent - even in northern lattitudes where the freeze/thaw cycle reaches 40-50 inches deep.

Got my soil sciences lesson today.

John Mc

 

[Rara Avis]

In Vermont...our freeze-thaw cycle goes a little deeper...
Frost Penetration Prediction Using Simulation With GIS

 

[Nuru]

Ok, so, where can one find out the load per wheel? Or what is the easiest method to calculate it?

 

[RalphVa]

Where you get compaction is from the treads on a skidder. They'll put a huge amount of pressure on the soil. Same with Ag treads on a tired tractor. This is the benefit of using turf treads. They likely put less pressure on the soil than the heal of your shoe while you're walking.

Ralph

 

[John_Mc]

In Vermont...our freeze-thaw cycle goes a little deeper...

Interesting article. I live less than 10 miles from one of their test points (New Haven). I wouldn't dispute that the frost here can extend below the 40-50" cited in the article (even here in Vermont's "banana belt"), especially under pavement -- it tends to freeze quicker and deeper than soil. I'd guess this is at least in part due to the fact that the pavement is kept clear of snow, which would act as an insulator if left on.

The article I cited indicated that the soil compaction at least in the "lower subsoil" (which they defined as below 20" depth) was not affected by the freeze-thaw cycle. It was essentially permanent... even in places where the freeze-thaw cycle goes to 40 or 50 inches.

 

[Rara Avis]

Well, when I ran a waterline out to the barn...I was done by a lifelong Vermonter...he dug it 7 feet deep...as he said just in case we have one of those really cold winters...

 

[Z-Michigan]

Well, when I ran a waterline out to the barn...I was done by a lifelong Vermonter...he dug it 7 feet deep...as he said just in case we have one of those really cold winters...

I have to visit Vermont sometime... sounds weird. Michigan gets pretty cold and snowy too. In my area frost depth for building code is 52". While it is slightly less in the southern tip of the state, as you would expect, strangely it is significantly less in the northern lower peninsula - which is both colder and snowier. Apparently in many areas up there the snow is reliable enough to insulate the ground and reduce frost depth, despite low temps that may be 10+ degrees lower than where I'm at, and much more consistent freezes. (My area often thaws every 1-2 weeks even in January.)

 

[Rara Avis]

I have to visit Vermont sometime... sounds weird. Michigan gets pretty cold and snowy too. In my area frost depth for building code is 52". While it is slightly less in the southern tip of the state, as you would expect, strangely it is significantly less in the northern lower peninsula - which is both colder and snowier. Apparently in many areas up there the snow is reliable enough to insulate the ground and reduce frost depth, despite low temps that may be 10+ degrees lower than where I'm at, and much more consistent freezes. (My area often thaws every 1-2 weeks even in January.)

What happens up here is as the thaw happens on the surface ground...the frost is driven deeper...

 

[daTeacha]

Getting back to the original topic, there was an article I read somewhere in the past -- sorry, I'm no librarian so I can't tell you where -- about the different ways compaction is calculated in Europe and on this side of the pond. The European method takes into account some different factors and I believe measures compaction to a greater depth as well to either side of the tire. I know someone here will have the thing at their fingertips. I think it might have been on a tire manufacturer's website or something like that.

Along those lines, an energy efficiency article in the latest 'Ohio Farmer' states that most farmers run their tractor tires at too high a pressure, increasing slippage. It also emphasized the importance of the correct balance or front-rear weight distribution for maximum traction and efficiency.