how to check dubious jobsite soil for dump truck

[cfherrman]

Lol tool needed!

If your not sure drive in with your front first and watch your front tires, if your in trouble you can back out and be fine.

This is one example where I love an automatic, a manual will just keep going till your stuck, an auto you can feel how soft it is as you have to throttle to get through it and with some experience you know when it's too soft.

 

[damnescavator]

...
Sometimes it's better to lose a customer than damage a truck or piece of iron. What was the towing bill compared to the profit on the load of dirt?...

Not good I agree. I pulled it out using a winch and tackle. Problem with residential material delivery is so many customers want their stuff dumped in a bad area. Most do. They will mess you up sooner or later with their ignorance no matter how clever you are. I've been at this for years. TV cables, mud, water meters, branches, gates, buried plastic pipe, there's a lotta ways to get screwed doing this. I was just looking for an easy way to check dirt support. A dump truck pushes down 60psi+. That's a lot. That is why so few guys are willing to do this. And yes, it is best to refuse the money, not worth it usually. A hundred bucks per delivery but you lose an entire day if you get stuck. At best.

FWIW, I found an article from the US Army 50s that says soil compactor probes are a good way to test the "trafficability" of sandy roads.

 

[cfherrman]

All trucks put down 90-100 psi on the ground

 

[damnescavator]

"All trucks put down 90-100 psi on the ground"

You're not quite right for medium duty trucks at least. Here's a calculation.

An 11R x 22.5 tire is 11" wide pumped up to 100 psi and mounted on a medium duty single axle truck with 6 wheels weighs 33,000 lb loaded (mine).

1. Dry soil. If the truck is on dry soil, then tire patches form to about 10" length, tire patches are
10" x 11" = 110 sq in.
33,000#/(110 sq. in. * 6 tires) = 50 psi. per tire. Not 90.

2. Damp soil. On damp or wet soil the tire patches may extend to like 20" long.
20" x 11" = 220 sq in
33,000#/ (220 sq in * 6 tires) = 25 psi.

Class 8 dump trucks definitely have higher ground pressures. Medium duty dump trucks (class 7) are valued for lower ground pressures off road, and that is why I use one.

 

[cfherrman]

Check your tire pressure, that's your ground pressure. If your air down your tires your foot print increases as your tire pressure increases. You can weight your truck with some poster board, tire gauge, and and tape measure.

 

[MG84]

Class 8 dump trucks definitely have higher ground pressures. Medium duty dump trucks (class 7) are valued for lower ground pressures off road, and that is why I use one.

I'm not certain about that last part, depends on how the truck is spec'd and how heavy you're loaded in either case. I've run single axle class 6-7 dump trucks for years, with lots of off road miles. Just recently bought a tandem spec'd well for off-road use. It's got a fairly light engine/transmission combo and 385 floats on the front. Haven't got to run it off road yet, but the math says with a 14 ton load I'll have the same or less ground pressure than my single axle with an 8 ton load.

 

[damnescavator]

"Check your tire pressure, that's your ground pressure."

That's a rule of thumb for pavement. When the contact patch starts growing larger for dirt surfaces the pressure per square inch goes down. That's why I ran a calculation above.

As a thought experiment. What would the ground pressure be on pavement for a solid metal tire (does not exist but imagine)? It would form a very small contact patch much like a dozer track treadle. The weight on the small contact area would be high and it would probably bite into the pavement. This is why tracked machines leave indentations on asphalt. No internal tire pressure there, but a high ground pressure is exerted. Using the internal psi of a tire to guess ground pressure applies only to hard surfaces.

 

[cfherrman]

It applies to soft surfaces as well, otherwise tractors wouldn't use low psi to not compact the ground or trucks wouldn't lower the tire pressure not to compact the ground.

You probably would still get stuck with low or high psi in the tires in most cases with a truck

 

[damnescavator]

"I'm not certain about that last part, depends on how the truck is spec'd and how heavy you're loaded in either case."

True, here's some calcs to go with it. Assume our tandem truck has 11R24.5 tires at 100psi which are 11" wide. Assume our truck weighs 60,000 lbs. loaded.

Dry soil. For a contact patch on dry soil let's assume 11" wide tire x 10" length soil indentation,
110 sq in. per tire for contact patch. 60,000 / (110 x 10) = 54 psi. Similar to single axle.

Wet soil. For the contact patch on wet soil let's assume 11" wide tire x 20" length soil indentation,
220 sq in. per tire for contact patch. 60,000 / (220 x 10) = 27 psi. Similar to single axle as well.

Although the numbers are fairly similar for ground pressure between the two, you can't get around the fact tandems are heavier trucks and carry more material. They do more damage in a single pass than single axles trucks do although the actual ground pressure may be the same. In other words you would have to take two SA's through for every tandem load to equal the wear on the road.

 

[damnescavator]

"It applies to soft surfaces as well, otherwise tractors wouldn't use low psi to not compact the ground or trucks wouldn't lower the tire pressure not to compact the ground."

It does have a relation but it's not a direct or linear one. Weight over area is what determines psi at the ground, no getting past the fact. If you deflate a tire it will have a larger contact patch but it is not necessarily a linear relationship with psi inside the tire. It depends on the tire design. Truck tires do not deform all that much run a bit low on air, tractors are designed with flexible sidewalls that are intended to deform with low air pressure inside them.

 

[ps66x4]

There are those 4x8 sheets of plastic used for protecting the ground but I guess they could be a pain too.

 

[cfherrman]

"I'm not certain about that last part, depends on how the truck is spec'd and how heavy you're loaded in either case."

True, here's some calcs to go with it. Assume our tandem truck has 11R24.5 tires at 100psi which are 11" wide. Assume our truck weighs 60,000 lbs. loaded.

Dry soil. For a contact patch on dry soil let's assume 11" wide tire x 10" length soil indentation,
110 sq in. per tire for contact patch. 60,000 / (110 x 10) = 54 psi. Similar to single axle.

Wet soil. For the contact patch on wet soil let's assume 11" wide tire x 20" length soil indentation,
220 sq in. per tire for contact patch. 60,000 / (220 x 10) = 27 psi. Similar to single axle as well.

Although the numbers are fairly similar for ground pressure between the two, you can't get around the fact tandems are heavier trucks and carry more material. They do more damage in a single pass than single axles trucks do although the actual ground pressure may be the same. In other words you would have to take two SA's through for every tandem load to equal the wear on the road.

Junk in junk out.

Load the truck to 60k then measure the tire patch with poster board. Then do the calculations.

What's the tire contact patch of a 11" wide tire aired up to 100 psi and it's not mounted on a truck, just sitting on the ground upright.

First axle does 80% of the compaction

 

[MG84]

"I'm not certain about that last part, depends on how the truck is spec'd and how heavy you're loaded in either case."

True, here's some calcs to go with it. Assume our tandem truck has 11R24.5 tires at 100psi which are 11" wide. Assume our truck weighs 60,000 lbs. loaded.

Dry soil. For a contact patch on dry soil let's assume 11" wide tire x 10" length soil indentation,
110 sq in. per tire for contact patch. 60,000 / (110 x 10) = 54 psi. Similar to single axle.

Wet soil. For the contact patch on wet soil let's assume 11" wide tire x 20" length soil indentation,
220 sq in. per tire for contact patch. 60,000 / (220 x 10) = 27 psi. Similar to single axle as well.

Although the numbers are fairly similar for ground pressure between the two, you can't get around the fact tandems are heavier trucks and carry more material. They do more damage in a single pass than single axles trucks do although the actual ground pressure may be the same. In other words you would have to take two SA's through for every tandem load to equal the wear on the road.

I’ll have to report back on real world experience, single vs tandem off road. FWIW my tandem will only be 50k gross, and front tires are 15” wide. With that said, your calculations are off on tire contact patch. The actual tread width on an 11r22.5 is only 8.5-9” wide. If you’re sinking in to where you are reaching the 11” cross section of the tire you are already cutting some serious ruts.

 

[damnescavator]

"I’ll have to report back on real world experience, single vs tandem off road."

Ground pressure for a single axle truck may be similar to tandem but the closeness of the two axles ona tandem seems to beat down the soil more than a single axle. Just my perception, your mileage may vary.

"FWIW my tandem will only be 50k gross, and front tires are 15” wide. With that said, your calculations are off on tire contact patch."

My calculations were based on guesses for these tires, maybe so. However I just went out and tape measured my single axle dump truck on our parking lot and unloaded the tire tread is exactly 8.25" wide and the apparent tire patch (unloaded) is less than 6" long on dry pavement. Contact patch would be about 50 sq in here, but its an unloaded number and the total patch size increases a lot on soil especially soft soil.

"The actual tread width on an 11r22.5 is only 8.5-9” wide."

Correct. I stand corrected.

"If you’re sinking in to where you are reaching the 11” cross section of the tire you are already cutting some serious ruts."

Agree; but the whole purpose of this thread was to talk about ground pressure in soft risky soils. I still think the ground pressure goes (way) down when soil begins deforming which is exactly what it does right before you get stuck. The main question of this thread is, does anyone know how to test this? I think the only real way is to get our large butt out of our truck and push soil probes using a $200 Agratronix compaction tester.

 

[cfherrman]

Unless your using poster board to measure your footprint it's junk in junk out. Your measurement will match the tire pressure or you messed up.

It's very simple to test this, find a freshly plowed field, put 100 psi in your tires, drive in the field and out. Take tires down to 40 psi, drive into field in new spot and back out. Compare tracks.

If tire pressure didn't set ground pressure we wouldn't have tractors with 14 psi in the tires, all they do is drive on soft ground.

Look at your lawn mower with 10 psi on the tires, now change them for car tires with 50 and see how your lawn looks

 

[PeterG]

depends on how the truck is spec'd and how heavy you're loaded

Plain and simple, the softer or weaker the surface, and the heavier your total weight of truck and load is, and with aggressive tires slipping and turning, this will usually do the most damage. You can Discuss tire sizes, tire pressure, tire tread, axle spacing, number of axles (think DOT Bridge rules), but at the end of the day, what is certain is that more weight causes the most damage when the ground isn't hard enough. Construction crews often use ground protection mats to distribute a heavy load wider preventing ruts and to protect the surface when the ground is soft or fragile. If the ground is soft, even if you don't get stuck, you may end up leaving mud/dirt all over the driveway and road as you depart.