I got my tandem dump truck stuck up to the axles in a soft loam field. I had a load of damp clay in the box. The truck has a weird hoist with a solenoid operated valve that is finicky. I got the hoist up 2/3rds and unloaded about 1/3 of the clay. Then used the 931B with backhoe to get about 3/4 of the clay out. Put the box down and hooked up the Cat to try and pull it forward. Wouldn't move so I put the Cat in 1st gear spinning and hopped in the truck, no go. Then I took the hoe and pulled as much clay as I could away from the back of the truck. Finally I took my skid steer to get as much clay cleared out as possible and made a pad about 18' long to pull on. Then I hooked the Cat to the pintle hitch and it rocked the truck slightly. I thought I was stuck for good. I didn't want to dig more holes with the Cat so I gave it just enough throttle to before it would spin. Then it dawned on me I should engage the inter axle lock on the truck. The truck seemed to want to move so I gave a little throttle and it climbed up on my pad. I was so glad it came out. It's pouring rain here now and I think the truck would have been there all summer had I not got it out. Couldn't believe how much difference the inter axle lock made when buried up the axles. One of those things you regret 2 seconds after you realize the ground is soft.
15 years ago I had a customer with 5 Peterbilts with air leaf suspension, even with the interlock in they would get stuck on a banana peel, I put Detroit lockers in the rear diffs and then they would climb trees. That gave them true 3 wheel drive with the interlock engaged.
I've heard air bag suspension is horrible off road and not the best for a dump truck.
If a tandem dump truck is going off-road regularly, especially to dump, the rear suspension setup ought to look something like this IMO. Personally I wouldn't take anything with rear air bags off good solid asphalt.
The truck has Hendrickson walking beams like that.
The trucks I mentioned above were road tractors loading semi trailers in the field. I too would stick with walking beams on a dump.
I've been stuck on banana peals several times recently. Usually when empty. In every situation, I would've done well to remember the: The law of conservation of energy is one of the basic laws of physics and therefore Kinetic energy - energy of motion. Potential energy - energy of ``location'' with respect to some reference point. I keep saying; next time, I'll keep the velocity up, when off road. engaging the interlock early really helps too.
The best part about carrying more speed is that those times when you still get stuck, at least is is a worthy one to remember.
In so cal here, they always take used long haul trucks and convert them into dumpers. The current Pete I use has no sub frame on the chassis and about 8" ground clearance. The tanks are flat on the bottom, and we baby this thing. I haven't got stuck yet tho. 2 fav stories, I'll summarize.. 92 kw, on 6" of gravel all day in between pad footings, stocking dirt across site. The boss got it stuck after talking smack that I was going too slow. I was slipping the clutch so it wouldn't spin and make a bunch of holes everywhere. I had to shovel his ruts in. At a rebar yard, forklift tried to pick up a cage in a field and got stuck in a puddle of powder clay. The truck driver unhooked his semi, hooked the lift with a chain, and spun on a banana peel. He didn't have axle locks. I told him to unhook and hooked it the the empty trailer behind the ancient dodge Cummins I was in and pulled him out first tug. His truck with no trailer had zero traction on flat ground. We were all tickled. I teased him that the Prius in the parking lot could have pulled that lift out. That being said, mud is way worse. We rarely drive in mud or snow out here.