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Double and triple bottom dumps consist of a 2-axle tractor pulled by one single-axle or double axle semi-trailer and an additional full trailer (or two full trailers in the case of triples). These dump trucks allow the driver to lay material in windrows without leaving the cab or stopping the truck. The main disadvantage is the difficulty in backing double and triple units.
The specific type of dump truck used in any specific country is likely to be closely keyed to the weight and axle limitations of that jurisdiction. Rock, dirt and other types of materials commonly hauled in trucks of this type are quite heavy, and almost any style of truck can be easily overloaded. Because of this, this type of truck is frequently configured to take advantage of local weight limitations to maximize the cargo.
For example, within the United States the maximum weight limit is 40 tons of load throughout the country except for specific bridges with lower limits. Individual states, in some instances, are allowed to authorize trucks up to 52.5 tons. Most states that do so require that the trucks be very long, to spread the weight over more distance. It is in this context that double and triple bottoms are found within the US.
Bulk density of both fine and coarse normal aggregates varies within the range of 1450 to 1750 kg/m3 (90 to 110 lb/ft3). The bulk specific gravity of sand is 2.3 and gravel is 2.5.
Well, let me see. I am not going to go look up the density of western coal right now, but I can think of a 50 ton (payload) belly dump semi trailer that I saw a few of. It was on a tridem, however pulled by a 3 axle tractor for 6 axles total. If I think of how many yards it was rated I will post back.
Then I remember one particular triple set, grossed out at 300,000 lb. on 13 axles total. It was hauling some type of ore that I won't mention to protect the guilty. That was an off road haul, but the trailers themselves were used as singles at other times with 3 axle tractors to haul over the road loads (80K max).
Then there was another set of triple belly dumps that I remember that WAS over the road that grossed 180K on a special permit. That was coal too. When off the permit road, they dropped the third triple and operated as doubles grossing out at 120K.
Maybe these will give you some ideas of the relative capacities. The first was Al, the second and third ones mentioned were steel.
Tractors in each case weren't anything extra special-pretty much regular over the road rigs, but without sleepers.
One would think that the typical legal load limit (40,000 lbs.) divided by the commodity density (unit weight in lbs/cu. ft) would give the most realistic load capacity (volume in yards).
The legal weight for an eighteen wheeler is 80,000 lbs. If you assume a truck empty weighs 40,000 lbs., the legal load that the truck can carry is 40,000 lbs. Divide this by the density and you get capacity in yds.
