One reason the trucks are gaining ground on the scrapers is that they can do any type of earthwork.
Sand, no problem. Scrapers do sand, but if there is an odd rock in it, the apron can hang up on it and loose 1/3 of the load en route to the fill. It can be hard to get the rated load unless conditions are just right. Paddlewheels will fill every time, but unless it is sticky sand, you loose a lot en route also. The trucks haul a full load every time, and it stays in.
Rock, no problem. Scrapers, it depends. Paddlewheels are helpless in rock. Yes, the 353 and the 639 could, but you would still pay a price, and you can just about count the ones left in the world on fingers and toes. Push pulls in rock is hard on tire wear, and with tires for 637's around 10K a pop, you don't want to play that game too long. General maintenance on any push pull in rock is huge as well. Cutting edges at over $1,000 a set, and in rock, they might not last the week. Single engine with push cats seems the best in rock, but you still have cutting edges to change, and tire problems, though not as many.
Trucks, load it with a hoe. One machine with a bucket to work on and put teeth on. I have averaged between 100,000 to 300,000 yards per set of teeth on a 375 Cat, and that set costs just a bit more than ONE set of scraper edges. Scraper edges are usually done in by 50,000 yards.Keep the road clear and tire life will be at least 3 times what it is on a scraper, not to mention much cheaper. A set of 6 truck tires is about 35K, while a set of 4 scraper tires will run 40K, and last 1/2 or less. It don't matter how well your roads are kept, the scraper runs over the rocks as they are dumped. That is when most flats I see on well managed rock jobs occur. Trucks run on a good road, and shouldn't be driving onwhat they dumped till it is knocked down.
Good dirt. Trucks haul it with no trouble. Here the scrapers will usually be more productibve, but also usually with a high operating cost. Scrapers can scale up easier by adding units. Trucks need to be matched to the loading tool, so you run 1 spread, or 2, but not in between.
For a fixed location, you can select the perfect tool, but for a contractor, one job is dirt, the next rock, then sand, and back to rock again, etc. Trucks can go to any of them and perform.
Much is made of the added support needed for trucks when the scraper fans are talking. Except in ideal conditions, you will usually find some dozers maintaining the cut, ripping, digging corners out, etc. The trucks have a big hoe, and the dozer from the fill comes by now and then to clean up. In the fill, the scrapers spread, but unless you are just stockpiling, there is usually a machine spreading and mixing. The truck spread has a dozer with it the same way.
Now, for cost. 637 scraper hauls about 40 ton to 44 ton per load. Burns a bit over 20 gallons per hour doing it. My 40 ton Volvo's burn around 5 gallons per. Scraper ops are higher paid than truckies. The hoe burns fuel too, but so does the dozer maintaining the scraper cut. The parts bill for the truck spread is a pittance compared to the parts bill for the scraper spread. Same goes with the tires.
Now, I am not saying scrapers are going out, or there is no place for them. There are jobs that they are absolutely the best tool. But, for a contractor who don't know what the next job will bring, the argument for trucks instead of scrapers is pretty strong.
I have simplified things a bit, but I think you can get the picture.
