Our company is relative new to this stuff... trying to settle a debate on the proper size of dozer for pushing through the cut. We are running 35 yd pans being pulled by 745s - material is fat clays and sandy clays. Are most people running D9s or larger, or anyone running D8s. If so, why and when would you run a D8?
That data is all in the Cat Performance Handbook for their lines of scrapers. I've never ran scrapers pulled by ADT's but regular wheel tractor scrapers like a 631 are mated to a D9. A D10 or two D9's in tandem will load them even better and get you closer to maximum productivity. That fat clay can be jammed in there to make such a load that the scraper can't unload evenly and/or takes forever to unload. There is a point of diminishing returns when bulking it out. Experience will dictate how long to stay in the cut. One D9 can load a 631 but two of them will pack it in and compress it more so that you're not hauling a fluffy load. My answer to the D8 is yes, it will load them but take much longer and may not even get a full load. If you have a D8 anyway and don't want to pay for bringing anything bigger in there for the economic side of the equation, then yeah it will work but performance will suffer. Just a trade off.
Is there a reason why you need to push-load the scraper? Wet conditions or ground too hard? Most brands of ADT Scrapers would have no problem quickly self-loading a bowl of standard clay/sand mix.
For equivalent size scrapers (cat 631’s) we would use D10’s with pushblocks and sometimes run in tandem if our D9L equipped with rear “stinger” was onsite. We would use a D8 for loading only if we didn’t have anything larger. We would also use a D7 or D8 on extremely soft jobs or when opening a new borrow pit to push the scrapers back empty to the push cat.
Just don't get crazy and push with 2 bigger dozers if the front one has a normal trunnion mounted blade. A hard pushing back D8 will add enough force to the dozer arms on the front machine that you will bend the dead axles out on the front machine. There was a reason they used bush blocks or inside mounted blade and rear push blocks on all but the rear push Cat. They were designed to transmit the energy thru the frame.