In cases I have seen where landfill searches were successful (Jennifer Blagg from CO is one who comes to mind, though daughter Abby was never found) police knew the suspect had used the dumpster at his work for several items as he was caught on video, therefore they had an idea of where to search, based on where the dumpsters in that area had been taken within the landfill from that day. It was all mapped out on a computer, i.e which dumpster pick-ups went where, etc. Even then, it took over six weeks of daily searching, if I remember correctly, to locate Jennifer. I really think this type of searching can only be successful when LE is almost certain a dumpster was used, and which one. To go just go in and try to search without that info would be an almost impossible task, as well as a dangerous one for searchers. It is not done lightly. I think Lori Hacking was also found this way, but with a detailed confession, and again, a known dumpster.
Jmo