George has it. Every soil material has it's properties (to the engineer gravel, dirt, and the other stuff are usually "soils"). For instance, gravel from a crusher will hold at a steeper angle than gravel from a river even if the parent material is the same. The particles interlock and that is one of the reasons soils are usually compacted. In filling a trestle you would try to find the closest and best source and you would take into account drainage as water running down the fill is also a factor. You would almost always use gravels over silts and clays as the latter react to water in unwelcome ways (they hold water instead of letting it drain out for instance, and many clays swell with added moisture). Interestingly, you would try to choose a source that is "well graded", i.e. has different sized particles in a deliberate design because that will compact best. In practice I'm sure it got quite wild and wooly but even the early engineers (millennia back) were relatively sophisticated with soils. The consequences of getting it wrong were well known.
I also find the Canadian video interesting when it shows no compacting (as we expect now) other than running the fill trains over it. This is especially interesting to me because the knowledge of the principles of using fill in this kind of application goes back beyond the Romans into unrecorded history. All the people in charge here knew about fill and compaction. The folks in the video did not spend time compacting because someone calculated it was not necessary for what they wanted to accomplish. Since the railroads engineers were certainly checking this contractor and they ultimately paid him, you can be sure the decision was made there. And that makes me suspect they were after something other than just supporting trains.
Eventually over time even with no effort made to compact it soils will gradually settle back to the same condition (compaction) the undisturbed source material would have had. But that takes a long time. They would have minimized settling by choosing materials where the settling would be as minimal as could be had, and they would have known. The trestle would take a long time to rot out, at least decades. Indeed this discussion shows that even now they are probably not all rotted out. Remember they would choose materials that drained well, and the timbers would now be protected from the weather. Notice that in the examples we have been shown, when the fill is removed the timbers rot away very quickly, even though they have remained relatively whole buried for a lot longer time. The fill is protecting the timbers. The railroad engineers probably figured the majority of settling would have occurred before any significant deterioration and would be dealt with as previously suggested by adding material. If it was gradual enough, the periodic maintenance of adding ballast might have done it.
My suspicion is they filled them not so much as to hold up trains (the trestles were doing that), but for other reasons. I have mentioned elsewhere recently meeting an old logger who recalled his first time riding in the cab of the steam locomotive at the invitation of the engine crew. When they came to a trestle the drop in the structure and its movement unnerved him. To the delight of the engine crew, I'm sure, who I suspect invited him for maybe just that reason! The old guys on construction crews still have such fun with the new hires on scaffolding and other places.
Interestingly, I knew what the old logger was going to say. I am an old guy. Things move and the inexperienced do not know that. If you want to try the experiment, find a busy freeway overpass (or a railroad bridge if it has an open walkway) and stand on it during traffic. Railroaders moving heavy trains at speed will not tolerate any track movement they can minimize. Whatever else happened, once filled those trestles lost their side sway and flexure and they were not going to move significantly.
But there is more: the continual inspection and maintenance needed for exposed wood structures stopped with the fill and that was a big expense.
Finally, think of one more thing every railroad feared; fire. Steam locomotives and railroad crewmen and passengers all smoked and produced sparks. And the trestle/fill is in a forest. Filled trestles can't and don't burn. So they don't need continual fire prevention and mitigation expenses either. I have seen the still visible remains of the oak barrel to hold water for a trestle way back on a Southern Pacific wood spur in the southern Cascades. Someone had to choose good barrels and someone had to make sure they were full. And that was only one fire measure. I have pondered that; the barrel lasted longer than the trestle.....
Timothy
(who has spent some time learning from older men who knew soils and what to do with them)
Edited 3 time(s). Last edit at 05/23/2020 12:23AM by heatermason.
