Re: Pile Bent versus Timber Bent trestles

I'll take a SWAG that assuming a pile driver was available, the terain, or more correctly what's below the surface determined which method was used. I've noted that SVRy used driven piles on trestles in the valley floor where they crossed the Powder River. These area would have fairly deep river bottom gravels suitable for driving piles into. In other places such at the Alder Springs Trestle, they used mud sills. Mud sills are simply large timbers much larger in cross section than the bents laid on the ground horizontal perpendicular to the span of the bridge or trestle. The bottom of the bents rest on the mudsills. At Alder Springs, solid rock justs out of the hillsides on either side of the ravine. To place the mudsills it probably took a bit of black powder to remove enough rock to lay the mudsill on a bench. Nearer the bottom, there is soil, but probably not enough depth before reaching solid rock to drive piles into.