re: Engineers of the OB: Will this bridge support a loaded log truck?

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Pickup trucks rolling over it no problem.

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Logging truck access back there for like one week

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There will be videos correct? Hope all is well.

“And will weld some supports under the center span and sink them in concrete in the bottom of the creek.”

Puss.

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culvert and backfill. 80000 is a heaping pile!

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yep. i do this stuff all the time, and a culvert is your best bet, no wider and deeper than that ditch is (it's hard to tell for sure from the pics). a low water crossing would be next, and tbh, spanning such a small & shallow creek wouldn't even be on my radar if loaded log trucks were on the agenda. it's just not worth it, but if i did span it, then it'd be with a rail car and nothing else.

but as far as the culvert goes, since small creeks like that tend to flash flood, depending on the flow rate of the creek, there's one caveat to the culvert: make a wide enough relief cut on each side of the culvert so that the log trucks can not only cross the culvert, but also traverse the relief cuts while the cuts themselves protect the culvert in the event of the volume of flowage exceeding the culverts ability to handle it. regarding the cuts, think, "wide and shallow" relief cuts below the finished level of the top of the backfill, and finish the relief cuts off with packed limestone rock so that the creek doesn't decide it likes the cuts and changes course after scouring out the cut ( i've got that t-shirt). you would also do this with the rock for the benefit of the trucks and traction. in the event that you put the relief cuts in, i would also add some smaller aggregate for the purpose of it being a smoother crossing, but with the assumption that the aggregate would need to be periodically replaced after major flood events.

oh, and no deeper than the ditch looks to be compared to its width, i would look at putting in a wide box culvert instead of the appropriately sized and "taller" round culvert that you'd need to use in order to handle the flowage of the ditch while it's at action stage. among other things, the box culvert would provide better visibility and decrease the "hump" effect that a big round culvert would likely cause in such a shallow yet wide ditch. you want a loaded log truck to have as little "ups and downs" as possible so as not to hang up and drag, and a box culvert with a finished elevation not far above the top bank elevation with a shallow, wide relief cut on one or both sides would provide a smoother crossing and minimize the chance of hanging up in the low spots (the relief cuts).

if i did the box culvert, it would be fortified with large aggregate/rock/ recycled concrete
slabs on the upstream side (and preferably on the downstream side but definitely on the upstream side) since the culvert and backfill act as a sort of "levee" when the flowage exceeds what the culvert can handle. in the case of that event happening, as water begins to build up at the fortified upstream side of the culvert, since overtopping is the kryptonite of a levee, you have a solution: the water can flow through the relief cuts and around the critical backfilled area of the culvert.

if done correctly, the setup above would provide years of reliable use for damn near anything that needed to cross the ditch, but if you decide to span it with a bridge, especially in small creeks and rivers that flash flood quickly and violently, you're messing up if you don't build up the final elevation of the superstructure (the bridge itself) well above the top bank elevation. plan for the abutments to be as high above the top bank elevation as is needed to give clearance until the creek can reach flood stage and give relief to the channel's water elevation. if your bridge is sitting at top bank elevation, what'll happen is that the creek will get up to action stage before it spills out into flood stage, and if your span is not well above action stage then it will be at the mercy of the action stage's current, as well as any debris that might be coming down the line like, say, a tree. once a tree or two or 10 slams into the span, the span's integrity can/is/will eventually be compromised, and a compromised bridge is as useless as goose shite on a pump handle.

if it were me and my main concern was the crossing of log trucks, pickups, utv's, etc. and to be able to do it on a budget, no deeper and wider than the ditch looks to be, assuming it goes from flood stage to a trickle in a matter of hours, i would build a low water crossing. i would put in one or more smaller culverts with debris guards on them so as not to dam up the intermittent flowage of the ditch upstream of the low water crossing as much as possible and then backfill and put in large rock and mortar it in with smaller aggregate and maybe even cement. make the approach as long and gradual as is needed for trailer truck crossing, which shouldn't be much for that shallow of a ditch, and you would be able to cross it with just about anything and at just about any time except for major flood events (and even then, depending on the final elevation of the low water crossing you'd be able to traverse the creek during minor flowages) and at a minimum amount of cost.

or you can always find a guinea pig to see if what you have will work, but there's nothing more frustrating and useless than a bridge that won't fit the bill. i've always said that you don't realize how important and convenient a water crossing really is until you don't have one. given the size and depth of the ditch, the box culvert is what i'd do if i wanted to do it right and had the money and time, and the low water crossing is what i'd do if i didn't. i'd save the option of actually spanning the water body for the deeper and wider water bodies, because at that point spanning it is likely the only best option.