That really sucks. I hope you are able to get some form of restitution. I try to avoid any of these large trucks like the plague. Too many of them these days couldn't care less about the people they share the road with.
In hindsight, you should have called the police immediately so you could have at least gotten some kind of record of it. In one of your pictures it looks like the gravel is just pouring out of the back. That would definitely be a case of gross negligence for not properly securing their load.
Years ago, my family use to own a building supply business and lumber yard. We had a fleet of trucks, and if we ever caught a driver not properly securing their load, we'd fire them right on the spot. That isn't something you play around with, and if items are lost in transit due to negligence, it is basically like them reaching into your pocket.
Unfortunately, like someone else here mentioned, this carelessness isn't just limited to big commercial trucks. About 15 years ago, I was cruising along on the highway and came up behind pick-up with an unsecured full length wooden Adirondack chair in his bed. I am about 5 car lengths behind him, and the next thing I know, a gust of wind must hit it and the chair comes flying out of his bed directly at me as I am doing about 70 mph. Thankfully, I was in something sporty and just barely managed to pull into the other lane and around it as the chair hit the ground and smashed into pieces. The cars driving behind me broke off left and right like the parting of the Red Sea. If I had hit that thing at speed, I very likely would have died. Definitely one of the scariest experiences I've had in a vehicle.
