Half of the material has been dropped this morning already. It was a non event for the horses. No ace was used. Final answer: I was being paranoid 
In case anyone is curious here are more details about the situation. I currently have three paddocks that are useable. The dirt work is happening in the two larger paddocks closer to the barn. The third paddock is the smallest and has the least secure fencing (temp fencing on one side as we clear back to the property line to increase paddock size and replace property line fencing). We live in a busy agricultural neighborhood and the horses are used to all sorts of vehicles. The banging of the back gates of the dump trucks was my concern. I was worried my younger horse might blast around like a fool and my old (gimpy) horse might be hurt. Especially if I confined them to the small back paddock. What I did was put them in one of the larger front paddocks while they dumped in the other one and then swapped. Since the horses seem fine, I’ve got them in the small back paddock now while they dump the rest of the material and do the skid steer work.
Fortunately, the “dirt work” guy is accustomed to working around horses. He’s actually the preferred local arena builder in my area. Without me telling him I was concerned about the noise, he held each back gate as the dump trucks finished dumping to minimize noise and kept a sharp eye on the horses. 
We are feathering out the pad around the barn to reduce the steepness of the grade with base clay and then are topping the clay with footing, angular sand.
Here’s a picture for tax. Thank you everyone that replied. I really appreciate you all!