That looks like quite a few homebuilt trailers I've encountered. The axle looks to be mounted at the midpoint of the cargo box, which balances the gravel load all on the axle (mobile home axle, too). Slipper springs, and light ones at that. A properly built box trailer should have closer to 60% of the box in front of the axle, so that tongue weight increases as the load does. Proper tongue weight is important - too light will cause the trailer to whip at speed, and that's pucker time! Quite often results in a very bad day.