These are fill gravelbags made from birds eye gravel. The are filled in a poly polypropylene bag and weigh around 30-40 pounds per bag. The are used for erosion control just like sandbags but just filled with gravel instead. Some jobs require gravelbags since the gravel is less likely to erode over time. Another benefit is that over time the poly bags will get damages and less material will be washed onto the street since gravel is more coarse than sand.