I have used 2-part epoxy putty to create sandbags and it works great. Available from just about any good hardware store and is fairly cheap. It comes in two "sticks" of putty that when blended together will give you a period of time to shape and form your sandbags, contour them, and press them into the formation you wish.
What I do is combine the 2 parts, roll the whole thing out into a "worm" about the size of the sandbags I want to make, cut then to length, press them with course material to give them a texture, use the end of my knife to press stitched and seams in the end of the bags, and finally place them in the situation that I want. Wait for them to dry and paint and weather accordingly.
Happy Modelling and God Bless
Robert