Keystone Bags Fig 323

Keystone bags function in a bag arch the same way that a key stone does in a stone arch. Two to three bags are used as the keystone, depending on the type of arch being built. The purpose of the keystone bag is to force pressure down and out that is met with resistance by the walls on either side of the arch. (This is referred to as the buttressing of an arch.) This resistance directs the compression forces above the arch to the side of the opening and down to the ground. The gravitational forces of the earth embrace an arch. The dynamics of an arch are truly one of nature's magical feats of engineering. (For detailed instructions on keystone bag placement, refer to Chapter 6).

Bags use a stand. Tubes (continuous bag on a roll) use a chute. A tube chute can be made from a 20-inch (50 cm) long piece of sheet metal duct taped into a tube or a 20-inch (50 cm) section of sturdy cardboard, called a Sono tube, used as a form for pouring concrete foun-

3.23: It is stronger to have too narrow of a gap for the keystones to fit into than one too wide.

3.24: A tube chute acts as a sleeve to scrunch a length of tube onto, as well as a funnel that the dirt is fed into.

dation piers. Typically, we use an eight-inch (20 cm) diameter Sono tube for a 16-inch (40 cm) lay-flat width woven polypropylene tube. The poly tube is held onto the chute with a Bungee cord. You can make the tube chute any length that is comfortable for you. Use different diameter chutes for a variety of tube widths.

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