- 3 - sand. Concrete sand is the coarsest and most valuable, and is used to make concrete; mortar sand is less coarse and is used by bricklayers; and bank sand is very fine and is typically used only to stabilize pipe bedding and create foundations. Sand and gravel of different grades can also be mixed into an aggregate used in construction. Sand and gravel are both dry mined with a backhoe or wet mined with a dredge. Dredges are more expensive than backhoes, but they dig up more sand and gravel, and do it more quickly. Because the water table is so high along the San Jacinto, they’re also the equipment most often used. What comes out of a dredge, though, is a watery mix of sand and gravel, which has to be pumped to a plant where the water can be drained off and the sand and gravel sorted by size or combined for aggregate before sale. Dredges in the Houston area can mine sand and gravel to depths of 60 to 70 feet, but operators regard a deposit as mined out-- whatever its depth--when their dredges run into the thick layer of clay that lies beneath almost the entire San Jacinto floodplain. Everyone in the industry understands that even the sand and gravel above this clay cannot all be mined economically. One constraint is the need to set aside some land for the workplant, another is a legal requirement of setbacks for pit walls adjoining a public road, and a third is the common-law obligation not to undermine the property of one’s neighbor.Page: Previous 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 NextLast modified: November 10, 2007