At times it is necessary to build a foundation on a clay or shale base. It will then be advisable to proportion the base larger than usual and to anchor the block to the bedrock by drilling or boring a number of holes, " fanning" them out to cover as much area as possible. The holes should be expanded with powder and 1-in. rods that have been upset or barbed at the end cemented in the holes and left projecting into the cement superstructure. Even should the formation be solid rock, the block should be anchored by the same method, but with fewer rods put in more perpendicularly. The surface on which the concrete rests should be uneven to afford a better grip and should be wetted before pouring the concrete. Having excavated a hole about 2 ft. larger each way than the dimensions of the block, build the form of 2-in. planks and hang the bolts or the pipes which will receive the bolts, from cross-beams in the exact places they will occupy. Reinforcement of old steel cables or barbed wire may be used to .strengthen the structure. The pipes for the bolts must be wired firmly in place and the form must be well braced to prevent movement while tamping the concrete.

As the concrete is mixed and shovelled or dumped into the form it should be rammed with a heavy pounding iron or a 4 by 4 scantling so shaped that it can be easily handled. If spaces must be filled with boards to give the form the right shape, these blocks must be made so that they can be easily taken out when the concrete has set.

The top of the block should be finished with a couple of inches of cement mortar containing 1 part of cement to 2 of sand and, before the final set the surface, should be levelled. This plan is better than grouting concrete under the casting after it has been put in place; in fact many concrete structures have failed because of this latter method of finishing, because the concrete rammed under the casting makes a poor bond with the concrete block. Bearing in mind the fact that a slow setting concrete is finally stronger than one that sets rapidly, the structure should be covered with canvas and kept wet until the final set has taken place, ordinarily about 7 days. For patching worn spots in a tube mill with pebbles cemented in a matrix of sand and cement 36 hr. is sufficient time to allow for the set, but for any extensive repairs of this character at least 3 days should be allowed.