Clay puddle consists of clay and sand-made into a plastic mass with water. It is used principally to fill cofferdams, and for making embankments and reservoirs water-tight.

167. Quality of Clay

Opaque clays with a dull earthy fracture, of an argillaceous nature, which are greasy to the touch, and which readily form a plastic paste when mixed with water, are the best clays for making puddle. Large stones should be removed from the clay, and it should also be free from vegetable matter. Sufficient sand and water should be added to make a homogeneous mass. If too much sand is used, the puddle will be permeable; and if too little is used, the puddle will crack by shrinkage in drying. It is very important that clay for making puddle should show great cohesive power and also the property of retaining water.

A simple test to find the cohesive property, can easily be made. A small quantity of the clay is mixed with water and made into a roll about 1 inch in diameter and 8 to 10 inches long; and if, on being suspended by one end while wet, it does not break, the cohesive strength is ample. The test to find its water-retaining properties is made by mixing up 1 or 2 cubic yards of the clay with water, making it into a homogeneous plastic mass. A round hole is made in the top of the mass, large enough to hold 4 or 5 gallons of water. The hole is filled with water, and the top covered and left 24 hours; when the cover is removed, the properties of the clay will be indicated by the presence or absence of water.

168. Puddling

The clay should be spread in layers about 3 inches thick and well chopped with spades, aided by the addition of sufficient water to reduce it to a pasty condition. Water should be given a chance to pass through freely as the clay is being mixed. The different layers, as they are mixed, should be bonded together by the spade passing through the upper layer into the under layer. The test for thorough puddling is that the spade will pass through the layer with ease, which it will not do if there are any hard lumps.

When a large amount of puddle is required, harrows are sometimes used instead of spades. Each layer of clay is thoroughly harrowed, aided by being sprinkled freely with water, and is then rolled with a grooved roller to compact it.

Puddle, when finished, should not be exposed to the drying action of the air, but covered with dry clay or sand.