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12 4 Methods of wall construction in relation to underground leakage For the most part, wells in the Las Vegas area have bean drilled with cabla tools. It has been the usual practice to drill until trouble is experienced with caving and then to case the well with casing of the largest sizo that can b'e used. After the hole* is cased it is continued with a bit small enough to pass through the casing. The smaller hole is carried down until more trouble is experienced with caving and then another string of casing is placed. Sometimes the well is drilled to considerable depth by allowing the casing to follow the bit as the hole is deepened. In other wells casings of successively smaller sizes are used. As a result most of the wells in this area have several sizes of casing that overlap in them. In some wells the smaller sizes are cut off near the bottom of the next larger size and in other wells the smaller sizes are brought all the way to the surface. As a rule lead-seals or packers have not been used to make tight connec­tions between casings of different sizes. When the first wells were drilled in the area they were provided with a minimum amount of casing and some of them have as little as 40 to 60 feet. In recent years it has been the general practice to "cement" on the outside of the largest casing. This is done by pouring a mixture of sand and cement down the outside of the casing, aided to some extent by probing with a long stick or rod.