Casings for trenchless installations can be used under roads up to what diameter?

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Casings under roadways are chosen to protect the installed conduit while preserving the road’s structure and allowing the bore to be completed with practical equipment. The size is limited by what standard trenchless equipment can safely pull or push and by the need to minimize traffic disruption and road restoration effort. In typical practice, casings up to about four feet in diameter are used under roads because this size provides enough clearance for the conduit and the annulus, while still staying within the capabilities of common HDD or jacking rigs. Larger diameters, such as six feet, would require much bigger equipment and more extensive road work, making them uncommon. Smaller diameters (two to three feet) are possible but do not meet the needs of larger installations. Therefore, four feet is the standard maximum.