WASHINGTON, Aug. 22 — The Bush administration is set to issue a regulation on Friday that would enshrine the coal mining practice of mountaintop removal. The technique involves blasting off the tops of mountains and dumping the rubble into valleys and streams.

It has been used in Appalachian coal country for 20 years under a cloud of legal and regulatory confusion.

The new rule would allow the practice to continue and expand, providing only that mine operators minimize the debris and cause the least environmental harm, although those terms are not clearly defined and to some extent merely restate existing law.

The Office of Surface Mining in the Interior Department drafted the rule, which will be subject to a 60-day comment period and could be revised, although officials indicated that it was not likely to be changed substantially.