A Critical Look at Use of ``Rules of Thumb'' for Selection of Grout Injection Pressures

The quite different "rules of thumb" used by American and European grouting practitioners for selection of grout injection pressures have for many years been a subject of great controversy between the two groups. Practitioners who are not "married" to either rule have long suspected that each may be an artifact of the respective systems of measurement used. Although use of the "American rule" commonly is rationalized on the basis of protection against possible uplift of horizontal strata, application of the pressures that this rule seems to dictate may not be sufficient to cause grout to adequately penetrate and fill small openings in potentially permeable fractured rock. Conversely, these same pressures may be excessive for use in very weak rock. "European rule" pressures would, of course, be potentially damaging to weak rock, but they have been widely and successfully used in "average" bedrock foundation conditions. Nonetheless, even higher pressures than would be allowed by that rule also have been widely and successfully used. Therefore, it should seem evident that — rather than mindlessly following either "rule" — grouting practitioners should base their selection of grout injection pressures on site-specific factors, including — to the maximum extent feasible based upon grout hole logging and water test data — the conditions at each specific hole into which grout is to be injected.