Grouting to control leaks in Quail Creek dike may actually have contributed to the dike's eventual failure, according to a detailed report released by an independent review team.
Grouting involves drilling a hole and pumping concrete into the dike in order to plug holes. In the case of the Quail Creek dike, more than 150 grouting holes were drilled in 1986-88.The dike, about nine miles from St. George, failed on Jan. 1, releasing a flood that caused $12 million in damage. The independent review team was appointed by State Engineer Robert L. Morgan with the approval of Gov. Norm Bangerter.
On March 7, the team reported that the primary cause of the failure was that embankment materials were not protected from seepage erosion beneath the dike. The design was flawed because planners did not expect the amount of seepage that actually occurred along joints in the rocks, the team said.
The report added that remedial grouting was not a long-term solution for seepage control, "as demonstrated by the shifting locations of seepage emergence during grouting and the sporadic outbreaks of new seepage after completion of each episode of remedial grouting."
Team members have now released a 155-page report, filled with charts and glossy photographs, that adds new details to their findings.
One possibility is that the grouting actually harmed the dike.
"Grouting through embankments is not desirable but often necessary in remedial work," the report says. "In this case the rec-ords and field evidence indicate that drilling and pressure grouting in the embankment, foundation contact and (the) foundation immediately below the contact caused hydraulic fracturing of the embankment."
Elsewhere in the report, the experts said that three times during grouting, the embankment's crest cracked. These were on Sept. 1, Sept. 5 and Nov. 11, 1988.
At that time, a crack parallel to the dam crest was observed.
"The grouting operation was stopped and the cracking stopped," the report said. "When the grouting operation resumed the cracking started again."
This was reported as lateral spreading or hydrofracturing - that is, the separating of materials caused by forcing in water or grouting material under high pressure.
The report says it is now clear that the embankment was fractured from the bottom to the crest when grout was being injected.
The report says that whether the fracturing contributed to the failure is speculative.
But it "does show that caution must be exercised in grouting near the foundation contact and particularly where the grout pipe is pulled above the contact and the embankment is directly exposed to pressure grouting."