July 31, 2012

PACIFIC BLASTING & DEMOLITION LTD.

Explosive compaction was used at the Seymour Falls Dam project in British Columbia.

FEATURE | Site Services

Explosive soil compaction an economic alternative

PETER KENTER

correspondent

Traditional methods of mechanical soil compaction involve the application of energy to a building site — vibration, impact, kneading or pressure — and require a significant investment of time. An alternative technique, explosive compaction, offers an almost instantaneous form of the art of soil stabilization.

For more than 20 years, Pacific Blasting & Demolition Ltd., with head office in Burnaby, B.C. has been offering explosive compaction to construction and engineering clients as the exclusive service provider to Explosive Compaction Inc., of Surrey B.C. The company has applied the technique across Canada and the U.S. and as far away as Russia and Japan in projects involving soil volumes as large as 600,000 cubic metres.

“Explosive compaction is an economic alternative to other more traditional soil improvement techniques, especially on large-scale construction sites,” says Ron Elliott, vice-president, Blasting Division, with Pacific Blasting. “These are typically construction projects such as dams, bridges, highways, industrial building sites, and oil drilling platforms. We’ve also applied the technique on large warehouse sites and on the preparation of a five-acre hotel site on the waterfront in Kelowna, B.C.”

The technique involves drilling blast holes at a prescribed distance apart, with distance depending on project design. A typical blast hole contains five to seven decks of explosive, which are detonated sequentially from bottom to top. The series of small charges induces liquefaction in the surrounding soil, duplicating at a small scale what happens in an earthquake.

“You can’t use an explosive that’s too sensitive, because you want the rounds to detonate independently, not set the other detonators off,” says Elliott. “The soil briefly behaves as a viscous liquid, which allows the soil particles to settle into a denser state. The results are almost instantaneous. You can observe immediate ground settlement of up to 10 per cent, depending on soil conditions — the looser and more granular the soil, the more dramatic the results. I’ve watched ground levels drop more than 1.5 metres following densification.”

When the resettlement occurs, a significant amount of groundwater comes to the surface, often at a rate of hundreds of gallons per minute.

“You see sand boils filled with water that appear on the surface of the soil, which is pretty neat,” says Elliott. “Most of this water later drains back into the ground.”

The technique works best when deep compaction is required. Blast holes 30 to 40 metres deep provide much more dramatic results than holes drilled 10 metres deep. On an optimum project, relative soil density can be increased by 70 to 80 per cent.

Advantages of the technique include a sort of inoculation against future earthquakes, because the soil has already undergone the compaction that an earthquake might cause. By increasing the density of the soil, the technique can also increase the storage capacity of earth-filled tailings dams.

Applied over small areas in test blast holes, explosive compaction can also be used to evaluate the seismic liquefaction potential of soil.

Some of the company’s signature projects include preparing the site of the TransX logistics company warehouse in New Westminster, and the seismic upgrading of B.C.’s Seymour Falls Dam from 2004 to 2007.

“In the early ‘90s, we worked on projects on Mount St. Helen’s in Washington,” says Elliott. “In one case, they were building a highway that crossed some avalanche debris, but as the heavy rock trucks traveled over the soil, it would liquefy and the trucks would get stuck right down to the boxes. We pre-densified the soil in a strip across the debris flow to allow them to work.”

The company also densified the sand ballast in the Molikpaq, a semi-submersible oil rig located in the Beaufort Sea.

“The rig was designed by Canadian engineers, but built in Japan and towed into place,” says Elliott. “The rig is huge — about two football fields side by side. The ballast box of the rig is filled to a depth of about 75 feet with dredged sand.”

Engineers were concerned that earthquake vibrations or impact loading from the shifting ice pack might liquefy the loose saturated sand and possibly destabilize the rig. Pacific Blasting was able to densify the ballast material by about 25 per cent.

The rig was later sold to Russian company Sakhalin Energy, which wanted to use it as a production platform in the Sea of Okhotsk. The rig was increased in height due to the addition of four hull sections during refitting in Korea, and then towed to the Russian sea for final placement.

“We were called to Russia to compact the sand ballast a second time,” says Elliott. “We came full circle, because the explosive compaction technique was developed and pioneered by Russian engineers in the 1940s.”

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