[design '98]

Soil freezing technology is gaining ground


Water and mud flow downhill. This fact of nature can be an enormous problem in managing excavation and tunnelling projects.

One of the best ways to stop water is to freeze it. Freezing water in the ground is a growing application in construction emergencies, and is emerging as an engineering strategy in construction.

RKK-SoilFreeze Tech-nologies, is freezing ground in the Pacific Northwest in ways never thought cost effective before. It has recently completed five shoring/ground water cutoff projects.

Bryn Mawr utility improvement project in Renton

Frozen soil shoring for an access shaft at Bryn Mawr utility improvement project in Renton.

One of the completed projects was a frozen soil shoring wall around a 20-foot-deep excavation along Interstate 405 for retaining a vertical cut during construction of a large storm water detention vault. The second was a frozen soil shoring wall around a 10-foot deep basement excavation for a condominium located in Madison Park just 8 feet from Lake Washington.

Both involved a 4-foot-thick vertical wall of frozen soil to provide shoring and to cut off inflow of groundwater into the excavation.

Both worked very well which is not surprising since frozen ground is nearly twice as strong as concrete and is essentially impermeable.

The surprising part is that the cost of ground freezing using newer technology is competitive with conventional shoring methods such as sheet pile or soldier pile walls (typically $15 to $25 per square foot of wall).

However, ground freezing offers many advantages over conventional methods, including complete ground water cutoff, ability to go around buried utilities, virtually no ground vibrations and installation in all soil types (running sand, cobbles, peat, clay, bedrock).

More recently, RKK-SoilFreeze Technologies help-ed the Port of Seattle fix an 8-foot-diameter broken storm line buried 25 feet below the ground. A large rock had punctured the steel pipe allowing sand to fill the pipe as the tide rose.

To complicate matters, two active rail lines ran within 6 feet of the site.

Conventional shoring, dewatering and excavation to expose the pipe would have been expensive and could have settled the rail lines.

Instead, the Port chose to freeze an arch of soil around the sides and top of the buried pipe. This provided a stable condition so that workers could safely enter the pipe and fix it from the inside, without any excavation.

Frozen ground is nearly twice as strong as concrete and is essentially impermeable

They actually removed an entire section of pipe from the inside exposing the frozen soil arch with no problems. The fix was completed when they inserted a smaller diameter sleeve inside the original steep pipe.

Another emergency use of ground freezing involved the retrieval of a boring machine that became stuck in the ground between the Cedar River and the Renton Airport. The machine was boring a hole for the installation of a new four-foot-diameter storm line when it encountered abundant tree trunks, logs, cables and other debris.

The machine became inoperable. The only practical way to get it out was to freeze a shoring wall around and below it so that a safe excavation could be made in the wet, loose sand near the river.

Dewatering was not an option because there was no permitted discharge in this area. A frozen soil wall was completed and a dry hole made to safely recover the million dollar machine.

Presently ongoing is another shoring project in the Renton area. It is somewhat unique in that a frozen soil wall has been made over, around and below an existing 11-foot-diameter concrete pipe to cut off most of the ground water. The result is a stable, shored excavation with few dewatering problems.

Another boring machine then cut through the other side of the frozen soil wall from two blocks away in order to install a new three-foot-diameter storm line that will connect with the existing concrete pipe.

The frozen soil wall created a tight seal around the boring machine casing so that very little water or soil came into the excavation as the boring machine entered.

Freezing applications

Ground freezing has many applications in civil engineering.

It eliminates the need for structural shoring systems and dewatering. It also creates a very hard, durable surface for construction equipment, even in soft soils. Ground freezing provides a strong, stable support for existing or new foundations near excavations.

It freezes loose, wet sand to prevent liquefaction during an earthquake. In landslide mitigation, it creates strong points in the slope for stabilization but allows ground water to flow.

There are many environmental applications for soil freezing as well.

It can provide an in-situ barrier for containment of contaminated groundwater and a bottom barrier at landfills or other contaminated sites, and provides temporary shoring for construction of permeable barriers or excavation of contamination. It creates a dry, safe environment for construction and excavation. It also bonds soil and waste together to prevent dangerous mixing during removal, and even helps with safe retrieval of unexploded ordnance.

RKK-SoilFreeze is receiving many inquiries regarding the use of ground freezing, and new ideas for its use can be very interesting. For example, a government laboratory in New Mexico has asked for a proposal to freeze a block of soil. They plan to crash a bomb shell (without the explosive) into the frozen soil to evaluate the structural integrity of the shell after impact.

Another request came from a contractor working at the Port of Seattle to use ground freezing to temporarily freeze wet caving soil above a hole in a large buried storm pipe to allow workers to safely enter and repair the pipe.

Soil shoring wall

Frozen soil shoring wall around a 10-foot-deep basement excavation for a condominium in Madison Park.

Presently, RKK-SoilFreeze is pursuing a number of more conventional ground freezing applications such as shoring for pump stations, utility lines, and other deep excavations below water.

It also has several proposals for contamination containment at nuclear plants and former nuclear plants, including Hanford.

RKK-SoilFreeze Technologies was formed last year. However, it draws upon the experience of its two parent companies: RKK, Ltd. and GeoEngineers, Inc.

RKK, Ltd., in cooperation with the University of Washington, has successfully developed, tested and patented its ground freezing technologies for containing high level radioactive and mixed waste materials for the Department of Energy (DOE).

The DOE has named the technology (Cryocell) to its top five list of new technologies for containing wastes at its facilities throughout the United States, including Hanford. Soil freezing will be used by DOE at the 100-N Springs site at Hanford and several other DOE sites, including Savannah River, in nuclear sites in South Carolina, and Idaho.

In order to expand its operations and to develop a private market base, RKK, Ltd. formed a separate corporation (RKK-SoilFreeze Technologies) jointly with GeoEngineers, Inc.

With the backing of the two parent companies, RKK-SoilFreeze is developing the private civil market for ground freezing as well as pursuing its original goal of solving the contamination problems at DOE, Environmental Protection Agency Superfund and state Ecology sites.

Dan Mageau a geotechnicanl engineer and principal at GeoEngineers. He is also the vice president of operations at RKK-SoilFreeze.

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