When contractors prepare to dig tunnel shafts, foundations, or other large excavations, the ground must be shored to provide structural stability, prevent cave-ins, and block groundwater from entering the work area. Traditional shoring methods range from soldier piles and lagging to slurry walls and secant pile walls, but many of these approaches are time-consuming, expensive, and environmentally disruptive. One alternative that has gained traction in civil and commercial construction is artificial ground freezing, a technique that transforms the soil itself into a structural barrier. This article examines how ground freezing technology works, the specialized chilling equipment that makes it feasible, and the real-world applications that demonstrate its value. For a broader look at subsurface excavation methods, see Tunneling and Underground Construction Equipment Boring Machines Excavation.
The Principles of Artificial Ground Freezing
Artificial ground freezing was originally developed and deployed in mining operations as an alternative to conventional shoring and dewatering methods. The principle is straightforward: by circulating a refrigerant through a closed-loop pipe system installed in the ground, heat is extracted from the soil until it freezes into a solid, impermeable mass. This frozen ground serves simultaneously as a structural support system and a waterproof barrier, eliminating the need for separate shoring and dewatering systems.
How the Freezing Process Works
The installation process follows several key steps:
- Piping is installed vertically in the ground around the perimeter of the excavation site, forming a closed-loop freezing system.
- A flow system connects to these pipes and circulates a brine solution through a chilling system.
- The chilled brine pulls heat out of the ground continuously until the soil temperature reaches -20 degrees Fahrenheit.
- This initial freezing phase typically takes between four and eight weeks, depending on soil conditions and ambient temperatures.
- Once the target temperature is reached, the brine continues to circulate to maintain the frozen state throughout the excavation and construction process.
Excavation work can proceed within the protected frozen ring for durations ranging from a few months to two years or more. As Larry Applegate, President of SoilFreeze, explained, the definition of permafrost is soil that remains frozen for at least two consecutive years, adding that SoilFreeze has effectively created permafrost in locations such as Los Angeles. Such long-term, extensive ground freezing demands extremely reliable chilling equipment that can operate continuously without interruption.
Environmental and Cost Benefits
Ground freezing offers environmental advantages that distinguish it from alternative shoring methods. The ring of frozen ground creates a waterproof barrier that eliminates the need to pump groundwater out of the excavation site, treat it, and store it. This has several cascading benefits:
- No risk of spreading contaminated groundwater into previously uncontaminated areas, which is particularly important in drought-stricken regions such as California and in areas with dangerous soil contamination.
- Elimination of groundwater settlement risks, as removing groundwater can cause buildings in the surrounding area to settle and potentially sustain structural damage.
- Reduction in overall project timelines, since the excavation does not need to stop repeatedly to install shoring and waterproof new areas.
While the upfront freezing period adds four to eight weeks to the schedule, the time savings realized during excavation often result in a net reduction in total project duration. When combined with avoided dewatering costs and reduced equipment mobilization, the cost savings can be substantial. For a detailed comparison of excavation support methods, refer to Construction Equipment and Project Controls Equipment Selection Earned.
The Equipment That Makes Ground Freezing Possible
For decades after its initial development in the mining industry, artificial ground freezing relied on the construction of semi-permanent refrigeration equipment at each individual building site. This approach rendered the method prohibitively expensive for all but the largest and most long-term projects. The transformative shift came when portable, packaged chilling systems became available, allowing ground freezing to be deployed cost-effectively across a wider range of construction applications.
The SoilFreeze and Technical Systems Partnership
Since its founding in 1998, SoilFreeze has been at the forefront of introducing artificial ground freezing into the United States civil and commercial construction market. Prior to 1998, the method was primarily limited to mining and environmental control applications. SoilFreeze located Technical Systems, Inc. (TSI), a division of RAE Corporation, through a refrigeration expert and chose to purchase chiller technology from TSI because the company could custom-build units to their exact specifications.
The key specifications SoilFreeze needed included:
- Powerful refrigeration units achieving extremely low temperatures regardless of ambient conditions.
- Rapid startup capability to minimize mobilization time.
- Reliable long-term operation without failures for periods of two years or more.
- Easy portability to allow transport around the country for use across many construction projects.
These requirements represented a departure from previous ground freezing applications, which had relied exclusively on site-built chillers. As Applegate noted, SoilFreeze currently owns 15 RAE chillers of varying ages, and continues to purchase from TSI because the equipment has consistently met their demanding needs.
Technical Specifications of TSI Chillers
The packaged, air-cooled chiller refrigeration systems built by TSI are designed specifically for the ground freezing application. The following table summarizes the key design features and their functional benefits:
| Design Feature | Functional Benefit |
|---|---|
| Forklift and crane handling capability | Rapid loading, unloading, and positioning on job sites without specialized rigging |
| Microprocessor control system | Remote monitoring and control of refrigeration units from any location |
| Flooded condenser head pressure control | Enables operation at ambient temperatures as low as -20 degrees Fahrenheit |
| Industrial-grade construction standard | Superior durability compared to typical light commercial refrigeration products |
| Customizable dimensions | Ability to fit narrow or confined spaces such as tunnels |
| Easy-maintenance coil design | Simplified cleaning and servicing in dusty construction environments |
Jeremy Colvard, Vice President at RAE Corporation, emphasized that the microprocessor control system was specifically designed so that SoilFreeze personnel could remotely connect to and communicate with the refrigeration units. When equipment is left in place for weeks or months, having a technician on-site constantly is impractical. Remote connectivity allows the team to monitor performance and respond to problems without dispatching personnel. As Colvard explained, it is critical to SoilFreeze success that equipment runs smoothly because if the units fail and cannot be brought back online, weeks of work can be undone.
The Seattle Seawall Project: A Case Study in Scalable Ground Freezing
A powerful demonstration of what modern ground freezing equipment can achieve is the reconstruction of the Seattle Seawall, which separates the west side of downtown Seattle from Elliott Bay. The crumbling seawall being replaced is close to 100 years old and is being upgraded to comply with new earthquake safety regulations. For additional context on dewatering strategies in challenging conditions, see Understanding Deep Well Systems for Dewatering of Excavations.
Project Scale and Equipment Deployment
The Seattle Seawall project stands apart from typical ground freezing applications due to its massive scale:
- At peak operation, 12 to 13 chillers were running simultaneously on a single project.
- The frozen soil zone extended 35 feet deep across a stretch of more than half a mile.
- The project costs hundreds of thousands of dollars per day, making equipment reliability extremely critical.
- Operation on the Seattle construction site has been smooth, with no significant equipment-related delays.
Environmental and Economic Impact
Ground freezing proved especially valuable on the Seattle Seawall because of the extremely wet ground conditions along the waterfront. By eliminating the need for dewatering, the project avoided pumping, treating, and storing vast quantities of groundwater that would have been contaminated with marine sediments and urban runoff. Applegate estimated that SoilFreeze saved the city of Seattle millions of dollars by eliminating the need for dewatering and guarding against water contamination.
This case illustrates a broader principle: ground freezing becomes most valuable where conventional dewatering is difficult, expensive, or environmentally risky. Urban waterfront sites, areas with contaminated groundwater, and locations near sensitive structures all benefit from the freeze-and-excavate approach.
Design Considerations for Ground Freezing Projects
Key Factors Affecting Freeze Performance
Successful ground freezing requires careful planning and an understanding of the factors that influence freeze performance. For a detailed overview of the underlying science, see Ground Freezing Technique for Soil Stabilization Methods Advantages. Several variables determine how effectively the ground can be frozen:
- Soil type and moisture content: Sandy and gravelly soils freeze faster than clay soils because they have higher thermal conductivity. Sufficient moisture is needed to allow ice bonding between soil particles.
- Groundwater flow velocity: Moving groundwater can transport heat into the freeze zone, slowing or preventing ice formation. High-velocity flow may require additional freeze pipes or pre-treatment.
- Ambient temperature conditions: While modern chillers can operate in ambient temperatures as low as -20 degrees Fahrenheit, extreme heat can reduce efficiency. Equipment must be sized for the worst-case conditions expected.
- Excavation depth and geometry: Deeper excavations require thicker frozen walls to resist lateral earth pressures. The freeze pipe layout must create a continuous, uniform frozen barrier at the required thickness.
- Project duration: Longer projects require more robust equipment with redundant systems to ensure uninterrupted operation over extended periods.
Equipment Sizing and Configuration
The number of chillers required for a project varies dramatically based on scale. Small projects may need only a single chiller to achieve and maintain proper ground temperatures. Large infrastructure projects can demand as many as a dozen chillers running in parallel. The key is to match the total cooling capacity to the thermal load imposed by the soil volume, groundwater conditions, and desired freeze time. TSI has built chillers for SoilFreeze in a wide range of sizes, including a narrower, longer unit custom-built for a tunnel project. This willingness to customize has been a hallmark of the partnership.
Maintenance and Monitoring
Long-term reliability depends on proper maintenance and monitoring. The TSI chillers incorporate design features that facilitate this:
- Coils designed for easy cleaning in dusty construction environments, critical for maintaining heat transfer efficiency over extended operation.
- Remote monitoring through the microprocessor control system, allowing engineers to track performance and detect anomalies early without a site visit.
- Industrial-grade construction that withstands repeated transport, loading, and unloading across multiple projects over many years.
The combination of robust design, remote monitoring, and responsive support has enabled SoilFreeze to maintain an exceptional reliability record on projects where equipment runs continuously for two years or more. This reliability is the foundation of the economic case for ground freezing, because any extended equipment failure can undo weeks of frozen ground development and delay the entire construction schedule.
Artificial ground freezing has evolved from a niche mining technique into a viable option for civil and commercial excavation projects. The key enabler has been the development of portable, reliable, and customizable chilling equipment that can be deployed rapidly and operated continuously for extended periods. As construction projects push into challenging urban environments with strict environmental regulations and tight schedules, ground freezing will play an expanding role in the contractor toolkit. Projects such as the Seattle Seawall reconstruction show that the method can save millions of dollars while protecting the surrounding environment.