Brownfield Redevelopment RKK-SoilFreeze Technologies has developed an innovative method to use ground freezing to cost-effectively contain and stabilize a contaminated site so that it can be developed as a useful, revenue-generating property.
Conventional Containment Methods The problem with most contaminated sites is that conventional remediation techniques such as removal or barriers and capping are either too costly or make construction of buildings on the site very difficult and expensive. A conventional barrier system for a large, contaminated site near a waterway is illustrated at the left. Sheet piles are not applicable to sites underlain by cobbles, boulders, debris or dense soil layers. Slurry walls are difficult to install in loose, wet sands. Both types are generally limited to moderate depths. Another problem with both methods is that the bottom of the barrier may not adequately penetrate the aquitard (impermeable layer below the contamination) or may poke through the bottom, resulting in a breach in the barrier. Moreover, these types of barriers have historically been prone to leaking over periods of time. Another consideration at contaminated sites underlain by loose sand or soft silt or clay is the potential for settlement or liquefaction to occur. For these sites, buildings must usually be founded on piles bearing on more competent materials at depth. This is illustrated at the left. This method involves driving piles through an aquitard that will allow the contamination to migrate down to a lower ground water aquifer along the sides of the piling. Moreover, piling may not sufficiently reduce the impacts of liquefaction, such as large lateral spreading along the waterfront.
Two Ground Freezing Methods We have two applications of ground freezing for brownfield redevelopment. The first involves containing the contamination using a frozen ground barrier in a similar fashion as a slurry wall or sheet pile wall barrier, with some important advantages. This is illustrated to the left. Frozen soil walls can be more economical than conventional methods, particularly if the contamination is moderately deep (more than about 30 feet), contains a mixture of difficult contaminants, or if the site is underlain by difficult soils, including running sands, cobbles and boulders or debris. Frozen ground: is essentially impermeable for nearly all known contaminants has very low diffusion can be remotely monitored for integrity does not corrode or decay with time can be used in all soil types the maintenance cost for long-term containment is relatively small
A second, more innovative application is to completely freeze the ground to stablize both the contaminated ground water and the loose or soft soil. This is illustrated to the left. This option involves installed horizontal freeze pipes below the entire contaminated site, usually by directional drilling methods. Once the ground is frozen, the risk of offsite migration of contamination is mitigated. Frozen ground has the strength of concrete thereby eliminating liquefaction or settlement of the previously loose/soft soil. Moreover, with the addition of board insulation and a gravel pad on the frozen ground, new buildings may be suported on conventional footings which are substantially less expensive than piles. What all this means is that the overall costs of developing the site can actually be less than the conventional approach of a sheet pile barier and pile-supported buildings. An example of a cost comparison of the three options is shown below. The values shown in this table are for illustrative purposes only and are not complete; actual cost estimates for a particular site will require evaluation of all parameters that affect the development.
Brownfield Redevelopment: Cost Comparison of Options (3-1/2 acre site, contamination 50' deep
Development components controlled by soil conditions Sheet Pile Wall Barrier ($million) Frozen Soil Barrier ($million) Deep Frozen Soil Stabilization ($million) Barrier installation 2.0 - 2.5 1.2 - 1.6 1.5 - 2.0 Barrier Operation (20 years) -- 0.2 - 0.4 0.5 - 1.0 Board Insulation -- -- 0.2 - 0.3 Pile Foundation Costs 1.0 - 2.0 1.0 - 2.0 -- Total Costs 3.0 - 4.5 2.4 - 4.0 2.2 - 3.3 Long-Term Risks - Offsite Contamination  moderate very low   very low Seismic Risks - Liquefaction/Lateral Spreading high high low
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