Public Services and Procurement Canada
In situ solidification/stabilization systems are used to limit the spread of contaminants in soil and groundwater by stabilizing and/or solidifying the contaminated soil. However, this technology is not intended for the treatment or removal of the contaminated material. As such, these systems are applicable to many types of organic and inorganic contaminants, including non-aqueous phase liquids (NAPL).
Solidification/stabilization systems consist of mixing deep or shallow contaminated soil with stabilizers and/or binding agents such as Portland cement, pozzoland, ash, lime, bentonite clay. Considerations must be made with respect to the compatibility of the contaminants and the materials used. The soil and contaminants will be stabilized chemically (stabilization) and/or bound physically (solidification) which reduces or eliminates leaching of contaminants. These treatments prevent off-site contaminant migration and expansion of the contaminated zone.
Stabilization involves transforming the chemical properties of the contaminants within the soil matrix. The contaminants are transformed into compounds that have lower water solubility, mobility and toxicity. In certain cases, a binding agent may be added to the contaminated material to maximize the stabilization process.
Solidification involves transforming the physical properties of the contaminated soil by the addition of binding agents which compact the matrix, change the pore volume and reduce the hydraulic conductivity. The contaminants are then trapped in the soil and binding agent mixture. Solidification does not actively promote chemical changes in the contaminants.
There are typically two types of in situ solidification/stabilization techniques used:
The first involves mixing a binding/stabilizing agent with contaminated soils using an auger;
The second technique involves the high pressure injection of a solubilized binding/stabilizing agent to force the binder into the contaminated soil matrix, which involves forcing the binder into the soil pore space using high-pressure grout injection pipes.
If volatile elements are present or if there is a potential for the production of gas emissions during the stabilization/solidification process, a gas emissions collection and treatment system may be required.
Centre for Public Environmental Oversight (CPEO)—Techtree: Stabilization/Solidification—Chemical.
This technology relies on locking the contaminants in low permeability, high strength stabilized and/or solidified monolithic blocks. In situ solidification/stabilization activities may include:
Soil mixing for in situ solidification/stabilization can be accomplished with excavator buckets, rotary drum mixers or augers. Auger mixing provides the highest level of quality control and is the only method capable of stabilizing materials deeper than 4.5 m below the work platform. Dewatering might be required to allow proper soil mixing below water table level.
Soil amendments for in situ stabilization include additives such as Portland cement, pozzoland, bentonite, polymers and may also consist of organics (for example, biosolids, manure and compost), pH control amendments (for example, lime, wood ash, or coal combustion products), or mineral soil amendments (for example, foundry sand, steel slag, cement kiln dust, fly ash and gypsum).
In situ solidification is most commonly performed through the addition of Portland cement alone, or in combination with other additives: blast furnace slag, cement kiln dust, fly ash, bentonite clay and activated carbon to name a few examples. Generally, permeability reduction and strength increase are the most important factors; with their objectives of achieving permeability less than 1x10-6 cm/s and a strength greater than 345 kPa. The solidification process may or may not involve a chemical bonding between the toxic contaminant and the solidification additive.
This technology may require capping or covering, engineering controls, and/or institutional controls, especially if the solidified material contains radioactive contaminants, where a soil cover sufficiently thick to absorb gamma radiation is required.
Effectiveness of in situ solidification/stabilization relies on the successful implementation during the construction phase. The formulations developed based on treatability tests must be achieved in the field. It is important to ensure that the correct proportions are attained and that sufficient mixing is imparted to the material.
On-site storage is typically limited to small amounts of fuel and lubricant (daily fuelling is typically from a mobile tank) as well as miscellaneous construction site supplies
Storage of materials used may include stockpiles of solidification materials (blast furnace slag, cement kiln dust, fly ash, bentonite clay, activated carbon, etc.) and/or stabilization materials (Portland cement, pozzoland, polymers, biosolids, manure, compost, lime, wood ash, coal combustion products, etc.)
The stabilized contaminated soil is not generally considered as a residual, though it remains in situ. There is potential for liquid and gaseous residuals leaving the in situ solidification/stabilization system, however, the monitoring and management of these residues are required as part of the remediation system design
There is limited potential for windblown dust that can occur from the construction activities, trench excavation or stockpiles, which for example, may also deposit directly on downwind surfaces (stormwater can also be impacted by dust)
Diverted and collected/treated stormwater is typically passed into the local stormwater system
Regular inspection and monitoring is required due to the potential leaks from the solidification/stabilization system
There are no predefined procedures for this treatment approach. A treatability study is therefore recommended before treatment. Treatability studies provide information on the optimal type and quantity of stabilizers/binders to add to maximize the solidification/stabilization treatment according to the type of contaminated matrix and volume of material to be treated. Treatability studies will also enable to assess if emissions of contaminants during construction work will occur and require mitigation as well as to assess the volume increase that may occur as a result of adding the stabilizing/binding agents.
The solidification/stabilization technology is applicable to contamination within the saturated and vadose zones. As mentioned earlier, dewatering might be required to allow proper soil mixing below water table level.
The solidification/stabilization technology is generally easier to implement in sandy, silty or gravely soils, than in soils with high clay content as it is easier to achieve a uniformity of mixing in the former, while the latter may tend to leave residual clay balls of unmixed and untreated material.
The target contaminants are generally metals and free radicals. The application of this technology has been successfully performed to treat metals and radioactive materials as well as organic contaminants of concern including non-volatile and semi-volatile compounds such as chlorinated ethenes, petroleum hydrocarbon constituents, polychlorinated biphenyls, pesticides, and dioxins and furans. However, it has not been demonstrated to treat volatile organic compounds. This technology is in constant evolution and may be applicable to a wider range of contaminants in the future.
Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short seasonal work windows
The use of certain additives (surfactants for example) in conjunction with Portland cement as a stabilizing/solidifying agent has been demonstrated to help resist the negative impacts of the freeze/thaw cycles
Frequent rain and freeze/thaw cycles may reduce the lifetime of the stabilized/solidified material and may increase contaminant mobilization
Enforcement of institutional controls, if required, and long-term monitoring may be challenging and costly. Telemetry can be used for remote monitoring of site conditions
Institutional controls are frequently very applicable to remote northern sites provided that the underlying risk assessment accounts for northern lifestyles, cultures and unique ecological systems
For Phenolic compounds, applies to pentachlorophenol (PCP) only.
The estimated treatment time is largely dependent on the size of the contaminated area. It can take from a few days to several months depending on numerous site-specific factors, including:
quantity of mixing equipment available on-site;
diameter of auger (in case of auger mixing)
presence of subsurface utilities or debris
Long term monitoring of in situ solidification-stabilization system may be required to assess its effectiveness
Long term groundwater and vapor monitoring should be performed to confirm the integrity of the solidification/stabilization system over time
Site use may be modified or restrictions may apply (for example, planting deep-rooted trees or other subsurface activities would require approval). It may be warranted to place signs to indicate the solidified/stabilized area
There is no by-product production during in situ solidification/stabilization. Contaminants are not degraded by the treatment and are still present within the contaminated site after treatment. The contaminants are immobilized within the matrix and are not inclined to migrate as long as the integrity of the solidified/stabilized matrix is maintained.
Groundwater must be monitored over the long term to prevent any leaching of contaminants from the treated areas
Collected gas emissions may require treatment before being exhausted into the atmosphere
Application examples are available at these links:
Unites States Environmental Protection Agency. 2000. Innovative Remediation Technologies: Field-Scale Demonstration Projects in North America, 2nd Edition, Year 2000 Report. EPA 542-B-00-004.
Cement Association of Canada. Remediation Technology: Solidification/Stabilization.
Bates, E. and Hills C. 2015. Stabilization and Solidification of Contaminated Soil and Waste: A Manual of Practice. Hygge Media.
The performance of several stabilizers and binding agents is well documented, having been proven to prevent contaminant migration while being easy to apply at low cost.
Use of renewable energy and energy-efficient machinery for construction of the system
Schedule optimization for resource sharing and fewer days of mobilization
Minimizing site visits by the use of telemetry for remote monitoring of site conditions
Consideration of locally available and/ or recycled materials in the design
Use of waste or by-products of industrial processes, if appropriate, as additives or amendments (for example, cement kiln dust from cement manufacturing)
Unavailable for this fact sheet
Composed by : Martin Désilets, B.Sc., National Research Council
Updated by : Martin Désilets, B.Sc., National Research Council
Updated Date : April 1, 2008
Latest update provided by : Daniel Charette, P.Eng., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.
Updated Date : March 31, 2017