The International Information Center for Geotechnical Engineers

Stabilization/Solidification - 3.0 Field Setup


3.0 Field Setup

Stabilization/Solidification of contaminated soil may be performed using in-situ or ex-situ processes. In-situ stabilization involves in-place mixing of reagents and waste without removal from the contaminated site. Ex-situ processes are typically performed on-site but involve above-ground mixing; the stabilized waste is then returned to the site, placed in a landfill-type cell, or hauled away to an off-site disposal facility. 

3.1 In-Situ Treatment Processes

The procedure for an in-situ treatment process may utilizes the existing lagoon as a mixing area. Large-scale remediation projects are typically better suited for in-situ remediation, which is usually less costly in such situations. From a regulatory standpoint, in-situ treatment does not trigger Land Disposal Restrictions (Andromalos et al.). Advantages of in-situ stabilization are that a smaller area  is required for support and worker exposure to hazardous contaminants is minimized (Andromalos et al.). However, quality control may be difficult to achieve during in-situ mixing. 

The follow steps are typical for stabilization/solidification of waste in-situ (Cullinane et al. (1986):

  1. Addition of reagent, such as kiln dust or flyash, using mechanical or pneumatic application.
  2. Mixing with backhoe or excavator until stabilization/begins.
  3. Setting/gelling for 24-48 hours.
  4. Off-gas treatment to collect hazardous vapors

See Figure 3.1 and Figure 3.2 below for a diagram of a typical in-situ stabilization/solidification treatment process.



Figure 3.1: Deep in-situ S/S mixing setup, including gas cap to trap vapors. 




Figure 3.2. In-situ S/S mixing setup, with single auger. (


3.2 Ex-Situ Treatment Processes

Ex-Situ processes are typically performed on-site using mobile mixing equipment (NAVFAQ). Small batches of waste may also be mixed in drums. Alternatively, area mixing may be implemented, in which waste and reagent are mixed in layers at the final disposal site. Ex-situ processes provide better control over reagents, mixing, and sampling. They are more practical for shallow sites or sites without access for large machinery. Important considerations for ex-situ mixing include swelling during mixing, which can double the original volume of waste, and the generation of harmful odors, dust, and especially organic vapors that are difficult to capture.

3.2.1 Mobile Mixing 

Mobile mixing systems utilize mobile or fixed systems to handle, meter, and mix waste and reagents. Wastes are physically removed from their location using pumping or construction equipment, mechanically mixed with reagents, and deposited into a prepared disposal site. This method is best suited for liquids and highly liquid sludges that can be pumped. Special equipment may be used to employ the mobile mixing method on high solids content sludges and soils (Cullinane, 1986). Mobile mixing plant stabilization/solidification typically requires the following project sequencing:


1.Excavation to remove contaminated waste

2.Classification of wastes

3.Mobile plant setup and mixing

a. Preparation of remedial site for installation of mobile system, including necessary utilities/electricity

b. Preparation of final disposal site

c. Installation of raw and treated waste handling systems, including pumps or construction equipment

d. Screening of wastes to remove particles too large for treatment (typically 2 inches or greater in diameter) (EPA, 2012)

e. Mixing of binding agents, water, and waste

4.Off-gas treatment to collect hazardous vapors

5.Final Disposal

  See Figure 3.3 below for a typical schematic of mobile mixing processes.


Figure 3.3: Typical Mobile Mixing Plant Schematic. (Federal Remediation Technologies Roundtable)


3.2.2. In-Drum Mixing

Toxic and hazardous liquids and sludges are often disposed on-site in drums. In-drum solidification/stabilization methods aim to utilize these existing drums as both a mixing vessel and storage container for hazardous waste. When possible, reagents are added directly to the drum. However, because head space requirements include 30-50% of the drum volume, and because drum integrity degrades over time, additional drums may be required. This significantly increases treatment costs. Typical in-drum mixing processes involve the following steps:

  1. Evaluation/identification of contents in each drum
  2. Evaluation of drum condition and head space
  3. Preparation of materials handling location, including concrete pad or gravel surface, chemical storage, and mixing equipment
  4. Addition and mixing of solidification/stabilization chemicals using mixers or propellers
  5. Placement of drums in a secture area for curing
  6. Addition of inert material to any remaining head space and replacement of top to each drum
  7. Final disposal of drums

3.3.3 Area Mixing 

Area mixing provides an economical method for stabilization/solidification of waste liquids and sludges without the use of conventional, stationary mixing equipment. Waste is placed in a layer over the disposal area in 2"-24" thicknesses and is then overlaid with a layer of reagents. The two layers are then lifted and turned with a mechanized vehicle, similar to tilling. The mixture is air dried or compacted and additional waste/reagent layers may be added up to the determined final material height. The final layer is typically covered with earth and seeded as a final cap. Alternatively, the waste may be removed to another disposal site, but this may leave significant areas of hazardous waste to clean up from the treatment site. Area mixing processes typically proceed as follows:

  1. Selection/preparation of disposal area
  2. Excavation and transport of waste
  3. Spreading of waste in desired thickness over disposal area using construction equipment
  4. Spreading of required amounts of reagents over waste
  5. Mixing of materials using high-speed rotary mixer
  6. Compaction of mixed layer
  7. Repeat steps 2-7 until allowable solidified waste height has been attained

See Figure 3.4 for an example of area mixing. 


 Figure 3.4: Ex situ area mixing. (


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