Solidification and Stabilization Method
Solidification and Stabilization Method
Soil stabilization significantly changes the characteristics of a soil to produce long-term permanent strength and stability, particularly with respect to the action of water and quick-lime, either alone or in combination with other materials, can be used to treat a range of soil types. The mineralogical properties of the soils will determine their degree of reactivity with lime and the ultimate strength that the stabilized layers will develop. In general, fine-grained clay soils (with a minimum of 25 percent passing the #200 sieve (74mm) and a Plasticity Index greater than 10) are considered to be good candidates for stabilization. Soils containing significant amounts of organic material (greater than about 1 percent) or sulphates (greater than 0.3 percent) may require additional lime and/or special construction procedures.
The Chemistry of Lime Treatment
When lime and water are added to a clay soil, chemical reactions begin to occur almost immediately.
- 1. Drying: If quicklime is used, it immediately hydrates (i.e., chemically combines with water) and releases heat. Soils are dried, because water present in the soil participates in this reaction, and because the heat generated can evaporate additional moisture. The hydrated lime produced by these initial reactions will subsequently react with clay particles (discussed below). These subsequent reactions will slowly produce additional drying because they reduce the soil's moisture holding capacity. If hydrated lime or hydrated lime slurry is used instead of quicklime, drying occurs only through the chemical changes in the soil that reduce its capacity to hold water and increase its stability.
- 2. Modification: After initial mixing, the calcium ions (Ca++) from hydrated lime migrate to the surface of the clay particles and displace water and other ions. The soil becomes friable and granular, making it easier to work and compact. At this stage the Plasticity Index of the soil decreases dramatically, as does its tendency to swell and shrink. The process, which is called "flocculation and agglomeration," generally occurs in a matter of hours.
- 3. Stabilization: When adequate quantities of lime and water are added, the pH of the soil quickly increases to above 10.5, which enables the clay particles to break down. Determining the amount of lime necessary is part of the design process and is approximated by tests such as the Eades and Grim test (ASTM D6276). Silica and alumina are released and react with calcium from the lime to form calcium-silicate-hydrates (CSH) and calcium-aluminate-hydrates (CAH). CSH and CAH are cementitious products similar to those formed in Portland cement. They form the matrix that contributes to the strength of lime-stabilized soil layers. As this matrix forms, the soil is transformed from a sandy, granular material to a hard, relatively impermeable layer with significant load bearing capacity. The process begins within hours and can continue for years in a properly designed system. The matrix formed is permanent, durable, and significantly impermeable, producing a structural layer that is both strong and flexible.
Solidification and/or stabilization are relatively simple processes. The treated material (soil, sludge, etc.) is mixed with a binder or mixture of binders and received mass is then cured to form a solid matrix that contains the contaminants.
This term refers generally to processes reducing the risk posed by a waste by converting the contaminants into a less soluble, less toxic, and immobile form. This state is usually achieved purposeful chemical reactions. The physical character of the treated material is not necessarily changed.
This method refers to the processes that encapsulate the contaminants in a monolithic solid of high structural integrity. The encapsulation may be:
• of fine particles (micro-encapsulation);
• of a large block or container (macro-encapsulation);
• solidification resulting in a soil-like material.
Solidification does not necessarily involve a chemical interaction between the contaminants and solidifying agents. It may mechanically bind the treated material. Contaminant migration is restricted by vastly decreased the surface area that is exposed to leaching and/or by isolation the contaminants within an impervious capsule.
Most of the processes utilized in the application of S/S are modifications of proven processes and are directed at encapsulating or immobilizing the hazardous constituents and involve excavation + processing or in situ mixing.