Practical aspects of deep mixing
The objective of deep mixing is to improve the soil characteristics, e.g. to increase the shear strength and/or reduce the compressibility, by mixing the soil with some type of chemical additives that react with the soil. The improvement becomes possible by ion exchange at the surface of clay minerals, bonding of soil particles and/or filling of voids by chemical reaction products. Deep mixing is classified with regard to the binder utilised (cement, lime/cement and possible additives, such as gypsum, fly ash, etc.) and the method of mixing (wet/dry, rotary/jet-based, auger-based or blade-based).
The development of deep mixing was started in Sweden and Japan in the late 1960’s. Dry mixing, using granular quick lime (unslaked lime) as a binder, was put into practice in Japan in the middle of the 1970’s. Approximately at the same time, dry mixing originated in Sweden as lime (powdered lime) mixing to improve the settlement characteristics of soft, plastic clays. Wet mixing, using cement slurry as a binder, was also put into practice in Japan in the middle of the 1970’s. Deep mixing has since spread into other parts of the world. More recently, the combination of cement and lime with gypsum, fly ash and slag has been introduced.
Since its introduction, applications have diversified, equipment has been improved and hardening agents modified. As a result of substantial research efforts and accumulation of practical experience, deep mixing methods have become widely accepted in many countries. Growing environmental concern has initiated the use of deep mixing for remediation and containment of contaminated areas.
Recently, hybrid techniques have been developed by combining deep mixing with other soil improvement methods (such as jet grouting) or other machinery (surface mixing). The development of the technology in the past quarter century is summarised e.g. by Terashi (2001). The generic classification of the equipment is shown in Figure A.1.
A.2 Fields of application
A variety of applications for deep mixing exists for temporary or permanent works and either on land or marine, see Figure A.2. The main applications are reduction of settlement, improvement of stability and containment.
The execution consists typically of positioning, penetration and retrieval. During penetration, the mixing tool(s) cut and disaggregate the soil to the desired depth of treatment. During retrieval, the binder is injected into the soil at a constant flow rate, as the retrieval speed is kept constant. The mixing blades rotate in the horizontal plane and mix the soil and the binder. There are, however, some variations of machines, in which the binder is injected during the penetration phase and both in the penetration and retrieval phase.
Where the ground movement during execution has to be minimised, special mixing tools may be used.
Deep mixing can be carried out by two different methods: dry mixing where the binder is introduced by air and wet mixing where the binder is in slurry form.
In dry mixing the binder is usually a mixture of cement and lime (unslaked), or a combination of cement, lime, gypsum, blast furnace slag or pulverised fuel ash (PFA) in granular or powdered form. Air is used to feed (or incorporate) the binder into the soil. (The moisture content of the soil needs to be ≥ 20 %.)
In wet mixing the most common binder is cement.
Dry mixing is primarily utilised to improve the characteristics of cohesive soil, whereas wet mixing is applied also in order to improve the characteristics of granular material. For certain applications, such as prevention of liquefaction, dry mixing has also been used in loose granular soil.
Underground contamination or hazards that can affect the execution method, the work safety or the discharge of excavation material from the site can consist of old refuse heaps, industrial waste material, chemical waste products, etc. Obstructions, such as boulders and tree-root systems, can affect the efficiency of deep mixing. Before the construction work is started, the intended quality of the columns has to be ascertained. The process of execution of a deep mixing project follows the principles shown in Figure A.3.
A.3.2 Dry mixing
Dry mixing is normally carried out in accordance with some general principles, summarised in Figure A.4.
As can be seen in the flow chart, the binder is fed into the soil in dry form with the aid of compressed air. Two major techniques for dry mixing exist at present: the Nordic and the Japanese techniques.
The installation is carried out according to the following procedure, from left to right:
- 1) the mixing tool is correctly positioned;
- 2) the mixing shaft penetrates to the desired depth of treatment with simultaneous disaggregation of the soil by the mixing tool;
- 3) after reaching the desired depth, the shaft is withdrawn and at the same time, the binder in granular or powder form is injected into the soil;
- 4) the mixing tool rotates in the horizontal plane and mixes the soil and the binder;
- 5) completion of the treated column.
A.3.2.2 Nordic technique
Equipments used in the Nordic countries are able to install columns to a depth of 25 m with a column diameter of normally 0,6 m to 1,0 m. The columns can be inclined up to about 70° in relation to the vertical. The machines have one mixing shaft with the injection outlet positioned at the mixing tool. Mixing energy and amount of binder are monitored and in some cases automatically controlled to achieve uniform treated soil.
The mixing tool is drilled down to the final depth and a predetermined amount of binder is added through an inner tube with an opening at the mixing tool (during the retrieval phase). During the retrieval phase, the soil and binder are mixed by continued turning of the mixing tool. Both phases can be repeated for the same location, if required.
The rotation speed of the mixing tool and the speed with which it is withdrawn is adjusted to produce uniform mixing, sufficient for the purpose. The amount of mixing work involved in producing a dry-mixed column depends on the type of binder, quantity of binder and type of soil. When using cement as binder compared to lime only, a higher amount of mixing energy is required. Special equipment has been developed to contain air and dust.
A.3.2.3 Japanese technique
There are several variations of execution machines, which have either one or two mixing shafts. Each mixing shaft of these machines have several blades with a diameter of 0,8 m to 1,3 m and are able to install columns to a depth of 33 m. The binder, usually cement powder, is brought to the mixing machine by compressed air. A bellows covering the mixing shaft has the function to avoid scatter of the air that comes up from the ground. The mixing tool is composed of several stacks of mixing blades to achieve uniformity of the treated column. The injection outlets are positioned above and below the mixing blades at the mixing shaft. A steel bar fixes the distance between two mixing shafts. The bar, and sometimes additional freely rotating (undriven or counteracting) mixing blades, also function to prevent rotation of soil adhering to the driven mixing blades and shaft. Air pressure and amount of binder are automatically controlled to achieve homogeneity of the treated column.
The binder is injected during the penetration stage or both during the penetration and retrieval stages.
|Equipment||Details||Nordic technique||Japanese technique|
|Mixing machine||Number of mixing shafts||1||1 to 2|
|Diameter of mixing tool||0,4 m to 1,0 m||0,8 m to 1,3 m|
|Maximum depth of treatment||25 m||33 m|
|Position of binder outlet||The upper pair of mixing blades||Bottom of shaft and/or mixing blades (single or multiple)|
|Injection pressure||Variable 200 kPa to 800 kPa||Maximum 300 kPa|
|Batching plant||Supplying capacity||50 kg/min to 300 kg/min||50 kg/min to 200 kg/min.|
Typical execution values of the Nordic and Japanese techniques are summarised in Table A.2.
|Mixing machine||Nordic technique||Japanese technique|
|Penetration speed of mixing shaft||2,0 m/min to 6,0 m/min||1,0 m/min to 2,0 m/min|
|Retrieval speed of mixing shaft||1,5 m/min to 6,0 m/min||0,7 m/min to 0,9 m/min|
|Rotation speed of mixing blades||100 revolutions/min to
|24 revolutions/min to
|Blade rotation number1)||150 per m to 500 per m||≥ 274 per m|
|Amount of binder injected||100 kg/m3 to 250 kg/m3||100 kg/m3 to 300 kg/m3|
|Retrieval (penetration) rate||10 mm/rev to 30 mm/rev.||10 mm/rev to 35 mm/rev.|
|Injection phase||Typically during retrieval||Penetration and/or retrieval|