The Deep Soil Mixing (DSM) is an in situ soil treatment technology whereby the soil is blended with cementitious and/or other materials. These materials are widely referred to as “binders” and can be introduced in dry or slurry form. They are injected through hollow, rotated mixing shafts tipped with some type of cutting tool. The shaft above the tool may be further equipped with discontinuous auger flights and/or mixing blades or paddles. These shafts are mounted vertically on a suitable carrier, usually crawler-mounted, and range in number from one to eight (typically two or four) per carrier, depending on the nature of the project, the particular variant of the method, and the contractor. Column diameters typically range from 0.6 to 1.5 m, and may extend to 40 m in depth. In some methods, the mixing action is enhanced by simultaneously injecting fluid grout at high pressure through nozzles in the mixing or cutting tools.
The cemented soil material that is produced has a higher strength, lower permeability, and lower compressibility than the native soil, although total unit weight may be less. The exact properties obtained reflect the characteristics of the native soil, the construction variables (principally the mixing method), the operational parameters and the binder characteristics.
The original concept appears to have been developed more than 40 years ago in the United States, although contemporary deep soil mixing technology reflects mainly Japanese and Scandinavian efforts over the last three decades. The main applications in Japan involve ground treatment for transportation and harbor facilities in soft native or reclaimed soils, and examples of such applications have also grown in frequency in the Unites States, China and Western Europe during the 1990s. In these highly urbanized and industrialized countries, the value of DSM to implement hazardous waste control and seismic retrofit solutions has also been widely exploited.
Wet and dry mixing
Basically there are two different mixing methods. The existing soil which has to be improved can be mixed mechanically other with a slurry including binder (wet DSM) or with a dry binder (dry DSM). Jetting of slurry can be also used to enhance mechanical mixing.
The wet method is more appropriate in soft clays, silts and fine-grained sands with lower water content and in stratified ground conditions including interbedded soft and stiff or dense soil layers. The dry method is more suitable for soft soils with very high moisture content, and hence appropriate for mixing with dry binders. Stabilization of organic soils and sludges is also possible, but is more difficult and requires carefully tailored binders and execution procedures
Advantages of DSM
DSM technology is based on a stimulating concept of improving natural soils or brown field ground to match adopted design requirements, so eliminating problematic excavation and replacement or more expensive deep foundation methods. Wide application range and variable patterns of execution of soil mixing columns allow for obtaining safe and very economic ground engineering solutions. The use of non-toxic binders as soil additives, including industrial by-products, as well as reduced spoil volumes comparing with jet grouting or classical drilled piles, for example, allow to position DSM as an environmentally friendly technology.
The key benefits of DSM include:
1-3- Typical applications of DSM
Typical applications of DSM
Application of DSM for excavation stabilization
Application of DSM under oil tanks - FHWA
DSM under oil tanks to mitigate liquefaction
Exposed cut-off wall with secant DSM columns - Figure Courtesy of Keller
Exposed DSM columns for a bridge support - Figure Courtesy of Keller
Construction of 2 m DSM columns
DSM columns with diameter of 1.5 m in Bandar Gaz, Iran (Omran Ista)
Mixing tool with 3 drilling rig
1000 DSM columns with diameter of 1.3 m in Bandar Abbas shore, Iran (Omran Ista)
Construction of DSM in Bandar Gaz, Iran (Omran Ista)
Examples of DSM column patterns - Figure Courtesy of Keller
1- The wet DSM method
In wet deep soil mixing method a special mixing tool is inserted into the soil on site. This mixing tool comprises a drilling rod, transverse beams and a drill end with a head. The drilling causes no vibrations, and is assisted by cement slurry outflow from nozzles purposely located at the end of the soil auger. Once the depth specified by the design is reached, the construction phase of DSM columns commences. Basically the diameter of such columns may vary from 40 to 240 cm, depending on the application.
The mixing tool, which may also move up and down along the column length to improve homogeneity of soil-mix, assures throughout mixing of the slurry with the soil.
The composition and volume of injected slurry is adapted to the required properties of stabilized soil, taking into account the required strength and/or sealing functions. Tightness may be further enhanced by adding various components to the slurry, such as bentonite. Bending capacity of DSM elements can be increased with steel reinforcement inserted into fresh columns.
Wet DSM is also possible inside a tube. This method, called Tubular Soil Mixing (TSM), is mainly used for execution of high-quality columns applied for excavation control.
Construction process of the wet DSM method - Figure Courtesy of Keller
2- The dry DSM method
Contrary to the wet method, dry soil mixing is only possible in soils that have sufficient moisture content to allow chemical reaction of stabilizing binders injected in dry form with the soil and groundwater. The basic advantages of dry mixing is that stabilization effects can be obtained in deep deposits of very weak soils, including organic ones, with high productivity, almost no spoil and cost effective. Also operations at low temperatures are possible.
Typical equipment for dry DSM comprises stationary or mobile binder storage and feeding plant and a purposely designed drilling rig for installation of the columns, equipped with special mixing tool at the end of the mixing rod.
Typical column diameter is 60 to 80 cm, and the depth of treatment is up to 25 m. Charging and mixing of dry binder with soil takes place while the rod is withdrawn, with mixing tool rotation direction reversed to the penetration phase direction. The binder is transported from the shuttle to the rig through connecting hoses using compressed air. Binder quantity is adjusted by changing the rotation speed of the feeding wheel. Air pressure and the amount of binder are automatically controlled to supply the specified dosage of binder to the treated zone of soil. As a rule plastic clays and silts are strengthened by lime or cement with lime, while in organic soils mixes containing blast furnace slag are used.
Construction process of the dry DSM method - Figure Courtesy of Keller
Shallow soil mixing
Shallow dry mixing offers a cost-effective solution for ground improvement works or site remediation when dealing with substantial volumes of very weak or contaminated superficial soils with high water content, such as deposits of dredged sediments, wet organic soils or waste sludges. In this method special mixing tools are used, which are in most cases fixed to an excavator’s rig arm. Mixing is executed vertically or horizontally, with mixing tools that resemble screw propellers having a centrally provided nozzle for binder. The binder is fed from a separate unit which houses the pressurized binder container, compressor, air dryer and supply control unit.
Stabilization is executed in phases, according to the operational range of the drilling rig, which generally comprises an area of 8 to 10 m² and depth up to approx. 4 m. Once the required binder volume has been applied, mixing is continued to assure the optimum mixing properties.
Shallow soil mixing
Shallow soil mixing - Figure Courtesy of Keller
Figure Courtesy of Keller