Deep soil mixing method or DSM is one of the practical methods of ground improvement technique in the world, which produces cement soil columns and modification of soil characteristics by injecting stabilizing materials such as cement or lime using a mechanically hollow drill in the soil.
In this method, the soil characteristics are improved by mixing the soil with cementitious materials by deep mixers and creating cement soil columns. In addition, by overlapping the columns before full trapping, continuous walls can be constructed under the ground.
In Deep soil mixing method, by successively executing the columns and creating an overlap between them, it is possible to create dewatering walls or any arrangement under the foundation of the building, and after obtaining sufficient resistance, the foundation of the structure can be built on the executed columns. Sometimes, in order to create retaining walls, before the trapping of soil-cement mixture, even or single steel profiles are driven into the soil, and thus in excavations, in addition to the use of soil anchorage, the required strength is provided and protection of the excavation will be possible against lateral forces.
The mixing machine has single drills with a diameter of 0.6 to 2 m, and in terms of height, these columns have been executed in the United States to a depth of 20 m and in Japan to a depth of 60 m.
The obtained column resistance is a function of factors such as:
- Type of additive
- Soil type
- Execution method
History of deep soil mixing method or DSM
DSM or Deep soil mixing method was first used in 1971 near Handa Airport in Japan to improve the soil. In this project, lime was used as a soil stabilizer. But over time due to:
- Familiarity with the effects of cement on soil
- Cement availability
The use of cement as an additive became popular.
Since the mid-1970s, the method of DSM has spread from northern countries of Europe. This method was used in the United States due to the presence of specialized Japanese contractors. The first DSM project in the United States was carried out in 1986 under a dam on Lake Jackson. After the execution of this project, this method was widely used in soil improvement in this country, so that the Federal Highway Administration in the United States in 1997 to develop regulations (FHWA-RD-99- 138) and criteria for executing the DSM method.
Design basics of Deep soil mixing method
DSM columns are used to increase soil bearing capacity in compression and shear, settlement control, counteract liquefaction, reduce permeability and also in environmental fields to limit or fix harmful chemicals in soil. In each of the above goals, calculations are performed in accordance with the relevant relations.
The design basics of deep mixing columns are almost the same as the design basics of jet grouting columns, and the compressive strength of cement soil is directly related to the soil type and the amount of cement used in mixing.
DSM columns can be executed in different arrangements:
Deep soil mixing (DSM) execution method
As shown in the figure below, drilling is first done by Auger to the desired depth. Then the drilling is stopped and Auger slowly upwards and during the movement, the slurry is injected into the soil from the nozzles installed in the tips of the arms with high pressure. This movement and injection is in stages and after 1 meter upwards, it rotates in that level for 1 minute and injects the slurry and this work continues until it reaches the level of natural ground. Of course, the intervals and times mentioned vary according to the type of device and the type of soil. In addition, the top-down injection method can be done depending on the project conditions and the location of the injection nozzles
Additives for Deep soil mixing (DSM)
Cement slurry or dry cement is the most common additive in the Deep soil mixing (DSM) method. The volume of slurry used, depending on the case and soil conditions, is about 20 to 30% of unmodified soil volume. Lime is also a suitable additive that is sometimes mixed with cement and used simultaneously. In this case, the weight ratio of cement to lime is about 1 to 4 and their total is added to the amount of about 100 to 200 kg/m3 of unmodified soil. Other additives such as volcanic ash, furnace slag and other chemicals can also be used if possible.
Types of mixing in Deep soil mixing (DSM) method
- Wet soil mixing method:
Wet soil mixing is the usual method of Deep soil mixing (DSM) for retention of excavation, which was described in the previous sections.
- Dry soil mixing method:
If there is enough humidity to perform a chemical reaction between the injected mixture and the soil, the Dry soil mixingmethod can be used. In Dry soil mixing method, the injection mixture (cement, lime or a combination of them) is injected as a powder and mixed with soil. The dry material is mixed with the soil by applying high pressure air, which is discharged through nozzles at the lower end of the drill pipe. In this way, the injected material reacts with the soil humidity and columns similar to Wet soil mixing method are created. The diameter of the columns created by the Dry soil mixing method is smaller than the Wet soil mixing method and is between 60 to 80 cm. The maximum depth of soil improvement with this method reaches 25 m.
Shallow soil mixing
In cases where only the first few meters of the ground surface have large amounts of weak or contaminated soils with high humidity content such as sediment and dredged deposits, more organic soils or sludge soils, these several meters can be improved by using shallow soil mixing. In shallow soil mixing method, a special mixing tool is used, which in most cases is attached to the arm of the drilling machine.
Mixing in Shallow soil mixing method is done horizontally or vertically by a blade Auger and a nozzle in the center of which is installed to exit the injection mixture. The injection mixture is injected into the soil through a separate device consisting of a mixed pressure chamber, a compressor, an air dryer and a control unit. Depending on the specifications of the tools used, soil improvement in Shallow soil mixing method is possible up to a maximum depth of 4 m. After the required volume of mixture is injected, mixing is continued to obtain the desired design specifications.
Applications of Deep soil mixing method (DSM)
Depending on the figure, the deep mixing method is used to improve a wide range of inorganic soft soils and soils in which other ground improvement methods are not suitable, although in the case of organic soils, properties of the soil can be improved by considering measures such as adding sand to the mixture.
Deep soil mixing method is one of the most economical methods for improving soft soils. Economic, temporal, or environmental limitations sometimes make other improvement methods, such as preloading, or dynamic methods, uneconomical. On the other hand, these two methods are not very useful in urban areas due to vibrations and noise. The deep mixing method has proven its capabilities as a suitable alternative to these two methods as well as the deep foundation method (pile execution).
About 50 years after the invention of DSM method, due to the lack of technology needed to execute it, in a few cases Deep soil mixing method has been used in Iran. Due to the existence of coastal areas in the north and south of Iran, the importance of using this method becomes clearer. Especially in our country, due to high seismicity and the possibility of liquefaction in coastal areas, the use of deep soil mixing method reduces the potential for liquefaction and prevents problems. Also, the possibility of improving the soil from the barge in the construction of ports and marine structures is another advantage of this method for use in coastal areas.
Soil improvement in DSM method is different depending on:
- Type of additives
- Mixing energy level
In deep soil mixing method, by mixing the slurry with the soil, materials are made that become harder and stronger over time and can be used as engineering materials with geotechnical characteristics better than the local soil in the designs.
Some applications of DSM method in bed improvement are:
- Strengthening the foundations of various structures
- Reduction of liquefaction potential
- Strengthening the foundation
- Strengthening the foundation in road construction
- Foundation of oil reservoirs
- Dewatering wall
- Execution of vertical retaining wall
- Excavation slope protection
- Port development
- Tunnel stabilization
Quality control of Deep soil mixing (DSM) method
- Tests required for quality control of deep mixing columns:
- Quality control of water used (according to national standards)
- Quality control of cement used (according to national standards)
- Quality control of additives used (according to national standards)
- Quality control of slurry used
- Outlet effluent control
- Geometric profile control tests (diameter, direction, length, etc.)
These tests are usually performed as observational measurements, which include the following:
- Deviation in the plan
- Deviation in depth
- Deviation in diameter at depth
Advantages of Deep soil mixing (DSM) method
- The economics of this method compared to similar methods of improvement
- Very low cost of equipping the workshop compared to similar methods such as dynamic compaction or pile execution
- Decreasing soil permeability
- Increasing soil resistance
- Reduction of soil subsidence due to overhead
- Preventing liquefaction
- Correction of the ground
- Ground becomes part of the soil-structure system
- Ability to control and confirm the quality of work
- Very high execution speed
- Very low vibration and noise pollution compared to other common urban improvement methods
- Ideal in terms of environmentally
Related contents:
Leave a Reply