– Soil improvement – DSM or deep soil mixing – What is deep soil mixing or DSM? (Deep Soil Mixing)
rose roofing: Deep soil mixing or DSM is one of the practical methods of soil improvement in the world, which produces soil columns by mechanically injecting stabilizing materials such as cement or lime using a hollow-axis drill in the soil. cementing and modification of soil specifications. In this method, the characteristics of the soil are improved by mixing the soil with cement materials by stirrers in depth and creating columns of cement soil. In addition, by overlapping the columns before they are completely fixed, continuous walls can be built under the ground.
In this method, by successively implementing the columns and creating an overlap between them, it is possible to create watertight walls or perform any decoration under the foundation of the building, and after obtaining sufficient resistance, build the foundation of the structure on the executed columns. Sometimes, to create retaining walls, before the soil-cement mixture is set, even or single steel profiles are pushed into the soil, and in this way, in excavations, along with the use of anchoring, the required strength is provided and protected. It will be possible against the side forces.
The mixing machine has single drills with a diameter of 0.6 to 2 meters, and in terms of height, these columns have been implemented up to a depth of 20 meters in America and up to a depth of 60 meters in Japan.
The obtained column resistance depends on factors such as:
Type of additive
Soil type
Procedure
is.
Deep Soil Mixing
History of Deep Soil Mixing Method (DSM)
Deep soil mixing was first used in 1971 near Handa Airport in Japan for soil improvement. In this project, lime was used as a soil stabilizer. But over time due to:
Getting to know the effects of cement on soil
Availability of cement
The use of cement as an additive became popular. From the middle of the 1970s, the method of deep mixing of soil from the northern countries of the European continent was spread in this continent. In America, due to the presence of Japanese specialized contractors, this method was used. The first deep soil mixing project in America was done in 1986 under a dam located in Jackson Lake.
After the implementation of this project, this method was widely used in soil improvement in this country in such a way that the Federal Highway Administration (Federal Highway Administration) in the United States developed regulations in 1997 (FHWA-RD-99- 138) and criteria for implementing the deep mixing method.
Deep mixing columns, which are implemented in the form of overlaps
Basics of deep soil mixing (DSM) design
Deep soil mixing columns are used to increase the load capacity of the soil in pressure and shear, control settlement, deal with liquefaction, reduce permeability, and also in environmental fields to limit or fix harmful chemicals in the soil. In each of the aforementioned goals, calculations are made according to the relevant relationships.
The design principles of deep mixing columns are almost the same as the design principles of jet grouting columns, and the compressive strength of cement soil has a direct relationship with the type of soil and the amount of cement used in mixing.
As shown in the figure below, first, drilling is done by auger to the desired depth. Then the drilling is stopped and the auger slowly moves upwards, and during the movement, the slurry is injected into the soil with high pressure from the nozzles installed at the tips of the arms.
This movement and injection is staged and after traveling 1 meter upwards, it rotates in that level for 1 minute and injects the slurry and this work continues until it reaches the natural ground level. Of course, the mentioned distances and times are different according to the type of device and the type of soil. In addition, the top-down injection method can be done depending on the implementation conditions of the project and the location of the injection nozzles.
Steps to implement deep soil mixing (more method)
The steps of implementing the deep soil mixing method are as follows:
Placement of drilling machine and drill at the place of implementation of deep mixing column
Driving the drill rod at a predetermined rate into the soil by rotating the blades to the desired depth
Mixing the mortar with the soil for a specified period of time to achieve an almost uniform mixing of the soil at the desired depth
Taking out the mixing rod at a predetermined rate while mixing the mortar with the soil
Running a deep mixing column until reaching the desired level
Video Soil improvement by deep mixing method (DSM) – Bandar Anzali Free Zone Project
Additives of deep soil mixing method (DSM)
Cement slurry or dry cement is the most common additive in deep soil mixing method. The amount of slurry used is about 20 to 30% of the volume of unmodified soil, depending on the case and soil conditions. Lime is also one of the appropriate additives that are sometimes mixed with cement and used simultaneously.
In this case, the weight ratio of cement to lime is about 1 to 4 and their combination is added to the amount of about 100 to 200 kilograms per cubic meter of unmodified soil. Other additives such as volcanic ash, furnace slag and other chemicals can also be used if possible.
An example of a deep mixing dry method device
Types of mixing in deep soil mixing method (DSM)
More method: The usual method is the deep mixing of the soil, which was explained in the previous parts.
Dry method: If there is enough moisture to carry out the chemical reaction between the injected mixture and the soil, the dry method can be used. In this method, the injection mixture (cement, lime or a combination of them) is injected as a powder and mixed with the soil. The process of mixing the dry injection material with the soil is done by applying high pressure air
