Liquefaction Mitigation by Deep Soil Mixing Method (DSM)
Liquefaction is a process by which saturated soil loses its strength and stiffness due to a sudden increase in its pore pressure. During earthquakes, the cyclic loading of soil can cause pore pressure to increase rapidly, leading to liquefaction and subsequent settlement, tilting or sliding of structures. This can result in catastrophic damage and loss of life. One of the methods to mitigate liquefaction is the Deep Soil Mixing (DSM) method.
DSM is a ground improvement technique used to enhance the engineering properties of soils. It involves the in-situ physical and chemical modification of soil to increase its strength, stiffness, and permeability. DSM is achieved by mixing inorganic or organic binders, such as cement or lime, with the soil. The resulting mixture is homogenous and can be engineered to meet the specific requirements of a construction project.
DSM is suitable for a wide range of soil types, including soft clays, sandy soils, and organic-rich soils. The method can be utilized to improve the soil's stability, increase its bearing capacity, and reduce the risk of liquefaction. DSM can be implemented in various forms, such as deep soil mixing (DSM), shallow soil mixing (SSM), and column-supported embankment (CSE).
In DSM, the soil is mixed with a binder at a depth that is greater than the depth where the liquefaction is expected to occur. The method involves drilling a series of overlapping boreholes into the soil to form a grid pattern. A mixing tool, attached to a machine, is lowered into the borehole, and the soil is mixed with the binder as the tool is withdrawn. The final result is a column of mixed soil-binder mixture that increases the soil's stiffness and strength. The method can be applied to depths of up to 60 meters.
SSM is a method of mixing the soil's surface layers with a binder to improve the soil's stability. SSM is appropriate for shallow soils and can be applied to depths of up to 5 meters. In this method, the soil is mixed using a rotating horizontal or vertical mixing tool. The resulting mixed soil-binder layer forms a stabilized soil layer.
CSE is a method that uses the DSM technique to support the embankment. The method involves constructing a pile-supported embankment with DSM columns as lateral support columns. The DSM columns support the embankment and improve the soil's bearing capacity, reduce the risk of slope failure and consolidation, and mitigate liquefaction.
The DSM method is an effective technique that mitigates liquefaction by increasing the soil's strength, stiffness, and permeability. The DSM columns that are installed at a depth greater than or equal to the liquefaction depth ensure that the soil resists the excecssive dilation that occurs during the earthquake. The stiffness imparted by DSM limits the soil's deformation and prevents liquefaction.
The DSM columns also increase the soil's bearing capacity, which is the ability of the soil to support a load. The DSM columns can improve the soil's bearing capacity by increasing the effective stress that the soil can withstand before it undergoes shear failure. This is particularly beneficial for structures that are constructed on weak soils or areas prone to liquefaction.
Another advantage of the DSM method is that it can be tailored to meet the specific requirements of a construction project. The DSM columns can be designed to vary in diameter, depth, and spacing depending on the site-specific conditions. This flexibility allows the method to be used in a wide range of construction projects, including buildings, embankments, and infrastructure projects.
DSM is also a cost-effective method of soil improvement. The cost of the DSM method is relatively low compared to other ground improvement techniques, such as soil replacement or the installation of piles. This is due to the in-situ nature of the method, which avoids the cost of excavating and transporting soil offsite.
The DSM method also has minimal environmental impact. The method requires minimal excavation, which limits the amount of soil that needs to be excavated and disposed of. The method also uses minimal water, which reduces the need for water management and treatment. The use of binders, such as cement or lime, may raise environmental concerns due to their carbon footprint. However, recent developments in green binders, such as geopolymer, have alleviated these concerns.
DSM has been successfully used in various construction projects around the world to mitigate liquefaction. One of the notable examples is the construction of a new extension to the Helsinki metro in Finland. The extension was constructed in an area prone to liquefaction due to its proximity to the sea. DSM columns were installed to improve the soil's bearing capacity and mitigate liquefaction. The use of DSM allowed the construction of the metro extension to proceed without any damage caused by liquefaction during the earthquakes that occurred during the construction period.
In conclusion, liquefaction is a significant risk for structures constructed on saturated soils. The DSM method is an effective and cost-efficient method of mitigating liquefaction. The method involves the physical and chemical modification of soil to increase its strength, stiffness, and permeability. DSM can be applied in various forms, such as deep soil mixing, shallow soil mixing, and column-supported embankment. The use of DSM technique can improve the soil's bearing capacity and reduce the risk of liquefaction. The DSM method is also environmentally friendly due to its minimal excavation, water use, and carbon footprint. With its numerous benefits, the DSM method has become a popular choice for soil improvement in construction projects worldwide.