Soil Mixing Techniques: Sustainable & Economic Foundation Engineering
Fadi Haddad, Head BD Dams, Dikes, Mining, Ports, Bauer Spezialtiefbau GmbH, and Vidyaranya Bandi, Technical and Tendering, Bauer Engineering India, draw a focus on Soil Mixing Techniques and their economic and ecological benefits.
The construction industry has long been recognized as a major contributor to global carbon emissions, necessitating a shift towards more sustainable practices. One area where significant progress is being made is in special foundation engineering, particularly through the use of soil mixing techniques. These techniques, including Mixed-In-Place (MIP) and Cutter Soil Mixing (CSM), are being increasingly adopted for their ability to reduce environmental impact while maintaining the structural integrity of foundational elements.
The Challenge of Special Foundation Engineering
Figure 1: MIPSpecial foundation elements are essential in a variety of construction projects, from the construction of dams and dikes to the foundations of high-rise buildings and the stabilization of soils for ports and LNG tanks. Traditionally, these elements have been constructed using methods that involve significant excavation and the use of concrete, both of which contribute to a high carbon footprint. The need for deeper and more robust foundation elements in response to the increasing demands of modern infrastructure has further exacerbated the environmental impact of these construction projects.
One of the major challenges in special foundation engineering is the need to rehabilitate aging dams and dikes. These structures often require the installation of cutoff walls to limit seepage and erosion, a task traditionally accomplished using diaphragm wall techniques. However, these methods involve trenching, concrete pouring, and the use of bentonite slurry, all of which increase the carbon footprint due to the transportation of materials and the energy-intensive nature of the processes involved.
Sustainable Alternatives
Soil Mixing Techniques: Soil mixing techniques have emerged as a sustainable alternative to traditional excavation and concrete construction methods. These techniques involve the in-situ mixing of existing soil with cement slurry, creating a strong, durable wall without the need for extensive excavation or concrete. By using the soil already present on-site, these methods significantly reduce the amount of material that needs to be transported, thereby lowering the carbon footprint of the construction process.
Figure 2: MIP – Mixing principle & Figure 3: MIP by executing a retainng wall for an excavation pit in Germany
Mixed-In-Place (MIP) Technique: The MIP technique, developed by Bauer, is particularly effective for the construction of cutoff and retaining walls. This technique involves the use of three augers that mix the soil while injecting a suspension containing water, bentonite, and cement. The result is a strong and durable wall that eliminates the need for excavation, significantly reducing the environmental impact of the construction process.
One of the key advantages of the MIP technique is its ability to create a homogenous wall by vertically mixing in-situ soils, even those with layers of organic or high fine content. The use of reversible augers and a back-step sequencing method ensures the overall homogeneity and continuity of the wall, which can be reinforced with H-beams and anchors for added structural strength.
Cutter Soil Mixing (CSM) Technique: The CSM technique, also developed by Bauer, utilizes special cutting and mixing heads derived from diaphragm wall cutter technology. This method is highly versatile, capable of creating deep walls up to 40 meters in depth. The CSM technique is particularly effective in non-cohesive and cohesive soils containing sand and a small percentage of fines. The mixing wheels of the CSM machine break up and mix the soil with an injected suspension, creating a homogenous mix that is both strong and durable.
One of the significant advantages of the CSM technique is its ability to create vertical rectangular panels, resulting in fewer vertical joints compared to multiple shaft systems. This makes it ideal for linear in-situ structures such as cutoff barriers, retaining walls, and liquefaction mitigation cells. The CSM system also allows for complete instrumentation inside the cutter gearbox support frame, providing real-time control of the cutting head’s coordinates, ensuring complete overlap between panels.
Figure 7: Installation of Cutter Soil Mixing
Economic and Ecological Benefits
Both the MIP and CSM techniques offer substantial economic and ecological benefits compared to traditional methods. By using the existing soil on-site, these techniques reduce the need for material transportation, which in turn lowers carbon emissions. Studies have shown that these methods can reduce CO2 emissions by up to 30% compared to traditional diaphragm wall construction.
In addition to their environmental benefits, these soil mixing techniques also offer significant cost savings. The reduced need for excavation and concrete results in lower material costs, while the speed of construction is increased, leading to lower labor costs. Furthermore, the ability to reinforce MIP and CSM walls with steel sections and anchors allows these techniques to be used in a wide range of applications, from simple cutoff walls to complex retaining structures.
Figure 8: Comparison of CO2eq balances for the geotechnical works at QH Track, using the conforming solution (D Wall and Jet Grout Plug) in contrast to the variation (MIP retaining wall and LWS silicate gel grout plug)
The sustainability of these techniques is further enhanced by their ability to minimize the environmental impact of construction activities. The in-situ nature of these methods means that there is no need for extensive excavation, reducing the disruption to the surrounding environment. Additionally, the reduced transportation of materials results in lower noise emissions and less traffic congestion, contributing to a more livable environment.
Quality Control
Quality assurance is critical to the success of both the MIP and CSM techniques. Prior to execution, preliminary tests are conducted to determine the optimal binding agent suspension mix and the appropriate quantity based on soil and groundwater samples taken from the site. During the construction process, samples of the soil mix are collected and tested to ensure the desired strength and durability are achieved.
Figure 9: Comparison of traffic volume for the geotechnical works at QH Track using the conforming solution (D Wall and Jet Grout Plug) in contrast to the variation (MIP retaining wall and BAUER LWS silicate gel grout plugs)
Conclusion
The adoption of soil mixing techniques such as MIP and CSM represents a significant step forward in the pursuit of sustainable construction practices in special foundation engineering. By reducing the carbon footprint of construction projects while maintaining the structural integrity of foundational elements, these techniques offer a viable alternative to traditional methods. As the construction industry continues to evolve, the use of sustainable techniques like MIP and CSM will play an increasingly important role in shaping the future of infrastructure development.
