How to Select the Best Soil Improvement Method Based on soil types?

 

Selecting the appropriate soil improvement method is essential for ensuring the stability and durability of construction projects. The choice is influenced by various factors such as soil type, equipment availability, cost, construction period, skills, and past experiences. This article delves into different soil improvement methods and their suitability based on soil types, focusing on techniques like soil replacement, grouting, preloading, site strengthening, and geosynthetics.

1. Soil Replacement

1.1 Soil Replacement Method

Soil replacement is a common technique used for improving soft soils. This method involves either the removal and replacement of the soil or soil displacement.

Removal and Replacement: This technique is suitable for soft or weak soils, particularly when the soft soil layer is near the surface. It is most effective when the groundwater can be economically lowered or when the soil layer is above the groundwater. The technique is appropriate for weak soil layers up to 9 meters thick. The process involves excavating the soft soil and replacing it with a structural fill during grading operations.

Soil Displacement: This method is used to overload very soft soils until they shear and are displaced by firmer fill. It is a practical approach when dealing with very soft soil conditions.

2. Water Removal

Water removal techniques are crucial for sites with underlying compressible cohesive soil layers. There are four primary techniques for water removal: trenching, preloading, preloading with vertical drains, and electro-osmosis.

2.1 Trenching Technique

The trenching technique involves excavating trenches on the construction site to facilitate water drainage. It is suitable for soft, fine-grain soils and hydraulic fill and is effective up to a depth of 3 meters. The speed of this process depends on the trench spacing and the soil type. By removing water from the soil layer above the soft ground, this method helps reduce settlement on the construction site.

2.2 Preloading Technique

Preloading involves applying loads to consolidate the soil layer before starting construction work. It is suitable for normally consolidated fine-grained soils, organic soils, and fills. While it is cost-effective, it requires a significant amount of time to achieve the desired consolidation.

2.3 Preloading with Vertical Drain Technique

This technique combines preloading with vertical drains to expedite the consolidation process. It is suitable for normally consolidated fine-grained soils, organic soils, and fills, and can be used for depths up to 30 meters. The vertical drains help speed up the water removal process, making it more efficient than traditional preloading.

2.4 Electro-osmosis

Electro-osmosis is ideal for normally consolidated clays and silty clays. It is particularly useful in confined areas and small sites. Although it is a fast method, it is also expensive and not suitable for conductive soil layers.

3. Site Strengthening Methods

Site strengthening methods enhance the load-bearing capacity of the soil. The choice of technique depends on factors such as equipment availability, cost, and local experiences.

3.1 Dynamic Compaction

Dynamic compaction is a cost-effective soil improvement technique that uses high-energy impacts to densify weak soils. This method involves dropping a heavy weight (15-40 tons) from a height (10-30 meters). It is best suited for cohesionless soils and soils with fines. However, it is not recommended for cohesionless soils below the groundwater table, as it may damage nearby structures. The method is effective up to a depth of 18 meters.

3.2 Vibro-compaction

Vibro-compaction is appropriate for cohesionless soils with less than 20% fines and is effective up to 30 meters in depth. This technique uses vibration to rearrange soil particles, thereby increasing soil density and strength.

3.3 Vibro-replacement

This method is suitable for soft cohesive soils and involves using jetting and vibration to penetrate and remove soil. Then, compacted granular fill is placed into the created hole to form columns surrounded by undisturbed soil. Although vibro-replacement is effective, it is also relatively expensive.

3.4 Vibro-displacement

Vibro-displacement is used for stiffer cohesive soils. This technique uses jetting and vibration to penetrate and displace soil, followed by the placement of compacted granular fill into the hole to create columns. It is an effective method for improving the load-bearing capacity of stiffer cohesive soils.

4. Grouting

Grouting involves injecting grout into the ground to fill voids, joints, and fractures, thereby improving soil strength.

4.1 Injection of Grout

Grouting is applicable for a wide range of coarse and fine-grained soils. Expansive grouts are necessary for grouting finer-grained soils. This method reduces soil permeability and increases strength.

4.2 Deep Mixing

Deep mixing uses jetting or auger techniques to mix the soil with grout. It is effective for a broad range of coarse and fine-grained soils up to a depth of approximately 52 meters. However, deep mixing is not suitable for highly cohesive clays and some gravelly soils.

5. Thermal Method

Thermal methods, including heating and freezing, are used to improve soil properties.

5.1 Heating

Heating is suitable for cohesive soils and generates irreversible strength. However, the high energy requirements limit its practicality.

5.2 Freezing

Freezing reduces soil permeability and increases bearing capacity. It is effective for tunneling and excavation projects to reduce or stop groundwater inflow. This method is suitable for soil types below the groundwater table and cohesive soils above the groundwater table.

6. Geosynthetics

Geosynthetics are materials used for erosion control, filtration, drainage, water barriers, and soil reinforcement. These materials are effective filters for all soil types and provide reinforcement for soft soils.

Geosynthetics are versatile and can be used in various soil improvement applications. They enhance the stability and durability of soil structures by providing additional strength and reducing erosion.

Conclusion

Selecting the best soil improvement method is crucial for the success of construction projects. Factors such as soil type, equipment availability, cost, construction period, skills, and past experiences play a significant role in determining the appropriate technique. Understanding the suitability of each method based on soil type is essential for making informed decisions.

By carefully evaluating the specific conditions of the construction site and the properties of the soil, engineers can choose the most effective soil improvement method. Whether it involves soil replacement, water removal, site strengthening, grouting, thermal methods, or geosynthetics, each technique has its advantages and limitations. Proper selection and application of these methods will ensure the stability and longevity of the constructed structures.