How to Select Soil Improvement Method Based on Soil Types?

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The selection of soil improvement method is governed by several factors like soil type, equipment availability, cost, construction period, skills, and past experiences. Soil type is one of the critical factors that can influence the selection of one soil improvement method over another.

The soil type alters both the effectiveness and cost of the soil improvement technique. Some of the soil improvement methods are soil replacement, grouting, preloading, site strengthening, and geosynthetics.

How to Select Soil Improvement Method Based on Soil Types?

1. Soil Replacement

The soil replacement method is applicable for soft soil. There are two primary techniques for soil replacement: removal and replacement of the soil and soil displacement.

The former is suitable for any soft or weak soil, provided that the soft soil layer is near the surface, groundwater can be lowered economically or the soil layer is located above groundwater, and the weak soil thickness is not greater than 9 m.

It involves the removal of soft soil and replacing it with structural fill during grading operations. Soil displacement includes overloading very soft soil until it shears and is displaced by firmer fill.

2. Water Removal

The water removal method is appropriate for sites with an underlying compressible cohesive soil layer. There are four techniques by which water is removed from the soil to improve its strength:
1. Trenching technique
2. Preloading technique
3. Preloading with vertical drain
4. Electro-osmosis

2.1 Trenching Technique

Excavation of trenches on the construction site facilitates water drainage. It is suitable for soft, fine-grain soils and hydraulic fill. It is appropriate for a depth of 3 m. The speed of the process relies on trench spacing and the soil. The removal of water from the soil layer above the soft ground would reduce settlement on the construction site.

2.2 Preloading Technique

It includes the use of loads to consolidate the soil layer before the commencement of the construction work. Preloading is suitable for normally consolidated fine-grained soil, organic soil, and fills. It is cost-effective but needs a long time to achieve adequate consolidation.

2.3 Preloading with Vertical Drain Technique

It includes the use of loads combined with a vertical drain to consolidate the soil layer before the start of the construction work. Preloading with vertical drain is suitable for normally consolidated fine-grained soil, organic soil, and fills. It is useful for a depth not more than 30 m.

2.4 Electro-osmosis

It is suitable for normally consolidated clay and silty clay. It can be used in a confined area, and is the best choice for small areas. Electro-osmosis is fast but expensive at the same time. It is not suitable for conductive soil layers.  

3. Site Strengthening Methods

There are a number of site strengthening methods from which engineers can select a suitable technique based on equipment availability, cost, local experiences, etc. Different types of site strengthening methods are described below:

3.1 Dynamic Compaction

Dynamic compaction is an efficient and cost-effective soil improvement technique that uses the dynamic effect of high energy impacts to densify weak soil. The dynamic effect is generated by dropping a static weight (15-40 tones) from a defined height (10-30 m). It is the best choice for cohesionless soil, and possibly suitable for soils with fines.

Dynamic compaction is not appropriate for cohesionless soils below the groundwater table as it may damage adjacent structures. It is recommended to use this method for a maximum depth of 18 m.

3.2 Vibro-compaction

It is appropriate for cohesionless soils with less than 20% fines, and useful for 30 m depth.

3.3 Vibro-replacement

It is suitable for soft cohesive soil and is considerably expensive. The vibro-replacement technique includes the use of jetting and vibration to penetrate and remove soil, and then the placement of compacted granular fill into a hole to create columns surrounded by undisturbed soil.

3.4 Vibro-displacement

The vibro-displacement technique is suitable for stiffer cohesive soil and involves the use of jetting and vibration to penetrate and displace soil, and then the placement of compacted granular fill into a hole to create columns.

4. Grouting

Grouting is the injection of grout into the ground to fill underground voids, joints, and fractures and consequently improve soil strength.

4.1 Injection of Grout

It is applicable for a broad spectrum of coarse and fine-grained soils. Expansive grouts are necessary to grout finer grain soils. The injection of grouts reduces the permeability of the soil strata.

4.2 Deep Mixing

It includes the use of jetting or auger to mix the soil with grouts. It is a useful option for a broad spectrum of coarse and fine-grained soils for a depth of nearly 52 m. Deep mixing is a wrong choice for highly cohesive clay and some gravelly soils.

5. Thermal Method

Heating and freezing are two techniques to improve the soil properties. The former is suitable for cohesive soil, whereas the latter is applicable for soil types below the groundwater table and cohesive soil above the groundwater table.

The heating process generates irreversible strength, but the need for high energy restricts its practicality. Freezing reduces permeability and increases the bearing capacity of the soil. It is suitable for tunneling and excavation to reduce or stop the inflow of groundwater.

6. Geosynthetics

The geosynthetic materials can be used for erosion control, filter, drains, water barrier, and soil reinforcement. It is an effective filter for all soil types and reinforcement for soft soil.

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