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Selecting methods of improving bearing capacity of soils is necessary in many cases for the construction of foundation. Soil investigation is carried out to establish existing ground conditions and soil properties for selection of suitable foundation types for the structure.

When, the soil is found to be poor in bearing capacity or filled subsoil, some decisions have to be made by the designer based on available options.

1. Relocate the construction project: The first option for the designer is to relocate the construction project and find soil with suitable bearing capacity.

2. Remove and replace the poor ground: When the depth of poor soil is not very deep, this option can be selected. Poor soils can be excavated and replaced with good compacted soils. But this method has a risk of differential settlement of foundations. Using this option for depths more than 4m can be uneconomical.

3. Consolidation / Compaction by surcharge load: The ground can be surcharged with heavy loads from aggregates or other materials to speed up the settlement and improve bearing capacity of soil. But this option can delay the construction project, as consolidation or settlement of soil may require time from weeks to years based on soil type.

4. Vibration of ground surface: This method is used to strengthen the soil by rearranging and compacting coarse granular particles to form stone columns with the ground surface either by using the natural coarse granular soil or by replacement. This is carried out by a large poker vibrator which has an effective compacting radius of 1.5m to 2.7m.

This method is used to strengthen the ground to increase the bearing capacity of soil with a range of 200 to 500kN/m2.

5. Dynamic Compaction of soil: Dynamic Compaction method of improving bearing capacity of soil consists of dropping a heavy weight from a considerable height. This method is particularly effective in granular soils.

When water is present in the subsoil, it is first drained out before using dynamic compaction method. The pattern, size, weight and height of drop for dynamic compaction is selected to suit each individual site. Generally 3 or 4 drops are made in each position forming a crater up to 2.5m deep and 5m in diameter.

Improving bearing capacity of soil

Fig: Dynamic Compaction of soil

Use of dynamic compaction method creates vibrations in the surroundings due to free-fall of heavy weight. This can cause problems in nearby structures and buildings. Therefore, conditions of existing buildings should be considered before using this method for improving bearing capacity of soil.

6. Jet Grouting: This method of consolidating ground can be used in all types of subsoil. Jet grouting is done by lowering a monitor probe into a 150mm diameter pre-bored guide hole.

Jet Grouting

Fig: Jet Grouting (Source: HaywardBaker)

The probe consists of two jets, the upper jet blasts water, concentrated by compressed air to force any loose material up the guide to ground level. The lower jet fills the void with cement slurry which sets into a solid mass.