Improvement of Bearing Capacity of Soils

If unsuitable soil conditions are encountered at the foundation site of a proposed structure, one of the following three procedures may be adopted:

1. The unsuitable soil is bypassed by means of deep foundations extending to a suitable bearing material.
2. The poor material is replaced and either treated to improve and replaced or substituted by a suitable material.
3. The soil is treated in place to improve the properties.

Now-a-days various methods are available by which the characteristics of the construction site can be improved to facilitate construction operation, to allow increased bearing pressures or to reduce settlements. Soil improvement in its broadest sense is the alteration of any property of a soil to improve its engineering performance. The various techniques discussed are surface compaction, drainage methods, grouting and injection, chemical stabilisation, thermal stabilisation, soil reinforcement, and application of geotextiles and geomembrane.

1. Surface Compaction

One of the most widely used and the oldest technique of soil densification is compaction. Construction of a building on a loose foundation site needs a compacted base for laying the structures. If the depth to be densified is less then surface compaction may alone solve the problem. Surface compaction needs less skilled labour and is usually the most economical method.

2. Drainage Methods

Drainage method of densification of soil is lowering the water table temporarily or permanently by pumping using well-point systems.

3. Vibration Methods

Vibration methods comprise of vibro-compaction and vibro-displacement compaction. Another method of vibration method is heavy tamping.

The most basic and simplest way of compacting loose soil is by repeated dropping of a weight on the ground. This method, also known as deep dynamic compaction or deep dynamic consolidation, consists of allowing a very heavy weight (up to 400 kN) to fall freely on the ground surface from a height of 15–40 m. This leaves an impression on the ground. The tamping is then repeated either at the same location or over other parts of the area to be stabilised. In the case of non-cohesive soils, the impact energy causes liquefaction, followed by settlement as water drains. Fissures formed around the impact points sometimes facilitate drainage in some soils. This method can be adopted for densifying soils both above and below the water table. This method has been successfully used to treat various types of soils and fill deposits up to 20 m thick.

4. Pre-loading and Surcharge Fills

In this process, an earth fill or some other material is placed over the required site. The amount of fill is sufficient enough to produce a stress in the soil equal to the one anticipated from the final structure. The soft soil is allowed to consolidate prior to construction. Since the consolidation takes a very long time, the method is suitable only for stabilisation of thin layers.

The rate of pre-load and surcharge fill placement has to be controlled depending on the bearing capacity of the soil. If the bearing capacity of the soil is inadequate layers of fill can be placed only after a sufficient gain in shear strength is obtained. The two main requirements for preloading are enough space and availability of fill material. Heaping of fill is the most common method of pre-loading although pre-loading can be successfully effected by the weight of water or by lowering the water table. Among the fill materials, granular soil is the most desirable because it does not turn into mud during rains. Ores and industrial products are generally satisfactory, but clayey soils are less desirable.

5. Vertical Drains

For deep clay deposits, pre-loading alone will take more time because of the long drainage path available for consolidation. An efficient way to do this is by providing vertical drains. Vertical drains are continuous vertical columns of pervious materials installed in clayey soil for the purpose of collecting and discharging the water expelled during consolidation. Vertical drains in combination with pre loading will rapidly accelerate consolidation.

6. Grouting and Injection

Grouting is used for the following in connection with foundation:

1. Void filling to prevent excessive settlement.
2. Stabilising loose sands against liquefaction.
3. Strengthening existing foundation.
4. Reduction of machine foundation vibrations.

As discussed earlier suspension or solution grouts are used in the above cases depending on the field condition.

7. Chemical Stabilisation

Chemical stabilisation in the form of lime, cement, fly ash and a combination of the above is widely used in soil stabilisation to:

1. Increase bearing capacity
2. Decrease settlement
3. Expedite construction
4. Reduce permeability
5. Improve shear strength

Chemical stabilisation may be used for surface soils more successfully. Such a stabilisation technique is sparingly used for building foundation.

8. Soil Reinforcement

Soil reinforcement is the process of strengthening weak soil by providing high-strength thin horizontal membranes. The modern form of soil reinforcement was first applied by According to Vidal’s concept, the interaction between the soil and the reinforcing horizontal membrane is solely due to friction generated by gravity.

Reinforced soil is somewhat analogous to reinforced concrete. A wide variety of materials such as steel, concrete, glass fibre, rubber, aluminium and thermoplastic have been used successfully. High alloy steel, aluminium, glass-fibre reinforced plastics (GRP) and geosynthetics are non-corrosive and have long life.