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Soil improvement in its broadest sense is an alteration of any property of soil. to improve its engineering performance. It usually consists of increasing the shear strength and decreasing the compressibility of the soil so that the bearing capacity of the soil is increased and the settlements of the structures built on it are reduced.
Sometimes, the aim is to decrease the permeability of the soil.
A. Soil Improvement
General practice is to use a shallow foundation as far as possible. But if the topsoil is either loose or soft, the load from. the superstructure is to be transferred to deeper firm strata. In such cases, a deep foundation such as a pile or pier is the obvious choice. But another method at such a site condition may prove more economical which is foundation soil improvement. These are the methods which are implemented to improve the characteristics of soil at the site.
Soil improvement basically consists of:
i) Increasing the shear strength
ii) Reducing permeability,
iii) Reducing compressibility.
The method of soil improvement to be discussed here includes:
1) Mechanical compaction
2) Dynamic compaction
4) Sand and stone columns
5) Use of admixtures
6) Injection of suitable grouts
1. Mechanical Compaction
It is the process of increasing the density of soil by the application of mechanical energy such as temping, rolling and vibration, Mechanical in the air is achieved by forcing the particles close together with a reduction in air voids. The compaction of ground soil will thus help in reducing the void ratio and improving, the soil strength parameters like cohesion (C) and angle of internal friction (Φ)
The purposes of compaction are:
i) To reduce shear strength
ii) To reduce compressibility
iii) To reduce permeability
iv) To reduce liquefaction potential
v) To control swelling and shrinkage
vi) To prolong durability
For cohesive and moderately cohesive soil, the optimum moisture content should be determined and compaction should be done at or near optimum moisture content, to achieve a maximum dry density with sheep—foot rollers.
In the case of cohesionless soils, compaction can be best achieved by means of vibrations. When cohesionless soil deposits exist in a loose state near the ground surface, they can be compacted easily with surface vibration using a vibratory roller. The effectiveness of rollers however decreases with depth. Needle vibrators may of course help compact larger depth of soil.
2. DYNAMIC COMPACTION
For deeper layers of soils, dynamic composition methods are generally used. Three types of compaction for a deeper layer of soil are:
ii) Dropping of heavy weight
iii) Blasting Vibroflotation
The vibroflotation technique is used for compacting granular soil only. The vibrofloat is a cylindrical tube containing water jets at top and bottom and equipped with a rotating eccentric weight, which develops a horizontal vibratory motion.
The process is shown in the figure where firstly vibrofloat is sunk into the soil and then raised in successive small increments, during which the surrounding material is compacted by the vibration process. This method is very effective for increasing the density of sand deposit for depths up to 30 in. The relative density achieved in most cases is 70% or more.
Vibro compaction methods are however effective only when the silt content in the cohesion less soil is less than 12-15% and the clay content is less than 3%.
ii) Dropping of heavy weight
In this method, loose soils are compacted by repeated dropping of a heavy weight on the ground surface so as to cause compaction up to sufficient depths. This method can be used for compacting cohesive as well as cohesionless soils. In this method a crane is used to lift a heavy concrete or steel block, weighing up to 500 kN and up to a height of 40 to 50 m, from which height it is allowed to fall freely onto the ground surface.
The process is repeated to cover the entire area. The topsoil is then levelled and compacted using normal compacting equipment the depth of compaction, D is meters is approximately given by;
D = ½ (wh)2
where, w = Weight of falling mass in metric tons
h = Height of drop in meters
In compaction by blasts, buried explosives are used to density loose soils. In this method, a series of holes are bored and explosives are placed in them. The holes are filled back with soil. These explosives when detonated cause shock waves in the ground causing densification of the loose surrounding soil.
Because of harmful vibrations in adjacent structures and the safety precautions required, this method is commonly used for remote sites.
The modification of soil by applying the compressive load on site of intensity equal to or greater than a possible load of the proposed structure before the construction of the corresponding structure is known as preloading or pre-compression. The surcharge or load is kept on the ground for a long time to cause consolidation of the soil in the ground. The purpose of preloading is:
i) To ensure enough settlement of soil before construction so that settlement of soil after construction is within the permissible limits.
ii) To increase the bearing capacity of soil at the site so that performance of the proposed foundation can be improved or the deep foundation can be replaced by a cheaper shallow foundation.
The principle of preloading can be explained with the help of consolidation theory.
The following figure shows the effect of loading, unloading and reloading on soil. The virgin cane AB indicates the decrease in void ratio of soil during initial loading. The curve CFD indicates decrease in the void ratio when the soil is reloaded. During pre-compression, when the surcharge fill is applied to the virgin soil in the field, the soil follows initial loading curve AB.
When the surcharge fill is removed, unloading occurs along BFC. When foundation is constructed on the pre—compressed soil, the reloading occurs along the curve CFD. The decrease in void ratio during reloading is considerably smaller than that in the virgin soil in the initial loading. This result in decreased settlement and decreased compressibility of soil.
Accordingly the shear strength of the soil is increased and hence bearing capacity of the soil is improved.
Advantage of the Preloading method
1. It ensures uniformity of improvement of the ground because it eliminates local inhomogeneities and weak spots.
2. The progress of consolidation of soil can be easily monitored by installing simple settlement plates and piezometers.
3. It improves shear strength of the soil which results in improved bearing capacity.
4. It reduces the settlements and compressibility of soil.
Disadvantages of the Preloading method
i) The time of consolidation is long
ii) This method requires large space around the site of construction to raise surcharge fill.
iii) It may be expensive if surcharge fill is not locally available at a low cost.
These were the ways that can help you for soil improvement.
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