SOIL STABILISATION AND TYPES

Soil Stabilisation

Soil stabilisation is the process of improving the engineering properties of the soil and thus making it more stable.It is required when the soil available for construction is not suitable for intended purpose.In its broadest senses, stabilisation includes compaction, preconsolidation, drainage and many other processes.Generally, soil stabilization is a method of improving soil properties by blending and mixing other materials.A cementing materials or a chemical is added to a natural soil for the purpose of stabilisation.Improvements include increases the dry unit weight, bearing capabilities, volume change, the performance of in situ subsoils, sands and other waste materials in order to strengthen road surface and other geotechnical applications.

Soil stabilisation is used to reduce the permeability and compressibility of the soil mass in earth structures and to increase its shear strength.Soil stabilization is required to increase the bearing capacity of foundation soils.The main purpose of soil stabilisation is to improve the natural soils for the construction of highways and airfields.This process is accomplished using a wide variety of additives, including lime, cement and flyash.Other material byproducts used in stabilization include lime-kiln dust (LKD) and cement-kiln dust (CKD).There are various method and materials that can used to stabilising the soil.Following are the types of Soil Stabilisation:

1. Lime Soil Stabilisation
2. Cement Soil Stabilisation
3. Bitumen Soil Stabilisation
4. Chemical Soil Stabilisation
5. Electrical Soil Stabilisation
6. Thermal Soil Stabilisation

1. Lime Soil Stabilisation

Lime stabilisation is done by adding lime to the soil.It is very useful for stabilisation of clayey soils.When lime reacts with soil, there is exchange of cations in the adsorbed water layer and a decrease in plasticity of the soil occurs.The resulting material is more friable than the original clay and is therefore, more suitable as subgrade.Lime stabilisation improve the density and bearing capacity of the soil.

2. Cement Soil Stabilisation

Cement soil stabilisation is done by mixing pulverised soil and portland cement with water and compacting the mix to attain a strong material.The cement contains active ingredients that help in disintegration of soil particles.Besides that, the cement helps in concealing the soil.Other materials added into the mixture include lime, calcium chloride, sodium carbonate, sodium sulphate and fly ash.The amount of cement that's added into the mixture varies depending on the type of soil that's being stabilised.

3. Bitumen Soil Stabilisation

Bitumens are non-aqueous system of hydrocarbons that are soluble in carbon di-sulphide.Bitumen stabilisation is generally done with asphalt as binder.In any inorganic soil which can be mixed with asphalt is suitable for bituminous stabilisation.In cohesionless soils, asphalt binds the soil particles together and thus serves as a bonding or cementing agent.

4. Chemical Soil Stabilisation

In chemical soil stabilisation, soils are stabilised by adding different chemicals.These chemicals react with the soil which in return cause its structure to be changed.The chemical seal the space between particles, leaving no room for water to penetrate through.Among the most common chemicals that are used for soil stabilization include sodium chloride, calcium chloride and sodium silicate.

5. Electrical Soil Stabilisation

Electrical soil stabilisation of clayey soils is done by a process known as electro-osmosis.In this method a direct current is passed through a clayey soil, pore water migrates to the negative electrode (cathode).It occurs because of the attraction of positive ions (cations) that are present in water towards cathode.The strength of the soil is considerably increased due to removal of water. Electro-osmosis is an expansive method and is mainly used for drainage of cohesive soils.

6. Thermal Soil Stabilisation

Thermal soil stabilisation is done by heating the soil or by cooling the soil.When the soil heated, water content of soil decreases and strength of soil is increased.When soil is heated at high temperature some irreversible changes occurs which make the soil non-plastic and non-expansive.

When soil is cooling for stabilising the soil, a small loss of strength occurs in clayey soil due to an increase in interparticle repulsion.However, if the temperature is reduced to the freezing point, the pore water freezes and soil is stabilised.

WHAT IS SPALLING OF CONCRETE - ITS CAUSES AND REPAIR

 What Is Spalling Of Concrete ?

Spalling of concrete is a phenomenon where few parts of the concrete break itself free from the structure and expose the reinforcement or the insides of the concrete to the atmosphere.

Spalling is caused by certain chemical reactions inside the concrete which lead to the formation of foreign products which are of high volume. These new products due to lack of space inside the concrete will increase the internal pressure. As a result, cracks are formed to release this pressure outside. 

Civil Insta

Causes Of Spalling Of Concrete

1. Corrosion of Embedded Metals
2. Inadequate Cover
3. Freeze thaw Cycles
4. High Temperature
5. Alkali Aggregate Reactions
6. Improper Compaction
7. Improper Water Content
8. Sulphate Attack
9. Chloride Attack
10. Rough Finishes

1. Corrosion Of Embedded Metals

As described earlier, steel uses for the reinforcement, so the corrosion of the steel and other embedded material cause the deterioration of concrete. When steel rust the resultant corrosion occupies a large volume than the steel.

2. Inadequate Cover

Inadequate cover will increase the vulnerability of the reinforcement to the environment. Even the slightest crack on the surface will expose the reinforcement. When steel comes in contact with water and air, it corrodes. The products of the corrosion have high volume than that of steel causing high internal pressure and thereby causing spalling.

3. Freeze thaw Cycles

In cold weather regions, the seeped water will turn to ice which is higher in volume than that of water.

4. High Temperature

Explosive spalling may occur during fire or when concrete is exposed to high temperature which causes high pore pressure developed by the oversaturation. High strength concrete is more vulnerable to failure under high temperature than that of normal strength concrete due to the increased brittleness.

5. Alkali Aggregation Reactions

When the alkali in the cement reacts with silica in the aggregate in the presence of water, it will from the alkali silica gel which is higher in volume than the conventional products of hydration.

6. Improper Compaction

Improper compaction may leave the environment initiating corrosion and thereby spalling.

7. Improper Water Content

Low water content will reduce the workability and make it hard for compaction. This may lead to the formation of unintended air pockets which may later pave a way for crack formation. High water cement ratio will reduce the strength of the concrete thus making it more vulnerable to micro cracks and thereby spalling.

8. Sulphate Attack

It is caused by rich cement or the sulphate present in the atmosphere. The decomposition of the products of the hydration reaction leads to reduction in the strength of the concrete and in doing so causes spalling.

9. Chloride Attack

This occur in structures exposed to sea water. Chloride attack will directly cause corrosion to the steel reinforcement inducing the spalling concrete.

10. Rough Finishes

A rough finished surface structure tends to accumulate more water on the surface than a smooth surface finished structure. The rough finished structure will increase the seepage of water and may encourage spalling.

Repair Of Spalling Of Concrete

• Remove the concrete at the spalled areas to expose the corroded steel bars.
• Clean the exposed area of steel bars with wire brush to remove the rust from the bars.
• Clean the exposed area with compressed air.
• Apply two coats of anti-rust paint to the steel bars.
• Before patching the area, apply a bonding agent to the affected surface to ensure proper adhesion.
• The affected area shall be filled with normal concrete or polymer modified cement mortar.

SHEET PILE AND TYPES

 

Sheet Pile

Sheet pile are generally made of steel or timber. These piles are driven into the ground for either separating members or for stopping the seepage of water, they are not meant for carrying the vertical load. Sheet piles are sections of sheet material with interlock edges that are driven into the ground of providing earth retention and excavation support. However, sometimes reinforced cement concrete sheet piles are also used. The use of timber piles is generally limited to temporary structures in which the depth of driving does not exceed 3m. For permanent structures and for depth of driving greater than 3m, steel piles are more suitable.

Types Of Sheet Pile

1. Cantilever Sheet Pile
2. Anchored Sheet Pile

1. Cantilever Sheet Pile

Cantilever sheet piles are further divided into two types:

A. Free Cantilever Sheet Pile

It is a sheet pile subjected to a concentrated horizontal load at its top, there is no backfill above the dredge level. The free cantilever sheet pile derives its stability entirely from the lateral passive resistance of the soil below the dredge level into which it is driven.

B. Cantilever Sheet Pile

A cantilever sheet pile retains backfill at a higher level on one side. The stability is entirely from the lateral passive resistance of the soil into which the sheet pile is driven, like that of a free cantilever sheet pile.

2. Anchored Sheet Pile

Anchored sheet pile are held above the driven depth by anchor provided at a suitable level. The anchor provide forces for the stability of the sheet pile, in addition to the lateral passive resistance of the soil into which the sheet piles are driven. The anchored sheet piles are also of two types:

A. Free-Earth Support Pile

An anchored sheet pile is said to have free-earth support when the depth of embedment is small and the pile rotates at its bottom tip. Thus there is no point of contraflexure in the pile.

B. Fixed-Earth Support Pile

An anchored sheet pile has fixed earth support when the depth of embedment is large. The bottom tip of the pile is fixed against rotations. There is a change in the curvature of the pile, and hence an inflexion point occurs.

RETAINING WALL AND TYPES

 What is Retaining Wall ?

Retaining wall are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical or vertical slope). 

They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. The most important consideration in proper design and installation of a retaining wall is to recognize and counteract the tendency of the retained material to move downslope due to gravity. It is important to have proper drainage behind the wall in order to limit the pressure to the wall design value. Drainage material will reduce or eliminate the hydrostatic pressure and improve the stability of the material behind the wall. 

Types Of Retaining Wall

1. Gravity Retaining Wall
2. Reinforced Retaining Wall
3. Concrete Cantilever Wall
4. Buttressed Retaining Wall
5. Reinforced Soil Retaining Wall
6. Green Retaining Wall
7. Mechanical Stabilization Wall
8. Anchored Wall

1. Gravity Retaining Wall

Gravity wall depend on their mass to resist pressure from behind and may have better setback to improve stability by leaning back towards the retained soil. For short landscaping walls, they are often made from mortarless stone and segment concrete units. Dry stacked gravity walls is somewhat flexible and do not require a rigid footing.

2. Reinforced Retaining Wall

Reinforced concrete and reinforced masonry walls on spread foundation are gravity structures in which the stability against overturning is provided by the weight of the wall and reinforcement bars in the walls.

3. Concrete Cantilever Wall

A concrete cantilever retaining wall is one that consists of a wall that is connected to the foundation. A cantilevered wall holds back a significant amount of soil, so it must be well engineered. They are the most common type used as retaining walls. Cantilever wall rest on a slab foundation. This slab foundation is also loaded by back-fill and thus the weight of the back-fill and surcharge also stabilizes the wall against overturning and sliding.

4. Buttressed Retaining Wall

Buttressed retaining wall are cantilever wall straightened with counter forts monolithic with the back of the wall slab and base slab. The counter-forts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. To reduce the bending moments in vertical walls of the great height, counterforts and used, spaced at distances from each other equal to or slightly larger than one-half of the height counter forts are used for high walls with heights greater than 8 to 12 m.

5. Reinforced Soil Retaining Wall

Reinforced soil can also be used as retaining walls if they are built as an integral part of the design and to act as an alternative to the use of reinforced concrete or other solutions on the grounds of economy or as a result of the ground conditions.

6. Green Retaining Wall

Green retaining walls can be used to retain more gentle slopes. A Geo cellular structure such as a series of honeycomb cells can be embedded into the surface of the slope to stabilize it, and the individual cells can then be planed.

7. Mechanical Stabilization Wall

Mechanically stabilized earth walls are walls that can tolerate some differential movement. The wall face is infilled with granular soil whilst retaining the backfill soil.

8. Anchored Wall

An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additionals strength using cables or other stays anchored in the rock or soil behind. It usually driven into the material with boring, anchors and then expanded at the end of the cable, either by mechanical means or often by injecting pressurized concrete which expands to form a bulb in the soil.

 

WHAT IS CLAYEY SOIL - TYPES AND ADVANTAGES

What Is Clayey Soil

Clay is a fine-grained natural soil material containing clay minerals. Clay particles are the finest of all the soil particle measuring less than 0.002 mm in size. It consists of microscopic particles derived from the chemical decomposition of rocks. Clays develop plasticity when wet, due to a molecular film off water surrounding the clay particles, but become hard, brittle and non-plastic upon drying or firing. These soils are made of over 25 percent of clay. Most of the clay minerals are white or light in colour but natural clay show a variety of colours such as reddish or brownish because of the presence of small amount of iron oxide.

Formation Of Clayey Soil

Clays and clay minerals occur under a fairly limited range of geologic conditions. The environments of formation include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits, and weathering rock formations. Most clay minerals are commonly form when rocks are in contact with air, water or steam and are also form locally from hydrothermal activity. The clay minerals are formed by the acidic weathering of feldspar rich rock such as granite in hot climates tends to produce kaolin.

Types Of Clayey Soil According To Its Percentage

1. Silt Soil     0 - 10 % Clay
2. Clay Soil 10 - 25 % Clay
3. Clay Soil 25 - 40 % Clay 
4. Clay Soil 40% Clay

Advantages Of Clayey Soil

• It is very fertile.
• It retains water and nutrients.