A Report on The Behaviour of Black – Cotton Soil

 ABSTRACT

            Black Cotton Soil deposits in India re a boon to farmers.  In Civil Engineering aspect these soils are giving hazardous problems to Engineers.  With the rapid development in soil improvement procedures, various structures construction is taking place.  Though various constructions techniques are utilized, the cracking (Minor Cracking) is seen in the buildings.  For the site investigations, the behaviour of soil is important.  So in this report the behaviour of the soil is explained.

INTRODUCTION

In India, expansive soils are called as Black Cotton soil. The name “Back Cotton” as an agricultural origin.  Most of these soils are black in colour and are good for growing Cotton.  All the black soils are not expansive soils and all the expansive soils are not black in colour.  These soils passed high strength in summer and decreased rapidly in winter. The soil has a swelling property due to the presence of montmorillonite mineral.

The swelling soils of India have their origin in subaqueous decomposition of basalt rocks or weathering In-Situ.  It is very necessary to discuss about the Engineering behaviour of soils.  From this information civil structures can be prevented from the damages causing Expansive soils. Black soils are highly argillaceous and are relatively rich in Coco₃.

CHEMICAL COMPOSITION OF BLACK COTTON SOILS

            Black Cotton Soils are made of varying proportions of day minerals like Montmorllionite, illite and Kaolinite, Chemicals, like Iron oxide and Calcium Carbonate (in the forms of Kankar Nodules) and organic matter like humus.  Montmorllionite is a Predominant mineral of black cotton soils.  The swelling and shrinkage behaviour of black cotton soils originate mainly from this mineral. Clay minerals are hydra silicates of aluminum and magnesium.  They are made of sheets of silica and alumina stacked one above the other forming sheet like structure with expanding lattice.   The structure of some aluminum is by magnesium ions and the mineral becomes chemically active. The mineral has high activity, and has the Base Exchange capacity of more than 80ml/100g compared the – VC charges on the clay minerals. They attract water molecules (dipoles) and various types of hydrated cations to the surface causing the soil to increase the volume.

            Abundance of calcium in black cotton soils to yet another feature. It may be present in the form of saturating ions or as nodules of Caco₃ (Kankar).  Treatment with the Sodium about base exchange and the soils become softer and move plastic.  Organic matter in the form of humus makes these soils more plastic and compressible.  The dark colour of the black cotton soils is believed to be either due to humus or titanium oxide.  Black cotton soils are found to have the following chemical properties.

            P^H Value  Þ    8.9

            Organic Contest Þ 0.4 to 2.4 %

            Caco₃ Þ  5-15%

            Sio₂  Þ  50-55%

            Sio₂ / Al₂03 Þ 3.5 %

            Montmorllionite minerals Þ 30-50 %

Engineering problems:--

Þ                In rainy season, these soils become very soft by filling up of water in the cracks and fissures.  These soft soils reduce the bearing capacity of the soils.

Þ                In saturated conditions, these soils have high consolidation settlements.

Þ                These soils have high swelling nature.  Due to this structure causes damages.

Þ                When lands are applied on these soils in wet conditions.  These soils get Shrinkage.

TYPES OF DAMAGES:-

Þ                When the structure is built on the black cotton soil in the dry seasons, there may be no damage in the structure in that season.  When the rainy seasons is starts due to the swelling property the soil get expands and the strength in the foundation structure decreases, which causes uneven settlement leads to crack in the walls.

Þ                Due to uneven settlement the beam gets deflected which effects the plastering to the walls.

Þ                Small fissure cracks are seen on the floors and on the walls.

Þ                Due to high degree of expansive soils, the buildings may fail which will be very dangerous.

Properties of soils:-

            By conducting various tests the properties of the soils is identified.

Sieve Analysis:-

            The sample is taken and pulverized.  The soil is taken in a jar and water is added for soaked condition.  If necessary deflocculating or dispersing agent, Sodium hexametaphosphate is added to make the soil cohesion loss.

DRY SIEVE ANALYSIS:-

            The solution is passed in the 75 Micron Sieve.  Water is added to it and washed until the retained material is free from slurry.   The material retained on the 75 Micro Sieve is collected and dried in an oven.  And nit is then sieved through a set of sieves which are arranged standardly.

            In the case of black cotton soils the material retained on 75 micron sieve does not exceeds 20%.

WET SIEVE:-

            The solution passed through 75 micron sieve is taken as a container and sedimentation analysis is done by using hydrometer method.

PRINCIPLE:-

            Hydrometer measures the specific gravity of the soil suspension  at the center of its bulb.  The specific gravity of the soil suspension depends upon the mass of soils present, which is turn depends on the particle size.                                                                                                                                

ATTERBERG’S LIMITS:-

            The water contents at which the soil changes from one state to the other state are known as Atterberg’s Limits or consistency Limits.

LIQUID LIMIT:-

            The water content at which the soil changes from the liquid state to the plastic state is known as liquid limit.  To determine the liquid limit in the soil.  Liquid limit apparatus called Casagrande’s apparatus is used.  The liquid limit of soil depends upon the clay mineral present the stronger the surface charge and the thinner the particle, the greater will be the amount of absorbed water and therefore the higher will be the liquid limit.

            The device used in Casagrande’s method consists of a brace cup which drops through a height of 1 CM on a hard base when operated by turning the handle which raises the cup and lets its drop on the rubber base.

            The soil sample is prepared by taking the soil and mixed it water.  It is taken in a Casagrande’s container.  By using grooving tool the sample is grooved. And the handle is rotated.  By rise and fall mechanism the grooves comes closer.  And some of the sample near the groove is taken in a container.  Its dry and wet weights are noted.  From this moisture content is determined. 

            For the expansive soils the range of the liquid limits is 50 – 100%.

PLASTIC LIMIT:-

            Plastic limit is the water contest below which the soil stops behaving as a plastic material.  It begins to crumble when rolled into a thread of soil of 3MM diameter.

            For determination of the plastic limit of a soil, it is air dried and sieved through a 425 micron IS sieve.  About 30gm of soil is taken in an evaporation dish.  It is mixed thoroughly with distilled water till it becomes plastic and can be easily moulded with finger.  The mould is rolled with fingers on a glass plate to from a soil threat of uniform diameter.   By taking the dry weights and wet weights the water content is determined.

            For clay or Black cotton soils the plastic limit ranges from 20-65%.

SHRINKAGE LIMIT:-

            It is the maximum water content at which a reductions of water content will not cause a decrease in the volume of the soil mass.

            The sample mined with water is taken in a shrinkage dish and dried in an over.  The dry pat is formed with the help of mercury the weight of the dry soil and wet soil put is determined.  From this the shrinkage limit is determined.

            Shrinkage limit ranges from 9-14% for black cotton soils.

SELLING PROPERTIES:-

            The swelling behaviour of a soil would depend largely on the type of clay minerals that are present in these soils and the proportion in which they are present.  In order to determine these some laboratory tests are conducted they are :-

1)                  Differential thermal analysis:-

A specimen of the soil with the unknown mineral is heated continuously along with an inert substance in an electric over and a record of change in temperate of the mineral plotted against oven temperature is obtained.   By comparing this with the available records of several known clay minerals.

2)                  X-Ray Diffraction Method:-

Different minerals with different regular patterns of crystalline.  Structure will different X-rays to yield different x-ray diffraction patterns.   With the x-ray diffraction patterns of common clay minerals being known, it is possible to tell which types of minerals are present and is what proportions.

3)                  ELECTON MICRO SCOPY:-

The soil is observed under polarized light in an electron micro scope.

FREE SELL INDEX:-

            It is the test conducted to measure the degree or expansive ren of given soils.  The degree of expansiveness and possible damage to lightly loaded structure may be qualitatively addressed from table given below.

            The sample is taken in two containers.  One container is filled with water and another is filled with Kerosene and it is kept of 24hrs observation.  We see the expansive in water with this degree of expansiveness is determined. 

TABLE:-

Degree of Expansiveness	DFS, percent
Low	Less than 20
Moderate	20 to 35
High	35-50
Very high	Greater than 50

CONSOLIDATION TEST:-

            The consolidation test is a laboratory to study the compressibility of a soil.  It consists of a loading device and a cylindrical container called consolidation cell.

            The consolidometer has arrangements for the application of the desired load increment, saturation of sample and measure of change in thickness of the sample at every stage of consolidation process.

            The ring containing the sample is then placed, on the bottom porous store.  Filter paper is kept on the top of the sample and then top porous stone is placed.  The loading pad is placed on the top porous stone and then the consolidation cell is kept under the loading unit.  The dual gauge readings are noted at the initial petting pressures after primary consolidation etc. these realings are noted by an increment of load.  After the consolidation under the final wad increment is cuple, the load is reduced an swelling is allowed and the readings are noted.

            If the consolidation settlement is very high, then that ground may consist of clayey soils which assumed unsuitable for Civil Engineering Structure.

Triaxial Test:-

By conducting triaxial test the Engineering property of soil, i.e., Strength parameter is determined.  It is the test conducted to find the shear parameters of the soil i.e., Cohesion (C) and Shearing resistance (j  ) for these type of soils consolidated – undrained test is dare.

            In this test the sample is enclosed in a rubber membrane, which is slid over the specimen with the help of a membrane stretcher.  The specimen is placed in a triaxial cell and filled with water by connecting it to the pressure supply.  The drainage value is closed.  The sample is sheared by applying deviator stage by the loading machine.  The specimen is the recovered after removing the loaning cap and the top porous stone.  From the observations noted the mohr-circle is drawn. It gives the mohr’s columb failure line.

Þ                From the tests we can get the engineering behaviour of expansive soils.

Remedial Treatments:-

            To modify the properties of these soils various constructions techniques have been evolved like.

Þ                Pile Foundation:-

When the soil or near the ground surface is not capable of supporting a structure, deep foundation are required to transfer the lands to deeper strata.

A pile is a slender structural member made of steel, concrete or wool.  A pile is either driver into the soil or formed in-sity by excavating a hole and filling it with concrete.   A pile foundation is generally much more expensive than a shallow foundation.  It should be adopted only when a shallow foundation is not feasible.

Þ                Soil Stabilization :-

By stabilizing the soil with line or cement, the strength of the soil can be increased line as a stabilizing agent reduced the plasticity, shrinkage and swelling considerably.  Line stabilized black cotton soils can be safely used as a sub grade and sub-base material.  

Þ                Sand Replacement Method:-

The soil strata which has high water context is removed by excavation process up to the influence zone or active zone ends up and it is replaced by the sand upto some depth and concrete in the remaining part.  It is the waster process.

CONCLUSION

By conducting various tests the behaviour of the soil is logged out.  That the soil is not suitable for Civil Engineering structures.  But by various construction techniques this type of gravel is also used for advance construction.

REFERENCE

Soil mechanics and foundation Engineering – Arora

Geotechnical Engineer – C. Venkata Ramaiah