Tag: Soils

Foundations in Black Cotton Soils

Foundations in Black Cotton Soils:

The following methods are generally adopted to meet the characteristics of foundations in black cotton soils,

 

(1) Foundation loads are limited to 5 tonnes/sq.m if water finds access to the foundations, otherwise it may be about 10 tonnes/sq.m

 

(2) Foundations are taken down to such depths to which the cracks do not extend.

 

(3) Trenches are dug on either side of the foundation and filled with sand  or other material to prevent intimate contact of the black cotton soil with the concrete and masonry of the foundations.

If the thickness of the black cotton soil is only up to 120 cm it should be completely removed and foundation laid on the soil below.

 

(4) For important buildings raft or mat foundations of reinforced concrete are provided.

 

North Dakota Cone Test

North Dakota Cone Test

North Dakota Cone Test is a cone penetration test similar to the California Bearing Ratio (C.B.R.) test developed by the North Dakota State Highways Deptt. This test is simpler and more rapid than the C.B.R. but its use is restricted to fine-grained soils and is considered reliable only for clayey soils. The penetrometer consists essentially of a shaft with a sharp cone attached to one end. The cone is loaded during the test by placing weights on a disc fixed to the top of the shaft and its penetration measured. The test is made directly on the sub-grade.

California Bearing Ratio (CBR) Test on Soils

California Bearing Ratio (CBR) Test on Soils

California Bearing Ratio (C.B.R.) Test which is an ad hoc penetration test developed by the California State Highways Dept. (USA) for the evaluation of sub-grade strengths. It is a measure of the shearing resistance of a soil to penetration under controlled density and moisture conditions. The strength of a soil is found by causing a Plunger (or piston) of standard size to penetrate a specimen of the soil prepared to the density and moisture Conditions of the soil to be tested in a standard mould. The resistance to penetrations measured and then expressed as a percentage of the known resistance to penetration of the plunger in a crushed aggregate. The California Bearing Ratio Test can be made on nearly all soils ranging from clay to fine sand. This test is generally used for the design of road pavements.

California Bearing Ratio Test

Vane Test Procedure – Shear Strength of Soils

Vane Test Procedure – Shear Strength of Soils

The Vane test is a field shear test for clays in which a vane consisting of two or four blades fixed at right angles, is attached to the end of rod and pushed into the soil at the bottom of a borehole. The torque required to cause rotation or shear the soil is measured. This torque is approximately equal to the moment developed by the shear strength of the clay acting over the surface of the cylinder with a radius and height equal to that of the vanes. The vane test has the advantage over the unconfined compression test in that the shear strength of a soft and sensitive clay at considerable depths (say up to about 30 m) can be determined insitu without obtaining undisturbed samples. This test is still in the process of development.

Unconfined Compression Test of Soils Procedure

Unconfined Compression Test of Soils Procedure

Unconfined Compression Test is similar to a compression test performed on concrete cylinders and is made by applying an axial load to cylindrical or prismatic soil samples and measuring the deformation corresponding to the stress as the load is increased. When the stress reaches the ultimate strength, the sample may fail either by a gradual bulging or by a sudden rupture. The ultimate strength of a test specimen prepared from an undisturbed sample (at unaltered moisture content), is a relative measure of the ultimate bearing capacity of a soil. A comparison of the stress-strain characteristics of a soil tested in the undisturbed state, and then in the remoulded state at unchanged moisture content, is indicative of the structural damage caused by remoulding. A simple apparatus intended for field use has been developed. Unconfined Compression Test is generally the most convenient for immediate tests on saturated or nearly saturated clays.

 Unconfined Compression Test Apparatus

Triaxial Compression Test of Soils Procedure

Triaxial Compression Test of Soils Procedure

Triaxial Compression test is used where more precise values of the cohesion and angle of internal friction of a soil are required than determined from a shear box test. The specimen of the soil is subjected to three compressive stresses at tight angles to one another, and one of these stresses is increased until the specimen fails in shear. The test differs from the shear box test in that the stresses determine the plane of shear failure which is not predetermined.

Triaxial Compression Test

In this test a cylindrical specimen usually 40 mm dia. and 75 mm long is enclosed in a thin rubber membrane and is subjected to radial fluid (water or glycerine) pressure. Increasing axial stress is applied at the top until failure occurs. The test is repeated with different pressures and the results are plotted in the form of Mohr’s circles. The triaxial apparatus is probably the most useful for research into the fundamental properties covering the strength of soils but is elaborate.

The undrained triaxial test is, in general, used as a basis for estimating bearing capacity, earth pressure and slope stability of cohesive soils. Unconfined compression test is used for predominantly clayey soils which are saturated or nearly saturated.

Shear Box Test of Soils – Procedure

Shear Box Test of Soils – Procedure

In the shear box test, failure is caused in a pre-determined plane of the soil, the shear strength or shearing resistance and the normal stress both being measured directly, as it is a direct shear machine. The essential feature of the apparatus is a rectangular box divided horizontally into two halves, the lower half box is fixed and the upper half is movable. The soil to be tested is enclosed in the two half boxes and porous stone plates or metal plates are placed above and below the specimen. While a constant vertical compressive force is applied, a gradually increasing horizontal force is applied to the upper half of the box, thus causing the soil prism to shear along the dividing plane of the box. This measures the horizontal load required to shear a soil corresponding to any vertical normal compressive load. The test is repeated on other identical specimens under different vertical loads and the results are plotted as shearing resistance against normal vertical load (shear stress is plotted vertically and the normal stress horizontally) and straight line is drawn through the points. The equation of this line is

s=C+ n tan θ

where,

s=horizontal force divided by the area A of the cross section of the soil specimen, i.e., the unit shear resistance

C=cohesion per unit area=the horizontal shear force under no vertical load. Cohesion for a granular soil (dry sand) is zero. Can be read off from the graph;

n=vertical normal load per unit area

θ =angle of shearing resistance or the angle of internal friction. Can be read off from the graph.

Shear Test Apparatus

In the case of undrained saturated clays the angle of shearing resistance is zero. The true angle of internal friction of clay is seldom zero and may be as much as 26 deg.

Direct shear tests are of two kinds (1) Immediate tests, in which the horizontal load is applied as Soon as the normal vertical load begins to act, and the specimen is enclosed between metal plates. (ii) Slow tests, in which the soil is allowed to consolidate completely under each increment of vertical load. The specimens are enclosed between porous plates which allow the soil to drain.

The inter-relationship between cohesion, internal friction and stability are determined from the above equation. The unit shear resistance is composed of two parts, that furnished by the resistance of soil grains to sliding over each other and that furnished by the cohesion existing between the soil particles. By experiments the cohesion C in kg/sq. m and θ have been ascertained as given in the table below, from which the vertical load n in kg/ sq. m can be computed.

6-30 Table

The soils subject to the higher normal stresses will ha’ lower moisture Contents and higher bulk densities than those subjected to lower normal stresses and will thus have increase in shearing resistance and cohesion with increasing normal stress.

 

Density of Soils

Density of Soils

The density or true weight of a soil is equal to the specific gravity of the solid materials x 1000 (weight or density of water per Cu. m). A soil consists of solids, pores or voids and the moisture. The overall weight of the mass (including solid particles and the effect of voids whether filled with air or water) per unit volume, i.e., total weight of soil ÷ total volume of soil, is termed Bulk Density. Bulk density varies with the type of the soil, moisture content and its compaction. The weight of the dry solid matters contained in a unit volume of soil, i.e., weight of soil particles ÷ total volume of soil, (determined after the water has been dried without bulk volume change) is termed Dry Density.

The usual method of measuring compaction in the field is to determine the dry density of the soil in site. The maximum dry density of a soils is obtained by a specified amount of compaction at the optimum moisture content by the Proctor Compaction Test. For each compaction method, there is an optimum moisture content at which a given soil can be compacted to greatest density, and different soils have different maximum densities and optimum moisture contents. Dry density varies from about 2.00 grams/cu. cm for coarse grained well graded gravels and sands to about 1.45 grams/cu. cm for heavy clays, the corresponding moisture contents being about 4 per cent for the gravel and 26 per cent for the clay. The density of the solids alone is sometimes termed absolute density.

 

Compressibility of Soils

Compressibility of Soils

Gravels, sands and silts are incompressible, i.e., if a moist mass of these materials is subjected to compression, they suffer no significant volume change. Clays are compressible, i.e., if a moist mass of clay is subjected to compression, moisture and or air may be expelled, resulting in a reduction in volume which is not immediately recovered when the compression load is withdrawn. The decrease in volume per unit increase of pressure is defined us the compressibility of soils, and a measure of the rate at which consolidation proceeds is given by the ‘‘coefficient of consolidations’’ of the soil.

Compressibility of sand and silt varies with density and, compressibility of clay varies directly with water content and inversely with cohesive strength. Clays and other highly compressible soils are known to swell when overburden pressure is removed.

Resiliency of Soils

Resiliency of Soils

Resiliency of a body is regarded as the extreme limit to which it can repeatedly be strained without fracture or permanent change of shape.