Soils

Quick Sand Formation/Condition

Quick Sand Formation/Condition

Quick sand formation/condition is not a soil type. Quick sand formation/condition is created in saturated thick layers of loose fine sandy soils when disturbed either due to vibration such as from pile driving in the near by locality or due to the presence of flowing water. The particles in trying to achieve a closer packing will force the pore water upwards and out at the surface and if this has sufficient velocity to cause a flotation or boiling up of the particles, the sand particles begin to move horizontally and get lifted up, the bottom sand rising up and its space is occupied by the adjoining particles, thus making a regular movement. The finer the sand the more readily it is affected by a current of water, especially if it contains a little clay. A particular form of this known as piping is met within coffer dam failures. Under such conditions the material may be carried off from under a structure, which can result in the settlement of buildings at a considerable distance. Even if a full flow is not created, the stability of the soil is lessened due to the upward seepage pressure. The condition can be corrected by lowering head of water by underground drainage.

Quick Sand Formation/Condition

lf there is any chance of excavation or pumping on adjoining sites causing a “loss of ground” beneath the structure by releasing a  layer of running sand. this layer should be effectively confined by  sheet piling.

Running sand:   Sand below the natural ground water level. which is carried into the trenches, trial pits or boreholes by the flow of  ground water as excavation proceeds.

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.

 

Test for Sand – Fine Aggregate

Test for Sand – Fine Aggregate

The required test for sand are as follows,

1. Take a glass of water and add some quantity of sand in it. Then it is vigorously shaken and allowed to settle. If clay is present in sand,its distinct layer is formed at top of sand.

2. For detecting organic impurities in sand, take a container add some quantity of sodium hydroxide or caustic soda and also add small quantity of fine aggregate/sand stir the container. If the color of the solution changes to brown it indicates the presence of organic matter.

3. To find the presence of salts in sand, the sand is actually tasted.

4. Take a heap of sand and it is rubbed against fingers, in case if the fingers get stained then it clearly indicates the presence of earthy matter.

5. The color of sand will indicate the purity of sand, the grain sharpness and size can be observed by naked eye.

Other test for sand such as void ratio, durability are carried out by mechanical analysis.

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.

 

Plastic Limit Test of Soil

Plastic Limit Test of Soil

A sample weighting about 15 grams is taken from the material passing the 425 micron sieve and is thoroughly mixed with water on a glass plate until it is plastic enough to be rolled into a ball. The ball of the soil is then rolled between the hand and the glass plate so as to form the soil mass into a thread. When the diameter of the thread becomes less than 3 mm, the soil is kneaded together and rolled out again. This process is continued until crumbling of the thread occurs at a diameter of 3 mm. The portions of the crumbled soil are gathered together and the moisture content of this soil determined.

6-28 Plastic Limit Test of Soil