Sunday, January 8, 2012


Surveying is the art of determining the relative positions of distinctive features on the surface of the earth, by means of measurements of distances, directions and elevations. The branch of surveying which deals with the measurements of relative hei9ghts of different points on the surface of the earth, is known as leveling.

The main objective of surveying is the preparation of plans and maps of the areas. The science of Surveying has been developing since the very initial stage of human civilization according to his requirements. The art of surveying  and preparation of maps has been practiced from the ancient times. As soon as the man developed the sense of land property , he evolved methods for demarcation its boundaries. Hence, the earliest surveys were performed only for the purpose of recording the boundaries of plots of land. Due to advancement in technology, the science of surveying has also attained its due importance. The practical importance of surveying cannot be over-estimated. In the absence of accurate maps, it is impossible to lay out the alignments of roads, railways, canals, tunnels, transmission power lines, and microwave or television relaying towers. Detailed maps of the sites of engineering projects are necessary for the precision establishment of sophisticated instruments. Surveying is the first step for the execution of any project. As the success of any engineering project is based upon the accurate and complete survey work, an engineer must, therefore be thoroughly familiar with the principles and different methods of surveying and mapping. It is for this reason, the subject of surveying has been made compulsory to all the disciplines of engineering at diploma and degree courses.

The surveying may be primarily divided into two categories:
1. Plane Surveying
The surveys in which the earth surface is assumed as a plane and the curvature of the earth is ignored, are known as Plane Surveys.

2. Geodetic Surveying
The surveys in which curvature of the earth is taken into account and higher degree of accuracy in linear as well as angular observations is achieved, are known as Geodetic Surveying.

Wednesday, December 21, 2011

Slopes Stability

In a sloping soil mass, forces acting on it try to cause the soil mass to move from higher elevation to lower elevation. The forces which try to cause instability are force due to gravity, imposed loads and seepage forces. These forces produce shearing stress in the soil, and unless the shearing resistance of the soil is sufficient to withstand them the slope fails in shear along a well defined failure surface. The safety of a slope against failure is its stability. The failure of a mass of soil located beneath a slope is called a slide. To avoid failure, a thorough analysis is needed else the failure may lead to loss of human life as well as a loss of national economy.

The stability can be determined with a reasonable accuracy, if the geological cross section of the slope and the shear strength parameters of the soil are known. The accuracy of the result depends on the accuracy with which the shear strength is predicted.

Causes of Slope Instability
The causes of slope instability may be due to:
a. Increase in shear strength.
b. Reduction in shear strength.

These may be due to :
  1. Gravitational force
  2. Force due to seepage of water
  3. Erosion of surface due to flowing water
  4. The sudden lowering of water adjacent to a slope
  5. Forces due to earthquakes
Many slope failures are associated with the exceeding presence of water during heavy rainfall and flood. The introduction of the exceeding water content to the soil contributes to both increases in the shear stresses as well as the reduction in the shear strength due to the increase in the pore water pressure.

Remedial measures for Slope Stability
  1. Flattening the slope reduces the weight of the potential sliding mass and consequently the driving force, resulting in the increase of the factor of safety.
  2. The presence of a berm adjacent to the toe of the slope increases the resisting forces and consequently the factor of safety will be increased. This is especially useful when there is a possibility of base failure.
  3. Proper drainage of water is one of the most effective methods to increase the stability of earth slope. Surface drainage and sub drainage are provided to increase the stability of the slope.
  4. Densification of the ground increases the shearing resistance of the soil, thus increases the stability of the slope. a) Addition of chemical additives (lime or cement) for clay soils & b) Vibro-floatation (deep densification of cohesion-less soils)
  5. Construction of earth retaining structures (retaining walls, reinforced earth, etc.) at the toe of the slope increases the resistance of the potential sliding mass.
  6. Planting the surface of the slope is beneficial to protect the slope against shallow slides. The plants enhance the stability of the slope in two different ways:
  • Consolidation of the soil by a network of roots and therefore increasing the resistance to shear.
  • Drying out the surface layers by water suction by the roots, increasing the shearing resistance of the soil.

Monday, December 19, 2011

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Index Properties of Soil

Soil can exit in nature in innumerable varieties and these materials do not lend themselves to separate into distinct categories. Proper classification of soil is an important step in connection with any foundation job because it provides the first clue to the experiences that may be anticipated during and after construction. The laboratory tests, which provide information on physical properties of soil, are known as classification tests and numerical results of such tests are known as index properties. If the classification tests are properly chosen, soil materials having similar index properties are likely to exhibit similar engineering behaviour. On the basis of some laboratory tests, it has been found that soil can be classified into groups within each of which the significant engineering properties are somewhat similar.

The index properties are of a great value to the civil engineer in that, in one hand, they provide means in the correlation of construction experience and on the other hand they form a basis for information of the correctness of the field identification of a given material. It the material is improperly identified, the index properties indicate the errors and lead to correct classification. Index properties may be divided into two general types:
1. Soil Grain Properties
2. Soil Aggregate Properties

Soil Grain Properties are the properties of the individual particles of which the soil is composed of and are independent in the manner of soil formation. These properties can be determined from distributed samples. Soil Aggregate Properties, on the other hand, depend on the structure and the arrangement of the particles in the soil mass, whereas the soil grain properties are commonly used for soil identification and classification. The soil aggregate properties have a greater influence on the engineering behaviour of soil mass. The engineering behaviour of a soil mass depends on its strength, compressibility and permeability characteristics. The most important aggregate property of a coarse grained soil is its relative density while that of a fine-grained soil is its consistency.

Friday, December 16, 2011

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