What is Site Investigation & Soil Exploration
When started any construction activity the required of knowledge foundation design of conditions under the ground level to required this we carryout site investigation and sub-soil exploration at the any construction Project or work .
Content in this Articles :-
- What is site investigation & Soil Exploration ?
- Necessity of Site investigation & Soil Exploration ?
- Types of Exploration ?
- Method of Soil Exploration ?
Necessity of Site investigation .
Site investigation and subsoil exploration can be considered as bases on which the foundation design rests. Soil exploration is technical investigation by which the necessary information regarding various soil properties are obtained which helps in designing safe and economical construction.
Find out the nature and dimension of underlying soils is purpose of soil exploration. It also tells whether soil is gravel, sand clay silt etc. And the indicating of thickness of soil depth of soil and properties .
In the following purpose Site investigations and subsoil explorations are very important :-
1. For the purpose of bearing capacity for foundation design.
2. To know stratification.
3. For seepage control.
4. For treating problem soils.
5. For finding index properties.
6. Enhance the properties by compaction and stabilization of soil . site investigation and subsoil exploration are very necessary .
1.For the purpose of bearing capacity for foundation design :-
This is the most preliminary purpose of soil investigation, bearing capacity goes on increasing with depth in most soils. As we go deeper, we encounter soil with more and more bearing strength, the bearing strength required for a particular structure must be available. Thus, the bearing capacity is estimated at a particular depth and then it can actually be tested by various tests available, of which plate load test is an important one. This bearing capacity is such a complex phenomenon, that after determining bearing capacity, a high factor of safety of 3 to 5 is still applied to that value. In view of all this, the site investigations are very important for determining the bearing capacity of soil.
2. To know stratification:-
Soil contains different layers or strata below the surface. These strata are arranged in different ways. The strata may be continuous, displaced by a crack or fault, at different angles, separated by a dyke arranged in the form of folds etc. The arrangement of strata below the ground level is called stratification. Many properties of the soil like permeability or bearing capacity depend on the orientation and nature of the stratification. When we take boreholes at the foundation level, or when we dig bore holes or test pits, this arrangement is clearly visible by the side of the bore holes or test - pits. By taking numerous bore holes, a geological map of the underground strata can be prepared. This will give a complete idea of the stratification of soil. Thus, investigations enable us to find the hidden features below the ground level.
3. For seepage control:-
Seepage is another complex phenomenon in soil mechanics . Seepage is especially important in case of earth dams and weirs and aprons on permeable foundations . Soil which has the same permeability in y direction, i.e. in gravitational direction, and in x direction, i.e. direction perpendicular to gravity is called isotropic soil. Soil which has different coefficients of permeability K, and Ky is called anisotropic soil. The net effect can be considered as a two dimensional flow which can be analysed by Laplace equation and flow - net technique. The effective flow will be the seepage flow. During detailed site investigations of soil and subsoil, field tests will give realistic picture reinforced by laboratory tests conducted on soil samples, both disturbed and undisturbed, obtained from sub soil explorations Thus for seepage controls,the detailed investigation will give a fair basis for further analysis .
4. For treating problem soils:-
Problem soils are those which have uncommon behavior when confronted with various parameters whether natural or artficial mainly two conditions come under this category. The first is called the quick sand condition. When water seeps through sand and comes to be surface, the head with which it comes out of the surface is called the exit gradient. The other problem soil is known as the black cotton soil . The soil is not weak in bearing capacity , nor it is excessively permeable . The problem in black cotton soil is the disproportionate volume change which occurs upon change in moisture content . This soil swells to a very high degree , even upto double the original volume upon being saturated with moisture and shrinks equally largely when it loses the moisture . Hence special remedial measures must be taken when it is inevitable to build foundations on black cotton soil. This once again brings out the necessity of soil and subsoil investigations, as even small pockets of problem soils must be detected for safe foundations.
5. For finding index properties:-
Index properties of soil mean basic properties such as bulk density, specific gravity, void ratio, porosity, water content, degree of saturation etc. These properties come into play and affect the other more complex properties such as permeability, shear strength, compressibility, bearing capacity etc. Hence index properties form the important basic step of investigation. Most of the index properties are determined in the laboratory, but sometimes field tests are also considered to be important for collecting the samples for determination of index properties, the site investigation and the subsoil exploration is thus found to be necessary.
6. Enhance the properties by compaction and stabilization of soil :-
compaction is actually the simplest method of soil stabilization. The term stabilization is generally used when compaction is enhanced by some other method or when it is enhanced by adding some substance as an admixture. In any case, be it compaction or stabilization, the properties of soil such as Maximum Dry Density , Optimum Moisture Content , permeability drainage characteristics etc. must be determined to a fine degree. Doing such an investigation will achieve much desired effects to a large extent within the least expense. Even the decision of whether to go for compaction or not, whether simple compaction or stabilization is required etc. depend on Site investigations of soil and subsoil carried out in details to a meticulous degree of perfection.
Types of exploration
The subsoil explorations are categories in two parts :-
1. Preliminary Exploration
2. Detailed Exploration
1. Preliminary exploration -
1. Geological study of the site and site reconnaissance.
2. Study of local topography, existing excavations, cuttings, and drainage patterns
3. Study of other natural features like streams, lakes, hills etc.
4. Location of High Flood Level marks.
5. Depth and composition of soil strata.
6. Types of foundation will be selected depending upon the properties of soil.
2. Detailed Exploration-
1. Determining nature, thickness and sequence of various subsoil strata.
2. Position of ground water table.
3. Index properties of different layers.
4. Taking samples to the laboratory obtained from various depth below the ground level, with a view to conduct all the tests.
5. Conducting many field tests which will give lots of required information.
6. Shear strength.
7. Permeability
8. Compressibility
9. Density.
Method of Soil Exploration.
Generally two method of Soil Explopration :-
1. Open Excavation 2. Boring
1. Open Excavation - Open excavation is also called trial pit. Generally, a square or rectangular trial pit is dug, and by measuring the thickness of various layers on the open face of the pit, fairly well idea about subsurface geology can be formed. Soil excavated from trial pits is taken to the laboratory for testing. The location and number of trial pits is determined as per the requirement. Size is generally 1 m x 1m or less. Depth is not fixed and depends upon the soil strata, water table etc.
2. Boring - A hole in the soil from the open surface is known as boring.
There are four different methods of boring:-
1. Auger boring
2. Wash boring
3. Percussion boring
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| Auger boring |
4. Rotary boring
1. Auger boring - Boring by an auger is carried out by holding it vertically and pressing it down while it is rotated simultaneously. Due to the sharp edges, soil is cut and enters the annular space and due to spiral shape, rises up as the auger advances. After some depth the augur is brought up and cleaned, and then again inserted in the borehole.Upto 6m depth hand operated augurs can be used. Augurs give severely disturbed samples. Auger boring is suitable for shallow foundations, roads etc.
2. Wash boring - The method consists of driving a casing pipe usually through a heavy drop hammer supported on a tripod and a pulley, Water is forced under pressure through a hollow drill rod which may be rotated or moved up and down inside the casing pipe. the slurry flowing out can be analyzed for soil strata change in rate of progress and change in slurry characteristics indicating change of stratum.
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| Wash boring |
3. Percussion boring - Percussion boring is carried out by repeated blows of a heavy bit or chisel inside a casing pipe. The borehole is usually kept dry except for a limited quantity of water used to form the slurry of pulverized material. The pulverized slurry is bailed out using a bailer or sand pump. A casing pipe is necessary only when the sides of borehole are likely to cave in. This is the only method suitable for drilling boreholes in gravelly or bouldery strata.
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| Percussion boring |
4. Rotary boring - In this method, boring is effected by cutting action of a rotating bit which is kept in firm contact with the soil. This method is also used to deepen boreholes already done by other methods.
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| Rotary boring |
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