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Soil Investigations for Foundations

written by: Ramachandran V • edited by: Lamar Stonecypher • updated: 6/27/2010

Soil mechanics site investigations are conducted to discover the characteristics of the soil at the particular location. There are different methods of soil investigation. After the soil samples are analyzed, a report is prepared recommending the type of foundation to be used.

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    The Purposes of Building Foundations

    The foundation is the lowest part of a building. It transmits the load of the structure to the soil below. The main purposes of a foundation are:

    • To distribute the load from the building over a large area
    • To load the substratum evenly so as to prevent unequal settlement
    • To take the structure sufficiently deep into the ground to prevent overturning

    Before a foundation is decided, it is necessary to determine the characteristics of the soil at the site of construction. This is done by conducting soil investigations.

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    Soil Investigations for Building Foundations

    Soil investigations involve the following steps:

    1. Planning the details and sequence of operations
    2. Collection of soil samples from the field
    3. Conducting all field tests for determining the strength and compressibility characteristics of the soil
    4. Study of ground water level conditions and collection of water samples for chemical analysis
    5. Geophysical exploration if necessary
    6. Testing in the laboratory of all samples of soil, rock, and water
    7. Preparation of drawings and charts
    8. Analysis of the results of the tests
    9. Preparation of report
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    Methods of Soil Investigations

    The normal methods of soil investigations are:

    • Inspection
    • Test pits
    • Probing
    • Boring

    Inspection

    The first step in this connection is the inspection of the site and its vicinity to get a preliminary idea of the site conditions. This includes the study of the existing buildings in the neighborhood and if possible the type of their foundations. The cuts made in the nearby areas should also be looked into. The subject can be discussed with those persons who were associated in constructing buildings in the surroundings with regard to their experiences and difficulties encountered by them.

    Test Pits

    Test pits are dug by hand or by excavating machines. The size of the pit should be such that a person can easily enter the pit and have a visual inspection. Both disturbed and undisturbed soil samples are collected from the pit for detailed analysis.

    Probing

    This will give a rough idea of the underlying soil. In this, a steel bar of 25 to 40 mm (1 inch to 1.5 inch) in diameter is driven into the ground until a hard stratum is met with. The bar is driven by a hammer. The bar is then drawn out at intervals and the soil sticking to the bar is examined to get an idea of the type of thesoil. An experienced workman can assess the nature of the soil by observing the way the rod is penetrated into the soil.

    Boring

    In this process, bore holes are made in the ground and the soil samples collected. Boring helps in obtaining 1) extent of each strata of soil/rock 2) nature of each stratum and the engineering properties of the soils 3) location of ground water table. The depth and number of boreholes will depend upon the type of the structure and nature of thesoil as obtained from preliminary examination. The depth of boreholes is governed by the depth of the soil affected by the loading. As a rough estimate, it is advisable to investigate the subsoil to a depth of at least twice the width of the anticipated largest size of the foundation. In case of a pile foundation, the depth of boring should extend into the bearing stratum.

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    Methods of Boring for Soil Investigations

    The different methods of boring are:

    Auger Boring

    The examination of the soil for ordinary buildings can be done by a post hole auger. The auger is held vertically and is driven into the ground by rotating its handle. At every 30 cm of depth, the auger is taken out and the soil samples collected.

    Wash Boring

    Wash boring is commonly used for boring in difficult soil. The hole is advanced by an auger and then a casing pipe is pushed to prevent the sides from caving in. A stream of water under pressure is forced through the rod into the hole. The loosened soil in suspension in water is collected in a tub.

    Percussion Boring

    In this method, the substrata is broken by repeated blows by a bit or chisel. Water is circulated in the hole and then the slurry is bailed out of the hole.

    Core Drilling

    When rocks are to be penetrated for examination, core drilling is resorted to. In this process, a hole is made by rotating a hollow steel tube having a cutting bit at its end.

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    Bearing Capacity

    The bearing capacity of a soil is defined as the capacity of the subsoil to support the load of the structure without yielding. The bearing capacity of the soil depends upon the characteristics such as cohesion, friction, and unit weight. The bearing capacity can be determined in the field and also from the results of tests conducted in the laboratory on the soil samples.

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    Field and Laboratory Testing

    Field Tests

    The commonly adopted field tests are:

    Standard penetration test, Dynamic cone penetration test, Static cone penetration test, Plate load test, Vane shear test, and Pressure meter test.

    Laboratory Tests

    A set of laboratory tests are required to be done to obtain the soil parameters for the design of foundation. These tests are: Shear strength, Compressibility, Permeability, Chemical and Mineralogical Composition, and Soil Classification.

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    Synopsis

    The foundation is that part of the building which serves as a base and transmits the load to the soil. Before a foundation is designed, it is necessary to investigate the characteristics of the underlying soil. There are different methods to assess these characteristics.