What Factors Affects the Number of Boreholes and their Depths in Subsoil Investigation?


Explain the various factors that help decide the number and depth of boreholes required for subsoil investigation

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Answers ( 2 )

  1. When the depth and position of exploration are selected one has to comply with the following guidelines:

    • Points of exploration have to be distributed in such a way that foundation soil layering can be determined across the location
    • Points of exploration for a building or structure have to be positioned on the critical point of the structure in relation to shape, behavior and expected load transfer of the structure
    • For route structures, points of exploration have to be distributed on appropriate distances from the central line, depending on a total width of structure such as the width of embankment or width of a cut
    • For structures on slopes and cuts (including excavations) or in their vicinity, points of exploration have to also be distributed outside the design area and positioned so that the slope or excavation stability can be determined; if anchors are foreseen one has to take into account probable stresses in the vicinity of their load transfer area
    • Points of exploration have to be distributed so that they pose no danger for structures, construction works or the environment
    • The considered area has to stretch into the neighboring location. The length has to cover the expected negative influence in that location.
    • For points of underground water measurement, one has to take into account the possibility of usage, after the initial positioning, for the continuance of measurement during and after construction.

    If the conditions in the foundation soil are relatively equal or it is known that the foundation soil has satisfying stiffness and strength, less exploration point can be foreseen or they can be on bigger distances. In any case, that kind of selection has to be justified by experience.

    If structures are built on layers that satisfy stiffness and strength, depth of exploration can be reduced to za = 2,0 m unless equal geological conditions when at least one borehole should reach the depth of za = 5,0 m. If the bedrock is located on a depth that matches the foreseen base of the structure, that depth has to be taken as a referent point za. Otherwise, za is referred to as the solid bedrock.

  2. Number of Boreholes

    1-Time allocated for investigations

    2- Cost

    3- The structure for which the investigation is conducted

    4- and sometimes the availability of necessary equipment and individuals with adequate skills and experience.

    5- Geological Conditions

    Borehole numbers are assumed to be economical unless their cost would not surpass the amount of savings in foundation cost. Otherwise it will increase the total cost of the project which is not desirable.

    Determining Depth of Boreholes

    The depth of boreholes is controlled by the depth of the soil that is under the influence of bearing pressure of the foundation. The recommended borehole depth is equal to one to three times the width of the loaded area. This is because it is suggested that bearing pressure below this depth would cause detrimental effect on the structure.

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