HOW TO DECIDE DEPTH AND SPACING OF TRIAL PITS AND BORING?

Depth and distance of test pits and borings
Depth and distance of test pits and borings

The depth and distance of test pits or exploratory boring shall be such as to give an accurate picture of the underlying soil with respect to any major change in thickness, depth or properties of layers at the base area of ​​the structure and in the immediate area. Nearby. At first only a preliminary estimate of the vacancy is done. This may be increased or decreased depending on the information disclosed by Boring.

In no case should the boring be taken down to a hard level below which soft material is not expected.

When bed rock is found, the surface must be plotted by several borings or surroundings to locate any depressions. For this purpose, some boring and sub-surface sounds are first made over large intervals and intermediate values, if necessary, can be obtained by normal sounding, or obtained by further boring in the case of an unpredictable profile nature. can be done.

according to standard

The following specifications have been laid down in this regard:

  1. For a compact building site covering an area of ​​approximately 0.4 hectares (ie 4000 square metres)2One borehole or trial pit in each corner and one in the center (ie 5 boreholes in total) should be sufficient.
  2. A bore hole or trial pit in the center should also suffice for smaller areas and less important buildings.
  3. For very large areas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Dynamic or static cone penetration testing can be performed every 100 m by dividing the area into a grid pattern and checking for variations in the penetration curve to determine the number of bore holes or trial pits.
  4. For exploration purposes, the minimum depth is taken as 1.5 m as weathering processes affect the soil up to this depth. But where industrial processes affect soil characteristics, this depth may be greater.
  5. Generally the depth of exploration should be taken as 1.5 to 2 times the estimated width of the largest footings (single or combined) from the base level foundation; But in weak soils, exploration should be continued to a depth at which the load can be carried safely (usually up to the hard layer).

according to the rule of thumb

The following values ​​have been suggested:

  1. For initial estimation, boring spacing can be adopted as follows:
types of buildings very similar soil average soil
multi-storey buildings 30 – 60 square meters 15 – 30 square meters
one storey building 60 – 90 square meters 30 – 60 square meters
  1. A single rule regarding depth of exploration that is found to be satisfactory for most conditions is 2.5 m to 3.5 m of boring depth or its equivalent in loading, for each floor in the building.

pay attention:

The depth of boring depends on the type of structure proposed, its total weight and the type of sub-soil. Although usually the depth of boring can be taken as 1.5 times the width of the largest footing, at least the boring must be made deep enough to detect the existence of any extraordinarily narrow stratum, Preferably up to the rock.


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Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.