Number of Bore holes for building projects

''Do we have to do a soil test for this project''!

I do hear this question often in my consulting practice. Owners,Architects,site engineers and every one thinks soil testing is an additional cost atleast for a small project. Is it ok to ignore soil testing?

Geotechnical engineering is a separate discipline from structures.Geotechnical engineer has to decide this.However a basic understanding of soil mechanics is needed for a structural engineer as many solutions are forced up on the structural engineers especially in my part of the world.

First let me discuss the geotechnical engineering practice situation in India.


Status & scope of Geotechnical engineering in Building Projects in India

There are many graduates in civil & Postgraduates in Geotechnical who are practicing as Geotechnical engineers.Many a time a structural engineer also acts as a geotechnical engineer with some experience. Some are well experienced and can handle smaller to very challenging requirement. Some are limited to handle a general soil profiling and testing.

The challenges in soil engineering has increased in the building sector with most large buildings having deep and multiple basements.

When some of the complicated project gets the due attention and there is a geotechnical engineer on board, for many midscale and small projects, there either no soil testing or the scope of the consultants ends with just providing a soil report.


Do one need to test soil for small to medium projects?

All projects need to adhere to codes of practice.If one has extensive understanding of the soil profile of a place by the virtue of practicing for a long time,some experts may be able to conclude a generalistic idea about the soil type. However there can be variance in soil profile locally. All this can be fairly studied by doing soil testing. Its a myth that one can save money by not doing soil testing.A structural engineer will assume a value for the SBC if he is not provided with a report. This can result in larger size than needed at times and less than needed at times. So its always needed to rely on a soil testing and the reports.


What does the code say regarding the number of bore holes?

A summary of the code provision is explained below.However if you need a comprehensive understanding of these aspects,it is available in civilera's foundation design course.


IS 1892 is the code of practice for Subsurface investigation for foundations. It states that -

''The spacing of the test pits and bore hole should be such as to identify major changes in thickness, depth and properties of the strata over the base area of the building and its immediate surroundings.

The number and spacing of bore holes or trial pits will depend upon the extent of the site and the nature of structures coming on it. For a compact building site covering an area of about 0.4 hectare, one bore hole or trial pit in each corner and one in the centre should be adequate. For smaller and less important buildings even one bore hole or trial pit in the centre will suffice. 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. Cone penetration tests may be performed at every 50 m by dividing the area in a grid pattern and number of bore holes or trial pits decided by examining the variation in the penetration curves. The cone penetration tests may not be possible at sites having gravelly or boulderous strata. In such cases geophysical methods may be useful.''


Conclusion

Every aspect of engineering is important and each discipline is important. It is important that each stake holders understand and appreciate the importance of adhering to codes of practice.

Geotechnical and structures are many a time inseparable and a sound understanding of basic geotechnical requirement is a prerequisite for any structural engineer.


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