(b) What are the advantages of using a drilled shaft foundation when compared to

Question

(b) What are the advantages of using a drilled shaft foundation when compared to the option of using pile group in a large construction project? [10] (c) The shear strength parameters of soil may be determined in the laboratory by the direct shear test and triaxial shear test among others. Why is the triaxial test considered more reliable than the direct shear test? [41 Briefly describe the important construction procedures for shaft excavations and list (d) some of the safety precautions to be observed when inspecting the hole. [81

Explanation / Answer

(b)

Advantages of using drilled shaft foundations over pile group in large constructions :

1. Pier of any length and size can be constructed at the site
2. Construction equipment is normally mobile and construction can proceed rapidly
3. Inspection of drilled holes is possible because of the larger diameter of the shafts
4. Very large loads can be carried by a single drilled pier foundation thus eliminating the necessity of a pile cap
5. The drilled pier is applicable to a wide variety of soil conditions
6. Changes can be made in the design criteria during the progress of a job
7. Ground vibration that is normally associated with driven piles is absent in drilled pier construction
8. Bearing capacity can be increased by underreaming the bottom (in non-caving materials

(c)

Triaxial Test is considered more reliable than direct shear test because :

1. Failure plane is not predetermined and it is the weakest plane in triaxial test whereas in direct shear test it is predetermined and is not the weakest plane.

2. Pore Pressure can be measured with the given assembly of triaxial test but it cannot be measured in direct shear test.

3. There is complete control of drainge conditions in triaxial test which direct shear test doesn't have.

4. Stress distribution on failure plane is much more uniform but in direct shear test it is not uniform.

5. Mohr Circle can be drawn from a single observation in Triaxial test and we cannot draw it from single observation from direct shear test.

6. Test results are accurate in triaxial test which are not so accurate incase of direct shear test.

7. Triaxial test is suitable for all types of soil which direct shear test is not.

(d) Construction procedures for shaft excavations :

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