Shear parameters for checking initial stability of a footing on fully saturated clayey strata:

In order to check the initial stability of a footing on fully saturated clayey strata, shear parameters are required. The shear parameters can be obtained from different tests such as consolidated untrained test, unconsolidated drained test and unconsolidated untrained test with pore pressure measurement. Among these options, the correct answer is option 'C', which is the unconsolidated untrained test.

Explanation:

- Unconsolidated untrained test: In this test, the soil sample is not subjected to any confining pressure and is kept in an unconsolidated state. The test is conducted under undrained conditions, which means that the pore water pressure is not allowed to dissipate during the test. The shear strength parameters obtained from this test are the undrained shear strength (su) and the angle of internal friction (ϕ).
- Fully saturated clayey strata: Fully saturated means that all the voids or pores in the soil are filled with water. Clayey soil has a low permeability and high plasticity, which means that it can easily deform under stress.
- Footing: A footing is a foundation element that distributes the load of a structure to the underlying soil or rock. The stability of the footing depends on the shear strength of the soil beneath it.

Why unconsolidated untrained test is used:

- Undrained conditions: The unconsolidated untrained test is conducted under undrained conditions, which is appropriate for fully saturated clayey strata where the pore water pressure cannot dissipate quickly. This test provides the undrained shear strength (su), which is the shear strength of the soil when it is not allowed to drain or consolidate.
- Initial stability: The test is used to check the initial stability of the footing, which means that it provides the shear strength parameters for the soil before any deformation or failure occurs. This is important for designing the foundation and ensuring its stability under the expected loads.
- Clayey soil: The unconsolidated untrained test is suitable for clayey soil, which is commonly found in construction sites. This type of soil has a low permeability and high plasticity, which means that it can easily deform under stress. The undrained shear strength is therefore an important parameter for designing the foundation and preventing failure.

Conclusion:

In summary, the correct shear parameters for checking the initial stability of a footing on fully saturated clayey strata are obtained from the unconsolidated untrained test. This test provides the undrained shear strength (su) and the angle of internal friction (ϕ), which are important for designing the foundation and ensuring its stability under the expected loads.