Soil Mechanics and Foundation Engineering interview questions

Answer: Soil Mechanics is a branch of civil engineering that deals with the study of soil properties and behavior under various conditions. It includes the analysis of soil strength, compressibility, permeability, and other characteristics to design safe and stable foundations and earth structures.

• Compaction Test: Determines the optimal moisture content and maximum dry density of the soil.
• Permeability Test: Measures the rate at which water flows through soil.
• Shear Strength Test: Assesses the soil's resistance to shear stress, typically conducted using direct shear test, triaxial shear test, or unconfined compression test.
• Consolidation Test: Determines the rate and magnitude of soil settlement when subjected to a load.
• Atterberg Limits Test: Determines the plastic and liquid limits of soil to classify its consistency.

• Cohesionless Soils: Soils like sand and gravel that do not exhibit significant cohesive properties. Their shear strength is primarily due to internal friction.
• Cohesive Soils: Soils like clay that have significant cohesive properties due to inter-particle attraction. Their shear strength comes from cohesion and internal friction.

Atterberg Limits are used to classify fine-grained soils based on their water content. The Liquid Limit (LL) indicates the water content at which soil changes from a liquid to a plastic state. The Plastic Limit (PL) is the water content at which soil changes from plastic to semi-solid. The Plasticity Index (PI) is the difference between LL and PL, indicating the range of water content over which the soil remains plastic.

Soil compaction is the process of increasing soil density by reducing air voids through mechanical means. It is important because it enhances soil strength, stability, and load-bearing capacity, reduces settlement and permeability, and prevents soil erosion

The principle of effective stress, introduced by Karl Terzaghi, states that the strength and deformation behavior of soil are controlled by the effective stress, which is the difference between the total stress and pore water pressure. Effective stress = Total Stress - Pore Water Pressure. It determines how much of the total stress is actually carried by the soil skeleton.

Bearing capacity is the maximum load per unit area that the soil can support without failure. It is determined using theoretical methods (like Terzaghi's bearing capacity theory), empirical methods (like Standard Penetration Test results), or in-situ tests (like Plate Load Test).

• Shallow Foundations: Used when the soil has sufficient bearing capacity at shallow depths (e.g., spread footings, mat foundations).
• Deep Foundations: Used when the bearing capacity at shallow depths is insufficient, requiring deeper support (e.g., piles, drilled shafts).

Soil consolidation is the process of gradual reduction in soil volume due to expulsion of water under sustained load, leading to settlement. It occurs over time and is influenced by the soil's permeability and compressibility. Compaction, on the other hand, is the immediate densification of soil by mechanical means, reducing air voids.

The triaxial shear test involves placing a soil sample in a cylindrical chamber, applying confining pressure, and then increasing axial stress until failure. It allows for the measurement of soil shear strength under controlled drainage conditions and varying confining pressures, providing valuable data for understanding soil behavior under different stress states.