Soil Mechanics Interview Questions

Here are the top 50 commonly asked questions in Soil Mechanics interviews. Whether you’re just starting your preparation or need a quick refresher, these questions and answers will help you tackle your interview with confidence.

Basic Soil Mechanics Interview Questions with Answers

1. What is definition of soil in Soil Mechanics?

Soil is a three-phase system consisting of solid, water, and air which are intermixed with each other such that the relative constituents decide properties of soil.

2. What are the types of shear strength of soil in Soil Mechanics?

Shear strength is an engineering property of soil and it is very important to understand because, soil always fail in shear. It is of two types: i) Undrained: Strength of soil mass when water is present; ii) Drained: Strength of soil mass when water drained out.

3. What is Soil Mechanics?

Soil Mechanics is a branch of engineering that describes the behavior of soils. It classifies soils and also used to determine strength factors of soil mass.

4. What is the difference between index property and engineering property in Soil Mechanics?

Properties which are used to identify and classify the soil are known as index properties e.g., water content, specific gravity etc. whereas properties which govern the strength of soil are known as engineering properties e.g., shear strength, bearing capacity etc.

5. What is plasticity of soil in Soil Mechanics?

Plasticity of a soil is a property of soil by which soil can deform without rupture, fracture or volume change. It depends on: i) Type of mineral cation; ii) Amount of mineral cation; iii) Amount of water absorbed.

6. What is the term “Thixotropy” in Soil Mechanics?

Over the period of time, the strength of the soil lost upon remolding due to reorientation of water molecules is regained. This property of soil is known as thixotropy.


7. What is the term “Sensitivity” in Soil Mechanics?

Sensitivity is the ratio of unconfined compression strength in undisturbed state to the unconfined compression strength in remolded state of same soil mass. It is always greater than one because upon remolding unconfined compression strength decreases.

8. What is stress path in Soil Mechanics?

The path which shows the soil loading history is known as stress path. It is drawn when horizontal and vertical stresses are principal stress. In the stress path, the curve is drawn between maximum shear stress and normal stress acting on the maximum shear plane.

9. What are the types of soil on the basis of IS classification in Soil Mechanics?

There are two types of soil on the basis of Indian Soil Classification system: i) Coarse grained: If 50% or more soil mass retained over 75-micron sieve; ii) Fine grained: If more than 50% passes through 75-micron sieve, then soil is referred as fine grained.

10. Which types of stresses soil experience in Soil Mechanics?

Stress types: i) Total stress: It is the stress at any section of the soil due to load acting from above section; ii) Neutral stress: It is the stress which is applied by pore water on the soil solids. iii) Effective stress: Stress which is transferred by grain-to-grain contact.

11. Which factors affect permeability in Soil Mechanics?

Factors that affect permeability are: a) Degree of saturation; b) Diameter of soil solid particle; c) Void ration; d) Dynamic viscosity of moving fluid; e) Foreign impurities; f) Structure of soil; g) Type of mineral cation; h) Adsorbed water.

12. What is seepage discharge for isotropic medium in Soil Mechanics?

Total discharge of fluid through flow net is known as seepage discharge. It is expressed by the formula, Q = K*H*NF/ND where K is permeability, H is total head loss and NF/ND is shape factor of the flow net.

13. How particle size distribution is obtained in Soil Mechanics?

The particle size distribution for coarse-grained soil is obtained by performing a sieve analysis, whereby a sample of dry soil is mechanically shaken through a series of sieves and the percentage retained or passing through each sieve is weighed.

14. What is liquid limit and how it is tested in Soil Mechanics?

The liquid limit is the water content above which the soil behaves as a viscous liquid. It is tested for using an apparatus referred to as a Casagrande device. It is determined as the water content required for the soil to flow into the groove after being tapped 25 times.

15. Why is activity of soil in Soil Mechanics?

Activity relates the amount of water that can be bound to a particular clay surface,
A = IP/c where IP is plasticity index and c is % of clay fraction. The higher the soil activity, the greater the influence of the clay fraction and the more plastic clay behavior.


16. What is saturated and partially saturated soil In Soil Mechanics?

Soil lying below the water table may have only two phases: water and solids. This would be called a saturated soil. On the other hand, soil lying above the water table usually has all the three phases: air, water, and solids. This is a partially saturated or unsaturated soil.

17. What is compaction of soil in Soil Mechanics?

Compaction is the densification of a soil, by the application of mechanical energy. It involves the increasing of the soil density by packing the particles closer together and in turn reducing the volume of air.

18. How standard proctor compaction test is conducted on soil in Soil Mechanics?

The Proctor compaction test involves the dropping of a hammer several times on a molded soil sample. The compact effort produced by this process is dependent on the weight of the hammer used, the height of the hammer drops and the number of hammer drops.

19. What is plasticity index in Soil Mechanics?

The range of water contents over which the clay behaves in a plastic manner is known as plasticity index. It is calculated by formula, IP = WL – WP where WL and WP is liquid and plastic limit of soil respectively.

20. What is grain size distribution curve in Soil Mechanics?

Grain size distribution curve is a curve that represents percent finer verses logarithm of average grain diameter. This curve helps to identify the grade of the soil mass i.e., poorly-graded, uniform soil, and well-graded soil.


Intermediate Soil Mechanics Interview Questions with Answers

21. How does soil form?

Soil is formed by the dis-integration of rocks. It forms via two processes: i) Physical: Impact and abrasion due to water, air etc., Splitting action of ice; ii) Chemical: Carbonation, oxidation, reduction, hydrolysis, leaching etc.

22. What are the limitations of Stokes law for sedimentation analysis?

Sedimentation analysis is an experiment for particle analysis and it is based on Stokes law. Limitations of Stokes law: i) Clay particles stick to each other due to charge on their surface; ii) Clay particles are flaky in shape; iii) Finite medium.

23. What is consistency of soil and what are its stages?

The ease with which soil can be deformed easily is called consistency of soil. It defines degree of firmness of soil like hard, stiff, and soft. There are 4 stages of consistency: i) Liquid stage; ii) Plastic stage; iii) Semi-solid stage; iv) Solid stage.

24. What is consistency limit and it types?

The water content at which soil changes from one stage to the other stage of consistency is known as consistency limit. There are 3 types of consistency limit: i) Liquid limit; ii) Plastic limit; iii) Shrinkage limit.

25. What is liquid limit?

The minimum water content at which soil is in liquid stage of consistency or has a tendency to flow is called as liquid limit. It is the water content at which soil changes from plastic stage to liquid stage or vie-versa.

26. What is the difference between consistency index and liquidity index?

Consistency index is the ratio of difference of liquid limit and natural water content to the plasticity index of soil whereas liquidity index is the ratio of difference of natural water content and plastic limit to the plasticity of soil.

27. What is soil structure and its types?

Arrangements of soil particles in the soil is called as soil structure. There are 4 types of structure: i) Coarse-grained structure; ii) Honey combed structure; iii) Flocculant structure; iv) Dispersed structure.

28. What is gravity water?

Gravity water is the water which fills the voids continuously up to ground water table level. It is also referred as ground water. For the ground water, all the laws of hydrostatic are valid.

29. What is structural water?

Structural water is the water present inside the structure of soil solid. There is no significance of structure water because in field condition it can’t be extracted. It can be obtained by heating the sample at temperature above 1100 C.

30. What is hydroscopic water?

Hydroscopic water is the water which is attracted from the atmosphere by the soil solid and it is retained over soil solid due to adhesion.

31. What is slaking of clay?

When oven dried clay sample is immersed suddenly in the water, it disintegrates in a soft wet mass because of very high pressure by water in the void. This property of disintegration is called slaking of clay.

32. What are the laboratory methods for determination of permeability?

There are two laboratory methods for determination of permeability: i) Constant Head method: It is used for coarse grained soil like sand. ii) Falling Head method: It is used for fined grained soil like silt, clay etc.

33. What is quick sand condition?

When flow is upward for the soil with effective cohesion coefficient, c’ equal to zero, then soil lose all its strength and soil try to move along with the water in upward direction. This condition is referred as quick sand condition or piping condition.

34. Which factors affect the compaction curve?

Factors affecting compaction curve: i) Compacting effort: With increase in compacting effort, dry mass density of soil increases and optimum moisture content decreases; ii) Water content; iii) Type of soil; iv) Method of compaction.

35. Why is time factor in consolidation?

Time factor is a factor which relates degree of consolidation to the time required to achieve particular degree of consolidation. It is denoted by the formula, Tv = Cv*t/d2 where d is length of drainage path, t is time and CV is coefficient of consolidation.

36. What is the difference between smooth wheel roller and sheep foot roller?

In smooth wheel roller, the compaction is done by application of pressure and it is preferred for coarse-grained soil whereas in sheep foot roller, compaction is done by needling action and it is used for cohesive soil like clay, silty clay etc.

37. What is consolidation of soil?

Decrement in the volume of saturated soil due to expulsion of pore water from fully saturated soil is called as consolidation. It is a time dependent process.

38. What are the assumptions of Terzaghi’s theory of consolidation?

Assumptions of Terzaghi’s theory of consolidation: i) Soil is fully saturated and remain saturated during the process of consolidation; ii) Flow is one dimensional, only in vertical direction; iii) The soil is homogenous and isotropic; iv) No change in cross sectional area.

39. What are the merits of triaxial test?

Triaxial test is a laboratory test to determine shear strength of soil. Merits of triaxial test: i) Can be used both for drained and undrained condition; ii) Pore water pressure can be measured; iii) Can be used for any type of soil; iv) Uniform stress distribution on failure plane.

40. What is unconfined compression test?

Unconfined compression test is similar to triaxial test but cell pressure is equal to zero and rubber membrane is not used. It is expressed by the formula, σd=2ctan(45+ ∅/2) where c is intermolecular attraction due to cohesion/adhesion and ∅ is frictional resistance.

41. What is Mohr’s failure theory for soil?

According to Mohr’s failure theory, soil never fails on a plane subjected to maximum shear and maximum normal stress. Soil fails on a plane where the resultant stress makes maximum angle with the normal acting on that plane.

42. What is the difference between angle of obliquity and critical angle?

The angle which the resultant stress acting on a plane makes with respect to normal is called as angle of obliquity whereas critical angle is the angle which the failure plane makes with respect to major principal plane.

43. What is exit gradient in flow net?

Hydraulic gradient corresponding to last flow filed is referred as exit gradient and it assumes the maximum value out of all the gradient. It is calculated using formula, ie = ∆h/L where ∆h is equipotential drop and L is length of exit flow field.

44. What are the causes of resistance in the soil mass?

Resistance offered by the soil is due to following reasons: i) Frictional resistance between particles; ii) Interlocking between the particles; iii) Intermolecular attraction due to cohesion and adhesion.

45. What are the fundamental units of clay minerals?

Clay minerals consists of two units: i) Silica tetrahedral unit: All the oxygen atom at the base shares the oxygen atom with the 2 tetrahedron unit; ii) Octahedral unit: It consist of six hydroxide ions enclosed by Al+3 metal ion at the center of octahedral geometry.

46. What is coarse-grained soil (CGS) and its types?

If soil retained over 75-micron sieve is equal or greater than 50% by mass, then soil is referred as coarse-grained. It is two types: i) Gravel: 50% or more of the CGS is retained on 4.75mm sieve; ii) Sand: More than 50% of CGS passes through 4.75mm sieve.

47. What is plastic limit of soil?

The water content at which soil changes its state from semi-solid to plastic stage or vice-versa is known as plastic limit. It is the water content below which soil began to crumble when rolled into the thread of 3mm diameter.

48. What is shrinkage limit?

The minimum water content at which soil is saturated is called as shrinkage limit. It is calculated using formula, WS = 1/Gd – 1/G where WS is shrinkage limit, Gd is dry mass specific gravity and G is absolute specific gravity of soil.

49. What are the types of soil grade?

There are mainly 3 types of soil grade obtained from particle size distribution curve: i) Uniformly graded soil: Same size of soil particles; ii) Well graded soil: All particle sizes are present; iii) Gap graded soil: Few particle sizes are missing.

50. What is secondary consolidation?

When the water is no more expelling out and the volume of soil still decreases, then the decrease in volume is due to plastic readjustment of soil solids. This plastic readjustment of soil solid is called as secondary consolidation.

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