# Soil Mechanics Viva Questions

Soil Mechanics Viva Questions

1. Compare Alluvial Soil and Aeolian Soil?

The soils that are transported by running water is known as alluvial soil and the soil transported by wind is termed as Aeolian soil.

These type of soils are deposited at the bottom of the lakes.

Note: Soils can be derived from igneous rocks, sedimentary rocks and metamorphic rocks. The chemical weathering of soil is mainly due to oxidation, hydration, carbonation and leaching. We can say the residual soils are sands, silts and clays. Sands are the examples of cohesion less soils. A fine grained soil may or may not be plastic in nature. We know that 0.002 mm is the maximum size of the particles of a clay.

The void ratio of saturated soil = Water content x Specific gravity of solids.

Montmorillonite is the clay mineral that gives maximum swelling. We know that Gravel and sand is a non-cohesive coarse grained soil. The specific gravity of sandy soils is 2.7. The unit weight of soil mall is measured in N/ m3.

In a normal clay, the sensitivity range is about 2 to 4. In clays, the plastic limit exists. The clays exhibiting high activity contain montmorillonite and having high plasticity index. A flow net is used to determine the seepage flow, seepage pressure and the exit gradient. The critical gradient of all soils is normally has the value of 1.

3. What is a fine grained soil with little or no plasticity?

Slit

4. Discuss about black cotton soil?

A black cotton soil is inorganic in nature, it exhibits high compressibility and it contains large percentage of clay mineral.

5. What you know about consolidation?

Consolidation can be defined as the property of soil used in finding the settlement of structures.

6. Define dry soil and porosity?

In a dry soil, the pores of a soil are completely full of air only. The ratio of the volume of voids to the total volume of soil mass is termed as porosity.

7. Define air content?

Air Content = (Volume of air voids) / (Volume of voids)

8. Define specific gravity of a soil?

(Unit weight of soil solids) / (Unit weight of water) at 270C

9. How will you accurately determine the water content of soils?

By using oven drying method.

10. What types of corrections are required at the time of hydrometer analysis?

At the time of hydrometer analysis, it requires temperature correction, meniscus correction and dispersing agent correction.

11. What is the use of uniformity coefficient?

The range of particle size are measured by uniformity coefficient.

12.  What is the peculiarity of well graded soil?

In this type of soils, the particles are nearly of same size. This type of soils has uniformity coefficient more than 10.

13. Discuss about sensitivity and thixotropy?

Sensitivity = (Unconfined compressive strength of undisturbed soil) / (Unconfined compressive strength of soil in a remoulded state)

Thixotropy can be defined as the property of a soil that enables to regain its strength lost on remoulding in short times, without the change of the moisture content.

14. Define shrinkage limit?

For a saturated soil, the maximum water content at which a reduction in its moisture doesn't cause a decrease in volume of soil is termed as the shrinkage limit. The soil will be in solid state, if the water content in a soil is reduced beyond the shrinkage limit.

15. Define Plasticity Index?

Plasticity Index = The liquid limit - The plastic limit

16. Define the activity of clay?

Activity of clay = (Plasticity Index) / (Clay Fraction)

17. Define relative consistency?

Relative consistency = (Liquid Limit - The natural content of a soil) / (Plasticity Index)

18. When you say a soil has a flocculent structure?

If the particles of a soil are oriented 'edge to edge' or 'edge to face' with respect to one another, we can say that the soil has a flocculent structure.

19. Define permeability of soil?

The permeability of soil can be defined as the property of the soil mass which permits the seepage of water through its inner connecting voids. The value of permeability depends on the influence of grain size, void ratio and the degree of saturation.

20. What you know about seepage pressure?

Seepage pressure is the pressure exerted by water on the soil through which it percolates. The flow net in the seepage of water through a soil medium is by the network of flow lines and equi-potential lines.

21.  What are the properties of seepage force in soils?

The seepage force in soils is proportional to the head loss, proportional to the exit gradient and perpendicular to the equi-potential lines.

22. What are the factors affecting the effective stress on the soil?

The effective stress on the soil depends on the external load acting on the soil and the weight of water present in soil pores.

23. Give the equation of coefficient of volume compressibility?

Coefficient of volume compressibility can be defined as: (The change in volume of soil) / (Unit of initial volume due to a given unit increase in pressure)

24. What you know about the critical density?

The critical density is the density of sand where there has no change in volume under the pressure of shearing strain produced because of shear stress.

25. What is the use of density index?

The density index is used for estimating the angle of friction of a sandy soil.

26. What are the types of frictional resistances offered by sand?

Sliding friction, Rolling friction and Resistance due to interlocking.

27. Define saturation line?

The saturation line can be defined as a line, which shows the dry density as a function of water content for soil contains no air voids.

28. Which parameter is directly proportional to the square of the depth of soil?

Lateral earth pressure

29. What you know about the safe bearing capacity?

The safe bearing capacity is the maximum unit pressure that a soil can withstand without  rupture in shear or without the excessive settlement of the structure.

Note: the total settlement of isolated footings for cohesive soil should be 65 mm  as per the I.S code. The contact pressure of flexible footings on non-cohesive soils is high in centre as compared with the edges. At 600 the water content of a highly organic soil is determined. By the oxidation with hydrogen peroxide, the organic matter is removed in pre-treatment of soil. The particle size distribution curve is obtained in a gap graded with a hump.

The coefficient of curvature is between 1 and 3 for a well graded sand. The unified soil classification and the IS classification of soils are similar. Clays are the soils which are plotted above the A line  in the plasticity chart. As per the IS classification system, soils are classified into 18 groups. The plasticity characteristics of a clay are because of adsorbed water. The bulking of sand is usually between 20% and 30%. The permeability of soil usually varies with respect to the square of grain size.  0.2mm is the maximum particle size for applying Darcy's law.

30. Discuss the different parameters affecting the settlement of a footing in sand?

The settlement of a footing in sand mainly depends on the width of the footing, the relative density of sand and the stress deformation characteristics of the sand.

31. What you understand about soil compaction and soil stabilization?

Soil compaction is the process of obtaining increased density of soil in a fill. This is obtained by the reduction of its pore space by the expulsion of air.

Soil stabilisation is the process of maintaining or improving the performance of a soil as a constructional material. This is done by the use of admixtures.

32. What type of soils are formed due to physical disintegration?

Cohesion less Soils

33. Which method is used for determining the water content in sands?

Pycnometer Method

34. In which law,  the terminal velocity of the particle is directly proportional to the square of the radius of the particle?

Stoke's Law

35. In which analysis, the meniscus correction is positive and the dispersing agent correction is negative?

The hydrometer analysis for a soil mass

36. What is the maximum size of particle for slit?

75μ

37. If a soil has a liquid limit as30, then what is the corresponding plasticity index given by the A line?

7.3

38. Give the normal range of coefficient of permeability of clay?

10-5 & 10-8 mm/s

39. For coarse grained soils, which type of permeability test are used?

40. If a soil has a void ratio of 0.5 and a discharge velocity of 6x10-7m/s, Calculate its seepage velocity?

Seepage velocity = 18x10-7 m/s

41. What is the shape of phreatic line in a homogeneous dam?

Parabolic

42. What is the use of flow net?

A flow net is used for determining seepage, exit gradient and the uplift pressure.

43. What is a quick sand?

The condition in which a cohesion less soil loses its strength due to  the upward flow of water is termed as quick sand.

44. Which parameter of a soil increases with decrease in void ratio?

45. Give the relationship between the specific gravity of solids (G) and the hydraulic gradient ( i ) for a void ratio of 0.6 for the quick sand condition?

G = 1.6i + 1

Note:  The stress developed at a point in the soil exactly below a point load at the surface is inversely proportional to the square of the depth of the point. In homogeneous soils, the Westergaard's analysis for vertical stress is used. As the liquid limit increases, the compression index of soil also increases. The consolidation time for a soil sample increases with decrease in  permeability. As the compression index increases, the ultimate settlement of a soil deposit also increases. The lime stabilisation of clayey soils leads to Flocculation of particles.

46.  Name the curve which joints points of equal vertical stress?

Isobar

47. The vertical stress under any shape of loaded area can be determined by?

The Newmark's influence chart

48. What is coefficient of compressibility?

It is the ratio of change in void ratio to the change in effective stress.

49. What is drained test?

If a drainage is permitted throughout the tri-axial test, then the test is called as a drained test.

50. The shear strength of which soil is proportional to the tangent of angle of shearing?

Cohesion less soil

51. What is the angle of failure of plane with the major principal plane?

450  +  Ø/2

52. Which one is most suitable for the compaction of clays?

Vibratory rollers

53. What is the range of the mass and the drop of a hammer in a standard compaction test?

2.6 Kg and 310 mm

54. The relative compaction is same as relative density? True or False.

FALSE

55. Vibratory rollers are effective in the case of highly cohesive soils? True or False.

FALSE

56. Zero air void line and 100% saturation line are identical? True or False.

TRUE

57. What is the percentage of particles passing 75μ sieve for getting maximum dry density?

20%

58. What you understand about aquiclude?

Aquiclude is the geological formation which contains water but is not able to transmit.

59. Which test is used for measuring the shear strength of very soft and sensitive clays?

In-situ vane shear test

60. What is the corrected value of the standard penetration number (N), if the actual value of N is greater than 15 for fine sands?

15+

61. What is the ratio of height to diameter of on-situ vane shear test apparatus?

2.0

62. Which chart is based on the total stresses using the friction circle method?

Taylor's stability chart

63. What you know about applied shear stress?

In stability analysis, the term mobilised shear strength is termed as applied shear stress.

64. What is known as effective stress?

The active earth pressure coefficient Ka is generally refers to effective stress

65. What is the radius of friction circle in the analysis of earth pressure?

R.SinØ

66. In which case, the permissible settlement is maximum?

Raft on clay

67. When a negative skin friction on a pile developed?

when the surrounding it settles more than the pile

NOTE: For a bored pile in sand, the load carrying capacity is 1/2 to 2/3 times that of a driven pile. For driven piles in sand at close spacing, the group efficiency is greater than 100%. The contact pressure beneath a rigid footing resting on a cohesive soil is more at edges as compared to the centre.

68. What is the void ratio of the soil, if in a wet soil mass, air occupies one-fourth of its volume and water occupies one half of its volume?

3

69. Calculate the value of porosity of a soil sample, if the total volume of soil grains is equal to twice the total volume of voids?

33.33%

70. Give the correct sequence of parameters of earth pressure intensity in descending order?

Passive, at rest, active

71. What is the ratio of passive to active lateral earth pressure for a sand having internal friction of 300?

9

72. What is the inclination of failure plane to horizontal in direct shear test?

0

73.  Which factor governs the behavior of a clay?

Surface energy

74. What is the maximum value of stability number in a stability chart?

0.26

75. Which type of soil is practically incompressible?

Clay

76. Because of large lateral pressure, which type of soils are not good for backfilling?

Cohesive Soils

77. What is the value of the depth factor Df for toe failure of slopes?

1.0

78. When a body of soil is said to be in plastic equilibrium?

On the verge of failure

79. As the specific gravity increases, what also increases?

The critical hydraulic gradient of seepage water

80. When the vertical pressure distribution on a horizontal plane is negligible?

If the horizontal distance is equal to twice the depth.