CE 2305 - FOUNDATION ENGINEERING Two Marks Questions With Answers 2015

Anna University, Chennai

Fifth Semester

Civil Engineering CE 2305 - FOUNDATION ENGINEERING

(Regulation 2008)

TWO MARKS

1. What is meant by a non- representative sample? Name the laboratory tests that could be conducted on this sample.

Soil sample consists of a mixture of soil from different soil strata is called non- representative sample and the size of the soil grains and mineral constituents have changed.

2. Write the uses of Bore log Report.

Uses of Bore log report :-

(i) Used to record the change of layer’s depth.

(ii) Used to record the water level.

(iii)Used to record the water quality in deeper levels.

3. Define detailed exploration.

Detailed exploration:-

Detailed exploration follows as a supplement to general exploration when

large engineering works, heavy loads, and complex and costly foundations are involved. A detailed exploration is meant to furnish information about soil

properties such as shear strength, compressibility, density index, and

permeability.

4. Define punching shear failure.

Punching shear failure:-

Punching shear failure occurs where there is relatively high compression of

soil under the footing, accompanied by shearing in the vertical direction around the edges of the footing. Punching shear may occur in relatively loose sand with relative density.

5. What is mean by swelling potential?

Swelling potential:-

The swelling potential of an expansive soils is defined as the percentage

swell of a laterally confined soil sample, when tested in a consolidometer test,

when soaked under a surcharge load of 7kN/m2 after being compacted to

maximum dry density at O.M.C, (optimum moisture content)according to

AASHO compaction test.

2. In which situation are raft foundation used?

Raft foundation:-

Raft foundation is used where settlement above highly compressible soils,

by making the weight of structure and raft approximately equal to the weight of the soil excavated.

3. What are the factors governing the selection of piles?

Selection of piles:-

1. Soil condition.

2. Type of the structure (or) building

3. Adjacent site condition

4. Construction techniques availability

5. Location of ground water table

6. Durability etc..

4. Define negative skin pressure.

Negative skin pressure:-

Negative skin friction force for a single pile is equal to the sharing resistance times the surface area of the pile. Therefore the negative skin friction on a pile group is

F n = t L P + g LA for group

F n = n t L p d for individual

5. Write any three assumption of Rankine’s theory.

1. The soil mass is semi-infinitive, homogeneous , dry and cohesionless,

2. The ground surface is a plane which may be horizontal inclined

3. The back of the wall is vertical and smooth.

6. Write the types of retaining wall.

Types of retaining wall:-

The earth retaining walls are of the following types. (a) Gravity wall.

(i) Mass concrete (or) masonary wall

(ii) Wall on wells

(iii)Precast block wall

(iv) Two - row sheet pile wall

(v) Crib wall

(b ) Reinforced concrete wall

(i) Cantilever type -‘T’wall (or) ’L’ wall

(ii) Counterforted (or) butteressed wall

(c ) Sheet pile wall

(i) Cantilever sheet pile wall

(ii) Anchored sheet pile wall (or) Anchored bulkhead.

11. What is objective of site exploration?

Objective of site exploration:-

The object of site exploration is to provide reliable, specific and detailed

information about the soil and ground water conditions of the site, economic design and execution of the engineering work.

12. What is site reconnaissance?

Site reconnaissance is defined as the inspection of the site and study of topographical features , the soil and ground water conditions and in deciding the future programme of exploration.

13. What is ultimate bearing capacity?

The ultimate bearing capacity of the soil (q f) is defined as that pressure (or) load intensity, which causes the shear failure of the supporting soil, immediately below and adjacent to a footing.

14. What is net pressure intensity?

The difference in intensity of gross pressure after the construction of a structure and the original overburden pressure is called “Net Pressure”.

Net load on the base of foundation

Net pressure intensity =

15. What is safe bearing pressure?

Area of footing

In conventional design, the allowable bearing capacity should be taken as the smaller of the following two values:

(i) The safe bearing capacity based on ultimate capacity, and

(ii) The allowable bearing pressure on tolerable settlement.

16. What is total settlement of a footing?

Total settlement defined as the settlement due to primary settlement, consolidation settlement and secondary settlement. S = S p + S c + S s .

17. What are Anchor piles?

Anchor piles:-

Anchor piles are the type of the piles which provides anchor edge against

horizontal pull from the sheet pileing (or) other pulling forces.

18. What are fender piles?

Fender piles are the type of piles are used to protect water front structures against the impact from ships or other floating objects.

19. What is earth pressure at rest?

Earth pressure at rest:-

The earth pressure at rest is defined as the intensity of lateral earth pressure

when the lateral strain is zero and it is expressed as PR = KR × g′ × Z where

KR = co-efficient of earth pressure.

20. What are the limitations of hand augers in soil exploration?

Limitation of hand augers in soil exploration:-

1. Hand augers are not suitable for sands and gravels above the water table.

2. The sample is distributed and suitable for identification purposes only.

101. What are the guidelines in terms of inside clearance and outside clearance for obtaining undisturbed sample?

An undisturbed sample is that in which the natural structure and properties remain preserved. The inside clearance should lie between 1 to 3 percent and the outside clearance. The walls of the sampler should be smooth and should be kept properly oiled.

22. What is allowable bearing pressure?

Allowable bearing pressure:-

It is defined as the net loading intensity at which, neither the soil fails in shear nore there is excessive settlement detrimental to the structure (qa).

23. Sketch the contact pressure distribution for a rigid footing on sand and

clay.

24. What is proportioning of footing?

Proportioning of footing:-

Proportioning of footing is defined as the arrangement of footing in the combined footing system, in which, it is arranged in such a way that, the centroid of the area in contact with the soil lies on the line of action of resultant of the loads.

25. What are the limitations of dynamic formulae?

Limitations of dynamic formula:-

1. Dynamic formula give no indication about probable future settlement or temporary changes in soil structure

2. The formulae do not take into account the reduced bearing capacity of pile when in a group.

3. Energy loses due to vibrations, heat and damage to dolly or packing are not accounted for.

26. What do you understand by plastic equilibrium in soils?

Plastic equilibrium in soils:-

A body of soil is said to be in plastic equilibrium, if every point of it is on the verge of failure.

27. What is critical failure plane?

Critical failure plane:-

Critical failure plane defined as the plane along which the failure occurs in which the shear stress on the plane is less than the maximum shear stress.

28. How piles are classified based on method of installation?

1. End bearing pile,

2. Friction pile,

3. Compaction pile,

4. Tension pile,

5. Anchor pile,

6. Fender pile and dolphins

7. Batter pile,

8. Sheet pile

29. What are the limitations of the dynamic pile load formula?

1. It largely depends on the nature of the ground through which the pile was driven to get down to finished level.

2. It takes very little account of the effect of friction on sides of pile, and this friction tends only to develop later.

30. Why are retaining walls usually designed for active pressure?

From Rankine’s assumption, no-existence of frictional forces at the wall face, the resultant pressure must be parallel to the surface of the backfill. The existence of the friction makes the resultant pressure inclined to the normal to the wall at an angle that approaches the friction angle between the soil and the wall.

31. List the various methods of soil exploration techniques.

1. Pits and trenches

2. Boring a) augur boring b) wash or water boring c) rotary boring d) percussion boring

3. Geophysical methods a) seismic refraction b) electrical resistivity

4. Standard penetration test

5. Static cone penetration test

32. Write short notes on Augur boring.

An augur is a type of tool which is used for understanding the characteristics of the subsurface

soil. Generally there are two types of augurs, a) Manually operated augur

b) Mechanically operated augur

33. Define standard penetration number.

The number of blows required to penetrate 300 mm of the split spoon sampler beyond a seating

drive of 150mm is known as penetration number (N).

34. List the various corrections to be carried out in SPT test.

The two corrections are

a) Dilatancy correction ( Silty sand)

b) Over burden pressure correction ( Granular soil)

35. What are the uses of soil exploration?

a) To select type and depth of foundation for a given structure

b) To determine the bearing capacity of the soil of the selected foundation c) To investigate the safety of the existing structure

d) To establish ground water level

36. What is soil exploration?

The process of collection subsoil sample by an appropriate method to a needed depth and check

those samples for knowing the properties is called soil exploration.

37. List the different types of samplers.

a) Standard split spoon sampler

b) Shelby and thin walled tube sampler c) Denison sampler

d) Piston sampler

e) Scraper bucket sampler

38. List the various parameters affecting the sampling disturbance.

a) Area ratio

b) Inside clearance

c) Outside clearance d) Inside wall friction

e) Position of non return wall

f) Recovery ratio

g) Methods of applying force

39. Write the advantages of SCPT over SPT.

a) There is no need of hammering action, just pushing into the ground. b) No need of bore holes, it is carried out on the ground

c) Engineering properties of the soil like permeability, Shear strength, Compressibility can be

evaluated.

40.Write short notes on spacing of bore holes.

The spacing of bore holes depends upon the variation of subsurface soil in the horizontal

direction. The factors influencing the spacing of bore holes are, a) Type of soil

b) Fluctuation of water table

c) Load coming from structure d) Importance of the structure.

e) Economical feasibility.

41. Safe Bearing Capacity:

Maximum intensity of loading that the foundation will safely carry without the risk of

shear failure of soil irrespective of any settlement that may occur.

As per Rankine’s formula

D min= P/W[(1-∅)/(1+∅)]

2

Where,

P-safe bearing capacity in N/m2

W = Unit weight of soil in KN / m3 ∅-Angle of repose of soil in degrees

42. List the types of bearing capacity failure:

Vesic (1973) classified the bearing capacity failure into 3 categories

1. General shear failure

2. Local shear failure

3. Punching shear failure

43. Write the components of settlement:

There are three types of settlements are

1. Immediate (or)elastic settlement(Si)

2. Consolidation settlement(Sc)

3. Secondary consolidation settlement(Ss)

44. Write the assumptions in Terzaghi’s bearing capacity theory:

1. The base of the footing is rough

2. The footing is laid at a shallow depth (Df<B) or (Df=B)

3. The load on footing is vertical and uniformly distributed

4. The shear strength of soil is governed by mohr’scolumb equations

5. Z=C+tanϕ

The ratio of L/B is infinite.where L is the the length and B is the width of footing.

45. List the limitations of plate load test:

1. Size effect

2. Scale effect

3. Time effect

4. Interpretation of failure load

5. Reaction load

6. Water table

46. List the various methods of computing elastic settlement:

1. Based on the theory of elasticity

2. Pressure meter method

3. Janhu –Bjerram method

47. Schmentmann’s method7. Define general shear failure:

At a certain load intensity equal to qu (ultimate bearing capacity), the settlement increases

suddenly .A shear failure occurs in the soil at the load and failure extend to the ground

surface. This type failure is known as general shear failure. A heave on the sides is always

observed in general shear failure. This type of failure occur when a strip footing rest on stiff

clay or a dense sand.

48. Immediate settlement (Si):

Immediate or elastic settlement takes place during immediately after the construction of the

structure. It is also known as the distortion settlement, as it is due to distortions within the

foundation soil.

49. Consolidation settlement (SC):

This component of settlement occurs due to gradual expulsion of water from the voids of

soil. This component is determined using Terzaghi’s theory of consolidation

50. Write the limitations of Terzaghi theory:

1. Terzaghi theory is valid only for shallow fountations (Df<B or Df=B) .

2. The load on the footing won’t be vertical and uniformly distributed as assumed

by

Terzaghi