FE UNIT 1 foundation engineering marks and answer.docx

UNIT 2 – SHALLOW FOUNDATION
ANNA UNIVERSITY 2 MARKS AND ANSWERS
1. What are the various factors to be considered in fixing the depth of foundation?
Specify the minimum depth of foundation as per IS codal provision.
How Depth of footing is decided? (AUC MAY/JUNE – 2013)
Depth of footing is decided after considering the following factors
a) Frost depth
b) Zone of volume change
c) Slopping Ground
d) Adjacent footing / property line
e) Scour Depth
f) Root Holes
g) Underground Defect or utilities
h) Water table
i) Top soil
j) Minimum depth
IS 1904 – 1978 specifies that all foundation should extend to a depth of atleast 50 cm
below natural ground surface.
Minimum depth of foundation is determined using the following formula
Df (min) =
q
γ [1−sin❑
1+sin❑ ]
2
2. Specify maximum permissible settlement for isolated and raft foundation on
sand and clayey soil. (AUC NOV/DEC – 2006)
Maximum permissible settlement according to IS 1904 – 1966 is
Type of Foundation Settlement on sand Settlement on Clay
Isolated Foundation 40 mm 65 mm
Raft Foundation 40 – 65 mm 65 – 100 mm
3. A strip footing 0.8 m wide carries a load of 80 kN/m and is founded on a clayey
soil of unconfined compressive strength of 100 kPa. Find the factor of safety
against shear failure as per BIS – 6403 ignoring depth factor. (AUC NOV/DEC –
2006)
4. The settlement of plate of size 300 mm on sand for a particular loading intensity
is 12.8 mm. Find the settlement of foundation of size 1.5 m on the same sand for
the same loading intensity. (AUC NOV/DEC – 2006)
Sf = Sp [B(Bp+0.3)
Bp(B+0.3)]
2
5. What are the requirements to be satisfied for locating a footing in sloping
ground? (AUC MAY/JUNE – 2007)
a. If a footing is located adjacent to a sloping ground, the line joining the base of
adjacent footing should be inclined at an angle 300
with horizontal or its slope
is 1 vertical and 2 horizontal.
b. The minimum distance from the lower edge of footing to the sloping ground
surface should be 90 cm.

6. Write down the various components of settlement. (AUC MAY/JUNE –
2006&2009)
Define settlement and list its components. (AUC NOV/DEC – 2010) State the
three types of settlements considered for the calculation of the total settlement of
a footing. (AUC APRIL/MAY – 2008)
The downward vertical movement experienced by a structure when the soil below the
structure compresses is called settlement.
There are three types of settlements are
a) Immediate (or)elastic settlement(Si)
b) Consolidation settlement(Sc)
c) Secondary consolidation settlement(Ss)
S = Si + Sc + Ss
7. What is net pressure intensity? (AUC MAY/JUNE 2007 & NOV/DEC – 2008)
Define net ultimate bearing capacity. (AUC NOV/DEC – 2009)
It is the bearing capacity in excess of the effective overburden pressure.
qnd = qd – qo = qd – γDf
Where γ = unit weight,Df=depth of footing (or) depth of over burden soil,
qd=ultimate bearing capacity.
8. What is safe bearing capacity? (AUC MAY/JUNE 2007 & NOV/DEC – 2008)
It is the bearing capacity at which neither the soil foils in shear nor settlement exceeds
the allowable limit.
The least qna &qns is taken as allowable bearing capacity (or) safe bearing
capacity (qs)
(i) Qs=qns if qna > qns
(ii) Qs=qns if qns > qna
9. What is the influence of size on bearing capacity of a surface continuous footing
resting on a purely cohesive soil as per BIS 6403? (AUC NOV/DEC – 2008)
From table 2 of Is 6403 -1981 for continuous footing
Sc=1
Sq=1
Sγ=1
10. Say true or false and justify your answer:
In Terzaghi’s bearing capacity theory, as the shearing resistance above the base
of the footing is ignored/ neglected, the bearing capacity is independent of depth
of footing. (AUC NOV/DEC – 2007)
False, bearing capacity is dependent of depth of footing. Although the shearing
resistance of the soil is ignored, overburden pressure is taken into consideration as
γDf.
11. Plate load test is not applicable for heterogeneous soils. Why? (AUC NOV/DEC –
2007)
The result of the plate load test reflect the strength and settlement characteristics of
the soil within the pressure bulb. As the pressure bulb depends on the loaded area, it is
much deeper for the actual foundation as compared to that of the plate. The plate load
test does not truly represent the actual condition for large depth. Hence it is not
applicable for heterogeneous soil.
12. What are the methods of minimizing settlement? (AUC NOV/DEC – 2007)
a. Compacting pile
b. Grouting

c. Vibro Compaction
d. Dynamic Compaction
e. Precompression
f. Sand drain & Wick drain
13. What are the types of shear failure in foundation? (AUC NOV/DEC – 2007 &
2009)
What are the different types of foundation failure? Draw their sketches (AUC
NOV/DEC – 2008)
a. General shear failure
b. Local shear failure
c. Punching shear failure
14. What is the maximum allowable settlement in raft foundation for both clay and
sand? (AUC NOV/DEC – 2007)
Maximum allowable settlement for clay:- 45 – 65 mm
Maximum allowable settlement for sand:- 65 – 100 mm
15. What is gross pressure intensity? (AUC APRIL/MAY – 2008)
What is ultimate bearing capacity? (AUC APRIL/MAY – 2008)
The ultimate bearing capacity of the soil is the maximum bearing capacity of the soil
at which the soil fails by shear. It is also known as gross pressure intensity.
16. What is the reason for permissible settlement of foundation on clay is greater
than sand? (AUC NOV/DEC – 2008 & 2010)
The permissible settlement of clay is higher because progressive settlement on clay
soil permits better strain adjustment in the structural member.
17. Define punching shear failure. (AUC MAY/JUNE – 2009)
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.
18. What are the assumptions made in Terzaghi’s bearing capacity theory? (AUC
NOV/DEC – 2009)
a. The soil fails in general shear failure mode.
b. The base of the footing is rough.
c. The footing is laid at a shallow depth i.e Df < B
d. The shear strength of the soil above the base of the footing is neglected. The
soil above the base is replaced by a uniform surcharge, γDf.
e. The load on the footing is acting vertically and is uniformly distributed.
f. The footing is long i.e, L>>B
g. The shear strength of the soil is governed by Mohr – Column equation.
19. What are the factors affecting the bearing capacity of soil? (AUC NOV/DEC –
2010)
a. Nature of soil / Type of soil
b. Property of soil
c. Depth of footing or Foundation
d. Shape and Size of footing
e. Water table
20. Write any two limitations of plate load test. (AUC NOV/DEC – 2010)
a. Time Effect: Plate load test is essentially a test for short duration. For clayey
soil it does not give the ultimate settlement.

b. Interpretation of failure load: The failure load is not well defined except in
the case of a general shear failure.
21. How would you estimate the immediate settlements of foundations on clay?
(AUC MAY/JUNE – 2013)
Si = qB [1−µ2
Es
]
2
I
Where q - pressure intensity or bearing capacity
B - Width of footing
µ - Poisons ratio
Es - Young’s Modulus or Modulus of elasticity of soil
I - Influence factor
22. Differentiate between immediate and consolidation settlement
Description Immediate Settlement Consolidation Settlement
Definition
The settlement which occurs
immediately during or after
construction of structure or
application of load is called
immediate settlement
The settlement which occurs due
to the expulsion of water from
pores of soil is called
consolidation settlement
Type of soil
All types of soil Clay & Organic soil prone to
consolidation
Formula used
Si = qB [1−µ2
Es
]
2
I Sc = Cc
H
1+e
log
(σo+△ σ
σo
)
23. Give the reason why allowable or permissible settlement of raft foundation is
greater than isolated footing.
The permissible settlement of raft foundation is higher because the raft bridges over
soft patches of soil which results in reduction in differential settlement.
24. What are the factors affecting the settlement of soil? (AUC NOV/DEC – 2010)
a. Elastic property of soil
b. Shape of footing
c. Rigidity of footing
d. Contact pressure
e. Width of footing
f. Compressibility characteristics of soil
g. Initial condition of soil
h. Degree of saturation
i. Over consolidation ratio
j. Time available for settlement
k. Thickness of soil layer
l. Load dispersion angle
25. Differentiate between uniform and differential settlement
Uniform Settlement Differential Settlement
When the building’s foundation settles
equally in different areas after
construction then it is called uniform
settlement
When the building’s foundation settles
unequally in different areas after
construction then it is called uniform
settlement
It does not causes any damage to super
structure
It does causes damage to super structure due
to the additional moment developed

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