SUB: GEOTECHNICAL ENGINEERING
MODULE-52C
MODULE- 52C Foundation, types-foundation, design requirements. Shallow foundations-bearing capacity, effect of shape, water table and other factors
________________________________________________________________ 1.
The ultimate bearing capacity of a soil is 300 kN/ . The depth of foundation is 1m and unit weight of soil is 20 kN/ . Choosing a factor of safety of 2.5, the net safe bearing capacity is (A) 100 kN/
(B) 112 kN/
(C) 80 kN/
(D) 100.5 kN/
Ans:
Net ultimate Bearing capacity =300-20X1 =280 kN/ Net Safe Bearing Capacity = 280/2.5 =112 kN/
2.
Two footings, one circular and the other square are founded on the surface of a purely cohesion-less soil. The diameter of the circular footing is same as that of the side of the square footing. The ratio of their ultimate bearing capacities is (A) 3/4 (B) 4/3 (C) 1.0 (D) 1.3 Ans:
For circular footing,
For square footing,
As soil is cohesion-less is resting on surface.
and
as footing
Ratio=0.3/0.4 = 3/4
JH ACADEMY
1
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 3.
Figure shows the geometry of a strip footing supporting the load bearing walls of a three storied building and the properties of clay layer.
If the pressure acting on the footing is 40 kPa, the consolidation settlement of the footing will be (a) 0.89 mm (b) 8.9 mm (c) 89.0 mm (d) None of these Ans: As the load dispersion is 1:2, and at the Centre of clay layer, Area of spreading the load =1.2 +3=4.2
18 X1 +3X(20-9.81) = 48.57
0.31 m
JH ACADEMY
2
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 4.
Figure shows the geometry of a strip footing supporting the load bearing walls of a three storied building and the properties of clay layer.
If the elastic modulus and the Poisson's ratio of the clay layer are respectively 50×103 Kpa and 0.4 and if the influence factor for the strip footing is 1.75, the elastic settlement of the footing will be (a) 0.41 mm (b) 1.41 mm (c) 14.1 mm (d) None of these Ans:
5.
Two circular footings of diameters D1 and D2 are resting on the surface of the same purely cohesive soil. The ratio of their gross ultimate bearing capacities is (a) D1/D2 (c)
(b) (d)
1.0 D2/D1
Ans:
For circular footing,
If c=0, surface is 1
6.
and as the footings are resting on does not the Therefore it depend on diameter. So the ratio
There are two footings resting on the ground surface. One footing is square of dimension 'B'. The other is strip footing of width 'B'. Both of them are subjected to a loading intensity of q. The pressure intensity at any depth below the base of the footing along the centerline would be (a) (b) (c) (d)
Equal in both footings Large for square footing and small for strip footing Large for strip footing and small or square footing More for strip footing at shallow depth (< B) and more for square footing at large depth (>B)
Ans: (c)
JH ACADEMY
3
SUB: GEOTECHNICAL ENGINEERING MODULE-52C
7.
A strip footing (8m wide) is designed for a total settlement of 40mm. The safe bearing capacity (shear) was 150 kN/m2 and safe allowable soil pressure was 100 kN/m2. Due to importance of the structure, now the footing is to be redesigned for total settlement of 25 mm. The new width of footing will be (a) (c)
5m 12 m
(b) 8 m (d) 12.8 m
Ans: 8X
8.
= 12.8 m
The bearing capacity of a rectangular footing of plan dimensions 1.5 m x 3m resting on the surface of a sand deposit was estimated s 600 kN/ when the water table is far below the base of the footing. The bearing capacities in kN/ when the water level rises to depths of 3m, 1.5 m and 0.5 m below the base of the footing are (a) 600, 600, 400 (b) 600, 450, 350 (c) 600, 500, 250 (d) 600, 400, 250 Ans: If water table is below the 1.5 m or at the 1.5 m from the base of footing, the bearing capacity will be same.
, =40 Now water table is 0.5 m below the footing. Then =0.5 X 20 + 1.0 X 10 = 20 Let us take
9.
A column is supported on a footing as shown in the figure below. The water table is at a depth of 10m below the base of the footing.
The net ultimate bearing capacity (kN/m2) of the footing based on Terzaghi's bearing capacity equation is (A) 216 (B) 432 (C) 630 (D) 846 Ans:
= 18 X 1X 23 + 0.4 X 18 X 1.5 X 20 = 630
JH ACADEMY
4
SUB: GEOTECHNICAL ENGINEERING MODULE-52C
10. A column is supported on a footing as shown in the figure below. The water table is at a depth of 10m below the base of the footing.
The safe load (kN) that the footing can carry with a factor of safety 3 is (A) 282 (B) 648 (C) 945 (D) 1269 Ans:
= 18 X 1X 23 + 0.4 X 18 X 1.5 X 20 = 630 =
=
= 210 P= 210 X 1.5 X 3 = 945 KN
11. The unconfined compressive strength of a saturated clay sample is 54 kPa. The value of cohesion for the day is (A) zero (B) 13.5 kPa (C) 27 kPa (D) 54 kPa Ans: (C) 12. . If a square footing of size 4 m x 4 m is resting on the surface of a deposit of the above clay. The ultimate bearing capacity of the footing (as per Terzaghi's equalion) is (A) 1600 kPa (B) 316kPa (C) 200 kPa (D) 100 kPa Ans: Cohesion=27 KPa 1.3 X 5.7 X 27 = 200
13. Likelihood of general shear failure for an isolated footing in sand decreases with (A) Decreasing footing depth (B) Decreasing inter-granular packing of the sand (C) Increasing footing width (D) Decreasing soil grain compressibility Ans: (C)
JH ACADEMY
5
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 14. An embankment is to be constructed with a granular soil (bulk unit weight = 20 kN/m3) on a saturated clayey silt deposit (un-drained shear strength = 25 kPa). Assuming un-drained general shear failure and bearing capacity factor of 5.7, the maximum height (in m) of the embankment at the point of failure is (A) 7.1 (B) 5.0 (C) 4.5 (D) 2.5 Ans:
15. Four columns of a building are to be located within a plot size of 10 m x 10 m. The expected load on each column is 4000 KN. Allowable bearing capacity of the soil deposit is 100 . The type of foundation best suited is (A) isolated footing (C) pile foundation
(B) raft foundation (D)combined footing
Ans: (C) 16. Bearing capacity of a soil strata supporting a footing of size 3m will not be affected by the presence of ground water table located at a depth which is a) b) c) d)
1.0m below the base of the footing 1.5m below the base of the footing 2.5m below the base of the footing 3m below the base of the footing
Ans (d) 17. No tension should develop at the base of the rectangular well foundation or at any horizontal section within the well. For no tension at the base, the resultant of ( total active thrust) and W( weight of soil and well above the base) must pass through middle a) b) c) d)
Half of the base Third of the base Quarter of the base Edge of the base
Ans (b) 18. Consider the following statements associated with local shear failure of soils: 1. Failure is sudden with well-defined ultimate load. 2. The failure occurs in highly compressible soils. 3. Failure is preceded by large settlement. Which of these statements are correct? a)
1,2 and 3
c) 2 and 3
b) 1 and 2 d) 1 and 3
Ans (c)
JH ACADEMY
6
SUB: GEOTECHNICAL ENGINEERING MODULE-52C
19. Rafts resting on sands can be allowed double of the allowable soil pressure when a) b) c) d)
Permissible settlement is doubled Length is doubled Depth factor is increased Water table is lowered
Ans (b) 20. As per Terzaghi’s equation, the bearing capacity of strip footing resting on cohesive soil (c =10 KN/ ) for unit depth and unit width (assume as 5.7) is a)
47 KN/
b) 57KN/
b) 67KN/
d) 77KN/
Ans:
21. A raft of 6m 9m is founded at a depth of 3m in a cohesive soil having c=120KN/ . The net ultimate bearing capacity of the soil using the Terzaghi’s theory will be nearly a)
820 KN/
b)
920 KN/
c)
1036 KN/
d)
1067 KN/
Ans:
(
)
(
)
=820 KN/ 22. Two circular footings of diameters and are resting on the surface of a purely cohesive soil. The ratio =2. If the ultimate load carrying capacity of the footing of diameter is 200 KN/ , then the ultimate bearing capacity(in KN/ of the footing of diameter will be a)
100
b) 200
c) 314
d) 571
Ans: (b)
JH ACADEMY
7
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 23. The minimum bearing capacity of a soil under a given footing occurs when the groundwater table at the location is at a) b) c) d)
The base of the footing The ground level A depth equal to one-half the width of footing A depth equal to the width of footing
Ans: (b) 24. If two foundations, one narrow and other wide, are resting on a bed of sand carrying the same intensity of load per unit area, then which one is likely to fail early? a) b) c) d)
Narrow foundation Wider foundation Both will fail simultaneously Difficult to judge since other conditions are unknown
Ans: (b)
25. The contact pressure distribution under a rigid footing on a cohesion less soil would be a) b) c) d)
Uniform throughout Zero at centre and maximum at edges Zero at edges and maximum at centre Maximum at edges and minimum at centre
Ans: (c) 26. A footing of 3m 3m size transmits a load of 1800KN. The angle of load dispersion in soil 0.5 what is the stress created by the footing load at a depth of 5m? a) 26.12 KN/ b) 28.12 KN/
b) 27.12 KN/ d) 29.12 KN/
Ans: At 5 m depth dispersion length is 3+5=8 m. Stress =
JH ACADEMY
= 28.12 KN/
8
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 27. When a load test was conducted by putting a 60cm square plate on top of a sandy deposit, the ultimate bearing capacity was observed as 60KN/ . What is the ultimate bearing capacity for a strip footing of 1.2m width to be placed on the surface of the same soil? a)
75KN/
b) 120KN/
c) 150KN/
d) 160KN/
Ans:
28. In case of footing on the surface or shallow depth in very dense sand which one of the following types of failure is likely to occur? a) b) c) d)
Punching shear failure Local shear failure General shear failure Any of the above three Ans: (c)
29. The bearing capacity factors a) b) c) d)
and
are functions of
Width and depth of footing Density of soil Cohesion of soil Angle of internal friction of soil
Ans: (c) 30. Match list-I (type of soil) with list-II (suitable foundation) and select the correct answer using the codes given below the lists: List-I A. Strong soil in surface layer B. Weak surface layer followed by rock at shallow depth below ground C. Swelling soil in surface layer extending up to a few meters below ground level D. Weak heterogeneous surface soil layer List-II 1. Raft foundation 2. Isolated footing 3. End bearing pile 4. Under-reamed piles Codes: A B C D a) 1 4 3 2 b) 2 3 4 1 c) 1 3 4 2 d) 2 4 3 1 Ans: (b)
JH ACADEMY
9
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 31. In which one of the following zones is a logarithmic spiral shape of failure surface assumed in the case of bearing capacity analysis of csoils? a)
Active zone
b) passive zone
c) Radial shear zone
d) surcharge zone
Ans: (c) 32. The contact pressure distribution below rigid footing on the surface of a clay soil is a) b) c) d)
Uniform for the full width Maximum at the centre and minimum at the edges Maximum at the edges and minimum at the centre Of an irregular shape
Ans: (c) 33. Consider the following statements: 1. The bearing capacity of a footing on clay does not significantly get altered by the presence of water table. 2. The bearing capacity of a footing on saturated clay ( ) is a function of its size. Which of these statements is/are correct? a)
1 only
b) 2 only
c) Both 1 and 2
d) neither 1 nor 2
Ans: (d) 34. Consider the following statements: Criteria for satisfactory performance of footings are that the 1. Soil supporting the footing must be safe against shear failure 2. Footing must not settle more than a pre-specified value 3. Footing must be rigid 4. Footing should be above water table. Which of these statements are correct? a)
3 and 4
c) 1 and 3
b) 1 and 2 d) 2 and 4
Ans: (b) 35. For a proposed building, raft foundation, isolated footings and combined footings are being considered. These foundations are to be listed in the decreasing order of preference in terms of performance. Which one of the following is the correct order of listing? a) b) c) d)
Raft foundation-combined footings-isolated footings Isolated footings-raft foundation-combined footings Combined footings- raft foundation-isolated footings Combined footings-isolated footings-raft foundation
Ans: (a)
JH ACADEMY
10
SUB: GEOTECHNICAL ENGINEERING MODULE-52C
36. The net ultimate bearing capacity of a purely cohesive soil a)
Depends on the width of the footing and is independent of the depth of the footing b) Depends on the width as well as the depth of the footing c) Depends on the depth, but is independent of the width, of the footing d) Is independent of both the width and the depth of the footing Ans: (d) 37. A soil has a low allowable bearing capacity. The soil deposit contains compressive loess. A foundation is to be provided for a structure carrying a heavy load. Which one of the following foundation types is to be adopted? a) b) c) d)
Strap footing Continuous footing Raft foundation Combined spread foundation
Ans: (c) 38. Two footings, one circular and the other square, are founded on the surface of a purely cohesion less soil. The diameter of the circular footing is the same as that of the side of the square footing. The ratio between their ultimate bearing capacities will be a)
1.0
b) 1.3
c) 1.33
d) 0.75
Ans: (d)
JH ACADEMY
11
SUB: GEOTECHNICAL ENGINEERING MODULE-52C 39. A multistory building with a basement to be constructed. The top 4m consists of loose silt below which dense sand layer is present upto a great depth. Ground water table is at the surface. The foundation consists of the basement slab of 6m width which will rest on the top of dense sand as shown fig. for dense sand, saturated unit= 20KN/ and bearing capacity factors = 15 and =45. Effective cohesion is zero for both the soils. = 10KN/ . Neglect shape factor and depth factor average elastic modulus E & of depth sand =60 KN/ and 0.3 respectively. Using factor of safety =3, the net bearing capacity (in KN/ ) for the foundation? The foundation slab is subjected to vertical downward stresses equal to net safe bearing capacity derived in the above question. Using influence factor =2.0 and neglecting embedment depth and rigidity corrections, the immediate settlement of the dense sand layer will be
a)
610, 111
b)
320 ,58
c)
983, 126
d)
693, 179
Ans:
JH ACADEMY
12
