Test: Soil Mechanics- 8 - Civil Engineering (CE) MCQ

Percussion boring can be employed in most types of soil, including those containing cobbles and boulders. However, there is generally some disturbance of the soil below the bottom of the borehole, from which samples are taken, and it is extremely difficult to detect thin soil layers and minor geological features with this method.

The advantage of rotary drilling in soils is that progress is much faster than with other investigation methods and disturbance of the soil below the borehole is slight. Also typical core diameters are 41, 54 and 76 mm, but can range up to 165 mm.

Wash boring can be used in most types of soil and below water table but progress becomes slow if particles of coarse gravel size and larger are present.

The test boring using augers can be used for soil exploration work for highways and small structures. Information regarding the types of soil present at various depths is obtained by noting the soil that holds to the auger. The soil samples collected in this manner are disturbed, but they can be used to conduct laboratory tests such as grain size determination and Atterberg limits.

When clayey soils with high plasticity are treated with lime, the plasticity index is decreased. The fine clay particles get flocculated and indicate a different grain size distribution. There is slight decrease in liquid limit and considerable increase in plastic limit.

Undisturbed samples → if soil structure, mineral content and moisture content of the soil remains unchanged while sampling then it is called Undisturbed sample.

Representative sample → If soil structure is modified but mineral content and water, content remains unchanged while sampling then it is called representative sample.

Non – Representative Sample → If soil structure mineral content and water content all get modified then it is called non – representative sample.

To determine K, C, ϕ, C_c, M_v undisturbed sample should be used.

To determine consistency limit, specific gravity, particle size analysis either undisturbed or representative sample may be used.

Concept:

% inside clearance = 

where D₃ = Inside diameter of sampling tube

D₁ = Inside diameter of cutting edge shoe

Calculation:

C_i = 2.67%

It must lies between 1 to 3%

SKEMPTON’S Method:

 This theory is applicable only for C-soils but it can be applied for shallow and deep footings both. In this theory, base resistance and side resistance both are considered. The net ultimate bearing capacity is given by:

q_nu=CN_c

Where,

N_c = Skempton’s bearing capacity factor which depends upon D_f/B ratio

Hence for a clay layer, the net ultimate bearing capacity (By Skempton) at the base of the footing is directly proportional to the cohesion of the soil.

Case 1: When D_f/B = 0,
 

Nc = 5.0 (strip footing)

= 6.0 (Square/Circular/Rectangular/ Raft)

Case 2: When 0 D_f/B < 2.5

Case 3: When  
 

N_c = 7.5 (Strip footing)

Nc = 9.0 (Square/circular/rectangular/raft)

Therefore, when B = D_f , case 2 is valid

 

Local shear failure:

This type of failure is seen in relatively loose sand and soft clay.

Some characteristics of local shear failure are:

1. Failure is not sudden and there is no tilting of footing.

2. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed

3. Failure surface is not well defined

4. Failure is progressive

5. In load-settlement curve, there is no well-defined peak

6. Failure is characterized by considerable settlement directly beneath the foundation

7. A significant compression of soil below the footing and partial development of plastic equilibrium is observed.

8. Well-defined wedge and slip surfaces only beneath the foundation.

For eccentric condition, effective area is taken to be

Effective area (A') = L' × B'

L' = L – 2e_x

B' = B – 2e_y

A' = L' × B'

L' = L – 2e_x

L' = 3 – 2 × 0.2

L' = 2.6 m

B' = B – 2e_y

B' = 2 – 2 × 0.3

B' = 1.4 m

A' = 2.6 × 1.4

A' = 3.64 m²

Concept:

Ultimate bearing capacity given by Terzaghi for rectangular footing is:

Where,

B = width of footing

L = Length of footing

c = Cohesion (kN/m²)

N_c, N_q, N_γ = Bearing capacity factors

γ = unit weight of the soil

D_f = Depth of footing

Net ultimate Bearing capacity

q_nu = q_u - γD_f

Net safe Bearing capacity

q_u = 4560 + 655.5 + 172.9 B

q_nu = 5215.5 + 172.9 B - γD_f

q_nu = 5215.5 + 172.9 B – 19 × 1.5

q_nu = 5187 + 172.9 B

q_safe = 2074.8 + 69.16 B + 28.5

800 = (2103.3 + 69.16 B) B²

B = 0.61 m

L = 1.5 × 0.61 = 0.915 m

Area of footing = L x B = 0.558 m²

For raft Foundation [By Terzaghi’s]

Ultimate Bearing Capacity is given by:

For pure clay N_γ = 0

B = 3 m

L = 4 m

N_C = 5.7

N_q = 1.0

q_u = 1450 kN/m²
 C × 5.7 + 19 × 3 × 1.0 + 0

1450 = 1.225 × 5.7 × C + 57

C = 199.49 kN/m²

Now By Skempton:

Net ultimate Bearing capacity is given by:

q_nu = CN_C

where,
N_c = 5.0 × 1.15 × 1.2

N_C = 6.9

q_nu = CN_C

q_nu = 199.49 × 6.9

q_nu = 1376.48 kN/m²

Concept:

By Hiley’s formula

Ultimate load carrying capacity of the pile is given by:

 

Where,

η_n = efficiency of hammer = 1.0 (drop hammer)
η_b = efficiency of hammer blow = 
 

e = coefficient of restitution

p = weight of pile

w = weight of hammer

c = constant (elastic compression between pile and soil)

s = set (penetration of pile per blow of hammer)

h = height of fall

calculation:

 

W = 24000 × 9.81 = 235.44 kN

ep = 0.30 × 47.71 = 14.31 kN

w > ep

 

η_h = 1.0

s = 0.6 cm/blow

c = 2.3 cm

B = (n – 1)s + d

B = (6 – 1) × 0.8 + 0.4

B = 4.0 + 0.4 = 4.4 m

Ultimate load carrying capacity of single pile:

Q_up = q_b A_b + q_s A_s

Q_b = base resistance of pile = 9c

c = cohesion at base of the pile

ϕ = 0° (clay)

S_base = c = 200 kN/m²

q_s = average skin friction resistance

Q_up = 1206.37 kN

Ultimate load carrying capacity of pile group

 

= 9 × 200 × (4.4)² + 0.5 × 130 × 4 × 4.4 × 12

Q_ug = 48576 kN

Safe load on pile group = 

So, safe load on pile group will be lesser of two. Hence safe load = 14476.44 kN