THEORY EXAMINATION (SEM–VI) 2016-17 EARTH AND EARTH RETAINING STRUCTURES

EARTH AND EARTH RETAINING STRUCTURES – ECE022

B.Tech (SEM VI) | Section-wise Solved Answers

SECTION – A

(10 × 2 = 20 Marks)

(a) Types of dams

Dams are classified as gravity dams, arch dams, buttress dams, earth dams, and rock-fill dams. Earth and rock-fill dams are most common due to economical construction.

(b) Piping in dams

Piping is a form of internal erosion caused by seepage forces that carry soil particles from within the dam, leading to tunnels and possible failure.

(c) Types of earth retaining walls

Earth retaining walls include gravity walls, cantilever walls, counterfort walls, sheet pile walls, reinforced earth walls, and mechanically stabilized earth (MSE) walls.

(d) Seepage discharge in isotropic earth dam

Seepage discharge is calculated using Darcy’s law:

q=k⋅H⋅Nfq = k \cdot H \cdot N_fq=k⋅H⋅Nf​

where kkk is coefficient of permeability, HHH is head loss, and NfN_fNf​ is number of flow channels.

(e) Stability analysis of earth retaining structures

Stability analysis checks the safety of retaining structures against sliding, overturning, bearing capacity failure, and overall slope failure.

(f) Soil nailing

Soil nailing is a ground improvement technique where steel bars (nails) are inserted into soil to increase shear strength and stability of slopes or excavations.

(g) Preliminary section of an earth dam

The preliminary section is the initial design layout of an earth dam showing crest width, slopes, freeboard, and zoning before detailed analysis.

(h) Pressure ratio

Pressure ratio is the ratio of pore water pressure to total overburden pressure. It indicates the influence of pore pressure on soil stability.

(i) Soil–reinforcement interface friction

It is the frictional resistance developed between soil particles and reinforcement material, which governs load transfer and stability in reinforced soil systems.

(j) Fibre reinforced soil

Fibre reinforced soil contains discrete fibres mixed with soil to improve shear strength, ductility, and resistance to cracking.

SECTION – B

(Attempt any five – 5 × 10 = 50 Marks)

(a) Mechanically Stabilized Earth (MSE) retaining wall

An MSE wall consists of soil reinforced with metallic or geosynthetic strips. The reinforced soil mass acts as a gravity structure and resists earth pressure through tensile reinforcement.

(b) Failure of MSE walls & stability checks

Failures occur due to reinforcement rupture, pullout, sliding, overturning, or bearing failure.
Stability checks include internal stability (reinforcement strength and spacing) and external stability (sliding, overturning, bearing capacity).

(c) Components and embankment details of earth dam

Earth dams consist of impervious core, shell, filters, drains, and cutoff trench. These components control seepage and ensure stability. (Neat sketch recommended in exam)

(d) Effect of excess seepage & pore water pressure

Excess seepage increases uplift pressure and reduces effective stress, leading to slope instability, piping, and possible failure of earth dams.

(e) Effect of submergence on retaining structures

Submergence reduces effective stress and shear strength of soil, increasing lateral earth pressure and decreasing stability of retaining structures.

(f) Failure types of reinforced earth foundations

Failures include rupture of reinforcement, pullout failure, sliding, bearing failure, and global instability.

(g) Design criteria of reinforced soil foundation

Design is based on reinforcement type, spacing, embedment length, tensile strength, soil properties, and safety against shear and bearing failure.

(h) Need for sufficient reinforcement length

Extra reinforcement length beyond failure surface ensures anchorage and load transfer. Reinforcement increases bearing capacity by restricting shear failure and spreading load.

SECTION – C

(Attempt any two – 2 × 15 = 30 Marks)

(3) Factor of safety of embankment

Given:
c′=28 kN/m2c' = 28 \, \text{kN/m}^2c′=28kN/m2, ϕ′=15∘\phi' = 15^\circϕ′=15∘
Mobilized values:
cm=20 kN/m2c_m = 20 \, \text{kN/m}^2cm​=20kN/m2, ϕm=12∘\phi_m = 12^\circϕm​=12∘

Factor of safety w.r.t cohesion:

Fc=c′cm=2820=1.4F_c = \frac{c'}{c_m} = \frac{28}{20} = 1.4Fc​=cm​c′​=2028​=1.4

Factor of safety w.r.t friction:

Fϕ=tan⁡15∘tan⁡12∘≈1.25F_\phi = \frac{\tan 15^\circ}{\tan 12^\circ} \approx 1.25Fϕ​=tan12∘tan15∘​≈1.25

Using average effective normal stress = 120 kN/m², true factor of safety is computed by combining mobilized shear strength components.

(4) Effectiveness of reinforcing elements

Effectiveness depends on reinforcement length, tensile strength, stiffness, surface roughness, orientation, and soil type.
Example: Geogrids are effective in granular soils due to interlocking.

(5) Failure modes & bearing capacity improvement

Failure modes include shear failure, pullout failure, and excessive settlement.
Reinforcement improves bearing capacity by redistributing stresses and reducing settlement.
Reinforced soil shows higher load capacity and lower settlement compared to unreinforced soil.