(SEM V) THEORY EXAMINATION 2022-23 GEOTECHNICAL ENGINEERING

SECTION A – Short Answer Type Questions (2 Marks each)

(a) What do you understand by Index Properties of Soil?

Index properties are the basic characteristics that help classify and identify soils and understand their engineering behavior. They do not directly measure strength but indicate composition and consistency.

Major index properties include:

Moisture Content (w): Ratio of water weight to dry soil weight.

Specific Gravity (Gs): Ratio of soil solids’ density to water density.

Atterberg Limits:

Liquid Limit (LL) – water content at which soil changes from plastic to liquid state.

Plastic Limit (PL) – water content at which soil starts to crumble when rolled into threads.

Shrinkage Limit (SL) – lowest moisture content before further drying causes volume reduction.

Grain Size Distribution: Classifies soil as clay, silt, sand, or gravel.

Importance: Used in soil classification systems like IS, USCS, and AASHTO.

(b) Define Thixotropy.

Thixotropy is the property of certain clays to lose strength when disturbed (due to remolding or vibration) and regain strength over time when left undisturbed.

It occurs in sensitive clays, particularly in marine and organic deposits.

Caused by rearrangement of particles and resettling of water films after remolding.

Thixotropic recovery helps soil regain shear strength gradually without external loading.

Example: Soft marine clay near coastal regions exhibiting strength recovery after pile driving.

 SECTION B – Long Answer Type Questions (10 Marks each)

(a) Explain Flow Net and Its Applications.

A Flow Net is a graphical representation of two sets of orthogonal curves:

Flow Lines: Indicate the path followed by seepage water.

Equipotential Lines: Lines of equal hydraulic head.

Together they form curvilinear squares showing the pattern of seepage flow through soil.

Construction Steps:

Draw boundaries (impervious base, phreatic surface).

Assume approximate flow lines and equipotential lines so that they intersect nearly at right angles.

Adjust until approximate curvilinear squares are formed.

Applications:

Seepage calculation:

• q=k×h×NfNdq = k \times h \times \frac{N_f}{N_d}q=k×h×Nd​Nf​​

where kkk = permeability, hhh = total head loss, NfN_fNf​ = number of flow channels, NdN_dNd​ = number of potential drops.

Seepage pressure and uplift pressure beneath dams.

Determining pore water pressure in earth dams.

Analyzing hydraulic gradients for piping safety.

Flow nets are vital for seepage control and stability analysis of hydraulic structures.

(b) Explain the Process Involved in Determination of Compaction in IS Light Compaction Test.

The IS Light Compaction Test (Proctor Test) determines the optimum moisture content (OMC) and maximum dry density (MDD) of soil.

Procedure:

Equipment:

Mould of 1000 cc capacity.

Rammer (2.6 kg weight, 310 mm drop).

Soil Preparation:

Dry soil is mixed with varying water contents (about 5 samples).

Compaction:

Each sample compacted in 3 equal layers, each receiving 25 blows.

Measurement:

Weigh mould + compacted soil → calculate bulk density.

Determine moisture content of each sample.

Computation:

Calculate dry density for each moisture content.

Plot Curve:

Dry density vs moisture content → gives a parabolic curve.

The peak point indicates:

OMC = moisture content at maximum dry density.

MDD = corresponding dry density.

Applications:
Used for embankment construction, road subgrade design, and earth dams to ensure required soil compaction and stability.

SECTION C – Very Long Answer Type Questions (10 Marks each)

(a) Define Piping Failure and Its Measures for Rectification.

Piping failure occurs when seepage forces within soil exceed resisting forces, causing soil particles to be washed away, forming underground channels (pipes).
It usually occurs below hydraulic structures like dams and weirs where water flows from high to low head.

Mechanism:

Uplift pressure and hydraulic gradient increase under the structure.

Once critical hydraulic gradient (icr) is reached:

• icr=G−11+ei_{cr} = \frac{G - 1}{1 + e}icr​=1+eG−1​

where G = specific gravity, e = void ratio.

Soil loses contact and gets eroded by seepage flow.

Preventive Measures:

Reduce seepage gradient: Provide a long seepage path using impervious cutoffs or drainage filters.

Filter design: Prevent movement of fine soil particles.

Pitched stone protection: To resist exit velocity.

Relief wells and drainage blankets: Release uplift pressure safely.

Importance: Prevents catastrophic failure of hydraulic structures and loss of foundation material.

(b) Derive the Expression for Terzaghi’s One-Dimensional Consolidation.

Terzaghi’s theory explains how excess pore water pressure dissipates over time under constant total stress, leading to gradual settlement.

Assumptions:

Soil is homogeneous, fully saturated, and compressible.

Compression occurs only in the vertical direction.

Darcy’s law governs flow.

Deformation and flow are one-dimensional.

Derivation:
Let,

u=u =u= excess pore pressure,

z=z =z= depth,

t=t =t= time,

cv=c_v =cv​= coefficient of consolidation.

Differential Equation:

∂u∂t=cv∂2u∂z2\frac{\partial u}{\partial t} = c_v \frac{\partial^2 u}{\partial z^2}∂t∂u​=cv​∂z2∂2u​

where

cv=kmvγwc_v = \frac{k}{m_v \gamma_w}cv​=mv​γw​k​

(k = permeability, mvm_vmv​ = coefficient of volume compressibility).

Solution:
Using boundary and initial conditions:

uu0=4π∑n=0∞1(2n+1)sin⁡[(2n+1)πz2H]exp⁡[−(2n+1)2π2cvt4H2]\frac{u}{u_0} = \frac{4}{\pi} \sum_{n=0}^{\infty} \frac{1}{(2n+1)} \sin \left[ \frac{(2n+1)\pi z}{2H} \right] \exp \left[-\frac{(2n+1)^2\pi^2 c_v t}{4H^2}\right]u0​u​=π4​n=0∑∞​(2n+1)1​sin[2H(2n+1)πz​]exp[−4H2(2n+1)2π2cv​t​]

This gives the distribution and dissipation of pore pressure with time.

Practical Use:
Determines time rate of settlement and helps design foundations on soft clay layers safely.