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Master Soil Mechanics & Foundation Engineering for GATE Civil Engg

This article on Soil Mechanics & Foundation Engineering has been written by Rohit Sachdeva. He graduated from Delhi College of Engineering (now DTU) in 2012 (a gold medalist in his batch) in Civil Engineering branch. Then he appeared in Civil Engineering (CE) paper in GATE 2017 and secured an All India Rank (AIR) of 93.

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In this blog, I will be discussing THE MOST IMPORTANT subject of Civil Engineering: Soil Mechanics & Foundation Engineering. This subject carries highest weightage amongst all technical subjects in GATE. It is very interesting, highly conceptual & application based, and all chapters in this subject are very important.

The importance of this subject can also be thought as it is one of the most sought-after branches for M.Tech (Civil). Many questions are asked in various interviews on this subject. Any Civil Engineer is incomplete without the thorough knowledge of Soil Mechanics! The application of this subject in field/industry is huge and very critical, hence it is inevitable to understand all the concepts thoroughly.

 

It is highly advised to give maximum time to understand the concepts in this subject, along with at least 2 rounds of thorough revision. There is a lot of theory to be understood & many application based numericals to be solved. Once the concepts are understood clearly, the questions can be answered accurately, hence giving time to this subject is much beneficial. A good preparation here will definitely boost your confidence as well as score.

A rough breakup of questions of various topics in last 30 years in GATE is as follows:

S No Topic No. of Questions
1 mark 2 marks

SOIL MECHANICS

1 Origin of Soil 2
2 Definitions & Properties of Soils 11 10
3 Soil Structures & Clay Mineralogy 6
4 Index Properties of Soils 11 1
5 Soil Classifications 13 5
6 Permeability 7 13
7 Effective Stress 3 8
8 Seepage Pressure & Critical Hydraulic Gradient 5 5
9 Seepage Analysis 9 9
10 Stress Distribution 8 5
11 Consolidation 11 23
12 Compaction 6 3
13 Shear Strength 17 18
14 Earth Pressure Theories 13 10
15 Stability of Slopes 6 16

FOUNDATION ENGINEERING

16 Bearing Capacity of Shallow Foundations 20 18
17 Pile Foundation 8 17
18 Soil Exploration 8 6
19 Sheet Piles 1 1

An analysis of last 5 years of GATE papers reflect that Soil Mechanics & Foundation Engineering carries 13-15 marks, which is huge! Many concepts in Soil Mechanics are linked with each other at different places. It is advised to go slowly into the subject, understanding each concept clearly & then only moving on to the next. There are lots of formulas & empirical relations to by-heart, for which you should maintain formula book & short notes for future revision. Don’t think of skipping any chapter: everything is important & scoring if you study properly.

The textbook(s) to be referred are mentioned in a separate comprehensive blog which you should refer.

Time required for preparation

30-40 days (if you have 8-10 month of preparation) with 4-5 hours daily
17-20 days (if you have 4-5 months of preparation) with 8 hours daily

Let’s start our topic-wise discussion (most important concepts are bold & italics):

SOIL MECHANICS

1. ORIGIN OF SOIL (1/2 DAY)

Just read through the process of soil formation & different types of soils (not in detail).

2. DEFINITIONS & PROPERTIES OF SOILS (2 DAYS)

This is a very important chapter & all the terms defined here will be used throughout the subject. Therefore you should learn all the definitions & their relations by-heart.

Soil as a 3-phase system, basic definitions (water content, the degree of saturation, void ration, porosity, air content, % air voids), unit weights (bulk, dry, saturated, submerged), specific gravities (absolute & mass), relative density, inter-relationship b/w these properties. All these formulas should be noted down in a formula sheet.

Practice lots of numericals & applications like borrow pit-embankment questions & other inter-relationship based questions.

3. SOIL STRUCTURES & CLAY MINERALOGY (1 DAY)

Fundamental units of soil structure (Silica & Gibsite sheets), properties and structure of Kaolinite, Illite & Montmorillonite minterals, flocculated & dispersed structures, composite soils (coarse-grained skeleton & cohesive matrix-fine grained skeleton). Read through these things & revise once in end.

4. INDEX PROPERTIES OF SOILS (3 DAYS)

This is another important chapter with many applications in subsequent chapters also. Index properties are water content, consistency limits, in-situ density, particle size distribution, sensitivity & activity.

Determination of water content (oven drying & pycnometer are most important; read through other methods once).

In-situ density determination contains methods like core-cutter method & sand replacement method. Pycnometer is also used to determine Specific gravity (formula in sheet).

Particle size analysis contains sieve analysis & sedimentation analysis. Concept of % finer, D10, D30, D60, the coefficient of uniformity & curvature, and identification of grading of soil on a graph are important to understand. Pipette method & Hydrometer method (meniscus, temperature & dispersing agent corrections) form part of sedimentation analysis.

Consistency limits and tests to determine them, shrinkage ratio, degree of shrinkage, plasticity index, consistency index, liquidity index, flow index, shrinkage index, toughness index, activity & sensitivity; all are very important and formulas should be noted in formula sheet. Practice numericals.

5. SOIL CLASSIFICATIONS (1 DAY)

IS soil classification system (which adopts Unified soil classification system) is the most important topic in this chapter & is to be studied in detail. Sub-classification of coarse (GW, GP, SW, SP, GM, GC, SM, SC) & fine soils (CL, ML, OL, CI, MI, OI, CH, MH, OH, Pt) into 18 groups; use of plasticity chart (A-line & U-line). Then there are cases where dual symbols are used. Practice questions.

6. PERMEABILITY (2 DAYS)

Darcy’s law & coefficient of permeability, apparent velocity & seepage velocity, factors affecting permeability, intrinsic permeability, effective permeability calculation in stratified soils, laboratory tests to find permeability (constant head, variable head & capillary permeability tests), empirical relations (Kozney-Karman eqn, Allen-Hazen eqn, Llouden’s eqn & consolidation eqn). Learn the concepts very clearly and practice numerical questions.

*Well hydraulics section has been explained in HYDROLOGY blog.

7. EFFECTIVE STRESS (1 DAY)

This chapter forms the basis of chapters at Sl No. 8, 9, 11, 13-16, and is, therefore, very crucial.

Concepts of effective stress, total stress & pore water pressure, capillarity, calculation of effective stress and construction of total stress, pore water pressure & effective stress diagrams for different cases (both short-term & long-term duration).

8. SEEPAGE PRESSURE & CRITICAL HYDRAULIC GRADIENT (1 DAY)

Upward and downward seepage, seepage pressure and force, critical hydraulic gradient, quick sand condition, piping failure. Practice numericals that include the practical examples of calculation of depth of excavations to prevent quick sand condition.

9. SEEPAGE ANALYSIS (1 DAY)

Laplace equation, Flow nets (flow lines & equipotential lines), applications of flow net (to find seepage loss rate, shape factor, to find seepage and uplift pressures at different points, to find exit gradient), phreatic line & drainage filters, Kozney’s equation, permeability in anisotropic soils & modified laplace eqn. Practice numericals in applications of flow net.

10. STRESS DISTRIBUTION (1 DAY)

Methods to find vertical stresses due to external load: Boussinesq’s theory/equation (pressure bulb, horizontal & vertical stress distribution, circular loaded area), Westergaard’s theory/equation, Newmark’s chart & approximate method (2V:1H method). Learn the formulae and prepare formula sheet. Practice numericals from all topics.

11. CONSOLIDATION (4-6 DAYS)

This is one of the most important chapters in soil mechanics. You should understand the theory clearly to attempt numerical questions from this chapter. Theory of consolidation, coefficient of compressibility (av), coefficient of compression index (Cc), coefficient of volume change (mv), determination of Cc based on empirical eqn, classification of soils based on stress theory (normally consolidated, over consolidated), Terzaghi’s theory of 1-D consolidation {coefficient of consolidation (Cv), time factor (Tv) & degree of consolidation(U)}, oedometer test (height of solids method & change in void ratio method), stages of consolidation settlement (determination of initial settlement, primary settlement & secondary consolidation), contact pressure distribution in flexible and rigid footing in different soils. Practice maximum number of numericals from this chapter. Give importance to questions related to calculation of settlement (including that on re-compression index based on stress history), time for settlement in single and double drainage conditions (lab-field type question).

12. COMPACTION (1 DAY)

This is more of a theoretical chapter. Standard & Modified Proctor test, Indian standard light & Indian standard heavy compaction test (learn by heart the weight of rammer, height of fall, number of layers, number of blows per layer & total energy ratio in these tests), Optimum moisture content, Max dry density, compaction curve (comparison of curves for different tests & in different soils), bulking of sand, properties of soils in dry and wet of optimum and their applications, compaction equipment (read through, not in detail), relative compaction.

13. SHEAR STRENGTH (3-4 DAYS)

Basics of Mohr Circle (from SOM), angle of failure plane, Mohr’s theory, Mohr-Coloumb failure envelope, Terzaghi’s theory (shear strength is a function of effective parameters), Direct Shear test, Triaxial test (UU, CU, CD drainage conditions, Mohr circle for each condition for NC & OC soil, stress-strain curve for loose & dense soil, plots of shear strain vs volume change, void ratio & pore pressure, critical void ratio), unconfined compression test, vane shear test (learn formula), Skempton’s pore pressure parameters A & B, stress path, liquefaction.

This is a very important & conceptual chapter from both theoretical & numerical point of view. Practice lot of numericals and understand all the graphs with clarity.

14. EARTH PRESSURE (E.P.) THEORIES (2-3 DAYS)

This chapter deals with calculation of lateral earth pressure (at rest, active or passive). Coefficients of lateral E.P. (Ko, Ka and Kp), major & minor principal stresses and failure wedge for these 3 cases, calculation of Ko.

Rankine’s theory (calculation of Ka & Kp, construction of lateral E.P. diagrams for cases like surcharge, presence of water table, horizontal backfill), Bells’s equation for calculation of lateral E.P. (for c-ɸ soils), depth of tension crack & critical depth (max unsupported depth). Practice numericals for different cases. Just read through Coloumb’s theory & understand the difference b/w assumptions of Rankine’s theory.

15. STABILITY OF SLOPES (2 DAYS)

Forces causing slope failure, types of slopes (infinite or finite & types of failures), driving forces, resisting forces & FOS for different failures, analysis of infinite slopes (both cohesive & cohesionless soil) for different conditions of water table/drainage, analysis of finite slopes (Swedish circle method, friction circle method, ɸu=0 analysis, Taylor’s stability number method). Practice enough numericals of infinite slopes for different W.T conditions & use of different unit weights.

FOUNDATION ENGINEERING

This section is like an entire subject in itself & is very important from interview point of view also. There will be a lot of empirical formulas also, so it is mandatory to maintain a formula sheet for quick revision.

16. BEARING CAPACITY (B.C.) OF SHALLOW FOUNDATIONS (4-5 DAYS)

Understand this chapter conceptually rather than just trying to memorize equations. Maximum number of questions are asked from this chapter, both theoretical & numericals.

Definitions of gross/net bearing pressure, ultimate/net-ultimate/net-safe/safe bearing capacities, Types of shear failure (General/Local/Punching-their properties & comparison), Rankine’s theory for sands, TERZAGHI’s THEORY (Assumptions, zones of failure, B.C. for strip, square, circular & raft/rectangular footings, effect of water table in bearing capacity, modified parameters for local failure), Skempton’s theory for clays (Nc formula for strip/raft & square/circular/rectangular footing), Mayorhoff’s theory, IS Code recommendations. Practice lots & lots of numericals for determinations of B.C. for different W.T. levels & different shapes of footings.

Next section is determination of B.C. based on field methods. It includes Plate load test (bearing pressure & settlement for sands & clays), Standard penetration test & corrections. Methods of finding B.C. based on SPT number (Peck-Hensen eqn, Teng’s eqn, IS Code eqn, Mayorhoff eqn). Practice numericals of plate load test & SPT test properly.

17. PILE FOUNDATION (3-4 DAYS)

Classification of piles (read through), static formula for load carrying capacity of piles (for sands & clays), Dynamic formulas which are empirical (Engineering News Record, Hiley’s formula), Pile load test, Under-reamed piles, negative skin friction, Group action of piles & Pile group efficiency (converse-Labarre formula, Feld’s rule), Settlement of pile group (different types of strata).

18. SOIL EXPLORATION (1 DAY)

This is a theoretical chapter. It contains soil stabilization techniques (compaction, chemical stabilization & geo-reinforcement) and Soil exploration (auger boring, wash boring, percussion drilling, rotary drilling, disturbed, representative & undisturbed sampling, and design parameters of sampler like clearances, area ratio & recovery ratio). Just read this chapter 2 times & go through previous year questions.

19. SHEET PILES (1 DAY)

This uses concept of active & passive E.P. theories. It is based on force & moment equilibrium. Just practice 1 numerical each for cohesionless soil, cohesionless soil (with tie bar) & clays.

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