Soil Engineering: Introduction, Importance, Fundamentals, Problems and Limitations!
Contents:
- Introduction to Soil Engineering
- Importance of Soil Engineering
- Fundamentals of Soil Engineering
- Soil as Foundation Material
- Special Soil Engineering Problems
- Solution of Engineering Problems
- Limitations of Soil Engineering
Introduction to Soil Engineering:
The use of soil as an engineering material may be said to be as old as mankind itself. Since that time, man has been confronting many types of problems while dealing with soils.
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Excellent pavements – Egypt and India much before the Christian Era.
Some earth dams have been used for storage of water in India for more than 2000 years.
During the excavation at the early civilization sites of at Mohenjo-Daro and Harappa in the Indian subcontinent indicate the use of soil as foundation and construction material.
Egyptians used caissons for deep foundations even in 2000 BC.
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The hanging gardens at Babylon (Iraq) were also built during the period.
The leaning tower of Pisa was also built around same time. The tower has leaned on one side because of the differential settlement of its base.
In the 17th century, Leonardo da Vinci constructed a number of structures in France, and the London Bridge in England.
The Taj Mahal at Agra is built on masonry cylindrical wells sunk into the soil at close intervals.
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The builders were guided by the knowledge and experience passed down from generation to generation.
In 1773, a French engineer Coulomb gave the theory of earth pressure on retaining walls. Coulomb also introduced the concept that the shearing resistance of soil consists of two components – cohesion and friction.
Darcy in 1856 gave the law of permeability. This law is used for the computation of seepage through soils.
In the same year, Stokes gave the law for the velocity of fall of solid particles through fluids. This law is used for determining the particle size.
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O-Mohr in 1871 gave the rupture theory for soils. He gave a graphical method of representation of stresses. Popularly known as Mohr’s circle, it is extremely useful for determining stresses on inclined planes.
Boussinesq in 1885 gave the theory of stress distribution in a semi-infinite homogeneous, isotropic, elastic medium due to an externally applied load. The theory is used for determining stresses in soils due to loads.
Atterberg in 1911, suggested some simple tests for characterizing the consistency of cohesive soils. These limits are useful for identification and classification of soils.
Prof. Fellenins (Sweden) in 1913 studied the stability of slopes. Swedish circle method for checking the stability of Sweden slopes are popularly used.
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The modern era of soil engineering began in 1925, with the publication of the book, Eradbaumechanic, by Karl Terzaghi. He is fittingly called the father of soil mechanics.
His theory of consolidation of soils and the effective stress principle gave a new direction.
Proctor in 1933, did a pioneering work on the compaction of soils.
Taylor worked on the consolidation of soils, shear strength of clays and the stability of slopes.
Casagrande worked on the classification of soils, seepage through earth masses and consolidation.
Skempton did a pioneering work on pore pressure, effective stress, bearing capacity and the stability of slopes.
Mayerhof gave the theories of B.C. of shallow and deep foundations.
Hvorlov did a commendable work on sub-surface exploration and on the shear strength of re-moulded clays.
Importance of Soil Engineering:
Once it is accepted that soil is a structural material, its importance in Civil Engineering becomes paramount. A Geotechnical Engineer should have thorough knowledge of this material of structure as in the case of any other structural material.
Study of Soil Engineering is particularly important in respect of infrastructure development and constructions, viz., highway and airport pavements, foundations and underground structures, retaining walls and embankments and multistorey buildings.
Foundation is considered the most critical part of any structure and it is on its soundness that the stability of the entire structure depends. Since the load bearing capacity of the foundation has a direct relationship with the soil characteristics, the importance of soil investigation should not be underestimated.
Fundamentals of Soil Engineering:
The word ‘soil’ derives from the Latin word solium, which means, the upper layer of the earth that may be dug or powdered, specifically, the loose surface material of the earth in which plants grow.
The term ‘soil’ in soil engineering is defined as an unconsolidated material composed of solid particles, produced by the disintegration of rocks. The void space between the particles may contain air, water or both. The solid particles may contain organic matter. The soil particles can be separated by such mechanical means as agitation in water.
A natural aggregate of mineral particles bonded by strong and permanent cohesive forces is called a ‘rock’.
Application of laws and principles of mechanics and hydraulics to engineering problems in dealing with soil is usually referred to as Soil Mechanics. The term soil engineering is used to cover a much wider scope implying that it is a practical science rather than a purely fundamental or mathematical one. Hence, Soil Engineering is an applied science dealing with the application of the principles of soil mechanics to practical problems.
It includes site investigation, design and construction of the foundation, earth retaining structures and earth structures.
Soil as Foundation Material:
In the design of any foundation system, the central problem is to prevent the settlements large enough to damage the structure. Just how much settlements to permissible depends on the size, the type and use of the structure, the type of foundation, the source is the subsoil of the settlement, and the location of the structure. In most cases, the critical settlement is not the total settlement but rather the differential settlement, which is the relative movement of the structure.
Embankment on Soft Soil:
Even though a steel storage tank is a flexible structure, a settlement of 1.5 m is too large to be tolerated.
Soil engineering studies show that a very economical solution to the tank foundation problem consists of building on the earth embankment at the site to compress the soft soil, removing the embankment and finally placing the tank on the prepared foundation soil. Such a technique is termed preloading.
Foundation Heave:
In areas of arid regions, the soils dry and shrink during the arid weather and then expand when moisture becomes available.
Water—rainfall drainage—or from capillary.
When an impervious surface is placed on the surface of the soil, it prevents evaporation. Obviously, the lighter a structure, the more the expanding soil will raise it. Heave problems are commonly associated with light structures such as small buildings, dam spillways and road pavements.
To avoid heave problem first holes are augered into the soil. Steel shells are placed and then concrete base plugs and piles are poured.
Under the building and around the piles an air gap is left, which serves to reduce the amount of heave of the soil (by permitting evaporation) and also to allow room for such heave without disturbing the building.
Soil as a Construction Material:
Soil is essentially the only locally available construction material. Earth has been used for the construction of monuments, tombs, dwellings, transportation facilities and water retention structures.
(a) Earth dam and embankment
(b) Highway pavement
Slope and Excavations:
When a soil surface is not horizontal there is a component of gravity tending to move the soil downward.
Stability analysis has to be carried out.
Underground Structures:
Tunnels, shafts and conduits require evaluation of forces exerted by the soil on these structures.
Special Soil Engineering Problems:
1. Vibrations:
Certain granular soils can be readily densified by vibrations. A building may undergo a considerable settlement due to vibrations – (a) compressors (b) turbines.
2. Explosions and Earthquakes:
Effects on building of earth waves caused by quarry blasting and other blasting for construction purposes. Similar problems arise as a result of earthquakes.
3. Frost:
Frost heave problems – When in contact with moisture and subjected to freezing temperature, they can imbibe water and undergo a large expansion. Such heave exerts forces large enough to move and crack adjacent structures and can cause serious problems on thawing because of the excess moisture.
The civil engineer designing highways and airfield pavements in frost areas must either select a combination of base soil and drainage that precludes frost heave or design the pavement to withstand the weak soil that occurs in the spring when the frost melts.
4. Regional Subsidence:
Large scale pumping of oil and water from the ground can cause major settlements over a large area.
The first step in minimizing such regional subsidence is to locate the earth material that are compressing as the fluid is removed, and then consider method of replacing the lost fluid.
Solution of Engineering Problems:
Civil engineers encounter the construction on soil, in soil and of soil.
The interpretation of insufficient and conflicting data, the selection of soil parameters, the modification of a solution, etc., require experience and a high degree of engineering judgement.
While a sound knowledge of soil mechanics is essential for the soil engineer, engineering judgement is usually the characteristic that distinguishes the outstanding soil engineer.
Limitations of Soil Engineering:
Over the past century many advances in soil engineering have greatly improved our ability to predict the behaviour of geo-material. However, we still need to maintain skepticism.
Most of our analyses are handicapped by uncertainties introduced by the site exploration and characterization programme. In addition, our mathematical models of soil behaviour are only approximate, and often do not explicitly consider important factors.
The solutions obtained in most cases are for an idealized, hypothetical material, which may not truly represent the actual soil. A good engineering judgement is required for interpreting the results.