Civil Engineering

GEO-TECHNICAL ENGINEERING – The technical tale of the Soil

Introduction:

Civil Engineering structures are identical from ancient age, due to its large size and unique look. But, the irony always been seen that, a general visitor never notice about its base, which provide sufficient support and balance the structure, safely over hundreds of decades. Yes, the base or in Civil Engineering terminology the ‘Foundation’.

There are many wonderful structures all over the world and their foundations are magnificent as well, due to its art of execution, design complexity and gigantic dimensions. The structures from seven wonders, such as the Pyramids, the Great Wall of China, the Hanging garden of Babylon etc. are heavy in self weight and stand over a large foundation with a great dimension. Therefore, the intensity of load over the foundation was technically not huge. But there also such kind of structures having base of less dimensions like the Petra, the Qutub minar etc. producing a large intensity of load over its foundations, thus it was necessarily built over a site with well-conditioned soil strata and adequate bearing capacity. That’s why such sites need to found to construct such constructions and it was not too problematic as availability of land was there in those ages. Besides of these, the existence of pad footings and strip-and-raft foundations are found in ancient Greek structures. Though, Geo-technical engineering was obviously not prominent at that time.

History:

In Ancient India, there were some procedures, practiced to analyze soil condition before constructing over it. The procedures were used to determine the water absorption nature, penetration resistance of soil etc. Some traditional ground improvement techniques also applied. Among these, most famous was to use the land as the pasture land for the herd of sheep or cattle for a certain period, before starting the construction over it. The weight of this animal through their small-sized hoof create better intensity of loading over the land, hence it got compacted. The same mechanism is adopted in modern engineering in the form of Sheeps foot rollers, to compact clayey deposit in site.

            Now, let’s know about the beginning of the modern Geo-technical engineering and about the pioneers of this trade. In 18th century, the earliest advances occurred in the development of Geo-technical engineering regarding Retaining wall construction and “Theory of Earth pressure” was developed. Then, Henri Gautier, a French Royal Engineer, recognized the “natural slope” of different soils in 1717, an idea later known as the angle of repose or Angle of shearing resistance ‘Ø’ of soil. The value of ‘Ø’, which nothing but the angle of internal friction between same soil particles is significant not only for the Stability analysis of finite slopes or embankment, but also for evaluation of Co-efficient of earth pressure, unsupported length of clay, the Shear strength and also the Bearing capacity of soil. Further improvement in Geo-technical engineering occurred on 1773, when analyzing the principles of mechanics to soils, Charles Coulomb suggested that the maximum shear stress on the slip plane, for design purposes. After that, combining with Coulomb’s theory, Christian Otto Mohr suggested a graphical method based on 2D stress state, the Mohr-Coulomb theory was developed by inventing the graphical method of Mohr’s circle.

In the 19th century Henry Darcy, Joseph Boussinesq, William Rankine, Albert Atterberg developed the theory regarding Permeability through soil, Effective stress below ground, alternative theory to Coulomb’s earth pressure theory, Consistency of clay respectively. These theories are benchmark among the most classic theories of Civil engineering and much important as well, hence the short introductions of some of these are given.

Permeability: It is a basic property of soil, where it permits water to pass (percolate) through its own body. Practically, the water passes through the innumerable inter-connecting voids of soil, sometimes it functioned as capillary tubes. This concept was first developed by Henry Darcy. The natural ground water table generally exists at certain depth below the ground level (G.L.). But in monsoon, the water table getting charged and gradually rises through these capillary tubes, with assistance of Surface tension. As a result, the ground become saturated and affects the foundation, in terms of its settlement and bearing capacity. This, phenomenon is also applicable for constructing the core of Earthen dam, such as Maithon dam on river Barakar in West Bengal, Bhavanisagar dam on Bhavani river in Tamil Nadu. It is also applicable for the Well hydraulics, which is a crucial part of the irrigation.

Effective stress distribution: This theory established on the Elastic theory of Hook and the soil below earth surface considered as Elastic half space. The equation for the Vertical stress (Radial and Shear stresses also) on any point inside or upon the ground due to the Point load, externally applied on the nearby ground, was derived by Joseph Boussinesq and later by Westergaard. Further, the theory was extended from point load to Uniformly distributed load on straight line and regular areas. The evaluation of these stresses is crucial for estimate the settlement of foundations.

Earth pressure: The Theory of Plasticity of soil was investigated by William Rankine, especially at the verge of failure of the backfill soil in different conditions. Basis on this theory, the concept of retaining wall was developed to retain the backfill soil and maintain a difference in the elevation level of the ground with a vertical section. It is very usual case for roads at hilly areas, where the retaining walls are constructed to resist the lateral earth pressure, exerts from the elevated side-hill cuts at the road sides. Further, the possible Failure plane of the backfill was estimate by the graphical representations proposed by Rebhann and Culmann.

Shear strength: It is the basic phenomenon of the Mechanics of Solids and due to its presence, solid is not flown like liquid. It means, Shear strength (τ) is the skeleton of any solid matter and being solid, soil also not except from it. Charles Coulomb established that, the shear strength of soil is the sum of the soil cohesion, ‘c’, and frictional resistance ‘σ tan Ø’, where ‘σ’ is the normal stress on the slip plane and ‘c’ is the cohesion i.e. the attraction between two particles of clayey soil. Thus, the ‘c’ and ‘Ø’ are termed as Shear strength parameters, which are the main factor for the Bearing capacity of soil.

Present scenario:

The modern age was begun in the year of 1925, when Karl Terzaghi developed the principle of effective stress, and demonstrated that the shear strength of soil is controlled by effective stress. He also developed the theories of bearing capacity of foundations, and the theory for prediction of the rate of settlement of clay layers due to consolidation. Due to his outstanding contribution Terzaghi was honored as the ‘Father of modern Soil mechanics and Geotechnical engineering’.   

The common constructional problems related to Geotechnical engineering are, (i) the stability of excavation, (ii) Dewatering process, (iii) effect of adjoining structures. Beyond that, the problem regarding quick sand condition, the construction of well foundation beneath the bridge piers in river bed, installation of large sheet piles, constructing tunnel through the hills or under the water body etc. are becoming very common jobs with challenge now-a-days.               

Further studies:

Except from these jobs, the extensive study is required in several issues in practice. In our country, some of these are the construction of expansive soil, design the machine foundation with absorbing capacity of the vibration high amplitude, encounter the failure of structure due Liquifaction, design the seismic proof foundation etc. In near future these jobs are also appear as the common challenges in present scenario as the Nation progresses very fast.

Though there are large numbers of problematic situations, with time the Geotechnical Engineering become sophisticated and developed its excellence in all sector. The utility of chemicals and various geo-grids are used to stabilize the soil. In recent past, the creation of the artificial islands of large area with shape of a palm tree in sea-bed near the coast of Dubai, UAE. This Palm Islands is one of the finest examples in this age and it only become possible for the mankind and it exhibits the evolution of Geotechnical engineering.   

Now maybe the development of the Geotechnical engineering become saturated on planet Earth and the day will come soon when it may move on for our satellite Moon or may be beyond that….who knows! But I wish, at that time the word ‘Geo’ stands for Earth will remain with the term of Geotechnical engineering.

Conclusions:

It is a common thinking about the role of the Geo-technical Engineers deals with the design of the substructure or foundation of any type of construction. The loading type and its distribution, the conditions of sub base soil are the main aspects for designing its foundation which is too difficult as the challenges occur due the soil conditions are uncertain with variation of various aspects such as position of water table, seismic properties etc. And the hard part of the Geotechnical engineering is, soil hide its secrets in its heart, are always existed about millions of years, therefore it needs to discover as it is, not for invention.

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