Technological University of the Philippines Ayala Blvd. Ermita, Ermita, Manila
College of Engineering Department of Civil Engineering
CE 410 – 410 – 4A 4A Soil Mechanics
Assignment No. 1 Introduction to Soil Mechanics
Fesalbon, Mayson R. 10-205-041 June 25, 2013
Engr. Jesus Ray M. Mansayon Instructor
Soil Mechanics – Geotechnical Engineering – Foundation Engineering
Soil Mechanics
In general, Soil Mechanics is the branch of science that deals with the study of the physical properties of soil and the behaviour of soil masses subjected to various types of forces. [1] [2] Karl Terzaghi, the father of modern soil mechanics , defined soil mechanics as follows:
Karl Terzagh i (1883 - 1963)
Source: http://benriya.files.wordpress.com/ 2008/10/terzagi.jpg?w=198&h=300
Soil Mechanics is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they [3] contain an admixture of organic constituents.
It differs from fluid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and water) and particles (usually clay, silt, sand, and gravel) but soil may also contain organic solids, liquids, and gasses and other matter. Soil mechanics is used to analyse the deformations of and flow of fluids within natural and man-made structures that are supported on [4] or made of soil, or structures that are buried in soils. The term Soil Mechanics is now accepted quite generally to designate the discipline of engineering science which deals with the properties [3] and behaviour of soil as a structural material. Our main objective in the study of soil mechanics is to lay down certain principles, theories and [3] procedures for the design of a safe and sound structure. Geotechnic al Engineering
Geotechnical engineering is the sub-discipline of civil engineering that involves natural materials found close to the surface of the earth. It includes the application of the principles of soil mechanics and rock mechanics to the design of foundations, retaining structures and earth [1] structures. It requires knowledge of strength and stiffness of soils and rocks, methods of [5] analyses of structures and hydraulics of groundwater flow. F o u n d a t i o n E n g i n e er i n g
Foundation engineering is an engineering field of study that deals to the design of those structures which support other structures like buildings, bridges or transportation infrastructures under different soil and environment conditions. It is at the periphery of Civil, Structural and Geotechnical Engineering disciplines and has distinct focus on soil [3][6] structure interaction.
Major Periods of Geotechnical Engineering
[7]
The record of a person’s first use of soil as a construction material is lo st in antiquity. For years, the art of geotechnical engineering was based on only past experiences through successive experimentation without any scientific character. Based on experimentations, many structures were built – some of which have crumbled while others are still standing.
Date
Event
2750 B.C.
The five most important pyramids (Saqqarah, medium, Dashmur South and North and Cheops) were built in Egypt.
2000 B.C.
Dykes were built in the basin of the Indus to protect the town of Mohenjo Dara (Pakistan)
1120 B.C. to 249 B.C.
Dykes were built in China during the Chan dynasty for irrigation purposes.
68 A.D.
Thousands of pagodas were built in China during the Eastern Han dynasty.
1173 A.D.
The construction of the Leaning Tower of Pisa in Italy began.
th
12 Century
The Garisenda Tower was built in Bologna, Italy.
The Leaning Tower of Pisa weighs about 15,600 metric tons and is supported by a circular base having a diameter of 20 m. The tower has tilted in the past to the east, north, west and finally to the south. Investigations showed that the weak clay layer exist a depth about 11m below the ground surface compression which caused the tower to tilt. It was closed in1990 because of it was feared to collapse with more than 5m out of plumb. It has been stabilized by excavating soil from under the north side of the tower. About 70 metric tons of earth was removed in 41 separate extractions that spanned the width of the tower.
Leaning Tow er of Pisa
Source: http://4.bp.blogspot.com/3ijUwVQcmN0/UNAShgdfUBI/AAAAAAAAf88 /FtYVvmFciEw/s1600/Super+Goof+Pisa.jpg
Other structures below also exhibited tilting that may account for the soil-bearing capacity in the construction of the said structures.
Source: http://4.bp.blogspot.com/M4v3aWbx6Uc/TtfqwZ8zAJI/AAAAAAAABnI/q98m3tkzMeU/s1600/20111029_WOC000.gif
After encountering several foundation related problems during the construction over the past centuries, engineers and scientist began to address the properties and behaviours of soils in a th more methodical manner starting the early part of the 18 century. The time span from 1700 to 1927 was divided into four major periods based on the emphasis and the nature of study in geotechnical engineering. 1. 2. 3. 4.
Pre-classical (1700 to 1766 A.D.) Classical Soil Mechanics – Phase I (1776 to 1865 A.D.) Classical Soil Mechanics – Phase II (1856 to 1910 A.D.) Modern Soil Mechanics (1910 to 1927 A.D.)
Pre-classic al Period of Soil Mechanics (1700 to 1766 A.D.)
This period concentrated on the studies relating the natural slope and unit weights of various types of soil. Henri Gautier (1660 - 1737). Henri Gautier is a French royal engineer who studied in 1717 the natural slopes of soil when tipped in a heap for formulating the design procedures of retaining walls. According to the study, the following results were obtained:
Unit Weight
Natural Slope
kN/m
lb/ft
Clean Dry Sand
31⁰
18.1
115
Ordinary Earth
45⁰
13.4
85
Classification of Soil
3
3
Bernard Forest de Belidor (1671 - 1761). He published a textbook for military and civil engineers in France. In his book, he proposed a theory of lateral earth pressure on retaining walls as a follow-up on Gautier’s original study. He also specified a soil classification system shown in the table below.
Classification
Unit Weight kN/m
lb/ft
-
-
16.7 to
106 to
Compressible Sand
18.4
117
Ordinary earth (found in dry locations)
13.4
85
Soft earth (primarily silt)
16.0
102
Clay
18.9
120
Peat
-
-
Rock Firm or hard sand
Francois Gadroy (1705 - 1759). A French engineer who reported the first laboratory model test results on a 76-mm-high retaining wall built with sand backfill in 1746. He al so observed the existence of slip planes in the soil at failure.
Classical Period of Soil Mechanics – Phas e I (1776 to 1856)
Charles Augustin Coulomb (1736 - 1806). A French scientist who presented a report using the principles of maxima and minima to determine the true position of the sliding surface in soil behind a retaining wall in 1776. He also used the laws of friction and cohesion for solid bodies.
Jacques Frederic Francais (1775 - 1833) & Claude Louis Marie Henri Navier (1785 1836). Their study is related to inclined backfills and backfills supporting surcharge. Jean Victor Poncelet (1788 - 1867). An army engineer and professor of mechanics provides a graphical method for determining the magnitude of lateral earth pressure on vertical and inclined retaining walls with arbitrarily broken polygonal ground surfaces. He was also the first to use the Greek letter Phi for soil friction angle. He also provided the first ultimate-bearing capacity theory for shallow foundations. Alexandre Collins (1808 - 1890). In 1846, he provides the details for deep ships in clay slopes, cutting and embankments. He also observed that the actual failure surfaces could be approximated as arcs and cycloids. William John Macquorn Rankine (1820 - 1872). In 1852, he published a study on theory on earth pressure and equilibrium of earth masses that ends the Phase I of classical soil mechanics.
Classical Period of Soil Mechanics – Phas e II (1856 to 1910)
Henri Philibert Gaspard Darcy (1803 - 1858). A French engineer who published a study on the permeability of sand filters in 1856. He defined the term coefficient of permeability or hydraulic conductivity of soil. Sir George Howard Darwin (1845 - 1912). A professor in astronomy who conducted laboratory tests to determine the overturning moment on a hinged wall retaining sand in loose and dense state of compaction. Joseph Valentin Boussinesq (1842 - 1929). He published a development of the theory of stress distribution under loaded bearing areas in a homogenous, semi-infinite, elastic and isotropic medium in 1885. Osborne Reynolds (1842 - 1912). He demonstrated the phenomenon of dilatency of sand in 1887.
Mod ern Soil Mechanic s (1910 to 1927)
Albert Mauritz Attenberg (1848 - 1916). A Swedish chemist and soil scientist who defined clay-size fractions as a percentage by weight of particles smaller than 2 microns in size in 1908. In 1911, he explained the consistency of cohesive soils by defining liquid, plastic and shrinkage limits. He also defined the plasticity index as the difference between liquid limit and plastic limit.
Jean Fontard (1884 - 1962). A French engineer who carried out an investigation to determine the cause of failure of the 17-m high earth dam in Charmes, France that happened on October 1909. He conducted undrained double shear test on clay 2 specimens (0.77 m & 200 mm thick) under constant vertical stress to determine shear strength parameters. Arthur Langley Bell (1874 - 1956). A civil engineer who worked in the design and construction of the outer sea wall at Rosyth Dockyard. He developed relationships for lateral pressure and resistance in clay as well as bearing capacity of shallow foundations in clay. He also used shear-box test to measure the undrained shear strength of undisturbed clay specimens. Wolmar Fellenius (1876 - 1957). An engineer from Sweden who developed the stability analysis of saturated clay slopes (ᵩ = 0⁰) with the assumption that the critical surface of sliding is the arc of the circle. Karl Terzaghi (1883 - 1963). An Austrian who developed the theory of consolidation for clays. It was published in his book Erdbaumechanik in 1925. SOIL – Its Origin and Types The term Soil comes with different meanings, depending on the general field in which it is [8] considered and used. To an Agriculturist:
Soil is the top thin layer of earth within which organic forces are predominant and which is [8] responsible for the support of plant life. To a Geologist:
Soil is the material in the relative thin surface zone within which roots occur and all the [9] rest of the crust is grouped under the term rock irrespective of its hardness. To an ENGINEER:
Soil is the uncemented aggregate of mineral grains and decayed organic matter (solid [1] particles) with liquid and gas in the empty spaces between the solid particles. Soil includes all earth materials, organic and inorganic, occurring in the zone overlying [8] the earth’s crust. Soil is also defined as a natural aggregate of mineral grains, with or without organic constituents, which can be separated by mechanical means such as agitation of water. [10]
Origin of Soil In general, soils are formed by weathering of rocks. The physical properties of soil are derived primarily by the minerals that constitute the soil particles and, hence, the rock from which is [8] derived. From the knowledge that soils comes from the weathering of rocks, this section will discuss its process of weathering and the rock cycle. [9][10]
Weathering
Weathering is a process of breaking down rocks by mechanical and chemical processes into smaller pieces to form soil or loose particles at or ne ar the Earth’s surface. There are two types of weathering which are the mechanical weathering and chemical weathering . Mechanical Weathering. Mechanical weathering is a type of weathering where the physical characteristics of a rock have been changed like change in size, shape, texture, etc. It may be caused by the expansion and contraction of rocks from the continuous gain and loss of heat which results in ultimate disintegration. Other physical agents that help disintegrate rocks are glacier ice, wind, running water in rivers and streams, and ocean waves. Chemical Weathering. Chemical weathering is a type of weathering in which the original rock is transformed into new minerals by means of a chemical reaction. In this type of weathering, water plays an important role by providing oxygen and mobility of moving ions that will cause a chemical reaction. The rate of chemical weathering depends on three factors: temperature, surface area of the rock exposed and availability of water or natural acid. Thus, tropical environment experiences most severe chemical weathering.
Transportation of Weathering Products. The products of weathering may stay in the same place or may be moved to other places by different causes. The soils formed by the weathered products at their place of origin are called residual soils. An important characteristic of residual soil is the gradation of particle size. Fine grained soil is found at the surface, and the grain size increases with depth. At greater depth, angular rock fragments may also be found. Transported soil may be classified according to their mode of transportation and deposition. 1. 2. 3. 4. 5. 6.
Glacial soils – formed by the transportation and deposition of glaciers Alluvial soils – transported by running water and deposited along streams Lacustrine soils – formed by deposition in quiet lakes Marine soils – formed and deposited by the wind Aeolian soils – transported and deposited by the wind Colluvial soils – formed by the movement of soil by gravity
Types of Soil [14]
A c c o r d i n g t o t h e i r P ar t i c l e S i ze
Soils generally are called gravel , sand, silt or clay, depending on the predominant size of particles within the soil. Several organizations developed particle-size classification of soil.
Grain Size (mm)
Name of Organization
Gravel
Sand
Silt
Clay
Massachusetts Institute of Technology (MIT)
>2
2 to 0.06
0.06 to 0.002
<0.002
US Department of Agriculture (USDA)
>2
2 to 0.05
0.05 to 0.002
<0.002
American Association of State Highway and Transportation Officials (AASHTO)
76.2 to 2
2 to 0.075
0.075 to 0.002
<0.002
Unified Soil Classification System
76.2 to 4.75
4.75 to 0.075
<0.075
<0.075
Illustration o f Particle-Size of Soil
Source: http://selectsg.com/yahoo_site_admin/assets/images/dirt-sizes.275203602_std.jpg
[15]
Acco rding to their Origin
On the basis of origin of their constituents, soils can be divided into two large groups: Residual Soils. Residual soils are those that remain at the place of their formation as a result of the weathering of parent rocks. The depth of residual soils depends primarily on climatic conditions and the time of exposure. Its sizes of grains are indefinite. Transported Soils. Transported soils are soils that are found at locations far removed from their place of formation. The agents of transport of these soils are the same as the modes of transportation of the weathering products as discussed earlier on the topic about weathering.
Soils that are generally used in practice
[15]
B e n t o i t e is a clay formed by the decomposition of volcanic ash with a high content of
montmorillonite. It exhibits the properties of clay to an extreme degree. Varved Clays consist of thin alternating layers of silt and fat clays of glacial origin. They possess
the undesirable properties of both silt and clay. Kaolin, China Clay is very pure forms of white clay used in ceramic industry.
Bou lder Clay is a mixture of an unstratified sediment deposit of glacial clay, containing unsorted
rock fragments of all sizes ranging from boulders, cobbles and gravel to finely pulverize clay material. Calcareous Soil is a soil containing calcium carbonate. Such soil effervesces when tested with
weak hydrochloric acid. Marl consists if a mixture of calcareous sands, clays or loam.
Hardpan is a relatively hard, densely cemented soil layer, like rock which does not soften when
wet. Caliche is an admixture of clay, sand and gravel cemented by calcium carbonated deposited
from ground water. Peat is a fibrous aggregate of finer fragments of decayed vegetable matter. It is very
compressible and should be cautious when using it for supporting foundations of structures. Loam is a mixture of sand, silt and clay.
Shale is a material in the state of transition from clay to slate. When exposed to air or to take in
water, it rapidly decomposes.
ROCKS – Its Origin and Cycle Rock can be defined as a compact, semi-hard to hard mass of natural material composed of one
or more minerals. The rocks are encountered at the surface of the earth or beneath and are commonly classified into three groups according to their modes of origin: igneous, sedimentary and metamorphic rocks which are also inter-correlated to each other by a cycle called the r o c k [8] c y c l e . The Rock Cycle
Since soil came from weathered rocks, the rock cycle contributes a great part in the origin and formation of soil.
The Rock Cycle
Source: http://etap.org/demo/Earth_Science/es3/1003_rock_cycle.jpg
Igneous Rocks. Igneous rocks are formed from the solidification of molten magma ejected from within the earth’s mantle. After a volcanic eruption, some of the molten magma cools on the surface of the earth. Sometimes magma ceases its mobility below [11] the earth’s surface and cools to for m intrusive igneous rocks that are called plutons. [8] There are two main classes of igneous rocks. They are the following: 1. 2.
E x t r u s i v e – these are igneous rocks that poured out at the atmosphere I n t r u s i v e – these are igneous rocks that have been formed below the earth’s
surface. Sedimentary Rocks. Sedimentary rocks are formed when the products of the disintegration and decomposition of any rock type are transported, redeposited and partly [17] or fully consolidated or cemented into a new type of rock. Methamorphic Rocks. Methamorphic rocks are formed when an any type of rock [16] undergoes a process called metamorphism. Metamorphism is a process when a rock undergoes a complete or incomplete crystallization by high temperature, high pressures [16][17] and/or high shearing stresses without melting the rock.
Application of Soil Mechanics Soil mechanics is a discipline that applies principles from engineering mechanics to predict the mechanical behaviour of soils. Every man-made structure needs foundations to support the [18] forces applied to it. Any structure that is built lies on the ultimate foundation – the earth. That is why we need to study the behaviour of the soil and its interaction with the structures we [18] are going to build to ensure the safety of the people who will use it.
Civil Engineering. One major field of application of soil mechanics is in the field of civil
engineering. Consider the suspension bridge shown on the next page. In the analysis of a suspension bridge, the loads on the roadway that is suspended from two main cables by means of vertical hangers transfer to the main cables. The forces acting on the main cables will pass [19] over a pair of towers that are anchored into a solid rock or concrete foundation at their ends. Same principle will also be exhibited to other types of structures. The knowledge on the lateral earth pressure, soil-bearing capacity and slope stability of soil mechanics will be very much helpful in the construction of any structures.
G o l d e n G a t e B r i d g e , S an F r a n c i s c o B a y
Source: http://teachers.egfi-k12.org/wp-content/uploads/2010/03/800px-GoldenGateBridge-001.jpg
Soil mechanics plays a very important role in the design, construction, operation and maintenance of new waste disposal and containment facilities; and in the isolation of contaminated ground. The following must be considered in a landfill that requires [20] knowledge and application of soil mechanics: Environmental Geotechnics.
1. 2. 3. 4. 5. 6.
The landfill must be safe against several possible types of stability failure. The landfill must be able to withstand earthquake shaking, without gross stability failure, rupture of linear system or failure of leachate collection and removal system. The foundation soils must be able of supporting the tailing embankment. The dam must be stable under both static and seismic loading at different stages of construction and after completion to maximum height. Stability of slurry trenches during excavation Stresses and deformation of the ground adjacent to the slurry trench and their potential adverse effects on structures and facilities.
Geotechnical engineering uses principles of soil mechanics and mechanics to investigate subsurface conditions and materials; determine the relevant physical/mechanical and chemical properties of these materials; evaluate stability of natural slopes and man-made soil deposits; assess risks posed by site conditions; design earthworks and structure foundations; and monitor site conditions, earthwork and [21] foundation construction. Geotechnical
Engineering.
M i n i n g . There are many applications of unsaturated soil in the mining field. This include the
wetting up and drain down of initially dry surface waste rock dumps; the irrigation and drain down of heap leach materials; drain down, desiccation and rewetting of mine tailings; dewatering of mineral products such as coal; the strength and compressibility of stored mine wastes by way of [22] rehabilitation.
References: [1]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, pp.19-22
[2]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, p.10
[3]
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Dhanpat Rai and Sons, p.24
[4]
http://en.wikipedia.org/wiki/Soil_mechanics
[5]
nd
Atkinson, J. (2007). The Mechanics of Soils and Foundation, 2 Edition, Taylor and Francis Group, p.3
[6]
http://en.wikipedia.org/wiki/Foundation_engineering
[7]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, pp.17
[8]
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Dhanpat Rai and Sons, p.26
[9]
http://www.eng.fsu.edu/~tawfiq/soilmech/lecture.html
[10]
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Dhanpat Rai and Sons, p.28
[11]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, p.15
[12]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, pp.19-22
[13] [14]
http://www.engr.uconn.edu/~lanbo/CE240LectW012Rock2soil.pdf th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, p.24
[15]
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Dhanpat Rai and Sons, pp.29-3 0
[16]
th
Das, B.M. (2010). Principles of Geotechnical Engineering, 7 Edition, Cengage Learning, p.23
[17]
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Dhanpat Rai and Sons, pp .27
[18] [19]
http://wiki.answers.com/Q/What_is_the_importance_of_soil_mechanics_in_civil_engineering th
Kassimali, A. (2010) Strucutral Analysis, 4 Edition, Cengage Learning, p.8
[20]
Mitchell, J.K. (1995). The Role of Soil Mechanics in Environmental Geotechnics, The 3 Spencer J. Buchanan Lecture, pp.2-5
[21] [22]
http://en.wikipedia.org/wiki/Geotechnical_engineering Williams, D.J. Some Mining Application of Unsaturated Soil Mechanics, pp.1-20
rd
