Technological University of the Philippines

1000 Ayala Blvd., Ermita, Manila

College of Engineering

Civil Engineering Department

CE 410 – 4B

Soil Mechanics, Lec.

Assignment no. 1

Introduction to Soil Mechanics

Cuizon, Stephen A.

12-205-033

Date of Submission: June 25, 2015

Engr. Jesus Ray M. Mansayon

Instructor

1. SOIL MECHANICS, GEOTECHNICAL ENGINEERING AND FOUNDATIONSoil 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 contain an admixture of organic constituents.

The term Soil Mechanics is now accepted quite generally to designate that discipline of engineering science which deals with the properties and behavior of soil as a structural material. All structures have to be built on soils. Our main objective in the study of soil mechanics is to lay down certain principles, theories and procedures for the design of a safe and sound structure. The subject of Foundation Engineering deals with the design of various types of substructures under different soil and environmental conditions.

During the design, the designer has to make use of the properties of soils, the theories pertaining to the design and his own practical experience to adjust the design to suit field conditions. He has to deal with natural soil deposits which perform the engineering function of supporting the foundation and the superstructure above it. Soil deposits in nature exist in an extremely erratic manner producing thereby an infinite variety of possible combinations which would affect the choice and design of foundations. The foundation engineer must have the ability to interpret the principles of soil mechanics to suit the field conditions. The success or failure of his design depends upon how much in tune he is with Nature.

The word 'soil' has different meanings for different professions. To the agriculturist, soil is the top thin layer of earth within which organic forces are predominant and which is responsible for the support of plant life. To the geologist, soil is the material in the top thin zone within which roots occur. From the point of view of an engineer, soil includes all earth materials, organic and inorganic, occurring in the zone overlying the rock crust.

The behavior of a structure depends upon the properties of the soil materials on which the structure rests. The properties of the soil materials depend upon the properties of the rocks from which they are derived. A brief discussion of the parent rocks is, therefore, quite essential in order to understand the properties of soil materials.

In his practice the civil engineer has many diverse and important encounters with soil. He uses soil as a foundation to support structures and embankments; he uses soil as a construction material; he must design structures to retain soils from excavations and underground openings; and he encounters soil in a number of special problems. This chapter deals with the nature and scope of these engineering problems, and with some of the terms the engineer uses to describe and solve these problems. Several actual jobs are described in order to illustrate the types of questions that a soil engineer must answer.

Reference: Geotechnical Engineering: Principle and Practices of Soil Mechanics and

Foundation Engineering by VNS Murthy page 3 - 5 Soil Mechanics by Lambe and Whitman page 3

2. MAJOR PERIODS OF GEOTECHNICAL ENGINEERING Preclassical Period of Soil Mechanics (1700 –1776)

This period concentrated on studies relating to natural slope and unit weights of various types of soils, as well as the semiempirical earth pressure theories. In 1717 a French royal engineer, Henri Gautier (1660–1737), studied the natural slopes of soils when tipped in a heap for formulating the design procedures of retaining walls. The natural slope is what we now refer to as the angle of repose. According to this study, the natural slope of clean dry sand and ordinary earth were 31_ and 45_, respectively. Also, the unit weight of clean dry sand and ordinary earth were recommended to be 18.1 kN/m3 (115 lb/ft3) and 13.4 kN/m3 (85 lb/ft3), respectively. No test results on clay were reported. In 1729, BernardForest de Belidor (1671–1761) published a textbook for military and civil engineers in France. In the book, he proposed a theory for lateral earth pressure on retaining walls that was a follow-up to Gautier’s (1717) original study. He also specified a soil classification system in the manner shown in the following table.

ClassificationUnit Weight

kN/m³ lb/ft³Rock - -Firm or hard sand 16.7 106Compressible sand 18.4 117Ordinary earth (as found in dry locations) 13.4 85Soft earth (primarily silt) 16.0 102Clay 18.9 120Peat - -

The first laboratory model test results on a 76-mm-high (_ 3 in.) retaining wall built with sand backfill were reported in 1746 by a French engineer, Francois Gadroy (1705–1759), who observed the existence of slip planes in the soil at failure. Gadroy’s study was later summarized by J. J. Mayniel in 1808.

Classical Soil Mechanics—Phase I (1776 –1856)During this period, most of the developments in the area of geotechnical

engineering came from engineers and scientists in France. In the preclassical period, practically all theoretical considerations used in calculating lateral earth pressure on retaining walls were based on an arbitrarily based failure surface in soil. In his famous paper presented in 1776, French scientist Charles Augustin Coulomb (1736–1806) used the principles of calculus for maxima and minima to determine the true position of the sliding surface in soil behind a retaining wall. In this analysis, Coulomb used the laws of friction and cohesion for solid bodies. In 1820, special cases of Coulomb’s work were studied by French engineer Jacques Frederic Francais (1775–1833) and by French applied mechanics professor Claude Louis Marie Henri Navier (1785–1836). These special cases related to inclined backfills and backfills supporting surcharge. In 1840, Jean Victor Poncelet (1788–1867), an army engineer and professor of mechanics, extended Coulomb’s theory by providing a graphical method for determining the magnitude of lateral earth pressure on vertical and inclined retaining walls with arbitrarily broken polygonal ground surfaces. Poncelet was also the first to use the

symbol f for soil friction angle. He also provided the first ultimate bearing-capacity theory for shallow foundations. In 1846 Alexandre Collin (1808–1890), an engineer, provided the details for deep slips in clay slopes, cutting, and embankments. Collin theorized that in all cases the failure takes place when the mobilized cohesion exceeds the existing cohesion of the soil. He also observed that the actual failure surfaces could be approximated as arcs of cycloids.

The end of Phase I of the classical soil mechanics period is generally marked by the year (1857) of the first publication by William John Macquorn Rankine (1820–1872), a professor of civil engineering at the University of Glasgow. This study provided a notable theory on earth pressure and equilibrium of earth masses. Rankine’s theory is a simplification of Coulomb’s theory.

Classical Soil Mechanics - Phase II (1856 –1910)Several experimental results from laboratory tests on sand appeared in the

literature in this phase. One of the earliest and most important publications is one by French engineer Henri Philibert Gaspard Darcy (1803–1858). In 1856, he published a study on the permeability of sand filters. Based on those tests, Darcy defined the term coefficient of permeability (or hydraulic conductivity) of soil, a very useful parameter in geotechnical engineering to this day.

Sir George Howard Darwin (1845–1912), a professor of astronomy, conducted laboratory tests to determine the overturning moment on a hinged wall retaining sand in loose and dense states of compaction. Another noteworthy contribution, which was published in 1885 by JosephValentin Boussinesq (1842–1929), was the development of the theory of stress distribution under loaded bearing areas in a homogeneous, semiinfinite, elastic, and isotropic medium. In 1887, Osborne Reynolds (1842–1912) demonstrated the phenomenon of dilatency in sand.

Modern Soil Mechanics (1910 –1927)In this period, results of research conducted on clays were published in which the

fundamental properties and parameters of clay were established. The most notable publications are described next.

Around 1908, Albert Mauritz Atterberg (1846–1916), a Swedish chemist and soil scientist, defined clay-size fractions as the percentage by weight of particles smaller than 2 microns in size. He realized the important role of clay particles in a soil and the plasticity thereof. 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 (see Atterberg, 1911).In October 1909, the 17-m (56-ft) high earth dam at Charmes, France, failed. It was built between 1902 – 1906. A French engineer, Jean Fontard (1884–1962), carried out investigations to determine the cause of failure. In that context, he conducted undrained double-shear tests on clay specimens (0.77 m2 in area and 200 mm thick) under constant vertical stress to determine their shear strength parameters (see Frontard, 1914). The times for failure of these specimens were between 10 to 20 minutes.

Arthur Langley Bell (1874–1956), a civil engineer from England, worked on the design and construction of the outer seawall at Rosyth Dockyard. Based on his work, he developed relationships for lateral pressure and resistance in clay as well as bearing

capacity of shallow foundations in clay (see Bell, 1915). He also used shear-box tests to measure the undrained shear strength of undisturbed clay specimens.

Wolmar Fellenius (1876–1957), an engineer from Sweden, developed the stability analysis of saturated clay slopes (that is, ∅=0 condition) with the assumption that the critical surface of sliding is the arc of a circle. These were elaborated upon in his papers published in 1918 and 1926. The paper published in 1926 gave correct numerical solutions for the stability numbers of circular slip surfaces passing through the toe of the slope.

Karl Terzaghi (1883–1963) of Austria developed the theory of consolidation for clays as we know today. The theory was developed when Terzaghi was teaching at the American Roberts College in Istanbul, Turkey. His study spanned a five-year period from 1919 to 1924. Five different clay soils were used. The liquid limit of those soils ranged between 36 to 67, and the plasticity index was in the range of 18 to 38. The consolidation theory was published in Terzaghi’s celebrated book Erdbaumechanik in 1925.

Reference: Principles of Geotechnical Engineering 7th Edition by BM DAS page 4 - 7

3. Origin of SoilTo the civil engineer, soil is any uncemented or weakly cemented accumulation

of mineral particles formed by the weathering of rocks as part of the rock cycle (Figure 1.1), the void space between the particles containing water and/or air. Weak cementation can be due to carbonates or oxides precipitated between the particles, or due to organic matter. Subsequent deposition and compression of soils, combined with cementation between particles, transforms soils into sedimentary rocks (a process known as lithification). If the products of weathering remain at their original location they constitute a residual soil. If the products are transported and deposited in a different location they constitute a transported soil, the agents of transportation being gravity, wind, water and glaciers. During transportation, the size and shape of particles can undergo change and the particles can be sorted into specific size ranges. Particle sizes in soils can vary from over 100 mm to less than 0.001 mm. In the UK, the size ranges are described as shown in Figure 1.2. In Figure 1.2, the terms ‘clay’, ‘silt’ etc. are used to describe only the sizes of particles between specified limits. However, the same terms are also used to describe particular types of soil, classified according to their mechanical behavior (see Section 1.5).

The type of transportation and subsequent deposition of soil particles has a strong influence on the distribution of particle sizes at a particular location. Some common depositional regimes are shown in Figure 1.3. In glacial regimes, soil material is eroded from underlying rock by the frictional and freeze–thaw action of glaciers. The material, which is typically very varied in particle size from clay to

Figure 1.1 The rock cycle

boulder-sized particles, is carried along at the base of the glacier and deposited as the

ice melts; the resulting material is known as (glacial) till. Similar material is also deposited as a terminal moraine at the edge of the glacier. As the glacier melts, moraine is transported in the outwash; it is easier for smaller, lighter particles to be carried in suspension, leading to a gradation in particle size with distance from the glacier as shown in Figure 1.3(a). In warmer temperate climates the chief transporting action is water (i.e. rivers and seas), as shown in Figure 1.3(b). The deposited material is known as alluvium, the composition of which depends on the speed of water flow. Faster-

flowing rivers can carry larger particles in suspension, resulting in alluvium, which is a mixture of sand and gravel-sized particles, while slower-flowing water will tend to carry only smaller particles. At estuarine locations where rivers meet the sea, material may be deposited as a shelf or delta. In arid (desert) environments (Figure 1.3(c)) wind is the key agent of transportation, eroding rock outcrops and forming a pediment (the desert floor) of fine wind-blown sediment (loess). Towards the coast, a playa of temporary evaporating lakes, leaving salt deposits, may also be formed. The large temperature differences between night and day additionally cause thermal weathering of rock outcrops, producing scree. These surface processes are geologically very recent, and are referred to as drift deposits on geological maps. Soil which has undergone signifi-cant compression/consolidation following deposition is typically much older and is referred to as solid, alongside rocks, on geological maps.

The relative proportions of different-sized particles within a soil are described as its particle size dis-tribution (PSD), and typical curves for materials in different depositional environments are shown in Figure 1.4. The method of determining the PSD of

a deposit and its subsequent use in soil classification is described in Sections 1.4 and 1.5.

At a given location, the subsurface materials will be a mixture of rocks and soils, stretching back many hundreds of millions of years in geological time. As a result, it is important to understand the full geological history of an area to understand the likely characteristics of the deposits that will be present at the surface, as the depositional regime may have changed significantly over geological time. As an example, the West Midlands in the UK was deltaic in the Carboniferous period (~395–345 million years ago), depositing organic material which subsequently became coal measures. In the subsequent Triassic period (280–225 million

Figure 1.4 Particle size distributions of sediments from different depositional environments.

Figure 1.5 Typical ground profile in the West Midlands, UK.

years ago), due to a change in sea level sandy materials were deposited which were subsequently lithified to become Bunter sandstone. Mountain building during this period on what is now the European continent caused the existing rock layers to become folded. It was subsequently flooded by the North Sea during the Cretaceous/Jurassic periods (225–136 million years ago), depositing fine particles and carbonate material (Lias clay and Oolitic limestone). The Ice Ages in the Pleistocene period (1.5–2 million years ago) subsequently led to glaciation over all but the southernmost part of the UK, eroding some of the recently deposited softer rocks and depositing glacial till. The subsequent melting of the glaciers created river valleys, which deposited alluvium above the till. The geological history would therefore suggest that the surficial soil conditions are likely to consist of alluvium overlying till/clay overlying stronger rocks, as shown schematically in Figure 1.5. This example demonstrates the importance of engineering geology in understanding ground conditions. A thorough introduction to this topic can be found in Waltham (2002).

Reference: Craig’s Soil Mechanics, 8th Edition by Knappett and Craig page 3 - 6

4. Soil and RockGeotechnical engineering is highly empirical and is perhaps much more of an

“art” than the other disciplines within civil engineering because of the basic nature of soil and rock materials. They are often highly variable, even within a distance of a few millimeters. Another way of saying this is that soils are heterogeneous rather than homogeneous materials. That is, their material or engineering properties may vary widely from point to point within a soil mass. Furthermore, soils in general are nonlinear materials; their stress-strain curves are not straight lines. To further complicate things (as well as to make them interesting!) soils are non-conservative materials; that is, they have a fantastic memory – they remember almost everything that ever happened to them, and this fact strongly affects their engineering behavior. Instead of being isotropic, soils are typically anisotropic, which means that their material or engineering properties are not that same in all directions. Most of the theories we have for the mechanical behavior of engineering materials assume that the materials are homogeneous and isotropic, and that they obey linear stress-strain laws. Common engineering materials such as steel and concrete do not deviate too significantly from these ideals, and consequently we can use, with discretion, simple linear theories to predict their response under engineering loads. With soils and rock, we are not so fortunate. As you shall see in your study of geotechnical engineering, we may assume a linear stress-strain response, but then we must apply large empirical correction or “safety” factors to our designs to account for the real material behavior. Furthermore, the behavior of soil and rock materials in situ is often governed or controlled by joints, fractures, weak layers and zones, and other “defects” in the material; yet our laboratory tests and simplified methods of analysis often do not take into account such real characteristics of the soil and rock. That is why geotechnical engineering is really an “art” rather than an engineering science. Successful geotechnical engineering depends on the good judgment and practical experience of the designer, constructor, or consultant. Put another way, the successful geotechnical engineer must develop a “feel” for soil and rock behavior before a safe and economic foundation design can be made or an engineering structure can be safely built.

RockRocks are made from various types of minerals. Minerals are substances of

crystalline form made up from a particular chemical combination. The main minerals found in rocks include quartz, feldspar, calcite and mica. Geologists classify all rocks into three basic groups: igneous, sedimentary and metamorphic.

Igneous rocksThese rocks have become solid from a melted liquid state. Extrusive igneous

rocks are those that arrived on the surface of the Earth as molten lava and cooled. Intrusive igneous rocks are formed from magma (molten rock) that forced itself through cracks into rocks beds below the surface and solidified there.

Examples: granite, basalt, gabbroSedimentary rocksWeathering reduces the rock mass to fragmented particles, which can be more

easily transported by wind, water and ice. When dropped by the agents of weathering, they are termed sediments. These sediments are typically deposited in layers or beds

called strata and when compacted and cemented together (lithification) they form sediments rocks.

Examples: shale, sandstone, chalkMetamorphic rocksMetamorphism through high temperatures and pressures acting on sedimentary

or igneous rocks, produces metamorphic rocks. The original rock undergoes both chemical and physical alterations.

Examples: slate, quartzite, marble.

SoilThe actions of frost, temperature, gravity, wind, rain and chemical weathering are

continually forming rock particles that eventually become soils. There are three types of soil when considering modes of formation.

Transported soil (gravels, sands, silts an clays)Most soils have been transported by water. As a stream or river loses its velocity

it tends to deposit some of the particles that it is carrying, dropping the larger, heavier particles first. Hence, on the higher reaches of a river, gravel and sand are found whilst on the lower or older parts, silts and clays predominate, especially where the river enters the sea or a lake and loses its velocity. Ice has been another important transportation agent, and large deposits of boulder clay and moraine are often encountered.

In arid parts of the world wind is continually forming sand deposits in the form of ridges. The sand particles in these ridges have been more or less rolled along and are invariably rounded and fairly uniform in size. Light brown, wind-blown deposits of silt-size particles, known as loess, are often encounters in thin layers, the particles having sometimes travelled considerable distances.

Residual soil (topsoil, laterites)These soils are formed in situ by chemical weathering and may be found on level

rock surfaces where the action of the elements has produced a soil with little tendency to move. Residual soils can also occur whenever the rate of break-up of the rock exceeds the rate of removal. If the parent rock is igneous of metamorphic the resulting soil sizes range from silt to gravel.

Laterites are formed by chemical weathering under warm, humid tropical condition when the rain water leaches out the soluble rock material leaving behind the insoluble hydroxides of iron and aluminum, given them their characteristic red-brown color.

Organic soilThese soils contain large amounts of decomposed animal and vegetable matter.

They are usually dark in color and give off a distinctive odor. Deposits of organic silts and clays have usually been created from river or lake sediments. Peat is special form of organic soil and is a dark brown spongy material which almost entirely consists of lightly to fully decomposed vegetable matter.

Reference: An Introduction to Geotechnical Engineering by Holtz and Kovacs page 3

– 4

Elements of Soil Mechanics 7th Edition by GN Smith and Ian Smith page 2 – 4

5. Soil TypesIt has been discussed earlier that soil is formed by the process of physical and

chemical weathering. The individual size of the constituent parts of even the weathered rock might range from the smallest state (colloidal) to the largest possible (boulders). This implies that all the weathered constituents of a parent rock cannot be termed soil. According to their grain size, soil particles are classified as cobbles, gravel, sand, silt and clay. Grains having diameters in the range of 4.75 to 76.2 mm are called gravel. If the grains are visible to the naked eye, but are less than about 4.75 mm in size the soil is described as sand. The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed as silt and those that are finer than 0.002 mm as clay. This classification is purely based on size which does not indicate the properties of fine grained materials.

Residual and Transported SoilsOn the basis of origin of their constituents, soils can be divided into two large

groups:1. Residual soils, and2. Transported 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. In some areas, this depth might be considerable. In temperate zones residual soils are commonly stiff and stable. An important characteristic of residual soil is that the sizes of grains are indefinite. For example, when a residual sample is sieved, the amount passing any given sieve size depends greatly on the time and energy expended in shaking, because of the partially disintegrated condition.

Transported soils are soils that are found at locations far removed from their place of formation. The transporting agencies of such soils are glaciers, wind and water. The soils are named according to the mode of transportation. Alluvial soils are those that have been transported by running water. The soils that have been deposited in quiet lakes, are lacustrine soils. Marine soils are those deposited in sea water. The soils transported and deposited by wind are aeolian soils. Those deposited primarily through the action of gravitational force, as in land slides, are colluvial soils. Glacial soils are those deposited by glaciers. Many of these transported soils are loose and soft to a depth of several hundred feet. Therefore, difficulties with foundations and other types of construction are generally associated with transported soils.

Organic and Inorganic SoilsSoils in general are further classified as organic or inorganic. Soils of organic

origin are chiefly formed either by growth and subsequent decay of plants such as peat, or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.

Names of Some Soils that are Generally Used in Practice Bentonite - 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. The constituents of varved clays were transported into fresh water lakes by the melted ice at the close of the ice age.

Kaolin, China Clay - are very pure forms of white clay used in the ceramic industry.

Boulder Clay - is a mixture of an unstratified sedimented deposit of glacial clay, containing unsorted rock fragments of all sizes ranging from boulders, cobbles, and gravel to finely pulverized clay material.

Calcareous Soil - is a soil containing calcium carbonate. Such soil effervesces when tested with weak hydrochloric acid.

Marl - consists of 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. Boulder clays or glacial till is also sometimes named as hardpan.

Caliche - is an admixture of clay, sand, and gravel cemented by calcium carbonate deposited from ground water.

Peat - is a fibrous aggregate of finer fragments of decayed vegetable matter. Peat is very compressible and one should be cautious when using it for supporting foundations of structures.

Loam - is a mixture of sand, silt and clay. Loess - is a fine-grained, air-borne deposit characterized by a very uniform grain

size, and high void ratio. The size of particles ranges between about 0.01 to 0.05 mm. The soil can stand deep vertical cuts because of slight cementation between particles. It is formed in dry continental regions and its color is yellowish light brown.

Shale is a material in the state of transition from clay to slate. Shale itself is sometimes considered a rock but, when it is exposed to the air or has a chance to take in water it may rapidly decompose.

Reference: Geotechnical Engineering: Principle and Practices of Soil Mechanics and

Foundation Engineering by VNS Murthy page 7-9

6. Fields of Application of Soil MechanicsThe knowledge of soil mechanics has application in many fields of Civil

Engineering.Foundations

The loads from any structure have to be ultimately transmitted to a soil through the foundation for the structure. Thus, the foundation is an important part of a structure, the type and details of which can be decided upon only with the knowledge and application of the principles of soil mechanics.

Underground and Earth-retaining StructuresUnderground structures such as drainage structures, pipe lines, and tunnels and

earth-retaining structures such as retaining walls and bulkheads can be designed and constructed only by using the principles of soil mechanics and the concept of ‘soil-structure interaction’.

Pavement DesignPavement Design may consist of the design of flexible or rigid pavements.

Flexible pavements depend more on the subgrade soil for transmitting the traffic loads. Problems peculiar to the design of pavements are the effect of repetitive loading, swelling and shrinkage of sub-soil and frost action. Consideration of these and other factors in the efficient design of a pavement is a must and one cannot do without the knowledge of soil mechanics.

Excavations, Embankments and DamsExcavations require the knowledge of slope stability analysis; deep excavations

may need temporary supports—‘timbering’ or ‘bracing’, the design of which requires knowledge of soil mechanics. Likewise the construction of embankments and earth dams where soil itself is used as the construction material, requires a thorough knowledge of the engineering behaviour of soil especially in the presence of water. Knowledge of slope stability, effects of seepage, consolidation and consequent settlement as well as compaction characteristics for achieving maximum unit weight of the soil in-situ, is absolutely essential for efficient design and construction of embankments and earth dams.

The knowledge of soil mechanics, assuming the soil to be an ideal material elastic, isotropic, and homogeneous material—coupled with the experimental determination of soil properties, is helpful in predicting the behaviour of soil in the field. Soil being a particulate and hetergeneous material, does not lend itself to simple analysis. Further, the difficulty is enhanced by the fact that soil strata vary in extent as well as in depth even in a small area. A through knowledge of soil mechanics is a prerequisite to be a successful foundation engineer. It is difficult to draw a distinguishing line between Soil Mechanics and Foundation Engineering; the later starts where the former ends.

Reference: http://www.newagepublishers.com/samplechapter/001206.pdf