What Is The Stress-Strain Curve Of Brittle Materials?

Many of the characteristics of a material can be deduced from the tensile test. It is more convenient to compare materials in terms of stresses and strains, rather than loads and extensions of a particular specimen of a material.

The stress at any stage is the ratio of the load of the original cross-sectional area of the test specimen; the strain is the elongation of a unit length of the test specimen.

For stresses up to about 750 MN/m2 the stress-strain curve is linear, showing that the material obeys Hooke’s law in this range; the material is also elastic in this range, and no permanent extensions remain after removal of the stresses.

The ratio of stress to strain for this linear region is usually about 200 GN/m2 for steels; this ratio is known as Young’s modulus and is denoted by E. The strain at the limit of proportionality is of the order 0.003, and is small compared with strains of the order 0.100 at fracture.

FIG 1.5

We note that Young’s modulus has the units of a stress; the value of E defines the constant in the linear relation between stress and strain in the elastic range of the material. We have

for the linear-elastic range. If P is the total tensile load in a bar, A its cross-sectional area, and Lo its length, then

where e is the extension of the length Lo. Thus the expansion is given by

If the material is stressed beyond the linear-elastic range the limit of proportionality is exceeded, and the strains increase non-linearly with the stresses. Moreover, removal of the stress leaves the material with some permanent extension; h range is then bothnon-linear and inelastic.

The maximum stress attained may be of the order of 1500 MNlm’, and the total extension, or elongation, at this stage may be of the order of 10%. The curve of Figure 1.5 is typical of the behaviour of brittle materials as, for example, area characterized by small permanent elongation at the breaking point; in the case of metals this is usually lo%, or less.

When a material is stressed beyond the limit of proportionality and is then unloaded, permanent deformations of the material take place. Suppose the tensile test-specimen of Figure 1.5 is stressed beyond the limit of proportionality, to a point b on the stress-strain diagram. If the stress is now removed, the stress-strain relation follows the curve bc; when the stress is completely removed there is a residual strain given by the intercept Oc on the &-axis.

If the stress is applied again, the stress-strain relation follows the curve cd initially, and finally the curve df to the breaking point. Both the unloading curve bc and the reloading curve cd are approximately parallel to the elastic line Oa; they are curved slightly in opposite directions.

The process of unloading and reloading, bcd, had little or no effect on the stress at the breaking point, the stress-strain curve being interrupted by only a small amount bd, Figure 1.6. The stress-strain curves of brittle materials for tension and compression are usually similar in form, although the stresses at the limit of proportionality and at fracture may be very different for the two loading conditions.

Typical tensile and compressive stress-strain curves for concrete are shown in Figure 1.7; the maximum stress attainable in tension is only about one-tenth of that in compression, although the slopes of the stress strain curves in the region of zero stress are nearly equal.


What Is Lam´e’s Ellipsoid?

There are always three orthogonal principal directions in a stress state. It is therefore always possible to choose a rectangular Cartesian reference system which coincides with the three principal directions. In this case, the shearing components of the stress tensor vanish and it takes the form

In an inclined facet, with a semi-normal defined by the direction cosines l,m, n, the relation between the components of the stress vector and the principal stresses may be deduced from expression 9, yielding

Since the direction cosines must obey the condition l2+m2+n2 = 1, expression gives

If we consider a Cartesian reference system T1, T2, T3, this expression represents the equation of an ellipsoid, whose principal axes are the reference system and where the points on the ellipsoid are the tips P of the stress vectors
OP (T1, T2, T3) acting in facets containing the point with the stress state defined by expression 24 (point O, Fig. 9)

This ellipsoid is a complete representation of the magnitudes of the stress vectors in facets around point O. It allows an important conclusion about the stress state: the magnitude of the stress in any facet takes a value between the maximum principal stress σ1 and the minimum principal stress σ3.

It must be mentioned here that this conclusion is only valid for the absolute value of the stress, since in expression 26 only the squares of the stresses are considered.

From Fig. 9 we conclude immediately that if the absolute values of two principal stresses are equal the ellipsoid takes a shape of revolution around the third principal direction and if the three principal stresses have the same absolute value the ellipsoid becomes a sphere.

In the first case, the stress→T acting in facets, which are parallel to the third principal direction have the same absolute value. Besides, if these two principal stresses have the same sign, we have an axisymmetric stress state.


Civil Engineering License In Canada

Obtaining Licensure in Alberta 
Civil Engineers must be licensed by the Association of Professional Engineers, Geologists,  and Geophysicists of Alberta (APEGGA) to practice in Alberta. Internationally educated Civil Engineers can and should begin the application process before  they arrive in Alberta.

They should apply to APEGGA as a Foreign Licensee using the  APEGGA Application for Registration. Links to the Application Forms and other resources are at the end of this document. Complete the APEGGA Application for Registration, and provide detailed and accurate  answers to all questions.

When your application is complete with all documents, sign it and  send it to APEGGA at the following address:

APEGGA Head Office
1500 Scotia One
10060 Jasper Avenue NW
Edmonton, Alberta
T5J 4A2
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How to applyfor a Professional Engineer Licence in Ontario
Professional Engineers Ontario (PEO) is the organization that is mandated, under the Professional Engineers Act, with setting the standards for admission and with regulating the practice of professional engineering in Ontario. To practise as a professional engineer in Ontario, an individual must be licensed by PEO.

Licence Requirements
To be granted a licence to practise professional engineering, the applicant must:
be at least 18 years old;
be of good character;
hold an undergraduate engineering degree from a Canadian Engineering Accreditation Board (CEAB)-accredited program (or possess equivalent qualifications).

Refer below to sections
“Minimum Academic Requirement” and “Applicants who do not hold a degree from a CEAB-accredited program;”  successfully complete PEO’s Professional Practice Examination (PPE); and demonstrate at least 48 months of verifiable, acceptable engineering experience, at least 12 months of which must be acquired in a Canadian jurisdiction under a licensed professional engineer (P.Eng.).
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How To Make Trenches For Pipeline In Civil Engineering Project?

The hydraulic hoe or backacter is the machine most widely used for trench excavation for pipelines. In hard ground, rock or roads, the trenching machine might be used. Depths for water and gas pipelines are usually the pipe diameter plus 1 m.

For sewers, greater depths are often required to maintain falls. When flexible plastic pipes are used, especially in the smaller diameters, pipe joints can be made above ground, the pipe being snaked in. Bottoming of the trench can be achieved by using a straightedged bucket without teeth, and the backhoe can also place soft material or concrete into a trench on which to bed pipes or fully surround them.

Provided no men are allowed in the trench, timbering can thus be avoided. When large diameter steel pipes with welded joints have to be laid, a string of several pipes may be welded up alongside the trench, and dozers equipped with side lifting booms can lower the string of pipes into the prepared trench.

This reduces the amount of timbering and excavation of joint holes necessary which need only be arranged where successive strings have to be jointed together. The principal defects occurring on pipelines come from defective joints and pipe fracture due to settlement of a pipe on a hard band, large stone or lump of rock in the base of the trench.

The use of the hydraulic hoe makes the preparation of an even bed for the pipe easier to achieve, especially on suitable selected soft granular fill. However, the base of the trench and the bedding along each length of pipe must be carefully boned in before the pipe is lowered to ensure each pipe is fully supported along its body.

For non-flexible pipes of ductile iron, asbestos cement, steel or concrete it will be necessary to joint them after laying. Sufficient access is then required for the jointer to make the joint properly, and support to the trench sides will be essential in every case where there is not absolute certainty there can be no slip of material into the trench.

Falls of material into trenches are a major hazard in civil engineering, and adoption of a consistent, rigorously applied safety approach is the only way to prevent accidents. The damaging weight of even a small fall of earth must be borne in mind.

While it will be obvious that gravity sewers must be laid to a fall, it is sometimes not appreciated that pressurized trunk water mains should be laid to a minimum rise or fall. The preferred minimum gradients are 1:500 on a rising grade in the direction of flow; and 1:300 on a falling grade.

The former would be to an air release valve, the latter from the air valve to a washout or hydrant. Thus the levels of ground ahead of the pipelaying must be prospected to locate suitable high and low points, and intermediate points where an increase or decrease of grade is necessary.

The pipeline between such pre-determined points should follow an even grade. In flat ground it may not be possible to comply with the foregoing grades, but it is still advisable to give uniform rises to air valves and falls to washout positions.

In built-up areas pipelines can generally follow the requisite cover below ground surface because branches and connections will release air, and hydrants will be used as washouts. Backfill to pipes should always be of selected soft or fine granular material to 150 mm above the crown of the pipe.

Few contractors in UK would fail to do this, but on some contracts overseas the resident engineer may need to stop the contractor from dozing the excavated hard material straight back into the trench irrespective of the rocks it contains which would at the least damage the sheathing to pipes.


What To Do With Excavated Materials In Civil Engineering Projects?

For large open excavations, such as when road cuttings have to be made and the material tipped to form embankments, or for building an earth dam from open borrow pit areas, the motorscraper is the most economical machine for excavating, transporting and placing clays and clay-sand mixes.

But the gradients traversed need to be gentle and the motorscraper cannot pick up hard bands of material or rock, unless ripping beforehand can break up the material sufficiently.

If hard or rocky material has to be excavated, the face shovel loading to dump trucks has to be used, the trucks commonly having a capacity of 50–60 t, sometimes larger. However, neither scrapers nor dump trucks can traverse public roads.

If the excavated material has to be routed off site via public roads to some dumping area, the excavated material has to be carted away by tipping lorries licensed for use on the public highway. Tipping lorries have a lesser capacity than dump trucks, usually in the range 10–30 t.

A factor often having considerable influence when needing to transport material along public roads, is the reaction of the local road and public authorities who may object to the extra construction traffic and mud on the roads. If the local authority has also to give planning permission for dumping spoil on some given land, such permission may only be granted subject to restriction on the size of lorries used and their frequency of passage.

This situation cannot be left for tenderers to find out; the employer has to obtain the necessary permissions and the contract must reproduce exactly the conditions laid down by the planning or other authority concerned and require the contractor to conform to them.

If the restrictions limit the size and frequency of tipping lorries, the contractor may be forced to temporarily stockpile excavated material on site and double handle it in order to conform to his intended programme for construction and the haulage conditions laid
down. This will raise his costs for excavation.

Assuming there are no planning restrictions, the contractor needs to choose that combination of excavating plant and haulage vehicles which achieves the required excavation rate at lowest cost. The face shovel or backhoe output must match the timing of empty vehicles back from the dumping ground and their loading capacity.

This means that the excavator bucket size and loading cycle time must be such that one haulage truck is loaded and moving away by the time the next vehicle arrives. Hence, the cycle loading time for the excavator must be known.

Thus if 10m3 haulage vehicles return at 5 min intervals, and the cycle loading time is 1.5 min, only three cycles of loading are possible so an excavator bucket size of 3.3m3 is required.

Alternatively if the cycle loading time could be 1.25 min a 2.5m3 bucket would suffice. Allowance has to be made for the bulking factor and unit weight of the material to be excavated.

The bulking of granular or soft material may range 1.1–1.3, through 1.4 for hard clays, to 1.6–1.7 for broken rock. Clays, clay–sand mixtures, gravels and sands may weigh 1.6–1.9 t/m3 in situ while rock and hard materials may vary 1.9–2.6 t/m3 in situ.

The excavator bucket size has to allow for the bulking factor: for example, a 2m3 bucket may only lift and load 1.4m3 loose material at 1.4 bulking factor, so it will need seven loading cycles to fill a 10-m3 tipper wagon. If this is too long a loading time for the required rate of output, an excavator with a larger capacity bucket is required.

Correct assessment of the bulking factor is financially important to the contractor, particularly in relation to the use of tipping lorries for offsite deposition of material. Whereas dump trucks used on site can be heaped, tipping lorries have a limited cubic capacity and payload, neither of which can be exceeded.

Thus if a bulking factor of 1.2 applies, a 10m3 lorry will take away the equivalent of 8.3m3 net excavation; but if the bulking factor is 1.35 the 10m3 lorry will take away only 7.4m3 net excavation. If the contractor has based his price on the former but experiences the latter, he would find his price for disposal of material off site 12 per cent too low.

This could mean no profit on the operation or a large financial loss, since there may be many thousands of cubic metres of material involved. In practice a contractor’s past experience will guide him as to what plant to use, taking into account many other practical matters which apply, such as reliability of different types of plant, need for standby, margins for hold-ups, length and nature of haulage road, cost of transporting plant to and from the job, and hire rates for different sizes of excavator and haulage vehicles.


What Are The Corrosion Resistance Methods For Structural Steel?

Since steel contains three of the four elements needed for corrosion, protective coatings can be used to isolate the steel from moisture, the fourth element. There are three mechanisms by which coatings provide corrosion protection (Hare, 1987):

1. Barrier coatings work solely by isolating the steel from the moisture. These coatings have low water and oxygen permeability.

2. Inhabitive primer coatings contain passivating pigments. They are low-solubility pigments that migrate to the steel surface when moisture passes through the film to passivate the steel surface.

3. Sacrificial primers (cathodic protection) contain pigments such as elemental zinc. Since zinc is higher than iron in the galvanic series, when corrosion conditions exist the zinc gives up electrons to the steel, becomes the anode, and corrodes to protect the steel.

There should be close contact between the steel and the sacrificial primer in order to have an effective corrosion protection.

Cathodic protection can take forms other than coating. For example, steel structures such as water heaters, underground tanks and pipes, and marine equipment can be electrically connected to another metal that is more reactive in the particular environment, such as magnesium or zinc.

Such reactive metal (sacrificial anode) experiences oxidation and gives up electrons to the steel, protecting the steel from corrosion. Figure 3.32 illustrates an underground steel tank that is electrically connected to a magnesium sacrificial anode (Fontana and Green, 1978).

Above is a diagram on Cathodic protection of an underground pipeline using a magnesium sacrificial anode.


What Makes The Steel Corrode? What Is Steel Corrosion?

Corrosion is defined as the destruction of a material by electrochemical reaction to the environment. For simplicity, corrosion of steel can be defined as the destruction that can be detected by rust formation.

Corrosion of steel structures can cause serious problems and embarrassing and/or dangerous failures. For example, corrosion of steel bridges, if left unchecked, may result in lowering weight limits, costly steel replacement, or collapse of the structure.

Other examples include corrosion of steel pipes, trusses, frames, and other structures. It is estimated that the cost of corrosion of the infrastructure in the United States alone is $22.6 billion each year (corrosion costs web site, 2009).

The infrastructure includes (1) highway bridges, (2) gas and liquid transmission pipelines, (3) waterways and ports, (4) hazardous materials storage, (5) airports, and (6) railroads.

Corrosion is an electrochemical process; that is, it is a chemical reaction in which there is transfer of electrons from one chemical species to another. In the case of steel, the transfer is between iron and oxygen, a process called oxidation reduction.

Corrosion requires the following four elements (without any of them corrosion will not occur):
1. an anode—the electrode where corrosion occurs
2. a cathode—the other electrode needed to form a corrosion cell
3. a conductor—a metallic pathway for electrons to flow
4. an electrolyte—a liquid that can support the flow of electrons

Steel, being a heterogeneous material, contains anodes and cathodes. Steel is also an electrical conductor. Therefore, steel contains three of the four elements needed for corrosion, while moisture is usually the fourth element (electrolyte).

The actual electrochemical reactions that occur when steel corrodes are very complex. However, the basic reactions for atmospherically exposed steel in a chemically neutral environment are dissolution of the metal at the anode and reduction of oxygen at the cathode.

Contaminants deposited on the steel surface affect the corrosion reactions and the rate of corrosion. Salt, from deicing or a marine environment, is a common contaminant that accelerates corrosion of steel bridges and reinforcing steel in concrete.

The environment plays an important role in determining corrosion rates. Since an electrolyte is needed in the corrosion reaction, the amount of time the steel stays wet will affect the rate of corrosion.

Also, contaminants in the air, such as oxides or sulfur, accelerate corrosion. Thus, areas with acid rain, coal-burning power plants, and other chemical plants may accelerate corrosion.


What Is Torsion Test Of Steel?

The torsion test (ASTM E143) is used to determine the shear modulus of structural materials. The shear modulus is used in the design of members subjected to torsion, such as rotating shafts and helical compression springs.

In this test a cylindrical, or tubular, specimen is loaded either incrementally or continually by applying an external torque to cause a uniform twist within the gauge length. The amount of applied torque and the corresponding angle of twist are measured throughout the test.

Below shows the shear stress–strain curve.

The shear modulus is the ratio of maximum shear stress to the corresponding shear strain below the proportional limit of the material, which is the slope of the straight line between R (a pretorque stress) and P (the proportional limit). For a circular cross section, the maximum shear stress shear strain and the shear modulus (G) are determined by the equations:


T = torque
r = radius

J = polarmoment of inertia of the specimen about its center, for a solid circular cross section.

0 = angle of twist in radians
L = gauge length

The test method is limited to materials and stresses at which creep is negligible compared with the strain produced immediately upon loading. The test specimen should be sound, without imperfections near the surface.

Also, the specimen should be straight and of uniform diameter for a length equal to the gauge length plus two to four diameters. The gauge length should be at least four diameters.

During the test, torque is read from a dial gauge or a readout device attached to the testing machine, while the angle of twist may be measured using a torsiometer fastened to the specimen at the two ends of the gauge length.

A curve-fitting procedure can be used to estimate the straight-line portion of the shear stress–strain relation.


What Is Structural Steel Tension Test?

The tension test (ASTM E8) on steel is performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area. Typically, the test is performed at temperatures between 10°C and 35°C (50°F to 95°F).

The test specimen can be either full sized or machined into a shape, as prescribed in the product specifications for the material being tested. It is desirable to use a small cross-sectional area at the center portion of the specimen to ensure fracture within the gauge length.

Several cross-sectional shapes are permitted, such as round and rectangular, as shown in Figure 3.15. Plate, sheet, round rod, wire, and tube specimens may be used. A 12.5 (1/2 in.) diameter round specimen is used in many cases. The gauge length over which the elongation is measured typically is four times the diameter for most round-rod specimens.

Various types of gripping devices may be used to hold the specimen, depending on its shape. In all cases, the axis of the test specimen should be placed at the center of the testing machine head to ensure axial tensile stresses within the gauge length without bending.

An extensometer with a dial gauge or an LVDT is used to measure the deformation of the entire gauge length. The test is performed by applying an axial load to the specimen at a specified rate.

Mild steel has a unique stress–strain relation. As the stress is increased beyond the proportion limit, the steel will yield, at which time the strain will increase without an increase in stress (actually the stress will slightly decrease). As tension increases past the yield point, strain increases following a nonlinear relation up to the point of failure.

Note that the decrease in stress after the peak does not mean a decrease in strength. In fact, the actual stress continues to increase until failure. The reason for the apparent decrease is that a neck is formed in the steel specimen, causing an appreciable decrease in the cross-sectional area.

The traditional, or engineering, way of calculating the stress and strain uses the original cross-sectional area and gauge length. If the stress and stains are calculated based on the instantaneous cross-sectional area and gauge length, a true stress–strain curve is obtained, which is different than the engineering stress–strain curve.

The true stress is larger than the engineering stress, because of the reduced cross-sectional area at the neck. Also, the true strain is larger than the engineering strain, since the increase in length at the vicinity of the neck is much larger than the increase in length outside of the neck.

The specimen experiences the largest deformation (contraction of the cross-sectional area and increase in length) at the regions closest to the neck, due to the nonuniform distribution of the deformation. The large increase in length at the neck increases the true strain to a large extent because the definition of true strain utilizes a ratio of the change in length in an infinitesimal gauge length.

By decreasing the gauge length toward an infinitesimal size and increasing the length due to localization in the neck, the numerator of an expression is increased while the denominator stays small, resulting in a significant increase in the ratio of the two numbers.

Note that when calculating the true strain, a small gauge length should be used at the neck, since the properties of the material (such as the cross section) at the neck represent the true material properties. For various practical applications, however, the engineering stresses and strains are used, rather than the true stresses and strains.

Different carbon-content steels have different stress–strain relations. Increasing the carbon content in the steel increases the yield stress and reduces the ductility. Below shows the tension stress–strain diagram for hot-rolled steel bars containing carbons from 0.19% to 0.90%.

Increasing the carbon content from 0.19% to 0.90% increases the yield stress from 280 MPa to 620 MPa (40 ksi to 90 ksi). Also, this increase in carbon content decreases the fracture strain from about 0.27 m/m to 0.09 m/m. Note that the increase in carbon content does not change the modulus of elasticity.

Steel is generally assumed to be a homogeneous and isotropic material. However, in the production of structural members, the final shape may be obtained by cold rolling.

This essentially causes the steel to undergo plastic deformations, with the degree of deformation varying throughout the member. Plastic deformation causes an increase in yield strength and a reduction in ductility.

This figure demonstrates that the measured properties vary, depending on the orientation of the sample relative to the axis of rolling (Hassett, 2003). Thus, it is necessary to specify how the sample is collected when evaluating the mechanical properties of steel.


What Are The Different Cold-Formed Steel Shapes?

A wide variety of shapes can be produced by cold-forming and manufacturers have developed a wide range of products to meet specific applications.

 Figure 3.11 shows the common shapes of typical cold-formed steel framing members.

Figure 3.12 shows common shapes for profiled sheets and trays used for roofing and wall cladding and for load bearing deck panels.

For common applications, such as structural studs, industry organizations, such as the Steel Framing Alliance (SFA) and the Steel Stud Manufacturers Association (SSMA) have developed standard shapes and nomenclature to promote uniformity of product availability across the industry.

Figure 3.11 shows the generic shapes covered by the Universal Designator System. The designator consists of four sequential codes.

The first code is a three or four-digit number indicating the member web depth in 1/100 inches. The second is a single letter indicating the type of member, as follows:

framing member with stiffening lips
L = Angle or L-header
F = Furring channels
U = Cold-rolled channel
T = Track section
S = Stud or joist

The third is a three-digit numeral indication flange width in 1/100 inches followed by a dash. The fourth is a two or three-digit numeral indicating the base steel thickness in 1/1000 inch (mils).

As an example, the designator system for a 6'', C-shape with 1-5/8'' (1.62'') flanges and made with 0.054'' thick steel is 600S162-54.


What Are The Special Design Considerations for Cold-Formed Steel?

Structural design of cold-formed members is in many respects more challenging than the design of hot rolled, relatively thick, structural members. A primary difference is cold-formed members are more susceptible to buckling due to their limited thickness.

The fact that the yield strength of the steel is increased in the cold-forming process creates a dilemma for the designer. Ignoring the increased strength is conservative, but results in larger members, hence more costly, than is needed if the increased yield strength is considered.

Corrosion creates a greater percent loss of cross section than is the case for thick members. All cold-formed steel members are coated to protect steel from corrosion during the storage and transportation phases of construction as well as for the life of the product.

Because of its effectiveness, hot-dipped zinc galvanizing is most commonly used. Structural and non-structural framing members are required to have a minimum metallic coating that complies with ASTM A1003/A1003M, as follows:

■ structural members – G60 and
■ non-structural members G40 or equivalent minimum.

To prevent galvanic corrosion special care is needed to isolate the cold-formed members from dissimilar metals, such as copper.

The design, manufacture and use of cold-formed steel framing is governed by standards that are developed and maintained by the American Iron and Steel Institute along with organizations such as ASTM, and referenced in the building codes.

Additional information is available at


What Is Mohr's Circle? The Purpose Of Mohr's Circle

The shear strength of soil is generally characterized by the Mohr–Coulomb failure criterion. This criterion states that there is a linear relationship between the shear strength on the failure plane at failure (τff) and the normal stress on the failure plane at failure (σff) as given in the following equation:
                            τff = σff tanφ + c
where φ is the friction angle and c is the intrinsic cohesion. The strength parameters (φ, c) are used directly in many stability calculations, including bearing capacity of shallow footings, slope stability, and stability of retaining walls. The line defined by Eq. (17.1) is called the failure envelope.

A Mohr’s circle tangent to a point on the failure envelope (σff, τff) intersects the x-axis at the major and minor principal stresses at failure (σ1f, σ3f). For many soils, the failure envelope is actually slightly concave down rather than a straight line.

For a comprehensive review of Mohr’s circles and the Mohr–Coulomb failure criterion, see Lambe and Whitman [1969] and Holtz and Kovacs [1981]. But more online resource, below are links to articles that best explain and gives example on the application of Mohr's Circle:

Mohr's Circle Calculator
Given the stress components sx, sy, and txy, this calculator computes the principal stresses s1, s2, the principal angle qp, the maximum shear stress tmax and its angle qs. It also draws an approximate Mohr's cirlce for the given stress state. Continue reading...

Mohr's Circle for 2-D and 3-D Stress Analysis
After the data for the Mohr's circle are input, press the button "Draw", then the Mohr's circle can be created; press the button "fill", the Mohr's circle are created and filled with red color. If the Mohr's circles are too small, press the button "size 1" or "size 2" to enlarge them. Whenever the data for the Mohr's circle are modified, press "Draw" or "fill" button to get modified Mohr's circle. Besides, the paramters for the Mohr's circle and calculated principal stresses and maximum shear stress are given too. Continue reading...

Mohr's Circle Information
Mohr's circle, named after Christian Otto Mohr, is a two-dimensional graphical representation of the state of stress at a point. The abscissa, , and ordinate, , of each point on the circle are the normal stress and shear stress components, respectively, acting on a particular cut plane with a unit vector  with components. Continue reading...


What Are The Strength Classes Of Rocks Used In Construction?

Based on the scale effects and geological conditions discussed in the previous sections, it can be seen that sliding surfaces can form either along discontinuity surfaces, or through the rock mass. The importance of the classification is that in essentially all slope stability analysis it is necessary to use the shear strength properties of either the discontinuities or of the rock mass, and there are different procedures for determining the strength properties as follows:

• Discontinuity shear strength can be measured in the field and the laboratory.
• Rock mass shear strength is determined by empirical methods involving either back analysis of slopes cut in similar geological conditions, or by calculation involving rock strength indices.

As a further illustration of the effects of geology on shear strength, relative strength parameters for three types of discontinuity and two types of rock mass are shown on the Mohr diagram. The slope of these lines represents the friction angle, and the intercept with the shear stress axis represents the cohesion

A description of these conditions on Figure 4.7 is as follows:

Curve 1 Infilled discontinuity: If the infilling is a weak clay or fault gouge, the infilling friction angle (φinf ) is likely to be low, but there may be some cohesion if the infilling is undisturbed.

Alternatively, if the infilling is a strong calcite for example, which produces a healed surface, then the cohesive strength may be significant.

Curve 2 Smooth discontinuity: A smooth, clean discontinuity will have zero cohesion, and the friction angle will be that of the rock surfaces (φr). The friction angle of rock is related to the grain size, and is generally lower in fine grained rocks than in coarse-grained rocks.

Curve 3 Rough discontinuity: Clean, rough discontinuity surfaces will have zero cohesion, and the friction angle will be made up of two components. First, the rock material friction angle (φr), and second, a component (i) related to the roughness (asperities) of the surface and the ratio between the rock strength and the normal stress.

As the normal stress increases, the asperities are progressively sheared off and the total friction angle diminishes.

Curve 4 Fractured rock mass: The shear strength of a fractured rock mass, in which the sliding surface lies partially on discontinuity surfaces and partially passes through intact rock, can be expressed as a curved envelope. At low normal stresses where there is little confinement of the fractured rock and the individual fragments may move and rotate, the cohesion is low but the friction angle is high.

At higher normal stresses, crushing of the rock fragments begins to take place with the result that the friction angle diminishes. The shape of the strength envelope is related to the degree of fracturing, and the strength of the intact rock.

Curve 5 Weak intact rock: Rocks that are composed of fine grained material that has a low friction angle. However, because it contains no discontinuities, the cohesion can be higher than that of a strong intact rock that is closely fractured. The range of shear strength conditions that may be encountered in rock slopes clearly demonstrates the importance of examining both the characteristics of the discontinuities and the rock strength during the site investigation.


What Are The Types Of Civil Engineering Construction Insurance?

Commercial General Liability Insurance
This type of insurance provides coverage for claims by third parties against the CM/GC and all additional named insured parties. A pedestrian who has an accident while passing by a construction site would be covered under this type of insurance policy.

Builders Risk Insurance
This type of insurance provides coverage against the insured’s loss to the property during the construction process. A break in a water service in the building during the construction process, which damages electrical, mechanical, elevator, and plumbing systems would be covered under this type of insurance policy.

It is a primary coverage rather than against a claim from a third party. One needs to define the policy period for which it is in effect, which could be when the project is completed or a date thereafter.

Errors and Omissions Insurance
This is the professional liability insurance for the CM/GC and design professionals. CM/GCs cannot easily obtain architectural or engineering services Errors and Omission (E&O) coverage. The CM/GC must be careful with this type of insurance, especially if involved with a design-build project.

Environmental Liability Insurance 
This is a specialized insurance policy to cover pollution and hazardous material damages such as asbestos, mold, lead paint, medical waste, fuel oil, etc. The coverage under the commercial general liability (CGL) policy is very limited, and a separate policy with broader environmental coverage is often obtained to deal with these matters.

It is recommended that the owner directly hold the contract for environmental work and provide the insurance policy for the appropriate coverage, with the CM/GC as the additional named insured.

Workers’ Compensation Insurance
Workers’ Compensation (WC) coverage is a state mandatory insurance to provide coverage for the CN/GC’s workers if they are injured while performing their work. It provides for lost wages, medical coverage, and loss of partial or full ability to work because of an accident on the job.

Automobile Coverage
This provides coverage for all automobiles and trucks used in connection with the construction of the project, as well as transporting personnel and material to and from the project site. The insurance company will require the names and drivers license information for all drivers operating vehicles covered under the policy.

If a driver’s report from the Department of Motor Vehicles is not good, that person may be excluded from coverage under the policy, and thus should not drive company vehicles.


What Is The Size And Revenue Of Civil Engineering Contractors?

The construction industry is composed of about 710,000 businesses, mostly small companies, 91 percent of which have fewer than 20 employees. While the largest U.S.-based contractor had revenues of $22 billion in 2007, the overwhelming majority of builders had an annual volume of less than $10 million.

To provide a snapshot of the construction industry, it can be categorized as one in which building contractors range in size from a small family-owned business operating in a narrow geographic area to giant multinational firms.Finding the right one for your project is sometimes a confusing task but can be made somewhat easier by understanding how the industry works.

This is a business of high risk and relatively low profi t margins. The Construction Financial Management Association (CFMA) of Princeton, New Jersey, is a nonprofit organization serving the construction financial community; every year it surveys the 7,000 members in chapters across the country to obtain financial data and the major concerns of the industry.

The members include residential, nonresidential, industrial highway, and specialty contractors. The 2007 financial survey presented the following national overview as reported by the respondents:

■ The year’s hot topic was fi eld personnel recruitment and the ability to retain qualified workers, a concern that will continue for the immediate future. (This can impact owners, who may see a decrease in quality levels of workers.)

■ Construction jobs are good jobs, with the seasonally adjusted hourly rate of $21.08 per hour as of September 2007, a rate that reflects a 4.5 percent increase over the previous year for the same period. (Owners may find that labor increases in the construction industry exceed the overall inflation figures reported in the media.)

■ Material costs are a major problem. From December 2003 to September 2007, construction material producer prices indices increased 30 percent, more than double the 13 percent rise in the Consumer Price Index (CPI). Steel, cement, diesel fuel, and other petroleum-based products were at the top of the price
increase column.

The construction slowdown in 2008 in the United States has had a dampening effect on price increases of some materials, while worldwide demand has increased the prices of others. The projected building cost index for 2009, as reported by McGraw-Hill in December of 2008, refl ected a decrease of 0.5 percent, as opposed to an increase of 5.5 percent for the year 2007 – 2008.

■ In 2006, shipments of construction materials exceeded $500 billion, approximately 11 percent of the total shipments by U.S. manufacturers, and shipments of construction machinery topped $36 billion, 11 percent of all U.S. machinery manufacturers.

Due to the value of the dollar in relation to other world currencies, heavy equipment manufacturers like Caterpillar saw export sales rise during that period.

■ The typical construction establishment is a small company with an average employment of fewer than nine individuals.

■ Internal Revenue Service fi gures for 2004 show that the 700,000 corporations in construction had a net income of $47 billion, or 3.7 percent of total receipts of $1.3 trillion, considerably below the all-industry average margin of 4.9 percent.

■ Construction is a high-turnover industry. The Small Business Administration (SBA) showed that in 2004, 77,000 companies closed shop.

CFMA reviews the member responses and prepares a Best-in-Class composite for nonresidential and industrial building contractors.


What Are Deep Sea Clay? Deep Sea Clay Information

The clay materials formed in large parts of the deep sea and oceanic basins are, generally, quite distinct from terrigenous clays. This is because many such areas are so far removed from land that detrital terrigenous material becomes a minor, even insignificant, source of sediment.

As a result, the products of other processes make a more important contribution to the fine grained sediments that accumulate in these environments (Berger 1974). Globally, the most important of these are the minute skeletal components of microfossils which form a continuous pelagic rain from surface to deeper waters.

Their contribution to the fine grained sediment accumulating at the ocean floor depends upon the dynamic balance between the processes of their production in surface waters and their destruction by dissolution on their journey down through the water column following death of the organisms. The two most important biogenic components are calcareous and siliceous microfossils.

The calcareous microfossils include foraminifera and coccoliths composed of calcium carbonate (CaCO3) mainly in the form of calcite, whilst the silceous microfossils include diatoms and radiolaria composed of opaline amorphous silica (SiO2), in the form known as opal-A. The rate of production of these organisms in surface ocean waters depends on biological fertility.

Diatoms dominate in more fertile nutrient rich water whereas coccoliths dominate in less fertile regions. Since seawater is universally under saturated with respect to amorphous silica, most silica is dissolved and recycled
as the skeletons of opaline microfossils descend through the water column.

A further fraction arrives at the sediment water interface where more is dissolved but in regions of high productivity some is preserved and may accumulate. Thus the distribution of siliceous pelagic sediments mirrors the patterns of the most highly productive ocean waters such as in regions of oceanic divergence and upwelling where nutrient-rich deep ocean waters rise to the surface.

The fate of calcareous pelagic sediment is similar except that the degree of undersaturation of seawater with respect to carbonates increases with depth. This gives rise to a depth in the oceans, known as the Calcite Compensation Depth (CCD), below which calcite does not accumulate.

In the deepest parts of the Ocean basins (> 3500 m) below the CCD, sedimentation rates may be extremely slow and hydrogenous processes involving iron and manganese oxides take on an important role. Such areas accumulate deposits know as deep sea red clays (Glasby 1991).

Red clays are extremely fine grained with often more than 80% < 2 um in size. They cover about 30% of the ocean basins and are most prevalent in the Pacific Ocean. Most of the components of Pacific red clays are allogenic, the most important being aeolian dust.

Red clays accumulate very slowly with the highest rates of sedimentation coeval with Pleistocene glacial periods when aeolian dust production was at a maximum (Glasby 1991). Because of their fine grain-size and long term stability serious consideration has been given to using red clays as sites for radioactive waste disposal (Burkett et al. 1991).


What Are The Parts Of The Bulldozers?

Bulldozers are great help in civil and structural engineering constructions. It is one of the most visible and most prominent machines in the construction site.

Bulldozers are these machines consist of a track or wheel mounted power unit with a mould blade at the front which is controlled by hydraulic rams. Many bulldozers have the capacity to adjust the mould blade to form an angledozer and the capacity to tilt the mould blade about a central swivel point.

Some bulldozers can also be fitted with rear attachments such as rollers and scarifiers.

The main functions of a bulldozer are:-

1 . Shallow excavations up to 300 m deep either on level ground or sidehill cutting.

2. Clearance of shrubs and small trees.

3. Clearance of trees by using raised mould blade as a pusher arm.

4. Acting as a towing tractor.

5. Acting as a pusher to scraper machines

Bulldozers (‘dozers’) are used for cutting and grading work, for pushing scrapers to assist in their loading, stripping borrowpits, and for spreading and compacting fill. The larger sizes are powerful but are costly to run and maintain, so it is not economic for the contractor to keep one on site for the occasional job.

Its principal full-time use is for cutting, or for spreading fill for earthworks in the specified layer thickness and compacting and bonding it to the previously compacted layer. It is the weight and vibration of the dozer that achieves compaction, so that a Caterpillar ‘D8’ 115 h.p. weighing about 15 t, or its equivalent, is the machine required; not a ‘D6’ weighing 7.5 t which is not half as effective in compaction. The dozer cannot shift material very far, it can only spread it locally.

A dozer with gripped tracks can climb a 1 in 2 slope, and may also climb a slope as steep as 1 in 1.5 provided the material of the slope gives adequate grip and is not composed of loose rounded cobbles. On such slopes of 1 in 1.5 or 1 in 2 the dozer must not turn, but must go straight up or down the slope, turning
on flatter ground at the top and bottom.

It is dangerous to work a dozer (and any kind of tractor) on sidelong ground, particularly if the ground is soft. Dozers cannot traverse metalled roads because of the damage this would cause, and they should not be permitted on finished formation surfaces. Sometimes a flat tracked dozer (i.e. with no grips to the tracks) can be used on a formation if the ground is suitable.

A Typical Bulldozer and Its parts is presented below:

This article is part of power equipment basic information series.


Cement Hydration And Concrete Curing Information

Concrete curing is not simply a matter of the concrete hardening as it dries out. In fact, it is just the opposite. Portland cement is a hydraulic material. That is, it requires water for curing and can, in fact, fully cure to a hardened state even if it is completely submerged in water.

Portland cement is anhydrous—it contains no water or moisture at all. The moment it comes in contact with water, a chemical reaction takes place in which new compounds are formed. This reaction is called cement hydration.

The rate of hydration varies with the composition of the cement, the fineness of the cement particles, the amount of water present, the air temperature, and the presence of admixtures. If the mixing water dries out too rapidly before the cement has fully hydrated, the curing process will stop and the concrete will not harden to its intended strength.

Curing will resume if more water is introduced, but at a slower rate. Hydration occurs more rapidly at higher air temperatures. Cement hydration itself generates heat, too. This heat of hydration can be helpful during cold-weather construction, and potentially harmful during hot-weather construction.

The chemical reaction between water and cement first forms a paste which must completely coat each aggregate particle during mixing. After a time, the paste begins to stiffen or set, and after a few hours has lost is plasticity entirely.

The rate of this setting, however, is not the same as the rate of hardening. A Type-III high-early-strength cement may set in about the same time as a Type-I general-purpose cement, but the Type III hardens and develops compressive strength more rapidly after it has set.

Concrete normally cures to its full design strength in 28 days. Curing is slower in cold weather, and at temperatures below 40°F, the concrete can be easily and permanently damaged if it is not properly protected.

Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. Concrete that is not kept moist reaches only about 50% of its design strength. Figure 2-19 shows the differences in concrete strength for various periods of moist curing.

If it is kept moist for at least three days, it will reach about 80% of its design strength, and for seven days, 100% of its design strength. If the concrete is kept moist for the full 28- day curing period, it will reach more than 125% of its design strength.


What Are The Types Of Specification In Civil Engineering Projects?

Technical requirements may be specified in different ways, depending on what best meets the client’s requirements. One or more of the following types of technical specifications may be used for a building project.

Descriptive Specifications. These describe the components of a product and how they are assembled. The specification writer specifies the physical and chemical properties of the materials, size of each member, size and spacing of fastening devices, exact relationship of moving parts, sequence of assembly, and many other requirements.

The contractor has the responsibility of constructing the work in accordance with this description. The architect or engineer assumes total responsibility for the function and performance of the end product. Usually, architects and engineers do not have the resources, laboratory, or technical staff capable of conducting research on the specified materials or products.

Therefore, unless the specification writer is very sure the assembled product will function properly, descriptive specifications should not be used.

Reference Specifications. These employ standards of recognized authorities to specify quality. Among these authorities are ASTM, American National Standards Institute, National Institute of Standards and Technology, Underwriters Laboratories, Inc., American Institute of Steel Construction, American Concrete Institute, and American Institute of Timber Construction.

An example of a reference specification is: Cement shall be portland cement conforming to ASTM C150, ‘‘Specification for Portland Cement,’’ using Type 1 or Type 11 for general concrete construction. Reputable companies state in their literature that their products conform to specific recognized standards and furnish independent laboratory reports supporting their claims.

The buyer is assured that the products conform to minimum requirements and that the buyer will be able to use them consistently and expect the same end result. Reference specifications generally are used in conjunction with one or more of the other types of specifications.

Proprietary Specifications. These specify materials, equipment, and other products by trade name, model number, and manufacturer. This type of specification simplifies the specification writer’s task, because commercially available products set the standard of quality acceptable to the architect or engineer.

Sometimes proprietary specifications can cause complications because manufacturers reserve the right to change their products without notice, and the product incorporated in the project may not be what the specifier believed would be installed. Another disadvantage of proprietary specifications is that they may permit use of alternative products that are not equal in every respect.

Therefore, the specifier should be familiar with the products and their past performance under similar use and should know whether they have had a history of satisfactory service. The specifier should also take into consideration the reputation of the manufacturers or subcontractors for giving service and their attitude toward repair or replacement of defective or inferior work.

Under a proprietary specification, the architect or engineer is responsible to the client for the performance of the material or product specified and for checking the installation to see that it conforms with the specification. The manufacturer of the product specified by the model number has the responsibility of providing the performance promised in its literature.

In general, the specification writer has the responsibility of maintaining competition between manufacturers and subcontractors to help keep costs in line. Naming only one supplier may result in a high price. Two or more names are normally supplied for each product to enhance competition.

Use of ‘‘or equal’’ should be avoided. It is not fully satisfactory in controlling quality of materials and equipment, though it saves time in preparing the specification. Only one or two products need to be investigated and research time needed to review other products is postponed.

Base-Bid Specifications. These establish acceptable materials and equipment by naming one or more (often three) manufacturers and fabricators. The bidder is required to prepare a proposal with prices submitted from these suppliers. Usually, base-bid specifications permit the bidder to submit substitutions or alternatives for the specified products.

When this is done, the bidder should state in the proposal the price to be added to, or deducted from, the base bid and include the name, type, manufacturer, and descriptive data for the substitutions. Final selection rests with the client. Base-bid specifications often provide the greatest control of quality of materials and equipment, but there are many pros and cons for the various types of specifications, and there are many variations of them.


What Is Bearing Capacity?

Some designers, when in a hurry, tend to want simple ‘rules of thumb’ (based on local experience) for values of bearing capacity. But like most rules of thumb, while safe for typical structures on normal soils, their use can produce uneconomic solutions, restrict the development of improved methods of foundation design, and lead to expensive mistakes when the structure is not typical.

For typical buildings:

(1) The dead and imposed loads are built up gradually and relatively slowly.
(2) Actual imposed loads (as distinct from those assumed for design purposes) are often only a third of the dead load.
(3) The building has a height/width ratio of between 1/3 and 3.
(4) The building has regularly distributed columns or load bearing walls, most of them fairly evenly loaded.

Typical buildings have changed dramatically since the Second World War. The use of higher design stresses, lower factors of safety, the removal of robust non-load-bearing partitioning, etc., has resulted in buildings of half their previous weight, more susceptible to the effects of settlement, and built for use by clients who are less tolerant in accepting relatively minor cracking of finishes, etc.

Because of these changes, practical experience gained in the past is not always applicable to present construction. For non-typical structures:

(1) The imposed load may be applied rapidly, as in tanks and silos, resulting in possible settlement problems.
(2) There may be a high ratio of imposed to dead load. Unbalanced imposed-loading cases – imposed load over part of the structure – can be critical, resulting in differential settlement or bearing capacity failures, if not allowed for in design.
(3) The requirement may be for a tall, slender building which may be susceptible to tilting or overturning and have more critical wind loads.
(4) The requirement may be for a non-regular column/ wall layout, subjected to widely varying loadings, which may require special consideration to prevent excessive differential settlement and bearing capacity failure.

There is also the danger of going to the other extreme by doing complicated calculations based on numbers from unrepresentative soil tests alone, and ignoring the important evidence of the soil profile and local experience. Structural design and materials are not, as previously stated, mathematically precise; foundation design and materials are even less precise.

Determining the bearing capacity solely from a 100 mm thick small-diameter sample and applying it to predict the behaviour of a 10 m deep stratum, is obviously not sensible – particularly when many structures could fail, in serviceability, by settlement at bearing pressures well below the soil’s ultimate bearing capacity.

Bearing capacity
Probably the happy medium is to follow the sound advice given by experienced engineers in the British Standard Institution’s Code of practice for foundations, BS 8004. There they define ultimate bearing capacity as ‘the value of the gross loading intensity for a particular foundation at which the resistance of the soil to displacement of the foundation is fully mobilized.’ (Ultimate in this instance does not refer to ultimate limit state.)

The net loading intensity (net bearing pressure) is the additional intensity of vertical loading at the base of a foundation due to the weight of the new structure and its loading, including any earthworks. The ultimate bearing capacity divided by a suitable factor of safety – typically 3 – is referred to as the safe bearing capacity.

It has not been found possible, yet, to apply limit state design fully to foundations, since bearing capacity and settlement are so intertwined and influence both foundation and superstructure design. Furthermore, the superstructure itself can be altered in design to accommodate, or reduce, the effects of settlement. A reasonable compromise has been devised by engineers in the past and is given below.


What Are The Methods Of Classifying Soils In Foundation Design?

It is necessary for the foundation engineer to classify the site soils for use as a foundation for several reasons:

1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."

The Unified Soil Classification System (USCS) of Table 2-1 is much used in foundation work. A version of this system has been standardized by ASTM as D 2487 (in Volume 04.08: Soil and Rock; Dimension Stone; Geosynthetics). The standardized version is similar to the original USCS as given by Casagrande (1948) but with specified percentages of sand or gravel passing specific sieves being used to give the "visual description" of the soil.

The original Casagrande USCS only classified the soil using the symbols shown in Table 2-1 (GP, GW, SM, SP, CL, CH, etc.), based on the indicated percentages passing the No. 4 and No. 200 sieves and the plasticity data. The author has always suggested a visual description supplement such as the following:

It is evident in this table that terms "trace" and "with" are somewhat subjective. The soil color, such as "blue clay," "gray clay," etc., is particularly useful in soil classification.

In many areas the color—particularly of cohesive soils—is an indication of the presence of the same soil stratum as found elsewhere. For example the "soft blue clay" on the soil profile of Fig. 2-4 for Chicago has about the same properties at any site in the Chicago area.

In foundation work the terms loose, medium, and dense, , and consistency descriptions such as soft, stiff, very stiff, etc., are also commonly used in foundation soil classification. Clearly, all of these descriptive terms are of great use to the local geotechnical engineer but are somewhat subjective.

That is, there could easily be some debate over what is a "medium" versus a "dense" sand, for example. The D 2487 standard removed some of the subjectiveness of the classification and requires the following terminology:

< 15% is sand or gravel use name (organic clay, silt, etc.)
15% < x < 30% is sand or gravel describe as clay or silt with sand, or clay or silt with gravel
> 30% is sand or gravel describe as sandy clay, silty clay, or gravelly clay, gravelly silt

The gravel or sand classification is based on the percentage retained on the No. 4 (gravel) sieve or passing the No. 4 and retained on the No. 200 (sand) sieves. This explanation is only partial, as the new standard is too lengthy to be presented in detail.

Although not stated in D 2487, the standard is devised for using a computer program3 to classify the soil. Further, not all geotechnical engineers directly use the ASTM standard, particularly if their practice has a history of success using the original USC system.


What Are The 5 Major Factors That Affect The Engineering Properties of Soils?

Most factors that affect the engineering properties of soils involve geological processes acting over long time periods. Among the most important are the following.

1. Natural Cementation and Aging

All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed of considerable importance in cohesionless deposits as well.

The effect of cementation and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation. Care must be taken to ascertain the quantitative effects properly since sample disturbance and the small relative quantity of grains in a laboratory sample versus site amounts may provide difficulties in making a value measurement that is more than just an estimate.

Field observations have well validated the concept of the cementation and aging process. Loess deposits, in particular, illustrate the beneficial effects of the cementation process where vertical banks are readily excavated.

2. Overconsolidation

A soil is said to be normally consolidated (nc) if the current overburden pressure (column of soil overlying the plane of consideration) is the largest to which the mass has ever been subjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one.

The soil is said to be overconsolidated (or preconsolidated) if the stress history involves a stress state larger than the present overburden pressure.

Overconsolidated cohesive soils have received considerable attention. Only more recently has it been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is relatively easy to ascertain overconsolidation in cohesive soils but very difficult in cohesionless deposits.

The behavior of overconsolidated soils under new loads is different from that of normally consolidated soils, so it is important— particularly for cohesive soils—to be able to recognize the occurrence.

3. Mode of Deposit Formation

Soil deposits that have been transported, particularly via water, tend to be made up of small grain sizes and initially to be somewhat loose with large void ratios.

They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can support larger grain sizes.

These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes.

Soil deposits developed'where the transporting agent is a glacier tend to be more varied in composition. These deposits may contain large sand or clay lenses. It is not unusual for glacial deposits to contain considerable amounts of gravel and even suspended boulders.

Glacial deposits may have specific names as found in geology textbooks such as moraines, eskers, etc.; however, for foundation work our principal interest is in the uniformity and quality of the deposit. Dense, uniform deposits are usually not troublesome. Deposits with an erratic composition may be satisfactory for use, but soil properties may be very difficult to obtain.

Boulders and lenses of widely varying characteristics may cause construction difficulties. The principal consideration for residual soil deposits is the amount of rainfall that has occurred. Large amounts of surface water tend to leach materials from the upper zones to greater depths. A resulting stratum of fine particles at some depth can affect the strength and settlement characteristics of the site.

4. Quality of the Clay

The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur.

Where drying has produced shrinkage cracks in the deposit we have a fissured clay. This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing intact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity.

A great potential for strength reduction exists during construction where opening an excavation reduces the overburden pressure so that expansion takes place along any fissures. Subsequent rainwater or even local humidity can enter the fissure so that interior as well as surface softening occurs.

A clay without fissures is an intact clay and is usually normally consolidated or at least has not been over consolidated from shrinkage stresses. Although these clays may expand from excavation of overburden, the subsequent access to free water is not so potentially disastrous as for fissured clay because the water effect is more nearly confined to the surface.

5. Soil Water

Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any soil will reduce the shear strength.

A sufficient increase can reduce the shear strength to zero—for cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of considerable importance when considering major structures (such as power plants) in earthquake-prone areas.

When soil water just dampens sand, the surface tension produced will allow shallow excavations with vertical sides. If the water evaporates, the sides will collapse; however, construction vibrations can initiate a cave-in prior to complete drying.

The sides of a vertical excavation in a cohesive soil may collapse from a combination of rainfall softening the clay together with excess water entering surface tension cracks to create hydrostatic water pressure. In any case, the shear strength of a cohesive soil can be markedly influenced by water.

Even without laboratory equipment, one has probably seen how cohesive soil strength can range from a fluid to a brick-like material as a mudhole alongside a road fills during a rain and subsequently dries. Ground cracks in the hole bottom after drying are shrinkage (or tension) cracks.


What Are Structure Foundations?

Foundations may be classified based on where the load is carried by the ground, producing:

Shallow foundations—termed bases, footings, spread footings, or mats. The depth is generally D/B < 1 but may be somewhat more. Refer to Fig. 1-la.

Deep foundations—piles, drilled piers, or drilled caissons. Lp/B > 4+ with a pile illustrated
in Fig. l-\b.

Figure 1-1 illustrates general cases of the three basic foundation types considered in this text and provides some definitions commonly used in this type of work. Because all the definitions and symbols shown will be used throughout the text, the reader should give this figure careful study.

The superstructure brings loads to the soil interface using column-type members. The loadcarrying columns are usually of steel or concrete with allowable design compressive stresses on the order of 14O+ MPa (steel) to 1O+ MPa (concrete) and therefore are of relatively small cross-sectional area. The supporting capacity of the soil, from either strength or deformation considerations, is seldom over 1000 kPa but more often on the order of 200 to 250 kPa.

This means the foundation is interfacing two materials with a strength ratio on the order of several hundred. As a consequence the loads must be "spread" to the soil in a manner such that its limiting strength is not exceeded and resulting deformations are tolerable. Shallow foundations accomplish this by spreading the loads laterally, hence the term spread footing.

Where a spread footing (or simply footing) supports a single column, a mat is a special footing used to support several randomly spaced columns or to support several rows of parallel columns and may underlie a portion of or the entire building. The mat may also be supported, in turn, by piles or drilled piers.

Foundations supporting machinery and such are sometimes termed bases. Machinery and the like can produce a substantial load intensity over a small area, so the base is used as a load-spreading device similar to the footing.

Deep foundations are analogous to spread footings but distribute the load vertically rather than horizontally. A qualitative load distribution over depth for a pile is shown in Fig. 1-1 b. The terms drilled pier and drilled caisson are for the pile type member that is constructed by drilling a 0.76+-m diameter hole in the soil, adding reinforcing as necessary, and backfilling the cavity with concrete.

A major consideration for both spread footings (and mats) and piles is the distribution of stresses in the stress influence zone beneath the foundation [footing or pile tip (or point)].

The theoretical distribution of vertical stress beneath a square footing on the ground surface is shown in Fig. IAa. It is evident that below a critical depth of about 5B the soil has a negligible increase in stress (about 0.02qo) from the footing load.

This influence depth depends on B, however. For example, if B = 0.3 m, the critical stress zone is 5 X 0.3 = 1.5 m, and if B = 3 m, the zone is 15 m for a zonal influence depth ratio of 1 : 10. Because these B values are in a possible range beneath a large building, any poor soils below a depth of 2 m would have a considerable influence on the design of the wider footings.

Any structure used to retain soil or other material (see Fig. 1-lc) in a geometric shape other than that naturally occurring under the influence of gravity is a retaining structure.

Retaining structures may be constructed of a large number of materials including geotextiles, wood and metal sheeting, plain or reinforced concrete, reinforced earth, precast concrete elements, closely spaced pilings, interlocking wood or metal elements (crib walls), and so on. Sometimes the retaining structure is permanent and in other cases it is removed when it is no longer needed.

The foundations selected for study in this text are so numerous that their specialized study is appropriate. Every building in existence rests on a foundation whether formally designed or not. Every basement wall in any building is a retaining structure, whether formally designed or not.

Major buildings in areas underlain with thick cohesive soil deposits nearly always use piles or drilled caissons to carry the loads vertically to more competent strata, primarily to control settlement. Note that nearly every major city is underlain by clay or has zones where clay is present and requires piles or caissons.

Numerous bridges have retaining structures at the abutments and spread foundations carrying the intermediate spans. Usually the abutment end reactions are carried into the ground by piles. Harbor and offshore structures (used primarily for oil production) use piles extensively and for both vertical and lateral loads.


What Are The Different Types and Application Of Paints? 

Types of Paints
Depending upon their constituents there are various types of paints. A brief description of some of them which are commonly used are given below:

1. Oil Paint: These paints are applied in three coats-primer, undercoat and finishing coat. The presence of dampness while applying the primer adversely affect the life of oil paint. This paint is cheap and easy to apply.

2. Enamel Paint: It contains white lead, oil, petroleum spirit and resinous material. The surface provided by it resists acids, alkalies and water very well. It is desirable to apply a coat of titanium white before the coat of enamel is applied. It can be used both for external and internal walls.

3. Emulsion Paint: It contains binding materials such as polyvinyl acetate, synthetic resins etc. It dries in 1 1/2 to 2 hours and it is easy to apply. It is more durable and can be cleaned with water. For plastered surfaces, first a coat of cement paint should be applied and then the emulsion point. Emulsion paint needs sound surfaces.

4. Cement Paint: It is available in powder form. It consists of white cement, pigment and other additives. It is durable and exhibits excellent decorative appearance. It should be applied on rough surfaces rather than on smooth surfaces. It is applied in two coats. First coat is applied on wet surface but free from excess water and allowed to dry for 24 hours. The second coat is then applied which gives good appearance.

5. Bituminous Paints: This type of paint is manufactured by dissolving asphalt or vegetable bitumen in oil or petroleum. It is black in colour. It is used for painting iron works under water.

6. Synthetic Rubber Paint: This paint is prepared from resins. It dries quickly and is little affected by weather and sunlight. It resists chemical attack well. This paint may be applied even on fresh concrete. Its cost is moderate and it can be applied easily.

7. Aluminium Paint: It contains finely ground aluminium in spirit or oil varnish. It is visible in darkness also. The surfaces of iron and steel are protected well with this paint. It is widely used for painting gas tanks, water pipes and oil tanks.

8. Anti-corrossive Paint: It consists essentially of oil, a strong dier, lead or zinc chrome and finely ground sand. It is cheap and resists corrossion well. It is black in colour.

Application of Paint
Preparation of surface for application of paint is the most important part in painting. The surface to be painted should not be oily and it should be from flakes of the old paint.

Cracks in the surface should be filled with putty and then with sand paper. Then primer is applied.

Painting work should be carried out in dry weather. The under coats and first coats must be allowed to dry before final coat is applied.