Composite types of coarse soil are named in Table 1. Mixes of very coarse material with finer soils can be described by combining the descriptions of the two components, e. The state of compaction or stiffness of the in-situ soil can be assessed by means of the tests or indications detailed in Table 1. Discontinuities such as fissures and shear planes, including their spacings, should be indicated.
Bedding features, including their thickness, should be detailed. Alternating layers of varying soil types or with bands or lenses of other materials are described as interstratified.
Layers of different soil types are described as interbedded or inter- laminated, their thickness being stated. Bedding surfaces that separate easily are referred to as partings. If partings incorporate other material, this should be described. Table 1. Soil classification systems General classification systems in which soils are placed into groups on the basis of grading and plasticity have been used for many years. The feature of these systems is that each soil group is denoted by a letter symbol representing main and qualifying terms.
The terms and letters used in the UK are detailed in Table 1. The liquid and plastic limits are used to classify fine soils, employing the plasticity chart shown in Figure 1. The axes of the plasticity chart are plasticity index and liquid limit; therefore, the plasticity characteristics of a particular soil can be represented by a point on the chart. Classification letters are allotted to the soil according to the zone within which the point lies.
The chart is divided into five ranges of liquid limit. The four ranges I, H, V and E can be combined as an upper range U if closer designation is not required or if the rapid assessment procedure has been used to assess plasticity. The diagonal line on the chart, known as the A-line, should not be regarded as a rigid boundary between clay and silt for purposes of soil description, as opposed to classification.
A similar classification system was developed in the US [10] but with less detailed subdivisions. The letter denoting the dominant size fraction is placed first in the group symbol.
If a soil has a significant content of organic matter the suffix O is added as the last letter of the group symbol. The name of the soil group should always be given, as above, in addition to the symbol, the extent of subdivision depending on the particular situation.
If the rapid procedure has been used to assess grading and plasticity the group symbol should be enclosed in brackets to indicate the lower degree of accuracy associated with this procedure.
The alternative term M-SOIL is introduced to describe material which, regardless of its particle size distribution, plots below the A-line on the plasticity chart: the use of this term avoids confusion with soils of predominantly silt size but with a significant proportion of clay-size particles which plot above the A-line.
Fine soils containing significant amounts of organic matter usually have high to extremely high liquid limits and plot below the A-line as organic silt. Peats usually have very high or extremely high liquid limits. Any cobbles or boulders particles retained on a mm BS sieve are removed from the soil before the classification tests are carried out but their percentages in the total sample should be determined or estimated. A general classification system is useful in developing an understanding of the nature of different soil types.
However, in practice it is more appropriate to use systems based on properties related to the suitability of soils for use in specific construction situations. For example, the UK Department of Transport [6] has detailed a classification system for the acceptability of soils for use in earthworks in highway construction.
In this system, soils are allocated to classes based mainly on grading and plasticity but, for some classes, chemical, compaction and strength characteristics are also specified.
Example 1. The results of limit tests on soil D are: Liquid limit: Cone penetration mm The percentage water content, to the nearest integer, corresponding to a penetration of 20 mm is the liquid limit and is The plastic limit is the average of the two percentage water contents, again to the nearest integer, i.
The plasticity index is the difference between the liquid and plastic limits, i. This is a till, a glacial deposit having a large range of particle sizes. Phase relationships 17 Figure 1. In a completely dry soil there are two phases, namely the solid soil particles and pore air. A fully saturated soil is also two phase, being composed of solid soil particles and pore water.
A partially saturated soil is three phase, being composed of solid soil particles, pore water and pore air. The components of a soil can be represented by a phase diagram as shown in Figure 1. The following relationships are defined with reference to Figure 1. The water content w , or moisture content m , is the ratio of the mass of water to the mass of solids in the soil, i.
Drying should continue until the differences between successive weighings at four-hourly intervals are not greater than 0. A drying period of 24 h is normally adequate for most soils.
See BS The degree of saturation Sr is the ratio of the volume of water to the total volume of void space, i. The void ratio e is the ratio of the volume of voids to the volume of solids, i. Procedures for determining the value of particle density are detailed in BS Part 2 [2]. Particle density is used in preference to Gs in British Standards but it is advantageous to use Gs which is dimensionless in deriving relationships from the phase diagram.
From the definition of void ratio, if the volume of solids is 1 unit then the volume of voids is e units. The volume of water is thus wGs. These volumes and masses are represented in Figure 1. The following relationships can now be obtained. The maximum density is determined by compacting a sample underwater in a mould, using a circular steel tamper attached to a vibrating hammer: a mould is used for sands and a 2.
The soil from the mould is then dried in an oven, enabling the dry density to be determined. The minimum dry density can be determined by one of the following procedures. In the case of sands, a measuring cylinder is partially filled with a dry sample of mass g and the top of the cylinder closed with a rubber stopper.
The minimum density is achieved by shaking and inverting the cylinder several times, the resulting volume being read from the gradu- ations on the cylinder. Full details of the above tests are given in BS Part 4 [2]. Void ratio can be calculated from a value of dry density using Equation 1. However, the density index can be calculated directly from the maximum, minimum and in-situ values of dry density, avoiding the need to know the value of Gs.
After being completely dried in an oven the mass of the sample is g. The value of Gs for the soil is 2. Determine the bulk density, unit weight, water content, void ratio, porosity, degree of saturation and air content. In general, the higher the degree of compaction the higher will be the shear strength and the lower will be the compressibility of the soil. An engineered fill is one in which the soil has been selected, placed and compacted to an appropriate specification with the object of achieving a particular engineering performance, generally based on past experience.
The aim is to ensure that the resulting fill possesses properties that are adequate for the function of the fill. This is in contrast to non-engineered fills which have been placed without regard to a subsequent engineering function.
The degree of compaction of a soil is measured in terms of dry density, i. The compaction characteristics of a soil can be assessed by means of standard laboratory tests.
The soil is compacted in a cylindrical mould using a standard compactive effort. In BS Part 4 [2] three compaction procedures are detailed. In the Proctor test the volume of the mould is 1 l and the soil with all particles larger than 20 mm removed is compacted by a rammer consisting of a 2.
If the sample contains a limited proportion of particles up to In the vibrating hammer test, the soil with all particles larger than After compaction using one of the three standard methods, the bulk density and water content of the soil are determined and the dry density calculated.
For a given soil the process is repeated at least five times, the water content of the sample being increased each time. Dry density is plotted against water content and a curve of the form shown in Figure 1. This curve shows that for a particular method of compaction i. At low values of water content most soils tend to be stiff and are difficult to compact. As the water content is increased the soil becomes more workable, facilitating compaction and resulting in higher dry densities.
At high water contents, however, the dry density decreases with increasing water content, an increas- ing proportion of the soil volume being occupied by water. However, this degree of compaction is unattainable in practice. The experimental dry density—water content curve for a particular compactive effort must lie completely to the left of the zero air voids line. These curves enable the air content at any point on the experimental dry density—water content curve to be determined by inspection.
For a particular soil, different dry density—water content curves are obtained for different compactive efforts. Curves representing the results of tests using the 2. The curve for the 4. Thus, a higher compactive effort results in a higher value of maximum dry density and a lower value of optimum water content; however, the values of air content at maximum dry density are approximately equal. The dry density—water content curves for a range of soil types using the same compactive effort the BS 2.
In general, coarse soils can be compacted to higher dry densities than fine soils. Field compaction The results of laboratory compaction tests are not directly applicable to field compac- tion because the compactive efforts in the laboratory tests are different, and are applied in a different way, from those produced by field equipment. Further, the laboratory tests are carried out only on material smaller than either 20 or However, the maximum dry densities obtained in the laboratory using the 2.
A minimum number of passes must be made with the chosen compaction equipment to produce the required value of dry density. This number, which depends on the type and mass of the equipment and on the thickness of the soil layer, is usually within the range 3— Above a certain number of passes no significant increase in dry density is obtained.
In general, the thicker the soil layer the heavier the equipment required to produce an adequate degree of compaction. There are two approaches to the achievement of a satisfactory standard of compac- tion in the field, known as method and end-product compaction.
In the UK these details are given, for the class of material in question, in the Specification for Highway Works [6]. In end-product compaction the required dry density is specified: the dry density of the compacted fill must be equal to or greater than a stated percentage of the maximum dry density obtained in one of the standard laboratory compaction tests. Method compaction is used in most earthworks. End- product compaction is normally restricted to pulverized fuel ash in general fill and to certain selected fills.
Field density tests can be carried out, if considered necessary, to verify the standard of compaction in earthworks, dry density or air content being calculated from meas- ured values of bulk density and water content. A number of methods of measuring bulk density in the field are detailed in BS Part 4 [2]. The following types of compaction equipment are used in the field.
Smooth-wheeled rollers These consist of hollow steel drums, the mass of which can be increased by water or sand ballast. They are suitable for most types of soil except uniform sands and silty sands, provided a mixing or kneading action is not required. Smooth-wheeled rollers, and the other types of roller described below, can be either towed or self-propelled.
Pneumatic-tyred rollers This equipment is suitable for a wide range of coarse and fine soils but not for uniformly graded material. Wheels are mounted close together on two axles, the rear set overlapping the lines of the front set to ensure complete coverage of the soil surface.
The tyres are relatively wide with a flat tread so that the soil is not displaced laterally. This type of roller is also available with a special axle which allows the wheels to wobble, thus preventing the bridging over of low spots. Pneumatic-tyred rollers impart a kneading action to the soil. The finished surface is relatively smooth, resulting in a low degree of bonding between layers. If good bonding is essential, the compacted surface must be scarified between layers.
Increased compactive effort can be obtained by increasing the tyre inflation pressure or, less effectively, by adding kentledge to the body of the roller. Sheepsfoot rollers This type of roller consists of hollow steel drums with numerous tapered or club-shaped feet projecting from their surfaces. The mass of the drums can be increased by ballasting.
The arrangement of the feet can vary but they are usually from to mm in length with an end area of 40—65 cm2. The feet thus impart a relatively high pressure over a small area. Initially, when the layer of soil is loose, the drums are in contact with the soil surface.
Subsequently, as the projecting feet compact below the surface and the soil becomes sufficiently dense to support the high contact pressure, the drums rise above the soil.
Sheepsfoot rollers are most suitable for fine soils, both plastic and non-plastic, especially at water contents dry of optimum. The action of the feet causes significant mixing of the soil, improving its degree of homogeneity, and will break up lumps of stiff material. Due to the penetration of the feet, excellent bonding is produced between successive soil layers, an important requirement for water-retaining earthworks.
Grid rollers These rollers have a surface consisting of a network of steel bars forming a grid with square holes. Kentledge can be added to the body of the roller. Grid rollers provide high contact pressure but little kneading action and are suitable for most coarse soils. Vibratory rollers These are smooth-wheeled rollers fitted with a power-driven vibration mechanism. They are particularly effective for coarse soils with little or no fines.
The mass of the roller and the frequency of vibration must be matched to the soil type and layer thickness. The lower the speed of the roller the fewer the number of passes required. Vibrating plates This equipment, which is suitable for most soil types, consists of a steel plate with upturned edges, or a curved plate, on which a vibrator is mounted. The unit, under manual guidance, propels itself slowly over the surface of the soil.
Power rammers Manually controlled power rammers, generally petrol-driven, are used for the com- paction of small areas where access is difficult or where the use of larger equipment would not be justified. They are also used extensively for the compaction of backfill in trenches.
They do not operate effectively on uniformly graded soils. Moisture condition test As an alternative to standard compaction tests, the moisture condition test is widely used in the UK. This test, developed by the Transport and Road Research Laboratory [8], enables a rapid assessment to be made of the suitability of soils for use as fill materials. The test does not involve the determination of water content, a cause of delay in obtaining the results of compaction tests.
In principle, the test consists of determining the effort required to compact a soil sample normally 1. The soil is compacted in a cylindrical mould having an internal diameter of mm centred on the base plate of the apparatus.
Compaction is imparted by a rammer having a diameter of 97 mm and a mass of 7 kg falling freely from a height of mm. The fall of the rammer is controlled by an adjustable release mechanism and two vertical guide rods. The penetration of the rammer into the mould is measured by means of a scale on the side of the rammer. A fibre disc is placed on top of the soil to prevent extrusion between the rammer and the inside of the mould. Full details are given in BS Part 4 [2].
The penetration is measured at various stages of compaction. For a given number of rammer blows n the penetration is subtracted from the penetration at four times that number of blows 4n. The change in penetration between n and 4n blows is plotted against the logarithm to base 10 of the lesser number of blows n. A change in penetration of 5 mm is arbitrarily chosen to represent the condition beyond which no significant increase in density occurs. The moisture condition value MCV is defined as 10 times the logarithm of the number of blows corresponding to a change in penetration of 5 mm on the above plot.
An example of such a plot is shown in Figure 1. For a range of soil types it has been shown that the relationship between water content and MCV is linear over a substantial range of water content. Allot group symbols and give main and qualifying terms appropriate for each soil. The value of Gs is 2. Calculate the void ratio and degree of saturation of the soil. What would be the values of density and water content if the soil were fully saturated at the same void ratio?
When dried completely in an oven the specimen weighs What is the degree of saturation of the specimen? Calculate the dry density, void ratio, degree of saturation and air content. Would it be possible to compact the above soil at a water content of Plot the dry density—water content curve and give the optimum water content and maximum dry density.
The volume of the mould is cm3. The maximum and minimum dry densities, determined by standard laboratory tests, are 1. Determine the density index of the sand. Chapter 2 Seepage 2. The pressure of the pore water is measured relative to atmospheric pressure and the level at which the pressure is atmospheric i.
The level of the water table changes according to climatic conditions but the level can change also as a consequence of constructional operations. A perched water table can occur locally, contained by soil of low permeability, above the normal water table level. Artesian conditions can exist if an inclined soil layer of high permeability is confined locally by an overlying layer of low permeability; the pressure in the artesian layer is governed not by the local water table level but by a higher water table level at a distant location where the layer is unconfined.
Below the water table the pore water may be static, the hydrostatic pressure depending on the depth below the water table, or may be seeping through the soil under hydraulic gradient: this chapter is concerned with the second case.
Above the water table, water can be held at negative pressure by capillary tension; the smaller the size of the pores the higher the water can rise above the water table. The capillary rise tends to be irregular due to the random pore sizes occurring in a soil. The soil can be almost completely saturated in the lower part of the capillary zone but in general the degree of saturation decreases with height.
When water percolates through the soil from the surface towards the water table some of this water can be held by surface tension around the points of contact between particles. The negative pressure of water held above the water table results in attractive forces between the particles: this attraction is referred to as soil suction and is a function of pore size and water content.
Permeability 31 2. The coefficient of permeability depends primarily on the average size of the pores, which in turn is related to the distribution of particle sizes, particle shape and soil structure. In general, the smaller the particles the smaller is the average size of the pores and the lower is the coefficient of permeability. The presence of a small percentage of fines in a coarse-grained soil results in a value of k significantly lower than the value for the same soil without fines.
For a given soil the coefficient of permeability is a function of void ratio. If a soil deposit is stratified the permeability for flow parallel to the direction of stratification is higher than that for flow perpendicular to the direction of stratification. The presence of fissures in a clay results in a much higher value of permeability compared with that of the unfissured material.
The coefficient of permeability also varies with temperature, upon which the viscos- ity of the water depends. The values of k for different types of soil are typically within the ranges shown in Table 2. On the microscopic scale the water seeping through a soil follows a very tortuous path between the solid particles but macroscopically the flow path in one dimension can be considered as a smooth line. The soil specimen, at the appropriate density, is contained in a Perspex cylinder of cross-sectional area A: the specimen rests on a coarse filter or a wire mesh.
If a high degree of saturation is to be maintained the water used in the test should be de-aired. For fine soils the falling-head test Figure 2. In the case of fine soils, undisturbed specimens are normally tested and the containing cylinder in the test may be the sampling tube itself.
The length of the specimen is l and the cross-sectional area A. A coarse filter is placed at each end of the specimen and a standpipe of internal area a is connected to the top of the cylinder. The water drains into a reservoir of constant level. The standpipe is filled with water and a measurement is made of the time t1 for the water level relative to the water level in the reservoir to fall from h0 to h1.
The coefficient of permeability of fine soils can also be determined indirectly from the results of consolidation tests see Chapter 7. The reliability of laboratory methods depends on the extent to which the test specimens are representative of the soil mass as a whole. More reliable results can generally be obtained by the in-situ methods described below.
Well pumping test This method is most suitable for use in homogeneous coarse soil strata. The procedure involves continuous pumping at a constant rate from a well, normally at least mm in diameter, which penetrates to the bottom of the stratum under test. A screen or filter is placed in the bottom of the well to prevent ingress of soil particles.
Perforated casing is normally required to support the sides of the well. Water levels are observed in a number of boreholes spaced on radial lines at various distances from the well.
An unconfined stratum of uniform thickness with a relatively impermeable lower boundary is shown in Figure 2. A confined layer between two impermeable strata is shown in Figure 2. Frequent recordings are made of the water levels in the boreholes, usually by means of an electrical dipper.
The test enables the average coefficient of permeability of the soil mass below the cone of depression to be determined. Full details of the test procedure are given in BS Analysis is based on the assumption that the hydraulic gradient at any distance r from the centre of the well is constant with depth and is equal to the slope of the water table, i. This is known as the Dupuit assumption and is reasonably accurate except at points close to the well.
In the case of an unconfined stratum Figure 2. At distance r from the well the area through which seepage takes place is 2 rh, where r and h are variables. A hydraulic gradient is thus established, causing seepage either into or out of the soil mass surrounding the bore- hole and the rate of flow is measured. In a constant-head test the water level is maintained throughout at a given level Figure 2.
In a variable-head test the water level is allowed to fall or rise from its initial position and the time taken for the level to change between two values is recorded Figure 2. The tests indicate the permeability of the soil within a radius of only 1—2 m from the centre of the borehole. Careful boring is essential to avoid disturbance in the soil structure.
A problem in such tests is that clogging of the soil face at the bottom of the borehole tends to occur due to the deposition of sediment from the water. To alleviate the q hc W. Seepage theory 37 problem the borehole may be extended below the bottom of the casing, as shown in Figure 2.
The extension may be uncased or supported by perforated casing depending on the type of soil. Another solution is to install within the casing a central tube perforated at its lower end and set within a pocket of coarser material. Expressions for the coefficient of permeability depend on whether the stratum is unconfined or confined, the position of the bottom of the casing within the stratum and details of the drainage face in the soil.
If the soil is anisotropic with respect to permeability and if the borehole extends below the bottom of the casing Figure 2. If, on the other hand, the casing penetrates below soil level in the bottom of the borehole Figure 2. Values of intake factor F were published by Hvorslev [5] and are also given in BS [1].
The coefficient of permeability for a coarse soil can also be obtained from in-situ measurements of seepage velocity, using Equation 2. The method involves exca- vating uncased boreholes or trial pits at two points A and B Figure 2. The hydraulic gradient is given by the difference in the steady-state water levels in the boreholes divided by the distance AB. Dye or any other suitable tracer is inserted into borehole A and the time taken for the dye to appear in borehole B is measured.
The seepage velocity is then the distance AB divided by this time. The porosity of the soil can be determined from density tests. Seepage theory 39 Equation 2. Integrating Equation 2. Such curves are called equipotentials. If the function x, z is given a series of constant values, 1 , 2 , 3 , etc. These curves are called flow lines.
Referring to Figure 2. Seepage theory 41 Figure 2. The directions s and n are inclined at angle to the x and z axes, respectively. The solution is represented by a family of flow lines and a family of equipotentials, constituting what is referred to as a flow net.
Possible methods of solution are complex variable techniques, the finite difference method, the finite element method, electrical ana- logy and the use of hydraulic models. Computer software based on either the finite difference or finite element methods is widely available for the solution of seepage problems. Williams et al.
Relatively simple problems can be solved by the trial and error sketching of the flow net, the general form of which can be deduced from consideration of the boundary condi- tions. Flow net sketching leads to a greater understanding of seepage principles. However, for problems in which the geometry becomes complex and there are zones of different permeabilities throughout the flow region, use of the finite element method is usually necessary.
The fundamental condition to be satisfied in a flow net is that every intersection between a flow line and an equipotential must be at right angles.
The figure shows a line of sheet piling driven 6. On one side of the piling the depth of water is 4. The first step is to consider the boundary conditions of the flow region. At every point on the boundary AB the total head is constant, so AB is an equipotential; similarly CD is an equipotential. The datum to which total head is referred may be any level but in seepage problems it is convenient to select the downstream water level as datum. Then, the total head on equipotential CD is zero pressure head 0.
From point B, water must flow down the upstream face BE of the piling, round the tip E and up the down-stream face EC. Water from point F must flow along the impermeable surface FG. The first trial sketching of the flow net Figure 2. The estimated line of flow HJ from a point on AB near the piling is lightly sketched. This line must start at right angles to equipotential AB and follow a smooth curve round the bottom of the piling. Trial equipotential lines are then drawn between the flow lines BEC and HJ, intersecting both flow lines at right angles and forming curvilinear squares.
If necessary the position of HJ should be altered slightly so that a whole number of squares is obtained between BH and CJ. The procedure is continued by sketching the estimated line of flow KL from a second point on AB and extending the equipotentials already drawn.
The flow line KL and the equipotential extensions are adjusted so that all intersections are at right angles and all areas are square. The procedure is repeated until the boundary FG is reached. At the first attempt it is almost certain that the last flow line drawn will be inconsistent with the boundary FG as, for example, in Figure 2.
By studying the nature of this inconsistency the position of the first flow line HJ can be adjusted in a way that will tend to correct the inconsistency. The entire flow net is then adjusted and the inconsistency should now be small. After a third trial the last flow line should be consistent with the boundary FG, as shown in Figure 2.
In constructing a flow net it is a mistake to draw too many flow lines; typically, four to five flow channels are sufficient. Three or four flow lines, varying in shape between the extremes of the two boundaries, can be sketched before or together with the equipotentials. Subsequent adjustments are made until a satisfactory flow net is achieved. In the flow net in Figure 2. The equipotentials are numbered from zero at the downstream boundary; this number is denoted by nd. The point P is at a distance zp below the datum, i.
The highest hydraulic gradient and hence the highest seepage velocity occurs across the smallest square and vice versa.
Example 2. At low tide the depth of water in front of the wall is 4. Plot the net distribution of water pressure on the piling. The flow net is shown in the figure. The water level in front of the piling is selected as datum. The total head at water table level the upstream equipotential is 2.
The total head on the soil surface in front of the piling the downstream equipotential is zero pressure head 4. There are 12 equipotential drops in the flow net. The water pressures are calculated on both sides of the piling at selected levels numbered 1—7.
Figure 2. Flow nets 47 Table 2. Determine the quantity of seepage under the dam and plot the distribution of uplift pressure on the base of the dam.
The downstream water level is selected as datum. Between the upstream and downstream equipotentials the total head loss is 4. In the flow net there are 4. The choice of material involves an element of personal opinion but the contents of this book should cover the requirements of most undergraduate courses to honours level as well as parts of some Masters courses. It is assumed that the reader has no prior knowledge of the subject but has a good understanding of basic mechanics.
The book includes a comprehensive range of worked examples and problems set for solution by the student to consolidate understanding of the fundamental principles and illustrate their application in simple practical situations.
Both the traditional and limit state methods of design are included and some of the concepts of geotechnical engineering are introduced. The different types of field instrumentation are described and a number of case studies are included in which the differences between prediction and performance are discussed.
References are included as an aid to the more advanced study of any particular topic. It is intended that the book will serve also as a useful source of reference for the practising engineer. The reason is the electronic devices divert your attention and also cause strains while reading eBooks. EasyEngineering team try to Helping the students and others who cannot afford buying books is our aim. For any quarries, Disclaimer are requested to kindly contact us , We assured you we will do our best.
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