STANDARD OEDOMETER TEST The standard oedometer (consolidometer) test can be used to conduct consolidation tests on dredged materials and foundation soils, as shown in Figure I1(USACE 1986). Due to the soft, often fluidlike consistency of the sediment samples normally tested, the fixed ring consolidometer should be used, instead of the floating ring device, since extrusion of the sample from the device will be less likely in the fixed ring consolidometer. Sample preparation and loading method constitute the only modifications necessary for testing of dredged material in this device. Consolidation test procedures for use with soft dredged materials are outlined below; more detailed procedures are provided in USACE (1987), Poindexter (1988), Poindexter-Rollings (1990), and Palermo, Montgomery, and Poindexter (1978), and troubleshooting tips are provided in Rollings and Rollings (1996). Although the foundation soils under dredged
TRIAXIAL SHEAR TEST A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil, sand, clay, and other granular materials or powders. There are several variations on the test, discussed below.
BASIC CONCEPT
For loose granular materials like sand or soil, the material is contained in a cylindrical latex sleeve with a flat, circular metal plate or platen closing off the top and bottom ends. This cylinder is placed into a bath of water to provide pressure along the sides of the cylinder. The top platen can then be mechanically driven up or down along the axis of the cylinder to squeeze the material. The distance that the upper platen travels is measured as a function of the force required to move it, as the pressure of the surrounding water is carefully controlled. The net change in volume of the material is also measured by how much water moves in or out of the surrounding bath. The principle behind a triaxial shear test is that the stress applied in the vertical direction (along the axis of the cylinder) can be different than the stress applied in the horizontal directions (along the sides of the cylinder). This produces a non-hydrostatic stress state, which contains shear stress.
A solid is defined as a material that can support shear stress without moving. However, every solid has an upper limit to how much shear stress it can support. The triaxial tester is designed to measure that limit. The stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands. A motion where a material is deformed under shear stress is known as shearing. The geometry of the shearing in a triaxial tester typically causes the sample to become shorter while bulging out along the sides. The stress on the platen is then reduced and the water pressure pushes the sides back in, causing the sample to grow taller again. This cycle is usually repeated several times while collecting stress and strain data about the sample. During the shearing, a granular material will typically have a net gain or loss of volume. If it had originally been in a dense state, then it typically gains volume, a characteristic known as Reynolds' dilatancy. If it had originally been in a very loose state, then compaction may occur before the shearing begins or in conjunction with the shearing. From the triaxial test data, it is possible to extract fundamental material parameters about the sample, including its angle of internal friction, apparent cohesion, and dilatancy angle. These parameters are then used in computer models to predict how the material will behave in a larger-scale engineering application. An example would be to predict the stability of the soil on a slope, whether the slope will collapse or whether the soil will support the shear stresses of the slope and remain in place. Triaxial tests are used along with other tests to make such engineering predictions.
TYPES OF TRIAXIAL TESTS
There are several variations on the basic concept of triaxial testing. These are given the following labels (corresponding test standard in parentheses): • • •
CD — Consolidated drained CU — Consolidated undrained (ASTM D4767) UU — Unconsolidated undrained (ASTM D2850)
DIRECT SHEAR TEST A direct shear test is a laboratory test used by geotechnical engineers to find the shear strength parameters of soil. In the U.S., the standard defining how the test should be performed is ASTM D 3080. The test is performed on three or four specimens from a relatively undisturbed soil sample. A specimen is placed in a shear box which has two stacked rings to hold the sample; the contact between the two rings is at approximately the mid-height of the sample. A confining stress is applied vertically to the specimen, and the upper ring is pulled laterally until the sample fails, or through a specified strain. The load applied and the strain induced is recorded at frequent intervals to determine a stress-strain curve for the confining stress. Direct Shear tests can be performed under several conditions. The sample is normally saturated before the test is run, but can be run at the in-situ moisture content. The rate of strain can be varied to create a test of undrained or drained conditions, depending whether the strain is applied slowly enough for water in the sample to prevent pore-water pressure buildup.
Several specimens are tested at varying confining stresses to determine the shear strength parameters, the soil cohesion (c) and the angle of internal friction (commonly friction angle) (φ). The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the x-axis and the confining stress on the y-axis. The y-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle. ATTERBERG LIMITS The Atterberg limits are a basic measure of the nature of a fine-grained soil. Depending on the water content of the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state the consistency and behavior of a soil is different and thus so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behavior. The Atterberg limits can be used to distinguish between silt and clay, and it can distinguish between different types of silts and clays. These limits were created by Albert Atterberg, a Swedish chemist.[1] They were later refined by Arthur Casagrande.
Laboratory tests Shrinkage limit The shrinkage limit (SL) is the water content where further loss of moisture will not result in any more volume reduction. [2] The test to determine the shrinkage limit is ASTM International D427. The shrinkage limit is much less commonly used than the liquid limit and the plastic limit. Plastic limit The plastic limit (PL) is the water content where soil starts to exhibit plastic behavior. A thread of soil is at its plastic limit when it is rolled to a diameter of 3 mm and crumbles. To improve consistency, a 3 mm diameter rod is often used to gauge the thickness of the thread when conducting the test.
Liquid limit The liquid limit (LL) is the water content where a soil changes from plastic to liquid behavior. The original liquid limit test of Atterberg's involved mixing a pat of clay in a little round-bottomed porcelain bowl of 10-12cm diameter. A groove was cut through the pat of clay with a spatula, and the bowl was then struck many times against the palm of one hand. Casagrande subsequently standardized the apparatus and the procedures to make the measurement more repeatable. Soil is placed into the metal cup portion of the device and a groove is made down its center with a standardized tool. The cup is
repeatedly dropped 10mm onto a hard rubber base during which the groove closes up gradually as a result of the impact. The number of blows for the groove to close for 13 mm (½ inch) is recorded. The moisture content at which it takes 25 drops of the cup to cause the groove to close is defined as the liquid limit. Another method for measuring the liquid limit is the Cone Penetrometer test. It is based on the measurement of penetration into the soil of a standardized cone of specific mass. Despite the universal prevalence of the Casagrande method, the cone penetrometer is often considered to be a more consistent alternative because it minimizes the possibility of human variations when carrying out the test. CALIFORNIA BEARING RATIO The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation. The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO T193. The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100. = CBR [%] = measured pressure for site soils [N/mm²] = pressure to achieve equal penetration on standard soil [N/mm²]
HYDROMETER A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is
noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly. The operation of the hydrometer is based on the Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink. (See also Relative density and hydrometers.) Some historians credit Hypatia of Alexandria with the invention of the hydrometer although there is little evidence to support this. SOIL ANALYSIS
A hydrometer analysis is the process by which fine-grained soils, silts and clays, are graded. Hydrometer analysis is performed if the grain sizes are too small for sieve analysis. The basis for this test is Stoke's Law for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the grain diameter and the densities of the grain in suspension and of the fluid. The grain diameter thus can be calculated from a knowledge of the distance and time of fall. The hydrometer also determines the specific gravity (or density) of the suspension, and this enables the percentage of particles of a certain equivalent particle diameter to be calculated. PROCTOR COMPACTION TEST The Proctor compaction test and the related modified Proctor compaction test, named for engineer Ralph R. Proctor (1933), are tests to determine the maximum practically-achievable density of soils and aggregates, and are frequently used in geotechnical engineering. The test consists of compacting the soil or aggregate to be tested into a standard mould using a standardized compactive energy at several different levels of moisture content. The maximum dry density and optimum moisture content is determined from the results of the test. Soil in place is tested for in-place dry bulk density, and the result is divided by the maximum dry density to obtain a relative compaction for the soil in place. HISTORY AND ITS ORIGIN
Proctor's fascination with geotechnical engineering began when taking his undergraduate studies at University of California, Berkeley. He was interested in the publications of Sir Alec Skempton and his ideas on in situ behavior of natural clays. Skempton formulated concepts and porous water coefficients that are still widely used today. It was Proctor’s idea to take this concept a step further and formulate his own experimental conclusions to determine a solution for the in situ behaviors of clay and ground soils that cause it to be unsuitable for construction. His idea, which was later adopted and expounded upon by Skempton, involved the compaction of the soil to establish the maximum practicallyachievable density of soils and aggregates (the "practically" stresses how the value is found experimentally and not theoretically). In the early 1930s, he finally created a solution for determining the maximum density of soils. He found that in a controlled environment (or within a control volume), the soil could be compacted to the point where the air could be completely removed, simulating the effects of a soil in situ conditions. From this, the dry density could be determined by simply measuring the weight of the soil before and after compaction, calculating the moisture
content, and furthermore calculating the dry density. Ralph R. Proctor went on to teach at the University of Arkansas. SOIL COMPACTION Compaction is the process of increasing the bulk density of a soil or aggregate by driving out air. For any soil, for a given amount of compactive effort, the density obtained depends on the moisture content. At very high moisture contents, the maximum dry density is achieved when the soil is compacted to nearly saturation, where (almost) all the air is driven out. At low moisture contents, the soil particles interfere with each other; addition of some moisture will allow greater bulk densities, with a peak density where this effect begins to be counteracted by the saturation of the soil. DIFFERENT TESTS The original Proctor test, ASTM D698 / AASHTO T99, uses a 4-inch diameter mold which holds 1/30th cubic foot of soil, and calls for compaction of three separate lifts of soil using 25 blows by a 5.5 lb hammer falling 12 inches, for a compactive effort of 12,400 ft-lbf/ft³. The "Modified Proctor" test, ASTM D1557 / AASHTO T180, uses the same mold, but uses a 10 lb. hammer falling through 18 inches, with 25 blows on each of five lifts, for a compactive effort of about 56,000 ft-lbf/ft³. Both tests allow the use of a larger mold, 6 inches in diameter and holding 1/13.333 ft³, if the soil or aggregate contains too large a proportion of gravel-sized particles to allow repeatability with the 4-inch mold. To ensure the same compactive effort, the number of blows per lift is increased to 56. The California Department of Transportation has developed a similar test, California Test 216, which measures the maximum wet density, and controls the compactive effort based on the weight, not the volume, of the test sample. The primary advantage of this test is that maximum density test results are available sooner, as evaporation of the compacted sample is not necessary. There is also a test (ASTM D4253) which uses a vibrating table using standard vibrations for a standard time to densify the soil. This test method prevents particle breakage, but is only usable for granular soils. The test method also includes a method to determine the minimum density of the soil; density of soils in place are compared against the maximum and minimum to obtain a relative density.
R-VALUE (SOILS) The R-Value test, California Test 301, measures the response of a compacted sample of soil or aggregate to a vertically applied pressure under specific conditions. This test is used by Caltrans for pavement design, replacing the California bearing ratio test. Many other agencies have adopted the California pavement design method, and specify R-Value testing for subgrade soils and road aggregates. The test method states: The R-value of a material is determined when the material is in a state of saturation such that water will be exuded from the compacted test specimen when a 16.8 kN load (2.07 MPa) is applied. Since it is not always possible to prepare a test specimen that will exude water at the specified load, it is necessary to test a series of specimens prepared at different moisture contents.
R-Value is used in pavement design, with the thickness of each layer dependent on the Rvalue of the layer below and the expected level of traffic loading, expressed as a Traffic Index. Details of the pavement design procedure are given in Chapter 600 of the California Highway Design Manual
SIEVE ANALYSIS A sieve analysis (or gradation test) is a practice or procedure used (commonly used in civil engineering) to assess the particle size distribution (also called gradation) of a granular material. The size distribution is often of critical importance to the way the material performs in use. A sieve analysis can be performed on any type of non-organic or organic granular materials including sands, crushed rock, clays, granite, feldspars, coal, soil, a wide range of manufactured powders, grain and seeds, down to a minimum size depending on the exact method. Being such a simple technique of particle sizing, it is probably the most common. PREPARATION A gradation test is performed on a sample of aggregate in a laboratory. A typical sieve analysis involves a nested column of sieves with wire mesh cloth (screen). A representative weighed sample is poured into the top sieve which has the largest screen openings. Each lower sieve in the column has smaller openings than the one above. At the base is a round pan, called the receiver. The column is typically placed in a mechanical shaker. The shaker shakes the column, usually for some fixed amount of time. After the shaking is complete the material on each sieve is weighed. The weight of the sample of each sieve is then divided by the total weight to give a percentage retained on each sieve. The size of the average particles on each sieve then being analysis to get the cutpoint or specific size range captured on screen. The results of this test are used to describe the properties of the aggregate and to see if it is appropriate for various civil engineering purposes such as selecting the appropriate aggregate for concrete mixes, asphalt mixes, and foundations. The results of this test are provided in graphical form to identify the type of gradation of the aggregate. The complete procedure for this test is outlined in the American Society for Testing and Materials (ASTM) C 136[2] and the American Association and State Highway and Transportation Officials (AASHTO) T 27[3] PROCEDURE A suitable sieve size for the aggregate should be selected and placed in order of decreasing size, from top to bottom, in a mechanical sieve shaker. A pan should be placed underneath the nest of sieves to collect the aggregate that passes through the smallest. The entire nest is then agitated, and the material whose diameter is smaller than the mesh opening pass through the sieves. After the aggregate reaches the pan, the amount of material retained in each sieve is then weighed [5].
CROSSHOLE SONIC LOGGING Crosshole sonic logging (CSL) is a method to verify the integrity of drilled shafts and other concrete piles. This method is considered to be more accurate than sonic echo testing in the determination of soundness of concrete within the drilled shaft inside of the rebar cage. This method provides little indication of concrete soundness outside the cage. Also known as Crosshole Acoustical Testing, CSL normally consists of plastic pipe installed in the drilled shaft and tied to the rebar cage. After the shaft is drilled, and before the rebar cage is placed in the hole, plastic tubes are attached to the interior of the rebar cage and then the cage is lowered into the hole and the concrete is placed. After the concrete has cured, the tubes are filled with water as an intermediate medium, and a sound source and receiver are lowered, maintaining a consistent elevation between source and sensor. A signal generator generates a sonic pulse from the emitter which is recorded by the sensor. Relative energy, waveform and differential time are recorded, and logged. This procedure is repeated at regular intervals throughout the pile and then mapped. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the soundness of the concrete throughout the pile can be gleaned. A more advanced, higher-end analysis that creates a mock 3-dimensional graphical display of the concrete soundness throughout the pile is known as Crosshole Sonic Tomography.
