# Geophysical properties of soil Methodology

geophysical and geotechnical properties of fined-grained soil in this project the following procedure should be done. The Atterberg Limits test was used in order to find the liquid limit, plastic limit, and shrinkage limit. A compaction test for sandy clay soil to find dry density was also used. A Time-Domain reflectometry test was incorporated to find the water content of the soil. To calculate the shear strength of the soil, the shear box test was utilized.

Soil preparation

It was suggested that 40% English China Clay and 60% Leighton Buzzard Sand be granulated in the mixer at the laboratory and carefully pass through a 5.0 mm test sieve, but it retained a75?m (micrometre) test sieve according to BS8204 for sandy clay. There was 5Kg of soil that needed to be prepared for the test’s procedure.

Optimum dry density

Testing the soil at optimum dry density in order to experiment with different tests not done at the same time, as the TDR test cannot be done in one day, was considered.

Time-Domain reflectometry (TDR)

TDR makes use of the dielectric constant, ?, of water to determine the volumetric water content of soil.

The dielectric constant of a medium is defined by ? In the following equation (Weast, 1964, p. F-37):

…where, F is the force of attraction between two charges Q. And Q0 separated by a distance r in a uniform medium. The dielectric constant of a material is the ratio of the capacitance of a capacitor with the material between the plates to the capacitance with a vacuum between the plates (Shortley and Williams, 1971, p. 519).

Most of the solid components of soil have dielectric constants in the range of 2-7, and that of air is effectively 1 (? Of air=1.000590). Thus, a measure of the dielectric constant of soil is a good measure of the water content of the soil. We are going to measure a travel time, and by knowing the length of the rods (waveguides) in the soil, we are going to get a velocity (velocity=length/time).We are going to relate this velocity to the dielectric constant. Then, we will relate the dielectric constant to volumetric water content.

The TDR technique measures the velocity of propagation of a high frequency signal (1 MHz-1 GHz). The velocity of propagation is as follows:

…where V is the velocity of propagation in the soil, c is the propagation velocity of light in free space, c=3*10-8m/s and K0 is the dielectric constant of the soil. By determining the travel time, t, of the pulse traveling in the transmission line or waveguide of length L, one can get the velocity as L/t.

The above equation can be rearranged to give the apparent dielectric constant as:

…where, Ka is the apparent dielectric constant. However, we need to add a “2” to the denominator in the above equation, because the line length is the distance traveled, but commercial cable testers measure the length down and the echo (reflection). Hence, the distance measured is twice the line length. So we have:

Experimental results (Topp et al., 1980) have given the following relation between volumetric water content and the dielectric constant:

It has been shown to hold for many different types of soils. The relationship between volumetric water content (q) and the dielectric constant (Ka) is essentially independent of soil texture, porosity, and salt content.

TDR test preparation

First, we need the TDR equipment proper. This includes the pulser of voltage, a sampling receiver that receives both the pulse and the reflected pulse from the soil, a timing device that synchronizes the timing for pulser, receiver, and data display, and a data display that shows the time and voltage magnitude.

Second, we need rods (also called probes or waveguides). They can be either two-pronged or three-pronged. If they are two-pronged, we need a balun, which is an impedance matching transformer. The coaxial cable is 50 ?. The coaxial cable is connected to a 185 ? shielded television cable and a balun to provide a “balanced line” (Herkelrath et al., 1991).

Third, we need cables for connecting the TDR instrument and soil probes (Topp, 1993). Cable combinations between the TDR instrument and soil probes are determined by the type of probes used (two-pronged, three-pronged, or more).

Fourth, we need tools for installation of soil probes. Three procedures can be used for insertion. First, for short probes (which applies to most soils except the most resistant), we can insert the probes by hand, take a reading, and remove them and move on to the next spot. In this way, we can easily take many readings and quickly get a “feel” for the spatial variability. Second, for longer probes, it is necessary to make “preholes” with a dummy probe. This process could also be done to obtain repeated measurements in space, although once the longer probes are inserted, we tend to leave them in place connected to a multiplexer, or with caps on the coaxial connector if single measurements are to be made. Third, B.E. Clothier and S.R. Green (CITE THIS, POSSIBLY?) in New Zealand have made “direct-wired” probes that they insert horizontally into a face of a pit that they have dug. The probes are inserted horizontally, the hole backfilled, and the probes remain in place underneath undisturbed soil during the summer. One has to be careful when removing them at the end of the experiment, for it is easy to put a spade right through the connector cable when exhuming them (B.E. Clothier, personal communication, February 23, 1994).

Fifth, if we are automating measurements, we need a multiplexer. Baker and Allmaras (1990) describe a system for automated measurements using a multiplexer. Multiplexers are available commercially from Campbell Scientific Corporation (Logan, UT) (Buckley et al., 2010).

Test procedure:

The samples needed to comfortably house the 75 mm long TDR probes to allow for accurate measurements; hence 100 mm by 100 mm cylindrical sample pots were used. To avoid issues with electrical interference affecting the readings, the sample pots were constructed out of plastic. The soil samples were compacted into the cylinders five 20mm layers and left for 24 hours to allow pore water to equilibrate across the sample. This is a similar approach to that recommended for British standard tri-axial tests where it takes approximately 24 hours to equalize sample pore pressure with cell pressure before testing (BSI, 1990).

For fine-grained soil samples tested at higher water contents it took longer if the soil was more plastic, and occasionally had to be left overnight to allow the soil chunks to absorb the water. The required mass of water for a target VWC was calculated based on the field dry density, cylinder size, and the known dry weight of soil present. Therefore, the amount of water to be removed from the sample was calculated and the target mass of the sample was derived. To achieve the target mass, the sample was dried using an oven at 105 “C for short intervals (no longer than 0.5 h) and then stirred vigorously so that no crusts of dry soil were allowed to form.

It was vital that the soil water should be allowed to equalize before being packed into the cylinders. The entire sample was carefully removed from the container and allowed to equalize for 24 h.

Each sample was subject to three water content tests by removing subsamples at different stages during sample preparation and testing. One subsample was taken at the compaction stage and great care was taken to replace the removed volume of soil. Two [YOU HAVE TO SAY WHAT THIS TWO IS REFERRING TO] OR THE SENTENCE DOES NOT MAKE SENSE, when the sample was dismantled after testing, one from where the probe was inserted, and one using the rest of the sample. This meant that a representative VWC could be found for the entirety of the sample in the cylinder. The cylinder was sealed with silicone to stop evaporation, and fitted with a TDR probe that was secured and remained in place throughout the duration of the testing. To examine the effects of temperature, each sample was placed in an incubator at 0, 10, and 20 “C for 24 hours. Three permittivity and BEC readings were taken at each temperature, and the mean at each temperature was used for analysis. The GWC was determined after the TDR measurement for all samples.

Compaction test

The compaction test is a laboratory method of experimentally determining the optimal moisture content at which a given soil type will become most dense and achieve its maximum dry density.

Procedure:

These laboratory tests generally consist of compacting soil of known moisture content into a cylindrical mold of standard dimensions using a compactive effort of controlled magnitude. The soil is usually compacted into the mold to a certain amount of equal layers, each receiving a number of blows from a standard weighted hammer at a specified height. This process is then repeated for various moisture contents and the dry densities are determined for each. The graphical relationship of the dry density to moisture content is then plotted to establish the compaction curve. The maximum dry density is finally obtained from the peak point of the compaction curve and its corresponding moisture content, also known as the optimal moisture content.

Atterberg limit test

The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil, such as its shrinkage limit, plastic limit, and liquid limit. As a dry, clay-type soil takes on increasing amounts of water, it undergoes dramatic and distinct changes in behaviour and consistency. 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 consequently so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil’s behavior.

Procedure:

As a hard, rigid solid in a dry state, soil becomes a crumbly (friable) semi-solid when certain moisture content, termed the shrinkage limit, is reached. If it is an expansive soil, this soil will also begin to swell in volume as this moisture content is exceeded. Increasing the water content beyond the soil’s plastic limit will transform it into a malleable, plastic mass, which causes additional swelling. The soil will remain in this plastic state until its liquid limit is exceeded, which causes it to transform into a viscous liquid that flows when jarred.

Plastic limit

The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. The procedure is defined in ASTM Standard D. 4318.

If the soil is plastic, this thread will retain its shape down to a very narrow diameter. The sample can then be remolded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is defined as the moisture content where the thread breaks apart at a diameter of 3.2 mm (about 1/8-inch). A soil is considered non-plastic if a thread cannot be rolled out down to 3.2 mm at any moisture.

Direct shear test

A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses

Procedure:

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 http://en.wikipedia.org/wiki/Stress%E2%80%93strain_curve curve for each confining stress. 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 known as the frictionhttp://en.wikipedia.org/wiki/Friction http://en.wikipedia.org/wiki/Friction angle (). The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the y-axis and the confining stress on the x-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.

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 un-drained or drained conditions, depending on whether the strain is applied slowly enough for water in the sample to prevent pore-water pressure build-up. A direct shear test machine is required to perform the test. The test using the direct shear machine determinates the consolidated drained shear strength of a soil material.

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