The Laboratory Technique to Grade Soil - Lab Report Example

In geotechnical engineering different laboratory techniques/tests are used to determine the properties of the soil. The different laboratory techniques that mostly used are Compression test, Atterberg limit test, Cone penetrometer test, Consolidation test, Direct shear test, Moisture Content test, Permeability test, Triaxial test and sieve analysis. These tests and analysis help us to classify the soil and to determine soil permeability, friction angle, unit weight, water content, strength and other different characteristics. Soil Grading: In order to grade soil, the labortary technique we use is sieve analysis or particle size distribution. In sieve analysis we assess the particle size distribution we the annular material. It is the very simple grading technique which helps us to determine that how the material will behave when it will be in use. The soil consists of particles of different sizes and there sizes range is very large. The following figure shows the range of soil particles and particular names given to them. The particle size distribution is used for gravel and sand size (coarse) particles, which can be separated into different size ranges with a series of sieves of standard aperture openings. Soil sieving is not used for the very much smaller silt and clay (fine) particles. To determine the distribution of the finer particles (smaller than 0.075 mm in diameter) the sedimentation procedures are used instead and most common would be the hydrometer test of soil. Procedure: Sieve analysis consists of shaking the coarse grained soil sample through a series of woven-wire square-mesh sieves that have progressively smaller openings. So particles larger than the size of each sieve are retained on the sieve. First the soil is oven dried and then its weight is measured. Then all lumps are broken into small particle before they are passed through the sieves. The weight of each sieve should be known or measure it before passing the soil. At least 10 minutes of hand sieving is desirable for soils with small particles. After the completion of the shaking period the mass of each sieve should be measure again. Subtracting this weight to the weight of sieve before soil passing give us the mass of soil retained in each sieve. The sum of these weights should be checked against the original soil weight. (The figure at left shows sieves of different sizes while right one shows soil passing and retaining through each sieve) The percentage of each soil size is measured by weighing the amount retained on each sieve and comparing the weight to the total weight of the sample. Percentage retained on any sieve = x 100 Cumulative percentage retained on any sieve = Percentage finer than on each sieve= 100 % - After making these calculations the results of the sieve analysis are plotted on semi–logarithmic plots known as particle–size distribution curves. In this particle diameters are plotted in log scale, and the corresponding percent finer in arithmetic scale. Analysis of these curves helps us to determine the soil gradation of the particular soil. In the following figure some typical grading curves are shown A - a poorly-graded medium SAND B - a well-graded GRAVEL-SAND C - a gap-graded COBBLES-SAND D - a sandy SILT E - a typical silty CLAY Three basic soil parameter can be determined by this graph/curve which are Effective size Coefficient of uniformity Coefficient of curvature The diameter in the particle–size distribution curve corresponding to 10% finer is defined as the effective size, or D10. Coefficient of uniformity is given by the relation Cu = Where D60 is the diameter corresponding to 60% finer in the particle-size distribution Also coefficient of curvature is given by the relation Cc = Where D30 is the diameter corresponding to 30% finer in the particle-size distribution Question: (i) Determine the grading of the following soil Sieve Size (mm) 50 37.5 20 14 10 6.3 3.35 1.18 0.6 0.15 0.063 tray Mass retained (g) 0 15.5 17 10 11 33 114.5 63.3 18.7 17 10.5 3.5 The total mass of the sample was 315 grammes, plot the particle size distribution curve for the soil using the graph sheet provided Total mass=315 g Sieve size (mm) Mass retained (g) % retained in each sieve Cumulative retained (%) Percent finer 50 0 0 0 100 % 37.5 15.5 4.92 4.92 95.08 % 20 17 5.40 10.32 89.68 % 14 10 3.17 13.49 86.51 % 10 11 3.49 16.98 83.02 % 6.3 33 10.47 27.45 72.55 % 3.35 114.5 36.35 63.80 36.2 % 1.18 63.3 20.09 83.89 16.11 % 0.6 18.7 5.93 89.82 10.18 % 0.15 17 5.40 95.32 4.78 % 0.063 10.5 3.67 98.89 1.11 % tray 3.5 1.11 100 0 The particle size distribution curve is given below (ii) From the inspection of this curve, determine the effective size, coefficient of uniformity and the coefficient of curvature of the soil and finally classify the soil From the above particle distribution chart we have D10 = 0.6 mm D30 = 3.5 mm D60 = 4.2 mm Thus the effective size of the soil is D10 which is 0.6 mm Now as we coefficient of uniformity is given by the relation Cu = So, Cu = 4.2/0.6 = 7.0 And The coefficient of curvature is given by the relation Cc = So, Cc = = 4.86 From the curve of the graph we can observe that the soil is gravel, with most of it part in the fine region. So soil can be classified as fine gravel soil. Liquid and plastic limits: Liquid limit: The liquid limit is the moisture content at which the soil passes from the plastic to the liquid state as determined by the liquid limit test. Plastic limit The water content at which a soil will just begin to crumble when rolled into a thread approximately 1/8" (3 mm) in diameter. For the determination of liquid limit and plastic limit cone penetrometer test can be used. Cone Penetrometer test In this test method a cone that has a 60 point, facing downward, is pushed into the ground at a continuous rate (The rate of penetration is about 1 to 2 cm/sec). Resistance is measured by correlating the depth penetrated with the force applied. The resolution of the cone penetreometer test depends on the size of the cone tip. Procedure: A standard penetrometer apparatus is used for this purpose. Mix the sample with distilled water until a thick homogeneous paste is formed. Place the cone in the cone holder and raise cone assembly to the highest position possible. Fill the penetration container by placing a quantity of the cured test portion in the bottom of the container and exert adequate pressure on the spatula to displace the cured soil in an outward direction so as to remove air bubbles from the cured soil. Completely fill the container in this manner and then level off the surface of the cured soil with the spatula with the blade held almost flat. After this lower the penetrometer head until point of cone makes contact with the sand. Meanwhile depress the indicator rod of the dial gage until it touches the top of the shaft. Record this dial gauge reading. Release the penetrometer shaft and allow the cone to penetrate the soil for about 5 seconds and then restrain the penetrometer shaft. Again depress the indicator rod of the dial gauge and record the reading. Now the difference between the two dial gauge readings is the penetration value. After this add little more of the soil in the penetration container and measure more penetration values. The cone penetrometer test is helpful in determining of the liquid limit. For the calculation of the liquid limit what we have to do is to first plot the moisture contents against their corresponding penetration values on a linear graph with the percent moisture content on the horizontal axis and the penetration value on the vertical axis. Then we have to draw a straight line which best fits the points plotted. After plotting the graph we have to determine the value of the moisture content corresponding to the intersection of the line of best fit and the 20 mm penetration ordinate. The moisture content obtained at this value is the liquid limit of the soil. The following figure shows a standard penetrometer with different parts Question A British Standard cone penetrometer test carried out a sample of boulder clay gave the following results: Cone Penetration (mm) 15.9 17.1 19.4 20.9 22.8 Moisture Content (%) 32.0 33.1 34.5 36.0 37.0 Determine the Liquid Limit of the soil The graph between moisture content and cone penetration is given below The value of moisture content at 20 mm penetration will be the liquid limit. From graph the value of liquid limit comes out to be 35 %. If the plastic limit of the soil, measured by the tests, was found to be 15%, determine the Plasticity Index and hence classify the soil Plasticity index is given by the relation Plasticity index = liquid limit – plastic limit So, Plasticity index = 35 – 15 = 20 Plasticity index shows its soft sandy clay Read More