Introduction
Most projects require materials of varying characteristics that need to be considered before the execution of such projects. A good analysis criteria should be employed when determining the properties of the material in question. One of the ways to analyze a material such as soil is the Undrained shear strength (Suc). This report explains the analysis carried out on a soil sample and the resulting classification according to AASHTO and USCS standards.
Materials
Harvard compaction device
Compression device
Evaporating dish
Spatula
Squeeze bottle
Metal pan
Clayey soil
Procedure
A sample of about 500grams of soil finer than # 40 sieve is obtained. 10% of the moisture content is added to the soil sample and a Harvard compaction Device is used to form the sample. In the process, the soil are added and compressed in layers. The old layer of the soil is roughed up using a spatula before adding a new layer. This ensure a cohesive soil sample. The diameter (Di) and the height (Hi) is measured in the three separate sample axis. In case, the sample is not formed it is put back to the compaction device for reforming. The sample is then placed on the compression device and the gaps between the loading platforms and the sample is closed without loading
the sample. The ring gage and the strain gauge is then zeroed. This followed by loading the sample at a steady rate ad recording of the gage data at every whole number indicated on the gage without pausing the loading. The loading of the sample continues until there is no obvious cracks, failures or a decrease in the loading force. Some few more data points are obtained to fill the table1 below (Chen, Yang and Yang, 20)
Results
The following results shows the values obtained during the Uncompressional test of soil analysis. The values of height, diameter of the compacted soil were obtained. The rest of the values were obtained through calculations in from equations 1 to 5. In each experiment performed, 10% of the water was added to ensure proper formation of the soil during compaction
The total strain (ε) can, therefore, be calculated from the formula in equation 1 below
ε= ∆HHi 1
Where ∆H is the change in height of the specimen. The calibration factor is found by providing the ring calibration sheet obtained from the manufacturer. Equation 2 below is used to determine the corrected area Ac.
Ac= Aiε 2
Where Ai is the initial area of the sample. The unconfined compressive strength (qu) of the sample, also referred to as stress applied is calculated from equation 3 below.
qu= PnAn ..3
Where, An is the corrected area of the nth strain and Pn can be determined using the Proving Ring Utility chart. The Undrained shear strength can be calculated from the equation 4 below
Su= qu2 .. 4
Graph
Discussion
Modifying equation 3 gives the variables used to plot the graph
Pn= quAn 5 (Chen, Yang and Yang, 21)
A graph of the load against corrected area is plotted to obtain a straight line. From the graph, the value of the slope is the Unconfined Compressive Strength, qu
i.e Slope = (1.58-1.5)/(822-819) = 0.58/3 = 0.1933
But from equation 4, Su= qu2
Su= 0.19332 = 0.09667 (MITACHI, KUDOH and TSUSHIMA, 32)
A decrease in the undrained shear strength improves the stability of the slope.
Mohr’s Circle Diagram
If a Mohr’s circle is drawn, Su=c= qu2
Where c= cohesion
Unconfined compressive strength, qu = 0.1933 = 19.33kPa
Su = 9.665
In the Mohr’s circle, the peak gives the Undrained strength as shown above.
Shear stress 9.655kPa
Normal stress 19.33kPa
Soil Analysis
There are many ways of soil analysis in this experiment. One critical way is the use of plastic limit and liquid limit. In this case, the plastic limit is given to be 34.14% and, the liquid limit is 47.28%. Using the chart given, the soil sample type and properties can be given. The soil has a high plasticity, dry, soft, gray, rounded and angular in shape. The group name of this soil is CL with the following characteristics when used. First, it has a good to fair compaction properties and medium compressibility and expansion. Secondly, it as poor drainage (or no drainage at all) but good stability when used as a value fill. Thirdly, the application as pavement subgrade is fair to poor but can never be used as a base coarse for pavement.
Sources of Errors
The experiment might have had some faulty equipment which gave some deviation in the recorded values. Ordinary blunders could be another source of error in the experiment. These and other errors can be corrected be using more accurate and error free equipment. It also entails carefully following the procedure to avoid mistakes and doing several trials to gain confidence.
Conclusion
The experiment that aimed at determining the value of unconfined compression strength and undrained shear strength was done carefully by following instructions. This involved the placement of the soil sample in a compacting device and compacting them in layers. The parameters of the resulting shape are taken for used in calculating the required soil strengths. Despite the small deviations in values obtained due to some errors, the aimed of the experiment was obtained.
Work Cited
Chen, Dong Xia, Kai Yang, and Yan Yang. "Experimental Study on Unconfined Compression Strength of Compacted Granite Residual Soils". AMR 250-253 (2011): 2124-2128. Web.
MITACHI, TOSHIYUKI, YUTAKA KUDOH, and MASAKI TSUSHIMA. "Estimation of In-Situ Undrained Strength of Soft Soil Deposits By Use of Unconfined Compression Test with Suction Measurement.” SOILS AND FOUNDATIONS 41.5 (2001): 61-71. Web.
Appendix
Readings
USACE EM_1110-2-1906 Appendix XI [3]
ASTM D2166 [19]
