Shear Box Report
Abstract
This repot describes the Shear Box test based on two types of soil that are loose soil and dense soil. The test has shown that the use of dry river sand under different densities to underestimate the shear strength parameters, the soil cohesion and the angle of friction with regard to the relationship between soil density and shear strength.

Introduction
The aim of the Shear box test was to determine the shear strength of sand using the shear box teat, and to investigate the effect of soil density on the strength.
The shear strength of a soil is its resistance to shearing stresses. It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles. To investigate more about the behavior of the soil, a Mohr-coulomb theory has been introduced. This theory states that a material fails because of a critical combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone. the limiting shear stress( the shear strength, t) that may be applied to any plane in the soil mass is found to be given by an equation of the form：

τ = c + σ tan(φ) where σ = normal stress on the failure plane φ = angle of friction c = (apparent) cohesion
The parameters C and φ are not generally soil constants. They also depended on the use of total or effective stresses.
In terms of effective stress the failure criterion is written: τ = σ' tan(φ') + c' where σ' is effective stress. And σ' = (σ - u), σ is the total stress applied vertical to the shear plane; u is the pore water pressure acting on the same plane. c' and σ' are referred to as the effective (drained) strength parameters.
This repot describes the Shear Box test based on two types of soil that are loose soil and dense soil. The results are used to determine the relationship between soil density and shear strength by plotting the graph of shear stress ratio vs horizontal displacement and peak and ultimate shear stress vs normal stress.
Methods
The soil is sheared along a predetermined plane by placing it in a box and then moving the top half of the box relative to the bottom half. The box is a 60 mm x 60 mm square in plan. The image below shows the schematic representation of the shear box.

Image 1: Schematic representation of the shear box test.

The shear box placed in the shear apparatus in image 2 below. The locations of the 3 gauges, the two left side adjustment bolts and the right-side adjustment nut, and the hangar bolt are all loosened-off before installing the shear box and finger-tightened after the shear box has been positioned in the apparatus.

Image 2: Shear Box installed in the apparatus for performing shear.

Procedure to assemble a shear box is listed below: 1. start with the shear box out of the machine and disassembled. Make sure that everything seems reasonably clean. 2. Place the top onto the base with the red numbers lining up, and screw in the two pins on each side. Weigh the soil plus the container. 3. Pour the soil into the shear box evenly and gently. Weigh the empty container. 4. Level off the soil using the white plastic template. 5. Screw the small handle bolt into the top of the brass plate, pick up the plate and place it into position in the shear box, aligning the red numbers. Then unscrew the handle. 6. Place the loading plate into position, making sure the red numbers are in the same corner. 7. For dense samples, use the mallet and punch to compact the soil. 8. Go to the shear box machine. Loosen off the left-side adjustment bolts and the hanger bolt. 9. Locate the guides in the holding tray. Pick up the shear box by the handles and place it in the holding tray so that it fits snugly into the guides, and the arm slots over the right-side bolt.

10. Finger – tighten the left side adjustment bolts, right side nut and hangar bolt. 11. Underneath the machine, with the prop applied, add the weight (remove previous weights first), and then loosen the prop. While one person is watching the spirit level, the demonstrator will adjust the hangar bolt. 12. Measure the height D2 using the vernier calipers, and lastly remove the pins.

Operation and recording the test:
The demonstrator will start the motor. The test will continue until a horizontal movement of 8 mm is reached. One student will read the #1 gauge (horizontal movement). He/she calls out each time the horizontal movement is at a value in the table in the table in the results sheet. At each of these calls, a second students reads the #2 gauge (shear force) and records the value, and a third student reads the #3 gauge (vertical movement) and records the value.
After each test:
Wind up the prop to relieve the load, remove the weight, unscrew the side bolts and the hangar bolt, remove the shear box and dismantle it on the bench. Return the soil to the container. The demonstrator will rewind the machine.

Results:
The typical results about the river sand soil has been shown in the table 1 below, it indicates the value of voids ratio, peak stress ratio, ultimate stress ratio, peak shear stress and ultimate shear stress for both loose soil and dense soil.

Table 1
According to the table above, if the sample is initially dense, the shear resistance rises to a peak and then drops to and ultimate value accompanied by an upward vertical movement while volume increasing. For an initially loose sample, the peak and ultimate strength are identical and a downward vertical movement while the volume decrease accompanies shearing. Based on this, two strength are often given which are peak and the ultimate strengths.

Figure 1.1 summary plot of stress ratio versus horizontal displacement

Figure 1.2 summary plot of vertical displacement versus horizontal displacement

Figure 1.3 summary plot of shear stress versus horizontal displacement

Figure 1.4 Graph of peak and ultimate shear stress versus normal stress
The trend of all the graphs shown above underestimates the shear strength parameters, the soil cohesion and the angle of friction with regard to the relationship between different soil density (loose and dense) and shear strength.

From the Figure 1.4 above, equation with regard to the shear stress and normal stress can be addressed by the Mohr’s columb: τ = c + σ tan(φ) where c is the y-intercepts, and the friction angle can be obtained by calculating the slope of the curve. Therefore results from line of best fit has been showed below:

Discussion
Figure 1.1 shows the relationship between the shear stress ratio and horizontal displacement. At the beginning, the dense sand soil reaches it peak stress with relatively low horizontal displacement. However, it ends up with the same ultimate strength due to the same shear stress ration with the loose soil. This is due to the degree of interlocking between the particles. By increasing the volume between the particles of dense soil to help it get more space to expand which will produce a reduction of friction between particles. Therefore, it will make the dense soil turns into a loose soil property and having the same ultimate strength with the loose soil.

Figure1.2 shows the relationship between the vertical displacement and horizontal displacement of both dense and loose soil particles. During the test, the value gained from the vertical displacement of the loose soil is negative which is contrast with the dense soil. While the loose soil is under shearing, it also undergoes a disturbing process that will make the particles combined together and decrease the porosity between them. However, while the dense soil is under shearing, it will overcome the internal friction between the particles that will make them need space to shear. Therefore, particles will expand to get more voids by keeping the horizontal displacement. Furthermore, by increasing the compression stress of the total area of dense soil, it will decrease the vertical stress.

Figure1.3 shows the relationship between the shear stress and horizontal displacement. As shown in the graph, dense soil reaches its peak strength at relatively low horizontal displacement. However the loose doesn’t have the peak strength at all. This is due to dilation angle which indicates that dense soil would expand with positive dilate angle and loose soil would compress with the negative dilation angle.

Figure 1.4 shows the relationship between the peak and ultimate shear stress and normal stress. Those curves on the graph are linearly, which indicates the linear relationship between the ultimate shear stress and normal stress. As much as the vertical stress has been applied, it will increase the shear stress to make the sand fail. Furthermore, by increasing the load on the soil will also results in increasing the shear resistance of the soil. For densely packed, loading will make the soil particles to seek more space to expand which is increasing the interlocking between the particles and to make to hard to slide over each over. Therefore, it will need more shear stress to make sand fail. Also for the loosely packed soil, loading will compress the soil by reducing the voids between particles. It also will increase the friction between the particles and make it hard to fail that will results in a larger shear stress required.

As a soil test the shear box is far from ideal. Disadvantages of the test include(2): 1. Non-uniform deformations and stresses. The stresses determined may not be those acting on the shear plane, and no stress-strain curve can be obtained. 2. There are no facilities for measuring pore pressures in the shear box and so it is not possible to determine effective stresses from undrained tests. 3. The shear box apparatus cannot give reliable undrained strengths because it is impossible to prevent localised drainage away from the shear plane.
However, it has many apparent advantages(2): 1. It is easy to test sands and gravels 2. Large deformations can be achieved by reversing the shear box. This involves pushing half of the box backwards and forwards several times, and is useful in finding the residual strength of a soil. 3. Large samples may be tested in large shear boxes. Small samples may give misleading results due to imperfections (fractures and fissures) or the lack of them. 4. Samples may be sheared along pre-determined planes. This is useful when the shear strengths along fissures or other selected planes are required. 5. In practice the shear box is used to get quick and crude estimates of the failure parameters.

Conclusion

In conclusion, this shear box test is o determine the shear strength of sand using the shear box teat, and to investigate the effect of soil density on the strength. It highlights the shear strength effects of different density of soil by the horizontal displacement and vertical displacement changing. It also shown that the use of dry river sand under different densities to underestimate the shear strength parameters, the soil cohesion and the angle of friction by applying the mohr’s columb theory.

Reference

1. Airey, D., ‘CIVL 2410 Soil Mechanics 2011 Course Notes’, School of Civil Engineering, University of Sydney, Sydney.

2. ‘Lecture notes of CIVL 2410 Soil Mechanics Course’, semester 2 2011, School of Civil Engineering, University of Sydney, Sydney.

3. ‘Lab manual of CIVL 2410 Soil Mechanics Course’, semester 2 2011, School of Civil Engineering, University of Sydney, Sydney.

4. ‘Web of Shear Box of CIVL 2410 Soil Mechanics in WebCT’, School of Civil Engineering, University of Sydney, Sydney.
5. LI, Lianghao. (2011). CIVL2006 Soil Mechanic Shear Box. Available: http://www.scribd.com/doc/71703289/Shear-Box-Test. Last accessed 17th Oct 2012.
6. wikipedia. (2007). Direct shear test. Available: http://en.wikipedia.org/wiki/Direct_shear_test. Last accessed 17th Oct 2012.

http://www.scribd.com/doc/71703289/Shear-Box-Test http://en.wikipedia.org/wiki/Direct_shear_test https://elearning.sydney.edu.au/bbcswebdav/pid-1392855-dt-content-rid-8856440_1/courses/2012_Semester_2_CIVL2410/SoilMechanics/Lectures/L15-stress_strain.htm

Appendices