Geotechnics and Structural Design

Geotechnics and Structural DesignUnconsolidated Undrained Triaxial Test On ClayContents

Page No1. Introduction

3

2. Procedure

33. Results

44. Sample Calculations

65. Discussion

11

6. Classification of Soil

127. Relevance of the Triaxial test

128. Conclusions

139. References

14Introduction

The unconsolidated undrained triaxial test is when the specimen is subjected to a specified all-round pressure and then the principal stress difference is applied immediately with no drainage being permitted at any stage of the test (R.F. Craig, 1997) The value of shear strength for a particular soil is vital to acquire solutions to possible problems relating to the stability of a soil mass in determining the load that can be exerted on the soil from structures.

The procedure for the triaxial test follows the instructed method in BS 1377:1990 part 7. The purpose of this laboratory report is to test three samples of clay and determine the shear strength and shearing resistance of the samples. Procedure The triaxial test started with three samples of clay which had to extruded from the cylindrical metal tubes into the brass former so that the height of the sample was 76mm. A small amount of the excess soil was taken in order to calculate the dry density with the rest discarded. The brass former was removed from the sample and the sample placed within a rubber membrane for the purposes of the experiment. The sample is then placed on the triaxial testing pedestal whereby the top and bottom plates are inserted and held to the rubber membrane by means of a rubber O-ring. The outer casing is then placed over the sample and the vessel filled with water, making sure all the air is displaced through the air point at the top of the apparatus. The triaxial testing machine was set to a rate of 2%/min with every 0.5mm deflection recorded the proving ring reading for each of the three samples with a maximum deflection of 15.5mm. The cell pressure of the water was set 75kPa for the first experiment and increased to 150kPa then 300kPa for the last sample. Once all results had been noted the cell pressure was released and the water left to drain from the cell. Results Soil parameters for triaxial testSample Number123

Total Mass of Sample (g)184.46186.54185.8

Tin Number42957

Mass of wet soil and tin (g)64.4462.555.72

Mass of dry soil and tin (g)57.3855.750.58

Mass of tin (g)16.216.1816.3

Mass of water (g)7.066.85.14

Mass of dry soil (g)41.1839.5234.28

Moisture Content (%)17.117.215.0

Bulk density (t/m3)2.142.162.16

Dry density (t/m3)1.831.851.87

Specific Gravity2.712.712.71

Void ratio "e"0.480.470.45

Porosity "n"33%32%31%

Degree of saturation "SR"96%100%91%

Air content "A"1.3%0.1%2.7%

Proving Ring Number117-3-727

Proving Ring Calibration (N/div)1.55

Volume of sample (mm3)86192.7

Dimensions for soil samples

Diameter (mm)38

Length (mm)76

Original Area (mm2)1134.1

New Area (mm2)1424.7

Volume (mm3)86192.7

Brass Former (g)313.26

Ring calibration1.55

Proving Ring Number117-3-727

Sample Calculations

Void Ratio (e)

Porosity (n)

Degree of Saturation (SR)

Air Content (A)

Moisture Content (MC)

Bulk Density ()

Dry Density dry

Triaxial Test Results

Figure 1 shows the graph of stress versus strain for the three samplesStress Calculations for Failure Reading of sample 1Strain ()

Deviator Stress (1 - 3) at failure

Corrected Cross-sectional Area (mm2)

Coulombs Law of soil Shear Strength

For Sample 1

Sample 2

Sample 3

Make equations equal (Sample 1 = Sample 2)

Substitute in equation 1

Tabulated Results for Undrained Unconsolidated Triaxial testSample No123

Cell Pressure (kPa)75150300

1252.34328.43479.51

Undrained Shear Stength cu87.6988.7686.97

Using coulombs law of soil shear strength

For Sample 1. f = c + tan = 87.69+75tan(0.413) = 87.7

For Sample 2. f = c + tan = 88.76+150tan(0.413) = 89.84

For Sample 3. f = c + tan = 86.97+300tan(0.413) = 89.13

Mohr Circles

DiscussionIt can be observed from the picture 1 that the three samples were very similar in their constituents. The angle of 0.416 proved in the calculations shows the three samples differed very little. The graph in figure 1 also shows the similarity between results whereby the lines rise continuously with each other. The samples in picture 1 show slight barrelling of the clay, with no obvious failure shearing lines with the strain measured at 20%. The Mohr circles show the failure envelope of the soil and, although not obvious from the diagram, the angle of shearing resistance of the line. The cohesion is stated on the drawing at the value of 87.69 kPa

Picture 1 shows the three samples of clay from the triaxial testClassification of soilThe three clay soil samples were above the plastic limits since the samples could be rolled into rods equivalent to 3mm. The soil was a firm to stiff clay, according to the undrained strength classification table in BS EN ISO 14688-2:2004 Table 5. The samples were brown in colour and contained a small amount of sand. The sample was able to be dried out when handling showing a silty clay.

Relevance of the triaxial testThe purpose of the triaxial test is to determine the shear strength of the soil at different confining stresses in order to obtain the highest stress the soil can sustain. This governs the stability or collapse of any structure. materials that have strength can sustain shear stresses and the strength is the maximum shear stress that can be sustained. Only materials with strength can have slopes because shear strength are required to maintain a slope. (Atkinson, 1993) The triaxial test is carried out on small samples of the soil that the proposed structure is to be built on. The samples are examined in the triaxial test in order to determine the characteristics so that the estimation for how the foundation or embankment might deform or collapse, or how much reinforcement might be required in order to support the embankment or foundation. The test is representative of the soils in the construction site where the rate of construction of the foundation or embankment is relatively fast and where the pore waters do not have enough time to drain. This test shows how the soil will behave in practice under fully saturated conditions.

In dams or embankments the core is normally constructed of clay which minimises the amount of water that seeps through the embankment or dam. The outer casings tend to be sand, gravel and rock whereby the triaxial test can be used to analyse the shear strength of each of the embankment materials.

Conclusion

From the triaxial test the shear parameters of soil were found to be Cu=87.69kPa for cohesion and the value of =0.413 with both results obtained through calculation and graphically. The shear strength of the three samples were calculated which could be used in real life situations for the calculation of structural loads.The triaxial test is essential in understanding soil behaviour. From the test the parameters for strength, stiffness and the response of certain soils under increasing pressures are found.

Understanding of the soil behaviour added to proper assessment of the soil characteristics allows the engineer to enhance designs and reduce the risk of failure. References

British Standards, BS 1377-7, 1990. Soils for civil engineering purposes. 2nd ed. London: Board of the BSI.

R.F. Craig, 1997. Soil Mechanics. 6 Edition. CRC Press. John Atkinson, 1993. The Introduction to the Mechanics of Soils & Foundations. Edition. Mcgraw-Hill CollegeCu

Failure envelope =0.413

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