consolidated-drained triaxial compression test report

“Triaxial Test” Report Soil Mechanics

1

crdm21 January/2015

Consolidated-Drained Triaxial Compression Test Report (BS1377-8-1990:8)

1 - INTRODUCTION The aim of this experiment is to replicate field conditions in laboratory, in order to understand the soil behaviour under applied pressure and to demonstrate its shear strength. The triaxial test is widely used in Civil Engineering analysis of the soil, as it allows controlled drainage of the sample, enabling low permeability soils to consolidate in more rapidly manner. Thus, the drained triaxial test on sand was chosen, rather than undrained on clay for instance, so as to obtain a much faster approach to this investigation. Tests were prepared for different groups of students and four samples under different pressure conditions were observed. The results shared show a diagonal failure in most of the samples. Also, these tests suggest that the shear stress that the sample can sustain up to the failure is directly proportional to sample’s condition of Deviatoric Stress (q). This test consists in using triaxial apparatus (see “3 – Experimental Method” for more details) to impose constant horizontal stress, σ3 (note that σ2=σ3), by filling up the triaxial cell with a fluid around the cylindrical soil sample and varying the vertical stress, σ1, by increasing the compressive axial load applied on the top of the sample; while allowing free drainage of the sample from open drains on top and bottom of the sample (pore pressure assumed constant). Before the test, some information regarding the sample initial state is recorded, such as Initial diameter and height, mass and membrane thickness. During the test, variation of force applied and displacement of the top of the sample are recorded, until the sample failure in shear. Data collected can be compared, especially using Mohr’s circles plot. In order to achieve this, it is necessary to understand the principles and, consequently, the application of formulas used in this report. Firstly, is essential to be aware of the change in length, area and volume, thus the change in strain, as the sample is being axially compressed. Secondly, from the Force recorded, deviator stress can be obtained. See main formulas below:

Where ∆L is the change in Length, Lc is the Length of

specimen after consolidation, ɛ is the strain (length). As is

the Area of the specimen normal to its axis, ɛv volume strain (considered ɛv = 0 for this test, as no volume change were recorded) and Ac is the Initial cross sectional area of the specimen.

From the diagram of pressure on the specimen on the left, F is the Force recorded, A is the area of the specimen normal to is axis, (σ1 – σ3)m is the Applied Axial Stress. Note that in order to obtain the Corrected Deviator Stress, (σ1 – σ3), is necessary to subtract the membrane correction, σmb, from the Applied Axial Stress. No side drain fitted for those tests, so only membrane correction was needed. (See BS 1377-8-1990: 8.4)


2

crdm21 January/2015

Also, with the purpose of drawing Mohr’s circles, it was necessary to obtain Stress path parameters, in terms of effective stress, s’ (centre of the circle) and t’ (radius of the circle), see on the right:

2 - EXPERIMENTAL METHOD All tests were carried out based on methods detailed on BS 1377-8-1990, also following procedures for tests preparations available on BS 1377-1-1990.

Figure 2.1 (schematic of Triaxial apparatus) With the intention of compare the behaviour of the soil, different pore pressure and cell pressure conditions were simulated:

Group / Test 1 2 3 4

Target cell pressure (kPa) 300.0 300.0 350.0 250.0

Target pore pressure (kPa) 250.0 200.0 150.0 100.0

Several sets of Axial Force (N) and Displacement of the top of the sample downwards (mm) were recorded within regular intervals of seconds for every one of the four test conditions.


3

crdm21 January/2015

3 - RESULTS The sample was collected in sand dunes of Broad Haven in Wales. The material can be described as Medium Sand, mostly sub-rounded, moderately sorted, with about 50% of transparent grains (crystal like, perhaps quartz), 33% yellow-light brown grains, containing also some small portions of darker grains and micro-fragments of shells. The main data results obtained from the experiment and calculation are shown in the Appendices. From this set of data, it is, perhaps easier to visualise and compare with the aid of graphs. The Stress vs. Strain graph, for four different conditions given, (see below) shows similar pattern of Force vs. Axial Displacement graph (in appendices).

Graph 3.1 Deviatoric Stress vs. Strain above, shows the same pattern of Force vs. Displacement graph (see Appendices).

The main inconsistency on the data is on Test 4, which shows maximum deviatoric stress of 783kPa, when it, perhaps, should be about 562kPa. From the plot of Mohr’s circles in the same graph, it is possible to visualize that Test 4 should have a smaller circle (smaller deviatoric stress), in order to follow similar friction angle than the other results (ϕ

≈41 ). The plot of Mohr’s circles indicates some deviation in Test4 more clearly, see next page:


4

crdm21 January/2015

Mohr’s circles for tests 1, 2, 3 and 4 results.

4 - DISCUSSION AND CONCLUSION

The friction angle accepted for this type of sample was between 33 and 41 , so, most of the data collected

performed according or close to expected (about 41 ). The reason for the Test 4 presenting a greater deviatoric stress value is because the specimen was partially saturated and pressure from the sample (pore pressure) and cell pressure were naturally “trying” to balance, as the membrane was probably perforated by one of the sand grains. The hole in the membrane was not visible to naked eye, but was observed soon as the cell chamber was filled with water (see photos in appendices). The test was authorized by the lab assistant to be carried out, despite this observation. It was not possible to achieve the target cell pressure (intended to be 350kPa for test 3), so both pore pressure and cell pressure were reduced to Test 4 target pressures. Another evidence of this behaviour was the fact that the failure plane happened furthest away from the partially saturated area of the specimen, so the region partially saturated resisted more to change in volume than the rest of the specimen. The test was considered success by the straight line of failure and barrel shape of the sample. However, it is necessary to consider that in this partially saturated condition, the specimen would take longer than dried condition to fail and, thus, support more stress prior failure. Repeating the test for the condition 4 would be the best to state the accurate result. This exercise was very helpful in applying the theory, specially using sand, to ensure the experiment was realised within the restricted time. Even the divergence found on Test 4 helped to consolidate the content taught in Soil Mechanics lectures. Four sets of data were enough to provide information for comparison and understanding of soil shear strength once compressive stress is applied.


5

crdm21 January/2015

REFERENCES

Barnes, G. E. (no date) Soil Mechanics: Principles and Practice. United Kingdom: Palgrave Macmillan. Knappett, J. and Craig, R. (2012) Craig’s soil mechanics. United Kingdom: Taylor & Francis Ltd. Heath, A. (2014). AR20076 Soil Mechanics class notes. Bath: Department of Architecture and Civil Engineering.

APPENDICES

Picture showing the Triaxial Cell at the start of the Test 4 (the red circle highlights the water intrusion into the specimen).


6

crdm21 January/2015

Picture shows the specimen after the Test 4. Despite the water intrusion (shown by the red circle), the specimen formed barrel shape under axial compression and failed diagonally.


7

crdm21 January/2015

Force applied vs. Displacement graph above.

Mohr’s circles for each set of data, showing friction angles:


8

crdm21 January/2015


9

crdm21 January/2015

Table with main results:

Group / Test 1 2 3 4

Date 27.11.14 20.11.14 4.12.14 27.11.14

Target cell pressure (kPa) 300.0 300.0 350.0 250.0

Target pore pressure (kPa) 250.0 200.0 150.0 100.0

Initial sample height (mm) 123.81 123.94 120.00 133.07

Initial sample diameter (mm) 70.62 66.50 65.40 67.32

Sample dry mass (g) 711.1 691.0 546.0 695.0

Membrane thickness (mm) 0.343 0.315 0.360 0.393

Principal stress, σ1 (kPa) 521.31 664.53 1111.28 1033.14

Deviatoric Stress, q (kPa) 221.31 364.53 761.28 783.14

Vertical (Effective) stress, σ1' (kPa) 271.31 464.53 961.28 933.14

Horizontal (Effective) stress, σ3' (kPa) 50.0 100.0 200.0 150.0

Mohr's circle radius, t (kPa) 110.65 182.26 380.64 391.57

Centre of Mohr's circle, s' (kPa) 160.65 282.26 580.64 541.57

Friction Angle, ϕ (degrees) 43.53 40.22 40.96 46.30

Force at failure (N) 1010 1500 3090 3293

Displacement at failure (mm) 6.46 9.43 12.64 9.41

Angle of Failure (degrees) 66.78 65.10 65.48 68.16

consolidated-drained triaxial compression test report

Documents