LEEDS BECKETT UNIVERSITY
CIVIL ENGINEERING
Geotechnical Engineering: Application & Theory (BEng)
Laboratory Experiment:
Undrained triaxial compression test (without pore water pressure measurement) BS 1377: Part 7: 1990.
Object of Experiment:
To determine the undrained shear strength of a soil using the triaxial compression test.
Theory/Apparatus:
The apparatus consists of a cell, which is filled with water under pressure; the specimen is loaded vertically, via a proving ring to measure load.
Triaxial Cell
The vertical load on the specimen is increased until failure occurs, the vertical strain being recorded at the same time using a dial gauge. The test is repeated on different specimens from the same soil, using different values of cell pressure.
Stresses on specimen in Triaxial Cell
Cell Pressure Deviator Stress =P/A 1=3+P/A
1 = major principal stress
3 = minor principal stress
Therefore, P/A = (1-3) =Deviator stress
The deviator stress is the load on the specimen, P, divided by the cross sectional area of the specimen. However, as the sample is compressed during the test, the cross sectional area will increase. Therefore, in calculating the deviator stress an allowance for the change in area must be considered. For the calculation of deviator stress, it is assumed that the volume of the specimen remains constant and that the sample will deform as a cylinder, e.g.
Where:
P = vertical load, which is measured by a load cell (kN)
A = Area calculated using the following method;
Method:
1. Extrude the sample from the tube and trim to size – soil sample of 38mm diameter and 76mm long.
2. Sleeve the sample with the rubber membrane.
3. Put the sample on the pedestal at the bottom of the cell and seal with the rubber ring. Place the loading cap on top of the sample and seal with rubber ring, before securing top drainage tube.
4. Mount the cell over the sample and fill as per the
Flooding Triaxial Cell checklist.
5. Set-up the test with the Clisp Studio assistant, and complete the
Pressurising Triaxial Cell checklist before running the test stages.
6. When test stages are complete, end the test via Clip Studio and complete the Draining Triaxial Cell checklist.
Results and Calculations:
- Calculate the moisture content of the soil as per Appendix A.
- Calculate the results as follows:
(i) For each sample tested:
- Plot the Deviator stress (1–3) vs. strain.
- Find the failure values of Deviator stress and strain when the Deviator stress hits a peak value; this is denoted as (1–3)f and f; ensure you correct the Deviator stress for the membrane correction (Figure 11 in BS1377-7:1990).
(ii) Plot the Mohr circle for each of the samples.
(iii) Determine the apparent cohesion (cu) and the angle of shearing resistance (u) of the soil by using the best common tangent method.
(iv) Also determine the apparent cohesion (cu) and the angle of shearing resistance (u) by using the alternative method (to iii above) of plotting the topmost point of each circle.
(v) Plot the stress path for each circle.
Conclusion:
- Comment on the values (cu and u) for the sample you tested, noting the significance of methods detailed in iii and iv above on the values obtained.
- Comment on the failure modes of your samples.
- Comment on the use of this test and your findings.
Indicative Reading:
Manual of Soil Laboratory Testing Vol.1 – K.H. Head
Basic Soil Mechanics – Whitlow, R.
Laboratory Work in Soil Mechanics — B. Vickers (Chapter 4: Shear-strength test is available to download from My-Beckett).
Appendix A
Moisture Content
| Container No. | ||
| Mass of wet soil + container (m2) | g | |
| Mass of dry soil + container (m3) | g | |
| Mass container (m1) | g | |
| Mass of moisture (m2-m3) | g | |
| Mass of dry soil (m3-m1) | g | |
| Moisture content | % |
Triaxial Checklist
Flooding Triaxial Cell
Cell sealing nuts Tightened, cell securely sealed
Load Cell Securing Arm Engaged
Cell Inlet Valve Closed
Cell Drainage Valve Closed
Unit Supply Valve Open
Unit Return Valve Open
APC Valve Open
APC Volume Reading Approximately 225cc, no High/Low limit alarm
Air Bleed Valve Open
Cell Inlet Valve now safe to operate for flooding cell. Check for water leakage from cell during flooding.
If leakage is found… Immediately close Cell Inlet Valve, open Cell Drainage Valve
Pressurising Triaxial Cell
Load Cell Securing Arm Disengaged & clear
Load Cell Rod Tip Engaged with secured restraint frame
Air Bleed Valve Closed
Cell Drainage Valve Closed
Cell Inlet Valve Open
Unit Return Valve Open
Unit Supply Valve Closed
APC Valve Open
Free to pressurise cell for test.
Draining Triaxial Cell
APC Pressure Reading Reduced to approximately 20 kPa
APC Status APC stopped, status displaying “Idle”
APC Valve Closed
Cell Inlet Valve Closed
Unit Return Valve Closed
Load Cell Rod Tip Disengaged from restraint frame
Load Cell Securing Arm Engaged
Cell Drainage Valve Open
Air Bleed Valve Open
Once cell has drained…
Cell Drainage Valve Closed
Air Bleed Valve Closed