lap report (GEOTECHNICAl)

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.

254

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.

  100% o

X Strain

L     1 3

P Deviator stress

A  

    

 

where P = vertical load, which is measured by a proving ring (kN) A = Area calculated using the following method;

 ( ) )o o o oVolume V A L AL A L X   

255

   1 o o

o

V A or A or A

L X   

 

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:

• Sketch the failure mode of each sample.

• Calculate the moisture content of the soil as per Appendix A. • Calculate the results as follows:

(i) For each sample tested: • Find the failure strain (either the final value or the 20% value); this

is denoted . • 3 is the cell pressure. • Area at failure is given by A=Vo/(Lo-X) or A= Ao/(1 - ) • (1-3), the Deviator Stress, is given by (Proving ring divisions at

failure  constant) ÷ (Area at failure) (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.

256

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 2 3

3 1

m m w

m m

    

  %

Average moisture content %

257

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

258