Introduction Experimental Setup Data Analysis Results Theory

Unconsolidated Undrained (UU) Triaxial Test

Need and Scope:

To find the shear strength of soil by Unconsolidated Undrained Triaxial test. In UU test, soil specimen is not consolidated and thus drainage is not allowed either during application of cell pressure (confining pressure) or shearing. UU triaxial test is the fastest triaxial test to obtain shear strength parameters (c,Φ) of soil.

Concept:

A cylindrical soil specimen is subjected to three compressive stresses in mutually perpendicular directions and one of these three stresses being increased until specimen fails in shear. Initially, a confining pressure (σ_{3}) is applied through water around the specimen in an impermeable membrane. The vertical stress becomes major principal stress (σ_{1}) while the confining pressure σ_{3} acts in other two principal directions. The intermediate principal and minor principal stresses are equal to each other. Deviator stress (σ_{d}) is the difference of σ_{1} and σ_{3}, acts on specimen while its shear deformation.

Unconsolidated Undrained (UU) Triaxial Test

Experimental Setup:

1. Loading frame of capacity of 10 t, with constant rate of movement.

2. Proving ring of 0.01 kg sensitivity for soft soils; 0.05 kg for stiff soils.

3. Frictionless end plates of 38 mm diameter (Perspex plate with silicon grease coating).

4. Dial gauge (0.01 mm accuracy)

5. Soil specimen of 38 mm diameter & 76 mm length

6. O-rings

7. Latex membrane

Specimen Preparation:

__a)Undisturbed Specimen(UDS): __

1. Note down the sample number, bore hole number and the depth at which the sample was taken.

2. Remove the protective cover (paraffin wax) from the sampling tube.

3. Place the sampling tube (38 mm dia) extractor and push the plunger till a small length of sample moves out.

4. Trim the projected sample using a wire saw, and push the plunger until a 76 mm long sample comes out.

5. Cutout this sample carefully and hold it on the split sampler so that it does not fall.

6. Take about 10 to 15 g of soil from the tube for water content determination.

7. Note the container number and take the net weight of the sample and the container.

8. Measure the diameter at top, middle, and bottom of the sample. Find the average and record the same.

9. Measure the length and weight of the sample and record.

__ b) Remoulded Specimen(R): __

1. For the desired water content and the dry density, calculate the weight of the dry soil Ws required for preparing a specimen of 38 mm diameter and 76 mm long.

2. Add required quantity of water Ww to this soil.

W_{w} = W_{s} x W/100 gm

3. Mix the soil thoroughly with water.

4. Place the wet soil in a tight thick polythene bag in a humidity chamber.

5. After 24 hours take the soil from the humidity chamber and place the soil in a constant volume mould, having an internal height of 76 mm and internal diameter of 38 mm.

6. Place the lubricated mould with plungers in position in the load frame.

7. Apply the compressive load till the specimen is compacted to a height of 76 mm.

8. Eject the specimen from the constant volume mould.

9. Record the correct height, weight and diameter of the specimen.

Testing procedure (IS 2720 Part 11):

• The sample is placed in the compression machine and a pressure plate is placed on the top. Care must be taken to prevent any part of the machine or cell from disturbing the sample while it is being setup, for example, by knocking against bottom of the loading piston. The probable strength of the sample is estimated and a suitable proving ring selected and fitted to the machine.

• The cell must be properly set up and uniformly clamped down to prevent release of pressure or leakage of water during the test, making sure first that the sample is properly sealed with its end caps and rings (rubber) in position and that the sealing rings for the cell are also correctly placed.

• When the sample is setup water is admitted and the cell is filled until water escapes from the bleed valve, at the top, which is then closed. If the sample is to be tested at zero lateral pressure water is not required.

• The air pressure in the reservoir is then increased to raise the hydrostatic pressure in the required amount. The pressure gauge must be watched during the test and any necessary adjustments must be made to keep the pressure constant.

• The handle wheel of the screw jack is rotated until the under side of the hemispherical seating of the proving ring, through which the loading is applied, just touches the cell piston.

• The piston is then moved down by handle until it is just in touch with the pressure plate on the top of the sample, and the proving ring seating is again brought into contact for the beginning of the test.

• The machine is set in motion (or if hand operated the hand wheel is turned at a constant rate) to give a rate of strain 0.1% to 1% per minute. At particular intervals of strain dial gauge readings and the corresponding proving ring readings are taken, and the corresponding load is determined using proving ring chart. The experiment is stopped at the strain dial gauge reading for 15% of length of the sample or 15% strain.

• If soil specimen is collected at smaller depth (2-5 m), the UU tests can be performed at confining pressures of 0.5 kg/cm^{2}, 1.0 kg/cm^{2}, 1.5 kg/cm^{2}. The soil specimen collected at larger depths, the UU tests can be performed at confining pressures of 1.0 kg/cm^{2}, 2.0 kg/cm^{2}, 3.0 kg/cm^{2}.

Unconsolidated Undrained (UU) Triaxial Test

Observation Sheet:

*Test 1: Cell pressure = ___________ kg/cm*^{2}

Weight of Sample:___________________ In-situ density: __________________

Diameter:________________________ Area:________________________

Initial Water Content:_________________ Deformation rate:_ _________________

Least count of dial gauge:_______________Proving ring constant: _______________

Test 2: Cell pressure = ___________ kg/cm^{2}, Weight of Sample:___________________ In-situ density: __________________

Diameter:________________________ Area:________________________

Initial Water Content:_________________ Deformation rate:__________________

Least count of dial gauge:_______________Proving ring constant: _______________

Test 3: Cell pressure = ___________ kg/cm^{2}, Weight of Sample:___________________ In-situ density: __________________

Diameter:________________________ Area:________________________

Initial Water Content:_________________ Deformation rate:__________________

Least count of dial gauge:_______________Proving ring constant: _______________

Calculations:

1. A_{corr} = A_{0}/(1-ε); where A_{0} is initial cross-sectional area of the soil specimen, ε is the axial strain at that point loading.
Axial stress = (Proving ring reading x Proving ring constant)/A_{corr}

2. Axial stress versus axial strain curves are drawn for UU tests performed at three different confining stresses.

3. Deviator stress (σ_{d}) at failure needs to be obtained for UU tests for all three confining stresses (σ_{3}). σ_{1} is the sum of σ_{d} and σ_{3}.

4. Maximum deviator stress can be considered as the failure point of the specimen in its stress-strain relationship.

5. Modified Failure envelop (q-p curve) is drawn using UU test data, which provides a & ζ parameters. a is intercept of q-p curve and ζ is the slope angle of q-p curve.

q = (σ_{1} - σ_{3})/2, p = (σ_{1} + σ_{3})/2

6. Shear strength parameters (c & Φ) can be calculated by using following equations.

ccosΦ = a, sinΦ = tanζ

Unconsolidated Undrained (UU) Triaxial Test

Graphs:

1. Axial stress versus Axial strain relationship

2. Modified Failure envelop (q-p) curve

Example:

UU tests have been performed on silty clay specimens at confining pressure of 1.0 kg/cm^{2}, 2.0 kg/cm^{2}, 3.0 kg/cm^{2}. The specimen size was 38 mm diameter & 76 mm height. The specimen was sheared at deformation rate of 0.4 mm/min (strain rate = 0.5% per min). It is important to note that the soil sample in UU test was unsaturated. If saturated soil samples are used, it is mentioned in the analysis.

__Results:__

Cohesion (c) = 57 kPa

Internal friction angle (Φ) = 38 deg

General Remarks

• It is assumed that the volume of the sample remains constant and that the area of the sample increases uniformly as the length decreases. The calculation of the stress is based on this new area at failure, by direct calculation, using the proving ring constant and the new area of the sample. By constructing a chart relating strain readings, from the proving ring, directly to the corresponding stress.

• The strain and corresponding stress is plotted. The maximum compressive stress at failure and the corresponding strain and cell pressure are found out.

• The stress results of the series of triaxial tests at increasing cell pressure are plotted on a Mohr's stress diagram. In this diagram a semicircle is plotted with normal stress as abscissa and shear stress as ordinate.

• The condition of the failure of the sample is generally approximated by a straight line drawn as a tangent to the circles, the equation of which is τ = c + σ tan(Φ). The value of cohesion, c is read of the shear stress axis, where it is cut by the tangent to the Mohr circles, and the angle of shearing resistance (Φ) is angle between the tangent and a line parallel to the shear stress. In total stress analysis on saturated soil, the Φ value for UU test should come out to be zero.

Unconsolidated Undrained (UU) Triaxial Test

Theory:

A UU test is carried out almost exclusively on cohesive soils. No drainage is allowed at any stage of the test. The isotropic confining pressure is applied with the drainage valve closed. The entire cell pressure is carried by the pore water, if specimen is saturated. The specimen is loaded under undrained conditions. During the test, there will be pore water pressure development, which is not measured. Therefore, the effective stresses remain unknown. Mohr circles are only drawn in terms of total stresses, which enable the failure envelope to be drawn in terms of total stresses, giving shear strength parameters (c_{u} & Φ_{u}) under undrained loading conditions.

Being relatively quick and inexpensive, UU triaxial tests are quite popular in geotechnical engineering practice for obtaining shear strength of cohesive soil. However, this test does not provide the shear strength parameters in terms of effective stresses c' and Φ', which are required for carrying out an effective stress analysis.

The following sketch shows the three various phases of UU triaxial testing; sampling stage, Isotropic loading stage (application of confining pressure, σ_{3}), Shear stage (application of deviator stress, σ_{d}). Total stress, pore pressure and effective stresses are shown at each phase of UU triaxial testing in the given diagram. u_{r} is the residual pore water pressure entrapped inside the soil specimen after its collection from soil site using UDS (Undisturbed sampling) tube. σ_{v} and σ_{h} are the vertical and horizontal stresses respectively acting on soil specimen during UU triaxial testing. A is the pore pressure parameter due to the shear deformation of the soil specimen.