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Lab 3 -CIVL 3720

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    Hong Kong University of Science and Technology

    CIVL 3720 Soil Mechanics

    Lab 3 - Consolidated Drained/Undrained Triaxial Compression

    Test (CD and CU test)

    Experiment date : 20th

    March, 2013

    Report submission date : 10th

    April, 2013

    Group Members

    Name SID Contribution

    (%)

    Signature

    CHAN, Yik Hin 20035984

    CHAU, Lai Bun 20029284

    CHAU, Man Kit 20031134

    CHONG, Sing Pui 20031225

    CHONG, Wai Ho 20029375CHOW, Jun Kang 20020628

    FUNG, Hoi Tai 20030489

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    Introduction

    A widely used apparatus to determine the shear strength parameters and the

    stress-strain behavior of soils is the triaxial apparatus. The name is a misnomer since

    two, not three, stresses can be controlled. In the triaxial test, a cylindrical sample of

    soil, usually with a length to diameter ratio of 2, is subjected to either controlled

    increases in axial stresses or axial displacements and radial stresses. The sample size

    is kept in this ratio so that the stress is uniformly distributed or no buckling occurs.

    The axial stresses are applied by loading a plunger. If the axial stress is greater than

    the radial stress, the soil is compressed vertically and the test is called triaxial

    compression. For another case, if radial stress is greater than axial stress, the soil is

    compressed laterally and the test is called triaxial extension.

    In this experiment, 2 tests are performed- consolidated drained (CD) compression test

    and consolidated undrained (CU) compression test. The below table summarizes the

    features of these two tests.

    CD Test CU Test

    Purpose Determine cs, p and c.

    Effective elastic moduli for

    drained condition E and Escan be obtained too.

    Determine su, cs and p

    Loading stages 1st stage: Isotropic

    consolidation phase

    - Consolidating soil sample

    until excess pore water pressure

    dissipates.

    1st stage: Isotropic consolidation

    phase

    - Consolidating soil sample until

    excess pore water pressure

    dissipates.

    2nd stage: Shearing phase

    - Pressure in the cell is kept

    constant and additional axial

    loads or displacmentes are

    added very slowly until the

    soil. sample fails.

    2nd stage: Shearing phase

    - Axial load is increased under

    undrained condition and the

    excess pore water pressure is

    measured.

    Objective

    To determine the stress-strain-strength behavior of a dry medium-fine sand by

    consolidated drained/undrained triaxial compression test (CD and CU tests)

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    Equipment

    1 Triaxial device (WF machine)2 Pressure gauge3 Dial gauge4 Device for measuring volume changes5 PC installed with data acquisition systemProcedures

    Sample preparation and setup:

    Sand samples would be prepared and set up in the triaxial apparatus by lab technicians.

    The dimension of the specimen and detailed explanation of the experimental setup

    would be given by TAs.

    Degree of saturation Checkingby B-value (Skempton pore pressure parameters):

    B-value could be used as an indicator to check the degree of saturation of the

    specimen. Procedures below should be followed:

    1 Valve of back pressure source was closed.2 Valve of cell pressure source was opened.3 Cell pressure was adjusted slowly to a certain increment, for example 50kPA.4 The corresponding excessive pore pressure,u, was recorded.5

    The B value was calculated by the definition: B =u / 36 Step 7 was processed for a B-value larger than or equal to 0.95. For B-value

    smaller than 0.95, an increment of back pressure was applied to improve the

    degree of saturation. Cell pressure to the same increment as back pressure was

    adjusted and the effective confinement was kept unchanged. The back pressure

    valve was opened until equilibrium was reached. Steps (1) to step (6) was then

    repeated.

    7. The pressure increment was released to check the B-value.

    Consolidation

    1. Cell pressure valve was opened and the pressure was adjusted to the designedvalue (effective confinement).

    2. The valve connecting to the device of measuring volume changes was opened.The sample was allowed to be consolidated about 5 minutes and the water would

    flow out from the sample to the device.

    3. The volume change during consolidation, which was inferred from the amount ofwater flowing out, was recorded

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    Drained/Undrained Shear Test

    1. The loading ram (plunger) was brought in contact with the loading cap on the topof the sample.

    2. The LVDT was connected to measure the axial displacement during shearing.3. The rate of vertical displacement was set to 0.5mm/min.4. For a drained test, the drainage valve had to be opened to ensure a drained

    condition. Similarly, the closing of the drainage valve would create an undrained

    condition.

    5. The shearing of sample was started (vertical loading).6. The test was stopped until axial strain reaches 15%.Remarks

    Group 1 2 3 4

    Effective consolidation pressure

    (confinement)

    50 kPa 100 kPa 200 kPa 300 kPa

    *However, data processing and discussion would be done with last years results

    (effective consolidation pressure was 100 kPa, 200 kPa, 300 kPa and 400 kPa) due to

    time constraint.

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    Data Processing and Discussion

    For the drained and undrained test performed during your lab session, plot

    1. '1and 3 vs e (void ratio)2. q vs p and p3. q vs 1 (%)4. v(%) vs 1(%) for drained test and u vs 1 (%) for undrained test5.Identify peak and/or ultimate shear strength from your own tests.

    For the all tests (including results from other groups)

    6.In a p-q space, plot all the peak (for drained test only) and ultimate strengthpoints. Calculate the shear strength parameters of the soil.

    7.DiscussionIn compression test, we will denote the radial stress r as 3and the axial stress z as

    1. Besides, we will denote compression stress as positive. For volumetric strain,

    positive value indicates compression, negative sign indicates expansion in order to be

    consistent with analysis in the text book. (This is opposite with data have been

    recorded in the machine)

    Axial total stress: 1= Pz/A + 3 Deviatoric stress: 13 = Pz/A = q

    Axial strain: 1 = z/H0 Radial strain: 3= 2= r/r0 Volumetric strain: p= V/V0= 1+ 2+ 3= 1+ 23 Deviatoric strain: q= (2/3)*( 13)

    Where Pz = the load on the plunger

    A = cross-sectional area

    r0 = initial radius of the sample

    r = change in radius

    V0 = initial volume

    V = change in volume

    H0 = initial height

    z = change in height

    Correction of cross-sectional area AThe area of the samples change during loading at any given instance is

    = = =

    1 1

    =

    (1 )1 To get void ratio, e, we have to do some derivation

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    = = 1 + =1 + 1

    = 1

    1

    Where Gs = specific gravity of sample (assume as 2.70)

    w = density of water

    md = dry weight of the sample

    In order to draw the stress path (we only consider the stage 2 shear phase in report),

    we have to find the value of p, p and q. In triaxial test, we assume axisymmetric

    condition, therefore 2 = 3, 2 = 3. Therefore

    = + + 3 = + 23

    = + + 3 = +23 = 12 [ + + ]/ =

    12 [2 ]/

    = In order to determine shear strength parameters of soils, a critical state model (CSM)

    is used to interpret it. In this model, we transform Mohr-coulomb failure envelope

    from - space into p-q space. Derivation is made under the axisymmetric condition.(z= 1, r= = 3)

    For axisymmetric condition, we will keep 3as constant and increases 1. Then we

    are able to derive a relationship between friction angle and Mc.

    = =

    + 2

    3

    =3 1

    + 2

    = 1 + sin 1 sin

    Then, we are able to get the following equation:

    = 6sin

    3 sin , sin =36 +

    Similarly for axisymmetric extension, we are able to get the relationship between

    friction angle and Me. Everything remains constant except decreasing 3. Below are

    the derivations obtained:

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    = 6sin

    3 + sin , sin =36

    Consolidated Drained Test (CD Test)

    The graphs below show the results obtained.

    400

    401

    402

    403

    404

    405

    406

    407

    0.72 0.73 0.74 0.75 0.76 0.77 0.78

    '3(kPa)

    Void ratio, e

    Graph of'3 vs e

    0

    200

    400

    600

    800

    1000

    1200

    1400

    1600

    1800

    0.72 0.73 0.74 0.75 0.76 0.77 0.78

    '1(

    kPa)

    Void ratio, e

    Graph of'1 vs e

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    -200

    0

    200

    400

    600

    800

    1000

    1200

    1400

    0 200 400 600 800 1000 1200

    q(kPa)

    p,p' (kPa)

    Graphof q vs p and p'

    q vs p (TSP)

    q vs p' (ESP)

    -200

    0

    200

    400

    600

    800

    1000

    1200

    1400

    0 5 10 15 20

    q

    (kPa)

    1 (%)

    Graph of q vs 1(%)

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    From the graph of v vs 1, we could suggest that the soil sample is dense soil as

    dilation occurs. Although the peak in the graph of q vs 1 is not obvious

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