JGS 0526-2020

Japanese Geotechnical Society Standard (JGS 0526-2020)
Method for K0 consolidated-undrained triaxial extension test on soils with porewater pressure measurements
1

Scope

This standard specifies a test method to determine the consolidation properties of soil consolidated in the K0
condition, and the strength and deformation properties when the soil is subjected to axial extension in an
undrained condition following consolidation. The test shall be performed for saturated cohesive soil.
Note:

2

This standard may also be applied mutatis mutandis to sandy soil.

Normative references

The following standards shall constitute a part of this standard by virtue of being referenced in this standard.
The latest versions of these standards shall apply (including supplements). The soil specimens used for thistest shall be prepared and installed in accordance with the following standards.
JGS 0520 Preparation of soil specimens for triaxial tests
When performing this test, the following standards shall be referenced for matters not specified in this standard.
JIS A 1217 Test method for one-dimensional consolidation properties of soils using Incremental loading
JGS 0522 Method for consolidated-undrained triaxial compression test on soils (CU)JGS 0523 Method for consolidated-undrained triaxial compression test on soils with pore water pressure̅̅̅̅̅)
measurements (CUJGS 0524 Method for consolidated-drained triaxial compression test on soils
JGS 0525 Method for K0 consolidated-undrained triaxial compression test on soils with pore water pressure
measurements (K0 ̅̅̅̅̅CUC)

3

Terms and definitions

In addition to those specified in JIS A 0207, the main terms and definitions used in this standard are as follows:
Note 1:

As axial extension force is applied following K0 consolidation, the soil specimen will be in an extended state when

the axial stress becomes lower than the lateral stress (when the principal stress difference becomes negative).
Note 2: This test may be abbreviated to “K0 ̅̅̅̅CUE triaxial test”.

3.1

K0 consolidated-undrained triaxial extension (K0 ̅̅̅̅̅
CUE)

Application of axial extension force to achieve shear while simultaneously measuring pore water pressure in the
undrained condition after consolidation in the triaxial K0 condition.

3.2

Axial stress

The stress acting on the specimen in the cylinder axis direction.

3.3

Lateral stress

The stress acting in the radial direction of the specimen.

3.4

Principal stress difference

The difference between the axial stress and the lateral stress. The value of stress shall be defined at the midheight of the test specimen.

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3.5

Anisotropic stress state

A stress state with different axial stress and lateral stress.

3.6

K0 condition

An anisotropic stress state in which no displacement is produced in the radial direction of the soil specimen.

3.7

Cell pressure

The pressure applied within a triaxial cell.

3.8

Back pressure

The pressure applied to the pore water within the specimen (JIS A 1227).
Note:

In this Standard, the back pressure means the pore water pressure applied to the specimen to achieve a higher

degree of saturation of the test specimen while maintaining a constant effective stress.

3.9

Consolidation stress

The stress of soil element that induces consolidation.
Note:

In this Standard, the consolidation stress means the difference between the externally applied stress on the test

specimen and the back pressure during the consolidation process.

3.10 Axial consolidation stress
Consolidation stress in the long axis direction of the specimen.

3.11 Lateral consolidation stress
Consolidation stress in the radial direction of the specimen.

3.12 Undrained extensive strength
The minimum principal stress difference that can be applied to the soil specimen when no pore water is allowed
to flow in or out of the specimen.

4

Equipment

4.1

Triaxial test apparatus

The triaxial test equipment shall consist of a triaxial pressure cell, a system for applying cell pressure and backpressure, axial loading equipment, and measurement devices for load, displacement, volume change, and porewater pressure. The apparatus shall satisfy the following conditions. Figure 1 in JGS 0525 “Method for K0̅̅̅̅̅ C”
consolidated-undrained triaxial compression test on soils with pore water pressure measurements K0 CU
shows an example of the configuration of K0 consolidated triaxial test equipment that conducts tests throughautomatic control. A configuration in which the piston and cap are rigidly connected is suitable for the structureof the triaxial cell. A configuration that prevents the triaxial cell from separating from the loading equipment whentensile force is applied to the piston is needed.
a)

The apparatus shall have sufficiently high load-bearing strength and load capacity to handle the maximum
cell pressure, back pressure and maximum axial load to be applied to the specimen, and shall be capableof applying axial stress and lateral stress independently during consolidation.

b)

The apparatus shall be capable of continuously applying the specified cell pressure and back pressure
within ±2 kN/m2 of the target value for pressures up to 200 kN/m2, and within ±1 % of the target value forpressures of 200 kN/m2 or greater, for the duration of a single test.

c)

The apparatus shall be capable of applying axial displacement continuously at a constant rate up to 15%of the specimen height.

2© JGS 2016 – All rights reserved

JGS 0526-2020d)

The apparatus shall be capable of measuring the cell pressure, the back pressure and the pore waterpressure inside the soil specimen, with an allowable tolerance of 2 kN/m2 for pressures up to 200 kN/m2
and 1 % for pressures of 200 kN/m2 or greater.

Note:

Normally, a diaphragm type pressure transducer is used as the pore water pressure measurement device. If thevolume of the test specimen is V, and the volume change of the measuring system including the pressure transducer is ΔV
for a change in pore water pressure Δu, then a proper transducer shall be selected so that its performance satisfies(ΔV/V)/Δu < 5×10-6 m2/kN. Also, the measured values of the pore water pressure are affected by temperature changes, so
it is desirable that the change in temperature within the measurement room be controlled to within ±2 C.

e)

The apparatus shall be capable of measuring the axial force up to the specimen’s maximum axial force
within an allowable tolerance of 1 %. As a standard, a load cell shall be provided inside the triaxial cell, andin this case, any effect of cell pressure shall be calibrated, and the axial force measurement shall becorrected. In the case of test equipment in which the load cell is provided outside the triaxial cell, the friction
force of the piston and the sliding section of the pressure chamber shall be measured and the axial
compression force measurement shall be corrected.

f)

The apparatus shall be capable of measuring the axial displacement up to 15 % of the specimen height
within an allowable tolerance of 0.02 %.

g)

The apparatus shall be capable of measuring the volume change of the specimen up to its maximum value
within an allowable tolerance of 0.05 % of the specimen’s initial volume. When measuring lateraldisplacement and controlling K0 consolidation, the apparatus shall be capable of conducting measurementswith an allowable tolerance of 0.025 % of the initial diameter of the soil specimen.

5

Test method

5.1

Preparation and set-up of test specimen

The preparation and set up of the test specimen shall be in accordance with JGS 0520 Preparation of soil
specimens for triaxial tests. The standard height of the specimen shall be 1.5 to 2 times the diameter. The drain
filter attached to the soil specimen shall be made of a material that exhibits little compression or expansion
when the cell pressure is changed.
Note 1:

Correction should be performed when there is a great change in volume of the drain filter.

Note 2:

When performing a test for K0 consolidation only, the height of the soil specimen may be reduced.

5.2

Confirmation of degree of saturation

The pore pressure coefficient B (B value) shall conform to Section 5.2 in JGS 0525 Method for K0 consolidatedundrained triaxial compression test on soils with pore water pressure measurements (K0 ̅̅̅̅̅
CUC).

5.3

Consolidation process

The consolidation process shall conform to Section 5.3 in JGS 0525 Method for K0 consolidated-undrained̅̅̅̅̅C).
triaxial compression test on soils with pore water pressure measurements (K0 CU

5.4

Undrained extension process

The undrained extension process shall be carried out in accordance with the following procedure.
a)

Check and adjust the zero reading of the load cell and displacement transducer.

b)

Close the drainage valve.

c)Start extending the specimen continuously at a constant strain rate keeping the cell pressure constant. To
ensure that the pore water pressure is uniformly distributed in the specimen, the standard axial
compression rate shall be about -0.1 %/min for silty soils and about -0.05 %/min for clayey soils.

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JGS 0526-2020

d)

Record the axial force P (N), the axial displacement H (mm) and the pore water pressure u (kN/m2).

Note:

If the axial compression force, the axial displacement, and the pore water pressure are not recorded continuously,

the measurement time intervals should be sufficiently small to enable smooth curves for the principal stress difference –axial strain curve and the pore water pressure – axial strain to be drawn.

e)Terminate the extension either when an axial strain of more than -3 % has been reached since the minimum
axial load reading, when the load has fallen to about 2/3 of its peak value, or when an axial strain of -15 %has been reached.

f)Remove the specimen from the triaxial pressure cell, and observe and record its deformed shape, failuremode, and other features. The observation shall be made in the direction that captures the features offailure most clearly. If a slip surface is found, observe it from the direction in which the steepest gradient isdetermined and record it so that the gradient angle can be approximately read. Any heterogeneity of thespecimen and the presence of any foreign materials shall be observed and recorded.

g)

Measure the oven-dried mass of the specimen ms (g).

Note:This step in the process may be omitted if the water content of the specimen is determined from the trimmed wasteof the sample.

6

Processing test results

6.1

State of soil specimen before consolidation

Calculation of the state of the soil specimen before consolidation shall conform to Section 6.1 in JGS 0525
Method for K0 consolidated-undrained triaxial compression test on soils with pore water pressure measurements̅̅̅̅̅C).
(K0 CU

6.2

Pore pressure coefficient B

Calculation of the B value shall conform to Section 6.2 in JGS 0525 Method for K0 consolidated-undrained
triaxial compression test on soils with pore water pressure measurements (K0 ̅̅̅̅̅
CUC).

6.3

Consolidation process

Data calculation of the consolidation process shall conform to Section 6.3 in JGS 0525 Method for K0̅̅̅̅̅C).
consolidated-undrained triaxial compression test on soils with pore water pressure measurements (K0 CU

6.4

Undrained extension process

The method used to calculate and organize the undrained extension process shall be as follows.
a)Calculate the axial strain of soil specimen εa (%) using the following equation (compression is positive).
εa =

∆H
× 100
Hc

where

H: Axial displacement of the specimen (mm), where compression is defined to be positiveb)

Calculate the principal stress difference (σa – σr) (kN/m2) under axial strain εa (%) and the increase in porewater pressure resulting from axial extension ue (kN/m2), using the following equations.
σa − σr =P − P0εa(1 −) × 1000Ac100

ue = u − ub

4

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JGS 0526-2020

where
P: Axial compression force (N) acting on the specimen at the axial strain εa (%)
u: Pore water pressure (kN/m2) measured at the axial strain εa (%)
ub: Back pressure (kN/m2)
c)

Draw graphs with the principal stress difference and the pore water pressure on the vertical axis versus
axial strain on the horizontal axis, to obtain the principal stress difference – axial strain curve and the porewater pressure increment due to axial compression – axial strain curve.

d)Obtain the minimum principal stress difference (σa – σr)min in the axial strain range of 0 < εa ≤ 15 %, and thisvalue shall be taken as the extension strength (kN/m2), which shall be rounded to three significant digits.
Also, determine the corresponding axial strain εf from these figures, which shall be rounded to one digit
after the decimal point.

e)

Determine the ratio su/σ′ac of undrained shear strength su =│(σa – σr)min/2│ to effective axial consolidationstress σ′ac, which shall be rounded to three significant digits.

f)Calculate the effective principal stress σ′a (kN/m2) and σ′r (kN/m2) in the undrained extension process fromthe following equations, and draw an effective stress path diagram with σa – σr on the vertical axis and(σ′a+2σ′r)/3 on the horizontal axis.
σ'r = σr − u
σ'a = (σa − σr ) + σ'r

Note:

If necessary, draw an effective stress path diagram with (σa – σr) /2 on the vertical axis and (σ′a+σ′r)/2 on the horizontal

axis.

g)

Calculate the effective principal stresses at the minimum principal stress difference σ′af (kN/m2) andσ′rf (kN/m2), using the following equations, which shall be rounded to three significant digits.
σ'rf = σrf − uf
σ'af = (σa − σr )min + σ'rf
where
σrf: Lateral stress at minimum principal stress difference (kN/m2)
uf: Pore water pressure at minimum principal stress difference (kN/m2)

Note:

7

Obtain σ′a and σ′r at (σ′a/σ′r)min, if necessary.

Reporting

The following test results and other items shall be reported.
a)Method of specimen preparationb)

Dimensions of the specimen before the consolidationc)

Magnitude of the back pressure (kN/m2)d)Time needed for B value and pore water pressure to stabilize (min)e)

Stress loading method at stages in which consolidation is in progress (note the stress in the case of staged
loading)f)

Change in volume during consolidation process (mm3), axial displacement (mm), and relationship of σaf –ub (kN/m2), σrf – ub (kN/m2) and time (min)© JGS 2016 – All rights reserved

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JGS 0526-2020

g)Oven-dried mass (g) of soil specimen and the dry density (Mg/m3) after consolidation

Note:

Report the void ratio of the specimen after consolidation, if necessary.

h)Relationship of (σrf – ub) / (σaf – ub), εr (%) and elapsed time (min)i)

Relationship between (σrf – ub) / (σaf – ub) and σaf – ub (kN/m2)

j)

Axial consolidation stress σ′ac (kN/m2), lateral consolidation stress σ′rc (kN/m2) and stress ratio σ′rc/σ′ac (=K0)k)

Strain rate of undrained extension process (%/min)l)Principal stress difference versus axial strain curve, pore water pressure increase versus axial strain curve,
and minimum principal stress difference (kN/m2) and axial strain at that time (%)m) Effective stress path diagramn)

Failure mode of soil specimeno)Effective principal stress (kN/m2) and su/σ′ac at minimum principal stress difference

Note:

Report the values of σ′a and σ′r at (σ′a/σ′r)min, if necessary.

p)

When testing multiple soil specimens with the same sample, the relationship between undrained shear
strength and consolidation stressq)

If the method used deviates in any way from this standard, give details of the method used.

r)

Other reportable matters

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