JGS 0522-2020

Japanese Geotechnical Society Standard (JGS 0522-2020)
Method for consolidated-undrained triaxial compression test on soils
1ScopeThis standard specifies a test method to determine the strength and deformation characteristics of soils when
they are subjected to undrained compression after consolidation in an isotropic stress state. The test shall be
carried out on three or more specimens prepared from a same sample under different consolidation stresses
within the required consolidation stress range. The relationship between the volume change (and the axial
displacement if possible) and consolidation time should also be obtained. This standard applies mainly tosaturated cohesive soils.
Note 1: In this standard, testing is in principle carried out with back pressure. However, the standard does not preclude
testing with no back pressure. If testing is carried out without back pressure, all provisions that specify devices and methods
relating to back pressure are to be ignored and values for back pressure are to be read as zero.
Note 2: The measurement of axial displacement during consolidation process is assumed, but if axial displacement is notmeasured, provisions relating to axial displacement may be ignored.
Note 3: This standard is applicable also to saturated coarse-grained soil.

2

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 specimens to be used in this test shall be prepared and installed in accordance with the following standard.
JGS 0520 Preparation of soil specimens for triaxial tests
For coarse-grained soil with a maximum grain size exceeding about 20 mm, the specimens to be used in thistest shall be prepared and installed in accordance with the following standard.
JGS 0530 Preparation of specimens of coarse granular materials for triaxial tests

3Terms and definitions

The terms and definitions used in this standard are as follows:Note:

The test covered by this standard can be referred to in abbreviated form as CU triaxial test.

3.1

CU

Shearing soil in the undrained condition after consolidating it (Consolidated and Undrained: CU).

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.

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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 specimen.

3.5

Isotropic stress state

The stress state when the axial stress is equal to the lateral stress.

3.6

Cell pressure

The pressure applied to the triaxial pressure cell. The lateral stress is equal to the cell pressure.

3.7

Back pressureThe pressure applied to the pore water within the test specimen (JIS A 1227).
Note: In this standard, the back pressure means the pore water pressure applied to the specimen to achieve a higher degreeof saturation of the specimen while maintaining a constant effective stress.

3.8

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.

3.9

Undrained compressive strength of soil

The maximum principal stress difference that can be applied to the consolidated specimen when no pore wateris allowed to flow in or out of the specimen.

4Equipment

4.1

Triaxial compression test apparatus

The triaxial compression test apparatus shall consist of a triaxial pressure cell, a system for applying cellpressure and back pressure, a compression device, and measuring instruments for load, displacement, andvolume changes. The apparatus shall satisfy the following conditions. Figure 1 shows an example of theconfiguration of triaxial compression test apparatus. For testing without back pressure, the components
indicated by the broken lines in the figure are not needed. Figure 2 shows the schematic structure of triaxialpressure cell: (a) is an example in which the loading piston and cap are rigidly connected while (b) is an example
in which the two are not rigidly connected. Both forms must allow for mounting of a specimen with the same
diameter as the cap and pedestal between the cap and pedestal, covered with a rubber sleeve, and sealed withan O-ring, etc.
a)

The apparatus shall have high load-bearing strength and load capacity to handle the maximum cellpressure, back pressure and maximum axial compression force to be applied to the specimen.

b)

The apparatus shall be capable of maintaining the cell pressure and back pressure within ±4 kN/m2 of atarget value for pressures up to 200 kN/m2, and within ±2 % of the target value for pressures of 200 kN/m2or greater, for the duration of a single test.

c)

The apparatus shall be capable of applying and sustaining axial displacement up to 15 % or more of thespecimen height.

d)

The apparatus shall be capable of measuring cell pressure and back pressure with an allowable toleranceof 2 kN/m2 for pressures up to 200 kN/m2 and of 1 % for pressures of 200 kN/m2 or greater.

2© JGS 2015 – All rights reserved

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The apparatus shall be capable of measuring the axial compression force up to the specimen’s maximum
axial compressive strength within an allowable tolerance of 1 %. If the test apparatus is set up such thatthe load cell is located outside the triaxial pressure cell, the frictional force at the point of sliding between
the piston and the triaxial pressure cell shall be measured. This friction value shall be used to correct themeasured value of axial compression force. If the load cell is located inside the triaxial pressure cell, theeffect of cell pressure shall be determined and the measured value of axial compression force shall becorrected.

f)

The apparatus shall be capable of measuring the axial displacement up to 15 % of the specimen height
within an allowable tolerance of 0.1 %.

g)

The apparatus shall be capable of measuring the volume change of the specimen up to its maximum value
within an allowable tolerance of 0.1 % of the specimen’s initial volume. In a standard setup, when testing
saturated soil, the volume change of the specimen shall be determined as the amount of water expelledfrom the specimen during the test measured by using a burette or other devices with equivalent or higher
measurement accuracy. In the sections below, a burette is used as a representative device for measuring
the volume change.

5Test method

5.1

Preparation and set-up of test specimen

Preparation and mounting of the specimen shall be in accordance with JGS 0520 Preparation of soil specimens
for triaxial tests. Additionally, the specimen shall have a height not less than twice its diameter. For coarsegrained soils with a maximum particle diameter greater than about 20mm, the specimen shall be prepared inaccordance with JGS 0530 Preparation of specimens of coarse granular materials for triaxial tests.

5.2

Consolidation process

The consolidation procedure of the test shall be in accordance with the following requirements.
a)

Make zero adjustments to the displacement transducer, and ensure application of the prescribed backpressure ub. At the same time, take the initial reading of the burette. If the piston and cap are not rigidly
connected, bring the load cell, piston, and cap into contact before the above operation.

b)

Close the drainage valve connected to the burette. Increase only the isotropic stress so that the difference
between isotropic stress and back pressure is equal to the desired value of consolidation stress.

c)Open the drainage valve to start consolidation.

d)Record readings of the volume change ΔVt (mm3) and the axial displacement ΔHt (mm) of the specimen at
appropriate time t intervals during consolidation, and plot them. Consolidation shall be continued at least
until the end of primary consolidation. Measure the volume change ΔVc (mm3) (which, for saturated soil, isequal to the amount of water expelled from the specimen) and the axial displacement ΔHc (mm) due toconsolidation. The termination of consolidation is to be determined by the following method in principle:
Plot the measured values (Vt, logt) on logarithmic-normal paper and draw the Vt-logt curve. Locate thesteepest part of the curve. Draw a straight line parallel to the steepest part of the curve and at the 3t point
in time (termed the “3t line”). The point at which the Vt-logt curve touches the 3t line is defined as the point
at which consolidation terminates. If determining the end of primary consolidation is not possible, such as
when the specimen is in an excessively consolidated state, terminate consolidation at an appropriate point
after 100 minutes of consolidation for specimens with a diameter of 35 mm and 150 minutes for specimens
with a diameter of 50 mm or above. If the axial displacement cannot be measured during consolidation
because the piston is not rigidly connected to the cap, carefully bring the piston into contact with the cap,
while observing the load cell reading such that no additional force is applied to the specimen. Then takethe reading of the displacement transducer. This measured value is to be taken as the axial displacement© JGS 2015 – All rights reserved

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ΔHc (mm) due to consolidation.
Note:

In selecting a measurement time interval, use as a guide JIS A 1217 Test method for one-dimensional consolidation

properties of soils using incremental loading.

5.3

Axial compression process

The axial compression shall be carried out in accordance with the following procedure.
a)

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

b)

Close the drainage valve.

c)Start compressing the specimen at a steady rate under a constant cell pressure. The standard rate ofcompression shall be 1 % axial strain per minute.

d)

Record the axial compression force P (N) and axial displacement ΔH (mm) during compression.

Note:

If the axial compression force and axial displacement are not recorded continuously, the measurement time interval

shall be sufficiently small to enable a smooth curve for the principal stress difference versus axial strain to be drawn. It isrecommended, for example, to measure axial displacement at a maximum interval of 0.2 mm up to the maximum value ofthe axial compression force, and thereafter at a maximum interval of 0.5 mm.

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

6Processing test results

6.1

Initial state of specimen before consolidation

The volume V0 (mm3) and height H0 (mm) of the specimen before consolidation shall be calculated using thefollowing equations.

V0 = Vi − Vi
H0 = Hi − Hi
whereVi :Initial volume of the specimen (mm3)Hi :Initial height of the specimen (mm)
ΔVi : Volume change of the specimen between the initial state and prior to the start of consolidation(mm3), where compression is defined to be positive
ΔHi : Axial displacement of the specimen between the initial state and prior to the start of consolidation(mm), where compression is defined to be positive4© JGS 2015 – All rights reserved

JGS 0522-2020

6.2

Consolidation process

The test results from the consolidation process shall be processed as follows:a)

The volume of the specimen after consolidation Vc (mm3) shall be calculated using the following equation.

Vc = V0 − Vc
whereΔVc: Volume change of the specimen due to consolidation (mm3), where compression is defined to bepositive
Note:The void ratio of the specimen after consolidation (before axial compression) ec should be calculated using thefollowing equation, if necessary.

ec =Vc /1000 × ρs
−1ms

where
ρs: Density of soil particles (Mg/m3)b)

The height of the specimen after consolidation Hc (mm) shall be calculated using the following equation.

Hc = H0 − HcwhereΔHc: Axial displacement due to consolidation (mm), where compression is defined to be positive
Note:

If the piston is not rigidly connected to the cap and the measurement of the axial displacement due to consolidationΔHc (mm) is uncertain, the following equation should be used to calculate the specimen height after consolidation Hc (mm),assuming isotropic strain in the specimen. If the value of Hc is obtained using this equation, this fact must be clearly indicatedin the report.

 Vc   H0H c = 1 −3V0 c)

The specimen’s cross-sectional area after consolidation Ac (mm2) shall be calculated using the following
equation.
Ac =d)Vc
Hc

The dry density of the specimen after consolidation ρdc (Mg/m3) shall be calculated using the following
equation, which shall be rounded to two digits after the decimal point.
ρd0 =ms× 1000
V0

wherems:

Oven-dried mass of the specimen (g)© JGS 2015 – All rights reserved

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6.3

Axial compression process

Refer to Section 6.2 of JGS 0521 Method for unconsolidated-undrained (UU) triaxial compression test on soils
for the method of calculation for the consolidation process.

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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 cell pressure (kN/m2) and the back pressure (kN/m2)d)Relationship between the volume change (mm3) and time (min) in the consolidation process

Note:e)Report the relationship between the axial displacement and time in the consolidation process, if necessary.

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

Note:

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

f)Strain rate (%/min) in the axial compression processg)

Compressive strength (kN/m2) and strain at failure (%)h)

Principal stress difference versus axial strain curvei)

Failure condition of the specimenj)

Relationship between compressive strength and consolidation stressk)

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

l)

Other reportable matters

6© JGS 2015 – All rights reservedJGS 0522-2020Vacuum
regulator

Deaerated water supply tank

Water

Single-tube burette

Water

Cell pressure meter

Specimen

Triaxial pressure cell

Displacement
gauge(Back-pressure meter)(Double-tube burette)

Load cellRegulator(Back-pressure regulator)

Air pressure

Compression
deviceFigure 1 Example of configuration of CU triaxial test apparatus (back pressure apparatus
shown in broken lines)

Loading piston

Loading piston

Air release valve

Air release valve
Top plateCap

Top plate

CapO-ringO-ringRubber sleeve

Post

Pressure cylinder

Pressure cylinder

Post

Porous plate

Cell pressure
supply path

Specimen

Specimen

Rubber sleeve

Pedestal
Cell pressure
supply path

Porous plate

Drainage path

Base plate

Drainage path
Drainage path

Pedestal(a) Loading piston and cap rigidly
connected

Base plate

Drainage path(b) Loading piston and cap not rigidly
connected

Figure 2 Schematic structure of triaxial pressure cells© JGS 2015 – All rights reserved

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