JGS 0523-2020

Japanese Geotechnical Society Standard (JGS 0523-2020)
Method for consolidated-undrained triaxial compression test on soils withpore water pressure measurements
1

Scope

This standard specifies a test method to determine the strength and the deformation characteristics of a soil
specimen when it is subjected to undrained axial compression after isotropic consolidation, and to obtain theeffective stresses at the maximum principal stress difference. The test shall be carried out on three or more test
specimens prepared from the same test material, under different consolidation stresses within the requiredconsolidation stress range. In this test, the relationship between the volumetric change during the consolidation
process (and the axial displacement if possible) and time, and the relationship between the pore water pressureduring the axial compression process and the axial strain are obtained. This standard applies mainly to saturated
cohesive soils.
Note 1:In this standard, it is assumed that the axial displacement is measured, but if the axial displacement is notmeasured, provisions relating to axial displacement may be ignored.
Note 2: This standard is also applicable to saturated coarse granular 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 particle size exceeding about 20 mm, the test specimens to be usedin this test shall be prepared and installed in accordance with the following standard.
JGS 0530 Preparation of specimens of coarse granular materials for triaxial tests

3

Terms 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.

3.1̅̅̅̅̅̅
CU

Shearing soil under the undrained condition with pore pressure measurement after consolidating it̅̅̅̅̅).
(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 test 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 pressure

The 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

degree of 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 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 soil specimen when no pore water is allowed
to flow in or out of the specimen.

4

Equipment

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 measurement devices for load, displacement, volume
change, and pore water pressure. The apparatus shall satisfy the following conditions. Figure 1 shows anexample of the configuration of triaxial compression test apparatus. Figure 2 shows the schematic structure of
the triaxial pressure cell; (a) is an example in which the 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 having
the same diameter as the cap and pedestal between the cap and pedestal, covered with a rubber sleeve, andsealed with an O-ring, etc.
a)

The apparatus shall have sufficiently high load-bearing strength and load capacity to handle the maximum
cell pressure, back pressure and maximum axial compression load 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/m2
or 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 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.

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JGS 0523-2020

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 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, the volume
change of the test specimen shall be determined as the amount of water expelled from 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.

5

Test method

5.1

Preparation and set-up of test specimen

Preparation and set up of the test specimen shall be in accordance with JGS 0520 Preparation of soil specimens
for triaxial tests. However, the test specimen shall have a height not less than twice its diameter. For coarsegrained soils with a maximum particle diameter greater than 20 mm, 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 pore water pressure transducer, and confirm
that the pore water pressure measurement system is indicating the desired value of back pressureub (kN/m2) by opening the valve in the pore water pressure measurement system. At the same time, takethe initial reading of the burette.

b)

Close the drainage valve connected to the burette. Increase only the isotropic stress so that the difference
between the isotropic stress and the back pressure is equal to the desired value of consolidation stress. If
the loading piston is not rigidly connected to the cap, set the load measuring device, the piston, and thecap in contact with each other before this process.

c)Open the drainage valve to start consolidation.

d)Record readings of the volume change ΔVt (mm3) and the axial deformation ΔHt (mm) of the specimen at
appropriate time 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 is equal to the amountof water expelled from the specimen) and the axial deformation ΔHc (mm) due to consolidation. If the axial
displacement cannot be measured during consolidation because the piston is not rigidly connected to thecap, 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 take the reading of the displacement transducer. Thismeasured value is to be taken as the axial displacement ΔHc (mm) due to consolidation.

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Note:

Refer to Section 5.2(d) of JGS 0522 Method for consolidated-undrained (CU) triaxial compression test on soils for

deciding when to stop consolidation.

e)

Close the drainage valve connected to the burette, and increase the isotropic stress by the amountΔσ (kN/m2). Wait for the change in pore water pressure caused by this to stabilize, and measure the porepressure increment Δu (kN/m2) after stabilization and the time required for stabilization. A schematic
illustration to explain this process is shown in Figure 3. The standard amount of isotropic stress increment
Δσ shall be 10 to 50 kN/m2.

f)

Return the isotropic stress to its original value, wait for the value of the pore water pressure to stabilize,
and open the drainage valve.

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 deformation transducer.

b)

Close the drainage valve.

c)Start Compressing 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.

d)Record the axial compression force P (N), the axial deformation ΔH (mm), and the pore water pressureu (kN/m2) during compression.

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. It is recommended, for example, to measure the axial
deformation at a maximum interval of 0.2 mm up to the maximum value of the axial compression force, and thereafter at
maximum intervals 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 has fallen to about 2/3 of its peak value, or when an axial strainof 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

Initial state of the specimen before consolidation

Refer to Section 6.1 of JGS 0522 Method for consolidated-undrained (CU) triaxial compression test on soils forthe initial state of the specimen before consolidation.

6.2

Consolidation process

Refer to Section 6.2 of JGS 0522 Method for consolidated-undrained (CU) triaxial compression test on soils forthe method of calculation for the consolidation process.

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6.3

Pore pressure coefficient, B-value

The B-value of the test specimen after consolidation shall be calculated from the following equation and rounded
to two significant digits.
B=

∆u
∆σ

where
Δσ:
Δu:

6.4

Amount of isotropic stress increment (kN/m2)
Amount of pore water pressure increment (kN/m2) caused by Δσ

Axial compression process

The method of calculation for the axial compression process shall be as follows.
a)

The axial strain of the test specimen εa (%) shall be calculated from the following equation, wherecompression is defined to be positive.
εa =

∆H
× 100
Hc

where
ΔH:
b)

Axial deformation of the specimen in the axial compression process (mm), where compression isdefined to be positive

The principal stress difference (σa – σr) (kN/m2) and pore water pressure increment ue (kN/m2) due to theaxial compression at axial strain of εa (%) shall be calculated from the following equations.
σa − σr =

P
εa
(1 −
) × 1000Ac100

ue = u − ub
where
P:
σa:
σr:
u:
ub:

Axial compression force (N) applied to the specimen at the axial strain εa (%), where P=0 in theisotropic stress state
Axial stress (kN/m2) acting on the specimen
Lateral stress (kN/m2) acting on the specimen
Pore water pressure in the specimen (kN/m2)
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 maximum principal stress difference (σa – σr)max in the axial strain range of 0 < εa  15 % fromthe graph, and the corresponding value of axial strain shall be taken as the strain at failure (%), which shallbe rounded to one digit after the decimal point. The value of this maximum principal stress difference shallbe taken as the compressive strength, which shall be rounded to three significant digits.

e)Calculate the effective principal stresses, σ′a (kN/m2) and σ′r (kN/m2) in the axial compression process fromthe following equations, and draw an effective stress path diagram with (σa – σr)/2 on the vertical axis
versus (σ′a + σ′r)/2 on the horizontal axis.
σ'r = σr − u© JGS 2015 – All rights reserved

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JGS 0523-2020

σ'a = (σa − σr ) + σ'rf)Calculate the effective axial stress σ'af (kN/m2) and the effective lateral stress σ'rf (kN/m2) at the maximum
principal stress difference from the following equations, which shall be rounded to three significant digits.
σ'rf = σrf − uf
σ'af = (σa − σr )max + σ'rf
where
uf:

Note:

7

Pore water pressure at the maximum principal stress difference (kN/m 2)

Obtain σ'a and σ'r at (σ'a /σ'r )max, 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 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 test specimen and the dry density (Mg/m3) after consolidation

Note:

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

f)B-value and time (min) required for the stabilization of the pore water pressureg)

Strain rate (%/mm) in the axial compression processh)Compressive strength (kN/m2) and strain at failure (%)i)

Principal stress difference versus axial strain curve and pore water pressure increment versus axial strain
curve

j)

Effective stress path diagramk)

Failure condition of the specimenl)Effective principal stresses (kN/m2) at the maximum principal stress difference

Note:

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

m) Relationship between compressive strength and consolidation stressn)

If the method used deviates in any way from this standard, give details of the method usedo)Other reportable matters

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Air pressure

Vacuum regulator

Tank for supplying
de-aired water

Double-tube burette

Water

Specimen

Triaxial pressure cell
Water

Cell pressure
gauge

Pressure transducer
for measuring porewater pressure

Displacement
gauge

Regulator

Back pressure regulator

Load cellPressure gauge
for back pressure

Compression
deviceFigure 1 Example of the configuration of ̅̅̅̅̅
CU triaxial test apparatus

Loading piston

Loading piston

Air releasevalve

Air release
valve

Cap

Upper
plate

Upper
plate

Cap
O-ringRubber sleeve

Rubber sleeve

Post

Pressure cylinder

Pressure cylinder

Post

Porous plate
Pedestal

Path of applying
pressure to cell

Specimen

PedestalSpecimen

O-ringPorous plate

Path of applying
pressure to cell
Base plate

Path of drainage water

Path of drainage water(a) Loading piston and cap rigidly connected

Base plate

Path of drainage water

Path of drainage water(b) Loading piston and cap not rigidly connected

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

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JGS 0523-2020

Stress

Isotropic stress
Pore waterpressure

Back pressure (ub)[Drainage
valve](Open) (Close)[At the end of
consolidation](Open) Time t[Equilibrium of [Equilibrium of
pore waterpore waterchange]change](Calculate B-value)

Figure 3 Method of measuring B-values

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