ASTM D7181-20

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7181 − 11 D7181 − 20
Standard Test Method for
Consolidated Drained Triaxial Compression Test for Soils
This standard is issued under the fixed designation D7181; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of strength and stress-strain relationships of a cylindrical specimen of either intact
or reconstituted soil. Specimens are consolidated and sheared in compression with drainage at a constant rate of axial deformation
(strain controlled).
1.2 This test method provides for the calculation of principal stresses and axial compression by measurement of axial load, axial
deformation, and volumetric changes.
1.3 This test method provides data useful in determining strength and deformation properties such as Mohr strength envelopes.
Generally, three specimens are tested at different effective consolidation stresses to define a strength envelope. The stresses should
be specified by the engineer requesting the test. A test on a new specimen is required for each consolidation stress.
1.4 If this test method is used on cohesive soil, a test may take weeks to complete.
1.5 The determination of strength envelopes and the development of relationships to aid in interpreting and evaluating test
results are beyond the scope of this test method and must be performed by a qualified, experienced professional.
1.6 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice
D6026.
1.6.1 The methodsprocedures used to specify how data are collected, calculated, or recorded in this standard are regarded as
the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures
used do not consider material variations, purpose for obtaining the data, special purpose studies or any consideration of the end
use. for the user’s objectives; and it is common practice to increase or reduce the significant digits of the reported data to be
commensurate with these considerations. It is beyond the scope of this test methodstandard to consider significant digits used in
analysis methods for engineering design.
1.7 Units—The values stated in SI units are to be regarded as standard. The inch-pound units given in parentheses are
mathematical conversions, which are provided for information purposes only and are not considered standard. Reporting of test
results in units other than SI shall not be regarded as non-conformance with this test method.
1.7.1 The gravitational system of inch-pound units is used when dealing with inch-pound units. In this system, the pound (lbf)
represents a unit of force (weight), while the unit for mass is slugs. The slug unit is not given, unless dynamic (F = ma) calculations
are involved.
1.7.2 It is common practice in the engineering/construction profession to concurrently use pounds to represent both a unit of
mass (lbm) and of force (lbf). This implicitly combines two separate systems of units: that is, the absolute system and the
gravitational system. It is scientifically undesirable to combine the use of two separate sets of inch-pound units within a single
standard. As stated, this standard includes the gravitational system of inch-pound units and does not use/present the slug unit for
mass. However, the use of balances or scales recording pounds of mass (lbm) or recording density in lbm/ft shall not be regarded
as non-conformance with this standard.
1.7.3 The terms density and unit weight are often used interchangeably. Density is mass per unit volume whereas unit weight
is force per unit volume. In this standard density is given only in SI units. After the density has been determined, the unit weight
is calculated in SI or inch-pound units, or both.
1.8 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all
of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate
safety and health practices and determine the applicability of regulatory limitations prior to use.
This test method is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.05 on Strength and
Compressibility of Soils.
Current edition approved July 1, 2011Jan. 1, 2020. Published August 2011February 2020. Originally approved in 2011. Last previous edition approved in 2011 as
D7181 - 11. DOI: 10.1520/D7181-11.10.1520/D7181-20.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7181 − 20
1.8 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all
of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate
safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D422 Test Method for Particle-Size Analysis of Soils (Withdrawn 2016)
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D854 Test Methods for Specific Gravity of Soil Solids by Water Pycnometer
D1587 Practice for Thin-Walled Tube Sampling of Fine-Grained Soils for Geotechnical Purposes
D2166 Test Method for Unconfined Compressive Strength of Cohesive Soil
D2216 Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass
D2435 Test Methods for One-Dimensional Consolidation Properties of Soils Using Incremental Loading
D2487 Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
D2850 Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D4220 Practices for Preserving and Transporting Soil Samples
D4318 Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
D4753 Guide for Evaluating, Selecting, and Specifying Balances and Standard Masses for Use in Soil, Rock, and Construction
Materials Testing
D4767 Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils
D6026 Practice for Using Significant Digits in Geotechnical Data
D6913 Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis
D7263 Test Methods for Laboratory Determination of Density (Unit Weight) of Soil Specimens
D7928 Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer)
Analysis
3. Terminology
3.1 Definitions—Definitions: Refer to Terminology D653 for standard definitions of common technical terms.
3.1.1 For definitions of common technical terms, refer to Terminology D653.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 back pressure, n—a pressure applied to the specimen pore-water to cause air in the pore space to compress and to pass
into solution in the pore-water thereby increasing the percent saturation of the specimen.
3.2.2 effective consolidation stress, n—the difference between the cell pressure and the pore-water pressure prior to shearing the
specimen.
3.2.3 effective consolidation stresses, n—for anisotropic (9.4), the vertical and lateral stress applied magnitudes are not equal
by design, with lateral stress equal to the cell pressure minus pore-water pressure, and the vertical stress equal to the desired total
vertical stress (9.4.2) minus the pore pressure.
3.2.4 failure, n—a maximum-stress condition or stress at a defined strain for a test specimen. Failure is often taken to correspond
to the maximum principal stress difference (maximum deviator stress) attained or the principal stress difference (deviator stress)
at 15 % axial strain, whichever is obtained first during the performance of a test. Depending on soil behavior and field application,
other suitable failure criteria may be defined, such as maximum effective stress obliquity, σ /σ , or the principal stress difference
1 3max
(deviator stress) at a selected axial strain other than 15 %.
3.2.4.1 Discussion—
Failure is often taken to correspond to the maximum principal stress difference (maximum deviator stress) attained or the principal
stress difference (deviator stress) at 15 % axial strain, whichever is obtained first during the performance of a test. Depending on
soil behavior and field application, other suitable failure criteria may be defined, such as maximum effective stress obliquity,
’ ’
σ /σ , or the principal stress difference (deviator stress) at a selected axial strain other than 15 %.
1 3max
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D7181 − 20
4. Summary of Test Method
4.1 The test specimen, either intact or reconstituted, is mounted in the testing apparatus using either a dry or wet mounting
procedure. The test specimen is cylindrical in shape and dimensions are measured prior to mounting. The test specimen is then
back pressure saturated. After saturation, the specimen is isotropically or anisotropically consolidated. The test specimen is then
axially loading at a constant rate and with the drainage lines open to allow the sample to drain.
5. Significance and Use
5.1 The shear strength of a saturated soil in triaxial compression depends on the stresses applied, time of consolidation, strain
rate, and the stress history experienced by the soil.
5.2 In this test method, the shear characteristics are measured under drained conditions and are applicable to field conditions
where soils have been fully consolidated under the existing normal stresses and the normal stress changes under drained conditions
similar to those in the test method.
5.3 The shear strength determined from this test method can be expressed in terms of effective stress because a strain rate or
load application rate slow enough to allow pore pressure dissipation during shear is used to minimize result in negligible excess
pore pressure conditions. The shear strength may be applied to field conditions where full drainage can occur (drained conditions),
and the field stress conditions are similar to those in the test method.
5.4 The shear strength determined from the test is commonlycan be used in embankment stability analyses, earth pressure
calculations, and foundation design.
NOTE 1—Notwithstanding the statements on precision and bias contained in this test method, the precision of this test method The quality of the result
produced by this standard is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies
that meet the criteria of Practice D3740 are generally considered capable of competent testing. and objective testing/sampling/inspection/etc. Users of
this test method standard are cautioned that compliance with Practice D3740 does not ensureassure reliable testing.results. Reliable testing dependsresults
depend on severalmany factors; Practice D3740 provides a means of evaluating some of those factors.
6. Apparatus
6.1 The requirements for equipment needed to perform satisfactory tests are given in the following sections. See Fig. 1
6.2 Axial Loading Device—The axial loading device may be a screw jack driven by an electric motor through a geared
transmission, a hydraulic loading device, or any other compression device with sufficient capacity and control to provide the rate
FIG. 1 Schematic Diagram of a Typical Consolidated UndrainedDrained Triaxial Apparatus
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of axial strain (loading) prescribed in 8.4.29.5.2. The rate of advance of the loading device should not deviate by more than 61 %
from the selected value. Vibration due to the operation of the loading device shall be sufficiently small to not cause dimensional
changes in the specimen.
NOTE 2—A loading device may be judged to produce sufficiently small vibrations if there are no visible ripples in a glass of water placed on the loading
platform when the device is operating at the speed at which the test is performed.
6.3 Axial Load-Measuring Device—The axial load-measuring device shall be an electronic load cell, hydraulic load cell, or any
other load-measuring device capable of the accuracy prescribed in this paragraph and may be a part of the axial loading device.
The axial load-measuring device shall be capable of measuring the axial load to an accuracy of within 1 % of the axial load at
failure. If the load-measuring device is located inside the triaxial compression chamber, it shall be insensitive to horizontal forces
and to the magnitude of the chamber pressure.
6.4 Triaxial Compression Chamber—The triaxial chamber shall have a working chamber pressure capable of sustaining the sum
of the effective consolidation stress and the back pressure. It shall consist of a top plate and a base plate separated by a cylinder.
The cylinder may be constructed of any material capable of withstanding the applied pressures. It is desirable to use a transparent
material or have a cylinder provided with viewing ports so the behavior of the specimen may be observed. The top plate shall have
a vent valve such that air can be forced out of the chamber as it is filled. The base plate shall have an inlet through which the
pressure liquid is supplied to the chamber and inlets leading to the specimen base and provide for connection to the cap to allow
saturation and drainage of the specimen when required.needed.
6.5 Axial Load Piston—The piston passing through the top of the chamber and its seal must be designed so the axial load due
to friction does not exceed 0.1 %0.5 % of the axial load on piston at failure and so there is negligible lateral bending of the piston
during loading. For triaxial cell with internal load, cell piston friction is not as important.
NOTE 3—The use of two linear ball bushings to guide the piston is recommended to minimizereduce friction and maintain alignment.
NOTE 4—A minimum piston diameter of ⁄6 the specimen diameter has been used successfully in many laboratories to minimizereduce lateral bending.
6.6 Pressure and Vacuum-Control Devices—The chamber pressure and back pressure control devices shall be (a) capable of
applying and controlling pressures to within 62 kPa (0.25 lbf/in. ) for effective consolidation pressures less than 200 kPa (28
lbf/in. ) and to within 61 % for effective consolidation pressures greater than 200 kPa, and (b) able to maintain the effective
consolidation stress within 2 % of the desired value (Note 5). The vacuum-control device shall be capable of applying and
controlling partial vacuums to within 62 kPa. The devices may consist of pneumatic-pressure regulators, combination pneumatic
pressure and vacuum regulators, or any other device capable of applying and controlling pressures or partial vacuums to the
requirednecessary tolerances. These tests can requirehave a duration of several days, therefore, an external air/water interface is
recommended for both the chamber-pressure or back-pressure systems.
NOTE 5—Many laboratories use differential pressure regulators and transducers to achieve the requirements for small differences between chamber and
back pressure.
6.7 Pressure- and Vacuum-Measurement Devices—The chamber pressure-, back pressure-, and vacuum-measuring devices shall
be capable of measuring the ranges of pressures or partial vacuums to the tolerances given in 5.66.6. They may consist of electronic
pressure transducers, or any other device capable of measuring pressures, or partial vacuums to the stated tolerances. If separate
devices are used to measure the chamber pressure and back pressure, the devices must be normalized simultaneously and against
the same pressure source. Since the chamber and back pressure are the pressures taken at the midheight of the specimen, it may
be necessary to adjust the zero-offset of the devices to reflect the hydraulic head of fluids in the chamber and back pressure control
systems.
6.8 Volume Change Measurement Device—The volume of water entering or leaving the specimen shall be measured with an
accuracy of within 60.05 % of the total volume of the specimen. The volume-measuring device is usually a burette connected to
the back pressure but may be any other device meeting the accuracy requirement. The device must be able to withstand the
maximum back pressure and of sufficient capacity for the performance of the test. Volume changes during shear are often on the
order of 620 % or more of the specimen volume. Either allowing for resetting of the system during shear or having a total capacity
capable of measuring the entire change may meet the requiredneeded capacity.
6.9 Deformation Indicator—The vertical deformation of the specimen is usually determined from the travel of the piston acting
on the top of the specimen. The piston travel shall be measured with an accuracy of at least 0.25 % of the initial specimen height.
The deformation indicator shall have a range of at least 20 % of the initial height of the specimen and may be a dial indicator, linear
variable differential transformer (LVDT), extensometer, or other measuring device meeting the requirements for accuracy and
range.
6.10 Specimen Cap and Base—The specimen cap and base shall be designed to provide drainage from both ends of the
specimen. They shall be constructed of a rigid, noncorrosive, impermeable material, and each shall, except for the drainage
provision, have a circular plane surface of contact with the porous disks and a circular cross section. It is desirable for the mass
of the specimen cap and top porous disk to be as minimal as possible. However, the mass may be as much as 10 % of the axial
load at failure. If the mass is greater than 0.5 % of the applied axial load at failure and greater than 50 g (0.1 lb), the axial load
must be corrected for the mass of the specimen cap and top porous disk. The diameter of the cap and base shall be equal to or
only nominally greater than the initial diameter of the specimen. The specimen base shall be connected to the triaxial compression
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chamber to prevent lateral motion or tilting, and the specimen cap shall be designed such that eccentricity of the piston-to-cap
contact relative to the vertical axis of the specimen does not exceed 1.3 mm (0.05 in.). The end of the piston and specimen cap
contact area shall be designed so that tilting of the specimen cap during the test is minimal. The cylindrical surface of the specimen
base and cap that contacts the membrane to form a seal shall be smooth and free of scratches.
6.11 Porous Disks—A rigid porous disk shall be used to provide drainage at each end of the specimen. The coefficient of
-4 –5
permeability of the disks shall be at most equal to greater than that of fine sand (1 × 10 cm/s (4 × 10 in./s)). The disks shall
be regularly cleaned by ultrasonic or boiling and brushing and checked to determine whether they have become clogged.
6.12 Filter-Paper Strips and Disk—Filter-paper strips are used by many laboratories to decrease the time requiredneeded for
testing. Filter-paper disks of a diameter equal to that of the specimen may be placed between the porous disks and specimen to
avoid clogging of the porous disks. If filter strips or disks are used, they shall be of a type that does not dissolve in water. The
-5 -6
coefficient of permeability of the filter paper shall not be less than 1 × 10 cm/s (4 × 10 in./s) for a normal pressure of 550 kPa
(80 lbf/in. ). To avoid hoop tension, filter strips should cover no more than 50 % of the specimen periphery. Many laboratories have
successfully used filter strip cages. An equation for correcting the principal stress difference (deviator stress) for the effect of the
strength of vertical filter strips is given in 10.3.3.111.3.3.1.
NOTE 6—Grade No. 54 Filter Paper has been found to meet the permeability and durability requirements.
6.13 Rubber Membrane—The rubber membrane used to encase the specimen shall provide reliable protection against leakage.
Membranes shall be carefully inspected prior to use and if any flaws or pinholes are evident, the membrane shall be discarded. To
offer minimum restraint to the specimen, the unstretched membrane diameter shall be between 90 and 95 % of that of the specimen.
The membrane thickness shall not exceed 1 % of the diameter of the specimen. The membrane shall be sealed to the specimen cap
and base with rubber O-rings for which the unstressed inside diameter is between 75 and 85 % of the diameter of the cap and base,
or by other means that will provide a positive seal. An equation for correcting the principal stress difference (deviator stress) for
the effect of the stiffness of the membrane is given in 10.3.3.211.3.3.2.
6.14 Valves—Changes in volume due to opening and closing valves may result in inaccurate volume change and pore-water
pressure measurements. For this reason, valves in the specimen drainage system shall be of the type that produces minimum
volume changes due to their operation. A valve may be assumed to produce minimum volume change if opening or closing the
valve in a closed, saturated pore-water pressure system does not induce a pressure change of greater than 0.7 kPa (60.1 lbf/in. ).
All valves must be capable of withstanding applied pressures without leakage.
NOTE 7—Ball valves have been found to provide minimum volume-change characteristics; however, any other type of valve having suitable
volume-change characteristics may be used.
6.15 Specimen-Size Measurement Devices—Devices used to determine the height and diameter of the specimen shall measure
the respective dimensions to four significant digits and shall be constructed such that their use will not disturb/deform the
specimen.
NOTE 8—Circumferential measuring tapes are recommended over calipers for measuring the diameter.
6.16 Data Acquisition—Specimen behavior may be recorded manually or by electronic digital or analog recorders. If electronic
data acquisition is used, it shall be necessary to calibrate the measuring devices through the recording device using known input
standards.
6.17 Timer—A timing device indicating the elapsed testing time to the nearest 1 s readability, shall be used to obtain
consolidation data (8.3.39.3.3).
6.18 Balance—A balance or scale conforming to the requirements of Specification D4753 readable to four significant digits.
6.19 Water Deaeration Device—The amount of dissolved gas (air) in the water used to saturate the specimen shall be decreased
by boiling, by heating and spraying into a vacuum, or by any other method that will satisfy the requirement for saturating the
specimen within the limits imposed by the available maximum back pressure and time to perform the test.
6.20 Testing Environment—The consolidation and shear portion of the test shall be performed in an environment where
temperature fluctuations are less than 64 °C (67.2 °F) and there is no direct exposure with sunlight.
6.21 Miscellaneous Apparatus—Specimen trimming and carving tools including a wire saw, steel straightedge, miter box,
vertical trimming lathe, apparatus for preparing reconstituted specimens, membrane and O-ring expander, water content cans, and
data sheets shall be provided as required.necessary.
7. Test Specimen Preparation
7.1 Specimen Size—Specimens shall be cylindrical and have a minimum diameter of 33 mm (1.3 in.). The average-height-to-
average-diameter ratio shall be between 2 and 2.5. An individual measurement of height or diameter shall not vary from average
by more than 2 %. The largest particle size shall be smaller than ⁄6 the specimen diameter. If, after completion of a test, it is found
based on visual observation that oversize particles are present, indicate this information in the report of test data (11.1.412.2.5).
NOTE 9—If oversize particles are found in the specimen after testing, a particle-size analysis may be performed on the tested specimen in accordance
with Test Method D422D6913 to confirm the visual observation and the results provided with the test report (observation.11.1.4).
D7181 − 20
7.2 Intact Specimens—Prepare intact specimens from large intact samples or from samples secured in accordance with Practice
D1587 or other acceptable intact tube sampling procedures. Samples shall be preserved and transported in accordance with the
practicespractice for Group C samples in Practices D4220. Specimens obtained by tube sampling may be tested without trimming
except for cutting the end surfaces plane and perpendicular to the longitudinal axis of the specimen, provided soil characteristics
are such that no significant disturbance results from sampling. Handle specimens carefully to minimize have negligible
disturbance, changeschange in cross section, or change in water content. If compression or any type of noticeable disturbance
would be caused by the extrusion device, split the sample tube lengthwise or cut the tube in suitable sections to facilitate removal
of the specimen with minimum disturbance. Prepare trimmed specimens,specimen, in an environment such as a controlled
high-humidity room where soil water content change is minimized. kept to a minimum. Where removal of pebbles or crumbling
resulting from trimming causes voids on the surface of the specimen, carefully fill the voids with remolded soil obtained from the
trimmings. If the sample can be trimmed with minimal disturbance, a vertical trimming lathe may be used to reduce the specimen
to the requirednecessary diameter. After obtaining the requirednecessary diameter, place the specimen in a miter box, and cut the
specimen to the final height with a wire saw or other suitable device. Trim the surfaces with the steel straightedge. Perform one
or more water content determinations on material trimmed from the specimen in accordance with Test Method D2216. Determine
and record the mass and dimensions of the specimen using the devices described in 5.166.16 and 5.206.20. A minimum of three
height measurements (120° apart) and at least three diameter measurements at the quarter points of the height shall be made to
determine the average height and diameter of the specimen.
7.3 Reconstituted Specimens—Specimens by Compaction—Reconstituted specimens shall be prepared at the conditions
specified for the test. Soil required for Reconstituted specimens used for a reconstituted specimen shall be thoroughly mixed with
sufficientenough water to produce the desired water content. If water is added to the soil, store the material in a covered container
for at least 16 h prior to compaction. Reconstituted specimensspecimen may be prepared by compacting material in at least six
layers using a split mold of circular cross section having dimensions meeting the requirements enumerated in 6.17.1. Specimens
may be compacted to the desired density by either: (1) kneading or tamping each layer until the accumulative mass of the soil
placed in the mold is compacted to a known volume; or (2) by adjusting the number of layers, the number of tamps per layer, and
the force per tamp. The top of each layer shall be scarified prior to the addition of material for the next layer. The tamper used
to compact the material shall have a diameter equal to or less than ½ ⁄2 the diameter of the mold. After a specimen is formed, with
the ends perpendicular to the longitudinal axis, remove the mold and determine and record the mass and dimensions of the
specimen using the devices described in 5.146.14 and 5.176.17. A minimum of three height measurements (120° apart) and at least
three diameter measurements at the quarter points of the height shall be made to determine the average height and diameter of the
specimen. Perform one or more water content determinations on excess material used to prepare the specimen in accordance with
Test Method D2216.
NOTE 10—It is common for the density or unit weight of the specimen after removal from the mold to be less than the value based on the volume of
the mold. This change occurs as a result of the specimen swelling after removal of the lateral confinement due toprovided by the mold.
7.4 Reconstituted Specimens—Specimens by Other Methods—Prepare reconstituted specimens in the manner specified by the
requesting agency. These methods will usually require a forming jacket used to form the specimen directly on the base pedestal.
The forming jacket will allow the specimen to be prepare in the membrane. There are steps in Section 8 that may not be required
by some of the methods. Common methods include:
7.4.1 Pluviation Through Water Method—For this specimen preparation method, a granular soil is saturated initially in a
container, poured through water into a water-filled membrane placed on a forming mold, and then densified to the required density
by vibration; refer to reference by Chaney and Mullis.
NOTE 11—A specimen may be vibrated either on the side of the mold or the base of the cell using a variety of apparatus. These include the following:
tapping with an implement of some type such as a spoon or metal rod, pneumatic vibrator, or electric engraving tool.
7.4.2 Dry Screening Method—For this method a tube with a screen attached to one end is placed inside a membrane stretched
over a forming mold. A dry uniform sand is then poured into the tube. The tube is then slowly withdrawn from this membrane/mold
allowing the sand to pass through the screen forming a specimen. If a greater density of the sand is desired the mold may be
vibrated.
7.4.3 Dry or Moist Vibration Method—In this procedure compact oven-dried, or moist granular material in layers (typically six
to seven layers) in a membrane-lined split mold attached to the bottom platen of the triaxial cell. Compact the weighed material
for each lift by vibration to the dry unit weight requirednecessary to obtain the prescribed density. Scarify the soil surface between
lifts. It should be noted that to obtain uniform density, the bottom layers have to be slightly under compacted, since compaction
of each succeeding layer increases the density of sand in layers below it. After the final layer is partially compacted, put the top
cap in place and continue vibration until the desired dry unit weight is obtained.
Chaney, R., and Mulilis, J., “Wet Sample Preparation Techniques,” Geotechnical Testing Journal, ASTM, 1978, pp. 107-108.
D7181 − 20
7.4.4 Tamping Method—For this procedure tamp air dry or moist granular or cohesive soil in layers into a mold. The only
difference between the tamping method and the vibration method is that each layer is compacted by hand tamping with a
compaction foot instead of with a vibrator, refer to reference by Ladd, R.S.
7.4.5 After the specimen has been formed, place the specimen cap in place and seal the specimen with O-rings or rubber bands
after placing the membrane ends over the cap and base. Then apply a partial vacuum of 35 kPa (5 lbf/in. ) to the specimen and
remove the forming jacket. If the test confining-pressure is greater than 103 kPa (14.7 lbf/in. ), a full vacuum may be applied to
the specimen in stages prior to removing the jacket.
8. Mounting Specimen
8.1 Preparations—Before mounting the specimen in the triaxial chamber, make the following preparations:
8.1.1 Inspect the rubber membrane for flaws, pinholes, and leaks.
8.1.2 Place the membrane on the membrane expander or, if it is to be rolled onto the specimen, roll the membrane on the cap
or base.
8.1.3 Check that the porous disks and specimen drainage tubes are not obstructed by passing air or water through the appropriate
lines.
8.1.4 Attach the pressure-control and volume-measurement system and a pore-pressure measurement device to the chamber
base.
8.2 Depending on whether the saturation portion of the test will be initiated with either a wet or dry drainage system, mount
the specimen using the appropriate method, as follows in either 7.2.18.2.1 or 7.2.28.2.2. The dry mounting method is strongly
recommended for specimens with initial saturation less than 90 %. The dry mounting method removes air prior to adding
backpressure and lowers the backpressure needed to attain an adequate percent saturation.
NOTE 12—It is recommended that the dry mounting method be used for specimens of soils that swell appreciably when in contact with water. If the
wet mounting method is used for such soils, it will be necessary to obtain the specimen dimensions after the specimen has been mounted. In such cases,
it will be necessary to determine the double thickness of the membrane, the double thickness of the wet filter paper strips (if used), and the combined
height of the cap, base, and porous disks (including the thickness of filter disks if they are used) so that the appropriate values may be subtracted from
the measurements.
8.2.1 Wet Mounting Method:
8.2.1.1 Fill the specimen drainage lines and the pore-water pressure measurement device with deaired water.
8.2.1.2 Saturate the porous disks by boiling them in water for at least 10 min and allow to cool to room temperature.
8.2.1.3 Place a saturated porous disk on the specimen base and after wiping away all free water on the disk, place the specimen
on the disk. Next, place another porous disk and the specimen cap on top of the specimen. Check that the specimen cap, specimen,
and porous disks are centered on the specimen base.
NOTE 13—If filter-paper disks are to be placed between the porous disks and specimen, they should be dipped in water prior to placement.
8.2.1.4 If filter-paper strips or a filter-paper cage are to be used, saturate the paper with water prior to placing it on the specimen.
To avoid hoop tension, do not cover more than 50 % of the specimen periphery with vertical strips of filter paper. The filter paper
should extend to porous disks on top and bottom of sample.
8.2.1.5 Proceed with 7.38.3.
8.2.2 Dry Mounting Method:
8.2.2.1 Dry the specimen drainage system. This mounting method may be accomplished by allowing dry air to flow through the
system prior to mounting the specimen.
8.2.2.2 Dry the porous disks in an oven and then place the disks in a desiccator to cool to room temperature prior to mounting
the specimen.
8.2.2.3 Place a dry porous disk on the specimen base and place the specimen on the disk. Next, place a dry porous disk and
the specimen cap on the specimen. Check that the specimen cap, porous disks, and specimen are centered on the specimen base.
NOTE 14—If desired, dry filter-paper disks may be placed between the porous disks and specimen.
8.2.2.4 If filter-paper strips or a filter paper cage are to be used, the cage or strips may be held in place by small pieces of tape
at the top and bottom.
8.3 Place the rubber membrane around the specimen and seal it at the cap and base with two rubber O-rings or other positive
seal at each end. A thin coating of silicon grease on the vertical surfaces of the cap and base will aid in sealing the membrane. If
filter-paper strips or a filter-paper cage are used, do not apply grease to surfaces in contact with the filter paper.
8.4 Attach the top drainage line and check the alignment of the specimen and the specimen cap. If the dry mounting method
has been used, apply a partial vacuum of approximately 35 kPa (5 lbf/in. ) (not to exceed the consolidation stress) to the specimen
through the top drainage line prior to checking the alignment. If there is any eccentricity, release the partial vacuum, realign the
Ladd, R.S., “Preparing Test Specimens Using Under-Compaction,” Geotechnical Testing Journal, ASTM, Vol. 1, No. 1, March, 1978, pp. 16-23.
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specimen and cap, and then reapply the partial vacuum. If the wet mounting method has been used, the alignment of the specimen
and the specimen cap may be checked and adjusted without the use of a partial vacuum.
9. Procedure
9.1 Prior to Saturation—After assembling the triaxial chamber, perform the following operations:
9.1.1 Bring When possible, bring the axial load piston into contact with the specimen cap several times to permit proper seating
and alignment of the piston with the cap. During this procedure, take care not to apply an axial load to the specimen exceeding
0.5 % of the estimated axial load at failure. When the piston is brought into contact, record the reading of the deformation indicator.
9.1.2 Fill the chamber with the chamber liquid, being careful to avoid trapping air or leaving an air space in the chamber.
9.2 Saturation—The objective of the saturation phase of the test is to fill all voids in the specimen with water without
undesirable prestressing of the specimen, allowing the specimen to swell, or causing migration of fines. Saturation is usually
accomplished by applying back pressure to the specimen pore water to drive air into solution after saturating the system by either:
(1) applying vacuum to the specimen and dry drainage system (lines, porous disks, pore-pressure device, filter-strips or cage, and
disks) and then allowing deaired water to flow through the system and specimen while maintaining the vacuum; or (2) saturating
the drainage system by boiling the porous disks in water and allowing water to flow through the system prior to mounting the
specimen. It should be noted that placing the air into solution is a function of both time and pressure. Accordingly, removing as
much air as possible prior to applying back pressure will decrease the amount of air that will have to be placed into solution and
will also decrease the back pressure required for saturation. In addition, air remaining in the specimen and drainage system just
prior to applying back pressure will go into solution much more readily if deaired water is used for saturation. The use of deaired
water will also decrease the time and backpressure required for saturation. Many procedures have been developed to accomplish
saturation. The following are suggested procedures:
9.2.1 Starting with Initially Dry Drainage System—Increase from partial vacuum acting on top of the specimen to the maximum
available vacuum. If the final effective consolidation stress is less than the maximum partial vacuum, apply a lower vacuum to the
chamber. The difference between the partial vacuum applied to the specimen and the chamber should never exceed the effective
consolidation stress for the test and should not be less than 35 kPa (5 lbf/in. ) to allow for flow through the sample.specimen. After
approximately 10 min, allow deaired water to slowly percolate from the bottom to the top of the specimen (Note 15).
9.2.1.1 There should always be a positive effective stress of at least 13 kPa (2 lbf/in. ) at the bottom of the specimen during
this part of the procedure. When water appears in the burette connected to the top of the specimen, close the valve to the bottom
of the specimen and fill the burette with deaired water. Next, reduce the vacuum acting on top of the specimen through the burette
to atmospheric pressure while simultaneously increasing the chamber pressure by an equal amount. This process should be
performed slowly such that the difference between the pore pressure measured at the bottom of the specimen and the pressure at
the top of the specimen should be allowed to equalize. When the pore pressure at the bottom of the specimen stabilizes, proceed
with back pressuring of the specimen pore-water as described in 8.2.39.2.3. To check for equalization, close the drainage valves
to the specimen and measure the pore pressure change until stable for at least 2 min. If the change is less than 5 % of the effective
stress, the pore pressure can be assumed to be stabilized.
NOTE 15—For saturated clays, percolation may not be necessary, and water can be added simultaneously at both top and bottom.
9.2.2 Starting with Initially Saturated Drainage System—After filling the burette connected to the top of the specimen with
deaired water, apply a chamber pressure of 35 kPa (5 lbf/in. ) or less and open the specimen drainage valves. When the pore
pressure at the bottom of the specimen stabilizes, according to the method described in 8.2.1.19.2.1.1, or when the burette reading
stabilizes, back pressuring of the specimen pore-water may be initiated.
9.2.3 Applying Back Pressure—Simultaneously increase the chamber and back pressure in steps with specimen drainage valves
opened so that deaired water from the burette connected to the top and bottom of the specimen may flow into the specimen. To
avoid undesirable prestressing of the specimen while applying back pressure, the pressures must be applied incrementally with
adequate time between increments to permit equalization of pore-water pressure throughout the specimen. The size of each
increment may range from 35 kPa (5 lbf/in. ). A minimum of three height measurements (120° apart) and at least three diameter
measurements at the quarter points of the height shall be made to determine the average height and diameter of the specimen ),
up to 140 kPa (20 lbf/in. ), depending on the magnitude of the desired effective consolidation stress, and the percent saturation
of the specimen just prior to the addition of the increment. The difference between the chamber pressure and the backpressure
during back pressuring should not exceed 35 kPa (5 lbf/in. ) unless it is deemed necessary to control swelling of the specimen
during the procedure. The difference between the chamber and back pressure must also remain within 65 % when the pressures
are raised and within 62 % when the pressures are constant. To check for equalization after application of a backpressure
increment or after the full value of backpressure has been applied, close the specimen drainage valves and measure the change in
pore-pressure over a 1-min interval. If the change in pore pressure is less than 5 % of the difference between the chamber pressure
and the back pressure, another back pressure increment may be added or a measurement may be taken of the pore pressure
Parameter B (see 8.2.49.2.4) to determine if saturation is completed. Specimens shall be considered to be saturated if the value of
B is equal to or greater than 0.95, or if B remains unchanged with addition of backpressure increments. The B Parameter could
also be checkchecked following consolidation stage.
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NOTE 16—Although the pore pressure Parameter B is used to determine adequate saturation, the B-value is also a function of soil stiffness. If the
saturation of the sample is 100 %, the B-value measurement will decrease with increasing soil stiffness. Therefore, when testing softstiff soil samples,
a B-value of 95 % or even below may indicate a saturation approaching 100 %.
NOTE 17—The back pressure requiredneeded to saturate a specimen may be higher for the wet mounting method than for the dry mounting method
because of the added difficulty of flushing out the air before back-pressure saturation and may be as high as 1400 kPa (200 lbf/in. ).
9.2.4 Measurement of the Pore Pressure Parameter B—Determine the value of the pore pressure Parameter B in accordance
with 8.2.4.19.2.4.1 – 8.2.4.49.2.4.4. The pore pressure Parameter B is defined by the following equation:
Δu
B 5 (1)
...