ASTM D4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils, Thí nghiệm nén ba trục CU

ASTM D 4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils

ASTM D 4767 Phương pháp thí nghiệm nén ba trục cố kết không thoát nước CU

  1. Scope

1.1 This test method covers the determination of strength and stress-strain relationships of a cylindrical specimen of

either an undisturbed or remolded saturated cohesive soil.

Specimens are isotropically consolidated and sheared in compression without drainage at a constant rate of axial deformation (strain controlled).

1.2 This test method provides for the calculation of total and effective stresses, and axial compression by    measurement of axial load, axial deformation, and pore-water pressure.

1.3 This test method provides data useful in determining strength and deformation properties of cohesive soils such as Mohr strength envelopes and Young’s modulus. Generally, three specimens are tested at different effective consolidation stresses to define a strength envelope.

1.4 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.

1.5 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D 6026

 

  1. Referenced Documents

2.1 ASTM Standards:

D 422 Method for Particle-Size Analysis of Soils2

D 653 Terminology Relating to Soil, Rock, and Contained Fluids2

D 854 Test Method for Specific Gravity of Soils2

D 1587 Practice for Thin-Walled Tube Sampling of Soils2

D 2166 Test Method for Unconfined Compressive Strength of Cohesive Soil2

D 2216 Method for Laboratory Determination of Water (Moisture) Content of Soil, Rock, and Soil-Aggregate

Mixtures

D 2435 Test Method for One-Dimensional Consolidation Properties of Soils2

D 2850 Test Method for Unconsolidated, Undrained Compressive Strength of Cohesive Soils in Triaxial Compression2

D 3740 Practice for Minimum Requirements for Agencies Engaged in the Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction2

D 4220 Practices for Preserving and Transporting Soil Samples2

D 4318 Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils2

D 4753 Specification for Evaluating, Selecting, and Specifying Balances and Scales for Use in Soil and Rock

Testing2

D 6026 Practice for Using Significant Digits in Geotechnical Data

 

  1. Terminology

3.1 Definitions—The definitions of terms used in this test method shall be in accordance with Terminology D 653.

3.2 Definitions of Terms Specific to This Standard:

3.2.1 back pressure—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

3.2.2 effective consolidation stress—the difference between the cell pressure and the pore-water pressure prior to shearing the specimen.

3.2.3 failure—the stress condition at failure for a test specimen.

3.2.2 effective consolidation stress—the difference between the cell pressure and the pore-water pressure prior to shearing the specimen.

3.2.3 failure—the stress condition at failure for a test specimen.

4.3 Using the pore-water pressure measured during the test, the shear strength determined from this test method can be expressed in terms of effective stress

4.4 The shear strength determined from the test expressed in terms of total stresses (undrained conditions) or effective stresses (drained conditions) is commonly used in embankment stability analyses, earth pressure calculations, and foundation design

 

  1. Apparatus

5.1 The requirements for equipment needed to perform satisfactory tests are given in the following sections. See Fig.1 and Fig. 2

5.2 Axial Loading Device

5.3 Axial Load-Measuring Device

5.4 Triaxial Compression Chamber

5.5 Axial Load Piston

5.6 Pressure and Vacuum-Control Devices

5.7 Pressure- and Vacuum-Measurement Devices

5.8 Pore-Water Pressure-Measurement Device

5.9 Volume Change Measurement Device

5.10 Deformation Indicator

5.11 Specimen Cap and Base

5.12 Porous Discs

5.13 Filter-Paper Strips and Disks

5.14 Rubber Membrane

5.15 Valves

5.16 Specimen-Size Measurement Devices

5.17 Recorders

5.18 Sample Extruder

5.19 Timer

5.20 Balance

5.21 Water Deaeration Device

5.22 Testing Environment

5.23 Miscellaneous Apparatus

ASTM D 4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils
ASTM D 4767 Phương pháp thí nghiệm nén ba trục cố kết không thoát nước CU

  1. Test Specimen Preparation

6.1 Specimen Size

6.2 Undisturbed Specimens

6.3 Compacted Specimens

 

  1. Mounting Specimen

7.1 Preparations

7.2 Depending on whether the saturation portion of the test will be initiated with either a wet or dry drainage system

7.2.1.4 If filter-paper strips or a filter-paper cage are to be used,

7.2.1.5 Proceed with 7.3.

7.2.2 Dry Mounting Method:

7.2.2.1 Dry the specimen drainage system.

7.2.2.2 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.

7.2.2.3 Place a dry porous disk on the specimen base and place the specimen on the disk

7.2.2.4 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.

7.3 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.

7.4 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

  1. Procedure

8.1 Prior to Saturation

8.2 Saturation

8.3 Consolidation – The objective of the consolidation phase of the test is to allow the specimen to reach equilibrium

in a drained state at the effective consolidation stress for which a strength determination is required. During consolidation, data is obtained for use in determining when consolidation is complete and for computing a rate of strain to be used for the shear portion of the test.

8.4 Shear—During shear, the chamber pressure shall be kept constant while advancing the axial load piston downward against the specimen cap using controlled axial strain as the loading criterion. Specimen drainage is not permitted during shear.

 

  1. Removing Specimen

9.1 When shear is completed, perform the following:

9.1.1 Remove the axial load and reduce the chamber and back pressures to zero.

9.1.2 With the specimen drainage valves remaining closed, quickly remove the specimen from the apparatus so that the specimen will not have time to absorb water from the porous disks.

9.1.3 Remove the rubber membrane

 

  1. Calculation

10.1 Measurements and calculations shall contain three significant digits.

10.2 Initial Specimen Properties

10.3 Specimen Properties After Consolidation

10.4 Shear Data

10.5 Principal Stress Difference (Deviator Stress) and Induced Pore-Water Pressure versus Strain Curves

10.6 p’ – q Diagram

10.7 Determine the major and minor principal stresses at failure based on total stresses

10.8 Mohr Stress Circles

ASTM D 4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils
ASTM D 4767 Phương pháp thí nghiệm nén ba trục cố kết không thoát nước CU

  1. Report: Test Data Sheet(s)/Form(s)

11.1 The methodology used to specify how data are recorded on the data sheet(s)/form(s), as given below, is covered in 1.5.

11.2 Record as a minimum the following general information (data):

11.2.1 Identification data and visual description of specimen

11.2.2 Values of plastic limit and liquid limit, if determined in accordance with Test Method D 4318,

11.2.3 Value of specific gravity of solids and notation if the value was determined in accordance with Test Method D 854 or assumed,

11.2.4 Particle-size analysis, if determined in accordance with Test Method D 422,

11.2.5 Initial specimen dry unit weight, void ratio, water content, and percent saturation,

11.2.6 Initial height and diameter of specimen,

11.2.7 Method followed for specimen saturation (that is, dry or wet method),

11.2.8 Total back pressure,

11.2.9 The pore pressure Parameter B at the end of saturation,

11.2.10 Effective consolidation stress,

11.2.11 Time to 50 % primary consolidation,

11.2.12 Specimen dry unit weight, void ratio, water content, and percent saturation after consolidation,

11.2.13 Specimen cross-sectional area after consolidation and method used for determination,

11.2.14 Failure criterion used,

11.2.15 The value of the principal stress difference

11.2.16 Axial strain at failure, percent,

11.2.17 Rate of strain, percent per minute,

11.2.18 Principal stress difference (deviator stress) and induced pore-water pressure versus axial strain curves as described in 10.5,

11.2.19 The p8 − q diagram as described in 10.6,

11.2.20 Mohr stress circles based on total and effective stresses, (optional),

11.2.21 Slope of angle of the failure surface (optional),

11.2.22 Failure sketch or photograph of the specimen, and

11.2.23 Remarks and notations regarding any unusual conditions

ASTM D 4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils
ASTM D 4767 Phương pháp thí nghiệm nén ba trục cố kết không thoát nước CU

  1. Precision and Bias

12.1 Precision

12.2 Bias

 

  1. Keywords

back pressure saturation; cohesive soil; consolidated

undrained strength; strain-controlled loading; stress-strain relationships; total and effective stresses

 

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ASTM D 4767 Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils
ASTM D 4767 Phương pháp thí nghiệm nén ba trục cố kết không thoát nước CU

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