Determination of Shear Strength Parameters by Triaxial Compression Test

The Triaxial Compression Test is one of the most reliable ways to measure the shear strength of soil. Here I will explains how shear strength parameters of soil are determined using the triaxial compression test. It outlines the purpose of the test, the equipment involved, and how controlled loading helps measure the soil’s response under different stress conditions. This method is widely used in geotechnical engineering to evaluate stability and predict how soil will behave in the field.

Shear Strength Parameters by Triaxial Compression Test

Scope

•       This test method covers determination of the strength and stress-strain relationships of a cylindrical specimen of either undisturbed or remolded cohesive soil. Specimens are subjected to a confining fluid pressure in a triaxial chamber. No drainage of the specimen is permitted during the test. The specimen is sheared in compression without drainage at a constant rate of axial deformation (strain controlled).

 

•      It provides data for determining undrained strength properties and stress-strain relations for soils. This test method provides the measurement of total stresses applied to the specimen i.e. the stresses are not, corrected for pore water pressure.

Terminology

Failure: It refers to the stress condition at failure for a test specimen. Failure is often taken to correspond to the maximum principal stress difference (deviator stress) attained or the principal stress difference (deviator stress) at 15% strain, whichever is obtained first during the performance of a test.

Significance and Use

•        In this test method, the shear strength of a soil is determined in terms of the total stress, therefore, the resulting strength depends on the pressure developed in the pore water during loading. In this method, water flow is not permitted from or into the soil specimen as the load is applied, therefore the resulting pore pressure, and hence strength, differs from that developed in the case where drainage can occur.

•        If the test specimen is 100% saturated, consolidation cannot occur during application of confining pressure nor during shearing stage since drainage is not permitted. Therefore, if several specimens of the same material are tested, and if they are all at approximately the same water content and void ratio when they are tested, they will have approximately the same undrained shear strength. The Mohr failure envelope will usually be a horizontal straight line over the entire range of confining stresses applied to the specimens if the specimens are fully saturated.

 

 

•       If the test specimens are partially saturated or compacted specimens, where the degree of saturation is less than 100%, consolidation may occur when the confining pressure is applied and during shear, even though drainage is not permitted. Therefore, if several partially saturated specimens of the same material are tested at different confining pressures, they will not have the same undrained shear strength, thus, the Mohr failure envelope for UU triaxial tests on partially saturated soils is usually covered.

•     The UU triaxial strength is applicable to situations where the loads are assumed to take place so rapidly that there is insufficient time for the induced pore water pressure to dissipate and for consolidation to occur during loading period (i.e. drainage does not occur).

•    Shear strength determined using the procedure may not apply where loading conditions in the field differ significantly from those used during testing.

 

Axial loading device: The axial loading device shall be screw jacked 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 of loading prescribed. The rate of advance of the loading shall not deviate by more than ±5% from the selected value. Vibrations during application of loading shall be sufficiently small not to cause dimensional changes in the specimen.

NOTE: A loading device may be said to provide sufficiently small vibrations if there are no visible ripples in a glass of water placed on the loading platen when the device is operating at a speed at which the test is performed.

Axial Load Measuring Device: The axial load measuring device shall be a load ring, electronic load cell, hydraulic load cell, or any other load measuring device capable of measuring the axial load to an accuracy of 1% of the axial load at failure and may be a part of the axial loading device.

Triaxial Compression Chamber: It should consist of a top plate and a base plate separated by a cylinder. The cylinder shall be constructed of any material capable of withstanding the applied pressure. It is desirable to use a transparent material or have a cylinder provided with viewing ports so that the behavior of the specimen may be observed. The top plate shall have a vent 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 fluid is supplied to the chamber to apply cell pressure.

Axial Load Piston: The piston passing through the top of the chamber and its seal must be designed so that the variation in axial load due to friction does not exceed 0.1% of the axial load at failure.

Pressure Control Device: It shall be capable of applying and controlling the chamber pressure to within ±2kPa (0.25psi) for pressures less than 200kPa (28psi) and to within ±1% for pressures greater than 200kPa (28psi). This device may consist of a reservoir connected to the triaxial chamber and partially filled with the chamber fluid (usually water), with the upper part of the reservoir connected to a compressed gas supply; the gas pressure being controlled by a pressure regulator and measured by a pressure gauge, electronic pressure transducer, or any other device capable of measuring to the prescribed tolerance. However, a hydraulic system pressurized by deadweight acting on a piston or any other pressure maintaining and measurement device capable of applying and controlling the chamber pressure to a tolerance prescribed in this section may be used.

 

Deformation Indicator: The vertical deformation of the specimen shall be measured with an accuracy of at least 0.03% of the specimen height. It shall have a range of at least 20% of the height of the specimen, and may be a dial indicator, linear variable differential transducer (LVDT), extensometer or other measuring device meeting the requirements for accuracy and range.

Rubber Membrane: It is used to incase the specimen and shall provide reliable protection against leakage. They shall be carefully inspected prior to use, and if any flaws or pinholes are evident, they shall be discarded. To offer minimum restraint to the specimen, the upstretched membrane dia shall be between 90-95% of that of the specimen. Its thickness shall not exceed 1% of the diameter of the specimen. Shall be sealed to the specimen base and cap with rubber O-rings for which the unstressed inside dia is between 75-85% of the dia of the cap and base or by any method that will produce a positive seal.

Sample Extruder: It shall be capable of extruding the soil core from the sampling tube in the same direction of travel in which the sample entered the tube and with minimum disturbance of the sample. If the soil core is not extruded vertically, care should be taken to avoid bending stresses on the core due to gravity. Conditions at the time of sample removal may dictate the direction of removal, but the principal concern is to keep the degree of disturbance minimal.

Specimen Size Measurement Device: Devices used to measure the height and dia of the specimen shall be capable of measuring the desired dimension to within 0.1% of its actual length and shall be constructed such that their use will not disturb the specimen.

Miscellaneous Apparatus: Specimen trimming and carving tools including a wire saw, steel straightedge, miter box and vertical trimming lathe, apparatus for preparing compacted specimens, remolding apparatus, water content cans and data sheets shall be provided as required.

Test Specimens

Specimen Size: They shall be cylindrical and have a min dia of 3.3cm (1.3”). The H/D ratio shall be between 2-2.5. The largest particle size shall be smaller than one-sixth the specimen dia. If, after completion of the test, it is found based on visual observation that oversize particles are present, indicate this information in the report of test data.

 

NOTE: If oversize particles are found in the specimen after testing, a particle-size analysis may be performed in accordance with the test method D422 to confirm the visual observation and the results provided with the test report.

Undisturbed Specimens: Prepare undisturbed specimens from large undisturbed samples or from samples secured in accordance with practice D1587 or other acceptable tube sampling procedures. Specimens obtained by the tube sampling may be tested without trimming except for cutting the end surface 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 disturbance changes 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 length vise or cut the tube in suitable sections to facilitate removal of the specimen with minimum disturbance. Prepare trimmed specimens, in an environment such as controlled high humidity room where soil water content change is minimized. 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. When the sample condition permits, a vertical trimming lathe may be used to reduce the specimen to the required dia. After obtaining the required dia, 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.

Compacted Specimens: Soil required for compacted specimens shall be thoroughly mixed with sufficient water to produce the desired water content. If water is added to the soil, store the material in a covered container for at least 6hrs prior to compaction. Compacted specimens may be prepared by compacting material in at least 6 layers using a split mold of circular cross section having dimensions meeting the requirements enumerated in the next section. Specimens may be compacted to the desired density by either: o kneading or tamping each layer until the accumulative mass of the soil placed in the mold is compacted to a known volume:

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 nest layer. The tamper used to compact the material shall have dia equal to or less than ½ the dia of the mold. After a specimen is formed, with the ends perpendicular to the longitudinal axis, remove the mold and determine the mass and dimensions of the specimen. Perform one or more water content determinations or excess material used to prepare the specimen in accordance with D2216.

NOTE: It is common for the unit weight of specimen after removal from the mold to be less than the value based on the volume of the mold. This occurs because of specimen swelling after removal of the lateral confinement due to the mold.

Procedure

•        Place the membrane on the membrane stretcher. Place the rubber membrane around the specimen and seal it with the cap and based with O-rings. A thin coating silicon grease on the vertical surfaces of the cap or base will aid in sealing the membrane.

   

Membrane Stretcher with suction tube, rubber membrane, O-rings, porous disc

 

•        With the specimen encased in the rubber membrane, which is sealed to the specimen to the specimen cap and base positioned in the chamber, assemble the triaxial chamber.

•        Place the chamber in position in the axial loading device. Be careful to align the axial loading device, the axial load-measuring device, and the triaxial chamber to prevent the application of a lateral force to the piston during testing. Attach the pressure-maintaining and measurement device and fill the chamber with the confining liquid. Adjust the pressure- maintaining and measurement device to the desired chamber pressure and apply the pressure to the chamber fluid. Wait approximately for 10min after the application of the chamber pressure to allow the specimen to stabilize under the chamber pressure prior to the application of the axial load.

NOTE: In some cases the chamber will be filled and the chamber pressure applied before placement in axial loading device.

NOTE: Make sure the piston is locked or held in place by the axial loading device before applying the chamber pressure.

NOTE: The waiting period may need to be increased for soft or partially saturated soils

 

                            

•              Apply the axial load to produce axial strain at a rate of 1%/min for plastic materials and 0.3%/min for brittle materials that achieve max deviator stress at approximately 3-6% strain. Continue the loading to 15% axial strain, except loading may be stopped when the deviator stress has peaked then dropped 20% or the axial strain has reached 5% beyond the strain at which the peak in deviator stress occurred.

•        Record load and deformation values to 3 significant digits at about 0.1, 0.2, 0.3, 0.4 and 0.5% strain; then at increments of about 0.5% strain to 3%; and, thereafter at every 1%. Take sufficient readings to define the stress-strain curve; hence, more frequent readings may be required in the early stages of the test and as failure is approached.

•        After completion of the test, remove the test specimen from the chamber. Determine the water content of the test specimen using the entire specimen, if possible.

•        Prior to placing the specimen (or its portion) in the oven to dry, take a snap of the specimen showing the mode of failure (shear plane, bulging etc.).

   Calculations

•        Measurements and calculations shall contain 3 significant figures.

•        Calculate the axial strain for a given applied axial load, as follows:

Ԑ = ∆H/Ho

Where: ∆H = change in height of the specimen as read from the deformation indicator

                                      Ho = Initial height of the specimen minus any change in length                                              prior to loading

•  Calculate average cross-sectional area, A, for a given applied axial load as follows.

A = Ao / (1 - Ԑ)

Where: Ao = Initial average cross-sectional area of the specim.

Calculate the principal stress difference (deviator stress), σ1 – σ3 for a given applied axial load as follows:

σ1 – σ3 = P/A Where:

P = measured applied axial load

 Stress Strain Curve: Prepare a graph showing the relationship between principal stress difference (deviator stress) and axial strain, plotting deviator stress as ordinate and axial strain in % as abscissa.

Calculate the major and minor principal total stresses at failure as follows:

σ3 = major principal stress = chamber pressure σ1 = major principal total stress=deviator stress at failure plus chamber pressure = ∆p+ σ3.

Calculate initial degree of saturation of the specimen using initial mass and dimensions. 

NOTE: The Gs determined in accordance with D854 is required for calculation of the saturation. An assumed Gs may be used provided it is noted in the test report.

•        Plot Mohr’s circle, failure envelope and determine C and ϕ.

 

 

Conclusion        

The triaxial compression test provides a reliable way to determine the shear strength of soil under controlled stress conditions. By measuring how the sample responds to increasing axial load, the test helps define key parameters like cohesion and the angle of internal friction. These values are essential for predicting soil behavior in real-world engineering projects, ensuring safer designs for foundations, slopes, and earth structures.