ML16305A306
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Triaxial Compression Test Unit CLINTON POWER STATION UPDATED SAFETY ANALYSIS REPORT Figure 2.5-299 Sheet 1 of 2 UNCONFINED COMPRESSION AND TRIAXIAL COMPRESSION TESTS (METHOD)
NOTES FOR FIGURE 2.5-299 Methods of Performing Unconfined Compression and Triaxial Compression Tests The shearing strengths of soils are determined from the results of unconfined compression and triaxial compression tests. In triaxial compression tests the test method and the magnitude of the confining pressure are chosen to simulate anticipated field conditions.
Unconfined compression and triaxial compression tests are performed on undisturbed, or remolded samples of soil, approximately six inches in length and two and one-half inches in diameter. The tests are run either strain-controlled or stress-controlled. In a strain-controlled test the sample is subjected to a constant rate of deflection and the resulting stresses are recorded. In a stress-controlled test the sample is subjected to equal increments of load with each increment being maintained until an equilibrium condition with respect to strain is achieved.
Yield, peak, or ultimate stresses are determined from the stress-strain plot for each sample and the principal stresses are evaluated. The principal stresses are plotted on a Mohr's circle diagram to determine the shearing strength of the soil type being tested.
Unconfined compression tests can be performed only on samples with sufficient cohesion so that the soil will stand as an unsupported cylinder. These tests may be run at natural moisture content or on artificially saturated soils.
In a triaxial compression test the sample is encased in a rubber membrane, placed in a test chamber, and subjected to a confining pressure throughout the duration of the test. Normally, this confining pressure is maintained at a constant level, although for special tests it may be varied in relation to the measured stresses. Triaxial compression tests may be run on soils at field moisture content or on artificially saturated samples.
The tests are performed in one of the following ways: Unconsolidated-undrained: The confining pressure is imposed on the sample at the start of the test. No drainage is permitted and the stresses which are measured represent the sum of the intergranular stresses and pore water pressures. Consolidated-undrained: The sample is allowed to consolidate fully under the applied confining pressure prior to the start of the test. The volume change is determined by measuring the water and/or air expelled during consolidation. No drainage is permitted during the test and the stresses which are measured are the same as for the unconsolidated-undrained test. Drained: The intergranular stresses in a sample may be measured by performing a drained, or slow, test. In this test, the sample is fully saturated and consolidated prior to the start of the test. During the test, drainage is permitted and the test is performed at a slow enough rate to prevent the buildup of pore water pressures. The resulting stresses which are measured represent only the intergranular stresses. These tests are usually performed on samples of generally non-cohesive soils, although the test procedure is applicable to cohesive soils if a sufficiently slow test rate is used.
An alternate means of obtaining the data resulting from the drained test is to perform an undrained test in which special equipment is used to measure the pore water pressures. The differences between the total stresses and the pore water pressures measured are the intergranular stesses.
CLINTON POWER STATION UPDATED SAFETY ANALYSIS REPORT Figure 2.5-299 Sheet 2 of 2 UNCONFINED COMPRESSION AND TRIAXIAL COMPRESSION TESTS (METHOD)
NOTES FOR FIGURE 2.5-300 Method of Performing Direct Shear and Friction Tests Direct shear tests are performed to detemine the shearing strengths of soils. Friction tests are performed to determine the frictional resistances between soils and various other materials such as wood, steel, or concrete. The tests are performed in the laboratory to simulate anticipated field conditions.
Each sample is tested within three brass rings, two and one-half inches in diameter and one inch in length. Undisturbed samples of in-place soils are tested in rings taken from the sampling device in which the samples were obtained. Loose samples of soils to be used in constructing earth fills are compacted in rings to predetermined conditions and tested.
Direct Shear Tests A three-inch length of the sample is tested in direct double shear. A constant pressure, appropriate to the conditions of the problem for which the test is being performed, is applied normal to the ends of the sample through porous stones. A shearing failure of the sample is caused by moving the center ring in a direction perpendicular to the axis of the sample. Transverse movement of the outer rings is prevented.
The shearing failure may be accomplished by applying to the center ring either a constant rate of load, a constant rate of deflection, or increments of load or deflection. In each case, the shearing load and the deflections in both the axial and transverse directions are recorded and plotted. The shearing strength of the soil is determined from the resulting load-deflection curves.
Friction Tests In order to determine the frictional resistance between soil and the surfaces of various materials, the center ring of soil in the direct shear test is replaced by a disk of the material to be tested. The test is then performed in the same manner as the direct shear test by forcing the disk of material from the soil surfaces.
CLINTON POWER STATION UPDATED SAFETY ANALYSIS REPORT Figure 2.5-300 Sheet 2 of 2 DIRECT SHEAR AND FRICTION TESTS (METHOD)
NOTES FOR FIGURE 2.5-301 Methods of Performing Pulsating Load Triaxial Tests Pulsating axial load tests are performed to evaluate the dynamic properties and the liquefaction potential of the soils under simulated anticipated field loading conditions.
Pulsating load tests are stress controlled and are performed on undisturbed or reconstituted samples of soil approximately six inches in length and two and one-half inches in diameter. The samples are encased in a rubber membrane, placed in a test chamber, and subjected to confining pressure throughout the duration of the test. The tests may be run on soils at field moisture content or on artificially saturated samples. The triaxial equipment acting through a Bellofram system applies a pulsating axial load. The cycling speed of the load can be varied between one-half to five cycles per second to simulate the field loading frequency. Dynamic Properties Determination To evaluate the dynamic parameters, the soil sample is loaded in cyclic compression. The load and deflection are recorded on two channels of a recording oscillograph. By tappi ng the output of the load and deflection transducers and applying these to vertical and horizontal plates, respectively, of a cathode ray oscilloscope, a hysteresis loop is produced. This loop is photographed, and the photograph is used to evaluate the damping value present. The procedure is repeated at various strain amplitudes to evaluate the dynamic properties in the range of interest on a particular sample. The load and deflection values obtained from the oscillograph are used to evaluate the dynamic moduli of elasticity.
Liquefaction Potential To evaluate the liquefaction potential, the soil sample is subjected to axial cyclic loading, the magnitude, frequency, duration and sequence of loading is determined on the basis of past earthquake records. The load deflection, and pore pressure are recorded on three channels of a recording oscillograph. These records are used to evaluate the liquefaction potential for that particular soil type under the test conditions.
CLINTON POWER STATION UPDATED SAFETY ANALYSIS REPORT Figure 2.5-301 Sheet 2 of 2 PULSATING LOAD TRIAXIAL TEST (METHOD)