Influence Of Stress Paths On Mechanical Characteristics Of Saturated Sand At High Pressures

Most of the research in the field of soil mechanics has focused on soil behavior at lowpressures, because where most geotechnical engineering problems tend to arise. However, inmany practical geotechnical engineering problems，such as deformation of high earth-rockfilldams, quicksand during mining under unconsolidated layers, shaft lining fracture in thickunconsolidated layers, soil mass deformation during subsidence, soils are subjected to pressuresabove1MPa, even above10MPa. Apparently, understanding the behavior of soils subjected tohigh pressures is one of important foundations of the solution to these types of geotechnicalengineering problems.In this paper, using GDS high pressure triaxial apparatus, saturated specimens of dense sandwere tested in undrained conventional compression and reduced extension tests and undrainedand drained reduced compression tests at effective confining pressures ranging from1.0to7.0MPa. The effects of confining pressures and stress paths on the stress-strain, pore pressure,strength and effective stress path characteristics of dense sand were investigated duringundrained shear at high pressures. The effects of confining pressures on the stress-strain,volumetric strain and strength characteristics of dense sand in drained reduced triaxialcompression tests were also studied. Over the range of confining pressures investigated, the mainconclusions and understandings are as follows:(1) Confining pressures and stress path tests have significant influences on the undrainedbehavior of dense sand. There are different “critical confining pressures”, at which thestress-strain, pore pressure and effective stress path characteristics of dense sand change indifferent undrained stress path tests. The critical confining pressure is1.0to2.0MPa forconventional compression tests,2.0to4.0MPa for reduced extension tests,3.0to4.0MPa forreduced compression tests.(2) In the undrianed stress path tests, the maximum deviator stresses occur at very largeaxial strains (>10.28%) at confining pressures below the critical pressure, and arise at low axialstrains（<4.4%） above the critical pressure, because rapidly developing pore pressures quicklydecrease the effective confining pressures. In the same stress path tests, with the increase ofconfining pressures, the initial tangent moduli increase and the initial slopes of the effectiveprincipal curves become gentler. The maximum effective principal ratios are in a very narrowrange from3.345to3.907for all tests.(3) In undrianed conventional compression and reduced extension tests, pore pressures firstincrease, then decrease or tend to be stable. In undrained reduced compression tests, pore pressures first decrease, at confining pressures above the critical pressure, then start to increaseat certain axial strains, finally tend to be stable. At the same confining pressures, the magnitudeand rate of positive pore pressure development are the biggest in conventional compression tests,and the next in reduced extension tests，while the smallest in reduced compression tests. In thesame stress path tests, the magnitude and rate of positive pore pressure development increasewith the increase of confining pressures. Under the same confining pressures and stress paths,pore pressure coefficient A is larger at the maximum deviator stress than at the maximumeffective principal stress ratio. Effective stress paths under different undrained stress paths are different.(4) The stress-strain curves for undrained conventional and reduced compression tests canbe characterized with hyperbolic model at confining pressures below their critical pressures, butcan’t above their critical pressures. The stress-strain curves for undrained reduced compressiontests don’t fit hyperbolic model over the range of confining pressures investigated.(5) With the increase of confining pressures, effective secant internal friction angles at themaximum deviator stress decrease in undrained conventional and reduced compression andremain substantially constant in undrained extension. However, Effective secant internal frictionangles at the maximum effective principal stress ratio remain substantially constant underdifferent undrained paths, respectively. Under the same confining pressures and stress paths,effective secant internal friction angle is larger at the maximum deviator stress than at themaximum effective principal stress ratio. Effective internal friction angles in undrainedconventional and reduced compression tests are approximately equal, and that in undrainedreduced extension tests is much smaller. At the same confining pressures, the initial tangentmodulus is maximum in reduced compression, and second in reduced extension, while minimumin conventional compression.(6) Stress-strain curves of drained reduced compression go well with hyperbolic model.With the increase of confining pressures, dilatation decreases and shrinkage increases duringdrained shear. The effective internal friction angle of drained reduced compression tests is biggerthan that of undrained reduced compression tests.