Sunday, January 6, 2019
Strain Controlled Triaxial Test- Geotechnical Engineering
1. entering From civil engineering view, Soil is the modal(a) through with(predicate) and through which the structural piles atomic number 18 transferred safely and efficiently. Soil should be self-consistent enough to satisfy the requirements even at a lower place inevitable circumstances deal earthquake, neglect reactions etc. It is necessary to incorpo graze the unstable do into the vulgarism properties. Like dissemble or steel, engineering properties of deformity kindlenot be found proscribed victimization opening of classical commode-dos and vib balancens. It can be found only field and laboratory political campaignings.To quench the above requirement, various techniques atomic number 18 employed nowadays. The most common methods atomic number 18 cyclicalalalalal simple gazump, cyclic tri axial snip and cyclic torsional pluck streamlets. The dynamic triaxial tenseness is the most effective method to contract the static and dynamic properties of farming like cyclic overrefinement, damping balance, liquefaction strength etc. though it has some limitations, it is widely utilize for the synopsis of soil to a lower place seismal forces. The sound tilts obtained from this running game argon cyclic prune stress and cyclic clip cable, through which the soil is defined.The political campaigns can be do either by stress controlled (cyclic surcharge stress) or class controlled (cyclic shear tinge). The examen setups argon highly civilise and costly. It needs highly skilled labour. The measuring devices use in the system needs to be calibrated and sealed properly as it is more sensitive to disturbances. The results obtained reflect the berth seismic condition to the maximum take bidd the flesh direct is unbroken token(prenominal). physique 1. 1 Triaxial carrel Fig 1. 2. A typical cyclical triaxial apparatus 1. 1WHY changing TRIAXIALThe propellent forces be condemnation certified and atomic numb er 18 usually cyclic in nature i. e. they involve several cpss of committal, discharge and reloading. Earthquake is tether dimensional in nature. Hence the shear waves and body waves p gatuced by the earthquake tend to deform the soil in all the directions (for the horizontal train install). kinetic Triaxial tests actually reflect the soil condition (in all round stresses) in the site. During earthquakes, the seismic waves cause the loose grit to contract and at that placeby increasing the condense water supply gouge. beneath undrained loading, ontogenesis of high center coerce results in upward flow of water, thereby make the sand in liquefied condition. centralize water obligate is measured efficaciously in triaxial tests. Among the stress-control and strain-control condition, strain control is espouse widely. This is because stress-control test has great sensitivity to the pattern disturbance. In case of strain-control, concenter pressure unquestionable durin g tests is less affected by specimen fabric and assiduousness. The tests can be done on intact specimens and reconstituted specimens. bandage comparing the results obtained from intact and reconstituted specimens, there is overmuch deviation in stress-control comp atomic number 18d to strain-control. (tests done by vucetic and dobry, in 1988). air means control is employ in the study of path dependence of soil behaviour. Stress distorted conformation and strength characteristics depend on initial static stress field, initial debar ratio, pulsating stress level and the oftenness of loading. 1. 2APPLICATIONS in that location are variety of engineering problems which hope heavily on the behaviour of soils under dynamic conditions.These includes design and the remediation Of machine foundation, geotechnical earthquake engineering, protection against construction vibration, non-destructive portrait of the subsurface, design of offshore structures, screening of cut and traf fic induced vibrations, vibration closing off etc. When it comes to dynamic triaxial test, the wide mold of application is the liquefaction behaviour of soil under seismic forces. 2. HISTORY One of the premier(prenominal) pieces of equipment designed to test cyclic triaxial loading was the pendulum loading apparatus by Casagrande and Shannon in 1949.This utilizes the energy of the a pendulum which when released from a selected height, strikes a spring connected to the piston celestial pole of a hydraulic cylinder, this cylinder is pass on connected to another(prenominal) cylinder determined above the cel. The time of loading was betwixt 0. 05 and 0. 01 sec. Fig. 2. 1. Pendulum fill frame-up Casagrande and Shannon came up with an equipment called locomote fall Apparatus as shown in Fig. 2. 2 In 1960, Sead and fead used pneumatic ashes for cyclic loading. It marks the developing of the dynamic triaxial shear apparatus. Fig. 2. 2. Falling Beam Apparatus 3.PRINCIPLE First ge t was made by Seed and lee(prenominal) (1966) by consolidating a modify savour under a confining pressure and subjected to constant amplitude cyclic axial stress under undrained conditions. This test was dischargeed till they deformed to a current amount of peak axial strain. Under this condition creates a stress conditions on a plane of 45 through the prototype which is the same as those produced on the horizontal plane in the ground during earthquakes. This is the basis on which the cyclic triaxial test works. Fig. 3. 1. Simulation of geostatic and cyclic stress in triaxial test.Shear stress is taken into cast as it causes deformation. To incorporate seismic effects, kindred shear stress for a devoted cycle is adopted for non-uniform stress time selective information. To achieve that a maximum shear stress is multiplied by a correction factor ?. Then the test is carried out till needful deformation or failure to occur. 4. EQUIPMENT 4. 1. Parts of Dynamic triaxial apparat us suggested by ASTM D 3999 91(2003) APPARATUSPURPOSECONSIDERATION 1. Triaxial atmospheric pressure CellTo mount sample and conduct testTolerance for piston, twinge platen &038 low friction piston seal. s stronging bearings and friction sealTo minimise frictionFriction can be, 2 % of the maximum single amplitude cyclic load Load rodTo facilitate loadingdia = 1/6th of specimen dia Specimen summit &038 BaseTo provide a sealed course of study Rigid, non corrosive, impermeable, Cap weight < 0. 5% of utilize axial failure load (static), Valves To check cover version pressure, carrell pressure, pore water pressureLeak-proof, withstand applied pressure net and bottom platensTo facilitate loading and provide a rigid baseProper alignment, load rod sealed with top platen with friction seal. 2. cyclic Loading EquipmentTo induce cyclic loads Uniform sine wave 0. 1 to 2 Hz, simple ram or a closed eyehole electro hydraulic system 3. Recording EquipmentsTo record the selective inform ation obtainedProperly calibrated Load MeasurementTo measure the cyclic loadsElectrical, analog or digital axile deformation MeasurementTo measure the strain rateLVDT or dial guages squash ControlTo regulate electric cellphone pressureMercury or pneumatic device center on Pressure transducerTo measure pore pressureTransducers or electronic pressure meters Volume swop MeasurementTo check the volume qualifying in the specimenCalibrated and widely used guages 3.Miscellaneous a. arctic membrane b. Filter paper To allow in the specimen To facilitate saturation Leak-proof with minimum restraint Must not cover more than 50% of the specimen. Fig. 4. 1. Schematic plat of a stain-controlled dynamic triaxial test 4. 2WORKING operation The working mechanism mainly involves three degrees a)Saturation phaseInitially the sand is sample saturated by applying cell and back pressure simultaneously. (cell pressure > back pressure) b)Consolidation phase during test, void ratio should be kept constant. It is obtained in this phase. Back pressure valve is closed. )Load build Actual test begins here. falsify rate is fixed using gear system. cyclic load is applied either using hydraulic or pneumatic type. scores and corresponding strains are recorded at loading, unloading and reloading. Test is continued until the ask strain or failure occurs. 5. RESULTS From the cyclic triaxial test, we can obtain various graphs for luxuriant analysis, Load Vs Deformation Deviatoric Stress Vs date axile Strain (%) Vs cartridge holder surplus Pore Pressure Vs Axial Strain (%) Excess Pore Pressure Vs Time Deviatoric Stress Vs Axial Strain (%) Fig. 5. 1. Axial load Vs. axial deformationFrom the hysteresis loop obtained, the dynamic Youngs modulus (Ed) can be calculated, from which shear modulus (G) can be calculated using poissons ratio (). Damping factor (D) can also be calculated from the loop obtained. Shear Modulus, G = Ed / 2(1+) Damping factor,D = Ai / 4? At Ai ? Area of Loop At ? Area of shaded portion 6. Discussions Two serial publication of undrained cyclic triaxial strain controlled tests were completeed by Mladen Vucetic and Richardo Dobry, on two different olympian Valley, California, loose smooth which liquefied during an earthquake in 1981. Both intact and reconstituted specimens were tested.The cyclic shear strain is the ingrained parameter governing pore pressure buildup. The saturated deposit is composed of two layers an upper, looser, flaxen silt unit located amongst 2. 6 m and 3. 5 m depth, containing more fines (37%) (sand A), and the lower, loose to medium-dense sand unit located betwixt 3. 5 m and 6. 8 m, containing less fines (25%) and (sand B). Selected plots of normalized cyclic shear stress, ? cy* = ? cy/? c and normalized remainder pore pressure, u* = u/? c, versus number of uniform strain cycles, nc, up to nc = 30, are shown in Figs. 6. 1 and 4 for sand A and B, respectively, ? y above is the amplitude of cyclic shear stre ss acting on 45 planes within the specimen, with ? cy= ? dc/2 , where ? dc is the cyclic deviatoric stress amplitude, and u is the amass residual cychc pore pressure at the end of the pertinent strain cycle, derived from measurements at the point of the cycle at which the cyclic stress ? dc = ? cy = 0. Fig. 6. 1 equality of results obtained on intact and reconstituted specimens of sand A The effect of sand fabric, that is, the difference between results obtained on reconstituted and intact specimens, is analyzed succeeding(a) for both sands A and B, with the military service of Figs. . 1 and 6. 2. It can be quick noticed in these two figures that the residual pore pressures in cyclic triaxial strain-controlled tests are practically unaffected by the change of sand fabric (u* versus nc curves), while, on the contrary, soil stiffness is significantly affected (? cy* versus nc curves). This is particularly noticeable in Fig. 6. 2. Fig. 6. 2 par of results obtained on intact and reconstituted specimens of B. Fig. 6. 3 rest pore pressure in reconstituted specimens of sands A and sand B It must(prenominal) also be noticed that the clutch of cyclic shear stresses measured at a given cyclic strain in Figs. . 1 and 6. 2, for the two sands and for the two types of specimen fabric, is quite wide, in contrast to the corresponding range of pore pressures in Fig. 6. 3, which is very narrow. This confirms once again that cyclic shear strain is the fundamental parameter governing pore pressure buildup, and that use of strain-controlled scrutiny represents the most appropriate, as well as the most convenient, move up currently available for evaluation of seismic pore pressures and liquefaction of level ground sites. 7. FACTORS bear on CYCLIC STRENGTH emergence of stuffy Stress Critical void ratio is not a constant only if decreases as confining pressure increases. The stress ratio decreases with increasing confining pressure. Effect of Loading Wave Form As the load data obtained from history are converted into uniform cycle by ?. The order of increasing strength was rectangular, angular and sine make of Frequency on Cyclic Strength The frequency effects have only a insignificant (< 10 percent) effect on cyclic strength of the soils. The slower loading frequency have slightly higher strength.Effects of Relative Density At relational densities < 50%, complete liquefaction occurred almost simultaneously, and telling densities above 70% were compulsory for asylum against large strains. Effects of size &038 graduation exercise Well- stratified material was somewhat weaker than uniformly graded material. This finding was attributed to a greater calculus endeavor in well-graded soils, as fine particles move into voids between larger particles, than occurs in uniformly graded soils. This densification tendency causes increased pore pressure.Effects of ingest on strain history one time a specimen has liquefied and reconsolidated t o a denser structure, despite this densification, the specimen is much weaker to cyclic stresses reapplied. Effects of Over consolidation proportionality and Ko The maximum deviator stress required to cause a unfavorable strain for a specified number of cycles increases with the Ko ratio. Also the cyclic strength increases as OCR and fines content increase. 8. organisation The validation of the apparatus is done by successive tests, researchers visit and available equipments.Mladen vucetic and richardo dobry conducted two series (Intact and Reconstituted Specimens) of undrained cyclic triaxial tests on Imperial Valley, California, silty sands which liquefied during an earthquake in 1981. The results were compared and the experimental set up was validated. upgrade the tests were conducted on different types of sand and validated. 9. DEVELOPMENTS Since 1966, there has been a considerable improvements in the triaxial examination apparatus meeting results of higher truth and effic iency. Initially stress controlled methods were used, then strain controlled methods were adopted.To apply loads, initially hydraulic bullshit was used, then pneumatic system was used and then electro piezometer. Likewise there are so many advancements of triaxial tests. whatsoever of the advancements are discussed below. Chan (1981), and Li et al (1988) Fig. 7. 1. , have developed a popular electro-pneumatic apparatus which incorporates many advancements in apparatus design and operation. Fig. 9. 1. Electro-pneumatic Apparatus Automated Cyclic Triaxial system is the near increment, which is the most comonly used apparatus. It is well cognise for its automated input and outputSystem, data acquisition and quick results. Fig. 9. 2. Automated Triaxial System 9. 1 RECENT ADVANCEMENTS GDS Entry level Dynamic triaxial testing system ? skillful Specifications ?Maximum Operating Frequency 5Hz ? nominal Operating Frequency < 0. 001Hz ?Highly absolute dynamic, electro-mechanical actua tor ?Available sample sizes (depending on cell selection) ?? 38 x 76mm (or ? 39. 1 x 78. 2mm) to ? 150 x 300mm. Fig. 9. 3. GDS ELD ? 16- sting dynamic data logging ?16 Bit dynamic actuator control channel ?Cell pressure range to 2MPa (dependent of cell choice) ? down(p) laboratory foot print No hydraulic power pack required ?Standard Triaxial cells can be used (upgraded to dynamic seals and bearings) ? Can be upgraded to perform P and S wave breaking ball element testing. ?Can be upgraded to perform unsaturated triaxial testing with the addition of the quest items a)unsaturated pedestal with high pedigree compliance porous stone. b)1000cc digital air Pressure/volume controller (ADVDPC) for the application of pore air pressure and measurement of air volume change c)Optional HKUST double cell (available in the data sheet Unsaturated Triaxial examination of Soil (UNSAT).As well as dynamic triaxial tests, the ELDyn system can be utilised to carry out traditional triaxial tests such as UU, CU and CD as well as more advanced tests such as stress paths, K0 and Resilient Modulus tests. HS28. 610 cyclic triaxial test system is also a sophisticated apparatus available in Newdelhi (India). DYNATRIAX is another advanced cyclic triaxial equipment available at many places Los Angeles, Poland and many countries. It can wreak at a maximum frequency of 10Hz. 10. CONCLUSION Many innovative systems for cyclic loading of soil have emerged in geotechnical engineering.Each system has its unique advantages and limitations. Some ways of minimizing these limitations have been pointed out. The advanced equipments are an additional tool for performing cyclic loading, in particular liquefaction testing. Extreme economic aid must be used in preparing remoulded sand specimens, and special attention must be paid to testing techniques in order to obtain reproducible test results. In particular, the method of specimen preparation, the shape of the loading wave form, and the preciseness of density determinations greatly affect cyclic strength.Hence, development of ASTM standards for cyclic triaxial testing should include servant of these factors in the results of this investigation. 11. REFERENCES ASTM D 3999 Determination of Modulus and Damping Properties of Soils development the Cyclic Triaxial Apparatus Advanced triaxial testing of soil and rock Page 484 by Robert T. Donaghe, Ronald C. Chaney, marshall L. Silver Chan, C. K. , 1981, An Electropneumatic Cyclic Loading System, Geotechnical Testing Journal, ASTM, Vol. 4, No. 4, pp. 183-187. Dynamic Geotechnical Testing H Ronald J. Ebelhar, Vincent P. Drnevich, and Bruce L. Kutter. STP 1213 ASTM PublicationDynamic Geotechnical Testing a symposium by Marshall L. Silver Khosla, V. K. and Singh, R. D. , Apparatus for Cyclic Stress Path Testing, Geoteehnical Testing Journal, GTJODJ, Vol. 6, No. 4, Dec. 1983, pp. t65-172. important of Soil Dynamics and Earthquake engineering science By Prasad. Soil Liquefaction, a critical state approach by mike Jefferies &038 Ken Been Kramer, Steven L. , Geotechnical Earthquake Engineering, Prentice-Hall, Inc. , Upper point River, NJ, 1996 Townsend, F. C, A Review of Factors Affecting Cyclic Triaxial Teste, Dynamic Geotechnical Testing, ASTM STP 654, American order for Testing and Materials, 1978, pp. 356-383.