Effect Of Sample Preparation Method On Tri-axial Test Results Of Sand With Different Grain Gradation

Geotechnical testing plays an important role in engineering practice and soil mechanics research.Soil is a natural loose aggregation consisting of mineral skeletons.The physical and mechanical properties of the soil must be studied through special geotechnical tests.The static tri-axial test is an important test to measure strength and deformation characteristics of soil.The tri-axial test is inseparable from the preparation process where saturated sand samples are often prepared by dry,moist and water pluviation sampling methods.Usually,a conventional static tri-axial apparatus and a static tri-axial apparatus is equipped with the whole-surface digital image measuring system so that it can conduct tri-axial consolidation drainage shear tests(CD tests)with the material,artificially prepared sand with different grain gradations,using different sample preparation methods.In this way,we can explore the effects of sample preparation method and grain gradation on the deformation and strength characteristics of sand tri-axial specimens.Based on Fujian standard sand,six kinds of different grain gradations sand samples with uneven coefficient C_u=2.0,3.0,5.0,6.0,10.0,15.0 were arranged by artificial screening test.A medium-density sample with a relative density of 50%was prepared using different preparation methods.Under a confining pressure condition with 100kPa,200kPa and 300kPa,a series of CD tests were carried out.First of all,The CD tests of saturated sands with C_u=2.0?3.0 and 5.0 were carried out by ordinary static tri-axial instrument and static tri-axial apparatus equipped with the whole-surface digital image measuring system using dry and moist sampling methods.And the stress-strain relationship curve?effective internal friction angle?_d and the Duncan-Chang model modulus parameters K?n of the sands were analyzed.The results showed:the different sample preparation methods had a great impact on the stress-strain curve of the samples,and the peak principal stress difference of the dry samples were significantly higher than the value of the moist samples;The stress-strain curve obtained by the dry samples were basically independent of the uneven coefficient,but the stress-strain curve peaks of three different particle grading sands obtained by moist samples decreased with the increase of uneven coefficient;The effective internal friction angle?_d of the dry samples and moist samples increased with the increase of the uneven coefficient C_u,and the effective internal friction angle?_d of the moist samples were smaller than the value of dry samples under the condition of the same uneven coefficient;The value of K obtained by dry samples were larger than the value of the moist samples by using two instruments,and the value of K?n obtained by the whole-surface digital image measuring system were larger than the value of the ordinary static tri-axial instrument.And then,based on the static tri-axial apparatus equipped with the whole-surface digital image measuring system,a series of CD tests were carried out on saturated sand samples with C_u=6.0,10.0 15.0 preparing by dry and water pluviation sampling methods.The results showed:the peak principal stress difference obtained by dry samples were significantly lower than the value of the water pluviation samples;The stress-strain curve obtained by the dry samples were basically independent of the uneven coefficient,but the stress-strain curve peaks of three different particle grading sands obtained by the water pluviation samples increased with the increase of uneven coefficient;The effective internal friction angle?_d of the dry samples and the water pluviation samples increased with the increase of the uneven coefficient C_u,and the effective internal friction angle?_d of the dry samples were smaller than the value of water pluviation samples.The value of the K obtained by the dry sampling methods were slightly smaller than the value obtained by the water pluviation sampling methods,but the values of n were significantly larger than the value obtained by the water pluviation samples.In order to further explore the reasons of the influence for the different sampling preparation methods and grain gradation on CD test results of sands,the axial and radial strain fields of samples with C_u=3.0,5.0,10.0,15.0 under 300 kPa confining pressure were drawn at different times by software“surfe”using different sample preparation methods.The results showed:the moist samples were constrained by the ends of the ends obviously,so the axial deformation was mainly concentrated in the middle portion,and the value of the axial deformation from the middle to the both ends was gradually decreased,and it had obvious stratification.The deformation of the axial strain field of the dry and water pluviation samples were relatively uniform,but the bottom deformation of the water pluviation sample was significantly smaller than the middle and upper parts.For radial strain fields,there was no significant difference in the development of the deformation of moist sample,dry sample and water pluviation sample,their radial deformation was mainly concentrated in the middle,gradually decreasing toward both sides.There was no significant impact on radial strain field with different sample preparation methods and grain gradations.