砂土小應(yīng)變剪切模量各向異性試驗研究及數(shù)值模擬

1、砂土小應(yīng)變剪切模量各向異性試驗研究及數(shù)值模擬【中文摘要】本文采用干燥的Toyoura和Leighton Buzzard(E)砂土,應(yīng)用能產(chǎn)生小應(yīng)變的彎曲元,基于真三軸系統(tǒng)分別研究結(jié)構(gòu)和應(yīng)力引起的各向異性小應(yīng)變剪切模量變化規(guī)律,以及時間對小應(yīng)變剪切模量的影響。并以試驗為依托,利用離散元法研究了這種變化規(guī)律產(chǎn)生的原因,目的是從微觀力學(xué)角度揭示小應(yīng)變剪切模量各向異性產(chǎn)生的力學(xué)機制。通過試驗和離散元數(shù)值模擬研究發(fā)現(xiàn):等向壓力條件下,各向上的小應(yīng)變剪切模量與壓力的乘冪成正比;非等向壓力條件下,小應(yīng)變剪切模量主要受剪切面內(nèi)應(yīng)力的影響,而與剪切面法向上的應(yīng)力關(guān)系不大,并且小應(yīng)變剪切模量與剪切面內(nèi)的應(yīng)力的乘冪

2、也呈正比關(guān)系。研究還發(fā)現(xiàn),應(yīng)力歷史對小應(yīng)變剪切模量也有明顯的影響。本文通過試驗,還研究了時間對砂土小應(yīng)變剪切模量的影響,得到了小應(yīng)變剪切模量強化度隨應(yīng)力大小,偏應(yīng)力比等變化的規(guī)律?；陔x散元數(shù)值模擬研究不僅印證了試驗的可靠性,還從微觀力學(xué)的角度揭示了小應(yīng)變剪切模量各向異性產(chǎn)生的力學(xué)機制。無論是結(jié)構(gòu)還是應(yīng)力引起的各向異性,其微觀力學(xué)角度上都可以同一為土顆粒之間法向接觸力和接觸法向分布的各向異性。);【Abstract】 Many previous researches have found that: soil, as a kind of engineering materials, exhib

3、its anisotropy in physical and mechanical properties. It can be divided into two categories due to different causes: fabric-induced anisotropy and stress-induced anisotropy. Fabric-induced anisotropy is dependent on the virgin fabrics which are governed by the minerals and the particle characteristi

4、cs during the soil sedimentation. While stress-induced anisotropy refers to the changes of mechanical properties due to stress state variations when the soil is subject to complex loading conditions. Although the two kinds of anisotropies demonstrate in the identical manner in macro scale, soil mech

5、anical parameters vary with directions, and the rules of such variations of the two types of anisotropies are different. This * studies the variation rules of strength anisotropy caused by fabric and stress changes based on experiments and numerical simulation-Discrete Element Method (DEM).Existing

6、studies have found that the relationship between shear-modulus (G) and shear-strain () is nonlinear, which implies that it is inadequate to determined shear -modulus (G) through classical stress-strain (-) curve. It is generally accepted that, under small shear strain (less than 10-4) conditions, sh

7、ear-modulus (G) virtually keeps constant. As a significant parameter in soil dynamics, small strain shear-modulus G0 is indispensable in seismic safety assessment of any construction site.However, due to the special characteristics of soil (relative to other materials, such as metal) and also limita

8、tions of appropriate experimental devices, studies on the problem are not easy. In view of the fact that shear wave induced strain (10-5) falls in the range of small strain, and shear wave velocity can be related to small strain shear-modulus, this * uses shear wave velocity to measure the small str

9、ain shear-modulus of soil. In conventional effective stress seismic reaction analysis, the Hardin empirical formula is usually used to calculate the small strain shear-modulus. However, due to soil anisotropy, the small strain shear-modulus is also anisotropic. Therefore, it is crucially important t

10、o take account of soil anisotropy if physical-mechanical parameters that can accurately reflect soil properties are to be obtained.In view of the two reasons of soil anisotropy, this * studies the small strain shear-modulus in different directions and its variation rules with stress of Toyoura sand

11、( mainly comprises of sharp angled particles) prepared by air pluviation under isotropic loading conditions in a true tri-axial apparatus. The effect of stress history was also considered. It is found that the small strain shear-modulus differs in the vertical and horizontal directions. Under normal

12、 consolidated conditions, small strain shear-modulus in all directions are proportional to exponential values of isotropic confining stress, which agrees with many other research findings in literature. Further study also shows that the values of exponents in all directions are similar, falling in t

13、he range of 0.42-0.45. However, when soil is over-consolidated, the exponent values (0.2-0.3) in all directions are significantly smaller than those for normal consolidated soil. This manifests that the small strain shear-modulus of soil is obviously impacted by stress history.Under anisotropic load

14、ing, this * studied the variation rule of small strain shear-modulus with stress state using Leighton Buzzard (E) sand, which mainly comprises of quasi-round particles. It is found that the shear-modulus is essentially controlled by the stress in the shear plane, whereas shows no obvious link to the

15、 stress out of plane (normal to shear su*ce). It is observed that the exponent value of the exponential proportional relation of small strain shear-modulus and deviatoric component of stress in the shear plane ranges from 0.6 to 0.65, while the counterpart value is 0.14-0.17 for the small strain she

16、ar-modulus in the normal direction.For natural soils, the formation process often lasts a long geological period. The time effect on soils physical-mechanical parameters is obvious. Therefore, the time effect on soil small shear-modulus is also included in this *. The findings are: under stable load

17、ing conditions, shear-modulus in all directions increase with time until reaching stable values; for relatively low level of isotropic loading, small strain shear-modulus in the horizontal direction increases faster than in the vertical direction, however, under relatively high isotropic loading, sm

18、all strain shear-modulus increases at similar rate with time in all directions.For sand, as the granular material, the stress history effect on small strain shear- modulus and stress and fabric induced anisotropy are investigated by the software of PFC3D based on the discrete element method. The sim

19、ulation reveals the micro mechanism of the stress and fabric induced anisotropic phenomena which are shown in the experimental tests. For fabric induced anisotropy, with the increase of the ratio of particle length to width, contact normal force between particles becomes more evenly distributed, whi

20、ch in turn make more contact normal along the preferential direction of the particle arrangement. For stress induced anisotropy, both contact normal force and contact normal tend to align with the direction of deviatoric stress component. Therefore, from the micro mechanics point, the fabric and stress induced soil anisotropy can b