Comparison Study on Dynamic Properties of Bachu Sand and Tangshan Sand Through Resonant Column Test
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摘要: 巴楚地震液化砂土标贯击数与锥尖阻力指标普遍偏大，却仍然发生广泛液化，原因至今没有查明。选取唐山地震和巴楚地震典型液化砂土，进行室内土工试验，对比两种砂土物性指标差异，并分别按相对密度30%、50%、70%、80%制样进行共振柱试验，试验发现：（1）相同相对密度下，唐山砂土干密度大于巴楚砂土，孔隙比小于巴楚砂土，试样中固体颗粒更多，孔隙更少。因此，唐山砂土的动剪切模量和阻尼比都高于巴楚砂土。然而，共振柱激振作用会导致土样内部结构发生变化，由于巴楚砂土颗粒更加均匀且颗粒比重更大，其动力稳定性高于唐山砂土。（2）随着固结应力的增大，唐山砂土动剪切模量比与动剪应变关系包络线范围逐渐收窄，符合一般规律。巴楚砂土在300 kPa的固结应力下，包络线范围反而变得更宽，体现了巴楚砂土明显的剪胀效应。（3）在同样的固结应力下，常规相对密度范围内，巴楚砂土剪切波速小于唐山砂土。然而，随剪应变增大，巴楚砂土动力稳定性更强。这表明仅用剪切波速判别土体液化存在问题，因为剪切波速代表土体小应变时的状态，而液化则是土体在大应变时的状态。从这个角度来说，标准贯入试验和静力触探试验中土体都处于大应变状态，与液化状态更相似，可以建立更好的相关关系。Abstract: SPT blow count and CPT cone tip resistance of extensive liquefied sands in 2003 Bachu earthquake are too large to liquefy, the reasons have not been identified. Take typical liquefied sand from Tangshan and Bachu respectively, laboratory tests were carried out, physical properties of two sands were compared, resonant column test were carried out for sand samples with relative density 30%, 50%, 70%, 80% respectively. Test results show that, (1) with same relative density, dry density of Tangshan sand is greater than that of Bachu sand, and void ratio is less. Therefore, dynamic shear modulus and damping ratio of Tangshan sand are higher than that of Bachu sand. However, vibration action of resonant column test will adjust internal structure of soil sample. As a result of more uniform particles and larger specific gravity, dynamic stability of Bachu sand is higher than that of Tangshan sand. (2) With increase of consolidation stress, the range of dynamic shear modulus ratio versus shear strain of Tangshan sand is gradually shrunk, which is in line with the law. Under consolidation stress of 300 kPa, the range of Bachu sand becomes wider, which reflects obvious shear dilatancy. (3) Under same consolidation stress and relative density, shear wave velocity of Bachu sand is less than that of Tangshan sand, but with increase of shear strain, dynamic stability of Bachu sand is stronger. This indicates that VS-based liquefaction evaluation methods are misleading, because VS are test at small strain, whereas, liquefaction are high strain and destructive phenomena. From this perspective, SPT and CPT are measured at large strain, more similar to liquefaction phenomena, can be established a better relationship.
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Key words:
- Bachu sand /
- Tangshan sand /
- Resonant column test /
- Dynamic shear modulus /
- Shear strain /
- Shear wave velocity
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图 5 4种相对密度下所有试样Gd-γd关系曲线
Figure 5. Gd-γd curves of all samples with different relative density
图 6 4种相对密度下所有试样λ-γd关系曲线
Figure 6. λ-γd curves of all samples with different relative density
图 7 不同相对密度砂土试样Gmax-σc关系曲线
Figure 7. Gmax-σc curves of samples with different relative density
图 9 3种固结应力下所有试样Gd/Gmax-γd关系曲线
Figure 9. Gd/Gmax vs γd curves of all samples under different σc
图 11 不同固结压力下两种砂土VS-e关系曲线
Figure 11. VS-e curves of two sand samples under different σc
表 1 砂土基本物理力学指标
Table 1. Basic physical and mechanical properties of two sands
土样 不均匀系数Cu 曲率系数Cc 粉粒含量/% 颗粒比重Gs 最大干密度ρdmax/(g·cm−3) 最小干密度ρdmin/(g·cm−3) 最小孔隙比emin 最大孔隙比emax 巴楚砂土 1.847 1.020 22.56 2.719 1.669 1.299 0.630 1.094 唐山砂土 2.280 0.818 3.62 2.651 1.771 1.439 0.496 0.842 
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表 2 共振柱试验工况
Table 2. Test program of resonant column test
原料 相对密度/% 孔隙比e 干密度ρd/(g·cm−3) 固结应力/kPa 试样编号 巴楚砂土 30 0.955 1.391 100/200/300 BC-30 50 0.862 1.460 100/200/300 BC-50 70 0.769 1.537 100/200/300 BC-70 80 0.723 1.578 100/200/300 BC-80 唐山砂土 30 0.738 1.525 100/200/300 TS-30 50 0.669 1.588 100/200/300 TS-50 70 0.600 1.657 100/200/300 TS-70 80 0.565 1.694 100/200/300 TS-80
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