Study On Mechanical Characteristics Of Loess And Liquefaction Discrimination Methods For Widespread Fines-Containing Soils

Loess liquefaction potential evaluation is a difficult problem in engineering practice for a long time. Water content of most loess of construction sites is low, and shear wave velocity is high, however, in laboratory, loess sample should be saturated before test, the influence of saturation process on the structure of soil is not determined. Besides, the influence of consolidation ratio on soil stiffness is still inconclusive. Accordingly, the relationship of loess liquefaction dynamic triaxial test results with field mechanical index has not been established. On the other hand, loess belong to the category of fine-grained soils, so liquefaction discrimination methods for fine-grained soils should be a reference for loess, but because of relative density, soil skeleton strength and fine particles existence form of soil specimen prepared in laboratory are difficult to control, so there has been significant controversy and confusion concerning the effects of fines content on soils liquefaction resistance.This article focused on the need of engineering and research work. The author designed a series of intact loess bender element tests under isotropic and anisotropic consolidation in dynamic triaxial apparatus, and shear wave velocity test of loess sample in saturation process and after saturated, and in-situ tests at a saturated loess site. Reviewing current SPT-based liquefaction discrimination methods, indices for fines-containing soils were specifically analyzed, and preliminary discrimination criteria for fine-grained soils and SPT-based formula for fines-containing sandy soils had been improved. At last, the reliability of CPT-based liquefaction discrimination methods had been tested by case history data.In detail, this study includes topics as follows:1. Using cyclic triaxial apparatus along with bender element system, shear wave velocity of Lanzhou loess under isotropic and anisotropic consolidation are tested, the reliability of previous experiments is verified. By comparing soil sample axial deformation under isotropic consolidation with that under anisotropic consolidation, the influence mechanism of consolidation ratio on soil stiffness is analyzed.2. Using cyclic triaxial apparatus along with bender element system, firstly, shear wave velocity of undisturbed loess is tested during saturation, secondly, shear wave velocity and axial deformation of undisturbed loess and saturated loess were tested under the same step by step compression process, and lastly by comparing with SPT blow count and shear wave velocity of loess layers those up and down to groundwater level at a saturated loess site, the soften characteristics of wet and saturated loess are analyzed.3. Comparing the statistical relationship between shear wave velocity and SPT blow count of saturated silts in Tangshan, Haicheng earthquake with that of sands, it is found that SPT blow count for saturated silts are small due to thixotropy, yet shear-wave velocity is a small strain and non-destructive test, finally for saturated silt and sand with an identical shear wave velocity, the SPT blow count of saturated silt is significantly less than that of sand. Combining NCEER recommended SPT-based and VS-based liquefaction discrimination methods, the relationship of critical shear wave velocity with critical SPT blow count for three fine contents are established, and the thixotropy of fines-containing soils is confirmed.4. Reviewing fines-containing soils liquefaction research findings at domestic and abroad, and using liquefaction data from the 1975 Haicheng, 1976 Tangshan, 1999 Kocaeli and Chi-Chi Earthquakes, the advantages and limitations of existing liquefaction discrimination methods for fines-containing soils are comparatively analyzed, and finally prelimilary discrimination criteria for fine-grained soils and SPT-based formula for fines-containing sandy soils are improved.5. Reviewing NCEER recommended CPT-based liquefaction discrimination method which proposed by Robertson and that proposed by Olsen, using Chi-Chi earthquake data, the identification results of CPT-based and NCEER recommended SPT-based method are compared, analysis results show that the prediction success ratio of CPT-based methods are overall lower than that SPT-based method, however, soil type chart is the greatest advantage of CPT.