2015 4th International Conference on Materials Engineering for Advanced Technologies (ICMEAT 2015) ISBN: 978-1-60595-242-0 Study on EVP Model and Consolidation-creep Test of Soft Clay Soils Under Consolidation by Preloading Method Sheng-xia Hu 1,2 1School of Transportation Science and Engineering, Beihang University, Beijing, China; 2Architectural and Civil Engineering Institute, Yanan University, Yanan, China; Yu-min Chen College of Civil and Transportation Engineering, Hohai University, Nanjing, China ABSTRACT: In order to explore the mechanism of post-construction settlement of a soft foundation improved by reloading, creep tests under triaxial consolidation condition were carried out. The soft clay samples were consolidated under the condition of vacuum, surcharge and vacuum-surcharge combined preloading, respectively. The microstructure tests of soft clayey soils were taken before and after the triaxial tests. Comparisons of the creep characteristics and the change of micro-structure were analysed. In the consolidation process, the largest settlement was found under the condition of vacuum-surcharge combined preloading, followed by that of surcharge preloading, and the smallest under the condition of vacuum preloading. In the process of creeping, the largest settlement was found under the condition of surcharge preloading, followed by that of vacuum-surcharge preloading, and the smallest under the condition of vacuum preloading. The coupling effects of triaxial consolidation and preloading creep on the characteristics of soft clayey soils were analysed. The change rate of the microstructure parameters was similar with that of macroscopic parameters. The specific stress path of change was weakened in EVP model. An elastic visco- plastic model which parameter could be determined by the micro-structure parameters was proposed. The improvement EVP model not only can calculate the elastic-visco-plasticity deformation of soft soils, but also consider the microscopic structure changes of soft soil, which can better reflect the essence of soil deformation. 1 INTRODUCTION mechanical tests should be carried out to propose a reasonable constitutive model. Vacuum-surcharging is a method of soft soil The micro-structural parameters of soft the soil improvement combined with vacuum preloading and sample were gotten by the microstructure test before surcharge preloading. It is widely used for and after triaxial test which simulating different improving the strength and reducing settlement of preloading method. On this basis, a viscous-elastic- soft foundation all round the world [1][2][3]. According plastic model which considering the quantitative to Soft Foundation Design of Highway and calculation of microstructural parameters into it with Construction Standard in china, the allowable post- deduced. construction settlements in highways and first-class highway are: the adjacent place between abutment and embankment ≤ 10 cm, general section≤30cm. 2 TEST CONTENT AND RESULTS The mechanism of the post construction settlement is still difficult to be exposed. In recent 2.1 Soil samples years, the important process was made in the Undisturbed soil sample was taken from the vacuum research on the microstructure of soils. Introduce the preloading construction site of Ningbo in Zhejiang quantitative study of microstructure of soil into the province in China. The soft foundation soil has high study of the soil constitutive relationship is the core water content, large pores, low-density, high- issue in the study of contemporary soil mechanics. [4] compression and other characteristics. It is a typical Shen et al. emphasized that soil’s constitutive mucky soil. The steel tube with 50cm length and a should be proposed in a framework of structural little larger than 10cm diameter was used to get the model based on comprehensive understanding of [5] undisturbed soil sample. The depth of the soil microstructure changing. Xie reckon that not only samples in test is around 4m. The results of indoors the macro-structural tests, but also the micro- tests are comparable and reliable. 336 2.2 Test procedure and equipment 2.4 Test results and analysis When the test was preparing, undisturbed samples in 2.4.1 Change of physical and mechanical thin-walled sample tube was pushed out and then parameters of soil samples before and after makes it into two cylindrical samples with 200mm triaxial test in height and 100mm in diameter. One is used for triaxial test and the other is for routine tests. After In order to judge and evaluate the effects of triaxial test, soil samples were taken for routine preloading for reinforcing the soft soil, in-situ test tests. and indoor soil test are carried out to calculate the Stress-controlled triaxial creep equipment which physical mechanics index. The changes of soil developed by GeoHohai in China was used to carry macro physical property can be contributed to out the triaxial test[6]. Sample’s height is 200mm, determine whether the effect of reinforcement has diameter is 100mm, and it is a cylinder. Before and been reached and the effect of strengthening can be after each triaxial test, soil samples were cut off to evaluated. Before and after the three triaxial tests, make regular samples and cylindrical soil stripes at the routine tests have been done and the physical the same time. For cylindrical soil with indexes have been gotten as shown in table1. 15mm×15mm×50mm size, snapping it from middle till it dried naturally, then ladder-shaped micro Table 1. Comparative table of changes in physical sample of soil was made. Then sent it into an SEM and mechanical parameters of samples before and sample preparation room for plating gold by after the triaxial compress tests. vacuum, and using electron microscopy scanner compressi water wet permeability preloading void on (type is JSM-5610LV)) to capture photographs of content density coefficient method ratio e modulus( SEM. In order to reflect the microstructure of soil (%) (g/cm3) (cm/s) fully and can be comparable, SEM photographs MPa) were magnified 1200 times. vacuum before 32.92 1.67 1.18 6.96*10-6 4.13 preloading after 28.14 1.85 0.89 2.78*10-6 5.76 2.3 Test Content change 80kPa 14.52 10.78 24.58 60.06 39.47 Three groups of triaxial consolidation and creep tests rate were carried out. Soil samples were consolidated for before 33.29 1.68 1.17 2.54*10-5 2.52 surcharge after 30.82 1.81 0.98 2.02*10-6 3.32 three days under cell pressure of 80kPa. After that, preloading change soil samples were loaded by 80kPa vacuum, 80kPa 80kPa 7.42 7.74 16.24 92.05 31.75 surcharge and 80kPa vacuum-surcharge preloading rate respectively till it becomes relatively stable. Then before 35.59 1.66 1.14 1.13*10-5 2.18 combined creep tests under the condition of different after 29.53 1.87 0.88 2.78*10-6 6.45 preloading preloading were carried out. change 80kPa 17.03 11.23 22.81 75.39 95.87 Microstructure characteristic parameters were rate obtained by Geoimage Software.Geoimage Software Table1 shows that the water content in soil is software for soil sample microstructure analysis, samples reduced, the void ratio decreased, the wet which is produced and developed by Geohohai in density and the modulus compression both increased China. Microstructure characteristic parameters such after reloading, which means the engineering as texture parameters, shape characteristic properties of weak soil has been changed. And also parameters and other parameters were worked out by the change rate of combined preloading is bigger through quantitative calculation after enhancing than single vacuum preloading and surcharge image, parting and other process. Compare preloading. By comparing the change rate of water Geoimage with Scion Image (Scion Image software content after the vacuum reloading and surcharge is a free image processing and analysis tools) in their preloading, the former is almost the double value of results of processing binaryzation image and find out the latter. After the reinforcement by three different two images are similar. Compared with Scion preloading, the parameters of samples are different Image, Geoimage has more advantages in processing from one another, which means the mechanism and and analyzing the microstructure of soil. It can part effects of strengthening the soft foundation with the particles, pores and linker in micro images. Texture three methods are different. features parameters and shape features parameters of soil microstructure can be reflected fully. 2.4.2 Triaxial consolidation creeps test settlement and settlement ratio curve Soft soil was consolidated by vacuum preloading, surcharge reloading and vacuum-surcharge combined preloading through drainage. Due to the different reinforcement mechanism of three preceding methods, the settlement in 337 2.4.3 Results of microstructure test and analysis We took micro photograph at 1200 times Magnification of each sample before and after triaxial compression test. SEM photos as examples were shown in Figure 3. Figure 1. Settlement versus elapsed time (consolidation). The consolidation process is different. Figure 1 is Fig 3-(1) Microstructure Fig 3-(2) Microstructure the settlement curve of the consolidation process photo of soft soils before photo of soft soils after vacuum preloading test vacuum preloading test under the loading of three different preloading （1200 times）. （1200 times）. methods. It can be seen in the process of Figure 3 Microstructure photos of soft clayey soils before and consolidation, the largest settlement of the sample is after the triaxial compress tests combined preloading, it is 20.2mm; secondly is surcharge preloading, it is 17.8mm; the smallest is using vacuum preloading, it is 3.4mm. Before and after the triaxial test, SEM photos for of the soil sample have been achieved and managed with Geoimage programme and micro parameters were extracted. As shown in Table 2. Meanings of each micro characteristic parameters are as follows: ① perimeter: Perimeter describe the basic ② characteristic of area size.