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 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 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 , 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 ≤ 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 , large pores, low-density, high- issue in the study of contemporary . [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 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 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 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. Circularity: circularity is used for describing the level of similarity between the target shape and circular; the ③ value is larger, the closer to circular. Shape complexity: describes the perimeter size of the regional unit area. Value of it is larger, the shape is ④ complex. Degree of orientation: expresses the Figure 2. Settlement versus elapsed time (creep). ordering level of particles’ arrangement. The smaller it is, the ordering level is better. ⑤ Distribution Figure 2 shows the settlement curve of the fractal dimension: distribution fractal dimension of sample’s creep process after three different particle is smaller, soil particles are more scattered. preceding methods. From figure 2, we can see in creep process, settlement of surcharge preloading is Table 2 Microstructure index of samples before and largest. Secondly, is vacuum-surcharge preloading. after triaxial test. The smallest is vacuum preloading. From the results of these triaxial tests, it can be index index of soil known that under the three different preloading shape degree distribution loading perimecircula settlement volume of combined preloading is the comple of fractal method ter rity largest in consolidation process and the smallest is xity orientation dimension vacuum preloading. In creep stage, due to the before 20.95 0.68 18.39 0.4 1.72 working of consolidation process, settlement volume vacuum after 38.51 0.43 29.53 0.66 2.04 of surcharge preloading are the largest, and vacuum preloading change 45.6 36.76 37.72 39.39 15.7 preloading is the smallest. Thus, soft soil’s creep the rate consolidation process and interacting. Bjerrum [7] before 27.01 0.56 22.62 0.48 1.84 reckons that creep throughout the consolidation process; consolidation and deformation of soil are surcharge after 38.38 0.43 28.95 0.66 2.03 [8] preloading change elastic-viscoplasticity. Tang’s study also shows 29.63 23.21 21.87 27.27 9.36 that instantaneous consolidation, primary rate consolidation and secondary consolidation of soft before 23.86 0.83 15.14 0.54 1.99 soil are all occurring simultaneously under stress. combined after 37.65 0.44 28.5 0.66 2.04 Preloading change 36.63 46.99 46.88 18.18 2.45 rate

338 From Table 2, after reinforced by three the change of macro property and the change of the preloading methods, perimeter of particles are microstructure are in the complementary longer. For three different preloading methods, relationship. The results of soft the foundation being because vacuum preloading is most apt to drain out reinforced by reloading drainage can be judged and the water, so the change rate of particle’s perimeter evaluated according to the change preloading of soil is the largest. However, when surcharge preloading engineering property and physical property consolidating soil, it cannot drain out the water parameter. At the same time, it can be judged by the completely in pores and its strength enhanced with soil microstructure changes under the certain dissipated of water pressure the pores. So change the preloading. rate of particle’s perimeter in surcharge preloading is the smallest. Through three preloading, the circularity of particles is reduced; complexity is 3 ELASTIC VISCOPLASTIC MODEL OF SOIL increased. For most microstructure indices, change RHEOLOGY rate of combined preloading larger than of vacuum preloading and larger than surcharge preloading. Soil is discovered that its deformation is elastic visco-plastic by a large number of practical and 2.4.4 Discuss the relationship between the triaxial experimental studies. Using Bjerrum’s timeline tests, physical tests and microstructure tests model and on the basis of indoors isotropic triaxial Triaxial test results show that settlement volume of tests, Yin and Graham [9] proposed a three-dimension combined preloading is the largest. The settlement EVP model. This model separates strain of soil into of surcharge preloading is larger than vacuum two parts including elastic strain and viscoplastic preloading. According to all regular tests of samples strain. Considering it need to work out the before and after the triaxial tests, the change rate of constitutive model among stress-strain-time which is combined preloading is the biggest. And at the same not influenced by different instantaneous applied time, the change of the micro-parameter was similar stress and specific change process of stress, the to macro-parameter indexes before and after the model uses “reference timeline”, “equivalent time” reinforcing. To visualize the etch rate of change of and other mathematical methods and finally deduced physical parameters and microstructure parameters religion constitutive model. of soil samples before and after the triaxial 3.1 Basic formula of EVP model compression test and the relationship between them, the rate of change in drawing a histogram, as shown The EVP model proposed by Yin and Graham in Fig 4. (1999) deduced Elastic visco-plastic model on the basis of “reference timeline” and a series of mathematical expression. According to the statement of Yin and Graham in paper [9] [10], the only one relationship of effective

stress pm′ , strain p′m and equivalent time te can be expressed as:

λ  p′  ψ  + tt  r vp r  m   0 e  (1) vm vm vm εεεε vm0 +=+= ln  + ln  V  ′m0  Vp  t0 

There are six parameters in the formula (1): κ V ， ， ， r ， ， λ V ψ V ε vm0 pm′ 0 and t0 . Among them, ， parameters κ V λ V and ψ V are related to characteristics of soil. r Figure 4. Rates of change rodshaped figure of micro-structure ε vm is reference timeline, can be expressed by and macrostructure index before and after the triaxial tests of following logarithmic form: soft clayey soils. λ  p ′  r r  m  (2) vm εε vmo += ln   V  p ′  Can be seen from Figure 4, before and after the mo ε r is volumetric strain of soil when ′ = pp ′ , improvement by different preloading method ,the vmo m mo λ /V 、 p′ and ε r are three parameters. rate of change of soft soil’s macrostructure index mo vmo t is equivalent timeline. After the reference and microstructure index has similar properties. e Generally, the change rate of combined preloading is timeline be determined, the whole group of most obvious. The changes of the macro-parameters equivalent timeline can be confirmed by following a such as water content, void ratio, and the changes of formula:  p′   micro-parameters such as degree of the order of the  r λ m  V e +−= ttt 00 exp vm εε vm0 −− ln   sample being loaded by vacuum preloading are V p′ ψ  m0   (3) larger than by surcharge preloading. It can be seen 339 For formula(1), strain rate of soil’s visco- parameters of microstructure been used. For elastoplastic on one point can be expressed as: example, Lei [12] introduced structural strength the  ′   • • ψκ λ p V parameters; established relationship between ε p += expε r + ln m −ε   (4) vm ' m  vm0 ′ vm  microstructure quantitative parameter and Vpm Vt0  V p ψ   m0   mechanical parameter and then proposed a model Formula (4) a is isotropic three-dimensional EVP which considering the stress-strain relationship model which has a universal meaning. model of structural influence. On the basis of isotropic three-dimension EVP model, Zhu Jungao[10] proposed anisotropic three- 4.2 EVP model considering microstructure dimension the EVP model in his doctoral thesis. In EVP model, the determination of [11] Zhoucheng proposed three-dimension the EVP κ V and ψ V are through matching the date of model of consolidation of K0, But the report of consolidation test. However, under the pressure of microscopic coupling parameters are rare. each level, micro image of SEM or CT could be get 3.2 Determination of EVP model parameters through certain micro test. And then through the corresponding micro image process and computer From what has mentioned above, isotropic three- analysis technology, corresponding micro image dimension EVP model formula (1) has a universal process can get. Accordingly, we can establish mathematical relationship for strain between meaning. There are six parameters: ， ， κ V λ V differential pressure and micro parameter. ， r ， ， ψ V ε vm0 pm′ 0 andt0 . According to methods in reference [10] [11], there are two ways to determine 4.2.1 Elastic compression micro parameterϑ these parameters. In EVP model, κ V is a related parameter to elastic (1) According to test results of creep test property of soil. V is special volume, its formula is (contains one-dimensional consolidation creep test, V=1+e. From its definition, κ V is related to void three-dimensional isotropic test, and so on), on the ratio of soil which is also called by structure of soil. basis of matching of tests data and using the basic On the other hand, EVP model is semi-empirical formula of EVP model works out parameters. For model because of some parameters are worked out example, in reference [10], author gets the parameters by matching empirical formula and test data. which are needed in EVP model through a group of Under different tests, variation and sensitively of isotropic three-dimensional test result data. soil’s microstructure parameters are different. (2) The determination of six parameters in the κ V reflect the size of instantaneous strain under the EVP model can be worked out through conventional pressure. As to the micro images of soil which are consolidation test and results of creep test, such as taken by different ways, using different software analyse them and get micro parameters which reflect reference [10] [11] expressed: k ＝ 0.434 C ； λ ＝ r microstructure property and changes of soil. ＝ 0.434 Cc ; ψ = 434.0 Cae ; t0 Parameters ϑ where choose to explain the soil’s r microstructure change under different pressure p, 1440min; = or εεε )( ; ′ = pp ′ . vm 24,0 hv ,EOPv 0 cm and then work out the variation of elastic

Cc and Cr are original pressure index and compression parameters ϑ under each level. As repressing index. It can be worked out by shown in following formula: dϑ ϑ −ϑ conventional indoors compression test. ϑ n == ,, inin +1 (5) n − ppdp ε 24, hv andε ,EOPv are time is 24h and strain capacity + ,1, inin 、 In this formula, ϑ ,in ϑ in +1, are one of the when initial consolidation finished. pc′ is the strain parameters at the time p ,in and p in +1, . ϑn is the capability of early consolidation pressure, Cαe is variability of one micro parameter under unit coefficient of secondary consolidation and the compression pressure, it is called an elastic compression micro parameterϑ . formula is: Cαe = ∆ε vm ∆ logt . 4.2.2 Micro parameter of visco-plasticityϖ Generally speaking, capability of soil creep is 4 AN ELASTIC VISCOPLASTIC MODEL influenced by two aspects. One is soil’s properties, CONSIDERING THE MICROSTRUCTURE - such as mineral composition etc. The other is soil IMPROVEMENT OF EVP MODEL microstructure. Sills, G and Eduardo E. Alonso[13] etc. studied consolidation creep of soil from 4.1 Microstructure parameters. microstructure, and they thought soft soil is With the development of microstructure test composed of pores among structure cells; creep of technology and computational analysis, analysis soil is the process of flowing of pore-water in methods of soil microstructure are different from

340 structure cells, flow out and constantly adjust of soil images and statistic of microstructure parameter particles. data. Similar to the formula of elastic compression micro parameter ϑ ,micro parameter of visco- plasticity before and after the stable of crap to 5 CONCLUSION expressing parameters’ change in creep process under different stress. Assume ϖ representing Tests for simulating vacuum preloading, surcharge ， micro parameter of visco-plasticity, ξ ,in ξ in +1, are preloading and vacuum-surcharge combined the typical micro parameters of time ti and ti+1 under preloading were carried out. Results show that soil’s pressure P, ti can be the start time of creep, ti+1 can consolidation and creep process have coupling be the end time of creep. Thus, the formula of micro effect. Photos of canning electron microstructure parameters in viscoplasticity is: (SEM) for the soil samples before and after each ξ − ξ triaxial test have been achieved and managed with ϖ = + ,1, inin (6) − tt Geoimage programme. The character changes of the +1 ii soft soil sample under the different preloading After two parameters-- κ V and ψ V of EVP method have been studied from microstructure view model are determined by microstructual test, which and the change of macrostructure indices. The microstructual parameter including elastic ϑ change rate of the microstructure parameters was compression micro parameter and visco-plasticity similar to that of macroscopic parameters. The creep micro parameter ϖ , formula (4) can be constitutive model based on the uniform of points of expressed as: • stress state was established by EVP model. An • p ϖ  λ p'  1  elastic visco-plastic model which parameter could be ϑε m += expε r + ln m − ε   (7) vm '  vm0 ' vm  determined by the micro-structure parameters was pm t0  V pmo ϖ  proposed. It not only calculates the elastic-visco- 4.3 Determination of parameters in improved EVP plasticity deformation of soft soils, but also reflects model the change of soil’s microstructure. Which can explain the deformation process of soil and the inner Original state soil which considering the micro mechanism of consolidation-creep from physical parameter ϑ in the test of vacuum-surcharge nature. preloading, assuming through early works it can be determined that soil’s microstructure parameter— prticle’s area is the most typical microstructure ACKNOWLEDGEMENT parameter. At 100kPa, 200kPa, particle’s area is 23.1% and 24.9%. Based on a formula (5), there is: This work was supported by China National Natural dϑ −ϑϑ ,, inin +1 − 1.239.24 ϑ = = = ＝0.018 Science Foundation(Study on Creep Characteristic −200100 − ppdp 100 + ,1, inin and VEP Model of Unsaturated and Undisturbed Similarity to considering the determination of ;No.51309199） and by an open research fund micro parameterϑ , assuming through early works, it project(Study of Creep Characteristics and VEP can be determined that soil’s micro parameter – Model of Soft Soils Under Consolidation by complexity of particle is the most typical Preloading Method;No.GH201101) of Key microstructure parameter of soil, and then work out Laboratory of Geomechanics and Embankment creep micro parameterϖ according by formula (6). Engineering, Ministry of Education of China. Assuming for 80kPa vacuum preloading, duration time is around 360h, the complexity of particles are 29.5 and 18.4, according to formula (6), there is: REFERENCES − ξξ − 4.185.29 ϖ = +1, ,inin = ＝0.0308 v80 − tt 360 [1] Chai, J.C., Carter, J.P and Hayashi, S. 2005. Ground +1 ii deformation induced by vacuum consolidation, The Above example shows the method of Geotechnical and Geoenvironmental Engineering, Vol. 131, calculating the plastic creep parameters by No. 12:1552–1561. microstructure parameter and then illustrates the [2] Hayashi, H., Nishikawa J., Nishimoto, S. and Sawai, K. feasibility of the improved EVP model in 2003. Performance of vacuum consolidation and prefabricated vertical drain in ground, Proceedings of considering the determination of micro parameters, 37th annual meeting, The Japanese Geotechnical Society: but it still lack of systematic and integrity. When 2–9 : Japanese. focus on a specific problem, it should list the [3] Chai, J.-C., Carter J. P. and Hayashi S. 2006. Vacuum reasons, the process and the result of specific and its combination with embankment worked out by one microstructure parameter through loading, Can. Geotech. Vol. 43, No. 10:985-996. a large number of comparisons of soil’s micro [4] Shen Zhu-jiang. 1998. Fundamental problems in the modern soil mechanics, Mechanics In Engineering, 20(6):1- 6. 341