SIMULATING THE STRESS AND STRAIN BEHAVIOR OF LOESS VIA SCC MODEL

Transcription

1 SIMULATING THE STRESS AND STRAIN BEHAVIOR OF LOESS VIA SCC MODEL M.D. LIU Faculty of Engineering, Univerity of Wollongong, Autralia, J. LIU Faculty of Engineering, Univerity of Wollongong, Autralia, S. HORPIBULSUK Civil Engineering, Suranaree Univerity of Technology, Thailand, W. HUANG College of Mechanic and Material, Hohai Univerity, China, The behaviour of collapible oil (loe) i tudied in thi paper. Simulation of the tre and train behaviour of the oil are made via the theoretical framework of Structured Cam Clay, with the effect of glutinou ingredient in loe uggeted to be modelled a cementation effect. Baed on the imulation, the capacity of the model for repreenting the behaviour loe i invetigated, and dicuion on modelling the behaviour of collapible oil in general are given. INTRODUCTION Loe, found in many part of the world, ha been proved to be engineering challenge becaue of it collapible nature [e.g., 1, 2, 3]. The oil poee feature of tructured oil, and undergoe dramatically detructuring when it i wetted, diturbed, or loaded. Compared with natural clay, the tudy on the collapible nature of the mechanical propertie of loe both laboratory tet and theoretical modelling are relatively le invetigated. Recently, there have been ueful advance in contitutive modelling of natural oft oil [e.g., 4, 5]. In thi paper, the mechanical propertie of loe are invetigated. A primary tudy i performed on the capability of the Structured Cam Clay modified for cementation effect propoed by Horpibuluk et al [6] for repreenting the behaviour of loe and ome dicuion on model parameter for loe are alo made. MECHANICAL PROPERTIES OF NATURAL LOESS Loe i tructured oil and frequently trongly tructured. The original tructure of loe may undergo dramatically detructuring when it i wetted, loaded or diturbed, normally leading to the reduction of the trength and tiffne of the oil. Baed on the examination of ome available experimental data [e.g., 1, 2, 3, 7, 8], ome pecial mechanical propertie of loe are found. (1) The tructural trength of loe i mainly contributed by cementation among oil. However, the cementation link can be diolved in water; thu coheion trength diminihe when the oil i fully aturated. The cementational trength can alo be removed by loading. (2) Qualitative imilarity in the mechanical behaviour between loe and natural clay can be een a follow. Firtly, the void ratio for a natural loe i higher than that of the recontituted oil of the ame mineralogy and the virgin yielding tre of the oil i alo higher that of the correponding

2 recontituted oil. Secondly, the tre and train behaviour of the tructured loe appear to be aymptotic to the curve for the recontituted oil, i.e. the influence of oil tructure tend to diminih with loading. (3) A baic feature of the behaviour of a loe i it dependence on the degree of aturation of the oil. STRUCTURED CAM CLAY MODEL EXTENDED FOR CEMENTATION EFFECT Becaue loe poee both cementation tructure and particle-arrangement tructure, the Structured Cam Clay Model Extended for cementation effect i employed a a bae to decribe the oil. A brief introduction of the extended model i given below, and detail of the model can be found in paper by Liu and Carter [9] and Horpibuluk et al [6]. Elatic behaviour Virgin yielding Void ratio e e e* e e i Recontituted oil: e* p' y,i Mean effective tre lnp' Structured oil: e = e* + e (a) Compreion behaviour of tructural oil p' q C/M p' o M p' CSL Structural yield urface Equivalent yield urface (b) Structural and equivalent yield urface M p' Figure 1. Materialization for the Structured Cam Clay Model Modification of the mean effective tre cementation A fundamental aumption in the extending the SCC model for the effect of cementation i taken into conideration a the reinforcement of the confining mean effective tre; and a modified mean tre parameter i propoed a follow, p p C Eq. (1) where p i the mean effective tre, i the lope of the failure envelope of cemented clay, and C i a parameter related to the hear trength contributed by cementation. It hould be pointed out that C i generally not contant, but a function of oil tructure. The coheion trength of loe i trongly dependent the degree of aturation. In thi tudy, the value for C for loe with a given aturation i identified. Then it i aumed that the value fo C remain contant before the peak trength. After peak failure tate, C gradually decreae due to the cruhing of oil tructure, and finally C = 0 will be reached at a critical tate of deformation. Material idealization In the extended SCC model, cemented clay i idealized a an iotropic material with elatic and virgin yielding behaviour. The yield urface varie iotropically with platic volumetric deformation. Soil behaviour i aumed to be elatic for any tre excurion inide the current tructural yield urface. Virgin yielding occur for a tre variation originating on the tructural

3 yield urface and cauing it to change. During virgin yielding, the current tre of a cemented clay tay on the tructural yield urface. The idealization of the mechanical behaviour of cemented clay i illutrated in Figure 1. In thi figure, e repreent the void ratio for a cemented clay, e* i the void ratio of the recontituted clay at ame tre tate with the ame yield urface, p y,i i the mean effective tre at which virgin yielding of the cemented oil begin, and e, the additional void ratio, i the difference in void ratio between the cemented clay and the recontituted clay at the ame tre tate. Hence, the virgin compreion behaviour of a cemented oil can be expreed by the following equation propoed by Liu and Carter [11], b p C y i e e e e ei c, * * c Eq. (2) p where b and c are oil parameter decribing the additional void ratio utained by cementation. e i i the value of the additional void ratio at the tart of virgin yielding (Figure 1a). Parameter c i that part of the additional void ration that can not be removed completely by loading, The yield urface of a cemented clay in p-q pace i alo aumed to be elliptical and i decribed a (Figure 1b). The ize of the yield urface i denoted a p. Elatic behaviour For tre excurion within the yield urface, only elatic deformation occur. For implicity, elatic deformation of cemented clay i aumed to be decribed by Hooke law, i.e., 31 2 dp E, d e v 2 1 dq Eq. (3) 3 E d e d where i the Poion ratio and E i the Young modulu. Virgin yielding behaviour For tre tate on the yield urface and with dp > 0, virgin yielding occur. The platic volumetric train increment for the original SCC model wa derived from the aumption that both hardening and detructuring of clay are dependent on volumetric deformation, with a conideration of the detructuring aociated with hearing [e.g., 9, 10]. The incremental tre and train relationhip of the Structured Cam Clay i e dp d v d v * b e c 1 Eq. (4) 1 ep e 2 dp d d d d * b e c 1 Eq. (5) 2 2 p ep o 1 1 p

4 where * and * are the compreion and the welling indice of recontituted clay, repectively, i a oil parameter decribing the detructuring aociated with hearing, i a model parameter, and p o i the ize of the equivalent yield urface with conideration of the influence of c, the part of the additional void ration that can not be removed completely by loading [6]. Softening behavior and breakdown of cementation Baed on experimental obervation, the oftening behaviour for cemented clay occur after the oil reache the peak trength tate ( M and e 0 ). The cruhing of oil-cementation tructure alo take place during thi tage. For implicity, it i aumed that the cruhing of oilcementation tructure commence after the peak tate to reach the final critical tate of deformation. During cruhing, the effective tre tate tay on the line defined by M but may travel along the line either upward or downward, depending on hardening or oftening, repectively. Baed on trial and error, the function for the cruhing of oil-cementation tructure i propoed, dc 2 C C in dp q p Eq. (6) where C in i the value of the initial cementation trength. SIMULATING THE BEHAVIOUR OF MA LAN LOESS In thi ection, the Structured Cam Clay Model Extended for cementation effect i employed to imulate the behaviour of Ma Lan loe. The experimental data were reported by Li and Yao [8]. Value of model parameter identified for imulation are lited in Table 1. Table 1 Value of model parameter of Malan loe * * * e IC * * b C Becaue limit data are available for the oil from thi paper and previou publication, the author are unable to determine the ize of the initial yield urface p y,i. By trial and error, the an empirical equation i propoed to determine p y,i baed on the undrained peak hear trength q peak. 1.85q peak p y, i Eq. (7) * A comparion of the effective tre path for Malan loe in undrained conventional triaxial tet between imulation and tet reult are hown in Figure 2 and 3. Six tet are imulated and compared with experimental obervation. They are four hearing tet tarting from iotropic tre tate, and 2 tet from aniotropic tre tate. The confining tree vary from 200 kpa to 750 kpa. It can be een that the general feature of the effective tre path of the loe are decribed well both qualitatively and quantitatively. The imulated deviatoric tre and train relationhip i hown in Figure 3, which i conitent qualitatively with experimental obervation

5 [2, 8]. It i een that overall model imulation are in conitency reaonable well with experimental obervation. From thi tudy, the following point on modelling the tre and train relationhip of loe are oberved. (1) Unlike that of recontituted oil, there are two additional factor contributing the oftening for loe: (a) the cruhing of oil tructure, and (b) decementation. For the cruhing of oil tructure, tre path keep the ame tre ratio a M* and move downward along the critical tate line. For decementation, cementation trength C reduce gradually to zero and the tre ratio alo decreae. It i een here that the oftening on loe i largely influenced by decementation. (2) It may be concluded from the comparion that the baic feature of the behaviour of loe can be captured by the Structured Cam Clay with cementation in particular, though more work i needed on apect uch a the determination of model parameter, the variation of coheion with aturation and the removal of oil tructure by loading. (3) It can alo be een from the behaviour of loe can be captured by conventional elatoplatic model in general, and the two key factor in uch a model are the tructure of the oil contituent and the uction force. Shear tre q (kpa) kpa 300 kpa 400 kpa 500 kpa 760 kpa 625 kpa 1-D CSL Simulation Mean effective tre p' (kpa) Fig 2. Effective tre path for Malan loe meaured and imulated Shear tre q (kpa) kpa 500 kpa 760 kpa Deviatoric train d Fig 3. Deviatoric tre and train relationhip of Ma Lan loe imulated

6 CONCLUSION Mechanical propertie of loe are invetigated and imulation of the tre and train behaviour of the oil are made via the the thoretical framework of the Structured Cam Clay, and thee imulation were compared with experimental obervation. It i concluded that accurate prediction of the behaviour of loe i highly challenging becaue of complexity of the cementation amoung oil particle and collapible nature of the tructure with water a well a loading. It i een that the Structured Cam Clay can provide approximate etimation of the behaviour of the loe. Therefore loe i a frictional material and it behaviour i dependent on the arrangement of and bonding amoung oil partice, eentially the ame a other natural clay. To improve the accuracy of the imulation, further reearch within the Structured Cam Clay i needed in accurate repreentation of the development and removal of the cementation and tructure of the oil with time, water and loading. An empirical equation for etimating the ize of the initial tructural yield urface i alo uggeted. REFERENCES [1] Chen C., Zhu Z., and Gao P, Reearch on relationhip between tructure and deformation property of intact loe, Rock and Soil Mechanic, Vol. 27, No. 11, (2006),pp [2] Shao S., Luo A., Yu Q., and Zhou F., Structural damage propertie of Q 3 loe under triaxial loading and moitening, Chinee Journal of Geotechnical Engineering, Vol. 28, No. 12, (2006),pp [3] Leroueil S. and Hight D., Compacted oil: from phyic, to mechanic to hydraulic and mechanical behavior. Firt Pan-American Conf. on Unaturated Soil, Cartagena de India, Colombia, (2013). [4] Gen, A. and Nova, R., Conceptual bae for contitutive model for bonded oil and weak rock. Geotechnical Engineering of Hard Soil-Soft Rock, Balkema, (1993). [5] Taiebat M., Dafalia Y. F., Peek R., A detructuration theory and it application to SANICLAY model, Int. J. for Numerical and Analytical Method in Geomechanic. Vol. 34, No. 10, (2010),pp [6] Horpibuluk S., Liu M. D., Liyanapathirana S. and Suebook J., Behaviour of cemented clay imulated via the theoretical framework of the SCC model, Computer and Geotechnique, Vol. 37, No. 1, (2010),pp.1-9. [7] Hu Z., Shen Z., and Xie D., Deformation propertie of tructural loe, Chinee Journal of Rock Mechanic and Engineering, Vol. 23, No. 24, (2004),pp [8] Li J. and Yao Y., Critical tate model of Ko comolidated tructure loe, J.Xi an Univ. of Arch. & Tech.(Natural Sience Edition), Vol. 42, No. 4, (2009),pp [9] Liu M. D. and Carter J. P., Structured Cam Clay Model, Canadian Geotechnical Journal. Vol.39, No. 6, (2002),pp [10] Liu M. D. and Carter J. P., The volumetric deformation of natural clay, International Journal of Geomechanic, ASCE, Vol. 3, No. 3/4, (2003),pp [11] Liu M. D. and Carter J. P. (2000), Modelling the detructuring of oil during virgin compreion, Géotechnique, Vol. 50(4), pp