Journal of Chemical and Pharmaceutical Research, 2014, 6(3): Research Article

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1 Avalable ole Joural of Chemcal ad Pharmaceutcal Research, 204, 6(3): Research Artcle ISSN : CODEN(USA) : JCPRC5 Dscrete elemet smulato o mechacal propertes of modfed sady gravel Le Zhag, Xaohu Lag 2 ad Lu Zhag Cha Uversty of Geosceces, Bejg, Cha Bejg Urba Costructo Explorato & Surveyg Desg Research Isttute CO..LTD Zhagle_cugb@26.com ABSTRACT Through the laboratory expermets, the mechacal propertes of sady gravel are obtaed. The, based o the slump tests, a modfed sady gravel model s developed by PFC 3D (partcle flow code of three dmesos), to whch relevat mcroscopc parameters are assged to umercally smulate the laboratory expermets. The laboratory test results are compared wth that of the umercal smulato by PFC 3D, whch shows ther perfect agreemet. Thus the partcle flow method s able to better smulate the process of the laboratory expermets of sady gravel. A ew approach for the study of the physcal ad mechacal propertes of modfed sady gravel s proposed the research to overcome the lmtatos of covetoal tests. Key words: dscrete elemet smulato; modfed sady gravel; traxal test; mortar model INTRODUCTION I sad gravel layer, the stratum structure s loose ad ot cemeted, ad has poor self-stablty, besdes, the cotet of gravel the layer s hgh ad the partcle sze s ueve. The sad gravel layer s a typcal ustable layer wth hgh sestvty[]. It s very hard to arch the sol formatos the sheld costructo process. Whe the cutter s rotatg, t s easy to destroy the orgal stablty of the formatos, causg collapse ad large dsturbace of surroudg rocs. Thus, there are a lot of techcal problems the sheld costructo process wth earth pressure balace, such as stablty of excavato face, promotg resstace, serous cutter wear, occluso of pressure postos, cag ad spewg, eve surface collapse[2]. Dstct Elemet Method s proposed by Cudall P.A[3]. It s a dscrete body of a partculate materal aalyss method, t s based o the prcple of molecular dyamcs ad t s frst used roc mechacs. O ths bass, Cudall ad Strac put forward the rgd dsc graular model 978 ad put the model to applcato sol mechacs, the results are cosstet wth photoelastc expermet results proposed by Drescher[4-5]. I the process of sheld costructo, the cohesve force of sol c ad teral frcto agle φ are two crtcal cotrol parameters, the ratoalty of the value ad the veracty of the value play decsve roles to the determato of excavato face support pressure ad the stablty of the excavato face. Sol partcles sady gravel are depedet from each other. Complex attrbutes dsplayed the course of the force, whch has yet to establsh approprate costtutve models. Developmet ad valdato of costtutve relatoshp requre a lot of physcal expermetal data as a bass for research, however the large dameter traxal tests the hgh cost, complcated operato, Addtoally, the large dscreteess of test results caused by the complexty ad ucertaty of the mcroscopc structure of sady gravel sol s also the problem accurate descrpto of ts mechacal behavor. Therefore, ths paper from the vew of mcrocosmc, tag PFC 3D as the tool, accordg to the sady gravel gradato curve costructo ste has self-programmg to obta the umercal model, lad the foudato for the study of the mechacal propertes of sady gravel. Through the laboratory expermets, the mechacal propertes of sady gravel uder dfferet stress 806

2 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): levels are obtaed. A modfed sady gravel model based o the slump tests s establshed to explore the basc mechacal propertes of modfed sady gravel, layg a sold foudato for the study of sol mprovemet ad umercal model tests. BASIC THEORY CALCULATION CYCLE OF PFC The calculato cycle PFC 3D s a tmesteppg algorthm that requres the repeated applcato of the law of moto to each partcle, a force-dsplacemet law to each cotact, ad a costat updatg of wall postos. Cotacts, whch may exst betwee two balls, or betwee a ball ad a wall, are formed ad broe automatcally durg the course of a smulato. The calculato cycle s llustrated Fg. At the start of each tmestep, the set of cotacts s updated from the ow partcle ad wall postos. The force-dsplacemet law s the appled to each cotact to update the cotact forces based o the relatve moto betwee the two ettes at the cotact ad the cotact costtutve model. Next, the law of moto s appled to each partcle to update ts velocty ad posto based o the resultat force ad momet arsg from the cotact forces ad ay body forces actg o the partcle. Also, the wall postos are updated based o the specfed wall veloctes. The calculatos performed each of the two boxes of Fg ca be doe effectvely parallel[6]. Fg. : Calculato cycle PFC 3D FORCE DISPLACEMENT LAW The partcle flow method whch s based o the dscrete elemet method s to smulate the movemet ad teracto of rgd spheres. PFC 3D s oe of dscrete elemet programs, the basc ut s a pure sphercal partcle. The force-dsplacemet law relates the relatve dsplacemet betwee two ettes at a cotact to the cotact force actg o the ettes. For both ball-ball ad ball-wall cotacts, ths cotact force arses from cotact occurrg at a pot. For ball-ball cotact, a addtoal force ad momet arsg from the deformato of the cemetatos materal represeted by a parallel bod ca also act o each partcle[6]. ()Cotact model The ormal cotact force ad the relatve dsplacemet betwee the two spheres cotact wth each other s calculated by: F = KU () Where K s the ormal stffess, U s the overlap the ormal drecto, The shear elastc force-cremet vector s calculated by: s s s s the ut ormal. F = K U (2) s K s the shear stffess at the cotact. (2)Sldg frcto model Durg the loadg process, there s frcto betwee sady gravel partcles, therefore the sldg frcto model troduced to smulate the sldg behavor betwee partcles allows partcles to sld wth the scope of the shear stregth. The cotact s checed for slp codtos by calculatg the maxmum allowable shear cotact force: 807

3 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): = m (3) s Fmax F Where µ the frcto coeffcet, If F F s ormal cotact force. s s s s > Fmax, the slp s allowed to occur, ad F = Fmax LABORATORY EXPERIMENTS TRIAXIAL TEST The test strumet s the large-scale traxal apparatus. Its operatg prcple s that actg a axal force o samples through axal system ad actg a crcular force o samples through pressure cell to smulate the sample falure uder three dmeso pressures, from whch the stress-stra curves of the tests are obtaed. The traxal apparatus parameters are as follows. Table Traxal apparatus parameters Sample sze Maxmum axal compresso Axal dsplacemet Maxmum cofg pressure Maxmum pore water pressure Φ300mm 600mm 2000N 0~300mm 0MPa 0MPa The sady gravel sampled from the Bejg subway costructo ste s used to prepare the specme. The stadard The sady gravels tae from the Bejg subway costructo ste are used to prepare the specme. The stadard screes whch has sx d of apertures(6cm 4cm 2cm cm 0.5cm 0.2cm)are used to scree the fully dry sady gravel.repeat screeg 4 tmes the calculate the average, the dstrbuto of the sady gravel partcle sze rage s show table 2. Table2 Sady gravel partcle sze dstrbuto Partcle dameter (mm) 40~20 20~0 0~5 5~2 0.2 Mass percetage (%) Accordg to table 2, the sady gravels of dfferet partcle sze are weghed ad mxed, the put the steel cylder whch the samples are dvded to 5 layers order to esure the compactess. Isotropc cosoldato draed shear (CD) test method s used, wth a shear rate of 2mm/m. The specme fals whe ts dsplacemet reaches 5% heght of the specme. I order to study the stregth ad deformato propertes of sady gravel, tests wth 3 cofg pressures (0.MPa, 0.2 MPa ad 0.3MPa) are coducted durg whch the stress ad stra data are recorded. SLUMP TEST The slump test s a commoly used for the test method for measurg the worablty of fresh cocrete. Through modfcato of the cotet of dfferet mud of modfed sady gravel slump test, the relatoshp betwee dosage of mud ad the lqudty of modfed sady gravel has bee establshed. The basc data to determe the model of rheologcal parameters of mortar s obtaed. The stadard slump coe as show Fg 4: 200mm 300mm 300mm Fg 4: The stadard slump coe Before the slump test, the modfed sady gravel should be prepared at frst. The modfed sady gravel s maly composed of sady gravel, mud ad water. Sady gravel sols accordace wth table 2, whch s eeded 40 g (approxmately 5 L) accordg to the volume of the mxg drum. The addtve amout of mud every tme s 200ml, The slump tests are carred out accordace wth the stadard test method after mxg evely, utl the plastc 808

4 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): flow of the sol was too large so far[7]. NUMERICAL SIMULATION TEST NUMERICAL SAMPLE OF TRIAXIAL TEST The cyldrcal specme of the traxal test umercal model s 60cm heght ad 30cm dameter. The upper ad lower plaes are both loadg board whose stffess s slghtly larger tha the ormal stffess of the partcle. The cylder wall s subjected to flexble costrats ad ts ormal stffess s smaller tha that of the partcles. Accordg to the gradg curve, the Fsh laguage embedded PFC 3D s used to establsh the umercal model. I order to esure computatoal effcecy, the mmum sze of partcles s cotrolled to 0 mm. The dstrbuto of the partcle sze rage of umercal samples s show table 3, ad the umercal sample s show Fg 3. Table 3 Partcle sze dstrbuto table Partcle dameter (mm) 40~20 20~0 0 Mass percetage (%) Fg3: Numercal sample The geerato process s dvded to the followg steps, tag group (40~20mm) for example: () Frst the total volume of the partcle aggregate s calculated by the equato below: D Vs = V = π H 2 2 (4) Where s the porosty of the sample, V s the total volume of the sample, H s the sample heght ad D s the sample dameter. (2) N partcles are geerated wth a rage of the partcle radus (40~20mm), the radus complace wth Gaussa dstrbuto. (3)The total volume of all the partcles of ths group ca be calculated as follows: N 3 4 d Vs = π 3 = 2 (5) Where d s the dameter of the partcles. (4) Whe V V P P s s ε >, all partcles the gra group are deleted. Where P s mass percetage of ths group, ad ε s the cotrol precso, ε=0.0. (5) N =N +, repeat steps (2)~(4) (6) Repeated steps ()~(5), utl the codto s satsfed requremets: 809

5 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): V V P P s s ε (6) The the partcles are geerated ext group. The partcle umber of each group s show table 4. Table 4 Partcle umber of each group Partcle dameter (mm) 40~20 20~0 0 Partcle umber NUMERICAL SAMPLE OF SLUMP TEST Three groups of the slump test of modfed sady gravel was executed, the addtve amout of mud s 5%, 0% ad 5% respectvely. Accordg to the results of traxal test, the model parameters s determed the umercal model of slump test s show Fg 5. The mortar ths model s replaced wth a dameter of 5mm partcle, the model parameters of sady gravel partcles ad the mortar partcles are evaluated separately. Fg 5: The umercal model of the slump test RESULTS AND DISCUSSION The data uder a cofg pressure of 0.Mpa of the umercal test are compared wth that of the laboratory test. After repeated adjustmets, a set of perfect value of the smulato parameters s obtaed as show table 5, whch are the used for the umercal smulato test wth cofg pressures of 0.2Mpa ad 0.3Mpa.The stress-stra curves of umercal smulato ad laboratory test are compared as show Fg6. (N/m) Table 5 Values of mcro parameters / s p (N/m) c (N/m) µ v (m/s) ρ (g/m 3 ).5e7 2.0 e8 5e Fg 6: Stress-stra curves of umercal smulato ad laboratory test The Fg6 shows that the laboratory test results ad the umercal smulato by PFC 3D are perfect agreemet. As the crease of cofg pressure, the pea stregths of umercal test rase. There are a few devatos betwee the stress stra relato uder dfferet cofg pressures from umercal smulato ad laboratory test, whereas the basc tred s cosstet. Whe axal pressure s large, the slde may occur ad the arragemet of the er partcles of the umercal model s ot compacted as that of the real sady gravel, thus the decreased terlocg causes the 80

6 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): devato. The umercal smulato ad the actual results of slump tests are show table 6. The fal state of tests s show Fg7. Table 6 Comparso of results betwee the ume rcal smulato ad the actual slump test No. Mud cotet /% Actual slump /cm Smulato slump /cm Errors /% (a) The slump of 5% mud cotet (b) The slump of 0% mud cotet (c) The slump of 5% mud cotet Fg 7: The fal state of slump tests The Fg7 shows that three group errors of the umercal smulato ad the actual slump test results are small. The umercal smulato reproduced the whole process of the slump test. Because the model of the mud partcles are 5mm dameter balls, the bte force betwee partcles are relatvely low, partcles are more proe to movemet, the results of umercal smulato s larger tha the actual. CONCLUSION Sady gravel s a partcle aggregate that cossts of partcles of dfferet szes. Base o the laboratory traxal tests ad the slump tests, the umercal models of traxal tests ad slump tests based o the graduato of the sady 8

7 Le Zhag et al J. Chem. Pharm. Res., 204, 6(3): gravel materals costructo ste s establshed by PFC 3D from mcroscopc perspectves the paper, ad the cocluso s made as follows by comparg two test results. ()The traxal tests of sady gravel are completed ad the stress-stra curves of sady gravel uder dfferet cofg pressures are obtaed. (2)The modfed sady gravel model s developed, whch s composed of gravel partcles ad mortar partcles. The slump test results of the umercal model are agreemet wth actual test. (3)The traxal model ad the modfed sady gravel model establshed here are able to smulate the physcal ad mechacal propertes of sady gravel accurately, whch overcome the lmtato of laboratory tests ad the dscreteess of the test results. REFERENCES [] Sog Kezh, Wag Bo, Kog heg, Yua Daju, Wag megshu. Chese Joural of Roc Mechacs ad Egeerg, v.24,.3, pp , [2] Aagostou G, Kovar K. Tuelg ad Udergroud Space Techology,v.,.2, pp.65-73, 996. [3] Cudall P A. Imperal College Lodo (Uversty of Lodo), 97. [4] Cudall P A, Strac O D L. Numercal Methods Geomechacs, 979,., pp , 979. [5] Drescher A, De Jossel de Jog G. Joural of the Mechacs ad Physcs of Solds,v.20,.5, pp , 972. [6] Cudall P A. Mesota: Itasca Cosultg Group Ic., [7] S. Clayto, T. G. Grce, D. V. Boger. Iteratoal Joural of Meral Processg, v.70,., pp.3-2,