LAYING METHOD OF GEOGRID MATERIAL AND REINFORCEMENT EFFECT OF SOFT GROUND

Transcription

1 LAYING METHOD OF GEOGRID MATERIAL AND REINFORCEMENT EFFECT OF SOFT GROUND Shgek ADACHI 1), Aknor HAZAMA ), Morto KUSABUKA 3) and Noro TAKEUCHI 4) 1) Graduate student, Department of Cvl and Envronmental Engneerng, Hose Unversty, Japan ) Mtsubsh Chemcal Functonal Product, INC, Japan 3) Professor, Department of Cvl and Envronmental Engneerng, Hose Unversty, Japan 4) Professor, Department of System Desgn Engneerng, Hose Unversty, Japan E-mal Address 04r510@k.hose.ac.jp ABSTRACT Ths paper presents the new layng methods of the geogrd to renforce a soft ground under embankments. The renforcement effect by the layng method s consdered based on the results of the small model experments n the laboratory and the fnte element method analyss. As a result, t s shown that the proposed method can expect the hgher renforcement effect than the conventonal layng methods. Mechancal vewpont of the layng method s the hgher axal force ntroduced nto geogrd, and we have been able to ntroduce the axal force by constranng axal dsplacement at the edges of geogrd. In the proposed layng method, the constrant of the axal dsplacement was acheved by penetratng the edge of geogrd nto the ground. We have tred the numercal analyss to consder renforcement effect of saturated clay by renforcement geogrd. Ths numercal analyss s used the fnte element method based on Bot consoldaton governng equaton. And the consttutve equaton of sol materal adopts the hyperbolc model of 3-parameters. Result of comparng experment value wth analyss value, we have shown that the proposed layng methods can consderably control the lateral flow wth sldng falure of a foundaton ground under embankments. 577

2 1. Introducton In Japan, many people lve as a feld of man economc actvty on the extremely soft ground. Cvl engneer has many techncal and economcal problems n soft ground whch we have to solve to develop nfrastructure development such as road works and ral works. For example, on the constructon of embankment structure on the soft ground, we have to develop the constructon technques to solve the problem about geotechncal engneerng such as a large consoldaton settlement and the fracture of a sol materal on the foundaton ground. In general, these techncal developments are needed many tmes and costs. On the other hand, n Japan, the cost reducton and the delvery of the new technques are very mportant n the constructon markets. The net method, layng geogrd on surface of soft ground, s developed to solve these problems, and ths method s generally usng n the world. In ths study, we notce ths net method, and try to the model experments and the numercal analyss about the layng methods of geogrd and the renforcement effects.. Proposed layng method The conventonal net methods have been expected the constrant effect of deformaton by the balance of ground settlement and heavng volume, or we expect the suspenson effect based on the tenson stress arsen nto the geogrd. In conventonal methods, the geogrd has been layng flat on the surface of soft ground. On the other hand, when the geogrd s lad on the transverse drecton of embankment shown n Fg.1, we have already proposed ths method penetratng the edges of geogrd nto the ground. In ths method, the geogrd on soft ground s constraned the axal dsplacement, and t s ntroduced the tensle stress nto the geogrd. Namely, the geogrd lad by the method works as a very useful renforcement materal to suspend embankment load. Embankment Press Tensle Force Fg.1 The penetratng method In order to expect a hgh renforcement effect by ths method, we need a larger layng wdth of geogrd than embankment wdth and we have to make arrangements a large constructon area. Therefore, t s not economc and practcal method. In ths paper, n order to avod ths economcal problem, as the second methods, we examned the method shown n Fg. whch s layng geogrd n longtudnal drecton of embankment. In ths layng method, one sde of the geogrd edge s fxed by buryng under the embankment area; the other sde edge of geogrd s penetrated n the ground of non-constructed area. 578

3 Embankment Embankment Drecton B=70mm n front of the loadng plate. The foundaton of the embankment was re-consoldated after fllng up Arakda clay mxed wth water. The materal propertes of the foundaton are shown n Table. The measurng ponts of the dsplacement were arranged 4 ponts on the surface of ground and ponts at the loadng plate as shown Fg.4. Fg. The second methods 3. Small model experments 3-1 Expermental procedures 500 We have carred out the small model experments n two-dmensonal plane stran condton as shown n fg.3. The expermental procedures are shown as follows: (1) We lad the renforcement nets on the soft ground consoldated by ntal pressure of 8.4 KN/m. () The net edge was fxed under loadng plate (Fg.4), and the other edge was penetrated nto the ground n front of the loadng plate. (3) The loadng was adopted the stepwse loadng, 3.6 KN/m Ç 1 tmes, and the total load was about 4.8 KN/m. (4) The dsplacements of the measurng ponts shown n fg.4 contnuously were measured durng 40 mnutes between preserved step loads. B Fg.3 Model experment Load ng D spl acement Gage Geogrd clay sand Fg.4 Expermental apparatus The layng length of the nets are 3 types of B=105mm, B=170mm and 579

4 3- Experment result The experment case s shown n Table1. Also, we show n the every experment case about the settlement of the loadng plate, where Case-1 s no renforcement, Case- s a layng length of 105mm, Case-3 s 170mm, Case-4 s 70mm. In comparson wth Case-1 (no renforcement) and Case- (penetratng method), Case- s shown the settlement restrant effect clearly. At the ntal loadng, the settlement s not dependent on the layng length, and the settlement rate s also small. But, when the loadng s over 17.9KN/m (after about 160 mn), the ground of no renforcement s settled down rapdly. On the one hand, the settlement of Case.~Case.4 s comparatvely small, and the fnal settlement s also controlled. Table.1 Test case Test Case Layng Length (B) Case-1 No Renforcement Case- 105mm Case-3 170mm Case-4 70mm 4. Consoldaton analyss We try the numercal analyss to consder renforcement effect of saturated clay by geogrd. The numercal analyss s used the fnte element method based on Bot consoldaton governng equaton. And the consttutve equaton of sol materal s assumed the nonlnear elastc model. 4-1 Governng equaton of consoldaton F Dsplcement (mm) Tme(mn) Case-1 Fg.5 Tme-Dsplacement Case-4 Case- Case-3 The dervaton of governng equaton coupled pore water flow and sol skeleton deformaton was frst ntroduced by Bot[1] but the present formulaton s elaborated upon n Zenkewcz et al.[]. We do not make any reference to the detal on these dervaton processes. The governng equaton usng n ths paper s standard type as fallows. The governng equaton about pore water flow can be wrtten by assumng Darcy s law: k γ w φ u φ ( ) + χ = 0 t t (1) 580

5 where u s the dsplacement component n the x drectons, φ s the potental functon, k s the coeffcent of permeablty, χ s the coeffcent of volume compressblty and γw s unt weght of water. The governng equaton of the sol skeleton assumng the elastcty can be wrtten as u G j j u j + (G + λ) j φ + F = p () where G s the shear elastc modulus, λ s the Lame s constant, φp s the pore water pressure, and F s the body force. To obtan the fnte element matrx equatons, we used the standard Galerkn s weghted resdual method to Eq.(1) and Eq.(). The geogrd ressts only axal force, so we assume as a truss element. 4- Consttutve model of sol materals In ths study, the consttutve equaton for the structure of sol skeleton was assumed the nonlnear elastc model wth 3-parameters, and then the hyperbolc model for shear stress-stran relatonshp was wrtten as γ oct τ oct = (3) A + Bγ oct octahedral shear stress-stran curve of sol materal. The materal parameter A and B are related to the ntal slope or tangent elastc modulus G T and the octahedral shear strength (τoct) ult,.e. can be wrtten as fallows: G T = 1 A 1 τ oct ) = (4) B ( ult The value of slope or tangent modulus E T at a pont on the octahedral shear stress-stran curves can be found by dfferentatng Eq.(3) wth respect to ç oct as E T A(1 + ν) = G T (1 + ν) = (5) (A + Bγ ) Snce the response of sol materals s dependent on hydraulc pressure p (=σ / mm/3) n effectve stress, for a gven stress path the behavor s often represented by a seres of a par of the parameter (A,B) for dfferent P[3]. However, the normal consoldated clay can assume that the ntal slope s nearly constant to P and the shear strength s dependent on P. So we assumed to be A = const and B = B(p ) n Eq.(4). The correlaton between the draned shear strength and consoldaton pressure p for normally consoldaton clay was gven by oct where τoct and γoct s octahedral shear stress and stran respectvely, A and B are materal parameters to determne the / σ ( τ oct ) ult = mp = m (6) 3 / 581

6 where m s the rate of shear strength ncrease. Substtuton of Eq.(6) and Eq.(4) n Eq.(5) yelds E T where M = 3/m. M(1 + µ ) σ mm = (7) (Aσ + Mγ ) mm We dvded the mesh, soparametrc 4-node element, as the two-dmensonal plane stran condton. Also, the loadng was ncreased stepwse as 3.6 KN/m x1 tmes = 4.8 KN/m, tme nterval of at the loadng stage s about t = 400sec. wthn the order estmated by l t = 1c v, c v oct k = (8) m γ where l s representatve edge length of a fnte element, c v s coeffcent of consoldaton, and m v s coeffcent of volume compressblty. 5. Numercal and expermental results The propertes of the materal are shown n Table. Table. Materal propertes of sol Densty of sol(g / cm 3 ).69 Water content (%) 40 Poson s rato 0.5 Modulus of elastcty(mpa) 10 Coeffcent of compresson 0.05~0. (normal consoldaton) Coeffcent of compresson ~ (over consoldaton) Strength ncrement rato 0.49~0.59 Coeffcent of consoldaton (cm ) 0.3-D~ 0.4-D v w Also, Fg.6~Fg.8 are compared the analyss results wth experment results about the settlement of loadng plate. The analyss value s comparatvely nferred to the experment value. In partcular, Fg.6 (wth no renforcement net) s shown Tme-dsplacement relatonshps, and the analyss value s comparatvely smlar to the experment value. But, about between Fg.7 and Fg.8 usng the nets, the errors may be observed, because the analyss value s slower than experment value on loadng nterval whch shows a rapd settlement. In other words, the generaton of the sldng surface s slow. Therefore, n the analyss, the renforcement effect usng the nets s deally generated from ntal stage. On the other hand, n the experment, when the net s lad, t s generated loose. So we thnk that t s dffcult to generate the renforcement effect at ntal stage. In ths regard, as t s shown by experment value n Fg.7 and Fg.8, we are able to assume that n former part of loadng the settlement rapdly advances, but the settlement rate doesn t advances n latter part. Except for the problem of ths experment, we consder that the error between analyss and experment value result from the dfferent causes. In partcular, the consttutve law of sol whch we convenently use, we are not able to express suffcently the complcated the consttutve formula of clay wth the consoldaton, and t s necessary to use the advanced consttutve law based on the crtcal state theory for us. 58

7 Dsplacement(cm) Dsplacement Dsplcement(cm) (cm) anal ys s exper ment Tme (mn) Fg.6 Tme-dsplacement: part1 Dsplacemet(cm) analyss experment Tme (mn) Fg.7 Tme-dsplacement: part anal ys s exper ment Tme(mn) Fg.8 Tme-dsplacement: part3 6. Concluson The man results of the proposed net methods can be summarzed as follow. (1) We carred out the small model experment about the layng method usng geogrd on the surface of soft ground. As a result, t was clear that we have the restrant effect by penetratng the edge of geogrd nto the ground. () The renforcement zone s the proper value, and we are needed the desgn consderng economcal problem, constructon cost and constrant of ste. (3) In comparson wth analyss value and experment value usng the consoldaton analyss program, we aren t able to get the deformaton as the contnued sldng surface s completely formed n the ground. (4) In the range of the suffcently controlled constructon condton, we showed that the analyss results are able to predct the actual phenomenon on the observed deformaton. As challenges for the future, about deformaton of sol skeleton structure at the numercal analyss, the effect of the stran s also mportant, but we try to consder preferentally on the relaton between the ncreasng strength of saturated clay and the consttutve law usng the consoldaton progress. 583

8 References 1. A.M. Brtto, M.J. Gunn: Crtcal state sol mechancs va fnte elements, Ells Horwood Lmted, John Wley & Sons, 1987, pp O.C. Zenkewcz & R.L. Taylor: The fnte element method, Vol.1, Butterworth-Henemann, MPG Books Ltd., 000, pp C.S. Desa & H.J. Srwardane: Consttutve laws for engneerng materals wth emphass on geologc materals, Pretce-Hall, Inc., 1984, pp T.Tokuhra, et al.: Vertcal bearng capacty test of very soft clay ground usng large sol contaner, sols & foundatons, Vol50, No.9, Ser.No.536, 00, pp C.Ghosh, et al.: Shape and deformaton patterns of vertcally loaded renforced sand, geosynthetcs engneerng journal, No.17, 00, pp Y.Do, et al.: Geotextle-renforced sol retanng wall: Reconstructon of a ralway embankment at Rokujzo Kyoto, Recent case hstores of permanent Geosynthetc-renforced sol retanng walls, 1994, pp A.Hazama, et al.: Comparng zgzag arrangement wth rectangular arrangement of geotextle-renforced sol wall, Jont ASME/ASCE/SES conference on Mechancs and Materals,