Pile-supported embankments on soft ground for a high speed railway - Load Transfer, Distribution and Concentration by different construction methods

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1 Ple-supporte embankments on soft groun for a hgh spee ralway - Loa Transfer, Dstrbuton an Concentraton by fferent constructon methos M. Rathel & A. Krchner Kempfert + Partner Geotechnk, Wuerburg, Germany H.-G. Kempfert Unversty of Kassel, Germany ABSTRACT: For the founaton of traffc nfrastructure n areas wth soft subsol often embankments supporte by ples are necessary ue to the settlement requrements. For suffcent loa transfer nto the ples fferent constructon methos are possble. For the constructon of a hgh spee ralway lne n Chna renforce concrete slabs, a cement stablaton of the embankment materal an geogr renforcement on top of the ples were use. The fferent methos are escrbe an mportant fel montorng results are presente. INTRODUCTION In areas wth soft subsol embankments have often to be foune on ples or columns, especally f restrcte settlement requrement exsts. Thereby a suffcent loa transfer nto the ples s necessary. Wthout any constructve elements for loa transfer an strbuton an archng effect n the embankment wll be present, ue to the hgher stffness of the ples compare to the soft sol. The mechansm of stress restrbuton can be moelle by a system consstng of several archng shells. In many cases ths s not suffcent for settlement reucton, especally f there s a rsk of hgh cyclc/ynamc loang. For a hgh spee ralway lne n Chna therefore three fferent constructon methos for a better an safe loa transfer, strbuton an concentraton were carre out: - Renforce concrete slab on top of the ples - Horontal geogr renforcement on top of the ples (so-calle geosynthetc-renforce ple-supporte embankment ) - Cement stablaton of the embankment materal In ths contrbuton the esgn an constructon of the ple-supporte embankments wll be presente. PROJECT Between Bejng an Tanjn 5 klometres of hgh spee ralway are beng bult by orer of the Chnese Ralway Department. The ralway lne wll go nto servce at the Olympc Games. Wth a esgn spee of 35 km/h, the superstructure of the lne wll be constructe wth the so-calle slab track system System Bögl evelope by Max Bögl GmbH & Co. KG - Germany. The Bögl slab track system conssts of laterally tensone, prefabrcate slabs wth mounte ral fastenngs. Fgure. Hgh spee ralway between Bejng an Tanjn. About % of the ralway lne wll run on a structure consstng of sngle-span an mult-span brges wth stanar span lengths of an 33 m an n some cases between an m. The remanng length of the track wll run on an embankment wth ple founatons. Due to subsol contons charactere normally by soft to stff slt an clay own to epths of 5 m an a grounwater level just beneath the groun surface, all structures were foune on ples wth lengths up to 7 m. Conserng the spee, the requrements concernng settlements of the founaton wth 5 mm after

2 constructon of the slab track are very hgh. In orer to etermne the actual bearng capacty of the ple founatons ple loa tests were conucte on specal test ples (see Rathel et al. ). To guarantee the necessary qualty, workng ples (auger- an rven ples) were also teste. transfer, strbuton an concentraton was use. On both ses a cantlever retanng wall was constructe. A schematc vew of the ple supporte embankment s shown n Fgure 3 an a pcture s gven n Fgure. 3 PILESYSTEM AUGER- AND DRIVEN PILES Axs embankement fll For the founaton of the embankments two fferent ple systems are use. The CFA-ples are rlle wth contnuous flght augers up to epth of m. The ameter of the auger s about cm an the ameter of the hollow stem s about cm. Atonal precast concrete rven ples (prestresse) wth ameter 5 cm an a total length up to 35 m were use. Accorng to our experences ths system proves very goo bearng capactes f the ples are properly prouce an nstalle. The couplng was acheve by wele jonts. - 3 m. -.3 m.5. renforce concrete slab ple.5. Fgure 3. Ple supporte embankment wth concrete slab on top of the ples. Fgure. Contnuous flght augers an centrfugally precast concrete rven ples. In orer to etermne the actual bearng capacty of the ple founaton, ple loa tests on rven ples as well as on auger ples wth lengths of 5 35 m each were conucte (Rathel et al. ). A problem s the group effect of the ple founaton, ue to the small separaton of the ples (stance of. to m). Therefore the settlements of the ple group are ncrease compare to the result of sngle-ple tests. The group effect of the ples was analye by snge ple tests an specal calculaton programs for efnton of the group effect. Further nformaton can be foun n Rathel et al. (). STANDARD CONSTRUCTION - REINFORCED CONCRETE SLAB Normally a renforce concrete slab wth a thckness of about 5 cm on top of the ples for a safe loa Fgure. Constructon of retanng walls. 5 HORIZONTAL GEOGRID REINFORCEMENT ON TOP OF THE PILES 5. General In recent years a new kn of founaton, the socalle geosynthetc-renforce an ple-supporte embankment (GPE) was establshe. Above the ple heas, the renforcement of one or more layers of geosynthetcs (mostly geogrs) s place, see Kempfert & Rathel (5). The applcaton of such solutons s recently evelope n Germany, see Alexew & Vogel (). In the followng the man prncples of calculaton an esgn an afterwars the constructon n Bejng are shown.

3 5. Calculaton an Desgn The stress relef from the soft sol results from an archng effect n the renforce embankment over the ple heas an a membrane effect of the geosynthetc renforcement, see Fgure 5 (Kempfert et al. ). Sol archng Geosynthetc layer behaves lke a tensone membrane Counter pressure from the soft sol Ple-element Bearng stratum Fgure 5. Mechansms of loa transfer an nteracton. Due to the hgher stffness of the columns n relaton to the surrounng soft sol, the vertcal stresses from the embankment are concentrate on the ples. Smultaneously sol archng evelops as a result of fferental settlements between the stff column heas an the surrounng soft sol. The 3D-arches span the soft sol an the apple loa s transferre onto the ples an own to the bearng stratum. The stress strbuton can be moelle n varous ways. Fgure shows, for example, a system consstng of several archng shells (Zeaske, Zaeske & Kempfert ). δ s ' n δφ δφ δ l r r k 3 n s p h Ths moel leas to a fferental equaton, whch s a functon of the escrbe vertcal stresses σ [] n the archng system (Zaeske ): σ Au+ ( σ + σ ) Ao () σ Φ δφ As sn m + γ V = For the areas above the arches a loa epenng stress strbuton s assume. The effectve stress on the soft sol stratum σ o results from the lmtng value conseraton wth t = heght of the loa epenng arch, so the followng equaton can be formulate. Smplfe σ o can also be erve from mensonless agrams (DGGT, 3). σ o h = λ χ p γ + h χ ( ) t λ λ + t λ + t λ + ( λ + t λ ) wth: (Kkrt ) χ = λ s χ s + s, λ = s χ () ϕ' λ = (s ), K krt = tan 5 + The loang of the renforcement s expresse by the fferental equaton of the elastc supporte cable, n whch the vertcal splacement W over the soft sol an the horontal force H, (Zaeske & Kempfert ) are the unknown varables, see Fgure 7. q k = + x H H wth: w, sw, w H = + (3) ' x + + ' x l ( + W ' ) x + ( + P ' ) W j j P x J s ' Geogr q,p s q,w H σ + σ A L x S δ φ m σ φ A s γ A o σ φ δ φ m y b Ers. Ple k s,p k s,w s - b o u ro σ x Fgure 7. Loang of the renforcement. s δ φ o r u Fnally the loang of the renforcement S can be calculate rectly as a functon of the elongaton ε (J = stffness) of the geosynthetc (for mensonless agrams see DGGT 3): δ φ u Fgure. Theoretcal archng moel. S[x] = ε [x] J = H + ' [x] ()

4 5.3 Sol archng uner cyclc loang Whereas the system behavour (sol archng an membrane effect n geosynthetc renforcement) uner statc loang s well-known, the bearng behavour an the settlements expecte uner cyclc loang are not yet fully unerstoo. Uner cyclc loang the archng effect can only be forme n a very lmte extent an part of the loa carre rectly by the ples can ecrease remarkably, whch results n an ncrease of the loa on the soft sol an on the renforcement. Due to the reucton of the sol archng, the strans n the geogr an the surface settlements ncrease conserably. Base on the results of moel tests an numercal nvestgatons, the man parameters whch cause a reucton of the archng effect have been entfe (Het ). Crtcal geometrcal an loang lmts were worke out, see Table. For system values beyon these lmts a mofe calculaton proceure s propose whch takes a sol archng reucton an an ncrease of the geosynthetc strans nto account (Het ). Table. Smplfe requrements for conseraton of cyclc effects on archng. reucton of Cyclc loang Geometry ) archng Large (ralway >.5 neglgble - about 3 kpa).5 partly up to complete Mle (roas - >.7 neglgble about 5 kpa).7 partly ) see Fgure 5 5. Constructon For the constructon of the ralway lne from Bejng to Tanjn a geosynthetc-renforce an plesupporte embankment (GPE) was bult n the cty area of Bejng. Fgure shows a pcture of the embee geogr. A schematc vew of the ple supporte embankment wth the geogr renforcement s shown n Fgure 9. Axs embankement fll top gr layer bottom gr layer Fgure 9. Geosynthetc-renforce an ple-supporte embankment Bejng. CEMENT STABILIZATION OF THE EMBANKMENT MATERIAL. Calculaton an Desgn In sectons wth very low embankments a cement stablaton of the embankment materal nstea of the geosynthetc renforcement s more reasonable, ue to the reucton of the archng effect an the ncrease of the forces n the geogr renforcement by a hgh cyclc loang. The cement stable embankment acts smlar to a slab over the ple heas, whereas the expensve renforcement can be save. For the esgn of the cement stable embankment a Fnte Element Moel FEM was use, conserng a so calle unt cell, whch means the conseraton of one ple wth the surrounng sol (nfnte ple gr) n an axally symmetrc moel. Fgure shows the stuaton before fllng. A schematc vew of the ple supporte embankment wth the cement stablaton s shown n Fgure. ple.. ~ m ~. m Fgure. Embee geogr. Fgure. Stuaton before fllng.

5 3.5 m Axs cement stable embankement fll December 7 the slab track was nstalle n all parts of the ralway lne. Fgure 3 shows typcal results for the settlements at fferent heghts n a cross secton wth a renforce concrete slab an Fgure the settlements of a secton wth a horontal geogr renforcement on top of the ples nstea of the concrete slab. In these sectons the embankment s ultmately about.5 an 5. m hgh (followng removal of the 3.5 m temporary loa), respectvely. ~ m.5.5 Fgure. Cement stablaton n Bejng. 7 MONITORING 7. Montorng concept an Measurement cross sectons Due to the settlement requrements a montorng program for verfcaton of the esgn an certfcaton of stablty an servceablty was nstalle. The fferent montore cross sectons have stances between 5 an 5 m. A large quantty of vertcal an horontal nclnometers an geophones were nstalle. Atonally, the settlements of the rals were measure. In a typcal measurement cross sectons one horontal nclnometer an up to four settlement plates are use for the examnaton of the eformaton behavour, see Fgure. In Fgure atonally the temporary overloa for mnmaton of the settlements after the constructon tme can be seen. Axs ple fll heght [m] settlement [mm] measurement calculaton ays [] Fgure 3. Settlements n a secton wth a concrete slab on top of the ples. fll heght [m] settlement [mm] measurement calculaton ays [] Fgure. Settlements n a secton wth horontal geogr renforcement on top of the ples. 3.5 m settlement plate temporary loa embankement fll top gr layer bottom gr layer tube for hyrostatc settlement montorng The settlements n the secton wth a cement stablaton of the embankment materal nstea of the geosynthetc renforcement or a concrete slab are very low (up to mm) ue to the very low embankment. Therefore these measurements are not comparable to the settlements shown above. Fgure. Typcal measurement cross sectons. 7. Measurement results In January, measurements ha been performe for about to months. The long-term montorng has confrme the stablty an servceablty. In CONCLUSION Accorng to the experences urng the constructon of the ralway lne Bejng-Tanjn an the experences n Germany a geosynthetc renforcement as well as a cement stablaton of the embankment

6 materal can be use nstea of a concrete slab to guarantee a suffcent loa transfer an strbuton. Base on German an nternatonal experences wth geogr-renforce ple-supporte embankments, practcal reasons, expermental results an the valty of the analytcal moel the followng recommenatons are establshe: The center-to-center stance s (see Fgure 5) of the ples an the ple ameter (ple caps) shoul be chosen as follows: - (s - ) 3. m: n the case of statc loas - (s - ).5 m: n the case of heavy lve loas - / s.5 - (s - ). (h - ) The stance between the renforcement layer an the plane of the ple/column/wall heas shoul be as small as possble, n orer to acheve maxmum effcency of the geogr membrane. However, t s recommene to have a safe stance (nterlayer) between the lowest renforcement an the ple heas n orer to prevent a structural amage of the renforcement because of shearng at the ege of the ple heas: - maxmum two renforcement layers - h geo.5 m for sngle layer renforcement - h geo.3 m for two layers renforcement - for two layers the stance between the geogr layers shoul be 5 to 3 cm - esgn value of the tensle strength R B 3 kn/m; ultmate stran %. - overlappng s generally allowe, but only just above the ple (caps) an only n the seconary bearng recton; length of overlappng. 9 REFERENCES Alexew, D. & Vogel, W.. Ralroas on ple embankments n Germany: Mlestone projects, Lanmarks n Earth Renforcements. In Ocha et al. (es), Swets & Zetlnger: 5-9. Empfehlung.9 3. Bewehrte Erkörper auf punkt- oer lnenförmgen Traggleern. Kaptel.9 für e Empfehlungen für Bewehrungen aus Geokunststoffen, EBGEO, DGGT (German Geotechncal Socety), raft. Het, C. & Kempfert, H.-G. 7. Bewehrte Erkörper über Pfählen unter ruhener un nchtruhener Belastung. Baungeneur, Ban : Kempfert, H.-G., Goebel, C., Alexew, D. & Het, C.. German recommenatons for renforce embankments on ple-smlar elements. Thr European Geosynthetc Conference, DGGT (German Geotechncal Socety), Volume : Kempfert, H.-G. & Rathel, M. 5. Sol Improvement an Founaton Systems wth Encase Columns an Renforce Bearng Layers. In Groun Improvement Case Hstores E. Buhma Inraratna an Chu Jan. Elsever B.V. Rathel, M., Krchner, A. & Knessl, A..Grünung er Hochgeschwngkets-Teststrecke Bejng-Tanjn auf Grunlage von verglechenen Probebelastungen an Großbohrpfählen, Rammpfählen un SOB-Pfählen. In Vorträge er Baugruntagung n Bremen, DGGT (German Geotechncal Socety). Rathel, M., Krchner, A. & Kempfert, H.-G.. German Recommenatons for renforce Embankments on Plesmlar elements. Proceeng of the th Asan Regonal Conference on Geosynthetcs, June 7 -, Shangha. Zaeske, D.. Zur Wrkungswese von unbewehrten un bewehrten mneralschen Tragschchten über pfahlartgen Grünungselementen. Schrftenrehe Geotechnk, Unverstät Kassel, Heft. Zaeske, D. & Kempfert, H.-G.. Berechnung un Wrkungswese von unbewehrten un bewehrten mneralschen Tragschchten auf punkt- un lnenförmgen Traggleern. Baungeneur, Ban h 3 hgeo tw Renforce embankment Geosynthetc layer Subgrae Softsol Bearng stratum Ple Bass of the Embankment Lve loa sx s sy Fgure 5. Geosynthetc-renforce ple supporte embankment (GPE) Defnton of parameters