Prefabricated Vertical Drain(Pvd)

Transcription

1 Internatinal Jurnal f Engineering Research and Develpment e-issn: X, p-issn: X, Vlume 10, Issue 2 (February 2014), PP Prefabricated Vertical Drain(Pvd) Sharath Paul 1, Eldh Varghese 2, Leni Stephen 3 B.Tech Student, Department f Civil Engineering, M A Cllege f Engineering, Kthamangalam, Kerala, India 1 Assistant Prfessr, Department f Civil Engineering, M A Cllege f Engineering, Kthamangalam, Kerala, India 2 Assciate Prfessr, Department f Civil Engineering, M A Cllege f Engineering, Kthamangalam, Kerala, India 3 Abstract:- At times, engineers are required t build n sites cntaining sft cmpressible sils in the cnstructin f buildings, infrastructures and ther structures. In such instances, it wuld be necessary t imprve the sft cmpressible sils befre cnstructing any structures n them.in saturated sils such as clay and silty clays, which have a large percentage f vids r pres usually filled with water, large settlements will ccur ver a lng perid f time. During this prcess called cnslidatin, the lad is gradually transferred t the sil particles as the vlume f the vids are reduced and this culminates in the frm f settlement. Presently, there are varius methds available t imprve the sft cmpressible sils. Sil imprvement with Prefabricated Vertical Drains (PVD) r smetimes knwn as wickdrains, is amng the mst ecnmical methd t expedite the cnslidatin prcess. I. PVD A SOLUTION FOR SLOW CONSOLIDATION CONSOLIDATION ccurs very slwly in saturated, fine grained sils because the lw permeability f these sils impedes the escape f pre water frm the sil vids. Even under large temprary surcharge lads, settlements can take many years because f this slw water mvement and the great distance the water must mve t exit the sil. Withut sme srt f sil treatment cnstructin is usually impractical due t unpredictable lng-term settlement. T achieve rapid cnslidated settlement it is necessary t create shrter paths fr mvement f the water. II. THE SOLUTION Since early 1970s prefabricated vertical drains (cmmnly called PVD, band drains r wick drains) have been used all ver the wrld t accelerate the cnslidatin f fine-grained sils. In India als PVDs have been used in several prjects especially in prts and infrastructure prjects in castal areas. The installatin f PVD creates artificial drainage paths, thereby reducing the distance the water must travel t exit the zne f grund subjected t excess pre-pressures and thus greatly accelerating the cnslidatin rate. III. INSTALLATION OF PVD A vertical drainage system can be installed in rder t accelerate the cnslidatin prcess and help t imprve discharge f excess pre pressure in the sub sil. In this way, a stable situatin is reached mre quickly, thus shrtening the filling time dramatically. At the same time, the settlement prcess is accelerated t such an extent that earthwrk finishing ff prcesses can be started earlier. All f this results in a substantially shrter cnstructin time which may be imprtant bth ecnmically and scially. The installatin f vertical PVC drains is usually dne with the aid f a dragline fitted with tams alng which the lance can mve up and dwn vertically. The length f drain t be installed determines the minimum length bth f the equipment and f the lance. The drains are wrapped arund a cil which is placed in a husing in the rig. Frm there, the drain passes ver a travel rller until it feeds int the tp f the rig and then via a secnd travel rller thrugh the lance dwnwards. Beneath the lance, a small anchr plate is secured t the drain. The lance is then installed t the desired depth either by vibratin r with the help f a static pressure and tensiner. Any slid layer encuntered can be pre-jetted. The anchr plate ensures that the drain, nce t depth, remains at that depth whilst the lance is being withdrawn. The drain is then clipped ff abve grund level, after which the entire prcedure can be repeated all ver again at the next drain lcatin. Depending upn sil cnditins, the length & width f the drain strip, an utput f 2000 t 6000 linear m per eight hur wrk day per rig can be achieved 50

2 Fig. 1 Installatin equipment fr wick drains. 3.1 Sequence f vertical drainage wick installatin All vertical drainage wicks will g t maximum allwable/anchrable depths r until refusal as per fllwing methd :- Thread the vertical drainage wick ff the wick rll/spl, up the wick tube, and ver the tp wick rller and dwn thrugh the mandrel. b) Place the drainage wick thrugh r arund the anchring device and tuck the lse end f the wick up int the mandrel abut 6 t 8 inches (150 t 200 mm). Pull the drainage wick s excess slack tight thrugh the mandrel and vertical drainage wick tube by reversing the vertical drainage wick rll by hand.by reversing the vertical drainage wick spl r wick rll, the anchring device will retract up tight against the bttm tip f the mandrel. This will prevent dirt r mud frm entering the mandrel during the insertin f the mandrel int the grund. c) Mve the machine/mandrel t the specified vertical drainage wick lcatin and insert the mandrel with anchring device in place using static frce (and /r vibratry frce if necessary) int the grund t the desired depth. d) Extract the mandrel, leaving the anchring device and the cmpleted r installed vertical drainage wick in place, uncntaminated and the prper depth. e) Cut the vertical drainage wick ff the cntract-specified length abve the wrking surface. Check the vertical drainage wick installatin machine mast t make sure it is plumb. Use hydraulic cntrls t crrect if nt within specificatin. 51

3 Fig. 2 Installatin f drain. 3.2 Splicing vertical drainage wick Vertical drainage wick is supplied n wick rlls. Each wick rll will hld abut 1000 ft. f drain r 305 m. Once the wick rll is used ff, a splice is necessary t add the next wick rll. T splice, cut the end f the previus wick rll at an angle and stuff it inside the end f the new wick rll. Then staple them bth tgether as shwn in figure. Fig. 3 Splicing f wick drains IV. PVD CONSOLIDATION PROCESS In saturated sils such as clay and silty clays, which have a large percentage f vids r pres usually filled with water, large settlements will ccur ver a lng perid f time when a lad such as a rad embankment is placed n tp f the sil. The lad will result in an initial increase in prewater pressure which will dissipate slwly as the prewater drains ff. During this prcess called cnslidatin, the lad is gradually transferred t the sil particles as the vlume f the vids are reduced and this culminates in the frm f settlement. 52

4 4.1 ACCELERATION OF CONSOLIDATION USING PVD Due t the very lw permeability f clay sil and silty clay sil, the cnslidatin prcess will take many years. In rder t accelerate and reduce the cnslidatin time, vertical drains are installed at regular clse spacing t the full depth f the cmpressible layer, t create artificial and shrter hrizntal drainage paths fr the pre water t drain ff. In additin, this will als take advantage f the higher permeability f the sil strata r sand lenses in the hrizntal directin. During this accelerated prcess, the sil will gain an increase in shear strength. V. WORKING OF PVD AND ITS MAKE 5.1 Hw des PVD wrk? By installing PVDs at a suitable spacing, the distance that water has t travel thrugh the lw permeability sil is reduced cnsiderably and cnsequently water drains ut in a relatively very shrt perid f a few weeks r mnths. Band drains can be installed vertically t depths exceeding 65 metres. The drains are usually placed in a square r triangular cnfiguratin f 1 t 1.5 m depending n the desired cnslidatin time. Under excess hydrstatic pressure, water has nly t travel the hrizntal distance t the nearest drain t reach a free drainage path. Cnsequently, the higher hrizntal permeability f the clay is als taken t advantage. The water flws thrugh the filter fabric f the drain and int the channels f the drain cre where it can flw vertically ut f the sil. This flw may be either up r dwn t intersecting natural sand layers r t the surface where a sand drainage blanket r prefabricated strip drains are prvided. Vertical drains are nrmally used in cnjunctin with prelading with surcharge embankment r vacuum pressure. 5.2 Prefabricated Vertical Drains (PVD) Prefabricated vertical PVC drain can be defined as any prefabricated material r prduct cnsisting f a synthetic filter jacket surrunding a plastic cre. Because f their shape,. they are als knwn as band r wick drains. They are manufactured in rlls f m and are inserted int grund t required depths using special drain stitcher rigs. Generally, installatin takes place up t full depth f cmpressible sils. PVDs have replaced cnventinal sand drains fr sil cnslidatin due t their easy & speedy installatin and unlike sand drains, they act as a integral unit during the prcess f cnslidatin. Prefabricated PVC wick drain 5.3 Cmpnents f Vertical PV Drains 53

5 There are tw cmpnents f pre-fabricated PV drain namely, cre and filter jacket. By cmbining the features f bth cre and filter jacket, pre-fabricated PV drain system prvides effective, fast and reliable perfrmance fr sil imprvement Cre It is als called drain bdy which is a unique, crrugated, flexible and made f plyprpylene specifically designed t prvide high discharge capacity, high tensile and cmpressive strength Filter jacket It is strng and durable nn-wven, thermically bnded plyprpylene fabric wrapped arund the cre. The fabric is f randm texture having high tensile strength, high permeability and effective filtering prperties. It acts as a filter t allw passage f grund water int the drain cre while eliminating mvement f sil particles and preventing piping. It als serves as an uter skin t maintain the crss sectinal shape and hydraulic capacity f the cre channels. VI. BENEFITS OF USING PVD High water discharge capacity t ensure sund safety factr The unique and flexible cre will nt pinch ff r flatten when the drains bend and flds during the cnslidatin f the sils High cmpressive strength f the cre t prevent the cllapse f the flw path Deep installatin exceeding 40m depth Light weight Custmized cre and filter jacket t suit particular sil cnditins Minimum disturbance t the sil during installatin Fast installatin Shrt cnslidatin perid Well prven perfrmance in many prjects under different sil cnslidatin. Gewrks PVD has been supplied and installed in varius cuntries in Asia. The type f prjects where it has been used successfully are: Cnstructin f rads, highways and expressways Treatment f ex-mining areas fr husing develpment Cntainer yards and harbr cnstructin Land reclamatin fr the cnstructin f airprt runways and buildings Cnstructin f tank farms Cmmercial develpment f ex-agriculture land. VII. CONCLUSION The technlgy surrunding the use and applicatin f prefabricated vertical drain is cntinually grwing. The cncept f vertical drainage utilizing prefabricated drains has been applied t many nntraditinal applicatins. The perfrmance f embankments cnstructed using prefabricated vertical drains (PVDs) and gesynthetic reinfrcement has been theretically examined. A tentative design prcedure has been develped t cnsider the benefits arising frm the use f bth PVDs and reinfrcement. The fllwing cnclusins are based n the analyses reprted herein. Partial cnslidatin f the fundatin sil during embankment cnstructin is significant because f the drainage imprvement prvided by prefabricated vertical drains and fundatin sils usually becme nrmally cnslidated at the end f embankment cnstructin. Cnsequently, the shear strength gain f sft fundatin sil due t partial cnslidatin can substantially imprve the stability f embankments such as thse examined. The rate f increase in shear strength f fundatin sil is a functin f cnstructin rate fr a particular reinfrced embankment and PVD system. Where PVDs are installed, the strength gain is relatively unifrm thrughut the thickness f the depsit and can be reasnably predicted using the prpsed methd. Fr a fundatin installed with PVDs, the embankment design based n assuming undrained cnditins and neglecting the shear strength increase in the fundatin sil during cnstructin is t cnservative. 54

6 REFERANCES [1]. Getextiles and Gemembranes 34 (2012) Degree f cnslidatin f clayey depsit with partially penetrating vertical drains C.Y. Ong, J.C. Chai, T. Hin Article histry: Received 7 February 2011 Received in revised frm 22 January 2012 Accepted 6 February 2012 jurnal hmepage: [2]. Numerical Mdeling Of Flating Prefabricated Vertical Drains In Layered Sil, Ikhya And Helmut F.Schweiger,Acta Getechnica Slvenica, 2012/2 [3]. IOSR Jurnal f Mechanical and Civil Engineering (IOSR-JMCE) ISSN: Vlume 4, Issue 1 (Nv. - Dec. 2012), PP [4]. A Cmparisn between Hrizntal and Vertical Drainage Systems (Include Pipe Drainage, Open Ditch Drainage, and Pumped Wells) in Anistrpic Sils Mhammad Valipur. [5]. Measured and predicted perfrmance f prefabricated vertical drains (PVDs) with and withut vacuum prelading,j. Sawapakpibn, Getextiles and Gemembranes 28 (2010)Thailand [6]. Getechnical Engineering fr Transprtatin Infrastructure, Barends et al. (eds) 1999 Balkema, Rtterdam.Settlement predictin f embankments stabilised 55