VIBRO-TECHNIQUES FOR GROUND IMPROVEMENT

Transcription

1 VIBRO-TECHNIQUES FOR GROUND IMPROVEMENT by Juan Manuel Rdríguez 1 ABSTRACT The Suth American cnstructin industry has been slwer than many f its Eurpean cunterparts t realise the technical and ecnmic advantages that vibr-techniques can prvide. In places where there is an abundance f sft alluvial and estuarine sils, many f these may be imprved sufficiently by vibr-techniques t allw standard fundatins t be cnstructed at relatively shallw depth, withut the need t resrt t deep piling. Where grund cnditins are suitable, the vibr-techniques have been shwn t be mre cst effective than traditinal fundatin techniques. While ther grund imprvement techniques, such as surcharging and band drains, can seriusly delay a cntract prgramme, vibr-techniques generally d nt prvide such a stumbling blck. Keywrds: grund imprvement, earthquakes, liquefactins, sft grund, vibr-techniques 1. INTRODUCTION Grund imprvement techniques are widely adpted in Eurpe, Asia and USA and are increasingly being used in Suth America and wrld-wide fr a vast variety f prjects with particular emphasis n infrastructure prjects such as prts and harburs. Grund imprvement schemes wrk with the existing grund rather than bypassing it. They can be tailred t meet specific bearing capacity and/r settlement requirements and are generally much mre cst-effective than ther traditinal methds such as piling. The prcess: Reduces fundatin settlement Increases bearing capacity Mitigates liquefactin ptential Prvides slpe stabilizatin Prevents earthquake-induced lateral spreading Alng this dcument, will be described sme f the mst used grund imprvement methds. These techniques, knwn as vibr-techniques are Vibr Stne Clumns and Vibrcmpactin with their different methds f applicatin. Vibr techniques cnsist f large depth vibratrs, typically abut 300 t 500mm in diameters, which penetrate the grund t suitable depth and then cnstructing cntinuus clumns up t the grund surface. Depending n sils type and water level table we can differentiate between Vibr Replacement, Stne Clumns frmed by wet technique, and Vibr Displacement, Stne clumns frmed by dry technique. Vibr stne clumns are cnstructed within the seabed where the added granular material is cmpacted using depth vibratrs. These relatively rigid clumns f stne then frm a cmpsite structure with the surrunding sils t achieve increased strength and reduced settlements under applied lad. The stne clumns develp their capacity frm the cnfining actin f the sils in which the clumns are cnstructed. Stne clumns are inclusins f material with high angle f frictin that can be used t enhance the stability f slpes. When the sils are f a clean granular nature, they will be densified by the hrizntal vibratins t prvide even greater clumn rigidity. The applied lads are then supprted by an array f generally strnger stne clumn springs surrunded by weaker springs representing the sil. The density f 1 Marine Cntracts Engineer, Pennine - BBGE, England, juan.rdriguez@pennine.c.uk 1

2 clumn cnstructin grid centres t residual vlume f sil will then dictate the degree f imprvement that can be achieved. Vibrcmpactin knwn als as Vibrfltatin in sme references and cuntries cnsist n the in-situ densificatin f lse granular sils, such as lse sand, gravel and hydraulic fills, using similar equipment but withut the additin f granular material. 2. VIBRO-TECHNIQUES 2.1. Fur basic systems available: 2.1.1) Wet Tp-Feed r Blanket Feed Methd The wet tp-feed system is the riginal cnstructin methd which has been largely replaced by the ther systems fr land based prjects but cntinues t be the mst useful system fr marine wrks. The wet tp-feed system (Figure 1) can be used fr the imprvement f sft sils belw the water table where larger clumn diameters are required. A layer f granular material is depsited ver the seabed layer; the vibrflt is lwered trugh this layer while the penetratin is assisted by water flushing. On attaining the required depth the vibratr can be partially withdrawn t then surge and flush ut t further increase the clumn diameter. Fllwing frmatin f the pen hle the water pressure is reduced and vibratr kept in the grund. Clean inert granular material was placed n the tp at seabed level. This then falls under gravity against the slight upward flw f water in the annulus between sides f vibratr and sil dwn t the base f the bre. The vibratr is then slightly withdrawn and pushed back int the stne at the base f the bre t cnstruct the stne clumns in a series f lifts. Figure 1: Blanket feed methd It is hwever imprtant that the water flw be maintained t ensure stability f the sils and t avid ptential cntaminatin f the stne clumn. The wet tp-feed system is nrmally used when imprving sils abve, and belw the water table f greater than abut Cu = 15kPa sil strength (Greenwd & Kirsh, 1983). Typical grading f the granular material is abut 20 t 75mm with stne clumn diameters f abut 600 t 1000mm being cnstructed. 2

3 2.1.2) Wet Bttm-Feed Methd ( Aquacaster System) The wet bttm-feed system cmbines the benefit f imprving very weak sils belw the water table with all the added advantages f the riginal and well prven water-flush cnstructin technique with the added ability ver dry bttm-feed t surge fr larger clumn diameter (Mseley & Priebe, 1993). When perfrmed ver water frm a barge the disadvantages encuntered n land such as prvisin f an adequate wrking platfrm, water supply, drainage ditches, settlement lagns, dispsal f effluent and cntaminatin f the site surface disappear. The Aquacaster vibrflt is specially adapted fr marine prjects. The vibrflt is penetrated t the required design depth and nce design depth is reached, water jetting is used t flush away lse materials and clear a 1m diameter cavity. As the water flush clears the cavity the aquacaster s hpper gate is pened t deliver a charge f stne that exits frm the tip f the vibrflt. The vibrflt is then re-inserted t cmpact the stne tightly int the seabed, befre being lifted slwly back t the surface. The sequence is repeated with the Aquacaster re-inserted and withdrawn until a dense stne clumn has been frmed t the surface as shwn n Figure ) Marine Vibrcmpactin Figure 2: Aquacaster System Shck waves and subsequent scillatin cause lse sands t act like a liquid - a prcess referred t as liquefactin where sils spread apart quickly and bearing capacity is severely reduced. Vibrtreatment can mitigate these effects by imprving sil density and cmpacting lse sands and granular particle. Vibr cmpactin is an established technique fr stabilising fine granular sils prevalent in marine envirnments, and is especially effective fr sand cmpactin (Figure 3). The methd is highly effective fr land reclamatin prjects and mitigating the risk f liquefactin in areas f the wrld subject t seismic activity. 3

4 Figure 3: Sand cmpactin With the vibratr mtr running, water is discharged frm the vibratr nse, and the vibratr slwly lwered t the required design depth under its wn weight. In the immediate vicinity f the vibratr the sil is saturated and liquefies lcally and temprarily under the influence f vibratins, the sils particles surrunding the vibratr rearranged free frm stress t a dense state f cmpactin as shwn schematically n Figure 4. Figure 4: Marine vibrcmpactin The area f sil affected by the vibratins, increase t a maximum value, depending n sil prperties such as particle size distributin and particle shape. This technique became increasingly mre difficult as the cntent f silt and clay fines increase and culd nly achieve the densificatin target if the fines cntent is nt higher than 10-12%. Brwn (1977) intrduced a suitability number (SN) t identify the sands that are suitable t treat by vibrcmpactin in relatin with fines cntents. 4

5 2.1.4) Dredged Trench Methd Fr marine structures requiring large bearing capacities and settlement perfrmance Pennine can install stne clumns within dredged grund t prvide additinal densificatin prir t placement f cncrete caissns. A trench is dredged frm the seabed t remve sft sils and sands befre being replaced with crushed rck. As shwn n Figure 5, this crushed rck fill is then densified t prvide a slid fundatin layer t supprt future marine structures. Figure 5: Dredged Trench Methd An verbuild layer f aggregate ballast is firstly depsited abve the filled trench befre a vibrflt is penetrated thrugh the verbuild stck and int the rck mattress. The vibrflt is lwered int the water and allwed t penetrate t the base f the rck mattress, with water jetting frm the tip f the vibrflt used t aid penetratin. The vibrflt penetrates t the required depth, and then is slightly withdrawn and pushed back int the stne at the base f the bre t cnstruct the stne clumns in a series f lifts in similar manner t the tp-feed system. By repetitin f this prcess, stne clumns are cnstructed within the trench t prvide a densified layer f rck mattress and the increased bearing capacities required fr large marine structures. 2.2) Vibr Design On any f the vibr techniques explained abve, the degree f imprvement will depends n many mre factrs including: sil cnditins, type f vibr equipment, cranes used, prcedures adpted, day & night shifts, and skills f site staff. The sil being treated, the degree f densificatin, the settlements required, lads, type f vibratr, prductin rates all have an influence n spacing required t use between clumns. The different design methds differ mainly n the evaluated imprvement results, sme f these methdlgies were set by Greenwd (1970), Priebe 1976, 1995), Balaam & Bker (1981), Van Impe & De Beer (1983), Balaam & Puls (1983) and Greenwd & Kirsch (1983). 5

6 Exist a wide range f sils where this techniques wrks successfully (Figure 6) Figure 6: Stne Clumns Vs Vibr Cmpactin 2.3) Equipment All fur systems may be perfrmed using similar types f hydraulically driven depth vibratr (Fig 7 & 8). The eccentric weight assembly is hused in the base f a heavy tubular steel casing with tapered nse t assist penetratin and vertical fins t prevent rtatin as shwn in the pictures belw. Figure 7: Aqucaster Vibrflt Figure 8. Series BD400 Steel fllwer tubes are then added t extend the length f the vibratr up t 60m. The vibratrs are suspended frm crawler cranes wrking frm barges r attached t the Pennine purpse built Stratacaster r Terrafirmer crawler rigs. 6

7 All the rigs can be equipped with the Pennine data lgger cmputer t mnitr each clumn installatin (Fig 9). Figure 9: Pennine Data Lgger System The lgger captures depth and hydraulic pressures. This data is prcessed in a lcal PVA database that allws QA/QC (Quality Assurance / Quality Cntrl) t verify that the cnstructins sequence fr each individual clumn was fllwed and prviding detailed recrds f the cnstructin (Fig 10). Figure 10: Clumns installatin Graphs 2.4) Testing The densificatin can be readily checked using standard penetratin test r cne penetratin test, cmparisns can be made between pre- and pst-cmpactins testing (Mseley & Priebe, 1993). Pennine can undertake cne penetratin testing (CPT) as a fast and safe methd t verify design accuracy. As with all cnstructin prjects, a gd site investigatin is essential, with infrmatin n cmpressibility and strength f the sils being particularly imprtant fr grund imprvement design. Gd supervisin and experienced site persnnel are essential t ensure that grund imprvement design is implemented crrectly in the field. 4. REFERENCES Greenwd, D.A. and Kirsch, K (1983), State f the art reprt n specialist grund treatment and vibratry and dynamic methds. Prceedings f the cnference n piling and grund treatment. Lndn, Thmas Telfrd Ltd. pp Ralph E. Brwn (1977), Jurnal f the Getechnical Engineering Divisin, Vibrfltatin Cmpactin f Chesinless Sils Mseley M.P. and Priebe H.J. (1993), Vibr techniques In Grund Imprvement (Mseley M.P. (ed.)). Blackie Academic and Prfessinal, Lndn, pp