Gahir, J.S.

More documents

Recommendations

Info

MAX. DSP=130mm MAX. DD=4.9mm WLD -15.9DMD CRESCENT NW SW SE MAX. DSP MAX. DD WLD Figure 4. Dewatering regime and cofferdam monitoring points. procured from Japan and had a2to3month delivery time. Used PU25 sections of some 10 m length were purchased locally and taken to an onsite yard at the top of the Spine where 20 m lengths were assembled. To determine the sheet pile drivability a trial was undertaken near the Spine portal close to a borehole location using PU series. The piles were spliced and driven to depths of 33 m using vibrohammer ICE815C (1250 kN max centrifugal force) without difficulty. The newly purchased SX27 sections were taken to Hamriya Port, about 35 km east of the project site, where they were chamfered and welded to form 30 m lengths. Hyperseal DPS-1000 was applied to the pile clutch. This material can expand up to 10 times its original volume in fresh water but its performance offshore is curtailed due to the salinity of sea water and it expands to half the figure. The piles were paired to form 1.2 m wide panels, so that each prepared offshore panel comprised of 4 sheet piles, before being barged to site. 5.3 Onshore sheet piling Sheet pile sections, 20 m long and 0.6 m wide, were pitched into a guide frame capable of accommodating 16 sheet piles and driven one by one with a 1.5 m stick up. Longer sheet piles could not be accommodated due to limitation of the crane boom. Ten metre long pile sections were hooked through the guide frame and spliced to the driven piles. Driving recommenced using vibrohammer ICE815C and progress was slow due to difficult driving. Some hard nodules present within the calcarenite and the wear and tear on the used piles were felt to contribute to difficult driving. Additionally two splices in the desired pile length did not always provide a plumb vertical section and this led to an increase in the frictional force in the pile clutch. At some locations especially on the Crescent piles were terminated 5m short of the required design penetration due to refusal with the driving system. A reassessment of the seepage inflow into the cofferdam was undertaken using k of calcarenite 2×10 −5 m/s which indicated that the anticipated inflow in this area increased 9 times locally with the nett effect that the overall infow into the cofferdam after draining increased from 820 m 3 /hr to 1600 m 3 /hr. To cope with the additional inflow contingency measures were put in place and comprised of (a) drive secondary short length sheet piles in local areas at the top of the tunnel trench excavation (b) increase the number of wells and (c) increase pump capacity. 5.4 Offshore sheet piling Offshore piling was undertaken from 2 barges, each with a 150T crane, and the prepared panels 30 m long and 1.2 m wide, were driven through a guide frame using a double clutch vibrohammer ICE1412 (2300 kN max centrifugal force). The contractor reported difficult driving conditions which required greater effort 1300
in getting the necessary penetration than had been anticipated initially. On average 2 to 3 panels were being achieved in a 12 hour shift. When the second barge, operating a week later on the opposite dyke, also experienced similar difficulties it was recognized that the problem was not local. Piling records indicated that the penetration rate decreased significantly below −17 mDMD in the second calcarenite layer. A review of the unconfined compressive strength results indicated weak rock and that although the presence of hard nodules may explain some of the driving difficulties, something fundamental had been overlooked. Pairing the piles doubled the area at the cutting edge and also doubled the friction from the fill and underlying rock, and this resistance could not be overcome by simply doubling the hammer capacity required for a single pile section.The decision to pair the piles was taken after discussion with a local piling contractor who had successfully driven paired piles but not to the depth required for this project. An offshore pile driving trial may have revealed the problem earlier, and more work in this area would have been useful. To overcome the difficulties in the short term predriving was undertaken with a panel connected to a water jetting system to −25 mDMD and withdrawn, and the final 1 m was driven with the permanent pile. Following the initial success with water jetting, a decision was taken to weld four 25 mm diameter steel pipes in the corners of all the sheet piles along its total length and connected to 2 mm diameter jetting nozzles located near the toe of the sheet pile. The pipes were hooked to a high pressure pump capable of delivering 250 l/min at 40 bar pressure. Simultaneous water jetting and driving with the same hammer overcame the driving difficulties. To make up for lost time a third barge was mobilized and a night shift introduced. In 86 working days 1290 paired sheet piles were successfully driven to the desired toe elevation. Towards the end a production rate of 5 to 6 pairs was being achieved in a single shift. 5.5 Cofferdam closure Seven onshore and 2 offshore closures were necessary to complete the perimeter. Sheet pile sections driven from the opposite ends were overlapped so that an open box was formed. A uPVC tube was inserted from the top of the piles to key into the top of the dyke and grouted, thus sealing the open box. After setting, a borehole was drilled through the grout, the dyke and into seabed extending to the same elevation as the toe of the sheet piles. A rapid setting chemical grout, colloidal silica with sodium chloride, was then injected at 0.5 m centres working from the bottom up. The colloidal silica when set has a very low permeability, thus effectively sealing the closure point. The offshore closure was undertaken during a Neap tide within a 4 hour window. Due to an increased flow velocity some scouring of the dyke, up to 2 m deep over an 8 m long section, was observed by divers deployed periodically during the course of the works. The dyke was repaired after closure. 5.6 Draining Some 4,350,000 m 3 of water inside the cofferdam (excluding seepage) was pumped out in stages to reduce the water level from +1mto−23 mDMD. This was undertaken using 9 high capacity pumps each fitted with a flow meter, of which 7 were operational at one time, connected via valves and manifolds to the discharge pipes and over the northern cofferdam wall and to sea. The pumps were located on two floating pontoons placed centrally at the deepest section of the trench excavation. Discharge was completed in 45 days and the rate of water level drop was maintained at less than 1 m/day. Some leaks in the sheet piles joints were sealed using underwater welding and sealants. 6 IN-SERVICE PERFORMANCE AND MONITORING 6.1 Summary of instrumentation installed To provide information on sheet pile deflections, the deformation of the dykes, and water levels in the cofferdam 6 total station targets with a mirror attachment, 8 inclinometers, 16 observation wells with ground water loggers, and settlement monitoring stations were installed at strategic locations and depths. Base readings were established on top of the sheet piles before draining and the instruments were installed shortly thereafter. Temporary bench marks were installed at 50 m intervals on the shoulder of the dyke for monitoring settlements. Trigger and action values were identified for each item along with an action plan should the values be exceeded, and to provide an early warning system in the event that unanticipated conditions arise.Typical results are included on Figure 4 and an aerial view of the cofferdam is shown on Figure 5. 6.2 Strengthening works A maximum total deflection of some 1000 mm and a relative deflection of 285 mm was recorded along a 200 m long section at the end of draining, which were in excess of the action values set. The measured deflections, which incorporates pile rotation, is in an area where the original dredged channel was the deepest and the dyke height maximum. CPT testing indicated the presence of very loose material with entrapped silty clay for which a horizontal modulus of subgrade reaction of 2000 KN/m 2 /m was appropriate. A best fit analysis using a 2D FEM analysis with 1301
Page 1 and 2: Underground Space - the 4 th Dimens
Page 3: under the fill, and is suspected to

Gahir, J.S.

Create successful ePaper yourself

Delete template?

Save as template?