Gahir, J.S.

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Figure 2. Tunnel alignments. Figure 3. Typical cross section and ground conditions. 2 PROJECT DESCRIPTION The total length of the tunnel structure is 1398 m and comprises of a 241 m long Spine portal approach, 969 m of reinforced concrete box section and 188 m of Crescent portal approach. The horizontal and vertical alignments were dictated by a maximum grade of 6% for a 60 kph design speed, in compliance with Dubai Municipality Geometric Design Manual for Dubai Roads (1999), and the need to ensure that the tunnel was sufficiently deep to maintain a 125 m wide arc-shaped navigation channel with a minimum draft of 10 m, above the 1.5 m thick layer of armour protection on the roof slab. The vertical alignment and the horizontal alignment, which is a reverse curve with a central straight section, are shown in Figure 2. Mean sea level is +1.0 mDMD (Dubai Municipality Datum, which is 0.09 m below Admiralty Chart Datum) with a tidal range of 1.2 m between MHHW and MLLW. The reclaimed islands are at +3 m and +4 mDMD at the Spine and Crescent, respectively. Seabed elevations before The Palm reclamation were in the range −11 m to −12 mDMD. Pre-construction surveys indicated that the minimum seabed elevation along the tunnel alignment and the cofferdam region was −15 m and −19 mDMD respectively, in areas where materials had been won for breakwater construction. A Design and Build contract was awarded to Taisei Corporation in 2004 for the construction of the vehicular tunnel. 3 GEOLOGY AND GROUND CONDITIONS The local geology in the vicinity of the project site has been shaped by the current hot arid climate and by fluctuations in sea level and is described by <strong>Gahir</strong> et al. (2006). Ground conditions were determined by sinking 13 offshore boreholes to a depth of 30 m into the seabed and 3 boreholes up to 30 m depth on the onshore reclamation. The minimum elevation explored is −45 mDMD. The offshore boreholes were drilled along the alignment of the dykes and are offset up to 230 m from the tunnel centre line. All boreholes were grouted after completion. The boreholes revealed that under a 12 m thick recent fill a 1 m to 2 m thick band of weak caprock is present which in turn is underlain by 2 m to 4 m thick layer of medium dense sand. The sand overlies an 18 m to 22 m thick sequence of very weak to weak interbedded calcarenite and sandstone. A conglomerate layer whose upper horizon is some −35 mDMD is present across the site (Figure 3). The UCS of rock encountered is in the range 1 to 3.5 MPa. In isolated boreholes a thin layer of soft silt was recorded 1298
under the fill, and is suspected to be the original seabed. In-situ permeability of the rock units, measured using standard Lugeon tests and variable head permeability tests, indicated k values ranging from 4×10 −6 to 5×10 −4 m/sec with lower values at depth. 4 DESIGN CONSIDERATIONS The cofferdam is required for some 17 months and at peak periods a workforce of about 1000 was expected within its confines and thus safety was of paramount consideration in the design process. The cofferdam outline was dictated by the following considerations (a) the dykes have a 12 m wide crest at −2 mDMD; an elevation adopted after a soil-structure interaction analysis was undertaken for different pile sections and various crest elevations, to provide a balance between the fill available and the structural performance of the piles (b) the sheet piles were planned 2 m from the seaward edge so that a 10 m wide corridor was available for a 2 lane temporary road inside the cofferdam after draining (c) the tunnel trench at its deepest is −26 mDMD and the trench slopes, located in interbedded sandstone and calcarenite, were planned at 1V:0.5 H (d) a 15 m minimum clearance was maintained from the top of the trench excavation to the toe of the dyke thus avoiding surcharging and undermining the stability of the trench slope and (e) a dyke slope of 1V:4 H from crest to −10 mDMD and 1V:7 H thereafter, was adopted from slope stability considerations based on recent experience gained in the construction of The Palm. A 2.4 km long cofferdam outline was thus identified having a plan area of some 300,000 m 2 and a maximum width of about 400 m. This outline involved 1550 m of offshore piles and 850 m of onshore piles. Local filling adjacent to the recently formed islands was undertaken to maximize land based operations. Both marine and land driven sheet piles are up to 30 m long and the top of the piles were planned at +3.3 mDMD for minimal overtopping under storm surge and wave conditions, and the toe at −26 mDMD based on seepage analyses which required 1 m minimum penetration into the lower less permeable sandstone which underlies the calcarenite. Bending stresses and deflections complying with Technical Standard and Commentary for Port and Harbour Facilities (2001) indicated that pile sections SX27 (section modulus, z = 2700 cm 3 /m) for the offshore piles, and PU25 (z = 2500 cm 3 /m) for onshore piles were appropriate. The estimated maximum deflections at the top of the marine piles, after draining, was calculated to be about 200 mm and the relative deflection between +3 m and −2 mDMD i.e. the cantilevered section, to be about 150 mm. These estimates are based on a sea level of +2.45 mDMD, to take account of storm surge, and a horizontal modulus of subgrade reaction of 10,000 KN/m 2 /m based on an empirical relationship to an SPT-N value of 5. Seepage analysis using 2D FLOW with an average k value determined for each substrata indicated that some 820 m 3 /hr could be expected under the cofferdam once steady state conditions had established. French drains at the base of the tunnel trench, with pits and submersible pumps of sufficient capacity, and lines of well point systems were designed to collect the inflow (Figure 4). The collected inflow was directed to two locations, one either side of the tunnel trench, before being pumped back to the sea over each dyke with due care taken not to erode the outer slope. 5 CONSTRUCTION METHODOLOGY 5.1 Dyke construction Cutter suction dredgers were used to excavate the tunnel trench, one for shallow and one for deep dredging below −15 mDMD, with operations commencing at the end of the alignment i.e. at the newly formed islands and progressing towards the centre where the trench is deepest.The dredgers lowered the seabed elevation by some 2 m in a single step until the desired elevation along the tunnel alignment was achieved within 0.5 m of the invert elevation, with the final grade being achieved using earthworks equipment in dry conditions. Seabed elevations were constantly monitored by surveying equipment on the dredger and from a survey boat. The dredged material was sucked into the dredger holds and placed at the dyke locations using a spread pontoon positioned by GPS and controlled by anchors and winches. Both dykes were built up concurrently and care was required to achieve the 1V:4 H slope by considerably reducing the volume discharged through the pipe. Total volume for the two dykes was 920,000 m 3 of which 660,000 m 3 was obtained from the tunnel trench excavation and the balance was obtained from a borrow source nearby, with a small quantity purchased from a supplier. A larger volume than had been estimated at the tender stage was necessary due to the flattening of the dyke slopes from 1V:4 H to 1V:7 H below −10 mDMD as already described in Section 4. Bathymetric surveys were carried out after filling to either confirm that the desired dyke profile had been achieved or identify local areas requiring additional fill. 5.2 Sheet pile procurement and preparation Procurement of the 12,500T of sheet piles was a lead item due to a shortage on the world market. Some 9500T of new SX27 sections in 15 m lengths were 1299
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Gahir, J.S.

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