South Outer Sea Wall Naos Dock (Arrecife, Lanzarote)
Description of the construction work
The construction works subject of this Project are necessary to build the “South Outer Sea Wall of the Naos Cruise Ship Dock”. The dock designed is aligned NW/SE, perpendicular to the Naos Cruise Ship Dock and has a total length of 322 m from the beginning of the Naos Dock until the tip of the dock where the access to the Port of Arrecife begins. This configuration provides the mouth of the port with a width of 432 m.
Along its length, the dock goes through natural sea-beds with increasing depth, starting at 5 m. and reaching 28 m. at the tip of it. To adapt to that morphology, two different types of wharfs were projected: a sloped dock with concrete blocks for depths of 10-12 m. and a vertical dock with prefabricated concrete caissons for greater depths. At the same time, the level of anchorage of the caissons will be adapted to the natural draughts to minimise the need to add quarried material to the support embankment.
The point at which one type transitions into the other has been determined by the intention of easing the connection between the future extension of the Cruise Ship Dock and the Outer Dock. Thus, the vertical type and the draught (10 m.) of the Cruise Ship Dock have been respected to ensure a simple transition between both construction projects.
In addition to the aforesaid, two other goals were pursued: not to have to dredge and to avoid the slope portion of the outer dock from caving in and covering the caisson where it connects with the future Cruise Ship Dock. As it was not possible to fulfil both objectives without making the construction much more expensive, the final decision was to avoid having to dredge.
Thus, the final length adopted for the vertical section is 118.1 m. which will be taken care of by four 29.45 m. long caissons and three 10 cm. wide joints between the caissons, which will be anchored at an increasing depth, the first starting at level -10 m. and the second at -15 m. and the two following ones at level -20 m.
The four caissons, differentiated only by their draught, comprise outside dimensions of 29.45 length, 17.50 shaft width and 13.50 m. 18.50 m. or 23.50 m. draught. The weight of the caissons is lessened by means of rectangular cells measuring 4.40 x4.40 m. or 4.40 x 3.475, all of which are bevelled at their vertexes. The inside walls separating the cells are 0.50 m. wide. The top of the caisson has been designed with a constant thickness of 0.90 m. and a 1.00 overhang offering a total width of 19.50 m.
The construction of the pilot section of this dock alignment is brought about by depositing 100 to 200 kg of rubble which makes up the core of the embankment, which has a clearance of 6.00 m. on both sides of the caisson. The cresting of the embankment is levelled at -20.00 m. or -10.00 m., evened off
with gravel so that it allows a regular support plane for the structure of the caisson. On the outside of the embankment there will be a 1-ton double layer of protecting rubble 1.44 m. thick.
Once the caissons have been anchored, the cells will be filled with granular material to level +3.50 m. To carry out the joint seals between the caissons, four polyethylene tubes of 0.50 cm. diameter filled with armoured concrete with six rods measuring 12mm. The tubes will be placed between the caisson overhangs on both the seaside and the basin side and the resulting gap will be fully filled with non-structural concrete. The rest of the joint gap in the middle of the caisson, will be topped off with rockfill.
The beginning of the construction of the dock wall will be carried out when the caisson is fully in service, in other words, that all cells are filled and the support embankment on the ocean side has been protected. The first phase of the dock wall will be laid at +3.50 level with an approximate width of 4.23 m. until level +5.10. The second phase will be trapezoid-shaped with a width of 2.75 m. at its base and 2.00 m. at its cresting, and a height of 3.70 m. from level +5.10 m. until +8.80 m.
Once the dock wall has been completed, the closure cope beam can be built. The beam has been designed with a rectangular section 1.50 m. wide with a 0.50 m. overhang at the outside of the caisson and 2.2 metres height, finished off with the edge of the dock at level +5.20 m.
The pavement of the stretch between the superstructure and the dock wall will be made of a layer of selected flooring approximately 1.20 m. thick, a 0.25 m layer of artificial gravel and a thin layer of 0.32 m. HP-40. The three layers will be sloped transversally at 1.70% towards the basin.
Regarding the slightly more than 200 m. long sloped section, a part of the present Naos Dock, at a point where the natural draught is about 5 m. and which and runs along increasing depths until it reaches between 10 and 12 m.
The pilot section is comprised of a nucleus of quarried matter cresting at level +3.30 m. above which on both of its sides there are two layers of rubble filter of 50 to 100 kg and 1 ton and 0.70 m. and 1.44 m. thickness respectively. The total width at level +4.00 m., above the first filter, is 10.00 m. On the outside face there is also a layer of two lines of 10-ton and 3.24 m. total thickness parallelepipedic concrete blocks. The protection mantle crests at level +8.68 m. with a berm of 4.22 m. width. The outside slope is designed on an angle of 3H/2V. On the inside, above the first 1-ton layer of rubble, a pack of flexible asphalt 4.57 m. wide, cresting at level +5.09.
From the encounter with the first caisson of the vertical section, the slope section falls over the caisson on both the ocean side and the basin side. On the ocean side the sizes of the blocks are larger in the protection mantle, which grown from 16-ton and 3.80 m. total thickness in both of the two layers.