Επιχειρησιακό Πρόγραμμα Εκπαίδευση και ια Βίου Μάθηση Πρόγραμμα ια Βίου Μάθησης ΑΕΙ για την Επικαιροποίηση Γνώσεων Αποφοίτων ΑΕΙ: Σύγχρονες Εξελίξεις στις Θαλάσσιες Κατασκευές Α.Π.Θ. Πολυτεχνείο Κρήτης 8.4.3 ΠΡΟΒΛΗΜΑΤΑ ΘΕΜΕΛΙΩΣΗΣ Θεόδωρος Χατζηγώγος Καθηγητής Τμήμα Πολιτικών Μηχανικών Α.Π.Θ. thechatz@civil.auth.gr
Περιεχόμενα της διάλεξης Γενικά χαρακτηριστικά υποθαλάσσιων σηράγγων Προετοιμασία της θεμελίωσης Χωματουργικά Προετοιμασία της θεμελίωσης Στρώμα έδρασης Αντοχή και μετακινήσεις ιαφορικές Καθιζήσεις Βελτίωση του εδάφους Θεμελίωση με πασσάλους
Immersed Tunnels Typically, an immersed tunnel is made by sinking precast concrete boxes into a dredged channel and joining them up under water. Tunnel sections in convenient lengths, usually 90 to 150 meters, are placed into a pre-dredged trench, joined, connected and protected by backfilling the excavation. The sections may be fabricated in shipyards, in dry docks, or in temporary construction basin serving as dry docks.
Once completed, an immersed tunnel is no different operationally from any other tunnel. However, it is built in a completely different way. The Construction technique: A trench is dredged in the bed of the water channel.
Tunnel elements are constructed in the dry, for example in a casting basin, a fabrication yard, on a ship-lift platform or in a factory unit.
The ends of the element are then temporarily sealed with bulkheads. Each tunnel element is transported to the tunnel site - usually floating, occasionally on a barge, or assisted by cranes.
The tunnel element is lowered to its final place on the bottom of the dredged trench.
The new element is placed against the previous element under water. Water is then pumped out of the space between the bulkheads. Water pressure on the free end of the new element compresses the rubber seal between the two elements, closing the joint.
Backfill material is placed beside and over the tunnel to fill the trench and permanently bury the tunnel, as illustrated in the figures.
Approach structures can be built on the banks before, after or concurrently with the immersed tunnel, to suit local circumstances.
Immersed tunnels have been in widespread use for about 100 years. Over 150 have been constructed all over the world, about 100 of them for road or rail schemes. Others include water supply and electricity cable tunnels. The examples given indicate the diversity of projects that have been realized.
Immersed tunnels do not suit every situation. However, if there is water to cross, they usually present a feasible alternative to bored tunnels at a comparable price, and they offer a number of advantages, such as: Immersed tunnels do not have to be circular in cross section. Almost any cross section can be accommodated, making immersed tunnels particularly attractive for wide highways and combined road/rail tunnels. Some examples of realised cross sections are shown below.
Immersed tunnels can be placed immediately beneath a waterway. In contrast, a bored tunnel is usually only stable if its roof is at least its own diameter beneath the water. This allows immersed tunnel approaches to be shorter and/or approach gradients to be flatter - an advantage for all tunnels, but especially so for railways.
Bored tunnelling is a continuous process in which any problem in the boring operation threatens delay to the whole project. Immersed tunnelling creates three operations - dredging, tunnel element construction and tunnel installation, which can take place concurrently, thus moderating programme risk considerably. Partly for this reason, an immersed tunnel is generally faster to build than a corresponding bored tunnel.
The perceived problems include: DREDGING Dredging technology has improved considerably in recent years, and it is now possible to remove a wide variety of material underwater without adverse effects on the environment of the waterway.
INTERFERENCE WITH NAVIGATION Interference with navigation: On busy waterways, it is sometimes assumed that construction of an immersed tunnel would be impractical as it would interfere with shipping. In fact, such tunnels have been successfully built in some exceptionally busy waterways without undue problems.
WATERTIGHTNESS It is often assumed that the process of building a tunnel in water, rather than boring through the ground beneath it will increase the likelihood of leakage. In fact, immersed tunnels are nearly always much drier than bored tunnels, due to the aboveground construction of the elements. Underwater joints depend on robust rubber seals which have proved effective in dozens of tunnels to date.
Υποθαλάσσιες σήραγγες
Case study: Øresund Link Strait crossing The Øresund link connects Copenhagen on Zealand, Denmark to Malmö in Sweden thus establishing a land traffic corridor from Scandinavia to the continent. The link comprises a 3.5 km immersed tunnel, 4 km artificial island, and 8 km bridge, including a 490 m span cablestayed bridge. The link incorporates approximately one million m³ of concrete, of which more than two thirds constitute the immersed tunnel. The Øresund Tunnel was motivated by the fact that one of the main shipping lane is very close to the Copenhagen international airport, making a high bridge over the nearest navigational channel unfeasible.
The tunnel cross-section accommodates two tubes for the two-track railway and two tubes for the four-lane motorway. A central installation gallery between the motorway tubes doubles as a safe and smoke-free escape route in case of emergency. The immersed part of the tunnel consists of 20 elements, each approximately 175 m long, resulting in a total immersed tunnel length of 3,510 m. Each element is made from 8 segments, joined together by temporary prestressing, and weighing approximately 56,000 t. The outer cross-sectional dimensions are 8.6 m by 38.8 m, the height being governed by the railway clearance profile. The track is fastened directly to the bottom slab, the omission of the ballast reducing the required tunnel height. The elements were placed in a predredged trench, and founded on a gravel bed. Backfilling along the sides and on the roof was designed to offer a permanent cover and protection of the tunnel in all situations. The final tunnel profile is in general below seabed level, and at the Drogden navigation channel the top of the cover is 10 m below water level. The rock cover was designed to withstand a falling or dragging anchor, or a sunken ship. Furthermore the protective layer is stable against scour and erosion caused by currents or ship propellers.
Σήραγγα Busan Geoje Κορέα Kasper,Rotwitt, Jackson, Massarsch Foundation of an immersed tunnel on marine clay improved by cement deep mixing and sand compaction piles
Βυθοκόρηση Τεχνικές βυθοκόρησης ιαστάσεις Γεωμετρικά στοιχεία Απόθεση Ανοχές (+/- 0.25 μέτρα)
Επιχωματώσεις Locking fill Engineering fill General fill Tunnel cover (scour protection, rock protection) Anchor release bands Filter layer
Έδραση της Θεμελίωσης Αμμώδες υπόβαθρο Sand jetting method
Sand flow method
Gravel layers screeding frames
Gravel layers - Scrading
Gravel layer gravel bed
Grouted foundations
Μελέτη της Θεμελίωσης Αντοχή Καθιζήσεις ιαφορικές καθιζήσεις ιαφοροποίηση του εδαφικού προφίλ Ανομοιογένεια του εδάφους ιαφοροποίηση των φορτίων
Άκτιο - Πρέβεζα
Aκτιο - Πρέβεζα
Βελτίωση- Ταξινόμηση των μεθόδων Μέθοδος Αντικατάσταση Λιθοπάσσαλοι (αντικατάσταση) Λιθοπάσσαλοι (συμπύκνωση) Αμμοπάσσαλοι συμπύκνωσης Ανάμειξη σε βάθος με τσιμέντο Ground replacement Stone columns (replacement) Stone columns (Displacement) Sand compaction Piles Soil mixing Compaction grouting Jet grouting Μηχανισμός βελτίωσης Κυρίως μεταφορά φορτίου Μεταφορά φορτίου και συμπύκνωση Συμπύκνωση Αύξηση της αντοχής του εδάφους
Βελτίωση λιθοπάσσαλοι Stone columns
Βελτίωση Αμμοπάσσαλοι συμπύκνωσης Sand commpction piles
Θεμελίωση με πασσάλους Σε σπάνιες περιπτώσεις Βασικό πρόβλημα η σύνδεση των πασσάλων με την θεμελίωση. Πιθανή λύση η περίπτωση εφαρμογής της θεμελίωσης των βάθρων της γέφυρας Ρίου- Αντιρρίου