Underwater construction

theory

1. UNDERWATER CONSTRUCTION

2. INDTRODUCTION During the construction of bridges, dams or any other structure where the foundation part of the structure is most likely to lie underwater , we have to opt for underwater construction Construction in water poses many difficulties especially in the places where there the depth is considerable. During underwater construction our main objective is to create dry and water free environment for working in such a manner that the structural stability of the structure is not compromised.

3. HOW IT ACT AS FOUNDATION Underwater construction works  as observed by the engineers and designers are considered as the most difficult  work . Caissons are sunk through ground or water to exclude water and semi-fluid material during the process of excavation of foundations and which subsequently becomes an integral part of the  substructure . Underwater foundations , also known as subaqueous  foundations , may be used in situations where the use of a cofferdam or caisson is prohibitively expensive or unfeasible. ... If the  foundation  is relatively short and in shallow water, bagged concrete or cement placed in layers may be used as formwork and left in position.

4. UNDERWATER CONSTRUCTION TECHNIQUES Dry Construction Wet Construction

5. DRY CONSTRUCTION 1.CAISSON Water tight retaining structure. Permanent in nature. Used in construction of bridges, piers,dams,etc. Constructed in a way water can pumped out. TYPES : Open Caisson Box Caisson Suction Caisson

6. ADVANTAGES OF CAISSON Economic. Slight less noise and reduced vibrations. Easily adaptable to varying site conditions. High axial and lateral loading capacity.

7. 2.COFFERDAMS Temporary structure. Built across body of water. Involves interection of structure,soil and water. Creates a dry work environment. COMPONENTS: Sheet Piling Brace Piling Concrete Seal

8. ADVANTAGES OF COFFERDAM Provides safe environment to work. Contractors have design responsibily. Steel sheet piles are easily installed and removed Materials typically reused on other projects.

9. WET CONTRUCTION Evaculate the overall cost of the work. Considering time Equipment needs The capability of a diver to accomplish the specific task. A more gross than fine nature.

10. TECHNIQUES FOR CONCRETING UNDER WATER Use pre-cast concrete units and lower into place Light enough to place Heavy enough to stay in place – or anchor Place wet concrete inside sacrificial bag Use a hopper with a bottom gate & skirt Use tremie pipe or flexible hose

11. TREMIE PIPE – FOR SMALL QUANTITIES ONLY Crumpled paper used to block tube initially Fresh concrete placed within existing mass Formwork required – can be pre-cast units Scour may be a problem Cofferdams can provide protection Can use flexible hose & pumped concrete Water level Sea bed level Fresh concrete in hopper

12. PUMP METHOD Pumping concrete directly into its final position , involving both horizontal and vertical delivery of concrete. Operational efficiency with potential savings of time and labour. For massive underwater concrete construction of navigation structures , the method is prohibited.

13. TOGGLE BAGS Used for the placement of small amounts of concrete. Bag is filled in dry condition with wet concrete. Used mainly for repairing works. Concrete is squeezed out from the bag by the diver.

14. SHAPE RELATED TO PRESSURE The biggest challenge for an underwater structure is withstanding the constant water pressure. To understand how these structures solve this problem we have to have a better understanding of the forces at play.

15. Pressure is a force divided by the area the force is acting on. The force of this pressure is exerted perpendicular to the surface on the object. The illustrations on the right are based on a gas pressure from inside the object, but the principles work the same with water pressure from the outside. The amount of water above the object and the resulting pressure on the object have a linear relationship. Domes and arches are derived from a spherical shape and are therefore used in architecture when dealing with large spans or high carry loads.

16. SHAPE RELATED TO MATERIAL Pressure from the outside compresses the material while pressure from inside of the object stretches the material. This difference is very important, because materials have different compressive and tensile strengths. Material used Steel concrete acrylic glass

17. SUBMERGED FLOATING TUNNEL LOCATION : Tokyo Bay Aqua – Line ( 9.6 km )

18. Submerged floating tunnel is basically making a tunnel to float underwater which is balanced by its buoyancy, self weight and constraint forces resulted from cable system and thus submerged to a certain depth underwater. The tube is placed underwater, deep enough to avoid water traffic and weather, but not so deep that high water pressure needs to be deal with, usually 20 m to 50 m is sufficient.

19. CONSTRUCTION A trench is dredged in the bed of water channel. 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 waterway.

20. PROCEDURE Construction of tunnel segments on dry dock. Transporting the tunnel segmenets to their final places and placing the underwater. Joinning of different tunnel segments by using rubber gasket. Anchoring the tunnel to the cables.

21. STEP 1 : PRECASTING Huge tunnel section are constructed on dry dock. The procedure consists same as that of precast construction. Dry dock is flooded and the panels are transported to their respective places. Sinking of tunnel is controlled by the use of ballast tank as in case of submarines.

22. STEP 2 : JOINTS After the submersion of different panels in water they are connected with one another by using a rubber gasket. Another procedure includes trapping of water between the joints and then removing it afterwards.

23. STEP 3 : FOUNDATION The application consists same as that of in caission foundation. A hollow chamber is penetrated down the sea bed as shown which evacuates the water trapped inside it by a valve present on its top surface. Such type of foundations are been used for the offshore oil rig plants.

24. STEP 4 : ANCHORING OF CABLES After the foundation work is completed, the cables are anchored to the floating tunnel which will avoid its movement and will place it firmly in alignment. This operation can be carried out by drivers. Finally the tunnel will be in position and ready to use.

25. MATERIAL USED As the tunnel is situated at a depth of 30 m. It should be perfectly water tight and secondly it should resist the salty sea water thirdly it should withstand against hydrostatic forces coming on it. It made of 4 layers. Outermost layer is constructed of aluminium to resists the salty sea water. Second and third layer is made of the foam to float the tunnel easily in water. Fourth layer is of concrete which gives strength to the tunnel.

26. The holland tunnel was built (1920-1924) by pneumatically pushing cylindrical shields through the river bottom. The shields not only dug through mud but also served as the shell beneath which the actual tunnel walls (built of iron rings filled with concrete) were constructed. THE HOLLAND TUNNEL

27. The Holland Tunnel opened in 1927 as the first underwater vehicular crossing of the Hudson River between Manhattan and Jersey City, New Jersey. It is the first tunnel constructed with a ventilation system specifically designed to handle automobile and truck exhaust fumes. By the 20th century, trains and cars had replaced canals as the primary form of transportation The Holland Tunnel, completed in 1927, was one of the first roadway tunnels and is still one of the world's greatest engineering projects. Named for the engineer who oversaw construction, the tunnel ushers nearly 100,000 vehicles daily between New York City and New Jersey.

28. Eleven different plans for the tunnel were proposed, including a design by General George W. Goethals that included a single tube lined with concrete blocks having two levels, each containing three traffic lanes. experiments were conducted to provide the data necessary to design the special ventilation system for motor vehicle exhaust . Holland's plan called for twin tubes lined with cast iron, each containing two lanes of traffic on a single deck, and was ultimately selected.

29. This type of longitudinal system would not work in a vehicular tunnel because it would require gale force winds of more than 70 miles per hour (113 km/h) a new type of ventilation system was devised in which clean air would be supplied throughout the tunnel through a fresh air duct located below the roadway with openings. An exhaust duct would be located above the roadway with openings at regular intervals to remove the diluted exhaust fumes out of the tunnel. large cylinder driven by 30 hydraulic jacks with a total force of 6,000 tons (5,400 t). excavated, 2.5 foot (76 cm) thick cast iron rings are added to support the walls.

30. Tunnel Construction: Soft Ground and Hard Rock Workers generally use two basic techniques to advance a tunnel. In the  full-face method , they excavate the entire diameter of the tunnel at the same time. The second technique, is the  top-heading-and-bench method .

31. In this technique, workers dig a smaller tunnel known as a  heading . Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench . One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.

32. Soft Ground (Earth) Workers dig soft-ground tunnels through clay, silt, sand, gravel or mud. In this type of tunnel,  stand-up time  -- how long the ground will safely stand by itself at the point of excavation -- is of paramount importance. Because stand-up time is generally short when tunnelling through soft ground, cave-ins are a constant threat. To prevent this from happening, engineers use a special piece of equipment called a  shield .

33. It carves a perfectly round hole and supports the surrounding earth. While workers remove debris and install a permanent lining made of cast iron or precast concrete.

34. Hard Rock Tunnelling through hard rock almost always involves blasting. Workers use a scaffold, called a  jumbo. The jumbo drills mounted to make several holes in the rock. The depth of the holes can vary depending on the type of rock, but a typical hole is about 10 feet deep and only a few inches in diameter.

35. Then they repeat the process. Fire-setting  is an alternative to blasting. In this technique, the tunnel wall is heated with fire, and then cooled with water. In this environment, extra support for the tunnel roof and walls may not be required. However, most tunnels pass through rock that contains breaks or pockets of fractured rock, so engineers must add additional support in the form of bolts, sprayed concrete or rings of steel beams. In most cases, they add a permanent concrete lining.

36. Tunnel Construction: Soft Rock and Underwater Tunneling through soft rock and tunneling underground require different approaches. Blasting in soft, firm rock such as shale or limestone is difficult to control. The circular plate is covered with disk cutters  -- chisel-shaped cutting teeth, steel disks or a combination of the two.

37. As the circular plate slowly rotates, the disk cutters slice into the rock, which falls through spaces in the cutting head onto a conveyor system . TBMs don't just bore the tunnels -- they also provide support . As the machine excavates, two drills just behind the cutters bore into the rock. Then workers pump grout into the holes and attach bolts to hold everything in place until the permanent lining can be installed. The TBM accomplishes this with a massive erector arm that raises segments of the tunnel lining into place.

38. Tunnels built across the bottoms of rivers, bays and other bodies of water use the  cut-and-cover method , which involves immersing a tube in a trench and covering it with material to keep the tube in place. Construction begins by dredging a trench in the riverbed or ocean floor. Long, prefabricated tube sections, made of steel or concrete and sealed to keep out water, are floated to the site and sunk in the prepared trench. Then divers connect the sections and remove the seals. Any excess water is pumped out, and the entire tunnel is covered with backfill Underwater

39. One of the most important concerns is ventilation -- a problem magnified by waste gases produced by trains and automobiles. His solution was to add two additional layers above and below the main traffic tunnel. The upper layer clears exhaust fumes, while the lower layer pumps in fresh air. Four large ventilation towers, two on each side of the Hudson River, house the fans that move the air in and out. Eighty-four fans, each 80 feet in diameter, can change the air completely every 90 seconds. Tunnel ventilation tower

40. Tunnel planning requires very different techniques than tunneling through hard rock or soft rock, such as shale, chalk or sandstone. For New York City officials, the solution was clear: Build an automobile tunnel under the river and let commuters drive themselves from New Jersey into the city. 51,694 vehicles made the crossing an average trip time of just 8 minutes. the challenge was an obsolete ferry system that strained to transport more than 20,000 vehicles a day across the Hudson River.

41. CONCLUSION Cofferdams are temporary structures and used in cases where the plan area of foundation is very large depth of water and for loose soils. At present,the tremie placement method is standard way of placing high quality concrete underwater. Underwater construction is useful for power generation,bridges,ship husbandry,water utilities,mining,pipeline,power and gas trasmission. And attracting places for tourits.