VISVESVARAYA TECHNOLOGICAL UNIVERSITY BELGAUM- 590 018
A SEMINAR REPORT ON “UNDERWATER CONSTRUCTION” In partial fulfillment of the requirements
For the award of the degree of
MASTER OF TECHNOLOGY IN
STRUCTURAL ENGINEERING SUBMITTED BY
RAKESH BORKER B S
UNDER THE GUIDANCE OF
Mrs. SHANTHI VENGADESHWARI R
DEPARTMENT OF CIVIL ENGINEERING DAYANANDA SAGAR COLLEGE OF ENGINEERING
BANGALORE-560 078 NOVEMBER- 2009
VISVESVARAYA TECHNOLOGICAL UNIVERSITY
Belgaum – 590 018
DEPARTMENT OF CIVIL ENGINEERING
Dayananda Sagar College of Engineering
Bangalore – 560 078
This is to certify that Mr. RAKESH BORKER B S has successfully completed the seminar report on “UNDERWATER CONSTRUCTION” in partial fulfillment of the requirement for the degree of Masters of Technology in Structural Engineering from VISVESVARAYA TECHNOLOGICAL UNIVERSITY, Belgaum during the academic year 2009-2010, under the guidance of Mrs. SHANTHI VENGADESHWARI R, Senior Lecturer, Dept of civil Engg., Dayanada Sagar College of Engineering, Bangalore.
Mrs.Shanthi Vengadeshwari R Dr K N Vishwanath Dr.B.S. Thandaveswara
Senior lecturer PG Co-Ordinator Professor and HOD
DAYANANDA SAGAR COLLEGE OF ENGINEERING
VISVESVARAYA TECHNOLOGICAL UNIVERSITY
I hereby declare that the work which is prescribed in the seminar report entitled “UNDERWATER CONSTRUCTION” in partial fulfillment of the requirement for the award of Masters of Technology in Structural Engineering from VISVESVARAYA TECHNOLOGICAL UNIVERSITY, Belgaum carried out during year 2009-2010 under the guidance of Mrs. SHANTHI VENGADESHWARI R, Senior Lecturer, Department of Civil Engineering, Dayananda Sagar College of Engineering, Bangalore.
RAKESH BORKER B S
2. CAISSON CONSTRUCTION2.1 INTRODUCTION 2.2 TYPES OF CAISSONS 2.2.1 BOX CAISSONS 2.2.2 OPEN CAISSONS 2.2.3 PNEUMATIC CAISSONS 2.2.4 SUCTION CAISSONS
3. COFFERDAM CONSTRUCTION3.1 INTRODUCTION 3.2 TYPES OF COFFERDAMS 1.2.1 DIKES
1.2.2 SINGLE WALL COFFERDAMS 1.2.3 DOUBLE WALL COFFERDAMS 1.2.4 CELLULAR COFFERDAMS
1.2.5 ROCK-FILLED CRIB COFFERDAMS 1.2.6 CONCRETE COFFERDAMS
1.2.7 SUSPENDED COFFERDAMS
4. UNDERWATER CONCRETING
4.2 CONCRETE PLACEMENT
4.3 UNDERWATER CONCRETE PLACEMENT METHODS 4.3.1 TREMIE METHOD
4.3.2 PUMP METHOD 4.3.3 TOGGLE BAGS 4.3.4 BAGWORK
5. A CASE STUDY ON BRIDGE FOUNDATION
6. CONCLUSION REFERENCE
Underwater concrete construction is a critical component of the entire project. It is technically demanding, usually on the critical path of the project. It is technically demanding, usually on the critical path of the project schedule, and involves complex construction logistics. Therefore, its significance in the project far beyond the concreting operations themselves, in essence, underwater concrete can be constructed with the same degree of reliability as above-water construction. But if it is not carried out properly, with the proper concrete mixture and placement procedure, underwater concrete construction can result in a major cost and schedule overrun. This is the area where sound design and competent construction planning can achieve a meaningful reduction in risk and cost.
For those used to concreting on dry land, concreting under water presents various challenges. Transporting, compacting, quality control, finishing and accuracy must all be carried out successfully in this different, and often difficult, environment. There are, however, many common aspects, chief of which is that air is not required for the setting and hardening of concrete – it sets and hardens just as well, and often even better, under water – but it must be fluid enough to flow into position and be self compacting as conventional vibration is not practicable under water.
The caissons and cofferdams are the techniques used for the construction of underwater structures. A caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships.
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. A cofferdam is an enclosure within a water environment constructed to allow water to be displaced by air for the purpose of creating a dry work environment. Commonly used for oil rig construction and repair, bridge and dam work, the cofferdam is usually a welded steel structure that is temporary, typically dismantled after work is completed. Its components consist of sheet piles, wales, and cross braces.
The caisson method is one method of producing deep foundations, i.e the load exerted by the building structure is transferred to firm, deeper strata. Caisson foundation is most commonly used underwater for a bridge, but sometimes used in building construction Caissons are normally made of reinforced concrete, various cross sections being possible, depending on the requirements. These are constructed such that the water can be pumped out, keeping the working environment dry. It is a large hollow structure that is sunk down through the earth by workers excavating from inside it, ultimately it becomes a permanent part of the pier. The outer walls of large caissons should be at least 1m thick, and the caissons should have horizontal and vertical stiffening walls. A round hole is dug or bored to a stable layer of earth and temporarily supported by a steel shell, then filled with concrete poured around a cage of reinforcing bars.
Uses of caissons:
1) To reach the hard bearing stratum for transferring the load coming on supports for bridge piers and building columns.
2) To serve as an imperious core wall of earth dams, when placed adjacent to each other. 3) To provide an access to a deep shaft or a tunnel.
4) To provide an enclosure below water level for installing machinery, pump, etc.
Materials used for the construction of caissons:
The common materials which are usually employed for the construction of a caisson are as follows,
2) Reinforced cement concrete 3) Steel
The cast-iron is suitable for caissons of open-well type. New segments of cast-iron are bolted as the caisson sinks. This material is unsuitable for pneumatic caissons as there is risk of
failure due to tension developed by the compressed air. The cost also works out to be more in relation to the steel or R.C.C.
The reinforced cement concrete is suitable for caisson shoes. This material has more weight and therefore it creates difficulties in handling and floating the caisson in the early stage of construction. It therefore becomes economical to construct a steel caisson with concrete filling.
The steel is found to be the most suitable material for the construction of a caisson. It is usually in the form of a double skin of steel plating and the hollow space is then filled with cement concrete.
The timber was used as a material for the construction of a caisson in the early stages of development of a caisson. But this material is now practically not adopted mainly because of its bulk and risk of fire.
2.2 TYPES OF CAISSONS
All caissons feature the shape of a tube, often with a cylindrical contour but it may also be rectangular, elliptical, or some other form. Some caissons are open at both ends, some are open only at the top, and some are open only at the bottom. It depends on the way each type of caisson is to be used.
The four main types of caisson are, 1. Box caisson
2. Open caisson 3. Pneumatic caisson 4. Suction caisson:
2.2.1 BOX CAISSON
This method is applied mainly in underwater tunneling, harbor, foundation for bridge pier constructions. Box caissons are prefabricated concrete boxes of various shape and comprise hollow bodies with water-tight floors and walls. After manufacture , they are launched, towed to the place where they are to be built in, sunk to the sea bed or river bed, which has been prepared accordingly, and finally ballasted. In this process, individual construction elements usually have to be connected and sealed off from each other. One problem with box caissons is that hollow concrete structures float and so they must be ballasted or anchored to prevent this until they can
be filled with concrete. Adjustable anchoring systems combined with a GPS survey allow engineers to position a box caisson with pinpoint accuracy. Elaborate anchoring systems may be required in tidal zone.
Fig 2.1 Schematic Diagram Of Box Caisson
2.2.2 OPEN CAISSON
The structure is open at the top and is manufactured wholly or partially at ground level. In that phase it rests on the shoe, the making of which requires special care and experience. The sinking process begins with the step-by-step removal of material inside structure under atmospheric pressure. First, above groundwater level, the material is excavated then, below ground water level, it is dredged. Interior and exterior groundwater levels must always be the same to prevent piping and to prevent material being sucked in from the outside as a result.
Friction between the structure and the surrounding soil is minimized by a gap around the structure filled with bentonite. As a rules it is 5 to 10cm wide and is produced automatically as the shoe projects beyond the outer wall of the caisson. Since the friction forces increase with increasing depth, the weight must be raised by additional loads.
When the final depth is reached, an under water concrete floor is built in to facilitate subsequent pumping of the interior, attention always being paid to safety against uplift. Open caissons used in soft grounds or high water tables, where open trench excavations are
impractical, can also be used to install deep manholes, pump stations and reception/launch pits for micro tuunelling.
Fig 2.2 Schematic Diagram Of Open Caisson
2.2.3 PNEUMATIC CAISSON
A caisson closed at the top and open at the bottom is a pneumatic caisson. This type of caisson generally is used in underwater construction projects. It can be used only if air is pumped in to produce a pressure greater than water pressure outside. Workers entering a pneumatic caisson must first pass through an intermediate chamber that allows their bodies to adjust from normal atmospheric pressure to the higher pressure within the caisson or vice versa. Pneumatic caissons can not be used at a depth of more than 120 ft (36.6 m). Beyond that point the air pressure needed inside the caisson to keep out water is too great for the human body to withstand.
As with the open caisson, the structure is manufactured wholly or partially( at least the working chamber and part of the vertical walls) at ground level. Material is excavated in the working chamber, the groundwater being kept out with compressed air. Due to the constant shear
failure below the shoe- as a consequence of the gradual material removal- the structure sinks in to the ground slowly under its own weight.
As with the open caisson, this process may be accelerated by friction minimizing measures and additional loads. The vertical walls are concreted up to the desired height during the sinking process. As soon as the final depth is reached, the working chamber is filled with concrete.
Advantages of pneumatic caisson:
2. The soil condition in the working chamber may be checked constantly. 3. The construction work is not hampered by groundwater rushing in. 4. The floor of the structure may be built in the dry.
Disadvantages of pneumatic caisson:
1. working in compressed air may leads to caisson disease.
2. locking in and out of material and equipment is a tedious process.
Fig 2.4 Working Chamber Of Pneumatic Caisson
2.2.4 SUCTION CAISSON
As offshore exploration and development of oil fields reach water depths of 1,000 to 3,000 m, novel methods of anchoring production platforms become attractive due to cost savings associated with installation. Surface production systems that are viable in these water depths include Tension Leg Platforms (TLP), spar platforms, and laterally moored ship-shaped and semi-submersible vessels. Possible anchor systems for TLP and spar platforms include the traditional driven piles, drag anchors and suction caissons.
Suction caissons become better alternatives to driven piles in deepwater because of technical challenges and costs associated with the installation equipment. In addition, suction caissons also provide a greater resistance to lateral loads than driven piles because of the larger diameters typically used.
Initial penetration of the suction caisson into the seabed occurs due to the self weight. Field observations have shown that the initial penetration of the pile in to ocean sediments under self-weight is substantial enough to develop an adequate seal to facilitate suction installation . subsequent penetration is by the suction created by pumping water out from the inside of the caisson( fig 2.5 ). A submersible pump attached to the top of the sealed caisson applies suction pressure. By evacuating water from the inside, a pressure differential is created. The limiting value of this pressure differential, such that cavitation does not occur, is the sum of the atmospheric pressure and hydrostatic pressure outside the caisson. In very deep waters, large
penetration or suction pressures can be created, which is only limited by the capacity of the pump. Once the required depth are reached, the pumps can be disconnected and retrieved.
Fig 2.5 Installation Sequence Of Suction Caissons (a) Touchdown Phase (b) Penetration Due To Self Weight/Ballast (c) Water Pumped Out To Create Suction Penetration
Advantages of caisson foundation:
2. Minimizes pile cap needs.
3. Slightly less noise and reduced vibrations. 4. Easily adaptable to varying site conditions. 5. High axial and lateral loading capacity.
6. Minimal handling equipment is required for placement of reinforcing cage.
“A cofferdam is a temporary structure designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built”. The cofferdam may or may not be pumped out completely dry. Cofferdams may be installed by driving sheet piles around a designated area or by submerging a pre-fabricated structure made of concrete, steel, or combination of concrete and steel. Cofferdams may even be formed by using inflatable rubber bags to surround a site, fabricated boxes to attach to existing structures, or, in the case of large dewatered areas, perimeter rock dikes or sand-filled structures. Now a days cofferdam will be fabricated steel structure hoisted in to place by a derrick barge powered by electricity, or a precast concrete structure floated in to place and set on bottom.
Following are some of the points which should be remembered in connection with the construction of cofferdams.
• Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility (structure) in the dry.
• A cofferdam involves the interaction of the structure, soil, and water. The loads imposed include the hydrostatic forces of the water, as well as the dynamic forces due to currents and waves.
• In construction of cofferdams maintaining close tolerances is difficult since cofferdams are usually constructed offshore and sometimes under severe weather conditions. Under these circumstances, significant deformations of cofferdam elements may happen during the course of construction, and therefore it may be necessary to deviate from the design dimensions in order to complete the project according to plan.
• The loads imposed on the cofferdam structure by construction equipment and operations must be considered, both during installation of the cofferdam and during construction of the structure itself.
• Removal of the cofferdam must be planned and executed with the same degree of care as its installation, on a stage-by-stage basis. The effect of the removal on the permanent structure must also be considered. For this reason, sheet piles extending below the permanent structure are often cut off and left in place, since their removal may damage the foundation soils adjacent to the structure.
• In cofferdam construction, safety is a paramount concern, since workers will be exposed to the hazard of flooding and collapse.
• Safety requires that every cofferdam and every part thereof shall be of suitable design and construction, of suitable and sound material and of sufficient strength and capacity for the purpose for which it is used, proper construction, verification that the structure is being constructed as planned, monitoring the behavior of the cofferdam and surrounding area, provision of adequate access, light and ventilation, and attention to safe practices on the part of all workers and supervisors, and shall be properly maintained.
3.2 TYPES OF COFFERDAM
Following are the most common types of cofferdams: 1. Dikes
2. Single wall cofferdams 3. Double wall cofferdams 4. Cellular cofferdams
5. Rock-filled crib cofferdams 6. Concrete cofferdams 7. Suspended cofferdams
A dike is an embankment of some material. The materials commonly employed for the construction of a dike are earth, rock and sand-bags. The dikes are generally employed for a short duration, particularly to enable the construction of a more durable cofferdam behind them.
4.2.2 SINGLE WALL COFFERDAMS
This type of cofferdam is suitable when available working space is limited and the area to be enclosed is small. A single row of piles is used on either side of the cofferdam.
4.2.3 DOUBLE WALL COFFERDAMS
When the area to be enclosed is large, it becomes essential to provide the double wall construction so as to give stability to the cofferdam.
4.2.4 CELLULAR COFFERDAMS
The cellular cofferdam is made of steel sheet piles and this type of cofferdam is proved successful in dewatering large areas.
4.2.5 ROCK-FILLED CRIB COFFERDAMS
A rock-filled crib cofferdam consists of timber cribs. A crib is a box or a cell open at the bottom and it essentially consists of a framework of horizontal timbers laid in alternate courses. The pockets thus formed are then filled with rock or gravel or earth to give stability to the crib against overturning and sliding.
4.2.6 CONCRETE COFFERDAMS
These are actually small concrete dams and they have been used economically on many jobs. The framework usually consists of pre-cast R.C.C. piles and sheets. The pre-cast R.C.C. sheet piles are provided with suitable edges and they are driven in a similar manner to steel sheet piles. The design of different units should be properly made. The main disadvantage of a concrete cofferdam is that it is costly. But when it is to be incorporated as part of a permanent structure, it proves to be economical.
4.2.7 SUSPENDED COFFERDAMS
Sometimes, a cofferdam is designed in such a way that a single unit of it is used several times. The cofferdam as such is lifted, floated and placed in another position as soon as its purpose is served. Such cofferdams are also known as the movable cofferdams.
4.3 COMPONENTS OF COFFERDAM
Fig 3.1: Components Of Cofferdam
3.3.1 SHEET PILING
Sheet piling is a manufactured construction product with a mechanical connection “interlock” at both ends of the section. These mechanical connections interlock with one another to form a continuous wall of sheeting. Sheet pile applications are typically designed to create a rigid barrier for earth and water, while resisting the lateral pressures of those bending forces. The shape or geometry of a section lends to the structural strength. In addition, the soil in which the section is driven has numerous mechanical properties that can affect the performance.
3.3.2 BRACING FRAME
These are structural members used for the lateral stability of cofferdam. These are made up of steel or wood.
3.3.3 CONCRETE SEAL
The typical cofferdam, such as a bridge pier, consists of sheet piles set around a bracing frame and driven into the soil sufficiently far to develop vertical and lateral support and to cut off the flow of soil and, in some cases the flow of water. The structure inside may be founded directly on rock or firm soil or may require pile foundations. In the latter case, these generally extend well below the cofferdam. Inside excavation is usually done using clam shell buckets. In order to dewater the cofferdam, the bottom must be stable and able to resist hydrostatic uplift. Placement of an underwater concrete seal course is the fastest and most common method. An underwater concrete seal course may then be placed prior to dewatering in order to seal off the water, resist its pressure, and also to act as a slab to brace against the inward movement of the sheet piles in order to mobilize their resistance to uplift under the hydrostatic pressure.
Advantages of Cofferdam:
Performing work over water has always been more difficult and costly than performing the same work on land. And when the work is performed below water, the difficulties and cost difference can increase geometrically with the depth at which the work is performed. The key to performing marine construction work efficiently is to minimize work over water, and perform as much of the work as possible on land. Below some of the advantages of cofferdams are listed.
1. Allow excavation and construction of structures in otherwise poor environment 2. Provides safe environment to work
3. Contractors typically have design responsibility 4. Steel sheet piles are easily installed and removed 5. Materials can typically be reused on other projects
Comparison between Cofferdam and caisson:
The main difference between a cofferdam and a caisson is that the former is a temporary structure while the latter forms the part of the permanent work. Following factors are to be considered while making a choice between cofferdam and caisson for a particular foundation work.
A cofferdam becomes uneconomical in cases where the plan area of the foundation work is small as compared to the depth of water. Under such circumstances, a caisson would prove to be the most suitable.
At places where the cofferdams cannot be dewatered successfully, the caissons are used. This may be due to the following reasons:
a) Depth of water
b) Nature of soil to be penetrated and
c) Permeability of soil below foundation level
The process of constructing a cofferdam is greatly simplified in cases of soils which allow easily the driving of sheet piles. The caissons, on the other hand, are useful where obstructions or boulders would prevent the successful driving of the sheet piles
For heavy foundation works which are to be provided at a depth of about 12 meters to 15 meters below the level of standing water surface, the caissons would prove to be more economical than the cofferdams.
4. UNDERWATER CONCRETING
For in-the-wet construction of the navigation structure, underwater concrete construction is a critical component of the entire project. It is technically demanding, usually on the critical path of the project. It is technically demanding, usually on the critical path of the project schedule, and involves complex construction logistics. Therefore, its significance in the project far beyond the concreting operations themselves, in essence, underwater concrete can be constructed with the same degree of reliability as above-water construction. But if it is not carried out properly, with the proper concrete mixture and placement procedure, underwater concrete construction can result in a major cost and schedule overrun. This is the area where sound design and competent construction planning can achieve a meaningful reduction in risk and cost.
4.2 UNDERWATER CONCRETE PLACEMENT
The technical requirements for underwater concreting cover the areas of placement method and technique, placement sequence, placement equipment layout, finishing, and protection of concrete. Concrete placement planning should include the relevant subjects of detail as well as the construction logistics( the relationship among various concreting operations and their relationship with other construction operations)
The choice of a proper placement plan for a specific project has to be ultimately determined by the site condition and engineering requirements, including the required in-place concrete properties, volume and thickness of the concrete placement, water velocity during concrete placement, presence of reinforcement or obstacles, availability of equipment, technical feasibility, and cost.
4.3 UNDERWATER CONCRETE PLACEMENT METHODS
1. Tremie method 2. Pump method 3. Toggle bags 4. Bagwork
For the underwater construction of navigation structures, the tremie method is the only sound method for placing high-quality underwater concrete. However, some contractors will request to use the pump method because it slightly reduces the labor cost. The following sections provide a critical examination of these two methods. The other placement methods are not appropriate for high-quality underwater concrete for major structures, although they may find application in special cases.
For placement of underwater concrete, the tremie method and pump method function in fundamentally different ways. Tremie placement deposits concrete solely by its own gravity in a open system where as the pump method employs surges of pump pressure to deliver concrete in a closed system. The technical difficulties and the inherent risk of failure with these two methods are substantially different.
4.3.1 TREMIE METHOD
The principle of this method is that concrete is poured down a pipe or tube from above the surface and is forced into the mass of concrete already in place by the weight of concrete in the tube. The tube is surmounted by a hopper (‘tremie’ in French) and the whole is suspended from a staging or frame, mounted so that it can be moved vertically when held by a crane. As the pour rises, sections of the tube can be removed to facilitate working. A convenient diameter for the tube is 8 to 16 times the maximum aggregate size and 250 mm is a common diameter. Figure shows a diagrammatical representation of a tremie.
Fig 4.1: Schematic Of A Tremie
Before starting the pour, a plug is inserted into the tube to stop the concrete and water intermixing. This plug can be purpose-made (similar to a bath plug), a sponge rubber ball or exfoliated vermiculite, which is the most common method in the UK.
At start-up the bottom of the tube should be on or very close to the sea or river bed, sufficient to allow the water in the tube to escape and to force the first load of concrete to spread out horizontally into a mound shape. The concrete pouring should be continuous with the bottom of the tube always inside previously placed concrete. If this immersion depth, normally at least 0.5 m, is not sufficient, a breakthrough will occur and the pour will have to be abandoned for the day. Any air that is in the concrete being placed will pass through the previously placed concrete and bubble to the surface, disrupting the settled concrete as it goes.
The flow of concrete in the tube is governed by gravity and friction with the tube wall, so the tremie has to be moved up and down to regulate the flow. A crane driver with a good ‘feel’ for this is useful. The tube should be restrained from lateral movement whilst placing concrete.
The placed concrete spreads out horizontally on the bed in a circle, with the top of the pour domed upwards.
Tremies are best used for thick pours of any area. For large area pours, multiple tremies are used, spaced at about 4–6 m apart, depending on the flatness required for the top level. The slope of the concrete surface from a tremie is likely to be in the range 1 in 9 for tremies close together to 1 in 6 for those spaced far apart as the slope increases with distance from the pipe.
The concrete flow pattern is dependent upon the consistency of the concrete mixture and the placement rate. In addition, the flow pattern is also affected by the thickness of the concrete placement and the tremie embedment depth in concrete.
4.3.2 PUMP METHOD
Pump method is defined as pumping concrete directly into its final position, involving both horizontal and vertical delivery of concrete in a closed system of discharge pipes. Pumping concrete has the advantage of operational efficiency with potential savings of time and labor. In recent years pump method has become increasingly popular for above-water structures due to the advancement of pumping equipment and techniques. Pumping concrete directly under deep water(>9m or 30ft in water depth) is a technically flawed procedure. Although pumping concrete in shadow water is feasible, it still involves significant risks and potentially poor concrete quality. For massive underwater concrete construction of navigation structures, the pump method should be prohibited. However, the pump method is an excellent way to deliver concrete horizontally to a tremie hopper. It is also an excellent way of placing grout or flowable sand underwater.
Pumping the mass concrete directly down to the structures on the riverbed or seafloor has several technical problems that will increase the risk of construction failure or poor concrete,
a) In tremie placement by gravity feed, the concrete flow rate can be controlled by the rate at which concrete is fed in to the tremie. On the other hand, the pump system fully fills the pumpline with concrete. For placement in deep water, the weight of concrete in the pumpline is much greater than the hydrostatic head from the water and concrete outside the pipe. Thus, the concrete exists the pipe at an uncontrollably high speed, causing
significant disturbance of already placed concrete and segregation of the concrete being poured.
b) A pump system is closed to the atmosphere. When concrete is pumped down to deep water, concrete may fall through and exit a pumpline at a rate faster than the pump output. Thus, a vacuum will be created in the line. The vacuum pressure so created will suck away the cement paste from aggregates, causing segregation and plugging of the line. c) Pressure surges from the pump can cause disruption of the concrete flow and plugging of
d) A concrete mixture optimized for pumping may not be the optimum concrete mixture for underwater applications.
e) Pumping in to confined space can potentially result in excessive pressures.
f) If the end of the pumpline is not adequately buried, excessive pump pressure surge can kick the pumpline out of the in-place concrete, causing mixing of the concrete with water.
4.3.3 TOGGLE BAGS
Where small amounts of concrete are required, such as in repair work, the toggle bag is ideal. The waterproof bag is filled in the dry with wet concrete and the mouth is closed with a tie rope and toggle. At the placing location the concrete is squeezed out by a diver and rammed into place. The use of a diver adds to the cost of the operation.
The type of bags used here are normally made from an open-weave material such as hessian. They should be half-filled with plastic concrete, sealed and then taken under water and
placed by a diver. Partial filling allows them to be moulded into shape and gives them good contact areas with adjacent bags. Grout from the mix seeps through the opentextured material allowing bond to be established with adjacent bags. For additional stability the bags can be spiked together with small-diameter reinforcing bars.
Divers prefer to handle bags of dry-mixed concrete and to grout up between bags. However, this system places too great a responsibility on the diver. The dry mix concrete is never fully wetted-out by water seeping in, the concrete cannot be fully compacted and contact surfaces are minimal.
Diver-handled bags are usually of 10 to 20 litres capacity but 1 m3 bags can be placed using a crane.
5. A CASE STUDY ON BRIDGE FOUNDATION
This section provides brief descriptions of the under concrete construction for Akashi Kaikyo bridge.
o The Akashi Kaikyo Bridge also known as the Pearl Bridge, is the world's longest suspension bridge (measured by the length of the center span of 1,991 metres). It is located in Japan and was completed in 1998.
o The two main tower foundations of the Akashi Kaikyo bridge, Japan, are large double-wall steel caissons filled with tremie concrete. The project required that a large volume of tremie concrete be placed up to 57m below the water surface. o The steel caisson is divided in to an inner core and 16 segments of the outer core.
While the tremie concrete in each segment of the outer core was placed at one time, the concrete in the inner circle was placed in 11 lifts. Each lift was about 3 to 4m in thickness and 54m in diameter.
o Prior to the construction, extensive tests were conducted to select the tremie concrete mixture. The concrete selected was self-leveling with a slump flow of 525mm. the concrete mixture selected was a ternary mixture with cement:slag:flyash proportions of 20:60:20. In addition, a significant portion of limestone powder was added to control the bleeding and improve the cohesion of the mixture.
o All the tremie concrete was produced on a floating batch plant.
o The concrete materials required for one lift of concrete (about 9,000m3) were
collected together on two material barges that were moored to each side of the caisson.
o Each tremie placement was carried out continuosly day and night for 3days. Each tremie pipe covered a 100-m2 area.
o The rate of placement in the inner circle was relatively slow at about 5 to 8cm/hr. due to the fluid characteristics of the concrete, the slow placement rate was necessary to prevent washout.
o The construction joints between the lifts were prepared with underwater robots and airlifting.
o A 3cm thick layer of antiwashout mortar was aced over the construction joint prior to placing another lift of tremie concrete. The total of 50,000m3 of concrete
was placed in the steel caisson.
• Cofferdams are temporary structures and used in cases where the plan area of foundation is very large, depth of water is less and for the soft soils, where soils allow easy driving of sheet piles.
• Caissons are permanent structures and becomes economical in cases where the plan area of foundation is small, large depth of water and for loose soils.
• Suction caisson anchors are gaining considerable acceptance in the offshore industry. The suction caisson is a highly versatile and efficient anchor concept that can be installed easily as compared to driven piles, especially in deep waters. The installation procedure is simple and requires no heavy lift vessel. The geometry to be used is dependent on the soil type.
• At present, the tremie placement method is the standard way of placing high-quality concrete underwater. The other placement method are not able to reliably place high-quality underwater concrete for major structures, although they may find application in special cases
• For massive underwater concrete construction of navigation structures, the pump method should be prohibited.
1. “Assessment of underwater concrete Technologies for in-the-wet
construction of navigation structures”- by Sam X. Yao, Dale E. Berner, Ben
2. “ Underwater concrete”- by Dr Jagadish R
3. “ Foundation engineering”- by R. B. Peck, W. E Hanson, T.H. Thornburn 4. “Concrete technology” – by M S Shetty`