1 COFFERDAM

2 INTRODUCTION The word "cofferdam" comes from "coffer" meaning box, in other words a dam in the shape of a box. Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent structure in the dry. A cofferdam involves the interaction of the structure, soil, and water and 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

3 TYPES OF COFFERDAM Earth fill cofferdams Sand bag cofferdamRock fill cofferdam Timber crib or Rock crib coffer dam Braced/ Sheet pile coffer dam 1.Single wall cofferdam 2.Double wall cofferdam 3.Cellular cofferdam Movable or suspended coffer dam

4 EARTH-TYPE COFFERDAM It is the simplest type of cofferdam. It consists of an earth bank with a clay core or vertical sheet piling enclosing the excavation. It is used for low-level waters with low velocity and easily scoured by water rising over the top.

5 SAND BAG COFFERDAM

6 ROCK FILL COFFERDAM

7 TIMBER CRIB COFFERDAM Constructed on land and floated into place.Lower portion of each cell is matched with contour of river bed. It uses rock ballast and soil to decrease seepage and sink into place, also known as “Gravity Dam”. It usually consists of 12’x12’ cells and is used in rapid currents or on rocky river beds. It must be properly designed to resist lateral forces such as tipping / overturning and sliding.

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9 BRACED COFFERDAM It is formed from a single wall of sheet piling which is driven into the ground to form a “box” around the excavation site. The box is then braced on the inside and the interior is dewatered. It is primarily used for bridge piers in shallow water ( ftdepth)

10 DOUBLE-WALLED SHEET PILEThey are double wall cofferdams comprising two parallel rows of sheet piles driven into the ground and connected together by a system of tie rods at one or more levels. The space between the walls is generally filled with granular material such as sand, gravel or broken rock

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12 CELLULAR COFFERDAMS Cellular cofferdams are used only in those circumstances where the excavation size precludes the use of cross-excavation bracing. In this case, the cofferdam must be stable by virtue of its own resistance to lateral forces

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14 COFFERDAM COMPONENTS 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 withone 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.

15 2.BRACING FRAME 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

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17 ADVANTAGES OF COFFERDAMAllow excavation and construction of structures in otherwise poor environment Provides safe environment to work Contractors typically have design responsibility Steel sheet piles are easily installed and removed Materials can typically be reused on other projects

18 COFFERDAM CONSTRUCTION SEQUENCE:For a typical cofferdam, such as for a bridge pier, the construction procedure follow the listed pattern. 1. Pre-dredge to remove soil or soft sediments and level the area of the cofferdam 2. Drive temporary support piles 3. Temporarily erect bracing frame on the support piles 4. Set steel sheet piles, starting at all four corners and meeting at the center of each side 5. Drive sheet piles to grade 6. Block between bracing frame and sheets, and provide ties for sheet piles at the top as necessary 7. Excavate inside the grade or slightly below grade, while leaving the cofferdam full of water 8. Drive bearing piles 9. Place rock fill as a leveling and support course 10. Place tremie concrete seal

19 11. Check blocking between bracing and sheets 12. Dewater 1311. Check blocking between bracing and sheets 12. Dewater 13. Construct new structure 14. Flood cofferdam 15. Remove sheet piles 16. Remove bracing Backfill

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22 SHEET PILES Sheet piles are thin piles, made of plates of concrete, timber or steel, driven into the ground for either separating members or for stopping seepage of water. It is not mean for carrying any vertical load It is driven into the ground with the help of suitable pile during equipment and their height is increased while driving by means of addition of successive installments of sheets

23 FUNCTIONS OF SHEET PILESTo enclose a site or part there of to prevent the escape of loose subsoil, such as safegaurd against settlement. To retain the sides of the trenches and general excavation. To protect river bank To prevent seepage below dams and other hydraulic structures

24 SHEET PILES MATERIALS Concrete sheet piles Timber sheet pilesSteel sheet piles

25 CONCRETE SHEET PILES Concrete sheet piles are reinforced precast unitsThe width of each unit may vary from 50cm to 60cm and thickness vary from 2cm to 6cm Typical sheet piles with proper jointing arrangements For important works prestressed precast concrete sheet piles are used

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27 TIMBER SHEET PILES It is used only for temporary work.The width of sheet may vary from 225 to 280mm and thickness should not be less than 50mm Jointed by either butt or V-joints Feet are bevelled and shod with sheet iron

28 STEEL SHEET PILES Steel sheet piles are most commonly usedIt is trough shaped and driven the piles are interlocked with alternate ones reversed. It is available in different shapes under different trade names It is made from steel sheets 20 to 30cm wide and 4 to 5m long

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31 CAISSONS Caisson is derived from french word caisse meaning chest or box. It is a box like structures, round or rectangular It is sunk from the surface is either land or water to some desired depth. It is divided into three types 1.Open caissons 2.Box caissons 3.Pneumatic caissons

32 OPEN CAISSONS Open caissons are concrete shafts that remain open at the top and bottom during construction. The bottom of the caisson of the caisson has a cutting edge. The caisson is sunk into place, and soil from the inside of the shaft is removed by grab buckets until the bearing stratum is reached. The shafts may be circular, square, rectangular, or oval. Once the bearing stratum is reached, concrete is poured into the shaft (under water) to form a seal at its bottom. When the concrete seal hardens, the water inside the caisson shaft is pumped out. Concrete is then poured into the shaft to fill it. Open caissons can be extended to great depths, and the cost of construction is relatively low. One of their major of disadvantages is the lack of quality control over the concrete poured into the shaft for the seal. The bottom of the caisson cannot be thoroughly cleaned out. An alternative method of open-caisson construction is to drive some sheet piles to form an enclosed area, which is filled with sand and is generally referred to as a sand island. The caisson is then sunk through the sand to the desired bearing stratum. This procedure is somewhat analogous to sinking a caisson when the ground surface is above the water table.

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34 BOX CAISSONS Box caissons are caissons with closed bottoms. They are constructed on land and then transported to the construction site. They are gradually sunk at the site by filling the inside with sand, ballast, water, or concrete. The cost for this type of construction is low. The bearing surface must be level, and if it is not, it must be leveled by excavation.

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36 PNEUMATIC CAISSONS Pneumatic caissons are generally used for depths of about ft (15-40 m). This type of caisson is required when an excavation cannot be kept open because the soil flows into the excavated area faster than it can be removed. A pneumatic caisson has a work chamber at the bottom that is at least 10 ft (≈3 m) high. In this chamber, the workers excavate the soil and place the concrete. The air pressure in the chamber is kept high enough to prevent water and soil from entering. Workers usually do not counter severe discomfort when the chamber pressure is raised to about 15 lb/in2(≈100 kN/m2) above atmospheric pressure. Beyond this pressure, decompression periods are required when the workers leave the chamber. When chamber pressures of about 44 lb/in2(≈300 kN/m2) above atmospheric pressure are required, workers should not be kept inside the chamber for more than 1122 hours at a time.

37 Workers enter and leave the chamber through a steel shaft by means of a ladder.This shaft is also used for the removal of excavated soil and the placement of concrete. For large caisson construction, more than one shaft may be necessary, an airlock is provided for each one. Pneumatic caissons gradually sink as excavation proceeds. When the bearing stratum is reached, the work chamber is filled with concrete. Calculation of the load-bearing capacity of caissons is similar to that for drilled shafts. Therefore, it will not be further discussed in this section.

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