Caisson

CAISSON OR WELL FOUNDATION

1. CAISSON PREPARED BY – BHAVIN PUNJANI 1

2. CAISSON Caisson is derived from French word “caisse” meaning a box. It is relatively large deep foundation. It is a box structure in shape of rectangular, round which is sunk from the surface of water or land to the desire depth. It is permanent structure and part of the sub- structure. PREPARED BY- BHAIVN PUNJANI 2

3. Uses Transferring the load of structure to the hard strata. Used for foundation of bridges, piers, abutments in river or lake. Used as impervious core wall of earth dams. To provide an access to a deep shaft or tunnel . PREPARED BY- BHAIVN PUNJANI 3

4. Material Following materials are used for caissons: RCC Steel Timber Cast Iron PREPARED BY- BHAIVN PUNJANI 4

5. types Box caisson (open at top and closed at bottom) Open or well type (open at top and bottom) Pneumatic type ( closed at top and open at bottom) 5 PREPARED BY- BHAIVN PUNJANI

6. Box caisson It is strong water tight vessel open at top and closed at bottom and made of timber, steel or RCC. It is build on land , cured and then float or launched to pier site where it is placed in position. Mainly it is used for shallow depth and for light weight. 6 PREPARED BY- BHAIVN PUNJANI

7. Box caisson 7 PREPARED BY- BHAIVN PUNJANI

8. Box caisson The caisson is sunk by filling sand, gravel, or concrete in the empty space inside. The place where the caisson base is to rest must be levelled and as such box caissons are used in places where the strata of sufficient bearing capacity is available near the ground. In normal practice, the soft natural bottom soil of the river bed is dredged out to some depth and the trench thus formed is filled with sand to have a levelled base. The function of the sand layer is to uniformly distribute the superimposed loads over the soil below and thus avoid tilting of the caisson. 8 PREPARED BY- BHAIVN PUNJANI

9. Box caisson 9 PREPARED BY- BHAIVN PUNJANI

10. Box caisson 10 PREPARED BY- BHAIVN PUNJANI

11. Box caisson 11 PREPARED BY- BHAIVN PUNJANI

12. Open or well caisson common shapes of caisson 12 PREPARED BY- BHAIVN PUNJANI

13. Well components Cutting edge Curb Steining Bottom plug Well cap Top plug Sand filling 13 PREPARED BY- BHAIVN PUNJANI

14. Well component function Cutting edge : provides sharp edge to cut the soil below during sinking operation. Curb : during sinking it acts as extension of cutting edge and provide support the well. It is made of RCC. Steining : it is main body of the well. Made of RCC or masonry with minimum thickness of 45 cm. Bottom plug : made of concrete and designed for an upward load equal to the soil pressure minus self weight of the bottom plug and sand filling. 14 PREPARED BY- BHAIVN PUNJANI

15. Well component function Top plug : it supports the well cap. Well cap : it transfers the load from pier to the well. Sand filling : it transfers a portion of load from well cap to the bottom plug. 15 PREPARED BY- BHAIVN PUNJANI

16. Single wall open caisson This is a box type structure having no top or bottom(during construction) and mainly consists of vertical walls. The method of construction of a typical single wall open caisson is describe below: Wherever necessary the site is made dry by dewatering the area by adopting suitable method (By constructing cofferdam, sheet piling etc.). A pit of suitable size and about 3 to 4 meter deep is excavated at the site where the caisson is to be sunk. The cutting edge is then placed at the bottom of the pit. Above the cutting edge, the wall reinforcements are suitably tied up and the shuttering for casting the concrete wall is erected. The caisson walls are then concreted in lifts to a suitable height. 16 PREPARED BY- BHAIVN PUNJANI

17. Single wall open caisson When the freshly laid concrete has gained sufficient strength, sinking operation is started. The caisson sinks due to its own weight when the soil from the space inside removed by use of clam shell or any other method. As sinking proceeds, additional sections of the caisson walls are successively cast. Sinking is stopped while each section is concreted and it is resumed only after the concreted section has gained sufficient strength. When the caisson is sunk to the required depth, its base is plugged by providing 15 to 45   m thick layer of concrete (concrete seal). The concrete for the seal is placed by use of a tremie pipe or by a bottom dump bucket. After the concrete seal has set, the water inside the caisson is pumped out and the empty space is filled with sand, gravel or concrete. 17 PREPARED BY- BHAIVN PUNJANI

18. Single wall open caisson The rate of sinking of the caisson is always slow because the downward moment of the caisson is resisted by the skin friction of the ground on its walls. At times the skin friction becomes so great that the caisson does not sink even after all the earth has been dredged out from the inside clean down to the cutting edge. In such situations, sinking is resorted to by loading the caisson with additional weights in the form of rails, ingots (block of steel) etc. which are removed afterwards. 18 PREPARED BY- BHAIVN PUNJANI

19. Cylindrical open caisson This may be defined as a cylindrical shell made up of timber, masonry, steel or reinforced concrete shod with a cutting edge and which is sunk by excavating the soil within the shell. The thickness of the caisson wall must be adequate so that when the inside soil is dredged out, it sinks under its own weight. To facilitate sinking of the caisson water jets are sometimes used around the sides which decrease the skin friction. Cylindrical open caisson is also known as well caisson. This type of caisson is similar in all respect to the single wall open caisson except that its wall is circular in plan. The method of construction of well caisson is exactly similar to that of a single wall open caisson described earlier. After the well is sunk to the desired depth its bottom is sealed with concrete. This type of caisson is commonly adopted for providing foundation for bridges and other structures to be built in rivers and waterways. 19 PREPARED BY- BHAIVN PUNJANI

20. Pneumatic caisson This type of caisson is closed at top and open (during construction) at the bottom. The water is excluded from the caisson chamber by means of compressed air. The construction of the pneumatic caisson is similar to the types described above, except that, the working chamber and shaft are made air-tight. In order that the workmen may carry out excavation work underneath the caisson and the water may not find its way inside from below, the pressure of the compressed air in the shaft is kept just higher than that of the water at that depth. Each caisson has two air locks. Through one air lock workmen go down for working while through the other excavated material is taken out. An air lock essentially consists of a steel chamber having two air-tight doors. 20 PREPARED BY- BHAIVN PUNJANI

21. 21 PREPARED BY- BHAIVN PUNJANI

22. Pneumatic caisson One door of this chamber serves as an entry for men and material from outside into the steel chamber and the other door leads to the air shaft. When a workman enters the airlock from outside, the pressure inside the airlock is the same as that of outside atmosphere. Thereafter the outside door is closed and the pressure inside the airlock is raised slowly. When the pressure inside the airlock becomes equal to the pressure in the caisson, the door of the airlock which leads to the air shaft is opened and the workman goes down the air-shaft with the help of a ladder installed therein. Exactly reverse procedure is followed when the workman comes out of the caisson. Air-shaft provides means of access for the worker from airlock down to the working chamber. 22 PREPARED BY- BHAIVN PUNJANI

23. Pneumatic caisson This method of providing foundation is complicated, expensive and very slow. However, in places where it is rather difficult to use bulky equipment required for alternative method, pneumatic caissons appear to be the only choice. The chief advantage of this method is that entire operation of sinking the caisson can be carried out under controlled conditions. This method provides better facilities for removing obstructions, inspection of work and concreting of foundations bed. This type of caisson is suitable for depths ranging from  25  m to 40 m. At higher depths, the persons working inside the caisson for sinking operation are liable to get caisson disease. 23 PREPARED BY- BHAIVN PUNJANI

24. Pneumatic caisson 24 PREPARED BY- BHAIVN PUNJANI

25. Pneumatic caisson sickness When workers working under compressed air inside the working chamber, they suffer certain type of disease when they return to the atmospheric pressure. This disease is known as “Caisson Sickness” or “Caisson Disease” . The main symptoms of the diseases are: Dizziness (vertigo) Double vision Headache Trouble to speaking Pain in body 25 PREPARED BY- BHAIVN PUNJANI

26. Pneumatic caisson sickness (Precautions) No person should work for more than one shift in a day. Shift should not exceed 12 hours. Temperature of the working chamber should be maintain 25 degree centigrade. The main locks should be well ventilated. Persons with strong heart, low blood pressure and good circulation should be employed on the work. Use of alcoholic drinks should be prohibited. Medical facility or chamber should be provided nearer to the work. 26 PREPARED BY- BHAIVN PUNJANI

27. Pneumatic caisson sickness (Precautions) The workers should take enough meal before the shift starts. All the passages, shafts etc. of the caisson should be well ventilated and properly lighted. The worker use man locks for entry, interval and exit. 27 PREPARED BY- BHAIVN PUNJANI

28. Sinking of caisson Mainly following methods are adopted: Air and water jets Blasting Loading Sand island 28 PREPARED BY- BHAIVN PUNJANI

29. Sinking of caisson Air and water jets: In this method, water jets are provided near the cutting edge level to reduce the skin friction. The air or water is forced through the jets which facilitates the sinking of caisson. Blasting : In this method, the explosives are used to remove any obstruction such as rock, boulder, etc. and facilitate the sinking of caisson. The cutting edge is cleared and suitable charge of explosive is provided at a depth of about one or two meter below the curb level. 29 PREPARED BY- BHAIVN PUNJANI

30. Sinking of caisson Loading : Applied uniform load on the top of the caisson to facilitate sinking. Sand island : This method is also known as an artificial island method. It is adopted when subsoil conditions are not in position to keep caisson stable. In this method sink a steel cylinder around the site of work and then filling this cylinder with sand or other dredged material. The caisson is sunk through this filling. 30 PREPARED BY- BHAIVN PUNJANI

31. Sinking of caisson Sand island 31 PREPARED BY- BHAIVN PUNJANI

32. Problems in well sinking The major problems in well sinking are: Sand blowing Tilting of wells Shifting of wells 32 PREPARED BY- BHAIVN PUNJANI

33. Problems in well sinking Sand blowing : The trouble of sand blowing takes place during the process of dewatering of the well passing through sandy strata. The ground around the well starts breaking up and wide cracks are formed. The fall of sand in the caisson is so sudden and huge in amounts to a depth of about 3 to 15m of sand. This may result in fatal accident. When sand blowing occurs, dewatering of caisson should immediately discontinued and bundles of grass or some such filling materials should be placed all around the steining to control the sand blowing. 33 PREPARED BY- BHAIVN PUNJANI

34. Problems in well sinking Tilting of wells: When a well sinks more or one side than the other, then it is known to have tilted. The tilting is mainly due to unequal dredging and non-uniform bearing power of soil. Following methods are used to control the tilting : Control of dredging Eccentric loading Water jetting Pulling the well Pushing by jacks 34 PREPARED BY- BHAIVN PUNJANI

35. Problems in well sinking Control of dredging : Dredging is done only near to the portion which has tilted. The portion which is higher than other is termed as tilted edge. This method is not very effective when the well has been sunk to a great depth. In such case, a hole is made in steining and by hooks, the rope of the dredge is pulled towards higher side to the maximum possible extent. The hole is made near the ground level. 35 PREPARED BY- BHAIVN PUNJANI

36. Problems in well sinking 36 PREPARED BY- BHAIVN PUNJANI

37. Problems in well sinking Eccentric loading : In this method heavy loads are placed on the tilted edge. The moment caused by heavy loads brings the well in true position. 37 PREPARED BY- BHAIVN PUNJANI

38. Problems in well sinking Water jetting : In this method, water jet is forced on the outer faces of the well towards the higher side so that skin friction is reduced towards the higher side. This method is effective in case the well is being sunk in sandy strata. 38 PREPARED BY- BHAIVN PUNJANI

39. Problems in well sinking Pulling the well: In this method, the well is pulled towards the higher side by placing one or more steel ropes round the well with vertical sleepers packed in between. This method is effective only in early stages of sinking. 39 PREPARED BY- BHAIVN PUNJANI

40. Problems in well sinking Pushing by jacks: In this method, the well may be pushed by jacks on the tilted side of wall. 40 PREPARED BY- BHAIVN PUNJANI

41. Problems in well sinking Shifting of wells: If a simple tilt is occurs at a certain depth and the sinking continued till designed foundation depth is reached, the shift at the bottom could be greater than a top. 41 PREPARED BY- BHAIVN PUNJANI

42. THANK YOU 42 PREPARED BY- BHAIVN PUNJANI