Chapter 4.2 coffer dam, well foundation-final1

coffer dam, well foundation (compiled)

1. Cofferdam- OVERVIEW INTRODUCTION OBJECTIVE TYPES OF COFFERDAM FACTOR AFFECTING DESIGN OF COFFERDAM ADVANTAGES

2. INTRODUCTION The word "cofferdam" comes from "coffer" meaning box, in other words a dam in the shape of a box. 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. 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.

3. INTRODUCTION CONT…. 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.

4. OBJECTIVE The main objectives of temporary structure “COFFERDAM” are :- Pile driving operation. Place grillage & raft foundation. Construct new structures. Enclose space for removal. Constructing without disturbances .

5. TYPES OF COFFERDAM Braced : It is formed from a single wall of sheet piling which is driven into the ground to form a “box” around the excavation site. 2. Earth-Type : It is the simplest type of cofferdam. It consists of an earth bank with a lay core or vertical sheet piling enclosing the excavation .

6. 3. Timber Crib : Constructed on land and floated into place. Lower portion of each cell is matched with contour of river. 4. Double-Walled Sheet Pile : They are double wall cofferdams comprising two parallel rows of sheet piles driven into the ground and connected together by a system of tie rod sat one or more levels.

7. TYPES OF COFFERDAM CONT…… 5. Cellular: 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.

8. Working inside a cofferdam

9. FACTORS AFFECTING DESIGN OF A COFFERDAM Hydrostatic head of water Dimensions of area. Sub-soil condition. Fluctuations of outside water level. Possibility of erosion. Presence of ice.

10. ADVANTAGES Work possible in poor environment. Safe environment for workers. Design responsibility Easy work. Re-usability of materials.

11. 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.

12. We l l Foun d a ti o ns ( Caissons )

13. Introduction Well foundations are being used in India from very early days. Taj Mahal was built on such foundations. Wells are classified as deep foundations. The main difference between a well and a pile foundation: Pile is f l ex i b l e like a beam under horizontal loads well undergoes rigid body movement under such loads

14. Types of Wells/ Caissons

15. Types of Wells We ll s have di f f eren t s ha p es a n d accordingly they are named as ( According to shape in plan) Circular Wells Dumb bell Double-D Wells Double Octagonal Wells S i n gl e an d Doub l e R e c t an g u l ar We ll s M u ltipl e Dre d g ed H o l ed We ll s ( According to Material Used for Construction) • Wood • Steel • Reinforced Concrete

16. Alternate fig.

17. Alternate fig.

18. Classification according to shape in cross section Classification according to cutting edge type

19. Advantages Large diameter, can be extended up to large depths. Similar to piles, but shorter If large bearing capacity soil (rock) is between 3-7 m, it can be very effective If loads are not too large, but the upper 3-7 m of the soil is not appropriate, can be applied effectively Cost of Construction is relatively less on bed level or lower side.

20. Disadvantages • Since the placing of concrete is done for concrete seal under water, it may not be satisfactory. • If any obstruction of boulders or logs are encountered, then progress of work becomes slow. • Through cleaning and inspection at the bottom of caisson is very difficult and hence not possible. • The help of divers may be required for excavation near haunches at the cutting edges.

21. C o m p onen t s of We l l Foun d a ti o n

22. C o m p onen t s of We l l Foun d a ti o n The various component of a well foundations are Cutting Edge Well Curb Bottom Plug Steining Top Plug Well Cap

23. Alternate fig.

24. Open Caisson

25. Cutting Edge: • Sharp edge which is provided at the lower end of the well or open and pneumatic caisson for accelerating sinking operation is called cutting edge. It is made up steel or it is made in R.C.C. Its angle to vertical is 30 0 and normally slope of 1 horizontal to 2 vertical given better result. • In concrete caissons, the lower part of the cutting edge is rigidly fastened with 12 mm steel plates. • The cutting edge should be sharp so as to penetrate into the soil and it should resist the various stresses caused by blows, boulders, blasting, etc. • A sharp vertical edge is generally provided to the outside face of the caisson. Edge facilities the rate of sinking.

26. Well Curbs It is made of concrete or brick. Cutting edge of well or caisson is attached to well curb. During sinking operation well curb impart to the well steining and facilities the formation of bottom

27. Bottom Plug A f t er f i nal g r ou n d i ng o f t he well to the required foundation level, a concrete plug is provided. The bottom plug transfer the entire load to the ground. The bottom plug functions as an inverted dome supported along the periphery of the steining. As it is not feasible to provide reinforcement at the bottom, it is generally made thick and a rich concrete mix (M20) is used.

28. Sand Filling The bottom plug concrete is cured and after curing , the well is filled with sand in saturated condition. Sand filling provides Stability to the bottom of the well . Eliminate the tensile forces at the base Cancels hoop stresses steining induced in

29. Steining • Steining is constructed in concrete or masonry work. • Use of steining is to provide dead load during sinking operation. Topping • Covering provided over the well or caisson is called as topping. • Sand is filled in between topping and bottom plug. Topping also acts as a part of shuttering for laying the well cap.

30. Top Plug The top plug is provided after the filling is completed. Top plug helps in transferring the load of the pier and superstructure to the steining. The thickness of the top plug is generally kept greater than 50 % of the smaller dimension of the dredge hole. If sand filling is used, the top plug is simply constructed using PCC of 1:2:4 otherwise it is reinforced with steel bars and lean concrete of 1:3:6 is used.

31. Well Cap As the shape of the well pier and cap are different, the well cap forms an interim (acting) layer to accommodate the pier. The well cap is so designed that the base of the pier is provided with a minimum all round offset. The centre of the well cap is made to coincide with that of the pier and not with that of the well. Such positioning nullifies the effect of the minor shifts which might have occurred during well sinking.

32. Construction Sequences

33. Well foundation> construction sequences

34. Stability Analysis of Well Foundations

35. Stability Analysis of Well Foundations A well foundation supporting a bridge pier is subjected to vertical and horizontal forces. The various forces acting on the well are Self weight of the well and its superstructure Live loads Water currents and buoyancy Temperature, wind and earth quake Breaking and tracking forces Resistance of the well walls Base and skin friction

36. IRC and IS Design Recommendations The IRC and IS 3955 publications recommend the following procedure for design of well foundations in sand deposits (for clay the expressions should be suitably modified ) Check the stability of well under working loads, assuming elastic theory 2. Find the factor of safety of the well against ultimate failure using ultimate load theory

37. Des ig n o f We ll s Design o f w e l l s b as i c a ll y i n v o l v es f i n d i n g De p th o f the w e l l S i z e o f the w e l l and Des i g n o f the ot h er c o m p o n ents .

38. Tilts & Shifts

39. C a uses of Tilt s and Sh i f t s Non uniform bearing capacity Obstruction on one side of the well Sand blowing in wells during sinking. It will cause sudden sinking of well Method of sinking: Material should be removed from all sides equally otherwise the well may experience tilt Sudden sinking due to blasting may also cause tilting of well Irregular casting of steining will cause less friction on one side leads to chances of tilting of well.

40. Precautions to be taken to avoid tilts and shift The cutting edge of caisson must be thick and sharp pointed The external surface of steining and caisson curb should be smooth. Dredging should be done uniformly on all sides and in all pockets of caisson. Caisson should be symmetrically placed. The diameter (D) of the curb must be placed from 40 mm to 80 mm or larger or more than external diameter of steining.

41. R ec ti f i ca ti o n of Til t E c c en t r i c g ra bbi ng E cce n t r i c l oa di n g Wa t er j e t t i n g Arres ti ng t he c u t t i ng e dg e P u l l i n g t he w e l l S t ru t t i n g t he w e l l P us h i ng t he w e l l b y j a c ks

42. Pushing the caissons or well with jack:

43. E cce n t r i c l oa di n g

44. Wa t er j e t t i n g

45. Providing temporary obstacles below the cutting edge:

46. Pulling the well or caisson

47. Strutting the caissons or well

48. Caisson Diseases

49. In case of sinking process of pneumatic caisson, workers or workmen have to work in working chamber under compressed air. If the compressed air pressure is less than 0.35 N/mm 2 to 0.4 N/ mm 2, then workmen may suffer from the following pains: Workmen may suffer from giddiness There is pains in ears of workmen There is breaking of ear drums of workmen There is bursting of blood vessels in the nose or ears of workmen The above mentioned pains are not that serious or fatal, but workmen is actually suffering during decompression and effect causing depression is called caisson disease.

50. Following are the caisson disease caused by decompression: Severe pains in joints leading to bends It may cause paralytic death Excessive oxygen get absorbed in the blood may cause bursting of vessels. If bubbles are developed in joints it causes bends If the bubbles are developed in spinal cord, it causes paralysis and if the bubble are developed in heart, it causes heart attack. Caisson diseases can be controlled by recompression followed by slow decompression.