Well foundations, also known as caissons, are deep foundations used to transfer structural loads through unstable soil layers to more competent soil or bedrock. They are constructed by sinking a watertight retaining structure (caisson) into the ground and then filling it with concrete. Key components include the cutting edge, well curb, bottom plug, steining, top plug, and well cap. Construction involves excavating inside the caisson while applying an air pressure differential to counter soil and groundwater pressures (pneumatic caisson). Workers are at risk of decompression sickness if pressure changes are not controlled slowly.

1. Well Foundations
( Caissons )

2.  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 flexible like a beam under horizontal
loads
 well undergoes rigid body movement under
such loads

4. Wells have different shapes and
accordingly they are named as
( According to shape in plan)
• Circular Wells
• Dumb bell
• Double-D Wells
• Double Octagonal Wells
• Single and Double Rectangular Wells
• Multiple Dredged Holed Wells
( According to Material Used for Construction)
• Wood
• Steel
• Reinforced Concrete

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

8. • 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.

9. • 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.

11. The various component of
a well foundations are
1. Cutting Edge
2. Well Curb
3. Bottom Plug
4. Steining
5. Top Plug
6. Well Cap

13. Open Caisson

14. 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.

15. 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

16. Bottom Plug
• After final grounding of
the 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.

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

18. 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.

19. 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.

20. 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 aminimum 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.

21. Construction
Sequences

22. Well foundation> construction sequences

24. A well foundation supporting a bridge pier is
subjected to vertical and horizontal forces. The
various forces acting on the well are
1. Self weight of the well and its superstructure
2. Live loads
3. Water currents and buoyancy
4. Temperature, wind and earth quake
5. Breaking and tracking forces
6. Resistance of the well walls
7. Base and skin friction

25. 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)
1 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

26. Design of wells basically involves finding
1. Depth of the well
2. Size of the well and
3. Design of the other components.

28. 1. Non uniform bearing capacity
2. Obstruction on one side of the well
3. Sand blowing in wells during sinking. It will cause sudden
sinking of well
4. Method of sinking: Material should be removed from all sides
equally otherwise the well may experience tilt
5. Sudden sinking due to blasting may also cause tilting of well
6. Irregular casting of steining will cause less friction on one side
leads to chances of tilting of well.

30. 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.

31. 1. Eccentric grabbing
2. Eccentric loading
3. Water jetting
4. Arresting the cutting edge
5. Pulling the well
6. Strutting the well
7. Pushing the well by jacks

32. Pushing the caissons or well with jack:

33. Eccentric loading

34. Water jetting

36. Providing temporary obstacles below the cutting edge:

37. Pulling the well or caisson

38. Strutting the caissons or well

39. Caisson Diseases

40. 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.

41. 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.