This work consists of construction of well foundation, taking it down to the founding level through all kinds of sub-strata, plugging the bottom, filling the inside of the well, plugging the top and providing a well cap in accordance with the details shown on the drawing. Well may have a circular, rectangular, or D-shape in plan and may consist of one, two or more compartments in plan. 

Well Components & their Function

In brief the function of various elements is as follows:

1. Cutting edge  

The mild steel cutting edge shall be made from structural steel sections. The cutting edge shall weigh not less than 40 kg per metre length and be properly anchored into the well curb, as shown in the drawing.

         When there are two or more compartments in a well, the bottom end of the cutting edge of the inner walls of such wells shall be kept at about 300 mm above that of outer walls.

 2. Curb

The well curb may be precast or cast-in-situ. Steel formwork for well curb shall be fabricated strictly in conformity with the drawing. The outer face of the curb shall be vertical. Steel reinforcements shall be assembled as shown on the drawings. The bottom ends of vertical bond rods of steining shall be fixed securely to the cutting edge with check nuts or by welds.

         The formwork on outer face of curb may be removed within 24 hours after concreting. The formwork on inner face shall be removed after 72 hoursIt is made up of reinforced concrete using controlled concrete of grade M-35.

3. Steining

The dimensions, shape, concrete strength and reinforcements of the well shall strictly conform to those shown on the drawings. The formwork shall preferably be of M.S. sheets shaped and stiffened suitably. In case timber forms are used, they shall be lined with plywood or M.S. sheets.  The steining of the well shall be built in one straight line from bottom to top such that if the well is tilted, the next lift of steining will be aligned in the direction of the tilt. After reaching the founding level, the well steining shall be inspected to check for any damage or cracks

4. Bottom plug  

Its main function is to transfer load from the steining to the soil below. For bottom plug, the concrete mix shall be design (in dry condition) to attain the concrete strength as mentioned on the drawing and shall contained 10 per cent more cement than that required for the same mix placed dry.

5. Sand filling

Sand filling shall commence after a period of 3 days of laying of bottom plug. Also, the height of the bottom plug shall be verified before starting sand filling. Sand shall be clean and free from earth, clay clods, roots, boulders, shingles, etc. and shall be compacted as directed. Sand filling shall be carried out upto the level shown on the drawing or as directed by the Engineer.

6. Intermediate plug

The function of the plug is to keep the sand filling sandwiched & undisturbed. It also act as a base for the water fill, which is filled over it up to the bottom of the well cap.

7. Top plug

After filling sand up to the required level a plug of concrete shall be provided over it as shown on the drawing, It at least serves as a shuttering for laying well cap.

8. Reinforcement

It provides requisite strength to the structure during sinking and service.

9. Well cap

It is needed to transfer the loads and moments from the pier to the well or wells below. A reinforced cement concrete well cap will be provided over the top of the steining in accordance with the drawing. Formwork will be prepared conforming to the shape of well cap. Concreting shall be carried out in dry condition. A properly designed false steining may be provided where possible to ensure that the well cap is laid in dry conditions

After water filling, pre-cast RCC slabs shall be placed over the RCC beams as per the drawings, as non-recoverable bottom shuttering for well cap. Initially built false wall shall act as outer shuttering for well cap casting. In case, there is no false wall, then steel shuttering is to be put from outer side.

For well Steining and well cap shuttering, permissible tolerances are as follows: -

• Variation in dimension                                      :         +50 mm to –10mm
• Misplacement from specified Position in Plan :          15mm
• Variation of levels at the top                              :         +/- 25mm

Depth of Well Foundation

As per I.R.C. bridge code , the depth of well foundation is to be decided on the following considerations:

• The minimum depth of foundation below H.F.L should be 1.33D, where D is the anticipated max. Depth of scour below H.F.L depth should provide proper grip according to some rational formula.
• The max. Bearing pressure on the subsoil under the foundation resulting from any combination of the loads and forces except wind and seismic forces should not exceed the safe bearing capacity of the subsoil, after taking into account the effect of scour. With wind and seismic forces in addition, the max. bearing pressure should not exceed the safe bearing capacity of the subsoil by more than 25%
• While calculating max. Bearing pressure on the foundation bearing layer resulting from the worst combination of direct forces and overturning moments. The effect of passive resistance of the earth on the sides of the foundation structure may be taken into account below the max. Depth of the scour only.

The effect of skin friction may be allowed on the portions below the max. Depth of scour. Accordingly for deciding the depth of well foundation we require correct estimation of the following:

1. Max. Scour depth.
2. Safe bearing capacity.
3. Skin friction.
4. Lateral earth support-below max. scour level.

It is always desirable to fix the level of a well foundation on a sandy strata bearing capacity. Whenever a thin stratum of clay occurring between two layers of sand is met with in that case well must be pierced through the clayey strata. If at all foundation has to be laid on a clayey layer it should be ensured that the clay is stiff.

Sinking of Wells

• In case of well sinking on dry grounds, an open excavation upto half a metre above subsoil water level is carried out and the well curb is laid. In case the wells are to be sunk in mid stream, a suitable cofferdam is constructed around the site of the well and islands are made.
• The islands in shallow water are formed by an edging of sand bags forming an enclosure filled with sand or clay. When the water depth is of the order of 3 to 5 m. the site is surrounded by sheet piling and the enclosure so formed is filled with clay or sand. The centre point of well is accurately marked on the island and the cutting edge is placed in a level plain. The wooden sleepers are inserted below the cutting edge at regular intervals so as to distribute the load and avoid setting of the cutting edge unevenly during concreting. The inside shuttering of the curb is generally made of brick masonry and plastered. The outer shuttering is made of wood or steel.
• Initially the well steining should be built to a height of 2m. Only. Later steining should not allowed to be built more than 5m. at a time. For this bridge the subsequent lifts were of 2.5 m. each.
• The well is sunk by excavating material from inside under the curb. Great care should be taken during well sinking in the initial stages because the well is very unstable. Excavation of the soil inside the well can be done by sending down workers inside the wells. When the depth of the water inside the well becomes more than one meter, the  excavation is then carried out by a Jham or a Dredger.
• The sump position at 8 equidistant locations along dredge hole sides & at well center are taken & recorded. The dredge water level is also recorded. 
• Vertical reinforcement of steining shall be bent & tied properly to facilitate the grab movement during sinking operations. 
• The position of the crane shall be such that the operation shall be able to see the signalman on the well top at all the times, & the muck is safely deposited away from the intermediate vicinity of the well.
• Grabbing process shall commence normally with the grabbing at the above designated sounding positions.
• If the well is not sinking after reasonable amount of grabbing is done, say after two rounds of grabbing, the sump position shall be checked accordingly, in combination with the tilt position, the grabbing pattern shall vary. The sump should not normally exceed 1.75m average. And thereafter, air jetting or water jetting shall be resorted to.
• The sinking operation shall be done in two shifts, day & night. In normal coarse, the sump and the dredge hole water levels shall be observed twice in each shift, and the cutting edge reduced level shall be checked by level at four positions at the end of the shift.
• As the well sinks deeper, the skin friction on the sides of the well progressively increases. To counteract the increased skin friction and the loss in weight of the well due to buoyancy, additional loading known as kentledge is applied on the well. The kentledge is comprised of iron rail, sand bags concrete blocks etc.

• Pumping out of water from the inside of the well is effective when the well has gone deep enough or has passed through a clayey stratum so that chances of tilts and shifts are minimized during this process. When the well has been sunk to about 10 m. depth, sinking thereafter should be done by grabbing, chiseling and applying kentledge. Only when these methods have failed dewatering may be allowed upto depressed water level of 5 m. and not more.

In case of sandy strata frictional resistance developed on the outer periphery is reduced considerably by forcing jet of water on the outer face of the well all round.