1200.Well foundation

Updated: Jul 22, 2020

1201. DESCRIPTION

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 as well cap in accordance with the details shown on the drawing

In case of well foundations of size larger than 12 m diameter, supplemental construction Specifications will be necessary.


1202. GENERAL

1202.1. Wells may have a circular, rectangular, or D-shape in plan and may

consist of one, two or more compartments in plan. The outer wall of the well is known as well steining, which may be cellular.

The process of taking down the well to be founding levels is known as well

sinking. After reaching the founding level, the hollow inside the well, (“dredge hole”) is plugged at the bottom by concrete (“bottom plug”). The dredge hole is then filled with approved filling upto the level indicated on the drawings and provided with a concrete plug (“top plug”).

To facilitate sinking of well, steel cutting edge is fabricated and connected to a

concrete well curb of required shape. On top of the well curb, adequate height of well steining is cast and the process of sinking is carried out. After a portion of the well has been sunk, another height of well steining is cast on top of the previous section and further sinking carried out. This process is continued till the bottom level of the well reaches the founding level.

At the top of the well steining, an adequately designed “well cap” is laid which

transmits the loads and forces from the sub-structure (piers or abutments) to the

foundations.


1202.2. At least one bore-hole must be available/carried out in accordance with

these specifications at each well foundation location, prior to commencement of work. The depth of bore-holes should extend upto a depth equal to one and a half times the outer diameter/least dimension of the well below the anticipated founding level. The results of soil exploration should be presented in accordance with clause 7.4 of IRC: 78. In case the well foundation is to rest on a rocky strata, it may be necessary to undertake additional borings/ probing prior to commencement of work to ascertain the actual profile and the quality of the rocky strata, at the level at which the well has to be seated, etc.


1202.3. Blasting may have to be resorted to in order to facilitate sinking through

difficult strata, such as boulders and rocks etc. In case blasting is anticipated,

protective/strengthening measures specified in clause 710.6 (IV) of IRC: 78 shall be

taken. The grade of concrete in bottom 3 metres of steining shall not be leaner than M 20 or as shown on the drawings.


1202.4. In case the bore hole data shows the presence of steeply dipping rock,

chiselling may have to be resorted to so as to obtain proper seating of the foundation. For this purpose, the well may require to be dewatered completely under high air pressure inside the well. This process is known as pneumatic sinking. Pneumatic sinking may also have to be resorted to in cases where obstacles such as tree trunks, large sized boulders or hard strata etc. cannot be removed by open dredging. The necessity of adopting pneumatic sinking shall be decided b the Engineer.

The curb and steining have to be specifically designed for special loadings when

pneumatic sinking is adopted.


1203. SETTING OUT AND PREPARATIONS FOR SINKING

1203.1. Necessary reference points shall be fixed, away from the zone of blowups or possible settlements resulting from well sinking operations. Such reference points

shall be connected to the permanent theodolite stations with the base line on the banks. The centre of the individual wells shall be marked with reference to these stations. The distance, wherever practicable, shall be checked with the help of accurate tapes and precision instrument.

Reference points shall also be fixed to mark X-X axis (usually traffic direction)

and Y-Y axis (normal to X-X axis) accurately.

A temporary bench mark shall also be established near the well foundation, away from the zones of blow-ups or possible settlement. The bench mark shall be checked regularly with respect to the permanent bench mark established at the bridge site.


1203.2. For wells which are to be pitched in water, an earthen or sand island

shall be constructed. Sand islands are practicable for water depths of about 5 meters under stable under stable bed soil conditions. For greater depths or in fast flowing rivers or for locations where soil is too weak to sustain sand island, floating caissons may have to be adopted.

The plan dimensions of sand islands shall be such as to have a working space of at least 2 metres all around the steining. The dimension of the sand islands shall however be not less than twice the dimension in plan of the well or caisson. Sand islands shall be maintained to perform their functions, until the well is sunk to a depth below the bed level at least equal to the depth of water.

Sand Island shall be protected against scour and the top level shall be sufficiently

above the prevailing water level to be decided by the Engineer so that it is safe against wave action.

While sand islands are constructed at well location, floating caissons are generally fabricated at or near the banks on dry land or dry docks. Floating caissons are towed into position in floating condition.

Floating caissons may be of steel, reinforced concrete or a combination of the two. They should have at least 1.5 m free board above water level and increased, if considered necessary, in case there is a possibility of caissons sinking suddenly owing to reasons such as scour likely to result from the lowering of caissons, effect of waves, sinking in very soft strata etc.

For floating caissons, a detailed method statement for fabrication, floating and sinking of caissons shall be prepared and furnished to the Engineer. Such statement shall include the total tonnage of steel involved, fabrication and welding specifications, list of materials and plant and a description of operations and manpower required for the work.The caisson shall be tested for leakages before being towed to site.

1203.3. Equipment

Equipment shall be deployed for construction of well foundation as required and

as directed by the Engineer. Generally, the following equipments may be required for the

work:

(a) Crane with grab buckets capacity 0.5 to 2.0 cum.

(b) Submersible pumps

(c) Air compressors, air locks and other accessories where pneumatic sinking of

well is anticipated.

(d) Chisels of appropriate sizes.

(e) Aqua-header for cutting rocky strata

(f) Diving helmets and accessories

(g) Equipment for concrete production, transportation planing and compaction

1204. CUTTING EDGE

1204.1. 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.

1204.2. The parts of cutting edge shall be erected on level firm ground.

Temporary supports shall be provided to facilitate erection and maintaining the assembly in true shape. The fabrication may be carried out in the shop or at site. Steel sections shall not be heated and forced into shape. However, “V” cuts should be closed by welding. Joints in the lengths of structural sections, unless otherwise specified shall be fillet welded using single cover plate to ensure the requisite strength of the original section.

1204.3. The cutting edge shall be laid about 300 mm above prevalent water

level.


1205. WELL CURB

1205.1. 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 hours.

1205.2. All concreting in the well curb shall be done in one continuous operations.


1206. WELL STEINING

1206.1. 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.

1206.2. Steining built in the first lift above the well curb shall not be more than

2 metres and in subsequent lifts it shall not exceed the diameter of the well or the depth of well sunk below the adjoining bed level at any time. For stability, the first lift of steining shall be cast only after sinking the curb at least partially for stability. Concreting of steining may be carried out in subsequent lifts of about 2 to 2.5 metres. Attempts should be made to minimise the number of construction joints. The concreting layers shall be limited to about 450 mm restricting the free fall of concrete to not more than 1.5 m. Liatance formed at the top surface of a lift shall be removed to expose coarse aggregates before setting of concrete at the proposed construction joint. As far as possible, construction joints shall not be kept at the location of laps in the vertical steining bars.

1206.3. 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. The work will be checked carefully with the aid of straight edges of lengths approved by the Engineer. Plumb bob or spirit level shall not be used for alignment. After sinking of a stage is complete, damaged portions if any, of steining at top of the previous stage shall be properly repaired before constructing the next stage.

1206.4. The height of steining shall be calibrated by making at least 4 gauges

(preferably in traffic direction and in a direction normal to traffic direction) distributed equally on the outer periphery of the well, with every metre mark shown in black paint.The gauges shall start with zero at the bottom of the cutting edge. Marking of the gaugesshall be done carefully with a steel tape.

1206.5. After reaching the founding level, the well steining shall be inspected to

check for any damage or cracks. The Engineer will direct and the Contractor shall execute the remedial measures before acceptance of the well steining. In case the well cannot be accepted even with any remedial measures, then the well shall stand rejected.


1207. WELL SINKING

1207.1. General

The well shall as far as possible be sunk true and vertical through all types of

strata.

Sinking or loading of the well with kentledge shall be commenced only after the steining has been cured for at least 48 hours or as specified in the drawings.

No well shall be permitted to be placed in a pre-dredged hole.

The well shall be sunk by excavating material uniformly from inside the dredge

hole. Use of water jetting, explosives and divers may be adopted for sinking of wells through difficult strata with prior approval of the Engineer.

Normally dewatering of well should not be permitted as a means for sinking the well. It also shall never be resorted to if there is any danger of sand blowing under the well. Dewatering shall however be done when well is to be founded into rock. Pneumatic sinking may have to be resorted to where obstacles such as tree trunks, large size boulders, etc. are met at the bottom or when there is hard strata which cannot be removed by open dredging. The necessity for pneumatic sinking shall be decided by the Engineer.


1207.2. Use of Kentledge as Sinking Load

Kentledge shall be placed in an orderly ad safe manner on the loading platform

and in such a way that it does not interfere with the excavation of the material from inside the dredge hole and also does not in any way damage the steining of the well.

Where tilts are present or there is a danger of well developing a tilt, the position

of the load shall be regulated in such a manner as to provide greater sinking effort on the higher side of the well


1207.3. Use of Water Jetting

Water jetting may be employed for well sinking wherever necessary.


1207.4. Use of Explosives

Mild explosive charges may be used as an aid for sinking of the well only with

prior permission of the Engineer. Blasting of any sort shall only be done in the presence of the Engineer and not before the concrete in the steining has hardened sufficiently and is more than 7 days old. When likelihood of blasting is predicted in advance, protection of the bottom portion of the well shall be done as per these specifications. After blasting operations are completed, the well curb and steining should be examined for any cracks and remedial measures taken.

If blasting has been used after the well has reached the design foundation level, normally 24 hours shall be allowed to lapse before the bottom plug is laid.

The charges shall be exploded well below the cutting edge by making a sump so as to avoid chances of any damage to the curb or to the steining of the well. A minimum sump of 1 metre depth should be made before resorting to blasting. Use of large charges, 0.7 kg or above, may not be allowed except under expert direction and with the permission from the Engineer. Suitable pattern of charges may be arranged with delay detonators to reduce the number of charges fired at a time. The burden of the charge may be limited to 1 metre and the spacing of holes may normally be kept as 0.5 to 0.6 metres.

All prevalent laws concerning handling, storing and using of explosives shall be

strictly followed.

All safety precautions shall be taken as per IS: 4081 “Safety Code for Blasting

and related Drilling Operations”, to the extent applicable, whenever blasting is resorted to.

There should be no equipment inside the well not shall there be any labour in the close vicinity of the well at the time of exploding the charges.

If rock blasting is to be done for seating of the well, the damage caused by flying

debris should be minimised by covering blasting holes by rubber mats before blasting.


1207.5. Use of Divers

Use of divers may be made both for sinking purpose like removal of obstructions,

rock blasting and for inspection. All safety precautions shall be taken as per any

acceptable safety code for sinking with divers or any statutory regulations in force.

Only persons trained for the diving operation shall be employed and shall be

certified to be fit for diving by an approved doctor.

They shall work under expert supervision. The diving and other equipments shall be of acceptable standard and certified to this effect by an approved independent agency.It shall be well maintained for safe use.

Arrangement for ample supply of low pressure clean cool air shall be ensured

through an armoured flexible hose pipe. Standby compressor plant shall be provided in case if breakdown.

Separate high pressure connection for use of pneumatic tools shall be made.

Electric lights where provided shall be at 50 volts (maximum). The raising of the diver from the bottom of wells shall be controlled so that decompression rate conforms to the rate as laid down in appropriate regulations.


1207.8. Tilts and Shifts

The inclination of the well from the vertical is known as tilt and the horizontal

displacement of the centre of the well at the founding level from its theoretical position is known as shift.

Unless otherwise specified, the tilt of any well shall not exceed I (horizontal) in 80 (vertical), and the shift at the well base shall not be more than 150 mm in any resultant direction.


1208. BOTTOM PLUG

For bottom plug, the concrete mix shall be designed (in dry condition) to attain

the concrete strength as mentioned on the drawing and shall contain 10 % more cement than that required for the same mix placed dry, to cater for another water concreting.

However, the total cement content shall not be less than 363kg/cu.m of concrete with a slump in the range of 150 mm to 200 mm. Admixtures may be added to the concrete to impart the required characteristics indicated here in.

Concrete for the bottom plug shall be laid by tremie pipe method. Tremie concrete when started shall be continued without interruption for full concreting in the bottom plug. The concrete production equipment and placement equipment should be sufficient to enable under water concreting within stipulated time. Necessary standby equipment should be available for emergency situation.

Before commencing plugging, all loose material from the bottom of the well shall be removed.

Concreting shall be done in one continuous operation till the dredge hole is filled upto the required height and thereafter sounding shall be taken upto ensure that the concrete has been laid to the required height.

Least disturbance shall be caused to the water inside the well while laying

concrete in the bottom plug.

Concrete shall not be disturbed in any way for at least 14 days.

In order to check any rise in the level of the bottom plug soundings should be taken at the close of concreting and once every day for the subsequent 3 days.

The soundness of the bottom plug may be tested by dewatering the well by 5 metres below the surrounding water level and checking the rise of water. The rate of rise shall preferably be less than 10 cms per hour. In case the rate is higher, suitable remedial measures as directed by the Engineer, shall be taken by the Contractor at his own cost.


1209. 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.


1210. TOP PLUG

After filling sand upto the required level a plug of concrete shall be provided over

it is shown on the drawing or as directed by the Engineer.


1211. WELL CAP

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.

The bottom of the well cap shall be laid preferably as low as possible, taking

account the water level prevalent at the time of casting. Bond rods of steining shall be anchored into the well cap.


1212. TOLERANCES

The permissible tilt and shift shall not exceed 1 (horizontal) in 80 (vertical) and

the shift at the well base shall not be more than 150 mm in any resultant direction.

For the well steining and well cap the permissible tolerances shall be as follows:

a) Variation in dimension : +50 mm –10 mm

b) Misplacement from specified position in plan : 15 mm

c) Surface irregularities measured with 3 m straight edge: 5 mm

d) Variation of levels at the tope : 25 mm


1213. TESTS AND STANDARDS OF ACCEPTANCE

The materials shall be tested in accordance with these Specifications and shall

meet the prescribed criteria.

The work shall conform to these Specifications and shall meet the prescribed standards of acceptance.

89 views0 comments

Recent Posts

See All