Specification Part 3 - Road & Bridge Works - Urban Administration ...

The well shall be uniformly seated at the founding strata. It shall be ensured by lest borings that
the properties of the soil encountered at the founding strata and upto a depth of one and a half times
the well diameter is identical to that adopted in the design. The procedure
for test borings shall satisfy the provisions of these specifications. In case the soil encountered is
inferior to that adopted in design, the well shall be re-designed by the Engineer adopting the soil
properties actually encountered and the founding level intimated to the Contractor, who shall carry out
the work.
In case of seating of wells in hard rocky strata, where the rock profile is steeply sloping,
pneumatic methods of sinking may be adopted to seat the well evenly as directed by the Engineer. The
decision of adopting pneumatic sinking shall be taken by the Engineer. The cutting edge may also be
embedded for a suitable depth in the rocky strata, as decided by the Engineer keeping in view the
quality of rock. As an additional measure of safety, the well shall be anchored to the rocky strata by
anchor bars provided in the steining of the well, as shown on the drawing irrespective of the fact that
tension develops or not at the base of the well under design loads. After the well has been evenly
seated on good hard rock, arrangements shall be made to facilitate proper inspection in dry and visible
conditions before the bottom plug is laid.
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 per cent more cement than that required for
the same mix placed dry, to cater for underwater concreting. However, the total cement content shall
not be less than 363 kg/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 herein.
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
he required height.
Least disturbance shall be caused to the water inside the welt 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 lev el 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 starling 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 as shown on
the drawing, or as directed by the Engineer.