Sinking of wells usually encounters difficulties in the form of tilting and shifting of wells. Tilting is the deviation of the vertical alignment of the well and shifting is the deviation from the horizontal alignment. Tilting and shifting are associative; shifting occurs if tilting is large and uncontrolled. Shifting can be eliminated, if the well is sunk in true vertical alignment.
Tilting and shifting of wells, if uncontrolled in the early stages, will aggravate and would undermine the stability of the foundation. It may be said that controlling the tilting and shifting of wells is one of the most essential elements of well sinking. Continuous and careful monitoring of the entire sinking process needs to be done and when tilting or shifting of a well is observed, adequate measures should be taken to rectify the same. Proper records of the well sinking, tilting, shifting, and their control should be maintained.
It is safer and economical to avoid tilting and shifting of wells by adopting the following preventive measures:
1. The outer surface of the well curb and steining should be level, straight, and smooth.
2. The radius of the well curb should be kept 2-4 cm more than the outer radius of the well steining.
3. The cutting edge should be sharp and of uniform thickness.
4. The steining should be built in lifts and the entire steining height should be built in one straight line from bottom to top at right angles to the plane of the curb.
5. Dredging should be uniform on all sides of the well. For a twin well, dredging should be uniform in both the wells.
6. The well should be constructed in stages of small height increments.
7. The magnitude and direction of sinking of wells should be properly and carefully monitored on a continuous basis to identify any tilt or shift and adopt appropriate corrective measures immediately to rectify the same.
8. If the well shows a tendency for tilting, dredging should be done on the higher side. If this does not bring required improvement, sinking should be stopped and should be resumed only after the tilting is corrected.
9. Dredged material should not be deposited unevenly around the well.
10. When a kentledge is used to provide additional sinking effort, it should be placed evenly on the loading platform.
Continuous monitoring should be done to identify if any tilt or shift of the well occurs and following measures should be adopted as deemed appropriate as per the decision of the Engineer-in-charge:
1. Controlled dredging should be done more on the higher side, that is, on the opposite side of the tilt, as shown in Fig. 21.12(a). This method is helpful only in the initial stages of well sinking.
2. Eccentric loading is applied by using extra downward pressure on the higher side, as shown in Figs. 21.12(b) and 21.12(c).
3. Pulling the well from the higher side in the opposite direction of tilting is also effective in rectifying the tilt. The pull can be applied by winding steel wire rope around the well through an anchor. Wooden sleepers are used during pulling to avoid damage to the well steining, as shown in Fig. 21.12(d). The pull can also be applied by using an anchor below the cutting edge on the higher side, as shown in Fig. 21.12(e).
4. Push can be applied to the well from the lower side to rectify the tilt. Care should be taken so that pulling or pushing does not cause a shift of the well.
5. Water jets may be used on the higher side, near the bottom of the well, to remove the soil partially, thereby reducing the frictional resistance on the higher side.
6. Temporary obstacles in the form of wooden sleepers may be placed below the cutting edge on the lower side to stop sinking on that side temporarily The continued sinking of the well on the higher side tends to adjust the tilt, as shown in Fig. 21.12(f).