Shifting and tilting occur generally during the sinking phase of well foundations. If proper care is not taken, they can cause grave problems which may lead to the weakening of the stability of foundations. Precautions to avoid shifting and tilting, limitations and rectifying methods are discussed below.
Shifting and Tilting of Well Foundations
- When the well is moved away horizontally from the desired position, it is called shifting of the well foundation.
- When the well is sloped against vertical alignment, it is called tilting of the well foundation.
Precautions to Prevent Shifting and Tilting
Following safety measures should be considered to prevent shifting and tilting of well foundations:
- The diameter of well curb should be more than the external diameter of steining. Generally, a
differenceof 40 to 80 mm is recommended.
well steiningshould be symmetrically placed over the curb.
- The outer surface of the
wellcurb and steiningshould be smooth.
- All the sides should be uniformly dredged.
- The cutting edge should be uniformly thick and sharp.
- The maximum tilt allowed in case of well foundation is 1 in 60.
- The shift in well foundation should not be more than 1 % of depth of sunk.
- Beyond the above limits, well foundation is considered as dangerous and in such a case, remedial measures to rectify shifting and tilting should be followed.
Rectifying methods for Rectification of shifting and tilting problems in well foundations are as follows:
- Eccentric loading
- Excavation on higher side
- Water jetting
- Pulling the well
- Using hydraulic jacks
- Using struts
- Excavation under cutting edge
- Wood sleeper under cutting edge
1. Eccentric loading
- The well tilt can be rectified by placing eccentric loading on the higher side. Higher side is nothing but the opposite side of tilt or lower side.
- A loading platform is constructed on the higher side and load is placed on it.
- This eccentric load will increase downward pressure on higher side and correct the tilt.
- The amount of load and eccentricity is decided based on the depth of sinking.
- Greater is the depth of sinking of well, larger will be the eccentricity and load.
2. Excavation on Higher Side
- When well is tilted to one side, excavation should be increased on the other side which is opposite to tilted side.
- This technique is useful only in the initial stages of well sinking.
3. Water Jetting
- Water jetting on external surface of well on the higher side is another remedial measure for rectifying tilt.
- When water jet is forced towards surface of well, the friction between soil and well surface gets reduced and the higher side of well becomes lowered to make well vertical.
4. Pulling the Well
- The well can be pulled towards higher side using steel ropes.
- One or more steel ropes are wound around the well with wooden sleepers packed in between well and ropes to prevent damage to the well steining by distributing load over to larger area of steining.
- Pull should be carefully done otherwise, shifting of well foundation may occur.
5. Pushing using Jacks
- Another method to rectify tilting and shifting of
wellfoundation is using hydraulic jacks or mechanical jacks, the tilted well can be pushed from lowerside to higher side.
- Neighbor vertical well foundations or suitable arrangements made will give support to the jack system.
- Care should be taken while pushing the well otherwise the well may shift.
6. Using Struts
- By providing struts as supports on the lower side or tilted side of well, further tilting can be prevented.
- Wooden sleepers are provided between struts and well steining to prevent damage to well steining and to distribute pressure to larger area.
- Struts are rested on firm base having driven piles.
7. Excavation under Cutting Edge
- This technique is used for hard strata soils. In this method, the well is de-watered first and open excavation is carried out exactly under the cutting edge on the higher side.
- If de-watering is not possible, soil strata is loosened using suitable equipment with the help of professional divers.
8. Wood Sleeper under Cutting Edge
- If tilting towards lower side is increasing, then wooden sleepers are placed under cutting edge on the lower side to control the tilting temporarily.
- When well is corrected to vertical level, these sleepers can be removed.