Measurement of Tilt and Shift of Well Foundation

yogendra borse

1. General

1.1 It is absolutely essential that the well steining shall be built up in one straight line from bottom to top. In no case the plumb bob shall be used for building up the well steining. In order to ensure vertically of steining, a straight edge shall be used and checked up at the four corners, diametrically opposite to each other. The central line of the wells from the centre line pillars should be checked up frequently to ensure verticality and positioning of the wells.

1.2. The height of the well steining is to be marked in the form of 100 mm, wide strips painted (oil paint) on the well with every 1/2 metre mark in red paint. Every well should have 4 such strips painted at the four corners diametrically opposite to each other. The gauge should start from the bottom of the cutting edge and the 1/2 metre marks are to be carefully made with the help of steel tape. After the well has been sunk to the desired depth and before plugging and finally before casting the well cap, the level as well as the central line of the top of well may be checked up and any deviation from the approved plan may be reported to the higher authority. The tilts and shifts of the well should also be measured and their direction noted as described below and the matter reported to Designs Office in case these exceed the permissible tolerance limits.

2. Measurement of Tilts The following methods are used for the measurement of tilts:

2.1 Four Gauge Method

2.2 Plumb Bob Method

2.3 Cross Staves Method

2.1 Four Gauge Method

a) As mentioned above, during building up the steining of a circular well four gauges (strips) are marked on its outer face. These gauges are to be located at the ends of the 2 diameters of the well. One diameter being along longitudinal axis of the bridge and other diameter at angles to it (in case of twin walls for a skew bridge, the two axes along which the measurements are to be taken, may be along the longitudinal and transverse axis of such a well for convenience. In transverse direction, the average of the values for each of the two transverse axes. One for each dredge hole may be taken). These gauges would extend right from base of curb to top of well and heights of the steining above curb marked in strips on all these 4 gauges.

b) A plane passing through the point at the same height marks on these 4 gauges will be parallel to the base of the well. When a well tilts planes passing through same height marks on the gauge will also tilt. To measure tilt, levels should be taken on the four gauges at the same height mark. The difference of levels at the 2 diametrically opposite point divided by the outside diameter of the well will give the tilt of the well in the direction of this diameter.

2.2 Plumb Bob Method

a) This is not so accurate a method, but it is useful. It gives the sinker an idea of the tilt directly during sinking. This method should not be used for measuring tilts in the final states of sinking.

b) At least 4 metal plumb bobs are hung on outside as well as inside of well. Of these, two will be diametrically opposite (one on the outer side and the other on the inner side) along the longitudinal axis of the bridge and the other two diametrically opposite (one on the outer side and the other on the inner side) along the axis at right angles to the former. The length of the string from the point of suspension to the bottom of the plumb bob, which should not be less than 2 lifts of concerting should be measured and also the distance from the steining surface (inner or outer as the case may be) to the tip of the bob. This distance divided by the length gives the tilt for that particular plumb bob. The average the 2 tilts given by the two plumb bobs located diametrically opposite along one axis, will give the tilt of the well along that axis.

c) If the well can be dewatered completely this method can even be used for ascertaining the final tilts of the well along both the axis before plugging as a choke on the tilts ascertained from method of four gauges given in para 2.1 above. In that case the length of the plumb bob will have to be the entire length of the well from top to bottom. In case there is appreciable difference between the tilt obtained by this method and the tilt obtained by four gauges method the accuracy of the marking of the gauges should be rechecked. This can be done by measuring the vertical height along the respective 4 points of the well on the inside of well by stretching a wire/rope from top of well to the bottom most point, such as, beginning of the inclined portion of curb, bottom of cutting edge etc. (if necessary, by sending a diver to hold one end of the rope/wire against such a point).

2.3 Cross Staves Method

a) This method is also useful for finding tilts during sinking operations so as to adopt timely corrective measures. This method should not generally be used for measuring tilts in the final of sinking.

b) Under this method a cross staff is lowered at the bottom of the well and another cross staff is introduced at the top of the well. The cross staff consists of either two timber pieces or structural steel members joined rigidly at right angles to each other and having length equal to internal diameter of the well. A wire is then tiled from the centre of the bottom cross staff to the upper cross staff and held right in position. The tilts of the well can then be found in any direction by holding a plumb bob along the wire in that direction and dividing the horizontal distance between the plumb bob and the wire by length of the plumb bob.

c) If the well can be dewatered completely this method can even be used for ascertaining the final tilts of the well along both the axis before plugging as a check on the tilts ascertained from method of four gauges given in para 2.1 above. In that case the length of the plumb bob will have to be the entire length of the well from top to bottom. In case of an appreciable difference between the tilt obtained by this method and the tilt obtained by the four gauges method. The accuracy of the marking of the gauges should be reached as described in 2.2 (c) above.

3. Measurements of Shifts of Well

For finding the shift of the well from its designed position, the position of the centre of the well as it should be as per design and that as actually obtained after sinking should be measured. Shifts of the centre from the designed centre should be measured in two directions i.e. one along longitudinal axis of the bridge and the other at right angles to it (for twin wells on skew bridges, the axis could be along the longer axis of the twin well and at right angle to it for convenience). This shift is usually measured at top of the well, with reference to the permanent centre line marks set up for each foundation in the two directions described above.

4. Reporting of Tilts and Shifts

The final observations regarding tilts and shifts should be elaborated by a sketch as indicated below while reporting the same to higher authorities. Wherever tilts and shifts exceed the permissible tolerances (which are usually 1 in 60 and 150 mm, respectively as per Specifications), the same should be reported for advice in case of wells on major bridges. The distances between longitudinal and transverse axis of top of the well and these as per design can be directly measured at top with reference to the permanent markings and centre line pillars constructed at four corners of the particular foundations. The distance between the two axes of the top of the well and the two axes of the bottom of the well can be worked out from the tilts in respective directions. From these two values, the distances between the axis of bottom of well and the axis of the well as designed can be computed. These distances should then be shown in sketch in the manner indicated below, while reporting the tilts and shifts in the proper directions to facilitate stress computations in the Designs Office.

Sketch for reporting tilts and shifts

Notes:

1. Desired position of well @ top and bottom shown by 'B'

2. Final position of the well @ bottom shown by 'C'

3. Final position of well top (bottom of well cap position) shown by 'A'.

4. Distances X₁ , Y₁ , X₂ , Y₂ should be reported this sketch.