Well+foundation 82951

Paper No.533

THE SINKING OF WELL FOUNDATIONS IN DIFFICULT SITUATIONS† GOURANGA PRASAD SAHA*

ABSTRACT Well foundations are quite appropriate foundations for alluvial soils in rivers and creeks where maximum depth of scour can be quite large. In India technology of well foundation for design and construction is quite well developed. Still there are situations where serious problems are encountered at site during construction of well foundations. Some of the typical problems have been identified and solutions adopted by the author and also elsewhere in some earlier jobs have been presented in this Paper.

1. INTRODUCTION

In the Indian subcontinent there are many rivers where the depth of alluvial deposits is very high and the scour around the pier foundations can be very deep if the piers are located within the active channel of river. For such condition well foundation is a very appropriate type of foundation. To the author’s knowledge one 3 km long Railway bridge crossing a creek was ed on pile foundations. After some years, the sway of the pile foundations was so large when the mail trains crossed the creek; the Railways not only reduced the speed of the trains substantially but constructed another bridge adjacent to this bridge on well foundations. Thus under certain situation, well foundation is a highly desirable type of foundation. In India the technology for the design and construction of well foundation is quite advanced. In all rivers, including large rivers with torrential flow of water currents, well foundations were provided. In the foundations of Howrah Bridge in Kolkata giant monoliths were provided. In Second Hoogly Bridge also in Kolkata with one of the longest span cable stayed bridges in the world having the central span of 457 m being same as that of Howrah Bridge, an ingenius solution was adopted. Instead of a monolith for the foundation of each tower of the cable stay bridge, 2 well foundations up to 23 m dia were placed side by side at 30 m center and interconnected by a 14 m deep beam. This solution was construction-wise easier and more controllable and quantity of material used was much less. These wells were constructed purely by gravity sinking method.

These well foundations under main towers were kept totally empty for the service condition. The entire well was designed as a water tight structure. At the junction of any two lifts of concreting of well steining, water stops were provided to prevent leakage of water through construction ts of well steining. The wells were constructed using gravity sinking method of construction. As stated earlier, the wells were kept totally empty and no water was inside the well during the service condition primarily to reduce the load of water on the foundations, since vertical loads were very large for the central spans of 457 m and side spans of 183 m. At the base of wells RCC slabs were constructed which were designed for the upward force from the base of the well. For of the slab and continuity of reinforcement, recesses were kept in the steining above the well curb where bond bar reinforcements were kept bent. A cover was provided with a steel plate which, were kept in position by bolts, to keep the bond reinforcement recesses free from blocking with mud’s. After the wells were sunk up to the designed level, the bottom plug was laid with colcrete. The wells were dewatered. It was noted that the colcrete bottom plug not only withstood the upward pressure but also was reasonably leakproof. Thereafter, those steel plate covers were removed and reinforcements were bent back to the designed positions. Reinforcements were laid and concreting was done. In this way a fully water tight design was done for these wells. Deep well foundation up to 67 m deep across the river Ganga in Varanasi and upto 68 m deep across the river Brahmaputra at Jogighopa were used. In Varanasi

* Executive Director, M/s Construma Consultancy Pvt. Ltd. Mumbai Email: [email protected] † Written comments on this Paper are invited and will be received upto 30th September, 2007.

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Bridge wells, no sand filling was done. These wells were filled with water only to reduce the weight at the founding level. In long span bridges or in bridges where the horizontal force is large, heavy foundation mass is desirable for stability and serviceability requirements point of view and hence well foundation is a highly appropriate solution. In spite of the excellent development of technology on well foundations there are still some areas where engineers face difficulty while sinking of wells, some of which are stated in this paper from the direct experience of the author. The author had the opportunity of having associated with construction of well foundations of many large and outstanding bridges including Second Hoogly Bridge, Varanasi Bridge, etc., while working with Gammon India Ltd., and Hindustan Construction Co. Ltd. 2. SINKING WELL THROUGH STIFF CLAY STRATA

If a very stiff clay layer is encountered during the sinking of a well, the engineer has to face a very tough and challenging situation if the well becomes stationery and does not move at all.

(i) Continue grabbing much below the cutting edge level of the well. (ii) Dewater well up to certain depth to reduce the buoyancy and thereby increase the weight of the well. (iii) Remove soil in with the outside surface of the well by grabbing to a certain depth. (iv) Flushing with a jet of water on the outside soil through holes, which are kept in the steining at certain intervals during the construction of the steining of the well. (v) By thumping on the water inside the dredge hole. (vi) By Kentledge loading on the well. The merits and demerits of each of the above measures are discussed below. (i) Some engineers continue grabbing much below the cutting edge level of the well

At that time due to the action of horizontal water current force, the well may start tilting. The well becomes more vulnerable due to tilt if a step is provided on the outside face of the well steining to reduce the thickness of the steining. At the initial stage if the tilt is not brought under control, the tilt goes on increasing making the situation very difficult to control. This leads to a very expensive and time-consuming affair for attempting to make the well straight and vertical within practical limits. This is a very serious situation and one should examine how this resistance is developed. A solution should be devised on the basis of insight into the causes for the resistance to sink. Referring to Fig. 1 it may be seen that the primary resistance comes on the sides and also by bearing at the base over the area of the well steining. The projection on the outside surface over the height of the kerb by 50 mm to 75 mm as recommended in IRC 78-2000 is very helpful to reduce the resistance due to stiff clay on the outside surface of the well. (a) To overcome this resistance to the author’s knowledge, the following measures have been tried by various engineers.

(a) PIPES PLACED PERPENDICULARLY

(B) PIPES PLACED VERTICALLY UPWARDS

Fig. 1. Showing location of pipes within well steining

HIGHLIGHTS THE 178TH COUNCIL MEETING THE SINKING OF WELLOF FOUNDATIONS IN DIFFICULT SITUATIONS One site engineer with vast experience in construction of well foundations and claimed in achieving success by adapting this method, continued grabbing to a depth of about 10 m below the cutting edge of the well situated in the river Ganga in a major project when the well became stationary in a very stiff clay layer. This particular well was partly in the water and partly in the bank. One day the well suddenly sank to a depth of nearly 10 m leading to a major catastrophe. This was a very unwise decision, which should never have been taken. IRC 78 : 2000 specifies that sumps made by dredging below cutting edge shall preferably not be more then half the internal diameter. (ii) Dewatering well as a device to sink a well The purpose of dewatering is to reduce the buoyancy. Thus the buoyant weight of the well is increased which increases the sinking effort of the well. But the major problem is the chance of occurring sand blow. If the sand blow occurs and if the sand blow is eccentric and the lateral force exerted on the well is high, the well may break into 4 segments along its height, which was noticed in many projects in this country. The author has developed a method for the design of a well foundation for resisting excessive sand blow forces, which was published in IRC Journal¹ in 1993. This method was used in a major bridge across river Ganga where the well foundations of 13 m diameter up to 67 m depth, which was the deepest well for a road bridge in India at that time was used. There is no literature available which specifies the evaluation of horizontal force for the design of a well foundation for the condition during construction when the well is like a hollow pipe and is in a most vulnerable condition. For such a large and deep well, IRC method of design was not used. Instead a rational and conceptual method developed by the author was used. The quantities of the well were nearly half even for such a deep well. It may be noted that all the wells were sunk to the designed founding level without any appreciable difficulty. According to the author for a major bridge with

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large and deep well foundations, the wells must be designed three dimensionally with a realistic lateral load during construction condition to avoid disastrous situation due to which the well steining may crack transversely. The cost of such an analysis and design is negligible compared with the unpleasant situation, which may become very expensive and delay the project badly. (iii) Remove soil in with the outside surface of the well If the top soil layers are soft, this does not help much. If the stiff clay layer exists at a shallow depth then it may help to some extent. (iv) Flushing with jet of water on the outside face of well In this process holes are kept in the well steining with outlets on the outside surface of the well. Two types of orientation of the outlets are shown in Fig. 1 (a) & (b). In Fig. 1 (a) the outlets are perpendicular to the vertical whereas in Fig. 1 (b), the outlets are inclined with the tip upwards. Experience shows that the outlet hole of the pipe gets blocked with the entry of the clay more easily than the inclined one. A plug was provided to prevent entry of sand or other softer material. This plug moves out with the force of water jet. When the well gets stuck in a very stiff clay layer, water is pumped from the top of the steining. Water coming out on the outer surface of the well is supposed to lubricate the surface and reduce the adherence of the clay on the surface of the well which consequently reduces the upward resistance against sinking down of the well. (v) By thumping on the water inside the dredge hole by dropping grab This also has been tried to overcome the resistance. (vi) By Kentledge loading on the well : The Kentledge loading is placed on the top of well either by sand bags or by concrete blocks leaving adequate space in the central zone of the dredge hole for grabbing. This weight helps

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SAHA ON increase the sinking effort of the well. In case of tilt of a well, eccentric Kentledge load is placed for the correction of tilt.

(b) The method which is different from the methods listed above had been tried by the author and briefly described below :In one project when the well became stationary in a stiff clay layer, this method was used. In those wells the outside projection of the well kerb was 75 mm. In the upper region the strata was of softer variety. Pipes were not kept in the well steining to inject water on the outside surface of the well.

clay over the entire inside area at the bottom of the well kerb. The well was observed to sink down when the inside clay soil was cut and removed. After the removal of the bearing still clay strata, the wells started moving down. Thus this method was found to be quite effective, safe and efficient method. 3. BOULDERY STRATA

The bouldery strata can be dealt with 3 ways. (i) When the boulders are lying loose, they will come

It has been stated earlier that the resistance to sinking comes from the outside surface of the well and also from the bearing on the stiff clay below the well kerb. In a large diameter well, the thickness of the steining is large and hence the resistance due to bearing on the stiff clay layer is quite substantial. To overcome this resistance veteran well sinking engineers continued grabbing inside the dredge hole much below the cutting edge level so that the soil below the well kerb fell off and this resistance was eliminated. The well of course used to sink down suddenly. In the design, designers used to design the well considering half the permissible tensile stress as specified in the IRC-78 to meet these eventualities . As stated earlier the method specified by the author for the design of a well three dimensionally can meet such a situation of sudden sinking down of a well. However, due to sudden sinking down of a well there may be catastrophic consequences, hence such method of continuous grabbing is not recommended. Instead the method which will directly break the clay layer and remove the bearing below the – kerb was adopted. This was done directly by cutting the stiff clay layer by a jet of water through a pipe using a high pressure pump. This pipe was ed on a circular frame. Actually 4 vertical pipes were fixed at 4 quadrants as shown in Fig. 2. The central area was kept free in order to facilitate grabbing. The water was pumped through pipes using one high pressure pump for one pipe. Two high pressure pumps were used at one time and water was injected at two diagonally opposite points at a time. After some time, the other two perpendicularly placed pipes were used for pumping water. After some time the frame was lowered and water was injected in order to cut the stiff

ELEVATION OF WELL AND PIPE LINES

Fig. 2. Showing clayey strata below well kerb and layout of pipe lines

HIGHLIGHTS THE 178TH COUNCIL MEETING THE SINKING OF WELLOF FOUNDATIONS IN DIFFICULT SITUATIONS out with normal grabbing. (ii) When they are cemented but not very firmly then underwater blasting may be effective. (iii) When the boulders are very firmly cemented and the thickness of the steining of the well is quite large then pneumatic sinking may have to be resorted to.

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method is stated briefly here below :How to achieve the condition that the weight of the well will be resisted through outside frictional force only.

In case of bouldery strata, the proper soil investigation should be done and what method of sinking is to be adopted must be decided. The design of the well must be done for such a condition. At least provision for limited blasting must be made and a three dimensional finite element analysis should be done for eccentric blasting force. The steining of the well must be designed not only in the vertical direction but also in the transverse direction providing appropriate design reinforcement. Steel strakes must be provided in such condition. It should be noted that in the past many well steining cracked due to blasting. The wells of Varanasi Bridge of 13 m dia, 2.5 m thick steining and up to 67 m deep were analysed for eccentric sand below condition by using 3-D Finite Element analysis by the author1. This became helpful to take care of adverse situation during sinking of wells. Formation of Heaves in the Dredge Hole even after the well reaches the Designed Founding Level. When a well es through soft strata over a considerable depth, the upward resistance acting on the outside surface of the well is less than the weight of the well. In that situation the well sinks down and quite often a heap is formed inside the dredge hole as shown in Fig 3. When the formation of heaves takes place after the well reaches the designed foundation level, then the engineers face the problem of laying the bottom plug. If the bottom plug is laid inside the dredge hole then the very purpose of laying the bottom plug is defeated, since the load from the well is transmitted to the foundation through the well steining and the bottom plug which lies just below the well steining. The author had developed a method² to overcome this problem. In a number of well foundations where large heaves were formed inside the well, the author used this method and the formation of heaves was stopped. Thereafter the bottom plug was laid within the well kerb region. For the benefit of readers this

Fig. 3. Showing the formation of heave

It is reckoned that no further concreting of the well steining should be done and sinking should be continued. In that case, the weight of the well will remain nearly same and the frictional force outside the well will increase very fast, the founding strata being in good soil. It is thought that the equilibrium of the well will reach with shorter depth of sinking below the designed founding level. The following method is used to evaluate the approximate depth of further sinking and also the height of water to be considered for the design of the cofferdam along the outer perimeter of the well at top. When the heave develops in the dredge hole, the equilibrium of the well takes place as per the following equation : W = F+qa

Eqn. ... (1)

F = frictional force in the vertical direction along the outside surface of the well.

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q = bearing pressure of soil.

depth of further sinking.

a = area of the well ed over the heave i.e. over the distance X, Fig. 4. X = tan A.

fl = p x (coefficient of friction) p = (pl + p2)/2

h = height of heave.

pl = active earth pressure at a depth H below the existing water level or ground level whichever is higher.

A = angle at the tip of well curb. The component qa is required to be balanced by the additional frictional force over the outside surface of the well, which will develop by sinking the well deeper. Thus fl al = qa

Referring to Fig. 4

Eqn. … (2)

Where fl = the frictional force acting on the outside surface of the well over depth of further sinking of the well per unit area which is evaluated below. al = area of the outside surface of the well over

p2 = active earth pressure at a depth H + h1 h1 = depth of further sinking. Substituting p in fl and fl in Eqn. 2 as shown in Fig. 4, hl can be evaluated. 4. FEW OTHER MEASURES WHICH CAN HELP RELATIVELY TROUBLE FREE SINKING OF WELLS

(i) Appropriate choice of cutting edge and adoption of

(a) POSITION OF WELL AT DESIGNED FOUNDING LEVEL WITH HEAVES IN DREDGE HOLE

(b) POSITION OF WELL AFTEER ADDITIONAL FURTHER SINKING WITH NO HEAVE IN DREDGE HOLE

Fig. 4. Showing additional sinking without any further concreting of steining

HIGHLIGHTS THE 178TH COUNCIL MEETING THE SINKING OF WELLOF FOUNDATIONS IN DIFFICULT SITUATIONS

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proper detailing Two types of cutting edges are of use for well foundations as shown in Fig. 5 (a) & (b) of IRC – 78 : 2000. The angle iron cutting edge works well when the well es through alluvial soil strata without any hard obstruction. There are instances when the cutting edge met some hard obstruction, it became dislodged and caused obstruction to the well. Compared with the angle iron cutting edge a V type cutting edge is more appropriate in meeting various obstructive situation provided correct detailing is adopted. The inclined plate should be stopped about 25 mm above the bottom tip of vertical plate and the welding of the t between the vertical plate and the inclined plate must be done as shown in Fig. No.5(b). The vertical and the inclined plates must not meet at the bottom most point and consequently the welding must not be done at the bottom tip. The author is aware that in many bridges in the Himalayan region where the strata are full of boulders the welding at tip came off and the plates were bent out and caused lots of obstruction to sinking leading to various problems including delay in progress of construction. Hence it is advised to adopt detailing as shown in Fig. 5(b). (ii) Adequate no. of Borelogs must be taken in the location of each well

Fig. 5 (b) :Typical details of V-shaped cutting edge

As per the present practice only one Borehole is taken at each foundation location. The soil properties of various strata are available along the depth. To obtain the complete information of the existence of type of soil/rock or large boulders if any covering a part of well foundation at least 3 boreholes should be taken. If the diameter of well is large, then at least 4 bore holes should be taken. With this knowledge the span arrangements be made so that wells can be sunk smoothly. Soil properties should be taken of one borehole for the design of foundations whereas the other boreholes will present information on the type of strata, locations and their depths. Presence of very large boulder covering a part of the well at some depth in the bridge over third Brahmaputra at Jogighopa and similar type of problems including sudden change of bed profile are encountered in various rivers in India for which a very expensive time consuming measures were adopted. This can be avoided if enough information of the soil strata is available at the time of design. (iii) Floating Caisson Sinking When the depth of water at the location of a well is more than about 5 m then construction of well on temporary sand island is not only difficult but also uneconomical. In this situation steel caisson is fabricated on the bank and floated in water either by tilting platform or by a gantry. If the caisson is very large and very heavy then tilting platform is more convenient. In other cases normally the caisson

Fig. 5 (a) :Typical details of angle iron cutting edge

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SAHA ON installed dead anchors at least at 6 locations as shown in Fig.6 with ropes with adjustable device. Each anchoring should also be done at 2 locations in elevation. By pulling or releasing the anchor rope the tilt and shift can be eliminated.

is brought to the water using a gantry. However, the choice of tilting platform or a gantry or by some other means should be made as per the locally available resources and convenience. After the floating caisson is brought to the designed location of the well, further building up of caisson is done. This work together with concreting inside the caisson to the designed depth is done till the cutting edge reaches the bed level of the river. This is a very critical stage. There is every possibility of tilt and shift occurring, since the caisson is subjected to unidirectional flow of water. In several projects, large tilt and shift occurred which subsequently created a lot of problem with that project. The tilt and shift must be checked very thoroughly before grounding and suitable corrective measures must be taken if any tilt or shift occurred. Some measures are suggested here below which if adopted judiciously should minimize the tilt and shift of the caisson. (a) For large diameter caissons •

•

Steel piles may be driven inside the dredge hole at 4 corners after ensuring the correct position of the well.

(b) For caissons with sinking sets The barges should be located towards upstream and downstream of the river and anchored at least at the locations as shown in Fig.7 with the device for adjustment so that the well can be positioned at the designed locations. The caisson is placed between the barges with very small clearance between any two barges and the caisson so that with the water current force acting on the caisson below the barges, tilting of the caisson is restrained. However, in case of any tilt or shift occurring the same is controlled by pulling or releasing the anchor ropes appropriately.

After taking the caisson to the designed location the caisson should be anchored to the already WINCH WINCH WATER LEVEL

WATER LEVEL WINCH WINCH WATER LEVEL

WATER LEVEL

RIVER BED LEVEL

RIVER BED LEVEL

ELEVATION

ELEVATION

DIRECTION OF RIVER FLOW

PLAN PLAN

Fig. 6

Fig. 7

HIGHLIGHTS THE 178TH COUNCIL MEETING THE SINKING OF WELLOF FOUNDATIONS IN DIFFICULT SITUATIONS

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5. CONCLUSION

ACKNOWLEDGEMENT

On this paper a list of difficult situations, which bridge engineers normally encounter during sinking of well foundations, is presented. In some cases the author himself devised some solutions to some perennial problems, which have been described in this paper. The problems like the formation of hump inside the dredge hole of a well foundation or wells cracking due to sand blow, or in a stiff clay layer wells becoming stationary and not sinking down are quite common situations. In several major projects where such situations developed the author as the Technical Head of that organization, developed methods and adopted them to overcome those problems and these methods are presented in this paper.

The author is indebted to Dr. T N Subba Rao for making many valuable suggestions by sparing his valuable time, to make this Paper more complete. REFERENCES

1. Saha G.P. “Analysis and Design of Varanasi Bridge Well Foundations – A Conceptual Approach” Journal of Indian Roads Congress Vol. 54-2. Paper presented in the Annual Session of IRC in November 1993. 2. Saha G.P. “Method of Stopping the Formation of Heaves Inside the Dredge Hole of a Well Foundation” Indian Highways, June 2002.