GB2297987A

GB2297987A - Directional ground dewatering to stabilise building foundation structures

Info

Publication number: GB2297987A
Application number: GB9503101A
Authority: GB
Inventors: Ozdemir Keskin
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-02-17
Filing date: 1995-02-17
Publication date: 1996-08-21
Also published as: GB9503101D0

Abstract

The ground below a building structure is isolated by constructing a diaphragm wall around the building structure with a tunnel formed around the base of the diaphragm wall. A series of water valves on the face of the diaphragm wall are opened to control the slow, directional drawdown of water from the isolated ground. The drawdown of water increases the bearing pressure of the ground while the movement of water brings with it minute particulate matter from the ground to bring about a slow, gradual and controlled subsidence.

Description

A. INTRODUCTION 1. The Tower of Pisa is an historic and world famous monument. The detached campanile was commenced in 1173 as the third and final stage of the city of Pisa's cathedral complex. It is a decorative, circular bell tower designed to stand some 56 metres in height and constructed of white marble. Following the brief initial construction period, work was halted for some one hundred years and only commenced for a short period after that and as result of the interrupted building campaigns the structure was not completed until 1350 when the belfry was added. Figure 1 shows the lay out and features of the cathedral complex (drawings taken from Micropaedia, Medieval Structure in Pisa, Italy) 2. The site where it is built in geological times was under the sea and later formed part of the river estuary and this has determined the ground conditions.There is an under stratum of soft clay laid down during the sea period which is overlaid by sand and silt laid down within the estuary period. Extensive work has been done to characterise the ground and one of the main contributory factors to the cause of the well known tilt of the tower is thought to be the distribution of sand and silt under the foundations. Structural problems with the building developed within the initial building stage and continued. Modifications to the block layers were incorporated over the decades in response to the movements in attempt to regain the vertical alignment, such that the completed tower is slightly banana shaped.

3. Within this century there have been numerous investigative committees set up to consider the preservation of the tower. The most recent proposal, of which the authors are aware of at the time of writing, concerns the placement of lead weights on the north side to alleviate the pressure on the south side where the movement is occurring. In the simplest terms this appears not unreasonable but it is theoretically structurally flawed and this action is not the end solution to the problem but only a temporary relief of symptoms of the ailment.

4. The tower's history indicates that extreme care must be taken if it is to be interfered with as whenever remedies have been applied in the past, the tower has tilted even more. In the 1920's, a walkway was dug round it and the tower tilted more; in the 1930's when Mussolini pumped 800 tonnes of cement into some 370 boreholes in the foundations, the tower tilted even more than before. The tower does not like to be disturbed within its own territorial area of ground, it is still trying to settle but because of the ground conditions, it cannot reach a comfortable equilibrium. The proposed method will stabilise the tower without arousing or disturbing it.

5. The present proposal outlines the fundamental structural principals and civil engineering works of the method to permanently stabilise the tower. A brief concept of how the project may be self financing is introduced.

B. STRUCTURAL OUTLINE Summary 6. A barrel mass of ground below the structure will be isolated by means of a diaphragm wall, then a concentric diaphragm wall will be formed to the outside and a trench dug between the two. A series of water valves on the face of the inside diaphragm wall will be used to control the slow, directional drawdown of water from under the earth mass of the structure.

The moving water will bring with it minute particulate matter from the soil and thus bring about slow, gradual and controlled subsidence. The resulting movement of the tower will therefore be at a controlled rate, direction and extent. The local landscape will be disrupted for less than a year during the wall and trench forming processes but following these, the area will be landscaped and the underground activities need not scar the area. The structural stability of the adjacent buildings will not be threatened.

Stage 1: Isolation of Pisa Tower by Diaphragm Walls 7. The diaphragm wall will be placed outside the tower's own ground territory. It will be formed by a modern technique appropriate to the prevailing conditions, to a depth to penetrate into an impermeable stratum. There are no reasons to envisage technical difficulties with wall formation as deep as 50 metres in ground of this type. The substructure and ground will be enclosed.

8. Even at this early stage of the works, on completion of the first diaphragm wall, the control and stabilisation of the structure has commenced as it is isolated from the outside underground movement.

9. When the above diaphragm wall has been completed, a second similar concentric diaphragm wall will be created to the outside, leaving a sufficient gap to satisfy the requirements of the trench which will be created between the two walls. Illustrated in Figure 2.

Stage 2 : Formation of Deep Trench Structure Round Tower 10. When the two walls have been completed, the ground between them will be excavated to a depth to be determined by calculation, based mainly on the structural dimensions and depths of the foundations. During this stage of the works, precautions will be taken to ensure the maintenance of the water table level within the inner and-outer ground areas. Peizometers will be incorporated at a number of locations to monitor the ground water levels. The excavation of the ground will not threaten the structural integrity of other buildings in the area as the ground and ground water will be stable.

11. An impermeable ring will be created, the working tunnel formed within the circular trench and then the trench will be filled in, up to adjacent ground level. The fill will provide additional lateral stability for both diaphragm walls. Figure 3 12. At this stage of the works, the Piazza ground surface level may be landscaped, including covering over the top of the ring trench, therefore the ground treatment can proceed for many years without scaring the appearance of the Piazza. Access to the trench will of course be retained.

Stage 3: Ground Water Control System 13. On the tunnel face side of the inner diaphragm wall, at a level well below the existing ground water table position, valves will be installed all the way round the circular wall, as illustrated in Figure 3.

14. Calculation and data base research will be required to determine the desired optimal flow rate of water from the valves and the distribution of the rates round the circular tunnel. A gushing water tap should not be envisaged, a slow dropwise flow rate will occur. The water will be collected and the particulate matter content measured to assess the progress of the ground subsidence in relation to the measured movement of the tower structure.

15. A working definition of subsidence is the actual downward movement of ground or ground particles, as opposed to settlement which occurs when the particles of the ground are compressed closer together but there is no actual bodily movement of ground. The tap valve flow maintenance will enable control of not only the rate of subsidence of the ground but also a directional control. Figure 4 shows a plan to illustrate the preferential removal of particulate matter from the north side ground.

16. The whole area enclosed within the inner diaphragm wall acts as an island therefore stabilising the tower as it is isolated from external groundwater variation. Dewatering will then bring about subsidence but due to the directional nature of the drawdown, there will be greater subsidence brought about on the north side, resulting in some vertical realignment of the tower. The extent of the corrective movement is anticipated to be at a rate equivalent to the movement experienced by the tower over past centuries. It will not be dramatic or traumatic.

The initial dewatering will continue until the existing ground water level reaches the valve level and may span over a time period of many years. To continue the corrective subsidence and realignment, replacement ground water can be injected through the pipes as illustrated in Figure 5.

C. CONCLUDING REMARKS 17. This simple method is intended to establish a stable, leaning tower and not to create a vertical tower. It will be accomplished without inflicting human disturbances on the actual tower but will use the harnessed forces of nature to control the forces of nature, reversing the movement 1 in harmony with nature.

18. The proposed structural and civil engineering works to stabilise the tower need not be made in a clandestine manner but should be extrapolated into a major event, thus in themselves attracting additional numbers of tourists, possibly in excess of the numbers lost when the tower was closed for safety reasons in 1990. If the tourist aspects are handled appropriately the whole project will easily be self financing.

19. The method employing a double diaphragm wall with tunnel and valve fittings on the inner diaphragm wall can be used in a number of situations to control the ground water level and ground conditions. The method can be applicable in such situations as football and cricket grounds where wet muddy playing conditions want to be avoided.

Claims

CLAIMS (reference paragraphs 10 to 14) The invention for which patent is sought is a method to stablise structures by control of ground conditions as specifically applied to the Tower of Pisa, Pisa, Italy, but is suitable for other tall slender structures and a range of varied situations Stage 1: Isolation of Pisa Tower by Diaphragm Wall

1. The diaphragm wall will be placed outside the tower's own ground territory. With consideration to the relatively near vicinity of the Cathedral, the wall need not be concentric round the Tower. The diaphragm wall will be formed by a modern technique appropriate to the prevailing conditions, to a depth to penetrate into an impermeable stratum. There are no reasons to envisage technical difficulties with wall formation as deep as will be required in ground of this type. The Tower substructure and ground will be enclosed.

2. The disturbances to the ground areas in the immediate vicinity to the Tower will be very short term and due to the modern methods employed, there will not be adverse effects to the other buildings in the Cathedral Complex or the Piazza area in general. The diaphraqm wal; works will take less than four months to complete

3. Even at this early stage of the works on completion of the diaphragm wall, the control and stabilisation of the structure has commenced as it is isolated from the outside underground water movement and other variations.

Stage 2: Formation of Tunnel Structure Round Outside of Diaphragm Wall

4. When the above diaphragm wall has been completed, a concentric underground tunnel will be constructed to the outside, near the base but at a depth determined by the ground conditions, creating a sufficient space to satisfy the requirements of the functions of the tunnel. The arrangement is illustrated in Figure 2.

5. During the works, precautions will be taken to ensure the maintenance of the water table level within the inner and outer ground areas. Peizometers will be incorporated at a number of locations to monitor the ground water levels. The excavation of the ground for the tunnel and creation of the tunnel will not threaten the structural integrity of the other buildings

6. At this stage of the works, the Piazza ground surface level may be landscaped, therefore the ground treatment can proceed for many years without scaring the appearance of the Piazza. Access to the tunnel will of course be retained.

Stage 3 : Ground Water Control System

7. On the tunnel face side of the diaphragm wall, at a level well below the existing ground water table position, valves will be installed all the way round the wall, refer to Figures 3 and 4

8. Calculation and data base research will be required to determine the desired optimal flow rate of water from the valves and the distribution of the rates round the tunnel. A gushing water tap should not be envisaged1 a slow dropwise flow rate will occur. The water will be collected and the particulate matter content measured to assess the progress of the ground subsidence in relation to the measured movement of the Tower structure.

9. The whole area enclosed within the diaphragm wall acts as an island therefore stabilising the Tower as it is isolated from external groundwater variation. Differentially water table in the intermediate sand stratum and will also increase the stability of the Tower base from an early stage.

10. The tap valve flow maintenance will enable control of not only the rate of subsidence of the ground but also a directional control. Figure 4 shows plan and elevations to illustrate the preferential removal of particulate matter from the north side ground.

11. Further realignment can be induced by introducing water into the upper strata ,via vertical pipes inserted to relevant depths, and using the drawdown and flush out technique. Due to the directional nature of the drawdown, there will be additional subsidence brought about on the north side, resulting in the further desired vertical realignment of the tower. The extent of the corrective movement is anticipated to be at a rate equivalent to the movement experienced by the Tower over past centuries. It will not be dramatic or traumatic. Figure 5.

Stage 4 : Conclusion

12. It is apparent that all attempts to correct the Tower's problem have been somewhat compact, encompassing only the immediate act of doing something superficial to the Tower and the ongoing works are repeating the same type of action. A permanent solution must be applied to the Tower.

13. This simple civil engineering method is intended to establish in the short time a permanently stable, leaning Tower and not to create a vertical Tower.

Additionally, it will facilitate slow and steady realignment of the Tower to any desired degree. It will be accomplished without inflicting human disturbances on the actual Tower but will use the harnessed forces of nature to control the forces of nature, reversing the movement of past centuries, in harmony with nature.

Tourism aspects

14. The structural work is at the core and forms the heart of the project but while proceeding with its development, supplementary aspects should be considered which will enhance the acceptability of the project both structurally and in general terms. A wider concept would benefit all, with the prestige side possibly having an influence. Engineering works are required, therefore advantages should be taken from the novel activities by creating an ongoing additional tourist attraction.

15. The proposed structural and civil engineering works to stabilise the Tower need not be made in a clandestine manner but should be extrapolated into a major event of history, thus in themselves attracting additional numbers, or even more, of tourists lost when the tower was closed for safety reasons in 1990.

16. This tourism suggestion is presented in part to enhance the overall desirability of the project. If the tourism aspects are handled appropriately the whole project will easily be self financing.

17 Bibliography, AGI by M. Jamiolkowski and C. Viggiani

16. This tourism suggestion is presented in part to enhance the overall desirability of the project. If the tourism aspects are handled appropriately, the whole project will easily be self financing.

Amendments to the claims have been filed as follows (reference paragraphs 10 to 14) The invention for which patent is sought is a method to stablise structures by control of ground conditions as specifically applied to the Tower of Pisa, Pisa, Italy, but is suitabie for other tall slender structures and a range of varied situations Stage 1: Isolation of Pisa Tower by Diaphragm Wall 1 The diaphragm wall will be placed outside the tower's own ground territory. With consideration to the relatively near vicinity of the Cathedral, the wall need not be concentric round the Tower, and may be circular, square or rectangular as best determined. The diaphragm wall will be formed by a modern technique appropriate to the prevailing conditions, to a depth to penetrate into an impermeable stratum.There are no reasons to envisage technical difficulties with wall formation as deep as will be required in ground of this type. The Tower substructure and ground will be enclosed.

2. The disturbances to the ground areas in the immediate vicinity to the Tower will be very short term and due to the modern methods employed, there will not be adverse effects to the other buildings in the Cathedral Complex or the Piazza area in general; Figure 1. The diaphragm wall works will take less than four months to complete

Stage 2: Formation of Tunnel Structure Round Outside of Diaphragm Wall

4. When the above diaphragm wall has been completed, a concentric underground tunnel will be constructed to the outside. near the base but at a depth determined by the ground conditions. creating a sufficient space to satisfy the requirements of the functions of the tunnel. The arrangement Is illustrated in Figure 2.

5. During the works, precautions will be taken to ensure the maintenance of the water table level within the inner and outer ground areas. Peizometers will be incorporated at a number of locations to monitor the ground water levels and the peizometric levels. The excavation of the ground for the tunnel and creation of the tunnel will not threaten the structural integrity of the other buildings

Stage 3 : Ground Water Control System

8. Calculation and data base research will be required to determine the desired optimal flow rate of water from the valves and the distribution of the rates round the tunnel. A gushing water tap should not be envisaged, a slow dropwise flow rate will occur. The water will be collected and the particulate matter content measured to assess the progress of the ground subsidence in relation to the measured movement of the Tower structure.

9. The whole area enclosed within the diaphragm wall acts as an island therefore stabilising the Tower as it is isolated from external groundwater variation. Differentially affecting the water table in the intermediate sand stratum, it will also increase the stability of the Tower base from an early stage.

10. The tap valve flow maintenance will enable control of not only the rate of subsidence of the ground but also a directional control. Figure 4 shows a plan to illustrate the preferential removal of particulate matter from the north side ground.

Stage 4: Conclusion

12. It is apparent that al attempts to correct the Tower's problem have been somewhat compact, encompassing only the immediate act of doing something superficial to the Tower and the ongoing works are repeating the same type of action. A permanent solution must be applied to the Tower.

Additionally. it will facilitate slow and steady realignment of the Tower to any desired degree It will be accomplished without inflicting human disturbances on the actual Tower but will use the harnessed forces of nature to control the forces of nature reversing the movement of past centuries. in harmony with nature.

Tourism aspects

14. The structural work is at the core and forms the heart of the project but while proceeding with its development. supplementary aspects should be considered which will enhance the acceptability of the project both structurally and in general terms. A wider concept would benefit all, with the prestige side possibly having an influence. Engineering works are required, therefore advantages should be taken from the novel activities by creating an ongoing additional tourist attraction.