Well-Pile Integrated Foundation For Signature Bridge

Er. Shishir Bansal, Chief Project Manager, DTTDC,
Er. R Prakash, President, M/s Gammon Infra Projects Ltd

A Cable Stay Bridge with 251 m main span and 154m high asymmetric pylon is under construction at Wazirabad, North-eastern part of New Delhi. The general arrangement of bridge is shown in Plate 1. The asymmetrical inclined shape of the pylon is a special feature of the bridge. With these arrangements, the pylon is supported by the weight of the Deck and front cables are supporting the Pylon, thus both the Pylon and Deck are complementing each other. The complete assembly is anchored at the backside with Foundation designated as P23 and the uplift force on the Foundation is as high as 12,000 t. During the soil investigation it was identified that the strata beneath the foundation at a depth of 50 m is very unfavourable. The rock beneath this location was sloping by 9 m in a well of 17 m dia. Due to such peculiar situation; this foundation is designed as integrated well pile foundation to transfer the forces to the slanting rock. The wells are filled up with iron ore to increase its density from 2.4 to 3.5 MT per cum.

The 8-lane bridge under construction is 575 m long with main span as 251 m and 9 spans each of 36 m. One of these 9 spans, two spans from P1 to P3 at 36 m each is in front of Pylon at P19 location. Further 7 spans from P19 to P26 at36 m each are behind the pylon.

Integrated Piles and Well Pile Foundation
Thus, the final structure concluded for foundation P23 was with two wells of 17 m diameter sunk up to the level 500 mm above the highest rock level with sixteen 1.20 m diameter piles inserted through the walls of the well embedded up to 6m into rock on each wells, a combined tie beam of 4.5 m deep and 42 m x 17.5 m sizes on top of the well connecting the wells, piles and piers constructed on top of the same. The final shape of the foundation is shown in Plate 2.

The wells were sunken up to the founding level by providing sixteen 1,360 mm dia. holes, equidistant in the steining and then the entire well was filled up with soil up to top to keep the well intact during execution of Piles. A temporary liner was inserted from top of the well up to rock level to arrest the surrounding soil between the bottom of well and rock. The temporary liner was anchored for 1m into rock level to give stability to the drilling tool during the drilling operation. A stool was fixed on top of steining wall suitably anchored from strong supports so that the drilling machine can be mounted on them. The drilling machine was placed on the stool and drilling was done up to 6 m into the rock. The reinforcement cage with structural member was covered by permanent liner from top of the rock level and up to 4 m above well curb. The bore was cleaned, and the drilling machine was removed before inserting the cage into the drilled hole and concreting up to 3 m above the top of well curb. The temporary liner was removed along with the stools and the concrete was completed up to top of well steining.

The sinking of the well works was started in Feb 2013 and completed by Aug 2014. The wells were provided with a temporary RCC wall 200 mm wide, in order cater the well sinking below ground level as well as to act as support to the RCD mounting tool. On completion of the well foundations, the wells were filled with soil up to 3 m from top of the well and 1,600 mm diameter stools made of 16 mm thick MS plates were mounted on the well and supported from two sides with the 200 mm thick wall. Further driving of temporary liners and anchoring them up to 1m depth in to the rock was completed for the initial four piles as the temporary liner was planned to be used repeatedly for the other bores after retrieving the same from the completed piles. As all the arrangements were already made as soon as the RCD rig were commissioned at site, the drilling activity started in Dec 2014 onwards (Plate 3).

Initially piles were having lots of setting trouble for installation of RCD Rig and after overcoming the all such troubles, first drilling was completed, and subsequently other troubles encountered are detailed hereinafter.

When the cage was to be inserted, the heavy weight to the tune of 45 MT and long length of the cage made it difficult to insert in designated bore. Then, it was decided to insert the cage in parts. First the structural element of the composite member was welded and made into one piece, and then the rebars were fitted over it suitably welded with ties at various locations followed by the insertion of Permanent liner into the temporary liner and hold it on top level with steel pins. Thereafter the reinforcement cage arrangement was inserted and welded with the permanent liner before lifting the entire arrangement and removal of the pins and lowering it till the rebar cage welding level. Then the second level of reinforcement cage was welded and finally the entire arrangement was lowered all together. This entire work process was finalized before starting the works and was smoothly carried for the first pile.

Plugging the Wells and Bottom Slab
After completion of the piling the entire soil was removed up to bottom of the well and all the soil stuck below the well curb was removed by sending the divers inside the well. Bottom plugging was done using the tremie pipes and continuous pouring was done for the entire quantity of 1,200 cum for each well.

As the foundation consists of RCC slab over the bottom plug of well, the entire water from the well was to be removed from the well to enable to clear the dowels and fixing of the rebars. A leakage was found 300 mm per hour after dewatering up to bottom. This warranted further plugging of permeability in the plugging. Providing nozzles sealed the holes and high viscosity epoxy grout was injected into the nozzles with high pressure. This reduced the ingress of water to a large extent and RCC slab was successfully completed on top of the plugging.

As the upward load on the structure is coupled with the uplift to the tune of 12,000 MT, the well should have enough weight to counter the same and any weight added above the scour level should have made the efforts less fruitful. In order achieve the weight requirement below the scour level at the defined height, normal concrete was not enough, and the concrete was provided with Iron ores as Aggregate instead of Normal Aggregate. This has increased the weight of concrete to 3.6 MT per cum.

Well Cap
Since the bottom level well cap is at 8 m below the ground level and 5 m below the ground water level, its executing was an uphill task. Firstly, both the wells must be sunk up to the required level that warrants the top of wells should be taken up to the bottom level of the well cap. Without providing any protective arrangement, it is not possible to arrest the water from entering in to the well area. A 200 mm thick wall was provided connecting to the well steining, which later helped in providing supports to the RCD drilling machines during piling.

After completion of the well works up to plugging of the well at the top, as the well cap must be made connecting both the wells, the false wall initially provided for sinking the well became an obstruction and needed to be removed. Removing of the same will invite water inside the well cap area and so an additional arrangement needs to be provided for casting of well cap.

The well cap must accommodate the rebars provided for the well cap itself, coupled with the rebars protruding out from the wells. The size of the well cap is so large that the heat emanating from the concrete at different levels of the well cap varies to large extent thereby causing differential temperature of more than 20°C. This was effectively controlled by covering the well cap with bad thermal conductors so that the well cap will not lose any temperature at any point the difference in temperature maintained less than 20°C.

Signature Bridge is one of the structures constructed on various complicated locations all over the world. Construction of Integrated pile Foundation over the sloping rock to resist around 12,000 MT of uplift force is unique and challenging job that required a dedicated team with right attitude for successful completion. In future this structure will provide a good reference to construct a similar type of foundation at such type of locations where conventional foundation is not enough to meet the structural requirement.