BORED CAST IN-SITU PILING FOR MARINE WORKS Mr. Ajit Kumar (B. E. Civil)
I. ABSTRACT
For rapid growth of the economy, sound infrastructure is the need of the day. India is the second fastest growing economy in the world today, with considerable progress in all the sectors. Ports play a vital role in the economic development of the country. A large number of ports have been constructed along the extensive Indian shoreline, and a number of ports are being developed in different parts of the country. The ports are equipped with berthing facilities for the vessels, provided by onshore marine structures like wharfs and jetties. These structures allow the vessel to stop at the port and facilitate for the loading/ unloading of goods and provision of other services. Gujarat has 41 ports and is the state with second highest number of ports in India, with continuous progress in this sector. Of the various types of piling techniques practiced, the method employed for piling is the Bored Cast In-situ technique. The piling is carried out by making use of piling gantries, which hold all the required equipment and labour to facilitate the construction process. It is done by the end-on method, and the gantry is moved sequentially to complete all the rows of piles. This technique has been, and continues to be one of the most popular techniques for marine projects. The paper presented here focuses on the concept of the bored cast in- situ technique of piling employed, with use of piling gantries, for Bridge construction at the Bhagirathi River, NH-34 Road Project.
II. PILE FOUNDATIONS
Pile foundations resist vertical, lateral and uplift load by carrying and transferring the load of the structure to the bearing ground. The main components of the foundation are the pile cap and the pile. Piles are long and slender members which transfer the load to deeper soil or rock by friction or bearing. The main types of materials used for piles are wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.
A. PILING TECHNIQUES There are a number of piles and piling techniques. The construction method adopted depends on the geology of the site. Broadly, the two basic methods of installing piles are: driving the pile into the ground or excavation of the ground, usually by boring, and filling the void with concrete. Driven piles are considered to be displacement piles. They may be driven by drop hammers, diesel hammers, vibratory methods or jacking methods. Bored piles (replacement piles) are generally considered to be non displacement piles, a void is formed by boring or excavation, in which concrete is filled to form a pile. Boring can be done by various methods such as Percussion method, Rotary drilling, Auger boring etc. The technique of Bored piles has been quite popular in India till date, whereas the most widely practiced method for piling is the Driven Pile technique throughout the world.
B. TECHNIQUES ADOPTED AT SITE: BORED CAST IN-SITU PILING The piling technique adopted at site is Bored Cast In-Situ Piling (making use of piling gantry). In this method, a liner (casing) is inserted and a bore is dug into the ground using Percussion (Chisel and Bailer) method. Thereafter, reinforcement is placed in the bore and cement concrete is filled in. the unstable ground requires temporary support, which is provided by the permanent liner (which goes upto a certain depth) and during the process, by use of drilling mud or bentonite, which prevents collapse of the sides of the borehole. This method is referred to as a replacement or non displacement type of piling. The bored cast in-situ piling method is opted for,
over the other techniques, for this project, as it is comparatively much more economical. It is ideally suited for piles with large diameters and depths, as is the case here. The soil removed during boring can be used for inspection. Also, this method would be quite convenient, as the client and the contractor have had considerable experience with this method in the past.
C. BORED CAST IN-SITU PILING: CONSTRUCTION METHODOLOGY A number of processes are involved in casting a pile by this technique. The chart 2 shown here, gives an overview of the activities involved in piling by the bored cast in-situ technique:
BARGE SHIFTING
CONCRETING
POINT MARKING &GUIDE FIXING
LINER LOWERING & PITCHING
FLUSHING
BORING TREMMIE LOWERING
REINFORCEMENT & CAGE LOWERING
Barge Shifting After completion of concrete of pile barge will be shifted to next location for piling work
Point Marking and Guide Fixing After the guide fixing is completed, the liner is lowered from its position. Extra liner units are welded to the bottom liner for liner extension, depending on the depth of the bed. When the liner to be lowered, it is released from its position and allowed to fall freely, so the liner drops into the
bore hole below and settles on the sea bed on its own, owing to its self weight and rests on it without any support. This is referred to as Liner Pitching. When the liner stops going inside the sea on its own and settles on the bed, it is driven into the sea bed. A removable driving cap is placed on the top of the liner to prevent any damage to the liner. A hammering tool called Monkey is used to drive the liner. It is dropped from a height of about 1- 1.5m above the liner to thrust it in. The liner is driven till it reaches the hard/ rocky strata, i.e. the refusal level, beyond which it cannot be driven. Image 9 shows driving of liner using hammer. Boring (using Chisel and Bailer) Boring is carried out to remove the soil and form the void for concreting, to form the pile. Once the liner is properly inserted below the sea bed level, boring is carried out using bailer, a steel vertical cylinder and a cross-chisel, a cutting tool. The initial boring (upto the refusal level i.e.23m) is done by bailer as the strata being sandy, can be cut easily. To facilitate the loosening of soil, seawater is made to flow into the pile, and then the soil is removed using mainly the bailer. Simultaneously, the saline water in the pile is replaced by pumping in mature bentonite underneath the water. Bailer then is lowered multiple times to remove the mud and wet soil from the bore hole to clean it, as shown in Image 11. Circulation of the bentonite slurry helps to prevent the sides of the bore hole from collapsing, due to its thixotropic action property. Boring is done till the required depth i.e. the founding level is reached. Sounding, with chain, is done at regular intervals during the process of boring and after boring is completed, to check the bore depth. The bore depth achieved is checked to ensure attainment of the required depth i.e. up to the founding level. Reinforcement Cage Lowering Before lowering the reinforcement cages, the borehole is thoroughly cleaned. Then the top of the liner is cut and removed in such a manner that the full- length cage can rest on its top.
The bottom reinforcement cage is lifted from the pontoon and is made to sit on the liner. A guide rope is tied to its bottom to control the lifting operation. The next cage is then lifted and positioned so that the bars of the two cages are properly aligned. The main bars are lapped and helical reinforcement are fitted in position. Then the bottom cage is lowered and another cage is lifted and welded. All cages are welded in this manner. The topmost cage has provisions of development lengths in the bars for embedment in the headstock (superstructure).Lifting of the reinforcement cage from pontoon is as shown in Image 12. Tremmie Lowering Tremmie lowering can be started immediately after completion of lowering of reinforcement cages. The tremmie pipes are attached to each other by male- female joints and are greased to facilitate their smooth attachment and removal. The tremmie is attached to the tremmie top which is suspended from the piling tower. The first tremmie is lowered and is kept suspended with the help of a tremmie fork. Then, tremmie top is detached from the first tremmie and is attached to the second tremmie, which is attached to the first tremmie at the other end. In this way, tremmie is lowered upto 300mm above the bottom of the bore hole. After all the tremmie pipes are lowered, pipe carrying bentonite slurry is attached to the tremmie top. Flushing Flushing is carried out to clean the rid of the unwanted matter prior to concreting. Fresh bentonite slurry having specific gravity of is circulated inside the bore hole. The specific gravity of bentonite should ideally be less than half the specific gravity of concrete, which actually increases when bentonite reaches the bottom i.e.1.14- 1.22. The bentonite mix at the bottom of the bore hole is checked by lowering a Mud Sampler and taking a sample from there. Hydrometer is used to check the specific gravity. If the density of bentonite is more than 1.25, flushing is continued. On completion of flushing, air flushing is done using air compressors, for a short period. Concreting
The concrete is prepared in a batching plant and is transported to the site in transit mixers, and is pumped into the bore. Since compaction of concrete inside the pile is not feasible, high slump of the concrete is maintained. Once the flushing is completed, a hopper is attached to the top tremmie pipe. The bottom opening of the hopper is initially plugged by a removable charge plate, which is attached to a separate winch wire rope. The tremmie hopper is filled with concrete. When the hopper is completely filled with concrete, the plug is pulled out and the concrete in the hopper rushes out of the tremmie pipe downwards, displacing the bentonite in the bottom end of the tremmie pipe. This is done in order to prevent mixing of concrete with the slurry. Now when the concrete is poured, it results in a pile shaft build up. A build- up of 4-5 m is always maintained in the pile. Once the build-up of 4-5m is obtained, the tremmie pipe is lifted out. The hopper is removed and the topmost tremmie pipe is detached. Thereafter, a smaller hopper is attached to the tremmie and the system is lowered into the pile and concreting is resumed. It is continued till the bore is filled with concrete. At the cut- off level of the pile, a window is cut on the liner to facilitate removal of the contaminated concrete by letting it overflow. About 1m3 concrete is allowed to flow out. ------------------xxxxxx------------------
