METHOD STATEMENT FOR INSTALLATION OF BORED CAST IN SITU PILES (DMC Method)

 

 INSTALLATION OF BORED CAST IN SITU PILES (DMC Method)

 

1. Site clearance Layout and Survey Work

The site shall be cleared of existing structures, trees, bushes, heap of earth etc. at the approximate location of pile groups on various bridges.

(1.1) The center line pillars and benchmark shall be established at the site of each bridge with the help of already available reference points and benchmarks. The center line pillars shall be masonry pillars of sufficient depth in order to have permanent reference during the entire construction of the bridge and of sufficient height so that height of pillar plus height of instrument can mark the center line right up to top of the superstructure. These center line pillars shall be duly checked & approved by the Engineer, before taking up any permanent works.

(1.2) With the help of above center line pillars center line of each pile group longitudinally as well as transversely shall be established.

(1.3) Based on above pile group layout a location of initial load test pile shall be selected in such a manner that if fairly represents the entire piling work on a particular bridge and area is reasonably leveled to create a platform to carry out the initial load test. This location shall be marked vis-à-vis the center line of the bridge and got approved from the consultant before taking up actual execution of the initial test pile.

(1.4) The setting out of various piles in a particular group shall be carried out with respect to the center of group and shape of the pile cap shall be marked. After setting out of each group Surveyor shall take spot levels (site levels) of existing ground before taking up excavation work for creating a working platform for installation of working piles. These spot levels shall be checked and jointly recorded with the consultant.

2. Earth Work / Excavation for working platform for installation of bored cast in situ piles.

Depending on depth of excavation for pile cap and unstable sub-soil conditions, suitable depth shall be excavated for each pile group in order to create a working platform for erection of piling equipment and installation / routine load testing of piles. Suitable shoring if required, shall be provided on the sides to avoid any collapse of sides. However, if such protection is required a sketch of the same shall be prepared and got approved from the Engineer before taking up the excavation as a safety requirement.

3. Installation of bored cast in situ piles.

(3.1) General.

It is proposed to carry out the boring of cast in situ piles with the help of a heavy duty tripod hoisting a DMC (Direct Mud Circulation) cutting tool as per the propriety design of the piling agency and operated with the help of diesel / electrically operated power winch.

In the DMC method of piling, the initial boring up to a depth of 1.5 m is done using the boring tools and guiding casing. The bore is then filled with drilling slurry and further boring up to the founding level is carried out by continuous slurry circulation with DMC boring tools. The purpose of a boring slurry is mainly to muck out the excavated material from the bottom of the bore to a settling tank on ground. In the DMC method the muck is carried out to the surface through boring slurry and is deposited in the settling tank through slurry coming out of the bore. The slurry is continuously circulated under the required pressure through DMC tubes. After the bore is completed up to the required depth, fresh slurry shall be pumped through DMC tool / tubes while DMC is resting at the bottom of the bore till such time that all the muck / loose materials are removed from the bore. Various steps of boring of piles shall be as follows:

(3.2) Installation of Temporary casing.

After setting out of the pile location by surveyor, a temporary casing shall be installed prior to the excavation to support the upper layer of weak soil. The length of the temporary casing shall be decided during installation of the initial test pile and generally be the same at least in a particular group. Casing installation shall be carried out with the help of a hammer attached to the tripod and winch. The control of verticality of piles within the allowable tolerances as prescribed by the Technical Specification, shall be performed during the installation of the temporary casing. 

(3.3) Boring Process:

After installation of temporary casing and checking of its location and verticality the boring DMC method shall be started. The drilling slurry consists basically of water and DMC powder in colloidal state. The drilling slurry will be prepared by mixing clean water with the appropriate DMC powder, to make a slurry which shall serve the purpose of a carrier for the excavated muck for the entire period of boring, during placement of concrete up to completion of the pile construction. High turbulence mixers shall prepare the slurry and, before it is used, it shall be left to season in a storage tank for at least 24 hours. Tests to determine density, viscosity and pH value shall be applied to the colloidal suspension pumped through DMC reds to the bore. The fresh slurry shall have the following properties. :

- Density g/ml < 1.05 g/cc

- Marsh Cone Viscosity 30-40 seconds

During the boring operation the drilling tool is lowered into the bore with percussion movement. Slurry of required specific gravity shall be pumped through the DMC rod by a vertical pump installed on the slurry tank. Simultaneously the DMC rod shall be moved up and down by winch. The slurry coming out from the bottom of the bore, shall be collected through a gravity flow channel in the slurry tank. During the flow in the channel the muck will settle and only slurry shall get collected in the tank.

Boring will be continued till the founding level is reached. If rock is encountered up to a maximum strength of 400 kg/cm2 a special rock chisel shall be used.

(3.4) Pile Base Cleaning:

Before placing concrete, measures shall be taken to ensure that there is no accumulation of sediments at the base of the bore. A sample of the slurry suspension shall be taken from the base of the bore using a sampling device and checked for its density and viscosity as per the specifications.

(3.5) Installation of Reinforcement Steel Cage:

The steel cage, pre-assembled in as many sections as needed and complete with the appropriate spacers and stiffeners will be lowered into the hole by the tripod. The quantity, type and size of steel will be according to the Good for construction drawings and specifications.

(3.6) Installation of the Tremie Pipe:

Prior to concrete pouring operation, a 250 mm internal diameter pipe of full length, assembled together from sections of 1.20 – 1.50 meter each, shall be lowered down to the bottom of the pipe and then lifted up by 200 to 300 mm. The top of the Tremie pipe shall be fitted with a feeding hopper to receive concrete either directly from a transit mixer or concrete pump. It shall be ensured that all the joints between the Tremie segments and between the funnel and Tremie are watertight.

(3.7) Concreting:

Concreting in pile under water shall be done with the use of the Tremie method to permit deposition of concrete in successive layers without permitting the mixing of water / slurry with the concrete.

Ready mixed concrete with a slump of about 150 mm to 180 mm, produced from the mixing plant with weigh batching shall be used to ensure a smooth flow of concrete in the Tremie pipe.

The toe of the Tremie pipe should always remain immersed at least 2m in concrete, during entire casting to prevent intrusion of drilling slurry / water as the case may be.

As the pouring operation continues, the rise of concrete level inside the bore shall be monitored and recorded. As the level of concrete rises inside the bore, the Tremie pipe shall also be raised by removing 1-2 sections of Tremie pipe. While raising the Tremie pipe it shall be ensured that the Tremie pipe always remains immersed @ 2m inside the concrete. Pouring of concrete shall continue until the designed level of pile is achieved and sufficiently above thereafter to allow for subsequent cutting of pile to get sound concrete at cut-off level. Depending upon the length of casing the temporary liner shall be extracted, if required