Method Statement For Construction Of Concrete Piles.
- INTRODUCTION.
This method statement briefly describes the proposed methodology for the installation of piles using long casing for a Project. Method statement shall be later subject to addition incorporating measures to improve performance or to suite site conditions / safety measures with approval by Engineer.
The objective of this method statement is to ensure all pile constructed in a controlled condition, according to applicable standards, code of practice and contract specifications.
- SCOPE OF WORK
Scope of Work for pile construction procedure shall be briefly depicted as below: -
Submission of relevant technical documents such as Method statements, designs and shop drawings as necessary complying project specification & contract drawings for the approval of Engineer / Authorities.
Supply & maintenance of tools / equipment and required manpower skilled/semi-skilled in the site to carry out the works.
Supply of materials required for execution of works with proper site management/logistics.
Construction of preliminary piles to carry out pile load test on non- working piles for enhancing /confirming the pile length / toe while analyzing geotechnical soil strata parameters.
Construction of bored piles as per approved documents after obtaining required permits / NOC etc.
Testing on working piles (Static, Dynamic, Sonic, Caliper & Integrity Test) i.e. as per project specs, approved method statement & interpretation
of results for ensuring quality of piles constructed.
- TOOLS, PLANT & EQUIPMENT.
The following tools & equipment’s shall
be deployed on site: -
Piling Rigs
Service Cranes, Vibro-Hammer
JBC/Low loader, Flat Trailers, Low beds, Hi-ab
Concrete pump, Mixers, Service Truck
Drilling Tools (Casing/Augers/drilling bucket/cleaning bucket)
Welding Machine / Gas cutting set / Portable grinder
Steel cutting
Air compressor / Water jet
Tanks, Pumps, Mixers, etc.,
- SITE ACTIVITIES.
Piling Works comprises of the following specific activities on site: -
Prepare & maintain a firm, dry and free working platform for execution of works.
Setting out pile center points
using rectangular
coordinate system / total
station.
Installation of temporary steel casing with vibro-hammer or twisting casing using piling rig.
Drilling of borehole using appropriate tools up to the toe-level.
Lifting and lowering of ready made steel rebar cage into the pile.
Concreting of
pile to be done using concrete pump/ tremie
tubes.
Extraction of temporary steel casing after casting of concrete.
Maintaining records of each pile while construction thereafter to Engineers approval.
Performing quality
control tests on working piles and submission
of test reports to Engineer’s approval.
- METHODOLOGY FOR THE PILE CONSTRUCTION.
- Pre-requisites to starting
on site
All site staff shall be fully conversant with approved method statement and the
drawings prior commencing piling work.
Piling rig stability is a critical priority. Rig overturning may be caused by the
following factors and these must be considered when positioning the rig and sequencing the works:
Poor ground conditions
Unsuitable platform
(unlevelled / steep/poor material/bad weather)
Poorly maintained
platform.
Sub surface voids
(manholes, pits, shafts, chambers etc.)
- Preparation of Working Platform.
The piles will be carried out from a proper-levelled working platform, hard soil
compacted for safe movement of equipment’s & heavy machinery such as rigs,
trucks and cranes. Working platform shall be free of any encroachments to allow
for free maneuvering of machines.
The piling platform is to be suitably a firm, dry and free working platform for
execution of works and well maintained to provide rig stability. Tracking should be
kept to a minimum. The rig is to be pulled off its position and the ground stabilized
if voids occur during piling.
- Carrying out Survey works.
Setting out piling position shall be carrying out by qualified surveyor, using rectangular coordinate system / total station based on the coordinates of piles as per layout drawing approved by Engineer.
The Surveyor will transfer the demarcation points to ground. Pile points shall be transferred to ground & located by driving pegs on the ground. Points fixed on the ground will be protected appropriately from moving machinery and other potential disturbances.
- Installation of Temporary Casing.
Long casing shall be installed using vibro-hammer up to the required level or
twisting method along with the rotary head of piling rig machine, casing can be
twisted until the bottom reaches the required level or depth.
The temporary casing used will be free from significant distortion and shall be of
uniform cross-section.
Using vibro-hammer suspended from a crane, temporary casing of appropriate
diameter shall be installed up to a minimum 0.5 m embedment in rock layer.
Otherwise using rotary head of piling rig, casing can be twisted until the bottom
reaches the target depth.
The installation of temporary casing will be carried out by qualified riggers. Check
during casing installation will be done by qualified riggers using spirit level and
two offset points at right angle to ensure that the position of casing is within the
tolerances allowed in project specifications. Surveyor will check the as built
coordinates vs design coordinates using Total Station after the completion of
installation of temporary long casing, exceeds allowable tolerances i.e. casing will
be withdrawn from the soil and will be re- installed again.
- Drilling of Pile bore up to Pile Toe-level.
After installation of the temporary guide casing, the borehole will be drilled using
appropriate drilling tools.The borehole will be drilled down to its final level by
means of bucket and auger.
The depth of borehole shall be measured using an end weighted measuring tape
and to be recorded in the record sheet.The bottom of borehole shall be clean and
settled sediments shall be removed using cleaning bucket.The drilling spoil from
boreholes shall be disposed as necessary to facilitate the works to appropriate
locations as to the satisfaction of Authorities.
- Lowering of Steel cages.
Suppliers will deliver steel to site and steel mill certificates will be provided. Steel
re-bars shall be preserved in storage yards in site and cages shall be fabricated as
required by skilled personnel/steel fixers as per Approved shop drawings for
inspection of Engineer.
Concrete spacers shall be fixed to the spiral of reinforcement cage to ensure the
specified concrete cover and to keep the reinforcement cage in position.
Sleeves are also installed on steel bars above the cut-off level for easy breaking.
Prior installation of steel cage, suspended sediments at the pile base to be cleaned
using cleaning bucket and ensure that pile toe-level is as per approved design.
The prefabricated steel cages will be lowered to the pile bore using cranes. They will be lifted by the service crane in a manner to avoids any permanent deformation while lifting or lowering into the pile.
- Casting of Concrete.
Concrete shall be delivered to site by suppliers and shall be checked for slump and
temperature on site. Maximum time elapse of casting concrete from batching of
concrete shall be 2 hours. Any concrete mix with slump and temperature out of
tolerance shall be rejected.
Concrete cubes will be casted to check the compressive strength as per required quantity complying specification and is detailed in clause 10.0 testing of concrete.
Tremie pipe / concrete pump pipe will be inserted in the borehole up to pile toe level. Pile concreting can be carried out by using concrete pump or by tremie pipe with bucket only, whichever is available. During concreting, tremie pipes will be inserted at the center of the pile to reach up to the toe and should always maintain an embedment of 2.0 m in concrete.
Prior concreting, site engineer should ensure that the required quantity of concrete for each pile is available / could be delivered on time to avoid any concrete cold joint. The concrete shall be delivered directly from the transit mixer or concrete pump to the funnel and thereafter to tremie pipe.
The concrete will be brought above cut-off level until platform level to avoid any concrete
contamination and to produce good quality of concrete at pile cut off level.
For piles with cut-off levels which are substantially lower than the working platform level, concrete shall be casted up to 2.0 m above cut off level of pile. However, in-situ soil shall be used for back-filling borehole for remaining borehole depth from platform level until top of concrete level (2.0 m above COL).
The finished concrete level in piles shall be maintained as per standards/specification above the top of bar level to avoid any concrete contamination and to produce good quality of concrete at pile cut off level.
- Extraction of Temporary Casing.
The temporary casing will be extracted using vibro-hammer or by twisting the casing followed by pulling out with extra care of monitoring the verticality of the casing during extraction. Collar provided with holes and groove at the top of casing helps to rotate / extract the casing during punching / extraction after concreting.
Wire sling / chain sling shall be fix on the top of the casing to support the vibro- hammer during the extracting of casing.
Extracted casing will be install on the other available points of the pile prior to start boring / drilling of the pile.
If required i.e. after fix / connecting of the wire / chain sling on the casing, crane will start to lift the casing and slowly lay down on the ground.
- PILE INSTALLATION TOLERANCES.
Pile Tolerance and verticality for the piles along with the column head shall
be check properly by Surveyor.
The tolerance variation of piles from the position on plan at cut-off level
shall not exceed 75 mm in any direction.
- TESTING OF CONCRETE.
All testing of fresh / hardened concrete will be conducted by an independent
testing agency and results will be presented accordingly as per standards
below. Testing of concrete (compressive strength / durability) shall be done by ITA.
- Type & Frequency of Concrete
Standards: To BS 8500
and BS EN 206-1
Strength Testing
(Cube Strength): BS EN 12390-3 and BS 1881
Compression Testing
of Concrete