METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION wadrillco original MS

Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd

METHOD STATEMENT FOR CONSTRUCTION OF BORED PILE
WITH TEMPORARY CASING AND BENTONITE SOLUTION
APPLIED STANDARDS:
1. Vietnamese Construction Standard TCXDVN 326:2004: Bored piles –
Standard for construction, check and acceptance.
2. ASSHTO of practice for foundations.
3. Technical Specification cast in place Bored piles ( Project : PHU MY.).
1.

STEP 1: ADJUST THE PILING MACHINE ON THE PILE AXIS

Prepare the drilling place, position the pile axis
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Given the soil base of the construction site, our principle in this case is to move
the piling machine mainly in an one-way movement, starting from side out,
without running over the completed bored holes.

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The pile construction steps, from drilling to concreting, must follow strictly the
sequence proposed for the week that has been approved by the Site Manager.
New holes must not be made too close to the just finished piles, as the latter
need time to mature. Specifically,
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The distance between 2 adjacent piles must be at least 0.8m centre to

centre of bored pile.
The construction time of 2 adjacent piles must be at least 24 hours.

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Having determined the pile ID numbers, the Contractor will proceed to locate the
pile axis, based on the surveyed marks and using use theodolite and steel
measuring tape. Preparations must be made in areas around the drilling place to
determine the pile centerline. Areas around the boreholes will be prepared to
facilitate operation of the piling machine, the muck removal and movement of
agitator – trucks, etc without affecting the construction of other piles.

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Once the pile axis has been determined, 3 equidistant points will be established
to serve as basis for the steel case positioning and pile axis check during the
drilling process.

Prepare the piling machine:

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
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Before drilling, the piling machine must be serviced and trial runs to ensure
smooth operation during drilling process.

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The drilling bucket: The opening between the drilling buckets will be smaller
than the nominal size of the drill shalf by 1÷ 4cm. The up and down hauling the
buckets along the pile shaft, coupled with the swaying of the kelly bar during the
drilling, will enlarge the bored hole at least to the nominal diameter. After having
completed a few initial bored piles, the rise level of concrete will be checked in
order to decide on the opening of the bucket opening.

Haul the piling machine to the drilling place:
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After positioning the pile axis, the piling machine is hauled to the drilling place.
The horizonal position of the machine is then adjusted using a surveyor’s level.
The kelly bar is adjusted to be vertically on the pile axis. The deviation from
vertical position of the kelly bar should not exceed 1%.
Verticality of the kelly bar is monitored using a plumb bob, or theodolite. With a
normal kelly bar length of 15 m, the deviation between its two ends must be less
than 15 cm, corresponding to ½ of its size.

2. STEP 2:
Casing work:
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HOLE OPENING – CASTING INSTALLATION

Casings are pre-fabricated in the workshop. Because of the casings, the actual
size of the bored hole is often larger than its nominal size ( (normally 80
mm÷100mm) by 8 mm÷10mm. the top end of the steel case to prevent it from
sinking more than expected depth.
Depending on the actual geological conditions, the steel cases may be placed at
6m÷9m. Based on our experience with test piling in this area, 6m long steel
case is appropriate.
Before placing, the steel casing must not be deformed that may affect the
movement of the drilling bucket within the casing.

Bentonite slurry:
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Bentonite must be calculated in sufficient quantity, and delivered to the
construction site.
The bentonite slurry must be prepared before drilling, using proper mixing
equipment, with sufficient quantity for the entire drilling process.
The slurry must meet the specifications in Table 1, before it is used in the pile
shafts.

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
Table 1: (in compliance with ASSHTO specification construction standard)
Physical property
Density
Viscosity (s)

Required data of slurry before using in
pile shaft
1.05÷1.15cm3
28 ÷ 45

PH
Sand content

8-11
<4%

Testing method
Density balance
Running time
through the
standard funnel
500/700 mm
PH Paper
Sand content
measuring device

The slurry may be re-used many times after treatment.
Hole opening, casing installation:
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Drilling buckets equipped with additional reamers are used to enlarge the pile
shaft for fitting the steel casings (since the casing size is larger than design
borehole diameter).
When the piling machine has been balanced, the kelly bar in a vertical position,
with the auger positioned to the pile axis, start drilling slowly.
When the bucket is filled, haul it slowly from the drilling level to the top of the
pile hole and swing to unload the dirt to the unloading area. The process is
repeated several times until the pile is completely drilled.
Hooks and suspension ropes have to be ready for placing the steel casings. Use
two ropes of equal length to ensure the verticality of the steel case, to prevent
damaging the pile base.
Using the service crane to place the steel case ensuring its alignment with the
pile axis and verticality until the top edges of the casing are about 30 cm÷60cm
above the ground level. Higher level will make it difficult to pour concrete by
trucks. Too low a level may allow penetration of dirt and mud from the vicinity
into the bored hole.
The placing of steel cases should be done in a way to ensure the following
allowable errors, when the casing is in the required place:
•
Grade:<1/50
•
Error in co-ordinates on a horizontal plane:<7.5 cm in all directions.
In order to meet this requirement, check the axis alignment of the casing during
installation using the triangulation points mentioned above, by means a steel
measuring tape. After finishing the casing installation, the axis alignment of it

may be checked by a theodolite. Check the verticality fo the installed steel case

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
using the inclination of the top end of the casing. The simplest way is using a
steel tape to measure the height of the casing top end form the bentonite slurry
level in the pile shaft.
3.
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STEP 3: DRILLING TO DESIGN DEPTH
After finishing installing the steel casing, reinforce it by backfilling before
resuming drilling to the design depth. During the drilling process, the steel
casing will prevent soil caving when the buckets move within the bored hole.
Frequently check the balance of the piling machine and verticality of the kelly bar
during drilling. Also, bentonite slurry must be pumped down the bored hole from
time to time to ensure it is always at a higher level than the underground water
outside the casting.
Frequently check the soil layers penetrated by the auger and compare with the
available geological data.
Frequently monitor the pile grade by checking the verticality of the kelly bar
using a plumb bob or theodolite. The maximum permissible vertical error is 1/50.
In order to ensure the stability of the bored hole without caving when drilling, all

impacts or other active forces to the hole base must be minimized.
The drilling process must be maintained continously and will not be stopped
unless the piling machine is out of service.
Check the bored hole depth using a plumb bob.

4. STEP 4:

1ST TIME CLEANING OF THE BORED HOLE BOTTOM

Upon reaching the design depth, stop drilling for 30÷60 minutes then start cleaning
the deposits at the bottom of the bored hole using the cleaning buckets. When the
buckets reach the bottom, the drilling speed is maintained at a low level to ensure
complete cleaning of the deposits. For higher efficiency of cleaning, extra fittings
may be fixed to the buckets, so that all dirt will be taken away from the hole bottom.
5.STEP 5
REBAR CAGE FABRICATION AND ERECTION- ERECTION OF
CONCRETE POURING TOOL
- Steel bars for use must comply with the designed size and type.
- Rebar cages are pre-fabricated using steel bars of standard length of all
segments after deducting the junctions will be the same as the designed length
of the rebar cage.
- After finishing drilling, all rebar segments will be assembled near the bored hole
for installation.
- The superimposed sections are normally basing on drawing, according to the
design requirements. The segments are jointed together firmly by steel wire of
Â1mmữÂ2mm and welding.
Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .

PHU CUONG Co., Ltd
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6
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The insertion of rebar cages must be made rapidly to mininmize the deposits at
the bored hole bottom and avoid caving.
Rebars must be inserted immediately after finishing cleaning of the bored hole.
In the event that the steel rebar segments must be jointed together by welding,
the welding equipment must be fully utilized to shorten as much as possible the
welding time.
Care should be taken in inserting the rebar cage in order to avoid impacts to the
bored hole wall and soil caving.
After the rebar has been placed to the required depth, it must be secured to the
steel case to prevent displacement during concrete pouring.
In order to ensure the alignment of steel bars with the pile axis, concrete
wedges of diameter about twice the thickness of the cage envelop, about
30mm÷50mm, will be placed on the rebar at length about 2m÷3m.
After finishing inserting the rebars, the tremie pipe will be isntalled . this is
composed by a number of thread-jointed pipe sections. The tremie pipe is not
placed to the bottom of the bored hole. This will be done after finishing the 2 nd
cleaning of the bored hole.
STEP 6

After the rebars and tremie pipe have been installed, if the bottom deposit

exceeds 10 cm depth (for friction piles+supports), or if the bentonite slurry
concentration is too high, a 2nd cleaning of the bored hole will be required. This
will be made by air-lift as described hereunder:
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7

2ND CLEANING OF THE BORED HOLE

Insert a blow pipe of small diameter(Ø90÷Ø100) down the tremie pipe, to
near the bottom of the boredhole. Compressed air is introduced to the
hole bottom to produce high pressure for pushing the deposits up along
the blow pipe, while additional bentonite slurry is pumped into the bored
hole.
The blow-cleaning is carried out until the outgoing bentonite slurry is
clean (i.e. sand conten < 6%, density ≤1.25) and the thickness of the
bottom deposits after cleaning does not exceed 100mm(for friction
pipes+supports).

The quality control of deposits is done by measuring the bored hole depths after
1st and 2nd cleanings.
STEP 7: CONCRETING-RECOVERING OF BENTONITE SLURRY

Type of concrete

Method Statement for Bore Pile works.

Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
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The concrete to be used is the commercial concrete supplied by the professional
concrete contractor. In the absence fo the design’s specification for the
aggregates, this will be taken care of by the contractor in order to meet the
following requirements:
 Compressibility of the concrete sample after 28 days≥N/mm2
 Slump at start of concrete pouring: 225 ± 25, 40 Mpa
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Additives:
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In order to improve the working properties of concrete, the supplier must
use various plasticizing and hardening retardant additives to produce a
concrete mix that meet the requirements of adopted technology.
The use of additives must comply with the manufacture’s instructions.
The additives must not contain any substances that are corroding or
harmful to the life time of concrete.

Transport of concrete
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Concrete must be transported by mixer-truck, when the delivery distance
exceeds 500 m.

Inspection of concrete before pouring
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Before concrete pouring, check the slump and visually check the quality of
concrete to make sure there is no coagulation and that the aggregates are of
the right size, to avoid obstruction of the tremie pipe during concreting.
Each pile must have at least 3 compressibility test data sets related to the
concrete sections at pile head, mid-section and base.

Concrete pouring
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General principle:
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Check the bored whole depth to insert the tremie pipe until its lower end
is about 20 cm above the whole bottom.
Tremie technique is used. The tremie pipe is topped by a funnel for
pouring the concrete. Before pouring, a soft foam ball having the diameter
equal to the inside diameter of the tremie pipe is placed at the top end of
the pipe. When concrete is poured into the funnel, the ball will go down
the pipe and push the bentonite slurry out to let the concrete fill up the
bored hole from its bottom. During concrete pouring, the tremie pipe is

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
hauled up and down to facilitate the release of concrete; and sections of
the tremie pipe will be dismantled until its depth in concrete is about
2m±3m.
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Concrete pouring tool:
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Tremie pipe:
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Concrete pouring is done using a vertical pipe.
The pipe is made of steel, with inside diameter of Ø20cm±30cm
The pipe is made up by sections having the same diameter, and
1m±6m. length
All joints of the tremie pipe must be firm and easy to assemble
/dismantle.
The tremie pipe must be sufficiently long for the pipe depth
Funnel:

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The funnel must be designed specially for tremie
pouring, enabling continous pouring without overflowing into the bored
hole.
The funnel grade must be reasonable so that none
of the concrete will remain on the funnel.
Water resistant ball:

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The ball is made of water resistant foam, with density <1 so that it
can float on the water and will not stay in the concrete.

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The ball must tightly fit the pipe, so that the bentonite slurry will be
pushed completely out of the pipe.
Before concrete pouring, the ball must be placed on top of the pipe.

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Concrete pouring:
The tremie pipe will be assembled and installed in the bored hole
along its depth.
Fix the concrete pouring tool on the operating platform.
To minimize the deposit layer and risk of soil caving, the concrete
must be poured into the bored hole as soon as the cleaning of the
hole is completed. Activities such as: check of the hole bottom
deposits, insertion of the rebars, insertion of the tremie pipe must be
carried out rapidly.

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
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STEP 8
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9.

TREATMENT OF BENTONITE SLURRY

Recovered bentonite will be treated for re-use
The treatment of bentonite slurry consists of:
 Sand treatment using desander or depositing tank
 Treatment of viscosity, density and PH by adding fresh bentonite or
certain additives, such as CMC.
The bentonite slurry will be consumed during drilling and additions of
fresh one will be made from time to time to meet the requirement. When
the recovered slurry after treatment fails to comply with the required
property, it will be removed from the construction site by tank trucks.
STEP 9: REMOVAL OF SOIL FORM THE CONSTRUCTON SITE AND
BACKFILLING

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Concrete is poured into the pipe must be in the concrete, about
2m±3m deep.
Minimize the time for assembly/dismantling of the pipe sections to
speed up the concreting.
During concrete pouring, avoid the overflow of concrete from the

funnel into the bored hole as this will affect adversely the viscosity of
the slurry and quality of the concrete.
During concreting, check frequently the level of concrete inside the
bored hole by means of steel tape and plumb bob to enable timely
adjustment fo the elevation of the pipe bottom end.
Before concreting check delivery capacity of all suplly sources to
ensure uninterupted supply of concrete during pouring.
The final level of poured concrete must be higher than level of
designed pile head.
Recovery of bentonite slurry
During concrete pouring, while the concrete fills in the bored hole,
bentonite slurry is pushed out of it. The rejected slurry is conducted to
a collection pit and pumped to a container for treatment.

Excavators and dump trucks should be constantly on stand-by at the
construction site. Excavated soils must be removed immediately from the site
to avoid muddiness.
For piles having poured concrete level lower thatn natural ground level, part of
the sellected sandy soil will be used for backfilling. This will be carried our
about 24 hours after the poured concrete has hardened.
STEP 10

MONITORING, RECORDING AND SAMPLING

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd

Samples will be taken from time to time, and the construction process is
monitored continously. The findings are stated in the inspection and
acceptance minutes approved by the parties.

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd

BORED PILING QUALITY CONTROL
1. Setting out control:
Precomputation and setting out of main-gridline will be conducted by Licensed
Surveyor to facilitate detailed setting out of pile position.
The actual position of bored pile is being set out by Land Surveyor of the piling
contractor.
The position of the piles set by the contractor shall be checked by Surveyor Officer
before commencement of piling.
2. Boring Accuracy Control:
Set the boring unit at a reasonably flat working platform.
The pile position surveyed shall be offset in two perpendicular directions to boring for
counter check of pile center.
Commence boring with boring unit. During the coarse of boring the following checks
shall be made:
- (a) Check center position of pile at about 1.5 m depth and
at proposed cut off level.
- (b) Check vertically of ca sing during installation using a
spirit level and plumb 1 line.
3. Borehole Excavation Control:
the borehole is drilled by using boring machine mounted on a crawler crane or using

hydraulic boring rig.
In the event hard formation is encountered before the required borehole depth is
reached, we could use either modified reversed circulation, rock auger, coring bucket
or pile redesign to resolve this.
During the excavation, bentonite slurry is fed continuously into the borehole so that
the excavation is always filled with slurry. The slurry level is constantly maintained
so that correction procedures promptly undertaken should the necessity arise.(for wet
hole).

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
Boring will stop when the agreed pile toe level is reach.
The base of borehole shall be leaned with a cleaning bucket. The sediment will be
drawn away by suction method before concreting.
4. Reinforcement Cage Fabrication and Installation Control:
Reinforcement cage is being fabricated using circular links and main bars of
appropriate size and spacing as per design.
Before placing of concrete, the reinforcing cage will be lower into the borehole to the
required cut off level.
Circular concrete spacers constructed to the same grade of concrete with basing the
drawing, cover is being adopted to provide corrosion protection.
Circular concrete space shall either be threaded though the links or part of the link
and tied to the main bars to provide 75 mm cover.
5. Placing of Concrete Control:
The placing of concrete is carried out by dry or tremie method.
For dry hole, concrete will be discharged directly into the hole with hoper. The
discharge shall be guided by the worker such that the concrete falls on the center of

borehole to minimize disturbance to the side of borehole. The concrete shall be cash
above the cut off level for at least 0.3 m.
For tremie hole, a 200mm or 250 mm diameter tremie pipe is lowered to the bottom
of the borehole and a hoper is attached at the top. The tremie pipe is the lifted up
about 150 mm from the toe of the hole to facilitate concreting. During concreting,
care should be taken to maintained the lower end of the tremie pipe at least 2.0 m
into the concrete to avoid discontinuity. The bentonite displaced by the concrete is
pumped from the borehole to a storage tank for either reuse or recycle. Concrete
shall be casted above the cut off level for at least 1.0 m so that the top portion
contaminated concrete can be hacked away at later stage to receive the pile cap.
The physical properties of the concrete must conform with the requirements of the
consultant but if shall not content coarse aggregates exceeding 20 mm in size and it
shall have a slump of 150-200 mm.

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
For tremie concrete, retarding agents should be used to delay the concrete set by
approximately four hours to ensure that the concrete at the tremie discharge point is
always free flowing.
During concreting ,the bentonite slurry displaced by the concrete is pumped from the
borehole to a storage tank for either reuse or recycle .

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd

POSSIBLE INCIDENTS DURING CONSTRUCTION AND
REMEDIAL ACTIONS
1.

STUCK BUCKETS
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2.

FALLEN BUCKETS
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The buckets may fall down to the bottom of the bore hole as a result
fo broken cables or bucket dowel. Thus in order to avoid this, the cables
and dowel must be checked from time to time.
When the buckets fall down to the bottom of the bore hole, they must
be recovered as soon as possible to avoid being buried by the dirt. If the
buckets can not be recovered as they are being stuck, the Site supervisor
must be promptly informed for remedial actions such as:

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3.

If the drilling has reached or come close to the design depth, the Site

supervisor’s agreement will be requested for cleaning and concreting the
bored hole ( to bury the buckets).
If the drilling has not reached the design depth, the Site supervisor’s
agreement will be requested for enlarging the bored hole by buckets fo
larger size. When reaching the stuck bucket, it will be removed and
drilling continues, until the design depth is reached, with the larger
bucket.

SOIL CAVING
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If the buckets get stuck, it must be released by a combination of
gentle pulling and turning movements, instead of sudden jerks.
Use a high pressure water jets to clear off the mud around the bucket
before pulling it up.
Otherwise, use a viabrator while pulling the bucket and kelly bar.

If this happens before insertion of the rebar cage: check again the bentonite
slurry. Replace it with fresh slurry and drill again.
If this happens after insertion of the rebar cage: Haul up the rebar, replace
the bentonite with fresh slurry and resume drilling: If the rebar gets stuck,
use the dirt clearing system t haul up the rebar from the bored hole.
INCLINED DRILLING

Method Statement for Bore Pile works.


Project : PHU MY BRIDGE .

PHU CUONG Co., Ltd
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Frequently check the verticality fo the kelly bar. If the drilling is made
with an inclination exeeding permissible limits, this will make it difficult or
even impossible to insert the steel bar and tremie pipe.
In the evenr of the inclined bored hole, and it is not possible to insert either
the rebar or tremie pipe , one of the following remedial actions can be taken:
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5.
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Backfill the bored hole with poor concrete. Let it stabilze for a few days
before resume drilling. While drilling this time, frequently monitor and
adjust the verticality of the kelly bar.
Enlarge the bored hole by bucket of larger size

OBSTRUCTION OF TREMIE PIPE
During concrete pouring the tremie pipe may be obstructed due to low quality
concrete, with inapproriate slump ( too thick or too thin). Also, this may be
caused by a large part of the tremie pipe is in the concrete. To avoid
obstruction, the tremie pipe must be constantly hauledup and down to
facilitate the release of concrete.
If the pipe is obstructed haul it up ( do not exceed the level of concrete

surface) and lower it quickly, coupled with tapping using hand hammer.
If the obstruction persits, pull the pipe out and remove the concrete by
tapping . Replace the pipe with a new one within 1,5 hours. Insert the tremie
pipe and follow the steps below:
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Place the lower end of the pipe about 20 cm above the surface of
poured concrete.
Pump concrete to fill up the funnel, let it fall down and continue
pumping concrete into the funnel.
When the concrete rises about 1 m within the bored hole, lower the
tremie pipe about 1±1.5 m more, while ensuring the funnel full of
concrete.
The bad concrete section of about 2m will be pushed up.

Upon the completion of concreting, the level of poured concrete must be at
least 2 m higher than the design level.

6. SIPHONING BORED HOLES
Soil caving may occur in the case of two adjacent bored holes, before concreting,
thus resultin gins two siphoning holes. When concrete is poured into one hole,
bentonite slurry in both holes will rise up. Thus, in this case, the concreting must
be carried out at both holes simultaneously to ensure equal the levels of concrete.
Method Statement for Bore Pile works.

Project : PHU MY BRIDGE .
PHU CUONG Co., Ltd
Alternatively, one of the holes will be backfilled and concrete poured into the
other hole. After finishing concreting the first hole, drill again the second hole
and start concreting.

Method Statement for Bore Pile works.