METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION wadrillco original MS

METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION Biện pháp thi công cọc khoan nhồi D800 METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION Biện pháp thi công cọc khoan nhồi D800 METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION METHOD STATEMENT FOR 800MM BORED PILE CONSTRUCTION Biện pháp thi công cọc khoan nhồi D800

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

- 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

- 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,

 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

- 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

- 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:

- Before drilling, the piling machine must be serviced and trial runs to ensure smooth operation during drilling process

- 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:

- 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: HOLE OPENING – CASTING INSTALLATION

Casing work:

- 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:

- 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

Table 1: (in compliance with ASSHTO specification construction standard)

Physical property Required data of slurry before using in

pile shaft

Testing method

Density 1.05÷1.15cm3 Density balance Viscosity (s) 28 ÷ 45 Running time

through the standard funnel 500/700 mm

Sand content <4% Sand content

measuring device The slurry may be re-used many times after treatment

Hole opening, casing installation:

- 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

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 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: 1 ST 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

¢1mm÷¢2mm and welding

- 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 2nd

cleaning of the bored hole

6 STEP 6 2 ND CLEANING OF THE BORED HOLE

- 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:

 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

7 STEP 7 : CONCRETING-RECOVERING OF BENTONITE SLURRY

Type of concrete

- 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



- Additives:

 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

 Concrete must be transported by mixer-truck, when the delivery distance exceeds 500 m

Inspection of concrete before pouring

- 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

- General principle:

 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

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

- Concrete pouring tool:

 Tremie pipe:

 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

 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:

 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

 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

 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

 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

8 STEP 8 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

9 STEP 9: REMOVAL OF SOIL FORM THE CONSTRUCTON SITE AND

BACKFILLING

- 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

10 STEP 10 MONITORING, RECORDING AND SAMPLING

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

BORED PILING QUALITY CONTROL

1 S etting 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)