Bored-Piling-Method-Statement-灌注桩施工方案.doc

Method Statement for Bored Pile

Table of Contents

1.0 General
2.0 Setting out
3.0 Drilling
4.0 Piles Termination
5.0 Clearing of Pile Base
6.0 Installation of Reinforcement
7.0 Placing of Concrete
8.0 Testing of Concrete
9.0 Possible Problems Encountered
10.0 Noise And Vibration
11.0 Hazard Analysis
12.0 Method of Locating And Avoidance of Utility Services
13.0 Bored Pile Record Sheets

1.0 General
The general sequence for bored pile works is shown as below: -

Figure 1: Flow Chart For Bored Pile Process

2.0 Setting Out
In house surveyor will set out the pile location with reference to the control point established by the licensed surveyor. Prior to boring, the pile point shall be checked and offset along the 2 main axis for checking any deviation during initial boring.

3.0 Drilling
Before boring, the supervisor/ project manager shall verify all associated technical information such as presence of services, pile co-ordinates, platform level, cut-off level, validity of drawings and etc.
The pile boring operations will be performed by the flight auger or boring bucket attached to a hydraulic drilling unit which is mounted on a crawler crane.
The verticality of bored hole will be checked from time to time from the top while advancing the bored hole. Sighting plumb line shall be used to check the verticality of the Kelly bar of boring machine against 2 major axis. The checking shall be carried out before and during boring process at regular interval to ensure boring hole is constructed and completed within tolerance.
Bored Hole

Steel Rod
Boring Machine
min. 1200mm HT

String

Kelly Bar

Plumb Bob
Sighting Point

45o

Detail of Sighting Point_

Steel casing of adequate thickness shall be used to prevent unstable soil from caving into the bored hole for the first 6~12m of the soil strata. The casing will be installed by vibrating hammer or driven by the boring rig. Water shall be utilised as a medium stabalising fluid in the process of forming the borehole.
During the boring process, the water level shall be kept as high as possible within the casing and well above the existing ground water level.
Rock drilling tools or chisels shall be used to drill into rock. The clerk of work or engineer shall be verified the length of actual length of boring into rock.
The bored length can be checked with measuring tape or length of kelly bar. Borehole stability shall be regularly checked by observing the bored depth and amount of soil removed.

4.0 Piles Termination
Before any permanent pile installation, preliminary load test shall be carried out to verify the design parameters used in design the pile length. Working load and integrity tests will also be carried out to ensure piles are properly installed.
Pile termination criteria shall be defined according to the design, such as socket length into rock, minimum length in soil only and founding strata shall be identified and stipulated.
Soil sample shall be collected at regular depth interval and compared with description in nearest soil investigation bored hole used in design of the pile. Inform resident engineer/COW if substantial difference observed.

5.0 Clearing Of Pile Base
After reaching the boring depth, the bottom of shaft shall be cleaned by mean of a mechanically operated cleaning bucket.

6.0 Installation of Reinforcement
Reinforcement steel cage will be supported centrally and firmly within the hole so that no movement occurs during concreting operation. Steel, concrete spacers will be used to provide interval support and ensured adequate concrete cover.
Reinforcing cage will be suspended centrally on top of the casing where structural requirement of the reinforcing cage is less than the deep and bored shaft.
If sonic logging test is required for the pile, the sonic logging tube shall be welded to the steel reinforcement and their end closed with mild steel cage. After test had been carried out, the sonic logging tubes shall be flushed clean by air lift method. Cement grout is then pumped through a poly-pipe into the tube after cleaning.

7.0 Placing Of Concrete
Concrete is poured into the bored hole through tremie pipes. The tremie pipes are 270mm diameter and are made up of coupled sections 0.5m, 1m, 2m, and 3m in length. As the level of concrete in the bored hole rises, the tremie pipe column is raised whilst always ensuring a minimum 2m embedment into the concrete in order to avoid mud inclusions.

During concreting, a log is kept of delivery time, volume and concrete level. Concrete cubes are taken to assess the concrete strength.

When concreting in dry hole condition, a conical hopper fitted with short length of pipe at bottom is placed centrally over the head of the shaft.
When the concrete is to be placed under water or drilling fluid, measures must be taken to ensure that:-
a) The tremie pipe shall be water tight throughout its length
b) High slump concrete (Tremie mix, slump = 175mm ± 25mm) will be used for the construction of the bored piles.
c) The tremie pipe will be lowered centrally to the bottom of the borehole.
d) The bottom of tremie pipe will be embedded at least 2 to 3m in the fresh concrete column. At all time, sufficient quantity of concrete shall be maintained within the tremie pipe to ensure the pressure formed exceeds that from the water or drilling fluid.
e) Where sectional tremies are used, they should be split at the required intervals in order to avoid excessive embedment and thus insufficient flow of concrete into the bore.
f) The diameter of tremie pipe shall not be less than 150mm and it shall have minimum external projections as to allow the pipes to pass through reinforcing cage without causing any damages, distortion or up-lifting of the steel cage. The internal face of the tremie pipe shall be free from any projection.
g) The spoil from the bored piling work shall be removed from site to an approved dump site.
In dry hole condition, the concrete level in the shaft shall be brought above the designed cut off level for 150mm. While in tremie condition, the concrete will be overcast minimum 500mm above the designed cut-off level.
The platform level is checked to establish the depth of the cut-off level. The top of the concrete is constantly being monitored during the concreting process with a measuring tape to ensure the required concrete level is achieved. The pile head of completed piles will be protected from subsequent damage by mean of over casting the pile above cut-off level. The holes of completed bored piles will be back-filled with earth for safety consideration and this is also to protect the piles.

8.0 Testing Of Concrete
Frequency of concrete testing shall be as follow: -
a) Slump test - Every load
b) Cube strength - One set of test cubes (6 nos. of cubes) for every pile.

Testing of cubes shall be performed for compressive strength at the following interval ages;

2 cubes at age of 7 days
2 cubes at age of 28 days
2 cubes as standby cubes or at age of 56 days (if required)
9.0 Possible Problems Encountered

Due to variable nature of soil, there may be some possible problems encountered. Some of the common problems are: -
a) Present of obstruction
For shallow obstruction that is within the reach of a hydraulic excavator, it shall be dug out with the hydraulic excavator. Otherwise, assuming that the obstruction is a boulder, chiseling is normally adopted.
b) Sudden loss of support fluid
As a precaution against the unlikely event of collapse water level is always maintained at 0.5m below casing. In the unlikely event of the sudden loss of support fluid, the boring will be immediately stopped and back-filled with clayey material or lean mix concrete until the loss of support fluid is stopped.
After remedial measures have been undertaken, boring will resume.
c) Verticality of excavation
The verticality is maintained by checking the Kelly bar against a plumb bob at two major directions. Should verticality be out of tolerance, the bored hole is back-filled and the verticality will be rechecked for re-boring.
d) Defective piles
If the completed piles is found (by sonic coring or other) not to be good quality over the design socket length, the remedial measures shall be: -
· The engineer shall assess whether the pile can be use after down grading.
· To install additional piles.
This problem can be prevented or minimized if the following measure is carried out:-
· Always plan the supply of concrete is timely so that the bored hole is not left long after excavation.
· During concreting, concrete curve chart must be maintained to check any collapse of soil.
· Adjacent piles close to the newly completed bored piles are not installed until at least 24 hours later.

10.0 Noise and Vibration
Unlike driven pile system, bored pile system generally will not have any problem with noise and vibration. However, noise monitor will be carried out during operation to check the noise-generated compliance to the local authority requirement.

11.0 Hazard Analysis
HAZARD PRECAUTIONS
a) Injury from falling defect Crane operator not allowed to lift items directly over workers.
All workers must wear safety helmet. Only bench men is allowed to stay near boring rig during boring operation

b) Injury from falling into borehole all open holes shall be covered if left unattended.

c) Injury from concreting Proper signal to crane operator and avoid standing at the direction of the concrete truck.

d) Entanglement with moving plant Stay out of danger zone of boring rig, crane and excavator.
e) Snapping of the wire rope Crane and Rig operator shall carry out daily safety and service checks.

12.0 Method of Locating and Avoidance of Utility Services
Before any piling operation being carried out, the following operations shall be carried out to locate any utility service: -
a) Based on licensed surveyor’s underground services survey mapping, we shall identify and peg the location of services. Those services need to be relocated or diverted shall be carried out prior piling commencement. Services that do not require relocation shall be clearly marked.
b) Visual inspection of the piling site shall be carried out to check any services markers that did not pick up by the surveyor’s mapping. Manual excavation or pilot trenching shall be carried out to determine the depth of services.
c) Cable detector will also be used to detect any underground cable.

13.0 Bored Pile Record Sheet

For every pile, it must be completed with the following record sheets: -

a) Inspection Request Form
b) Bored Pile Record
c) Concrete Progress Chart

Refer QA/QC procedures attachments for sample of record sheets.

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