Monday, April 7, 2008

Work method statement for Secant Pile

WORKS METHOD STATEMENT

SECANT PILE WORKS & RETAINING STRUCTURES




APPROVAL






REVISIONS RECORD

REV. DATE ISSUED REVISION DESCRIPTION APPROVAL STATUS








TABLE OF CONTENTS


1.0 WORK ORGANISATION
2.0 DESIGN CONSTRAINTS AND COORDINATION
3.0 CONTRACTING METHODS
4.0 PROCUREMENT METHODS
5.0 RESOURCE PROVISIONS
6.0 LOGISTICS PROVISIONS
7.0 MATERIAL MANAGEMENT SYSTEMS
8.0 CONSTRUCTION METHODS AND TECHNOLOGY
9.0 CONSTRUCTION PHASING AND WORKFLOW
10.0 TRAFFIC MANAGEMENT AND MONITORING
11.0 SAFETY PROGRAMME
12.0 QUALITY ASSURANCE AND CONTROL PROGRAMMES
13.0 INDUSTRIAL RELATIONS POLICIES
14.0 ENVIRONMENTAL MONITORING PROGRAMMES



1.0 WORK ORGANISATION

2.0 DESIGN CONSTRAINTS AND COORDINATION

3.0 CONTRACTING METHODS

4.0 PROCUREMENT METHODS

5.0 RESOURCE PROVISIONS

6.0 LOGISTICS PROVISIONS

7.0 MATERIAL MANAGEMENT SYSTEMS

8.0 CONSTRUCTION METHODS AND TECHNOLOGY

8.1 Construction of the Secant Pile

During the installation of the bored piles, the installation of the secant pile shall proceed simultaneously. The installation of the secant piles commences at four (4) locations as shown in Sketch 1. Boring records shall be made during the installation of secant pile record shall be made. The detailed method of statement is very similar to the method statement for bored piling works and shall be made available before construction commences on site.

Secant pile wall formed by the primary (female) and secondary (male) piles by drilling the piles to the specified diameter and required depth. The secant pile wall is constructed in two stages. The primary piles will be constructed at Stage 1 with the specified stregth of mass concrete and the secondary piles will be constructed at stages 2 with the required concrete strength after the reinforcement cage is lowered into the bored holes. The secondary piles are positioned between the primary piles and secant with the primary piles in order to form the interlocking joint.

1. Guide Wall
In order to ensure the alignment of wall and the extend of over cutting the primary piles and guiding the boring tools during initial boring works of bored hole, a guide wall shall be constructed at approximate 0.5m below the existing ground level prior to the commencement of boring works.

2. Bentonite
The GTC4 bentonite is delivered to site in 50kg per bags, which are stored under cover. The bentonite is mixed by high turbulence mixer and the slurry is stored in silors or pools for use in the bored hole.

Where boring of piles takes adjacent to the sea or in marine deposits, the propoerties of bentonite may be adversely affected by the organic or saline attack. In order to counteract any possible deleteroius effect in such environments, polymer additives may be used.

A laboratory is provided on site for regular testing of the slurry. The minimum frequency of testing and the acceptable range of physical characteristics of the bentonite slurry are given in Appendix 1. Laboratory reports are kept during the construction period.

Apparatus available in the site laboratory includes the following:-

1 mud balance (density test)
1 marsh cone (viscosity test)
1 filter press (fluid loss test)
1 sand screen set (sand content test)
Paper for measuring pH

Contaminated bentonite slurry will be discarded to an acceptable dump area.
8.2 Secant Pile Wall Construction

Boring
The boring plant will be moved to the intended pile position for boring works. The kelly bar mounted with an auger is placed just above the pile point. The vertically of the Kelly bar shall be checked by means of a spirits level and will be enchanced with the presence of the temporary guide wall. The auger is then lowered and boring operation commenced. During the boring operation, the bored hole shall be observed for its stability. The temporary steel casing shall be driven into stable stratum if the bored bole is found to be unstable. The bored hole shall also be filled with bentonite slurry as the drilling fluid; the bentonite is kept to within 0.4m from top of guide wall level in order to stabilized the bored hole. If hard material is encountered during the boring process, other coring tools may be utilized in order to penetrate into the hard stratum.

Upon completion of boring, the bottom of the bored hole is thoroughly cleaned with the cleaning bucket prior to recycling of the bentonite. A submersible turbine pump attached to a tremie pipe is lowered to the bottom of the bored hole. The bentonite, loaded with soil particles in suspensions, is drawn off from the bottom of the bored hole and re-cycled through a Caviem or equivalent recycling unit. The process is continued until the bentonite arriving from the base of bored hole satisfies the specification given in Appendix 1.

A. Primary Pile (Female Pile)
The female pile will be constructed as thew primary piles for the secant pile wall. The primary pile will be filled with the specified strength of mass concrete without any steel reinforcement.

B. Secondary Pile (Male Pile)
The secondary pile will be constructed in between the primary piles and overlap with the primary piles with the specified strength of concrete after the reinforcement cages are lowered into the bored hole. The secondary pile formed by cutting into the primary piles to form the interlocking joint by using the specified coring tools. The boring process for the secondary pile shall only commence upon initial set of the primary piles.

C. Reinforcement
Reinforcement cages shall only be installed into the primary pile as per the design requirement. Normally the reinforcement cages will be pre-fabricated on site; and upon completion of recycling the bentonite, the reinforcement cages will be lowered into the bored hole by means of as service crance. The reinforcement cages are equipped with round spacers to ensure that the specified minimum concrete cover to the reinforcement is maintained.

D. 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 section 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 bentonite inclusions.

During concreting, a log is kept of delivery times, volumes and concrete levels. Concrete cubes are taken to assess the concrete strength.

E. Extracting Temporary Casing
All temporary steel casing are to be extracted immediately after the completion of concreting process. A Vibro-Hammer will be used to extract the temporary steel casing.

F. Pre-cautions
In the unlikely event of the sudden loss of bentonite slurry, the boring process will be immediately stopped and backfilled the bored hole with soil by using the hydraulic excavator until the loss of bentonite slurry is stopped.
9.0 CONSTRUCTION PHASING AND WORKFLOW

10.0 TRAFFIC MANAGEMENT AND MONITORING
11.0 SAFETY PROGRAMME
12.0 QUALITY ASSURANCE AND CONTROL PROGRAMMES
13.0 INDUSTRIAL RELATIONS POLICIES
14.0 ENVIRONMENTAL MONITORING PROGRAMMES

Appendix
1. ITP
2. Checklist

No comments: