Monday, April 7, 2008

Work Method Statement for Diapharm Wall






This submission explains the method of construction to be adopted for the diaphragm walling at the project site.


Not applicable.


The conditions of Contract have been reviewed by PCSB and particularly those conditions in respect of Sub-PCSBs. The conditions are bespoke for this project and therefore be required to be reflected in any form of subcontract utilized on this project.

Please refer to PHOS-CN-010 to PHOS-CN-030 for the Contract Procedures.


Please refer to PHOS-CN-010 to PHOS-CN-030 for the Contract Procedures.


5.1 Manpower / Labour

The number of manpower / labour to be used during the activities have been identified in the Manpower Record (Weekly Progress Report).

5.2 Plant Equipment

~ Excavation Crane Liebherr 853
~ Service Crane Link Belt LS 108-BS Crawler Crane
~ 25T Mobile Crane
~ Miller Welding Machine
~ Hydraulic Excavator
~ 600mm diameter coring tools
~ 600mm Clamshell
~ Chisel TD600
~ Office / store container
~ Generator set 300kVA and 125kVA
~ Tremie pipes c/w Hopper
~ Tremie brake
~ Caviem Desander
~ Desilter
~ Desanding pump
~ 1m3 Digestuer
~ Tsurumi GPN3 Submersible pump
~ Silo
~ Mission pump
~ WSI Joint x 12m length


The logistic control shall be in accordance with the approved Site Logistic Plan.


Please refer to PHOS-CP-040 for the Material Control Procedure.


8.1 Details of the construction method are considered according to the following matters:

1. Outline Method
2. Guide Wall
3. Bentonite
4. Soil Investigation
5. Diaphragm Wall Excavation
6. Recycling of Bentonite
7. WSI Joints (Construction Joints)
8. Placement of Reinforcement
9. Placing of Concrete
10. Diaphragm Wall Equipment
11. Report Form
12. Precautions

8.1.1 Outline Method

The Diaphragm Wall technique consist of constructing reinforced concrete walls from existing ground level by first excavation a trench by grab. During excavation, the sides of the trench are supported by bentonite slurry.

Upon completion of excavation, a steel reinforcement cage is lowered into the slurry and concrete is then poured into the trench by the tremie method.

As the concrete level rises, excess bentonite is drawn off for re-use. WSI Joints are used to form the joints between adjacent panels.

8.1.2 Guide Wall

In order to guide the grab during initial excavation for each panel and plus ensure the position and verticality of the diaphragm wall, a guide wall is constructed at around 0.5m from ground level prior to commencement of diaphragm walling. The guide wall also provides support during suspension of the reinforcement cage.(Ref. Appendix 15.4)

8.1.3 Bentonite

The bentonite GTC4 is delivered to site in 50kg per bags, which are stored under cover. The bentonite is mixed by high turbulence mixers and the slurry is stored in storage reservoirs until used in the trench.

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 15.5. 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 sand screen set (sand content test)
1 shearometer of Fann viscometer (shear strength test)
Paper of measuring pH

Contaminated bentonite slurry will be discarded to an acceptable dump area.

8.1.4 Soil Investigation

Prior to commencement of diaphragm walling, soil investigation holes to assess the geological conditions will be drilled/provided.

From the site investigation information, the excavation techniques are finalized and should obstructions be anticipated, the most appropriate method of treatment is planned.

8.1.5 Diaphragm Wall Excavation General

Excavation utilizes rectangular grabs cable-operated by crane. During the excavation process, the bentonite slurry is kept to within 0.4m of the level of the top of the guide wall. The vertically of the trench is monitored by visual inspection of the crane cables during successive lowering of the grab into the trench.

The excavation crane would be maintained at a minimum distance of 4.5m from the edge of opened trench. Any movement of excavation crane will be supervised by the foreman to enforce this requirement.

Various types of panel are used, primary, successive and secondary (closing) panel.
The general arrangement of panels will be submitted separately later as shopdrawings. Removal of Earth During Excavation

Excavated earth from the trench will be temporary stockpile on the platform and will be removed by using hydraulic excavator load into truck and dump into the approved dump yard. Primary Panels

The design length of the primary panels (with two WSI joint formers) is consistent with either the minimum length possible of the size of grab necessary to excavate or full bites at each end of the panel with a small remaining to complete the excavation at the center of the panel. Successive Panels

The panels that are equipped with only one WSI joint former are successive panels. Secondary Panels

The panels that are to be constructed in the last stage upon completion of the previous primary and successive panels. No WSI joint former needs to be installed. Overcoming Obstacles

Dependent on the nature and size of the obstruction, several alternative methods can be adopted for removal of obstacles :-

a) By grabbing where the size of the obstructions is compatible with the size of the grab;
b) By using rock boring tools or down the hole hammer noting that chiseling is not allowed.

Rock boring tools or down the hole hammer shall be utilized to cut the boulder/rock into size that compatible with the jaw of grab before being grab out. Verticality Monitoring

During excavation, the operator can have an easy visual way to monitor possible deviations. The simple watch of the position of the suspension cable of the clamshell in relation to the guide walls is a clear and relatively accurate way to estimate any possible deviation.

8.1.6 Cleaning of Base and Recycling of Bentonite

Upon completion of excavation, the bottom of the trench is thoroughly cleaned with the clamshell prior to recycling of the bentonite. A submersible turbine pump attached to a tremie tube is lowered to the bottom of the panel. The bentonite, loaded with soil particles in suspension, is drawn off from the bottom of the trench and re-cycled through a Caviem or equivalent recycling unit. The process is continued until the bentonite arriving from the trench base satisfies the specification given in Appendix 15.5.

8.1.7 WSI Joint System

“WSI” Joint System in the past decade was developed for allowing the the execution of watertight joints between diaphragm wall panels. WSI Joint principle

The WSI Joint is a stop end extracted sideways when excavating the adjacent panels, thus bringing a positive answer to the problems encountered when extracting sliding forms. Installation

During the recycling of the bentonite after excavation is completed, WSI joints are installed at the end of the excavated panels, primary panels having a joint at both ends and successive panels at one end. The stop ends consist of separate sections bolted together and lowered successively into the trench until the WSI joint reaches the design depth, which is few meters below the future bulk excavation level or into the low permeability soil layer.

The WSI joint is a stop end form extracted laterally. A rubber water stop is incorporated into the joint prior to placing the WSI joint into the diaphragm wall panel. The WSI joint left in place at the end of the panel while the adjacent panel is being excavated. The excavation equipment is then guided by and removes the WSI during excavation of the subsequent panel.

A typical section of the joint is attached.(Appendix 15.6) WSI Joint System and clamshell

Due to their suspension by cables and their rectangular shape, the clamshells are very well suited for use in conjunction with the WSI system. The excavation tool is locked on the WSI at regular intervals throughout the excavation operation, bringing an immediate correction to any tendency to deviate. Advantage in using the WSI Joint

The use of the WSI Joint system brings four main advantages to the construction of better quality diaphragm walls. Stop end removal is totally independent from concrete placement operations, this allows better site efficiency organization and planning. This also alleviates the need for extending working hours beyond the end of the concrete pour. An excellent guide is provided for the excavation of the adjacent panel. It allows the installation of rubber water stop. As the WSI form is left in place at the end of the panel while the next panel is being excavate, it protects the concrete of the previous panel. Therefore the geometry, the cleanliness and the quality of the joint are excellent.

8.1.8 Placing of Reinforcement

Reinforcement cages are pre-fabricated on site and upon completion of recycling the bentonite and installation of WSI joint(s), the cage is lowered into the slurry trench by crawler crane. The cage is equipped with concrete spacer “skids” to ensure that the specified minimum concrete cover to the reinforcement is maintained.

The steel cages are generally composed of 12m long sub-cage elements connected by welding the designed lap length during lowering into the excavation trench. Once all the sub-cages are lowered, they are suspended to the required level from the guide walls by measured suspension bars connected with the calculated lengths in preparation for concreting.

Reservation for box-outs shall be fixed in the sub-cage and positioned by tape measurement from the top fo the corresponding sub-cage.

Where an inclinometer is required in a diaphragm wall panel, reservation in the form of welding connected steel pipe for inclinometer shall be provided into the reinforcement cage. Installation of the inclinometer access tubing can be carried out at a later stage after concreting of the diaphragm wall panel.

8.1.9 Placing of Concrete

Concrete is poured into the trench through tremie pipes. The tremie pipes are 270mm and are made up of coupled sections 0.5m, 1m, 2m, and 3m in length. As the level of concrete in the trench 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.

8.1.10 Diaphragm Wall Equipment

A list of equipment type necessary to carry out the works is given below:-
• Excavation crawler crane : model Liedherr 852 or equivalent;
• Excavating clamshell : in sufficient numbers;
• Bentonite mixing unit : digestor mixer withmission pump 3x4R;
• Bentonite recycling unit “ Caviem 100m2/hr;
• Bentonite storage : 1 to 2 Nos. pools each of about 200m3
• Service crawler cranes : cranes with capacity between 50-100 Tonnes.
• Various pumps and tremie pipes for recycling;
• WSI joints;
• Offices, workshops and changing rooms;
• Mud laboratory

The number of excavation rigs will be adjusted depending on the construction period and the soil condition.

8.1.11 Report Form

Various report forms used for diaphragm walling are enclosed in Appendix 15.7.

8.1.12 Pre-cautions

A number of pre-cautions will be undertaken prior to commencement of work and during the construction of diaphragm wall as follows :-

a. Trench Stability Analysis shall be carried out to determine the following:-

i. Maximum panel length
ii. Minimum bentonite density
iii. Minimum height of bentonite level in trench

b. Ensuring that the bentonite is not below the minimum level as required for stability. As a precaution against the unlikely event of collapse, bentonite level is always maintain at 0.4m below guide walls. As a good practice, the guide walls shall be constructed at least 1m above the ground water table and the bentonite level is kept near to the top of guide walls.

c. Maintaining density of bentonite so that it is above the minimum density required for stability.

d. Handling and treatment of bentonite so that its stabilizing properties is not affected.

e. Detracting of panel sizes so that is does not exceed the maximum panel size for such stability.

f. Stockpile of soil will be maintained on site as a contingent measured in case of backfilling is require due to some reasons.

g. In the event of a collapse, excavated trench will be backfilled with stockpile material until the situation is under control.

h. In the unlikely event of the sudden loss of bentonite slurry, the excavation will be immediately stopped and back-filled with stockpile material until the excavated trench is stable.

i. During desanding of bentonite while the bentonite is pumped from the bottom of trench to the desander, precaution has to be taken to ensure that the supply of treated fo fresh bentonite to the trench is regulated to ensure that the supply of treated or fresh bentonite to the trench is regulated to ensure that the level of bentonite does not fall below the required level for trench stability.


Please refer appendix 15.3


10.1 Maintenance of Existing Roads, Footpaths and Service Drains, Etc.

The construction team shall maintain all existing site access, roads, footpath, service drain, etc. and reinstate any damage caused by any reason whatsoever during the progress of the works.
It will be the Logistic Manager’s responsibility to ensure that ingress and egress to the site are kept free from obstruction brought about by the work on this site and in no way shall cause hindrance to traffic or ancillary works either by his own vehicles or by his work people, material, etc.


11.1 Safety Hoarding, Temporary Works and Public Safety

The Logistic Manager shall be responsible for submission of plans and drawings to the relevant Authorities for the construction of all the necessary temporary fencing and protection hoardings, temporary drains and desilting pits, safety nettings, screens, etc. for the works.

The Logistic Manager shall provide adequate signboards at all strategic positions warning the public to keep away from the work site and erect temporary fencing and barriers where necessary around the site to prevent unauthorized trespassing during works.

11.2 Pollution and Disturbance

The HSSE Manager shall be responsible to take necessary measures to ensure that noise and air pollutions are orderly controlled to satisfy the full requirements of the relevant authorities.

He shall also ensure that disturbance due to noise and air pollutions caused by the works to the neighbourhoods and public are kept to an absolute minimum.

In particular, the HSSE Manager shall ensure that the existing driveway areas are constantly wet to prevent excessive dust / air pollution during the demolition work.


Please refer to Appendix 15.1 for Inspection Test Plan and Appendix 15.2 for the Diaphragm wall checklist.


Not Applicable.


Please refer to PHOS-EMP-001 Environmental Management Plan.


Appendix 15.1 Inspection Test Plan
Appendix 15.2 Inspection Checklist
Appendix 15.3 Construction Flow (Step 1 to Step 8)
Appendix 15.4 Guide Wall Design Calculations & Typical Details
Appendix 15.5 Bentonite Slurry Testing Frequency
Appendix 15.6 WSI Joint Technical Data
Appendix 15.7 Reporting Form
Appendix 15.8 Job Safety Analysis

1 comment:

dew said...

I would appreciate if you can publish the appendices.