Method Statement – Slurry Method Page 1

METHOD STATEMENT FOR PIPE JACKING – SLURRY METHOD

1.0 Objective This method statement intends to outline the procedure that will be adopted for pipe jacking works using slurry method for the concerned project. Periodic reviews on the specific details at site shall be carried out and necessary adjustments may be made based on these findings. This statement shall not be deemed as conclusive nor shall it be regarded as a replacement of the original design and is solely proposed without prejudice.

2.0 Introduction

Pipe jacking, or commonly referred to as microtunnelling for smaller diameters, is a technique for installing underground pipelines, ducts and culverts. Powerful hydraulic jacks are used to push specially designed pipes through the ground behind a shield at the same time as excavation is taking place within the shield. The method provides a flexible, structural, watertight, finished pipeline as the tunnel is excavated.

The slurry method, as the name suggests, employs the use of a mechanical shield and slurry system that maintains a stabilised face pressure during the excavation process. The shield is remotely controlled from a control panel, usually placed next to the launching shaft. The excavated spoils are removed in the form of a slurry through the discharge slurry pipes to the separation chamber where the water is separated from the solids.

There is no theoretical limit to the length of individual pipe jacks although practical engineering considerations and economics may impose restrictions. Drives of several hundred metres in a straight line or to a radius can be achieved using appropriate guidance systems. Pipes in the range of 150mm to 3000mm in diameter can be installed with construction tolerances comparable to other tunnelling methods.

3.0 Preliminary Investigation A Desk study and field study shall be carried out prior to any physical work. The desk study shall include assessment of available utility plans, soil investigation reports and any other relevant information. This is essential in determining the feasibility of the proposed pipe jacking work. The field study shall include visual site investigation for existing services, speculation of likely ground conditions, any obstructions, accessibility to site, working space and traffic condition. Such studies may even include trial pitting or Macintosh probes.

Method Statement – Slurry Method Page 2

4.0 Construction of Operation Shafts A site survey shall be carried out and the proposed alignment shall be demarcated with wood pegs. The launching shaft, otherwise known as the jacking pit, shall be constructed in accordance to the alignment and designed invert level of the pipeline to be installed. A 1m deep trench shall be excavated at the proposed location of the launching shaft and the I-beams for the support waler shall be arranged and welded in the excavated trench. Where necessary, short diagonal struts shall be welded as additional support for the I-beam waler. Upon completion of the fabrication of I-beam waler, sheet piles of 12m length shall be driven into the ground around the waler using either a vibro-hammer. The jacking pit shall measure approximately 6m in length by 4m in width. Excavation within the trench shall then proceed and upon reaching the designed depth of the waler level, the waler shall be welded into place. Excavation shall continue until the next designed level of the second waler where the second waler shall be put into place. Upon excavating to the final required depth of about 6.5m, that is the designed invert level of the pipeline together with an additional space for the setup of the pipe jacking ancillary equipment, a mass concrete platform shall be constructed. The thickness of the concrete platform shall be of approximately 100mm in thickness. The reception shaft or receiving pit shall be constructed in the same manner as the launching shaft and is usually smaller in size. The receiving pit shall measure approximately 4m in length by 4m in width. Construction of the concrete base for the reception shaft is solely based on need-to-use basis.

5.0 Pipe Jacking Setting up of equipment shall be carried out in accordance to the level and alignment of the pipe jacking crossing, which shall be transferred into the launching shaft using survey equipment. The setting up work shall commence with the construction of a thrust wall, which shall serve as a reaction resistance to the jacking force. A mild steel plate of approximately 50mm thick shall be erected in front of the thrust wall together with the formwork. Ready mixed concrete of approximately Grade 20 shall be poured into the formwork to form the thrust wall. The erected mild steel plate shall serve to distribute uniformly any uneven or concentrated loads exerted by the hydraulic jacks during pipe jacking. The guide rails shall then be erected at the base of the shaft and welded firmly to the sheet piles to form a launching platform for the pipes. Appropriate water pumps and other ancillary equipment, which includes charge and discharge pumps, slurry tanks, slurry pipes, control panel and etc. shall be positioned accordingly as shown in the typical layout below. A joint measurement shall be carried out upon completion of the setting up works to confirm the accuracy of the alignment and level.

Method Statement – Slurry Method Page 3

The pipe jacking work commences with the insertion of the slurry shield into the ground. The shield is positioned on the guide rails and driven into the ground by the hydraulic jacks. Once there is sufficient space in the launching shaft, a 375mm dia. reinforced concrete jacking pipe is lowered on to the guide rails and jointed to the shield. To ensure that jacking forces are evenly distributed around the circumference of the pipe, a thrust ring is used to transfer the loads uniformly. Spacer blocks may be used to lengthen the stroke of the jacks for non-telescopic jacks.

The pipe is then driven into the ground together with the slurry shield. Once the first pipe is sufficiently jacked into the ground, another pipe shall be lowered on to the guide rails and jointed to the first pipe. As the pipeline proceeds, excavation of the soil takes place within the shield and the spoils are removed in the form of slurry via the slurry pipes. Frequent checks are carried out to ensure that the pipeline is advancing according to the designed line and grade. When the need arises, necessary adjustments at the slurry shield can be made to correct any deviations. Details such as the jacking force, the advance rate, the position of the slurry shield, charge and discharge pressure and etc. are recorded accordingly in the pipe jacking record form.

The process is repeated until the shield reaches the reception shaft. An exit on the shaft is then created to facilitate the retrieval of the shield. Once the slurry pipes and the cables are disconnected, the shield is lifted out of the reception shaft.

Method Statement – Slurry Method Page 4

6.0 Deepening of the Jacking Pit Once all pipe jacking work is completed, the equipment shall be dismantled and removed from the jacking pit. Hacking of the thrust wall and the concrete base shall also be carried out to make way for the subsequent deepening of the excavation depth, which is necessary to facilitate the construction of a backdrop manhole within the jacking pit. An additional layer of H-beam waler shall be welded into place around the level of the removed concrete base. Excavation shall then be resumed to deepen the pit to approximately 10m.

7.0 Safety & Health and Environmental Control Measures

Standard procedures for safety & health shall be observed during the pipe jacking work. This shall include, but not limited to the following: a. Erection of guard rails around all shafts to prevent accidental falls

b. Proper ladders with hand rails shall be installed to facilitate safe entry and exit from the

shafts

c. First aid kit shall be made available at site at all times

d. PPE shall be worn at all times by all personnel working on site

e. Weekly tool box meeting shall be conducted on site to create awareness of safety & health issues amongst site personnel

Environmental control measures shall include, but not limited to the following: a. Discharge of construction water into public drainage shall be filtered first by a silt trap prior

to release

b. Unsuitable excavated material shall be carted off-site to an approved disposal tip