Special Works
backwall a temporary works design was submitted and approved. The thrustwall restraint consisted of a rear block „tied‟ into the concrete, and an engineered formation of UC posts and struts as per the schematic below. The alignment of the structure had to be centre to the bore and the reinforced concrete elements were cast to +/- 10mm positional tolerance.
The thrust restraint was designed to accommodate a push pressure of 100t even though it was highly unlikely these pressures would be reached. Suitable time was left for the RC structures to cure in advance of Terminator equipment mobilisation.
The vehicles for the equipment were booked for delivery onto BFK‟s vehicle booking system and off- loaded, and set into the 30m deep shaft utilising a dedicated 200T crane and qualified banksmen/slingers. Positioning of the equipment within the thrustwall restraint structure was very challenging for several reasons. Firstly the dimensions between the backwall-frontwall were within a 30mm tolerance of the actual Terminator thrust-rig, which was very restricted, even though the thrust-rig was extendable to a certain degree. Secondly, the existing and numerous myriad of steel thrusts at different levels within the large SHW2 shaft (part of shaft construction) significantly limited the positioning of the equipment when the 200T crane is 30m above invert level. Cooperation and coordination with the crane driver, slinger above, and slinger below was paramount. Thirdly, the presence of remaining, steel anchors within the concrete floor, and manholes, caused positional difficultly which could only be overcome with removal of some of the anchors and re-positioning the equipment many times.
Once the Terminator thrust-rig was set to line and level, the Terminator Head was tested to ensure all three steering rams worked within the precision envelope of their control. The fluid jetting assembly was also tested, all service lines connected and the Terminator head attached to the initial flex-casing and installed through the grout wall and into the virgin ground.
The head has aggressive conical bits which cut the soil and a lead hollow central auger through which the soil is carried to the main casings, and larger augers to return the spoil. There is a target at the head which is viewed by theodolite/guidance system, thus negating the need for a pilot bore.
Once each auger is installed another is added and the process continues until breakthrough. Steering along the way is carried out by the controllable steering rams.
During the first 20m of the install, aggressive ground was encountered that was outside the n-value range expected. Also the stone content was higher and obstructions at 7m and 21m were encountered that deviated the machine. The casings and Terminator
were extracted and the damaged and badly worn head was sent to the workshop for refurbishment and modification to deal with harder ground.
4 No. shifts after re-launch and the target at the SCL tunnel was reached well within tolerances (only 3mm deviation vertically). The casing line was displaced with Naylor Denlock 300mm pipe, and successfully pressure tested to Crossrail specification. A special denlock adapter was manufactured to reduce the 300mm to 200mm for connection to existing pipework.
In order to achieve a Programme of 2 weeks and 2 days, 2 No. 12 hr shifts were employed Monday–Saturday to allow 24hr working. A fatigue and risk index calculation to develop a fatigue/risk index for the shiftworkers as per HSE Research Report 446 (2006) was carried out. The management of the workers fatigue was satisfied with an acceptable value of 2.33 through good coordination.
An air monitoring survey was carried out to assess exposure to total inhalable and respirable dust and elemental carbon around thrustbore site. The survey also aimed to establish potential exposure to diesel engine exhaust emissions (DEEE), which are classified by the IARC as carcinogenic. All plant within the shaft were fitted with diesel particulate filters (DPF‟s) and regularly maintained. The workplace exposure limits were within the guidelines.
The project was completed to time and budget 32 drain TRADER | September 2015 |
www.draintraderltd.com
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