Sewer Rising Main – Shoring Solution

THE CHALLENGE

The customer had to install an 800mm diameter polyethylene sewer rising main pipeline. Most of the pipeline was open trenched and benched at depths between 6 to 8 meters. There was a section near the Northern Motorway that could not be benched and had to be shored up. Here they had to push the pipe through, do a pressure test and a weld to join the polyethylene pipe that ran under the motorway. One end at the bottom of the excavation had to remain partially open for the pipe entry.

OUR SOLUTION

Two options were considered for this project - Shoring box or sheet piling the excavation. After looking at both options the shoring box was by far the most cost effective and quicker solution without jeopardising safety. What we offered was a 4.175m(l) x 4.175m(w) x 6.8m(d)(internal) box.

This was to be made up with 4m ultra shield panels and our corner joining brackets. Three of the 4m boxes were to be stacked on top of each other and joined using joiners designed for stacking the ultra-shields. The box of the bottom would have three solid sides with the fourth end open and struts installed.

These struts had to be manufactured specifically for this job to keep the box square in relation to the top boxes. Steel road plates were offered to cover up the exposed areas around the pipe at the bottom.

THE RESULT

The excavation, assembly and installation of the box only took a few hours in total using a 30-tonne digger and two workers. Each box was assembled safely above ground. The bottom and middle boxes were joined then lifted into the excavation and then the third top box was put into place and joined. Thereafter, three steel road plates were lifted into place at the bottom between the struts and the top box around the exposed pipe entry.  

For safe access in and out of the excavation an access platform with ramp was installed. The access platform has two gates, fall arrest and a bracket that secures a ladder to it for safe use of the ladder. This was installed just before the winter season in Auckland and once the rain started the ground conditions become unstable and shifted with the rising and falling water table. Even with these change in the conditions the box held and kept the workers safe inside.