A winning HDD project in South Africa

The recent replacement of a 1,300 m section of asbestos cement pipe used an innovative HDD, pipe reaming and HDPE solution in Tshwane, South Africa.

Subcontractor Trenchless Technologies says the project broke a pipe reaming record and was a HDPE installation first, earning it the 2017 Joop van Wamelen Award for Excellence.

In 2017, the City of Tshwane, South Africa completed a project to replace a 1,300 m stretch of DN 800 asbestos cement pipe. Construction required contractors to pass under operational electric cables and wooden electrical pylons while upgrading and extending the Temba Water Purification Plant.

The nature of the installation meant that the use of large construction equipment was not viable, so the city engaged engineering firm AECOM and specialist subcontractor Trenchless Technologies to design a trenchless solution.

While a number of relining and replacement technologies were considered, horizontal directional drilling (HDD) was selected as the ideal method. Pipe reaming was the most economical replacement technique and had the advantage of replacing the existing pipe with brand new high density polyethylene (HDPE) pipe.

Trenchless Technologies says the project resulted in two world firsts and earned the company the 2017 Joop van Wamelen Southern African Society for Trenchless Technology (SASTT) Award of Excellence. The contractor says the project was the largest diameter host pipe to be replaced using pipe reaming technology, as well as being the first single pass installation of DN 800 PN 16 HDPE pipe without filling the host pipe void with bentonite and reaming in stages.

Asbestos removal

One of the challenges of replacing the original asbestos cement pipe was planning for the safe removal and disposal of the contaminated waste created by the reaming process, which involves an abrasive reamer grinding up the host pipe while installing the HDPE.

The asbestos removal plan, which was first approved by the South African Department of Labour, was facilitated by Averda, a company specialising in waste management in cities, industries offices and homes across three continents.

Each receiving and catchment pits were lined with 250 micron plastic sheeting to contain the bentonite, spoil, asbestos and water, a Kosun KMSR-250 mud separation system was used to remove the elements contaminated with asbestos.

The bentonite and water mix was able to be reused in the pipe reaming process once the contaminated materials were separated.
As asbestos removal continued, it became apparent the amount of asbestos entering the bentonite mix could be controlled by the design and selection of the pipe reaming teeth, reducing the volume of contaminated waste.

The drive

At the receiving pit, a DD10 American Augers HDD rig’s 6.1 m long drill rods were threaded together to form a continuous drill stem and was pushed through the host pipe to the launch pit. Once in the launch pit, a pipe reamer was connected to the drill rods.

The DN 800 PN 16 HDPE pipe, which had a wall thickness of 76 mm, was delivered to the site in 18 m lengths, where it was butt welded into longer continuous sections of 150 m lengths using electrofusion couplings. A bull-nose was attached to the pipe reamer using a swivel, then connected to the HDPE pipe.

The rig then pulled the reamer from the launch pit, back to the receiving pit using pullback forces of up to 50 t and rotation forces of up to 982 Nm, with the pipe being pulled into place behind the bore.

As the reamer ground the existing asbestos cement pipe into smaller fragments, the pipe cuttings, surrounding soil and bentonite mud mix were taken into the Kosun mud system for separation.

The end point connections and scour valve used HDPE stub-ends and backing flanges, meaning a total HDPE solution was used for the project.

Award winning

This project demonstrated a number of features that contributed to its recognition as winner of the 2017 SASTT Joop van Wamelen Award for Excellence.

The project successfully replaced ageing pipeline infrastructure made of asbestos cement, without causing disruption to cables and powerlines. It demonstrated the advantages of the pipe reaming methodology, such as enabling large upsizes in pipelines and allowing single pulls of long lengths of pipe.

The project also made use of locally produced HDPE pipe, reducing import costs. It also demonstrated a possible solution for safely upsizing asbestos cement pipes, which many countries currently choose to reline. This development may eventually lead to solutions for the replacement of leaking asbestos cement water pipes on an international scale in the future.

This article was featured in the Fall edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.

For more information visit the SASTT website.

If you have a project you would like featured in Trenchless International contact Assistant Editor Chloe Jenkins at cjenkins@gs-press.com.au

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