Countering micro flooding on Innes Road

Installing underground services in urban environments requires detailed planning to minimise disruptions. A case in point is a complex stormwater project carried out for eThekwini Metropolitan Municipality, where horizontal directional drilling (HDD) proved to be the optimal approach in upgrading a network section along Innes Road in Morningside, Durban.

One of the hilliest areas within the city, the topography poses grade challenges, as well as various site constraints, which had to be carefully assessed and managed. These included positioning the HDD rig within the tight space available, and the need to drill alongside or under a series of buildings and structures along the route. Another key challenge was the need to drill under the cantilever foundation of a retaining wall running adjacent to approximately 80% of the drill line.

“However, the most challenging aspect of the works was tying up the levels at the entry and exit pits across the 17 m fall of the drill grade,” explains Byron Field, director, BLOC Contractors. BLOC was appointed as the main contractor to execute the HDD programme on behalf of the eThekwini Roads and Stormwater Department.

The drilling programme required the installation of the main stormwater trunk, which comprises a PN10 630 mm diameter HDPE pipe measuring 85 m in length. This now links all the properties in the immediate area with the city’s new main stormwater bulk line.

Technical aspects

“There were some interesting technical challenges on this drill that required some innovative thinking from all members of the team and the application of an array of skills to ensure the job was delivered,” Field explains.

Some of the challenges included:

• the drill length itself, which was in the order of about 120 m in length

• the drill grade, which was down a slope of approximately 10%

• the drill time required and the need to keep the drill head lubricated

• the excessive mud produced and the need to manage this

• the management of the traffic on busy Innes Road.

Some of the solutions applied included:

• the design of a vertical and horizontal site profile to help manage the drill line (see Figure 1 and 2)

• a 4 m deep pit constructed by BLOC for the changing of reamers and removal of the pullers

• a 315 mm puller specially engineered and manufactured by HDD Engineers

• a 630 mm to 315 mm pipe reducer designed and built by RHO-Tech

• a 10 000 ℓ water tank installed to ensure a constant supply of water to make the drilling mud

• mud management using a combination of a TLB, 5 m3 tipper lined with plastic, and a vacuum pump.

Piloting phase

The piloting process required a dedicated focus as the constraints had to be avoided both vertically and horizontally, while maintaining the correct grade throughout the drill programme.

“Furthermore, the drill had to enter pit 1 at the right location and exit at the right location at pit 2. This was not immediately achieved, as the retaining wall proved challenging as we navigated around the foundations, but after two days of careful and patient drilling we succeeded,” Field continues.

Reaming and pipe pull-back

The scope required the drill team to ream to a diameter of 800 mm, which also proved to be an intensive process, taking five days to complete. During this time, the mud produced had to be constantly collected and sent to an acceptable disposal site.

“The pipe pull-back was the only day we needed full road closure. Always the most stressful part of the HDD process, we prepped for road closure and then got the pipe in place. After tightly connecting the reamer and puller, the pull-back process began and was concluded by day’s end,” Field explains. Recognising that most of the drilling mud had been drained, BLOC’s main concern was that there would be a cavity between the 630 mm diameter pipe and the 800 mm ream. Since the specification clearly ruled out any evidence of settlement on any of the constraints or structures, it was decided to grout the pipe in.

A job well done

In the end, the project was successfully concluded on time, within budget and to the client’s satisfaction, with all the support networks in place.

As Field points out, the project’s ultimate success was based on exceptional teamwork and proactive stakeholder engagement. For eThekwini, it’s also a proven case study on how effective HDD is a trenchless option for complex works in densified urban areas.

“Growing the trenchless construction industry in South Africa is a major focus for BLOC – not only for our business to succeed, but for the development of a skills and technology base that can serve to ensure the better delivery of infrastructure projects across the country,” Field concludes.