Employer: ETHEKWINI WATER AND SANITATION

Engineer: Reudebaker Nel – eThekwini Water and Sanitation Department

Main Contractor: CSV Construction (Pty) Ltd/Tuboseal Services (Pty) Ltd Joint Venture

Contract value: R 108 198 359.80

Practical Completion date: 10 December 2021

 

Background

The existing 1350mm diameter concrete trunk sewer in Mahatma Gandhi Road was constructed in 1954 (74 Years Old) has a total length of 1.8km and carries most of the central business district (CBD) sewage in Durban. After an investigation the pipeline was found to be severely corroded, to an extent that it required immediate attention.

Project Scope:

The project consisted of the rehabilitation of the existing 1350mm – 1400mm diameter sewer in Mahatma Gandhi Road, Durban. The length of pipeline required to be rehabilitated was 1850m. A UV cured CIPP liner was chosen as the method of rehabilitation.

In addition, 80m of 1250mm diameter HDPE was laid at depths of 6.0m using open trench construction to complete the rehabilitation project.

Our longest installation was 139m manhole to manhole with the heaviest liner weighing in at 15 tonnes.

Diversion of Flow

The catchment of the pipe required to be rehabilitated had incoming sewer pipes of 860,1050 and 1100mm diameters at various locations that were required to be diverted to facilitate the rehabilitation by UV cured CIPP. The diversion of flows from different points along the 1350mm diameter pipeline presented a unique challenge due to the volumes of flow involved and the effect any diversion of flow will have on the rest of the sewer network in the area. In addition, there were four smaller lateral incoming flows on the line which needed to be diverted.

To increase capacity in the system to allow for the diversion of flow the adjacent sewers needed to be cleaned. Cleaning comprised of 1300m of the adjacent 860mm diameter sewer (with approximately 25% silt volume) which was cleaned under flow using jetting.

The decision was made to divert flows using diversions by building diversion walls in the existing manholes. Most of this work was undertaken at night in low flow conditions.

The diversion of flow for the project had to be sequenced based on the section of pipeline that was being rehabilitated thus points of diversion changed throughout the project.

In addition, 80m of new 1250mm HDPE weholite pipe needed to be installed by open trench excavation for this diversion to work. The pipe was installed at 6m depth using 8.0m long steel sheet piles with wellpoint dewatering. With the existing water table at 1m below ground level and the site being located a few 100 meters from the ocean this resulted in the open trench excavation being very challenging in terms of excessive groundwater in the trench.

 

Deep 8m excavation cut off piles were driven to facilitate excavation and installation, but the vast number of services encountered proved challenging.

The three incoming smaller lateral pipes were diverted by installing diversion pipelines to the adjacent sewer.

Cleaning of the host pipe

Bulk cleaning of the pipeline was required to enable profiling and a condition assessment of the existing pipe, which came with a few challenges due to the amount of silt in the pipe and the silts cementitious consistency which made the silt difficult to remove. The use of a high pressure jetting under flow was used to move the bulk of the material. Once the flows were diverted the pipe was cleaned (silt removed) by hand and profiled to determine the ovality.

Liner Design

The required liner thickness was calculated using ASTM F1216 – Appendix X1.2.2 . The liner was designed based on a fully deteriorated condition and onsite measurement of the ovalties along the pipeline . Largest ovality observed was 5% but mainly due to gas attack at the manholes due to flow turbulance. The liners installed  varied from a 10.8mm to 14.9mm thick liner using a reinforced UV cured liner supplied by Reline Europe.

These liners were shipped in batches from Europe to meet the staggered installation programme.

Installation

Numerous challenges were experienced in the installation mainly due to the weight of the liners (up to 15 tonnes) and large diameter.

The existing manhole shaft top slabs needed to be removed to facilitate the installation of the liner and limited space in these manholes made working conditions very difficult. In some cases sheetpiled shoring had to be deployed to get access to the manhole reducer slabs.

A Prokasro UV rig with local support from Bodotex was employed for the installations. Liners were winched into place and then blown up and cured in terms of the manufacturers strict protocols. A total of 17 liners were installed.

Testing of Installed Liner

After each liner was installed a sample of the installed liner was taken to an independent lab (SBKS Labs in Germany was used for mahatma gandhi) to get tested for its properties to determine if it complies with the document specification requirements.