Investigation on a distressed steel sewer pipe in Hymany Park adjacent to Randpark drive, Randpark ridge ext. 16
Gps coordinates s26o 06’ 05.4” e 027o 57’ 38.5”
Randpark Ridge Village Association (RRVA) – a Section 18A Public Benefit Organisation run on behalf of the residents of Randpark Ridge Ext. 16.
Mr. Phil Culham, Chairman of the RRVA requested that Roderick Rankine inspect the condition of a distressed steel sewer pipe which runs across the concrete lined stream channel adjacent to Randpark Drive in Randpark Ridge Ext 16, evaluate the risks of failure of this structure and to recommend remedial measures to mitigate further damage and save the structure.
Location of the structure
The location map shown in Figure 2 shows the position of this sewer pipe which crosses the stream coming from Hymany Dam (a tributary of the Klein Jukskei). This stream runs in an easterly direction under the N1 Highway and joins Pampoen Spruit in Sharonlea which eventually becomes the Klein Jukskei.
The writer visited the site on 23 August 2018, in the presence of Mr Culham and again on 15 November 2018.
The following documents had been supplied at the time of writing this report:
- An email by the RRVA (Randpark Ridge Village Association) Chairman Phil Culham addressed to the Management of Joburg Water and DA Ward Councillor Beverley Weweje entitled “Corroded and collapsing sewer pipe adjacent to Randpark Drive river bridge”, dated 21 February 2018.
- An email by Roderick Rankine addressed to Jane Eagle (City of Joburg Deputy Director: Open Space Planning) entitled “Environmental problems in Randpark Ridge Ext. 16”, dated 25 April 2018.
The following information was supplied:
The RRVA Chairman had reported the deteriorating condition of this pipe several times over many years. In 2017, he had been informed by Neels Coetsee of Joburg Water and Sanitation that plans to replace this pipe had been approved. To date nothing whatsoever had been done to further remediate or replace the pipe. Sewage now pours out of this pipe virtually continuously which is a health and environmental hazard.
Observations and comments
- This pipe had been installed on the western side of bridge crossing the stream in Randpark Drive in Hymany Park Randpark Ridge Ext 16. It appeared that the pipe had been installed at the same time as the concrete canal lining had been installed.
- The steel pipe in question was approximately 12 m long and 200 mm in diameter (internal) and had been constructed in a single length by helical winding and welding. The transitions to couple it to the sewer line (which is probably PVC) had been encapsulated within the concrete or soil and were not visible
- The steel pipe exhibited multiple perforations above the level of the invert. The worst corrosion was prevalent on the vertical sides of the pipe section and it manifested as a series of perforations either side of the “equator” of the pipe section. This is consistent with sewer corrosion in a pipe which operates half full much of the time. Vegetation, including a small tree, had taken root inside of the pipe and appeared to be flourishing off the nutrients – see Figure 3.
- The pipe was partially supported by two concrete bases projecting through the inclined sides of the concrete channel lining.
- A temporary repair by means of a stainless steel Cascade clamp had been made on the northern end of the pipe – between the northern concrete support base and the point where the pipe terminates into the northern inclined concrete channel lining – see Figure 4.
- There were manholes on both banks – approximately 3 m away from the northern end and approximately 30 m away on the southern end.
The choice of steel for this exposed suspended section of sewer pipe was apparently based on the need for flexural strength in the span over the stream between the two supports. Other common materials used for sewer pipes (such as vitrified clay and PVC) would not have had sufficient flexural capacity to safely span this distance without deflecting excessively and/or breaking in flexure.
Unfortunately, a steel sewer pipe in this application has a limited service life due to the problem of sewer corrosion. Figure 5 show schematically the mechanism of sewer corrosion. Sewer corrosion is caused by anaerobic bacteria (micro-organisms which thrive in the low-oxygen environment within a closed sewer) which reduce sulfur compounds present in the sewage into sulfides in those parts of the system where aerobic bacteria have depleted most of the oxygen. These anaerobic bacteria reduce sulfur compounds in the sewage into sulfides – some of which escape into the sewer atmosphere in the form of hydrogen sulfide gas. This hydrogen sulfide gas dissolves in any condensed moisture above the sewage flowline (invert level) by sulfur-oxidising bacteria in the presence of oxygen to form low pH sulfuric acid. The condensed water has no capacity to buffer the acid and acid corrosion causes severe deterioration and erosion of the steel above the invert.
Repair will be difficult due to the fact that the sewer flows constantly and the steel section passes beyond the concrete lining. The sewer above the channel crossing will have to be temporarily blocked or diverted to facilitate the replacement. The loss of cross-section is now too severe to consider further patching and/or a collapsible polyethylene lining. There is now no alternative but to replace the entire steel pipe. This needs to be done urgently before the entire pipe collapses and discharges all its contents into the stream – which would be an ecological and health hazard.
Enquiries among local suppliers of spiral welded steel pipe confirmed that it will probably not be possible to purchase a single 12 m length. Two 6 m lengths will therefore need to be prepared with flanges and bolted together in-situ.
This repair needs to be carefully planned and the execution well coordinated to coincide with offpeak sewer flow – probably in the very early hours of the morning or in the morning after rush hour. The use of a small mobile crane to lift and hold the weight of the pipe during fitment and alignment would be a great help.
Roderick G.D. Rankine Pr.Eng, ECSA Reg. No. 2000027, PhD Eng (Wits), MSc Eng (Wits), BSc Building (Wits), ACT Dip (UK), MSAICE, M.Conc.Soc. (UK), Certified Infrared Thermographer (ITC Stockholm, Sweden)
This report was reviewed by Wim van Steenderen BSc Eng (Civil) MSc Eng (Wits) MSAICE (Retired).