Environmental|

Investigation of a distressed concrete canal lining in Hymany park adjacent to randpark drive, randpark ridge Ext. 16

Gps coordinates s26o 06’ 05.4” e 027o

This report was drawn up for Randpark Ridge Village Association (RRVA) – a Section 18A Public Benefit Organisation run on behalf of the residents of Randpark Ridge Ext. 16.

Brief

Mr. Phil Culham, Chairman of the RRVA requested that Roderick Rankine inspect the condition of a distressed concrete channel lining to the stream running through Hymany Park 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.

figure 1

Fig 1. The unreinforced concrete channel lining to the section of the stream west of Randpark Drive in Hymany Park Randpark Ridge Ext. 16 looking East.

Location of the structure

The location map shown in Figure 2 shows the position of this concrete channel lining in 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.

Fig 2. Location of the concrete channel lining in Hymany Park adjacent to Randpark Drive.

Information supplied

The writer visited the site on 23 August 2018, in the presence of Mr Culham and again on 15 November 2018. No documents had been supplied at the time of writing this report.

Observations and comments

This lining comprised cast in-situ unreinforced concrete slabs approximately 75 mm thick – one slab cast horizontally on the stream bed and two inclined slabs cast on either embankment at a slope of approximately 45degrees.

  • The western upstream end of the lining had been severely undermined by the stream so that the regular flow of the stream passed beneath the westernmost five floor slabs – reappearing through the joint between the fifth and sixth slabs – see Figure 1 and Figure 3. This type of failure is called a piping failure – the water bypasses the structure by flowing underneath, rather
  • than on top of, the canal lining as intended. In the process it undermines the founding material and puts the entire canal in jeopardy.

Fig 3. Concrete channel lining viewed from the Randpark Drive Bridge looking west. Note the vegetation growing out of the construction joints and note the appearance of water only after the westernmost five panels on the far side of the image – prior to that point where the water wells up, it flows freely under the structure (called a piping failure). The vulnerable and severely corroded steel sewer pipe can be seen in the foreground crossing the stream.

  • The combination of undermining and flood pressure had caused severe full-depth cracking and some vertical faulting of the westernmost floor panel. A large deep cavity could be seen through these cracks and displaced portions – see Figure 4.

Fig 4. Two views of the cracked and displaced floor panel on the westernmost side. Note the
deep void under the slab (now totally unfounded) as well as how the stream has bypassed the
channel lining and is now flowing beneath the concrete.

  • The joints between the panels were completely unsealed. Vegetation had taken root through these joints and some of the joint edges had spalled appreciably – see Figure 5.

Fig 5. Vegetation growing through unsealed joints and joint edge spalling.

  • There was a scour cavity between the upstream leading edge of the concrete lining and the stream valley adjacent to it – see Figure 6.

Fig 6. Scour cavity upstream, and underneath, the leading edge of the concrete channel lining.
Note the complete absence of any upstream cut-off wall to combat scour undermining the lining.

  • The intact dry westernmost slabs were tapped with a heavy steel rod to detect hollow voids beneath the concrete. With a few isolated and small area exceptions, these slabs appeared to be well founded.

Discussion

This concrete channel lining is in a critical state. The broken concrete panel on the west end is liable to be lifted and washed downstream by a typical Highveld thunderstorm. Once this happens, a chain reaction may be initiated whereby adjacent panels are undermined and similarly displaced. If a large panel gets lodged under the bridge aperture, it could obstruct/block the flow of water passing under the bridge or possibly divert the stream flow to one embankment where the erosion could inflict severe structural damage. There is also a strong likelihood that such obstruction might destroy the steel sewer pipe adjacent to the bridge (which is in a very poor state of disrepair).

Recommendations

  1. Demolish and remove the cracked and displaced concrete panel at the west end. The soil underneath this panel should be cleared of as much organic matter and clay as possible and the level excavated by at least a further 100 mm. If possible try to compact this soil provided it is not totally waterlogged.
  2. Cast a new but thicker horizontal concrete slab (at least 200 mm thick) using 30 MPa concrete with a slump of approximately 75 mm.
  3. Fill the scour cavity shown in Figure 6 with a 30 MPa fluid flow grout. Try to encourage the grout to flow under the slab and to fill as much empty void space as possible.
  4. Construct a concrete upstream cut-off wall all along the leading upstream edge of the channel lining at least 300 mm thick and at least 1 m deep using 30 MPa concrete to combat further scour.
  5. Drill 30 mm diameter core cavities through the westernmost panels where water is piping under the structure and inject cementitious grout* into the voids beneath. This is a specialised operation that should be undertaken by a geotechnical engineering contractor with suitable experience in pressure grouting. If excessive grouting pressure is used, the hydrostatic force may lift/displace/crack the existing concrete lining.
  6. Remove vegetation rooted in the construction joints and fill the dry joints with a suitable elasto-plastomeric sealant such as Sikaflex Pro-3 (see Annexure A for datasheet) with a circular closed-cell polyethylene backing cord/rod. The bottom joints should only be sealed in the winter months if the stream dries up.

*Resist the temptation to inject polyurethane expansive foam filling supplied by specialists such as Uretek. This foam has very low density and is likely to become buoyant in waterlogged soil. This would greatly increase the risk of uplift of the concrete lining during a flood event – particularly if the voids filled below the lining are large.

By 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)

Peer Review

This report was reviewed by Wim van Steenderen BSc Eng (Civil) MSc Eng (Wits) MSAICE (Retired).

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