Murray Jones, secretary of Thirroul Village Committee, explores the history of land instability near Thirroul Plaza.
Please allow me to relay my concerns about two separate historical geological conditions that could impact on the new Thirroul Plaza DA.
In the late 1950s, whilst attending Thirroul School, I can recall the sight of a playground lined with leaning and broken retaining walls. One section was continuously damp with seepage from the hill above. Then, for what seemed like a whole year, the playgrounds were declared out of bounds whilst huge trenches were dug, in part, to drain the underground springs.
I was familiar with the problem because 200 metres north at home in Phillip Street a permanent spring slowly bubbled away in the midst of the foundations at a rate that remained constant during the heaviest rain or longest droughts. The source of these springs was supposedly rain falling on top of the escarpment. There were two other similar occurrences in roads close by.
I also recall when walking to school the sight of four neat homes lining Seafoam Avenue slowly decaying and contorting over a lengthy period, maybe several years. They were eventually demolished, the road closed and the area was assigned as dangerous. The site devolved into an overgrown swamp directly above and 300m to the west of the plaza site. This section of Seafoam Ave, once a road, is now a convoluted walking track.
In an attempt to verify my recollections, I googled “Landslip Seafoam Avenue” and found Uow060573.pdf, an appendix documenting landslip in the Illawarra(from a publication titled “Geotechnical Risks Associated with Hillside Development” authored by Walker, B., Dale, R., Fell, R., Jeffrey, A., Leventhal, A., McMahon, M., Mostyn, G and Phillips, A.)
The appendix contained a list of 131 references to media coverage of landslip in the Illawarra dating from 1879 to 1976.
Eighty-seven were references to landslip events from Bulli north and 44 from south of Bulli. Why do two out of every three significant land slippages in the Illawarra occur from Bulli north? Also, to my surprise, it showed that in 1950 Thirroul Primary School’s main two-storey building, which had stood for over 50 years, was lost to landslip.
The document also referenced an article in the Illawarra Mercury dated 25 November 1959 where heavy rain was listed as the reason why these four homes, which had stood firmly for around 40 years, started to slowly slip downhill. It can be argued that the cause of this landslip are slow-flowing springs as the swampy conditions are constant even in the recent drought.
Why is the Northern Illawarra so susceptible to landslip? Why is the same section of Phillip Street above the location of these four homes still slowly slipping down the same slope 70 years later (as shown in the photo above)? A shiny new layer of bitumen was laid recently in an attempt to repair the slippage cracks. After a few months, the cracks re-appeared. The road has dropped by about 4cm vertically and laterally since the latest repair.
I read the document further and was fascinated by a description of the ‘Scarborough Greasyback’.
Illawarra Mercury, 20 May 1950
Conference of NSW Government experts, State legislators and Wollongong
Council officials met to discuss the Wombarra-Scarborough subsidence and decided “there was no engineering solution to the problem” because it was caused by the geological formations. The problem was a stratum of clay subsoil. “The Scarborough ‘greasyback’ has been a problem to residents for many years but first caused extensive damage to homes and other property during heavy rains this year. A number of Wombarra and Scarborough residents have been forced to abandon homes which have been badly damaged by subsidence. Roads and electric services have also been damaged after each heavy rain.
Could “a stratum of clay subsoil” be an additional factor and relate to other local landslip problems that I have witnessed in my lifetime, like the tennis courts that fell into the sea at the end of Woodland Avenue Thirroul, 1km south of the DA site? Here, a half metre of land each year on average dropped into the sea, whereas barely 100m north similar erosion is not noticeable. Could a 1 to 2m-thick white clay seam – visible at the high tide mark under this site but above the high tide mark 100 metres north – be a factor?
It is interesting to note that this clay seam was quarried to produce white/grey house bricks 24 hours a day, 7 days a week at the Newbold factory on what is now McCauley Beach Estate. For over 50 years Thirroul Soccer Fields was part of a huge quarry. This quarry flooded before being filled in.
A geologist friend explained the most probable source of this clay was volcanic activity in the Kiama area around 250 million years ago. With the apparent help of southerly winds, several metres of volcanic ash was deposited over the whole of the Northern Illawarra. This was compressed by low-grade overburden over time to form white clay. Since then, rock and coal strata formed on top in various horizontal layers.
Further online investigations using the words “Thirroul springs” led me to a document called “Illawarra Springs”, published in 2014 by Dr. Joseph Davis of the Illawarra Historical Society.
It states: “A resident of Harbord Street, Thirroul, never has to water her gardens because she has a hose connected to her spring which happily pumps low volumes of water all year round.”
Harbord Street is 300m or so from the plaza site on the opposite side of Thirroul from the school. This spring would have flowed underground close to the Plaza site. Thus, is it fair to say that large parts of Thirroul, including the Plaza site, are underlayed by a combination of fresh water springs and seams of clay? Is Thirroul built on a greasyback that has yet to be antagonised?
Let’s now consider how these local geological abnormalities could physically relate to Thirroul’s first triple-height basement excavation, relative to the railway, as shown in the drawing above from the DA.
Could a 150m-long excavation that is probably three times as deep as any other basement in Thirroul, directly adjacent to the main South Coast rail line, cut through underground water and clay seams and antagonise a new “greasyback” and put at risk the South Coast rail link?
Is it wise to dig a huge pit directly below a known landslip area that is probably underlaid by wet clay whilst continuously shaking the area with the vibrations from heavy rail transport from Australia’s major steel manufacturing plant?
