Last week on the blog we introduced you to the 1881 South Belt sewer beneath Moorhouse Avenue: how it was built, how it got blocked, and how recently as part of SCIRT’s horizontal infrastructure rebuild program, their Downer delivery team and sub-contractors Donaldson Civil fixed the blockage. On this week’s blog installment, we look at what we found both above and below the sewer. Enjoy!
Whenever anyone digs down into a road to repair or replace damaged horizontal infrastructure we always find what I like to call ‘road trifle’ – but with layers of asphalt and aggregate instead of layers of custard and fruit. Actually probably more of a ‘road crème brulee’ – as our modern road surfaces (like fancy French desserts) have a hard top to break through first. At the best of times it’s almost impossible to tell whether any such layers exposed in the side of trenches are of the 19th century period, as we know that most city roads have been rebuilt many times in the past. This one had a layer of soft friable concrete below the asphalt, which was a bit different. Image: Hamish Williams.
They didn’t have any mechanical excavators back in 1881, which means that the Drainage Board contractors had to dig the sewer trench by hand, using pick, spade, and shovel. As well as digging, they also had to do cutting – we found lots of monster-sized tree stumps and tree roots with saw-cut ends mixed in with clay that had been used to backfill the sewer after construction. You can see some of these bits of stumps at the left of this photo. It’s not uncommon for us to find evidence of prehistoric swamp vegetation well preserved in the anaerobic clay at great depth below the city. Image: Hamish Williams.
We didn’t find much in the way of artefacts in the backfill atop the sewer, with the exception of broken taper bricks. These bricks had evidently fractured because of the large clinkerous inclusions contained within. Useless for construction, they were apparently left by the bricklayer beside the crown arch. Image: Hamish Williams.
Found in close association with these broken taper bricks was a small red pebble with an attractive white stripe. This is by definition not an artefact (it was not made or modified by humans) but is a manuport (it is a natural object transported from its original location and is otherwise unmodified). I reckon this pretty pebble must have been picked up somewhere by one of the sewer gang boys back in the day before it was lost on the job, maybe falling out of a pocket. I can’t think that such an awesome find would have been deliberately discarded, unless perhaps the peer pressure of a ‘manly man’ on the sewer gang who collects pretty pebbles in his spare time was a contributing factor? Archaeology can never answer such questions for us – but it’s nice to ponder them all the same. Image: Hamish Williams.
I thought that perhaps at depth we would find in situ 19th century trench shoring timbers, which would have been put in place to stop the sides of the trench collapsing (today we mostly use steel trench shields, but the work has to go super deep, sometimes interlocking sheet piles). This was not to be, but we did find the casts from where these would have once been in the outer faces of the concrete sewer invert. Pieces of 200 mm x 100 mm timber set some 200 mm apart were evidently used to support the walls of the trench. These had been pulled out and presumably used to line the next stretch while the concrete was still wet. Images: Hamish Williams.
Working in an area with a high water table was a challenge while working on fixing the sewer, even though there was a dewatering system of pumps set up to make the subterranean works area as dry as possible. How did they manage this back in the day when they built the sewer? Evidently with some difficulty – at the start of construction, the Drainage Board reported that “the [sewer] works on the South Belt are in very troublesome ground, with much water and running sand, and the progress is consequently slow” (Press 22/2/1881:3). They had pumps, yes, but these coal-powered steam-driven pumping devices were probably not as effective as the diesel powered pumps we have available today. They were clever, however – first installing a line of earthenware pipes within a smaller gravel filled trench, which we hope would have removed a great deal of the water away from where they were pouring concrete and laying bricks – presumably to a pumping collection point further down the trench line. You can just make out this 19th century dewatering system in place below the concrete invert in this photo. Image: Pieter White.
Not surprisingly, when one of these pipes was removed SCIRT found that it was choked with liquefaction silt. Image: Hamish Williams.
A very happy Ty Laskey from Donaldson Civil with one of these earthenware dewatering pipes that was recovered intact. Unlike earthenware pipes for sewerage applications, which are always glazed, these particular pipes were unglazed, and had been laid dry; that is to say without any cement mortar between the individual pipes to allow for the free infiltration of water. Image: Hamish Williams.
Many thanks to SCIRT, Downer and Donaldson Civil for a job well done, and especially to Moorhouse Avenue businesses and motorists for their patience while SCIRT has been working on fixing this and other damaged horizontal infrastructure in the area.