Culvert Construction for the GNR

Side-drains were an essential part of the road design but means for the water to be led away into gullies, creeks and valleys was of equal importance. For example, an uphill side-drain needed to be emptied across the road line onto the valley side. In a flatter countryside, both side drains had to empty somewhere; depending on the lie of the land, at least one side-drain would have to discharge under the road. The drains also had to be kept clean of soil, leaves and twigs. The Assistant Surveyor would have had to use his experience and judgement to decide how big the drain should be and how the flowing water could be conducted to a place where it could discharge without causing erosion to the road and on the roadside. The design of culverts for the road in many places became a matter for continuing consideration because it was necessary to construct them from the materials readily available at the particular site - stone, timber or a combination of these.

When work began on the road to the Hunter Valley in 1826, the walls and drainage structures were scarce, crude and poorly built. (Karskens, Ref.1). In 1827, the works were under the supervision of Lieutenant Jonathon Warner at Wisemans Ferry. The picture of a culvert outlet and the retaining wall shows the small scale of the outlet and the low standard of the work in both culvert outlet and wall. At least, the idea that drainage was a necessary part of the road construction was in place but more needed to be done to raise the standard of work. The impact of the transition to supervision by Lieutenant Percy Simpson was soon become evident in the structures that he put in place, with clear and active help from his overseers and road parties.

As work began under Lt. Percy Simpson and Heneage FInch, construction of the culverts and their inlets and outlets took on variations in design to meet the requirements of drainage, depending on the gradient of the roadway under development. Culverts of stone, such as on the Devine's Hill Ascent, were set either at right-angles to the line of the roadway or at an angle between 130 and 145 degrees to that line. As Karskens (Ref.1) notes, such oblique culverts would be between 25 feet (7.7 m) and 45 feet (13.8 m) in length across the road.

Webb (Ref.2) comments that the culverts ranged in size from large enough to crawl through to as small as 400 mm by 320 mm. In September 1830, the No.25 Road Party constructed on Devine's Hill a culvert 15 yards long walled both sides, flagged top and bottom and covered 2 feet in the road. After digging the trench, the bottom was lined with split stone slabs butted together to form a solid base. From the dimensions given, the slabs would have weighed around one and a half tonnes. It is not evident what method was used to carry, lift and place in position such huge slabs. Some form of lifting tackle must have been available since a block of stone of one and a half metres in length, a metre in width and 30 cm thick would not have been lifted into place by, say, six men shoulder to shoulder. But it was done and done accurately. The sides were also of split stone, set on each side of the trench, bedded onto the large slabs. Finally split stone slabs were set across the top to form the culvert. Webb comments that, during repair work in the mid-1980s, a culvert of this design was repaired. It was found that some of the top stone slabs had been bedded onto the side walls with clay in which small flat stones had been placed to prevent the top slabs rocking when vehicles passed over the road. This is the only known example of a form of mortar used as all the other blockwork in walls and bridges was dry laid. There were as many as 51 culverts of various sizes, with varying construction methods used, on the Devine's Hill Ascent.

Webb gives an interesting account of the use of timber for culvert construction where suitable stone was not available. Squared timber logs and split timber slabs were used in the construction, with the logs held together by iron spikes of around 20 mm in diameter and up to 500 mm in length. The sketch gives some idea of the construction of such culverts. He notes that only a few timber culverts remain intact to carry water under the road; most have collapsed with the timber rotting away (eaten by white ants?) and even burnt out by bushfires.

The inlet and outlet entrances to culverts gradually developed from the rather elementary designas shown above into quite sophisticated ones that are discussed next.

References:

1. Karskens, Dr. Grace, Four essays about the Great North Road, CTP Occasional Monograph 1998 2. Webb, Ian, Blood, Sweat and Irons, pp.50-51, Dharug & Lower Hawkesbury Hist. Soc. 1999