Archaeology Notes
Event ID 693870
Category Descriptive Accounts
Type Archaeology Notes
Permalink http://canmore.org.uk/event/693870
NT12SW 38 10648 23099 to c. 271 690
For Talla Reservoir (centred NT 1200 2140), see NT12SW 17.00.
The Talla aqueduct carries drinking water from the measuring house (NT12SW 17.06) at the Talla Reservoir to the Alnwickhill Filters (Edinburgh, Liberton, Alnwickhill Waterworks: NT28NE 87.00). It measures about 35 miles (56km) in length; this figure includes 9 miles (14.5km) of tunnel and about 12 miles (19.3km) of cut-and-cover, the remainder being bridges or cast-iron piping. As constructed, the aqueduct has a typical gradient of 1-in-4000, and is sufficiently large to carry the water not only from Talla, but also from the adjoining watershed of Menzion. Only a single line of pipes was laid, but provision was made for duplication if desired, ease of extension being a prime consideration in the design of the scheme.
There are 21 tunnels in all, the shortest being about 133 yards (121.7m) in length and the longest 1 1/3 miles (2.14km). They are driven through material of various types, ranging between hard rock (specifically greywacke) in the case of those between Broughton and the reservoir, and sand and gravel, in the case of those near Penicuik. One of the latter was driven by compressed air, without the use of a shield. Several were dug by both night and day shifts, the best progress achieved in driving those through hard rock being about 48 yards (43.9m) per month. Between Broughton and the dam, rock drills (driven by air compressors) were situated at convenient places alongside the construction railway (NT12NW 57.00, NT12SW 37 and NT13SW 85.00), where they were readily supplied with coal; the compressed air was then piped into the headings. It was originally intended to leave considerable portions of the tunnels unlined, but in the event they were all lined, either with cement concrete or with brickwork in cement. The least dimensions within the tunnels are 6ft (1.83m) in width by 7ft 6ins (2.3m) in height. Exceptionally, a brick section was used in heavy ground near Penicuik.
Cast iron pipes were used for the siphons across the valleys. These range in diameter between 27ins (0.69m) and 36ins (0.91m), according to the available fall. An automatic valve was provided at the beginning of each siphon, and set to close in the event of a burst within it.
[No map of route provided: fig. 1 names locations traversed, and notes syphons].
W A Tait 1905.
