Ash aqueduct (2K)
Engineering Aspects
The Ash Aqueduct

The project was managed by Surrey County Council's Highways and Transportation Department.

The aqueduct was designed by Surrey County Council's Engineering Consultancy Division.

The aqueduct, and the road, built by Alfred McAlpine Construction Ltd.

The Department of Transport.

Overal length: 134m

Weight: 3,500 tons

Width of canal: 5.5m

Depth of water: 1.3m

Height above road: 5.7m (minimum).
  INTRODUCTION - An overview  


From the mid-1960s it was realised that increasing traffic congestion on roads adjacent to the Blackwater Valley could not be addressed by small scale improvements. Therefore plans were developed for a new relief road, linking the M3 and A30 in the north to the A31 in the south, and a line for this Blackwater Valley Route was established which largely followed the course of the Blackwater river along the border of Surrey and Hampshire. This line was safeguarded to protect the route from development.

aerial photo of planned route (11K)
map of route (15K) THE CANAL

The line of the Basingstoke Canal, at that time abandoned and derelict, was crossed by this planned road. After the restoration of this section of the canal in the early 1980s, a crossing of the road by the waterway was needed. The first proposal was for a series of 3 locks down and 3 locks up, each side of the new road, but after strong representations by the Canal Society, proving that this was not a viable solution, plans for the present aqueduct were drawn up.


In order to minimise disruption of facilities that closure of the canal would cause, construction work had to be undertaken within a 9 month period during the winter of 1994/95. The contractor engaged for this work, (and also for the whole road construction project) was Alfred McAlpine Construction Ltd.

Since the 200 year anniversary of the canal was being celebrated in 1994, work on the aqueduct could not start until the end of September that year, and which therefore necessitated its completion by June 1995.

drawing of aqueduct (7K)

Initial designs were for a suspension type but due to objections to the resulting high tower, these were abandoned in favour of a pre-stressed post-tensioned form of construction, as the finished aqueduct, with water, would be extremely heavy. The post-tensioning was achieved by 4 steel cables inside each side of the structure.


The first operation was to remove a 150 metre section of the Ash Embankment, following which a layer of hard core was laid down in order to withstand the weight of the equipment to be used. Deep piles were then driven down to support the aqueduct structure.

removal of embankment (9K)

wood falsework (10K)

To make it aesthetically pleasing, the design stipulated a wooden appearance to the outside of the concrete and this was achieved by making falsework of wood strips over which the concrete would be poured.

Before this could be accomplished a large amount of steel reinforcement had to be incorporated.

steel reinforcement (14K)


Although the winter of 1994/5 was very wet, the construction was not delayed and Alfred McAlpine Construction achieved completion and the canal was re-opened within 6 hours of the deadline.

handshake in front of plaque (9K) OPENING

One month after completion, the aqueduct was formally opened on Saturday 29 July 1995 by former Chairmen of Hampshire and Surrey County Councils: Graham Smith (Hampshire CC) and Cecilia Gerrard (Surrey CC).


Only after the aqueduct was completed could work on the road underneath commence - but that is another story.... As is the diversion of the Blackwater river, landscaping, environmental protection, and the construction of the Centre Section of the road itself. (These will follow in a forthcoming section).

aerial photo of road construction (18K)

Photographs on this page by Philip Lane Photography from Surrey County Council's Brochure of the BVR Opening. Map by ADD. On other pages all photographs by ADD unless otherwise credited.

NEXT - the project in detail
The Design


Last updated November 2001