Chiswell case study: The Scheme
Raising the Weymouth road by 1.5m. Note the diverted traffic on the beach.   The Scheme index page		Raising of Weymouth Road 53. Hydraulic considerations of the open channel and the need to keep the Weymouth road open during time of flood, fixed the minimum level of the road at 3.5 m A.0.D., which meant filling over the existing pavement to a maximum depth of 1.5 m. 54. The alignment of the road was dictated by two factors: the existing land to the west was a site of Special Scientific Interest (SSSI) and was administered by the Crown Commissioners; that to the east was administered by the Ministry of Defence (MOD). The agreed cross-section for the road was 2.0 m footway on the west side, 1.0 m hard strip on the east side, and a 7.3 m carriageway. A design speed of 100 km/h was adopted for the horizontal and vertical alignment. These standards minimised encroachment on both the SSSI and MOD land, while meeting the principal requirements of the Highway Authority. 55. The Highway Authority required the minimum disruption to traffic. The most economical solution, which also resulted in the minimum disruption, was the construction of a temporary diversion road on the Beach. The geomorphological importance of the Beach necessitated that all the materials used for the temporary road could be removed completely. The specification for the temporary road included a geotextile layer so that the road might be 'rolled up' and removed on completion. 56. Within the area of the site is a police checkpoint used in the event of an escape of prisoners from the detention centre on Portland. An important element of the design was the maintenance of this facility throughout the construction period. Another novel aspect of the design was the provision of street lighting under the approach flight path of helicopters to HMS Osprey. Over a length of the road, column lights had to be reduced in height to 6 m (standard 10 m), with a corresponding reduction in spacing. 57. The highly permeable nature of the underlying Beach material ensured that earthworks could continue through the winter without regard to the weather. Consequently, traffic was transferred to the new road within the time permitted under the contract.
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