at the wearing course level of the road.
Such an assembly is suitable for situations in which the depth of the wearing course is within certain defined limits whereby strict regulations relating to the extent of overlap of the components of the assembly can be satisfied.
In particular, it is necessary to provide a minimum 50 mm vertical overlap of the cover with its supporting frame, and a minimum 50 mm overlap between the inner and outer frames.
The standard depth of the outer frame is 100 mm, the thickness of the base plate of the outer frame is 6 mm, and the recommended thickness of the wearing course is 40 mm. The overall relationship between the inner and outer frames of this known assembly is such as to provide aframe co-operating to support a cover for the assembly in a first, lowermost position associated with the base course level of the road, and a second inner frame which, on removal of the first inner frame from within the outer frame, can be located within the outer frame to support the cover at a higher position associated with the wearing course level of the road.
There may be a plurality of second inner frames, one associated with each of a number of different higher positions.
Alternatively, the position of the second inner frame relative to the outer frame may be vertically adjustable, for example by providing selectively variable support means reacting between the outer frame and the underside of the second inner frame.
In a first embodiment of the invention, the