the outer frame 2 of fig1 as in the embodiment of fig2 to 7 the cover 34 of fig8 and 9 is supported in its lowermost position by abutment with the outer frame 54 more particularly with the support portion 56 with the temporary inner frame referenced 58 also seating on the support portion 56 to fill the space between the cover 34 and the outer frame 54 the higher position of the cover 34 is determined by a permanent inner frame referenced 60 similar to the inner frame 42 having increased thickness support portions 62 at the corners thereof which seat on the support portions 56 of the outer frame 54 the function and operation of this embodiment conforms with that of the embodiment of fig2 to 7 in all embodiments the provision of separate temporary and permanent inner frames associated with the base course level and the wearing course level of the road respectively ensures that all british standard regulations relating to the extent of vertical overlaps of components can be readily met whilst at the same time enabling accommodation of a variety of thicknesses of wearing courses in excess of the conventional thickness of 40 mm clearly the precise construction and configuration of the various frames can be altered from that described and illustrated without departing from the scope of the basic invention while the material of the temporary frame can be chosen to suit particular requirements where this temporary frame supports as in fig1 the material may be steel in non supporting situations as shown in fig2 to 9 the temporary frames may be.