I have read with interest your article 'Alfred McAlpine to use slate waste as road sub-base' (NCE 2 May).
As I was Hyder Consulting geotechnical engineer on site for two years during the construction of the new A55 across Anglesey, I should point out that slate sub-base was not used on the main A55 in Anglesey.
The only areas where slate sub-base was used were at a limited number of side roads near the Llanfair PG end of the route (nearest to the Menai Strait).
The sub-base under the A55 in Anglesey consisted of limestones, granites and gneisses, sourced from site won materials, and local quarries.
However, where used, slate sub-base performed satisfactorily.
Sergio Solera (M), principal geotechnical engineer, Hyder Consulting, Firecrest Court, Centre Park, Warrington WA1 1RG