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Pyrites in Panama

Letters

I read with interest the article on enlarging the Panama Canal (NCE 31 May). Nearly 30 years ago I was consulted on a proposed bridge over the canal.

The excavation of the clay shales when the canal was excavated caused much difficulty. The bedrock was very pyritic. When pyritic rock is exposed to the air, the pyrite oxidises and generates sulphuric acid which destroys carbonate cementation in the rock.

In Panama I was surprised when hard cores were immediately placed in metal cylinders filled with water to stop them disintegrating.

Problems with pyritic shales occurred recently at the Carsington Dam in the UK. They are not fully understood. There is loss of strength and difficulties in deciding on design parameters. The difficulties at Panama were graphical described by Enock in 1914 at the time. He recognised the influence of pyrites from contemporary experience in mining.

The pyritic nature of the rock at Panama is potentially a significant problem, particularly in dry excavation if a water table is drawn down and the rock fissures are aerated. I understand from the article that some excavation like this is planned in the new work but the pyretic nature of the rock is not mentioned or what provision is being made for it.

Peter Vaughan, 101 Angel Street, Hadleigh, Ipswich, Suffolk, IP7 5DE

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