The purpose of concrete jacks

June 6th, 2005 § One comment

When I lived in Santa Cruz, I sometimes visited Seabright Beach and wondered why all those concrete jacks were piled up on the shore next to the yacht harbor. Thanks to “The Atlantic Generating Station,” from John McPhee’s book Giving Good Weight, I now know the answer:

East London’s breakwater had been weakened by rough seas some years earlier, and now the storm of 1963 tore off sixty per cent of the armor. Eric M. Merrifield, East London’s harbor engineer, wondered whether that would have happened if the armor had not been solid—had not been designed to accept on one plane in one moment the great force of the ocean. He decided to reconstruct the breakwater with porous armor, and in doing so he invented a momentous novelty in harbor engineering.

The idea was to cover the breakwater with objects of branching shape—like children’s jacks—that would engage with one another, clinging together while absorbing and dissipating the power of waves. There had been similar attempts. The French had tried a four-legged concrete form, a tetrapod, and it had worked well enough but had required an expensive preciseness in construction, because each one had to be carefully set in place in relation to others. Merrifield wanted something that could almost literally be sprinkled on the breakwater core. Eventually, he thought of dolosse.

Dolosse—the singular is dolos—were crude toys that had been used by South African white children since the eighteen-thirties, when they acquired them from tribal children in the course of the eponymous trek, the overland march of the voortrekkers from the Cape Colony to the Transvaal. A dolos was the knucklebone of a goat or a sheep, and might be described as a corruption of the letter “H” with one leg turned ninety degrees. The game that had been played with dolosse by voortrekker children, and by South African children ever since, was called knucklebone. As crude toys, dolosse were also thought of as imaginary oxen. Witch doctors had used them as instruments of magic power. Merrifield replicated them on a grand scale in concrete, making dolosse that weighed twenty tons apiece, and with these he armored his breakwater. When high seas hit them, the water all but disappeared—no slaps like thunder, no geysers in the air. The revised breakwater seemed to blot up the waves after breaking them into thousands of pieces.

One comment

  • Cdawg says:

    I was at Seabright on Memorial Day, and wondered, for the nth time, what those things were too! Thanks for the insight!

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