Karl Terzaghi: From theory to practice in geotechnical filter design

In a period of just over 10 years, Karl Terzaghi proved the idea of a weighted filter in simple laboratory tests while on faculty at the American Robert College in Turkey, patented the concept, developed empirical rules for the grain size distribution of filter through specialist consulting advice on relatively small weirs in Europe, and firmly established those rules as a basis for engineering practice on the Bou-Hanifia Dam in North Africa. The Terzaghi filter criteria are commonly attributed to experience gained on that latter project. However, an evaluation of unpublished letter reports, contract documents, and correspondence suggests the evolution of those criteria for soil retention and permeability occurred during his earlier collaboration with the Austrian consulting company of J. Pfletschinger and Co. This new insight appears to be confirmed by the uncovering of a draft patent claim, prepared in the name of Josef Pfletschinger, which sets out the precise details of those filter criteria and whose existence was acknowledged in contract documents for Bou-Hanifia.