Arthur Casagrande

Arthur Casagrande: The Man Who Made Dirt Measurable

There is a small brass cup, mounted on a cam, that sits in every soil laboratory in the world. You fill it with wet clay, cut a groove down the middle, and turn a crank so the cup drops a fixed distance, over and over, until the groove closes. The number of blows tells you something exact about the soil. Before Arthur Casagrande built that device, engineers judged clay by kneading it between their fingers and forming an opinion. Afterwards, two laboratories on different continents could test the same soil and get the same answer. That is roughly the whole story of how foundation engineering stopped being a craft.

Ajdovščina, Vienna, and a Violin

Casagrande was born on 28 August 1902 in Ajdovščina, then part of Austria-Hungary and now in Slovenia. He was a childhood violin prodigy before he turned to engineering, and he came from a family already in the profession. He attended his first year of school in Linz, moved to Trieste, went through Realschule, and graduated in civil engineering from the Technische Hochschule in Vienna in 1924, staying on as full-time assistant to Professor Schaffernak in the hydraulics laboratory.

His father died that same year. Post-war Austria-Hungary had collapsed and had no work for young engineers, so in 1926, over the objections of both his mother and his professor, he sailed for New York.

A Chance Meeting

America was not immediately kind. He worked as a steel detailer for Carnegie Steel in New Jersey. Then, while interviewing for a position at MIT, he met Karl Terzaghi — the Austrian engineer who had just invented the entire theoretical basis of soil mechanics — and was offered a job on the spot as Terzaghi's private assistant.

From 1926 to 1932 Casagrande was a research assistant with the US Bureau of Public Roads, assigned to MIT, running soil tests for Terzaghi. When Terzaghi returned to Vienna in 1929, Casagrande went with him to set up a soil mechanics laboratory, toured the other European laboratories, and came back to MIT knowing more about the practical side of the young field than almost anyone alive.

Making Soil Measurable

Terzaghi had supplied the theory. Casagrande supplied the instruments, and the difference matters more than it sounds.

Soil is not a manufactured material. Two clays that look identical can behave completely differently under a building, and until the 1930s there was no agreed way to describe the difference. Clay passes through states as it takes on water: brittle when dry, plastic in the middle, effectively liquid when saturated. The water contents at which it crosses between those states — the Atterberg limits — characterise it usefully. But measuring them depended on the tester's fingers and judgement.

Casagrande's laboratory at MIT produced a whole apparatus of objectivity: the liquid limit device, the hydrometer test, the horizontal capillary test, the oedometer for measuring compression, and the shear box. He brought the triaxial shear test to the United States, was among the first to study how soil changes volume as it is sheared, recognised what happens to the water pressure between grains during undrained shearing, and developed the procedure engineers still use to work out a clay's preconsolidation pressure — the greatest load it has ever carried in its geological past, which governs how much it will settle under a new one.

During the Second World War he turned all this into classification systems for the Army Corps of Engineers and the Bureau of Reclamation, so that any soil could be assigned a category that predicted its engineering behaviour. That principle survives as the Unified Soil Classification System.

Harvard

He joined Harvard in 1932 as an assistant professor, and on Terzaghi's insistence took a doctorate at the Technical University of Vienna in the same year, having until then had none. At Harvard he built a postgraduate programme in soil mechanics that grew from twelve students to more than eighty after the war, and in 1946 was given a newly created chair in soil mechanics and foundation engineering. The programme's emphasis on laboratory work and seepage analysis became the model copied worldwide. Between 1942 and 1944 he pushed roughly four hundred army officers through intensive four-week courses in airfield construction for the Corps of Engineers.

His most consequential act of organisation came earlier. In 1936 he convened the First International Conference on Soil Mechanics and Foundation Engineering — an idea Terzaghi himself thought too risky to attempt. It worked, it established soil mechanics as indispensable to civil engineering, and it produced what is now the International Society for Soil Mechanics and Geotechnical Engineering, which Casagrande went on to serve as president from 1961 to 1965.

Liquefaction and Dams

His 1936 paper on the characteristics of cohesionless soils affecting the stability of slopes and earth fills addressed a phenomenon he named for the field: liquefaction.

The mechanism is worth stating plainly, because it kills people. Loose sand below the water table sits as a skeleton of grains with water in the gaps. Shake it — an earthquake, a blast, a rapid loading — and the grains try to rearrange into a denser packing. The water cannot escape fast enough, so it takes up the load instead. Pressure in the pore water rises, the grains stop pressing against one another, and for a few seconds the ground has no strength at all. It behaves as a heavy liquid. Buildings sink; earth dams flow away.

Casagrande spent the rest of his career on this and on seepage, consulting on earth dams across three continents, on the foundations of Boston's Logan Airport, on the effect of atomic blast loading on the Panama Canal embankments, and on the investigation into the Teton Dam failure. Characteristically, he later argued for restricting the word liquefaction to cases of drastic strain-softening producing genuine flow, rather than letting it expand into a catch-all.

Why Arthur Is Called a Genius

The claim rests on a specific and underrated kind of intelligence: knowing that a science is impossible until its quantities can be measured the same way twice. Terzaghi produced the concepts; Casagrande produced the tests, the apparatus, the classification and the teaching programme that let ordinary engineers use them, and then built the international institution through which the field could argue with itself. Convening the 1936 conference against his mentor's advice was the pivotal judgement of his life, and it was right. His instruments are still on the bench ninety years on, which is a longer working life than most theories manage.

The honest counter-case is that he was, and largely remained, the second man. The founding theory of soil mechanics — effective stress, consolidation — is Terzaghi's, and the great conceptual leap was not Casagrande's to make. His own contributions are empirical: procedures, devices, classifications and correlations, immensely useful but not laws. There is something quietly telling in the fact that his most distinguished honour was to be the first recipient of the Karl Terzaghi Award, named for his teacher. He also arrived at Harvard without a doctorate and took one only because Terzaghi told him to. Set against inventors of theory he is a lesser figure; set against the question of whether buildings fall down, he may matter more.

Legacy

Casagrande died on 6 September 1981. He was the first Rankine Lecturer of the British Geotechnical Association and a Terzaghi Lecturer of the American Society of Civil Engineers, and the Arthur Casagrande Professional Development Award, which supports young geotechnical engineers, carries his name. The discipline he helped construct is now the reason it is unremarkable to put a tower block on soft ground. The brass cup is still there too, unchanged, doing exactly what he designed it to do.

Achievements

• Affiliated with Harvard University
• Educated at TU Wien
• Worked as civil engineer, geotechnical engineer and geologist