For a building to sink six inches in eighteen years causes some concern at M.I.T. If the science of soil mechanics had been developed earlier, the sinking would have been uniform at least. In the Harvard Engineering School laboratory of soil mechanics, Arthur Casagrande, assistant professor of Civil Engineering, a former member of the United States Bureau of Public Roads, has carried out experiments with the most modern equipment that go far towards explaining the reasons for the settlement.
Dr. Casagrande found that a hard dry piece of clay when compressed becomes soft mud because clay is made up of minute mineral particles and voids filled with water. Compression gradually forces the water out, the rate of consolidation depending on the depth of the strata and the weight supported. A classic example of the subsidence is the Tower of Pisa, which furnishes fairly accurate data on the factors of weight and shape. Mistakes in foundation engineering are responsible for more damage and loss of life than all other causes combined.
Wet sand, an entirely different substance, supports weight quite well when undisturbed, but when piles are driven into it the whole mass suddenly liquefies and the piles sink. Quicksand is thus merely the upward flow of water through sand. Dry sand acts even more strangely; when all the air is pumped out, it becomes as hard as rock. This fact explains why foundations set on deep piles are usually safe.
Effect of Frost
Frost draws water from the soil beneath and causes heaving of pavements; in the spring the liberated water filters through the soil again, making mud with no supporting power. Among the projects Dr. Casagrande has in mind is laboratory investigation of the action of frost in soils, rocks, and building stones.