Earth Pressures

AT the Friday evening discourse at the Royal Institution on Feb. 19, Prof. C. F. Jenkin discussed the mechanics of shifting sand. Prof. Jenkin explained why experiments intended to measure the pressure exerted by granular material, such as sand, fail, and showed that the failure is due to arching, a necessary consequence of the property of dilatancy, discovered by Osborn Reynolds. The latest design of apparatus for measuring the forces on a retaining wall was shown, and the results of a test made with it were described. With this apparatus the old earth pressure theories have been tested and the truth in them separated from the error. As a result, a revised wedge theory has been formulated. The apparatus was shown to be capable of measuring the forces not only on plane walls, of any batter, but also on stepped walls and on L-shaped walls, and also of measuring the forces under water. Finally, by an ingenious device, a wall of sand has been tested and the true friction angle between two sand faces measured. The reaction, that is, the force exerted by the wall on the sand, was described and its importance illustrated by an account of the partial failure of a great weir in Australia. The apparatus described can only deal with one-quarter of the whole problem of earth pressures; it cannot measure the forces under a foundation, and it cannot measure either horizontal or vertical forces exerted by clay.