The stability of quay walls on earth foundations

IN spite of the large amount of experience which has been gained in the construction of quay walls, it is still one of the most difficult problems in engineering to design a wall on an earth foundation with confidence that it will be stable when completed. A warehouse or a bridge can now be designed, not only with the assurance that it will bear its load, but also, with a knowledge of its factor of safety sufficiently accurate to satisfy the designer that material has not been wasted. But the same can certainly not be said of a retaining wall on a soft bottom-at all events not of a wall, say, 40 to 80 ft. high, such as is commonly needed to sustain the quays of a modern dock. Even if the designer of such a wall is assured that it will stand, he cannot with any confidence tell you what factor of safety it possesses. The cause of his uncertainty is of course the difficulty of ascertaining the actual lateral pressure imposed by an earth backing and the actual resistance offered by an earth foundation. His difficulties are thus quite different from those of the engineer who has to design large masonry dams. The latter structures are invariably placed on a foundation of solid rock, and the designer's chief care is that the stresses in the masonry of which the dam is composed shall not exceed a safe limit. The dock engineer on the other hand has to be anxious that his wall shall not move as a whole on the comparatively soft material on which such structures have in general to be placed. F. E. Wentworth-Shields