It is seldom a dock-wall fails from insufficient mass or incorrect design. The chief element of danger being imperfect foundations causing a forward movement of the wall, which may either result in a bulge or a complete advance of the whole section for a considerable distance, and usually when this forward motion commences it extends for a long length, and has an apex, and is not only of a disastrous character, but also difficult to quickly and permanently restore to a state of stability. The wall may be most carefully designed, and be amply sufficient to resist lateral thrust due to the pressure of the earth and any load upon the quay, and nevertheless, although as a wall it is perfect and unimpaired, the whole mass from want of sufficient depth of the foundations or hold in the ground frequently slides forward: and there are few extensive docks that have been constructed in any but the firmest earth that have been free from such a mishap. In order to limit the extent of a sliding movement, it might be said, reduce the lengths of the wall and make the unsupported from the front length as short as possible, and in doubtful earth it may be advisable to abandon all long straight walls and adopt the system of short jetties, with ample width at their heads for unloading and loading operations, for a vessel to lay alongside, for store-house room, and a double line of rails and the necessary free action of machinery; so as to avoid the expense of slips pushing forward the wall: but to do this may almost ruin the commercial prospects of a dock, for naturally owners and captains of ships will send their vessels where the greatest facilities are offered and the impediments to movement the least. Local considerations in each case alone can determine the shortest required clear length. However, in designing jetties to dock-walls it is advisable to so construct them that they may act also as face counterforts to a wall, and be located at any apparently weak place so as to give support where it is needed.