The allowance for lateral settlement should be liberal and be regulated by the nature of the earth, the height of an embankment, and other local conditions of situation and rainfall that affect earth, many of which are named in other chapters. It will vary greatly, and may be anything, from 5 per cent. to 100 per cent. additional width of formation. An addition of from 5 to 10 per cent. of the height of an embankment is usually sufficient, but in clay sand and such soils an embankment may be slowly washed away by rain until it has shrunk to half its required width. As an embankment settles or weathers the width of the formation should be maintained without having to steepen the slope, widen the top, or erect a wall upon the formation in order to hold the added earth. The additional formation width of embankments is also a provision against the effects which the “lurching” of an engine may cause by its weight temporarily acting upon one rail, and the pressure to be in the 4 feet 8½ inches gauge at a distance of about 2 feet 6 inches from the centre of the permanent way. For some distance upon each side of the point of junction of two high tip heads the top width should be increased, _vide_ Chap. IX., and in sidelong ground it is advisable to widen an embankment more on the upper side than the lower, as slips seldom occur upon the higher side; and in the case of railways, should the lower slope move, the rails can be placed towards the hill, and perhaps away from the slipped portion of the embankment.