SLIP, Earthwork. When in an embankment, or cutting, the materials move laterally in consequence of some dynamical action, they are said to slip, and they do so occasionally under circumstances which can only be overcome with great difficulty, and at great expense. Slips occur either when heavy loads are placed on incoherent materials, which under such circumstances are simply displaced ; or they occur when the earth is of such a nature as to absorb so much water as to become a semi-fluid mud ; or when there are intereallated between more impermeable strata beds of sand, or other open materials suscep tible of being removed. The former of these sources of danger can easily be avoided by carrying the foundations of the intended load to such a depth as to ensure their not exerting any detnisive fore( the latter sources of danger are far more complicated, and they requii to be dealt with very carefully, on account of the numerous cm aidarations attached to the change of state the materials often Amur before they get Into motion.
Uniformly permeable materials, such as broken stones, gravel, c shingle, are not exposed to slip after they lave once taken their awl of repose, because the gravity of the separate stones is sufficient t keep them in their places whilst the water falling upon their expose surfaces Is percolating through them. Very fine muds, however, at able to be rendered semi-fiuid under some conditions, and they the have no angle of repose, and are able to spread in every direction Some ;clays, such as the Oxford clay, the gault, and the London chi which seem to have originally been the alluvial muds of mania estuaries, subsequently dried and consolidated, are susceptible a absorbing again quantities of water able to bring them into their origing state; and it seems, from the experience of our railway and cans engineers, that many years must pass before cuttings executed in thos formations attain permanent conditions of stability. Instances hav been known lu which slips have taken place in the Oxford clay forma tion when the slopes have been even 10 to 1 (10 base to 1 iu vertigo height); and the New Cross cutting of the Brighton Railway, in)th London clay, slipped in the winter 1860-61, although it had fennel part of the cutting for the old Croydon canal executed nearly sixty years since. Slips, it ,may be added, occur in undisturbed uatura deposits exposed to the action of the sea, or of running water, and thi same class of accidents, above described as occurring in the cutting and enthankments executed by the hand of man, occasionally happet in consequence of the action of the ordinary laws of nature. Whet they do happen in coal mines, or in analogous positions, owing to the interference with the lateral support of the strata worked through, the slips are technically known under the name of " creeps," and they constitute by far the most dangerous, because the most irresistible of the accidents to which coal mining is exposed.
As slips are principally attributable to the effects of water upon the materials concerned, it must be evident that the only method o: preventing or remedying them must be to establish a perfect systerr of drainage in order to carry off superficial waters without allowing them to soak into the ground so as to dilute the soluble materials, or to set in motion the more minute particles of the intercalated permeable strata. The mode usually adopted near London in dealing with the earthworks to be formed around, or for the support of, water. works, where of course slips would be of very serious importance, is to carry up the embankment In layers, alternately of the clay in its natural state, and of the clay after having been burnt in a heap; vertical dykes or trenches filled in with burnt clay are carried from the top to the bottom of the bank ; and the top is dressed off with the longitudinal and transverse inclinations required to throw the rain water into a series of surface drains. Theoretically this system is the one to be aimed at in all such works, but it is too often the case that the cost of the precautions it involves Induces constructors to neglect some of them; and again it may happen that the materials of the banks are exceptionally impermeable, and thus justify a more economical mode of treatment. Nevertheless, it must be considered that, especially upon ground having a transverse fail, an embankment formed of the London clay is always exposed to slip, unless it should have been executed in some such manner as the one above mentioned. Broken stone or broken chalk may be used instead of the burnt clay ; but the condition to be aimed at in the application of those materials is that they should be perfectly permeable, and that the water they may remove should have a free outlet. In deep cuttings, the experience of the New Cross section of the Brighton line proves that the London clay, as It exhibits itself there, will not stand with slopes of leas than 4 to Leven when a good system of superficial drains has been executed. In no case will the stiff blue clays stand with slopes of less than 2 to 1, without giving rise to numerous slips; and those accidents can hardly be prevented, unless the inclination of the slopes is made in the ratio of 3 to 1. An essential precaution to be taken before establishing au embankment of clay is to clear its seating from the vegetable would which might be originally there, and to provide efficient lateral drains, in order to prevent any land waters from finding their way under the embankment. In order to prevent lateral displacement, or slipping on the bed, on eloping ground, it is frequently the case that toothings are cut in the natural surface, for the of increasing the friction, and thus of opposing the tendency to slip.