EMBANKMKXT. 



KMKAXKMKXT. 



are frequently used to lek ui> the fronts of acarped 

 fortification*, and to form the glacis, the cavalirm, banquettes, capo- 

 niere*, or other earthwork defence! ; but a* the punMMes to which 

 embankments are applied in civil engineering are perhaps more im- 

 portant than those connected with military engimering, the former 

 will alone be discussed in the present article. 



In designing a roadway, whether for ordinary carriage or railway 

 traction, or in designing a canal, it i desirable, as far as possible, to 

 provide in the work lUelf for tin- disposal of the whole of the earth it 

 may be necessary to remove, in order to secure the required conditions 

 of level of the finished surfaces ; or, in other words, to make the 

 embankment* balance the excavations. Thin condition very frequently 

 renders it necessary to form very lofty embankments ; and the move- 

 ment* these accumulation* of earth are likely to produce, or to under- 

 go, render it equally necessary to adopt great precautions in their 

 execution. If, for instance, an embankment should be formed on a 

 compressible or on a shifting subsoil, it may either produce a vertical 

 or a lateral displacement of that surface ; or again, if the embankment 

 should have been carried up with undue haste, the loose earth of which 

 it is composed will be exposed to subside, in all probability, in a very 

 unequal manner. Naturally, the precautions to be observed in the 

 various cases thus alluded to, will vary according to the peculiar cir- 

 cumstances affecting them ; but the leading principle is the same, in 

 all, namely, that the weight of the material of the embankment must 

 be made to act equally in a vertical direction, and that the tendency to 

 lateral movement should be effectually resisted. 



Kmbankments formed upon peaty subsoils are especially liable to 

 vertical compression ; and it is to be observed that, in the majority of 

 alluvial districts, layers of peat are intercalated between the layers of 

 sand, clay, or soft semifluid silt, which constitute the bulk of those 

 deposits. In diluvial, and in many of the clayey series of the older 

 formations, compressible silty beds occur between others of a more 

 resisting character; and if, by reason of the great superincumbent 

 weight, or by any accidental disturbance of the natural conditions of 

 equilibrium of the upper beds, the compressibility of the lower ones, 

 should be brought into action, the practical effects produced may 

 become of the most dangerous description. It is, therefore a matter 

 of great importance that borings should be taken nil over the area of 

 any intended embankment ; and that, if it should be of any great 

 height, the borings should be continued to a considerable dept 1 1 

 cially upon the banks of a river which might be likely to interrupt the 

 continuity of any of the subterranean strata. Very frequently it may 

 also happen that drainage operations may disturb the normal conditions 

 of equilibrium of the intercalated sandy or peaty beds of the alluvial 

 districts referred to above ; and in those cases, the subsequent addition 

 of any extraordinary weight, by reason of the execution of an embank- 

 ment, would require that measures be adopted to limit the range of action 

 of the respective disturbing causes, whether they act above or below 

 the natural surface. Perhaps the best commentary on the principles 

 above stated may be found in a brief description of the practical 

 methods adopted in various countries to ensure the stability of large 

 embankments. 



Upon peaty subsoils the whole of the action of the superincumbent 

 weight is frequently resolved into a direct vertical one by means of a 

 double row of sheet piles, which confine the influence of the weight to 

 the precise area so inclosed ; or again, as in the case of the embankments 

 executed in Bavaria for the railways carried across the peat bogs of that 

 country, pits are sunk (in the manner of the squares of a chess-board) 

 through the peat, and these pits are subsequently tilled in with incom- 

 pressible materials. In some instances, when heavy buildings (which, 

 of course, act in precisely the same manner as heavy embankments) 

 have been executed upon layers of soft mud of a practically unlimited 

 thickness, they have been laid either upon a wide bed of concrete, 

 or upon a thick bed of fine sand kept from spreading laterally ; or they 

 have been founded upon piles driven with their butt ends downwards, 

 as at Brest. The principle observed in all these instances is merely to 

 distribute the weight over a large base ; but in Holland, and in many 

 analogous districts, the engineers exercise great skill and ingenuity in 

 the adaptations of means for diminishing the weight of the heart of 

 the embankment, the portion which precisely would be the heaviest 

 if the bank were executed of solid earth, by the introduction of layers 

 of reeds, or of fascines. These materials, moreover, tend to distribute 

 the weight over a large area, and they thus produce a double benefit. 

 The sketch given in Miiiard's ' Traite" des Canaux,' &c., of the mode 

 of forming the dykes, or the embankments, across the smaller creeks 

 in the Dutch province of Zeeland, will illustrate the system adopted in 

 the countries above mentioned. 



Occasionally the tendency of new embankments to slip laterally is 

 resisted by forming steps in the earth of the subsoil, or by cutting deep 

 trenches at the feet of the respective slopes. It must, however, be 

 evident that the success of these operations must depend upon the 

 nature of the subsoil itself ; because, whenever the latter is susceptible 

 of lateral displacement it can serve but little purpose to connect the 

 superincumbent load with it Stepped Beatings, and deep footings have 

 at any rate this advantage, namely, that they resist the passage of 

 water between the natural surface of the ground, and the under surface 

 of the embankment ; and therefore, one or the other of them should 

 invariably be resorted to, if there should be any danger from the 



infiltration of water beneath the seat Under ordinary circumstances 

 it would be preferable, of course, to form ditches, or drains, for tin 

 purpose of intercepting these surface waters ; but the additional pre- 

 cautions of stepping, or trenching the feet of the bank, are rarely 

 u|K>rlliiou. In those cases wherein banks are formed for the purpose 

 of storing large bodies of water, it becomes especially necessary to 

 prevent the passage of the water under the seat of the bank itself ; and 

 in all such works (of any considerable magnitude) it is customary to 

 insert what is called a " puddle dyke," or a species of wall of imper- 

 meable materials, which is carried through the whole depth of the 

 bank and the porous materials of the subsoil, to the impermeable 

 strata beneath them. Some of the most remarkable works of this 

 description are those lately executed for .the catch-water reservoirs of 

 the Liverpool and of the Manchester Water-works, or those executed 

 in America for the Croton Water-works, wherein, by the way, the 

 resistance of the banks to lateral displacement was assisted by the 

 introduction of cribbed or cradle-work, to be noticed hereafter, under 

 WJ-IK. 



Embankments formed upon the sides of running-streams, have to 

 resist not only the tendencies to vertical compression, and to lateral 

 displacement already referred to, and the danger arising from the 

 infiltration of water under their seats, but they have to resist, Int. the 

 dxnamical action of the water at its maximum elevation : and _n<l. 

 the tendency of the current to carry away the materials of which the 

 banks themselves are composed. The fonner of these conditions will 

 naturally be affected by the height of the water, and, to a great extent 

 also, by its width; for the waves produced in any body of water 

 depend for their height, partially upon the breadth of the surface 

 over which the wind may blow. The latter condition will equally, 

 of course, depend upon the velocity of the current, and the materials 

 which are the most directly exposed to its action must be of such a 

 nature as to obviate any danger of their being displaced by that agent. 

 In the article HYDRODYNAMICS, there will be found a table of the 

 transporting powers of water at different velocities, and the various 

 modes of defending .embankments from the action of the waves, or 

 currents, will be discussed under SEA DYKES ; RIVER DEFENCES. Of 

 whatever materials such river or sea walls may be externally com- 

 posed, it is desirable that they should present a very flat slope towards 

 the land side ; and that they should be carried up above the highest 

 point to which the waves can reach. 



The compression of the materials of an embankment (which is, it 

 must be observed, one of the most important considerations in the 

 execution of such works), must depend mainly upon the mode adopted 

 for the transport and delivery of the materials. Formerly, embank- 

 ments of small height were executed either by barrow, or cart work, 

 and in regular layers of small thickness ; and this mode of execution 

 is still retained in many instances in hydraulic works. But when 

 railways created the necessity for the rapid formation of the colossal 

 embankments, some of which will hereafter be described, it became 

 necessary to introduce a more expeditious, even though less perfect 

 mode of execution. It was for the purpose of meeting this practical 

 requirement of the age that the system of transporting the materials 

 furnished by cuttings to the embankments, by means of railroads and 

 i!., and of depositing the materials over what is technically called 

 a " flying tip," was introduced. Upon the ancient system, the thick- 

 ness of the layers of an embankment varied from 18 to 36 inches; 

 whilst upon the modern one, that thickness hardly ever descends below 

 6 feet, even in forming banks for hydraulic works; and in many in- 

 stances railway embankments of 80 or 40 feet in height, are made 

 without the intervention of any method for producing a partial com- 

 pression of the ground. It cannot, therefore, be a matter of surprise 

 that the rate of compression in large embankments should vary between 

 the extreme limits of from J to s ' 5 of the finished cubical contents ; and 

 it thence becomes necessary both to keep the top of a new Kink, 

 formed in the modern style, above the proposed finished height, and 

 to provide for its ultimate compression, by frequent recharges. Espe- 

 cial precautions must be taken against danger from this cause in the 

 case of the embankments required for forming the dams of catch- 

 water reservoirs ; and indeed, it would seem to be indispensable that 

 Mich works should be executed in thin layers, and that each of those 

 layers should be carefully " punned," or rammed ; care being taken to 

 prevent the formation of any continuous horizontal bedding in the 

 materials of the bank. 



In calculating the respective cubical proportions of the embankments 

 and excavations of a particular work, this fact of the ultimate com- 

 pressibility of the materials furnished by the latter, and of their 

 primary expansion, must be carefully taken into account. Another 

 practical observation, which perhaps it may be as well to make here, is 

 that embankments formed upon the sloping sides of hills are especially 

 liable to lateral displacement ; and that in their construction it is there- 

 fore more than usually necessary to step, or trench, the footings or 

 seats, and to provide efficacious means for carrying off the surface or 

 spring waters. 



One of the most important details to be considered in forming an 

 embankment is the slope to be given to the sides, for this slope will 

 vary with every description of material employed. Thus, it is found 

 that even in executing the embankments required for sup|H>rting 

 ordinary roads or railways (or those which in fact have merely to sup. 



