Embankments have frequently to be carried over ground which is low, soft, and wet, but not boggy. If the culverts and drains are sufficiently large, and properly arranged, these places are not likely to cause much future trouble.
For a thoroughly soft deep bog, however, it is most difficult to make any accurate calculation as to the amount of
embankment filling which will be necessary to form a permanent foundation for the line; and the construction of a high heavy embankment across such a place is one of those undertakings which every engineer is most anxious to avoid. A large quantity of material may be tipped into the bog, and seem to stand fairly well for a time, and then suddenly disappear altogether. More material has to be brought forward, and will most likely disappear in a similar manner. The filling material being heavier than the bog on to which it is thrown, falls through, and displacing the soft semi-liquid matter, continues to sink down lower and lower until it is stopped by a harder stratum underneath. In a measure the operation somewhat resembles the tipping of earth into a lake; the material will go down until it meets with a solid bottom, and in going down it assumes its own natural slope, and forms for itself a width of base corresponding to its height. It will be readily understood what an enormous amount of filling material will be swallowed up in following out such a process. On a very soft bog, say 20 feet in depth, over which an embankment 20 feet high has to be formed, the extent of the actual earthwork filling will very probably closely approach the outline shown in [Fig. 63]. The upper portion, ABCD, representing the embankment proper, will contain about 133 cube yards to the yard forward, whereas the lower portion, CDEF, which has displaced the soft boggy matter, will contain about 266 cube yards to the yard forward, or, in other words, the filling which is out of sight will be double the filling which is in view above the section ground line.
Apart from the large amount of filling consumed in forming this semi-artificial island, the progress of the work itself is very perplexing. A long length of the bank may have been raised again, once or twice, to the proper height, and may have carried rails and earth-waggons for some weeks, and then sink all at once several feet. The sinking, too, may not be uniform, but may produce fissures, depressions, and separation of the earthwork which will necessitate much care when bringing forward fresh filling material. The bog may not be of the same consistency throughout, there may be some layers of harder material, such as imbedded trunks of trees, and these may sustain the filling for a time, and then yield under the increasing weight of the superincumbent mass. Even when the embankment is finished throughout, and shows no sign of sinking, it should be very
carefully watched for a long time for any indication of further movement.
When the bulk of the material has been taken out of an earth or clay cutting, the work of trimming the slopes should be put in hand, so that any surplus left on the wings, or sides, may be removed, and carried away before stopping the earth-waggons. The angle of slope having been decided, a battering rule of light wooden boards is made to correspond to the slope, and in form similar to that shown in [Fig. 64]. A plumb-bob is suspended from a fixed point, A; the lower end, B, is then held against a peg or mark which indicates the correct level and width of the cutting at the place, and the upper end, C, is raised or lowered until the plumb-bob string coincides with the vertical line marked on the rule from A to D, and the plumb-bob rests steadily in the space cut for it at D. With this battering rule a length of seven or eight feet, according to the size of the rule, is first trimmed to the correct slope, and by continuing the application of the rule up the side, a correct slope line is obtained from bottom to top of slope at that place. By repeating the process at convenient distances along the cutting, a series of correct slope lines are obtained, and the intermediate space can readily be trimmed to correspond.
The same form of battering rule and method of working is applicable for trimming the slopes of the embankments.
When the slopes of the cuttings and embankments have been trimmed, vegetable soil, which has been laid aside, or reserved as previously described, should then be spread evenly over the slopes to the uniform thickness of not less than four inches, and the whole sown with good grass seeds to form a strong sward.
The trimming, soiling, and sowing of the slopes not only gives a more finished appearance to the earthworks, but the strong grass, when once well grown, binds the surface together, and helps to resist the injurious effects of heavy rains and melting snow.
There are many places abroad where a neat finish to the earthworks is considered quite a secondary matter, or where it would be difficult to obtain suitable soil to spread on the slopes. The earthworks are hurried forward to allow the iron highway to be laid down as quickly as possible, the slopes of the cuttings and embankments are only roughly trimmed, and nature is left