Forms of cross-section of the retaining-wall may vary considerably. It may be rectangular, with a thickness about one-third its height (fig. r), or trapeziform (jig. 2), with the outer face inclined or curved (fig. 3). The last named affords the greatest stability with the least expenditure of material. To strengthen these structures, abutments are employed where their use seems to be required (fig. 5). Figure 4 shows the English practice of arching between such abutments.
commencing the construction of an embankment the ground must be cleared of such materials as vegetable mould, grasses, etc., which would retain moisture, and which cannot sustain the weight of the embankment. It is of the first importance, also, that culverts and cross-drains of ample dimensions should be provided for confining and carrying off the water of small streams, springs, etc., that may lie in the way of the proposed embankment. Larger streams will require bridging. Where marshy ground is to be crossed by a high embankment, two lateral ditches are dug, and the ground is compressed by the g-radual settlement of the embankment, until at length it has acquired the necessary stability. While this sinking is going on, the level of the embankment is maintained by fresh additions to its surface. In the case of a low embankment the weight of which is not sufficient to compress a deep marshy soil, the pre caution is necessary to build it somewhat higher than is actually required, leaving it to sink to its proper level in time; or if the marsh be a shallow one, the soft ground may be partially or wholly removed and replaced by sand, gravel, stone, etc.
Wherever it is practicable, advantage should be taken of the grades to drain the structures as thoroughly as possible by means of properly-disposed. channels communicating with the lateral ditches (Jo/. 24, fig. 3), which are provided to ensure thorough drainage. Where a cutting must be made through ground composed of layers of water-bearing strata (such as sand, gravel, and the like) alternating with layers impervious to water, like beds of plastic clay, danger from landslides is to be feared, and provision should be made to guard against such accidents by the liberal use of drain-pipes and channels. Where, as in the case of the railroad over the Brenner Pass, in the Tyrolese Alps, an entire mountain-side must be drained, a system of vertical shafts loosely charged with fragments of rock is constructed, communicating at the bottom with g-alleries into which the drainage-water is directed, and from which it is delivered far below the road. Where solid strata rest upon a yielding stratum the stability of the cutting is imperilled, aud the softer gronnd must be relieved from pressure by abutments inter posed at suitable intervals, being at the same time properly drained.