Thus an excavation of loam measuring 1,000 cubic yards will form only about 880 cubic yards of embankment, or an embankment of 1,000 cubic yards will require about 1,120 cubic yards measured in excavation to make it. A rock excavation measuring 1,000 yards will make from 1,500 to 1,700 cubic yards of embankment, depending upon the size of the fragments.
The lineal settlement of earth embankments will be about in the ratio given above; therefore either the contractor should be instructed in setting his poles to guide him as to the height of grade on an earth embankment to add the required percentage to the fill marked on the stakes, or the percentage may be included in the fill marked on the stakes. In rock embankments this is not necessary.
Classification of Earthwork. Excavation is usually classi fied under the heads earth, hardpan, loose rock, and solid rock. For each of these classes a specific price is usually agreed upon, and an extra allowance is sometimes made when the haul or distance to which the excavated material is moved exceeds a given amount.
The characteristics which determine the classes to which a given material belongs are usually described with clearness in the speci fications, as: Earth will include clay, sand, and loose gravel.
Hardpan will include cemented gravel, slate, cobbles, and boul ders containing less than one cubic foot, and all other matters of an earthy nature, however compact they may be.
Loose rock will include shale, decomposed rock, boulders, and detached masses of rock containing not less than three cubic feet, and all other matters of a rock nature which may be loosened with a pick, although blasting may be resorted to in order to expedite the Solid rock will include all rock found in place in ledges and masses, or boulders measuring more than three cubic feet, and which can only be removed by blasting.
Prosecution of Earthwork. No general rule can be laid down for the exact method of carrying on an excavation and dis posing of the excavated material. The operation in each case can only be determined by the requirements of the contract, character of the material, magnitude of the work, length of haul, etc.
Formation of Embankments. Where embankments are to be formed of less than two feet in height, all stumps, weeds, etc. should be removed from the space to be occupied by the embankment. For embankments exceeding two feet in height stumps need only be close cut. Weeds and brush, however, ought to be removed and if the surface is covered with grass sod, it is advisable to plow a fur row at the toe of the slope. Where a cutting passes into a fill all the vegetable matter should be removed from the surface before placing the fill. The site of the bank should be carefully examined and all deposits of soft, compressible matter removed. When a bank is to be made over a swamp or marsh, the site should be thoroughly drained, and if possible the fill should he started on hard bottom.
Perfect stability is the object aimed at, and all precautions neces sary to this end should be taken. Embankments should be built in successive layers, banks two feet and under in layers from six inches to one foot, heavier banks in layers 2 and 3 feet thick. The horses and vehicles conveying the materials should be required to pass over the bank for the purpose of consolidating it, and care should be taken to have the layers dip towards the center. Embank ments first built up in the center, and afterwards widened by dump ing the earth over the sides, should not he allowed.
Embankments on Hillsides. When the axis of the road is laid out on the side slope of a bill, and the road is formed partly by excavating and partly by embanking, the usual and most simple method is to extend out the embankment gradually along the whole line of the excavation. This method is insecure; the excavated material if simply deposited on the natural slope is liable to slip, and no pains should be spared to give it a secure hold, particularly at the toe of the slope. The natural surface of the slope should be cut into steps as shown in Figs. 23 and 24. The dotted line A B represents the natural surface of the ground, C E B the excavation, and A D C the embankment, resting on steps which have been cut between A and C. The best position for these steps is perpendicular to the axis of greatest pressure. If A D is inclined at the angle of repose of the material, the steps near A should be inclined in the opposite direction to A D, and at an angle of nearly 90 degrees thereto, while the steps near C may be level. If stone is abundant, the toe of the slope may be further secured by a dry wall of stone. On hillsides of great inclination the above method of tion will not be sufficiently secure; retaining walls of stone must be substituted for the side slopes of both the excavations and bankments. These walls may be made of stone laid dry, when stone can be procured in blocks of sufficient size to render this kind of con struction of sufficient stability to resist the pressure of the earth. When the stones laid dry do not offer this security, they must be laid in mortar. The wall which forms the slope of the excavation should be carried up as high as the natural surface of the ground. Unless the material is such that the slope may be safely formed into steps or benches as shown in Fig. 23, the wall that sustains the embank ment should be built up to the surface of the roadway, and a parapet wall or fence raised upon,it, to protect pedestrians against accident. (See Fig. 24.) For the formula for calculating the dimensions of retaining walls see instruction paper on Masonry Construction.