In both the preceding cases, to ensure stability and to prevent any protective works being pushed forward, the foundations must be below the formation or ground level. To lessen sliding action the foundation should incline at right angles to the face, and should have a batter either for the whole width or for a distance not less than about one-third to one-half of the bottom thickness in the case of low retaining walls. A wall having a steep batter upon each face, and therefore a wide base as compared with its area, causes the centre of gravity to be at a greater distance from the exposed face, and therefore the resistance to overturning is increased; but care must be taken that it does not slide forward. A foundation for such a wall upon clay or tenacious soil is to be preferred, provided there is no upheaval of the ground in front, but it should not be upon a thin stratum, or the latter may slide upon another. In towns, or where land is of considerable value, the two sides of a cutting can be made to support each other, and one of the following principal methods may be adopted. 1. An invert under the line. 2. A heavy flat platform, arched on plan or solid throughout, under the permanent way or formation, to prevent a forward movement of the toe. 3. Overhead arches and iron struts at intervals to resist and reduce the pressure upon a wall. In such a situation, should a tunnel or covered way be not required, any subsidence of the earth may cause the destruction of valuable property, and the erection of a retaining wall be imperatively necessary from a due consideration of prudential construction, and altogether regardless of the character or condition of the earth or its liability to become water-charged. In order to prevent movement of the earth different forms of support may be required. In some cases, owing to excessive pressure, a counterforted wall with an invert under the permanent way may be essential, or the invert may be flat and be arched on plan, thus supporting the toe of a retaining wall between the intervals, in combination with a vertical or inclined pilaster and a front counterfort system, and in addition flat overhead struts of iron or other material between the walls acting as an auxiliary support above the required traffic space may have to be adopted where the walls have to sustain considerable thrust, the thickness of the retaining wall being thus reduced; or a simple retaining wall may be sufficient. A non-jointed material as Portland cement concrete of equal character is to be preferred to a mass consisting of hard materials yet incapable of possessing a joint equal to their strength or durability.