In pulling down walls that have been built with offsets at the back it will usually be noticed that the filling does not rest upon them, but that hollows occur as the earth subsides; for this reason, a straight batter at the back is to be preferred. No support from the weight of the earth, which is supposed to be upon them, should be relied upon, although it may temporarily exist. The filling should be commenced at the wall, and the layers be so deposited that their slope approximates to that of being at right angles to the surface of the slope of the ground rather than parallel to it. When thought desirable and the natural ground is solid, it can be benched, and the backing be damped and rammed. The backing of a wall may consist not only of an inverted triangular piece of earth, but extend for some distance to the rear, and may have a top surface at a considerable height above it; the wall will then have to sustain a severe thrust. The angle of repose of the earth should be ascertained and the filling be thoroughly drained, or it may be disintegrated by moisture, the whole mass gradually become unstable, and finally push the wall forward with great force. CHAPTER XII. NOTES UPON SLIPS OF EARTH, SUBSIDENCES AND MOVEMENT IN FOUNDATIONS CAUSED BY “BOILS” OR AN UPWARD RUSH OF WATER IN LOOSE EARTHS.—CONSIDERATION OF SOME PRECAUTIONARY AND REMEDIAL OPERATIONS. With regard to “boils” in sandy soils and the general drainage of the site of dockworks, operations should be commenced as long as possible before the works of construction, so as to lessen the probability of the occurrence of slips, subsidences, and movement of earthwork. In order to reduce infiltration, it is well to make a trench round the area of any ground that has to be excavated, which may embrace the whole site. A system of drains and conduits should be established within it, and at the lowest level a sump at a convenient position, and to a depth a few feet below that of the work; it can then be made the chief pumping station of the dock; but all drawing away or flow of the earth must be prevented: and to obviate erosion of the drains, they may require to be rough lined with an impervious covering such as clay. The sump should consist of an iron cylinder with proper provision against a run of soil. In all loose earths the pumping station should be some distance from buildings or roads so as to avoid any settlement.