METHOD OF PLACING BACK-FILLING. The manner of depositing the back-filling has a very important effect upon the stability of a retaining wall, but usually receives little or no con sideration. If the back-filling is dumped so as to slide or flow toward the wall, the pressure against the wall is likely to be much greater than for a static load, since the flow develops surfaces of cleavage which cause large masses of earth to slide down against the wall as the back-filling settles. This tendency to form surfaces of separation may be due to a difference in the material of the back-filling, or to a difference in fineness, or to the effect of rain, or to all combined. When the back-filling is first deposited, the thrust is a maximum, because the cohesion is then a minimum, and usually the masonry is then weaker than it will be later, because the cement has not fully set; and hence it is particularly unwise to deposit the earth in such a manner as to greatly increase the thrust. The back-filling should be deposited in horizontal layers or be dumped so as to flow away from the wall. Not infrequently, by depositing the back-filling so as not
to slide against the wall in settling, a light wall of inferior materials and workmanship may be made to stand, where if the earth is dumped in such a manner as to slide against the wall a heavier section of superior materials and workmanship fails.
If the back-filling is deposited before the cement has fully hard ened, it is wise either (1) to tamp the earth in thin layers which are horizontal or slope away from the wall, or (2) to support the wall temporarily by shores solidly wedged up.
Particular care should be taken in depositing the back-filling of a wall built near a steep undisturbed slope of earth or rock, since either an excessively heavy earth wedge may be formed or the earth may arch and thereby throw a heavy lateral thrust near the top of the wall.
It is always more economical to increase the stability of a retain ing wall by drainage and care in depositing the back-filling than by increasing the cross section.