The Effect of Friction at the Strip Footings’ Underside on the Expansion of Plastic Zones in Subgrade

There exists horizontal friction besides vertically pressure at the foundations' underside. Considering the effect of friction on the expansion of plastic zones in subgrade is needed to accurately evaluate subgrade’s safety. A strip footing is chose as the research object. Assuming the distribution of friction at the strip footing’s underside is two symmetrical triangles. With the help of Flamant formula and via definite integration, the formulas of stress in subgrade induced by the friction are got. Setting the Coulomb-Mohr strength theory as the yielding criterion for the subgrade soil, through the comparison among the different friction angles in expansion characteristics of plastic zones, the research object is achieved. Two major conclusions as following: (1) the friction makes the plastic zones appear in advance, the initial critical load get smaller with the friction get bigger; (2) the plastic zones get broader in horizontal direction under the action of the friction, this makes the plastic zones in two sides beneath the footing run-through later and so enhances the subgrade’s ultimate bearing capacity.