Coulomb’s theory of earth pressure is still playing an important role in calculating soil earth pressure and is well known all the world. By analyzing the mechanical characteristics of the limiting equilibrium soil wedge behind the retaining wall

especially the force between the soil wedge and retaining wall

some modifications to traditional Coulomb’s theory of earth pressure are presented. Considering that it is not necessary for the force (which is defined as earth pressure) between the wedge and the retaining wall surface to be on the point of sliding or to be in a condition of limiting equilibrium; the direction of the earth pressure can not be determined

but it must be limited in an allowed range according to the friction angle. Therefore

the active earth pressure is defined as the maximum value of the force within the allowed range; and the passive earth pressure is the minimum value of the force within the corresponding allowed range. This paper also considers that the soil wedge and the retaining wall are two different objects; and the soil wedge is formed only because of the potential failure surface of the soil itself. In other words

it is not necessary for the force between the wedge and the wall surface to achieve a critical state. As a matter of fact

the force between the wall surface and the soil wedge is equivalent to the force exerted on an object (soil wedge) by another object (retaining wall); and even if the soil wedge slides along a failure surface

the two objects do not need to slide each other along their contact surface. The formulas of the modified active earth pressure are derived and an approximate solution is presented for the passive earth pressure calculation. Examples prove that the results of this study are significantly different from the classic Coulomb’s theory. The works of modifying Coulomb’s theory of earth pressure deserve much more attention.