When you specify some amount of passive pressure above the toe to neglect, the assumption is that it is because that soil may become frost-disturbed or otherwise insufficiently compacted to generate the specified passive pressure. But the assumption is also that the frost-disturbed soil will still be there as overburden to help the subsoil generate the specified passive pressure. As a result, when you specify some amount of passive pressure above the toe to neglect, the passive pressure distribution becomes trapezoidal. The trapezoid can be constructed by starting with a triangle with its vertex at the top of the soil on the toe side. Then remove a triangle whose vertex is at the top of the soil on the toe side and whose bottom is at the bottom of the layer of passive pressure that is to be neglected. The remaining pressure diagram describes the passive pressure distribution that is considered to be effective.