A rigid smooth retaining wall of height $7$ m with vertical backface retains saturated clay ass backfill. The saturated unit weight and undrained cohesion of the backfill are $17.2 \: kN/m^3$ and $20 \: kPa$, respectively. The difference in the active lateral forces on the wall ( in $kN$ per meter length of wall, up to two decimal places), before and after the occurrence of tension crack is _____