Seismic sliding with damp proof membrane

[bkal]

We are designing a reinforced concrete building on a foundation raft with a reasonably flat bottom surface. A damp proof membrane it so be provided between the foundation and the subgrade soil. There is no indication regarding a coefficient of friction which can be mobilised at this interface to resist sliding due to inertial seismic forces. One way out is to key-in the foundation into the soil, but this adding costs and programme. Is there any guidance on how can friction at this interface be enhanced or claimed.