Over- Consolidation

Question

Hi all, 
 I need some explanation for these basic questions. 
 01. Why the lateral soil pressure coefficient is high in OC soil when compare with the NC soil? 
 02. Why only OC soil has cohesion along with friction angle (NC soil has no cohesion, only friction angle)? 
 
 Thanks.

Faseel Khan · Answer

01. Over-consolidated soils have experienced a higher stress in the past than they are currently experiencing in their natural state. If for example, we have a 10 m high soil strata. Over time, say the top 5 m gets eroded away and only the bottom 5 m high soil strata remains. This bottom 5 m soil strata will be over-consolidated and will be denser compared to a normally consolidated soil of 5 m height with no past over-burden pressure. Simple reason is that the eroded top 5 m of overburden will have an added vertical and horizontal stress on the bottom 5 m of soil. This vertical and horizontal stress is calculated based on the Boussinesq's equations for different loading conditions. So compared to normally consolidated soil, over-consolidated soil will have a higher lateral soil pressure. 02. It is not necessary that over-consolidated soils alone exhibit cohesion. Normally consolidated soils do exhibit cohesion. It depends on the soil type and presence of any cementing agent. Normally consolidated clays for example will exhibit low to very high cohesion because cohesion depends on the mineral structure in the clay. Normally consolidated sands may exhibit apparent cohesion due to surface tension or if there is a cementing agent present.