A question by a visitor

Anonymous said...

I'm confused in calculating the effective vertical stress in a sandy gravel soil layer below the graound water level and it is between a sand layer on the top and a clay layer on the bottom.
What unit weight i have to consider the sand unit waight or I need the porosity(n)of the sandy gravel to get the unit waight?

Thanks

geotechie said...

suppose we have a situation like the following:

_____________________________________________

Sand wet unit weight = Ys,w
layer thickness = d1
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
water table @ depth dw, unit weight of water = Yw
Sand saturated unit weight = Ys,sat
______________________________________________

Sandy Gravel
saturated unit weight = Ysg
layer thickness = d2
______________________________________________

Clay
saturated unit weight = Yc
layer thickness = d3

______________________________________________

Effective Stress = Total Stress - Pore Water Pressure

Total Stress at the bottom of clay layer = (Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3)

Pore water pressure = (Yw)(d1-dw) + (Yw)(d2) + (Yw)(d3)

Therefore, Effective Stress =
(Ys,w)(dw) + (Ys,sat)(d1-dw)+ (Ysg)(d2) + (Yc)(d3) - (Yw)(d1-dw) - (Yw)(d2) - (Yw)(d3)

= (Ys,w)(dw) + (Ys,sat-Yw)(d1-Yw) + (Ysg-Yw)(d2) + (Yc-Yw)(d3)

Let me know if it helps (or if it does not).