Water Table and the Capillary Fringe

[Abuh001]

Hi I hope you are doing well.

Recently as a structural engineer I have been looking to improve my Geotechnical skills. I have opened up the old textbooks and looked over foundation design using the Brinch Hansen approach. One of terms in the equation is for effective overburdened pressure = unit weight of soil x height - pore water pressure


However for silts and clays the capillary fringe can extend 3m above the water table. The hand book of geo investigation and design tables by Burt Look suggests using saturated density for all soils in the capillary fringe zone. Does that mean I need to also assume pore water pressure in increases.

An example to illustrate

So dry density of silt, let's say 17Kn/m3
Saturated density = 19kn/3

If the water table is 5m below ground and I am looking at the overburden 2.5m below ground. Would I do to following.

Assume calipary fringe extends to 2m bgl
Pressure= (17x2.0+19x0.5) - (9.81x0.5).

Or am I overthinking this and you would just do
17x2.5.

 

[HTURKAK]

Does that mean I need to also assume pore water pressure in increases.

Just curious , what is the pore pressure at capillary rise zone?.

 

[EireChch]

just assume 17*2.5, dont over complicate it.

Check Das book on Foundation Engineering and Geotech engineering, it has a very good illustrative examples of what to do with water table.

ps- I never use saturated or dry unit weights, I typically just go with above or below the water table. Most of the time I use

18-loose -med dense SAND/Gravel.
19-dense SAND, maybe 20 for dense GRAVEL
15-17 for soft Alluvium
11-12 for PEAT
19-20 for stiff cohesive Till
all kn/m3