Retaining walll design

[marcpreston]

I have been asked to design a small retaining wall. Tests have shown effective cohesion to be 20kn/m2. Soil unit weight 18kn/m3

The problem is the retaining wall is only 1m high.

When calculating the thrust I am using the formula

= (Ka*soilunitweight*H)-(2*c*(Ka^1/2))
Bells solution

The second part of the equation is so high the thrust value becomes negative. For the worst case would it be best to ignore the second part because cohesion diserpates over time. Ignore the thurst all together or design as a cohseionless soil.

 

[ishvaaag]

Except in clayey-like soils (unlikely from the density) you can dismiss cohesion and size the wall. Meteorization might distroy mid to longterm the cementation. Do not forget weep holes.

 

[oldestguy]

Why not forget the strength calculations and simply make a gravity wall. Make the width, front-to-back,1/3 the height and be done with it. For that small a wall the effort to go for reinforcing and the like cost more than simply a larger block of concrete, not reinforced, no footing, etc.

Tilt it back and reduce the width a little if you like.

If you wish, then simple vector force analysis can be done taking the soil as a fluid at maybe 1/3 its unit weight. But, using the 1/3 rule you need no calculations.

 

[concretemasonry]

The low rigid retaining walls of poured concrete using footings are really a small part of the wall market unless there are specific reasons for that type of construction.

Although it reduces the engineering importance and fees, the dry stack gravity walls (SRWs) are practical alternative.

Most municipalities have standard design plates for SRW use in road and sidewalk realignments as long as the walls are under 4' high that can also work with inside and outside curves or corners. The problems with weeps and drainage are minimized, as are the frost heaving and cracks, since the systems are designed to permit small movements that can be allowed to relax to the original conditions when the freezing or moisture conditions return to normal.

The same units (and appearance)can be used on 40' high retaining wall if you do all the engineering.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[lamy2781]

effective cohesion = 20kn/m2 ??
are you sure it is not undrain shear strength?

If the effective cohesion = 20kn/m2, and you get -ve value for (Ka*soilunitweight*H)-(2*c*(Ka^1/2)), which means that the soil itself is stable for that 1 meter height. The retaining wall could be only for surface protection.

but you have really need to make sure your soil have effective cohesion = 20kn/m2 as which could be close to rock.