Segmental Retaining Wall at a Pond, Design Method? 1

[MWPC]

I am designing segmental retaining walls on a project. One portion of one wall skirts a retention pond. The NCMA design manual throws up a lot of red flags regarding "Submerged SRW Design" in section 8. Other designers feel that if backfilled with free draining granular material the water will flow out as quickly as the water recedes eliminating any additional pressure. Any help would be appreciated. Ken

 

[dik]

Have you checked with the engineering people at one of the wall suppliers? You might check with Risi, one of the manufacturers. They also have a wall design program, so they are well into the technical aspects. Off the top, the saturated soil would have a greater lateral load and the bearing material would have a reduced bearing capacity due to the reduced shear strength of the submerged soil.

You might have to live with the wall being constructed a short distance away from the pond to allow an impervious membrane to be installed between the wall and the pond. Membrane can be EPDM, Bentafix, or even a layer of impervious clay...

Just some suggestions.

Dik

 

[Boonie]

The critical factor in designing a segmental retaining wall that impounds water is rapid drawdown. Due to the fact you are designing a wall made from dry stacked concrete blocks, the water will obviously saturate the reinforced backfill during high water events. The FHWA design manual for MSE retaining walls describes the methodology of the design and recommends utilizing reduced factors of safety for the drawdown condition. We design segmental pond walls on a regular basis and always recommend backfilling the reinforced zone with a free draining material (3/4" minus washed, crushed stone)as an alternative to the rapid drawdown design. This is often a cause for debate among designers since the stone is not "required" by AASHTO nor NCMA.

 

[MWPC]

Thanks Boonie and dik. Very helpful. Ken

 

[MSEdesigns]

Ken,
I have designed several MSE walls in water front cases. Main concern is rapid drawdown as Boonie mentioned. Using 3/4" clean stone as reinforced backfill wraped behind with filter cloth have proved good enough in my experience in channel applications as well.

NCMA has a good resource if you want to consider the hidrostatic pressures. Click on the following link to get it:

http://secure.ncma.org/source/order...&CFTOKEN=44707207&continue=1&SEARCH_TYPE=find

Quite long link, but dont know how to make it shorter. Hope it works.

Good luck!
JL

 

[MSEMan]

Don't forget that when dealing with submerged structures you are also dealing with submerged granular soil densities abd properties. (i.e. unit weight less 10kN/m3 and saturated phi and c') Depending on other soil parameters, global stability (particularly sliding) can become an issue.

 

[80smetalfm]

MWPC
Have you checked the Australian standard AS4678? The main thing to confirm here is the type of segmental material used at this end of the project. Llso, design for effect of static & transient pore water pressures.

Special considerations shall be given to:

- drawdoen effects,tidal lag, residual WT
- pipping & material loss from the structure
- scour or erosion infront of structure
- wave action?
- loss of backfill
-durability of materials