Segmental Retaining Wall Design

[jaggars]

I have a situation whereby i am designing a Segmental Retaining Wall is Ht:- 1.575m it is holding up 2H:1V slope above i have designed tie back of 1.3m (0.83 base to ht ratio). The slope runs about 5.8m and then it tapers to Rail track. I am asked to account for rail load Cooper E90. I drew a 10H : 7V from the rail track. It just touches the back of Geogrid behind the wall. The structural engineer of the client recommends i use 2H : 1V as load influence from the rail track. How should i account for this extra load. I if draw 2H : 1V slope line from rail track the load line is hitting the Facing system at almost mid of wall.
Thanks for all your suggestions.

 

[VoyageofDiscovery]

Hi jaggars,

You should use Boussinesq formula for horizontal pressures from strip loading. Beyond this you need to ensure no slip circle failure.

HTH

VOD

 

[jaggars]

there was an earlier discussion on the same lines
thread255-150849.

 

[PEinc]

Most railroads tell you how you must apply the Cooper E80 surcharge. Usually, they require a Boussinesq analysis based on a rigid wall. Therefore, the pressure starts at the top of the wall unless the top of wall is higher than the bottom of the railroad ties.

 

[jaggars]

how does it apply to Segmental Retaining Walls in my case. Should i include the loads in my internal & external stability calculation. VoyageofDiscovery (Structural) made a good point that i have to include them in Global Stability Calculation. I use ReSSa software for my global stability calculation.I will apply this load as a strip load in my Global Stability calculation. I am using segmental retaining wall manufactures proprietary software for Internal and External stability calculation. This is based on NCMA methodology. NCMA does not have clear guidelines how to incorporate them in external & internal stability calculations. In my case top of the wall is at 2H: 1V drop from railroad tie and the distance from the rail is 5.8m. i was following the rule if the load is twice the ht of the wall far away (1.575 X 2)=3.15m < 5.8 there should be no influence of the loads in external and internal stability calculation on SRW wall.

Thanks for all your suggestions

Thanks for all your suggestions

 

[VoyageofDiscovery]

It appears from the thread you referred to, that in these situations of lateral pressures one needs to do the full analysis based on one's geometry versus using a rule of thumb.

Internally and structurally, you will need to ensure that the lateral pressure from the Cooper loading does not tend to bulge the facing from the attached reinforcing strips, presuming the strips can take the tensile forces, ie check the facing rebars and facing to strip connections.

Externally, you should apply the increased lateral pressure due to the lengthening of the strips (iterative) for overall sliding and overturning at this stage.

 

[VoyageofDiscovery]

should say

Externally, you should apply the increased lateral pressure due to the lengthening of the strips (iterative at this stage) for overall sliding and overturning.

 

[jaggars]

Thanks
In my case its polyester geogrids (Strata grid 200) but i guess the principle remains the same. Apply increased lateral pressure check external, internal stability of wall and also check global stability of entire wall slope geomerty.Based on the guide line i have attached i should apply resultant pressure Ph for base sliding calculation and Ph X hp for overturning calcualtion my other doubt do i have to apply correction in the equation for wall batter since in my case its flexiable system Segmental retaining wall.

 

[VoyageofDiscovery]

Okay, I think you have two questions here.

First, yes, statics should be followed for slip circle, max allowable bearing pressures, overturning and sliding.

Second, wall facing batter has no effect on the applied pressure. The Code you should be using is the AREMA code NOT the CANFEM. Last I checked, CANFEM is not a Code but a manual of Canadian practice. AREMA has the correct formulas for horizontal pressures from strip loading on flexible walls as well, the pdf you refer to shows point load only.

As you probably know, make sure that you are accounting for the upward sloping fill above the top of the wall. Ka values are notably high for this case. You must also account for earthquake as well using AREMA, even though this is in Canada, as CN and CP require this.

 

[MSEMan]

It would be almost criminal to ignore the load for overall stability considerations. Your wall is a very small part of an overall embankment condition and the Cooper E80 loading needs to be part of the analysis of that overall condition. I agree with others that for internal stress (sigma v) use a Boussinesq distribution and superimpose the surcharge stress on the normal stress at the locus of maximum tension. You don't need a program to do this.

I also worry that your grid length will be inadequate. Normal practice is 2.4m min although this can be reduced if small comapction plant is used. In AASHTO the minimum Le, (i.e. length in the resistant zone) should be 1m. From my experience, with such a large surcharge slope, relative to the very short wall height, I'd expect to be seeing L to H ratios of more like 1.2 or greater.

You will also want to specify very good fill, consider sub base or AASHTO #57 stone (sub base is probably better since the extra weight will help with global stability.)

Personally, I'm not 100% comfortable with NCMA, I think AASHTO LRFD produces a more reliabe design.