Geogrid installation questions 1

[glacialwhat]

Proposed benched/ terraced segmental wall.

1. Is there any geo/struct concerns with geogrid reinf from one wall extending laterally below the next benched wall and so on to meet its design length. The intent is to minimize the bench spacing thereby minimizing loss of backyard.

2. I have never seen this and will likely reject it but need basis: can a grid have a batter to it, and if so what are the limits. I expect it needs to be placed/taut in a flat position throughout.

 

[GeoPaveTraffic]

There is nothing wrong with having the upper wall over the grid of a lower wall, however, many wall manufacturers do not recommend installing benched or terraced walls due to settlement problems with the upper walls.

With respect to your second question, the grids should always be installed horizontally.

 

[Boonie]

The FHWA design manual deals with the design and construction of tiered walls. Tiered wall designs will more likely than not, result in the upper wall bearing directly on the reinforced zone of the lower wall. Something to keep in mind, internal and external stability calculations for the lower wall will not generate adequate grid lengths to meet the global stability requirements unless the upper wall is more than 2H away. Global stability will almost certainly drive the design. Depending on the application (proximity of the building, anticipated settlement) select fill may be prudent to reduce settlement in the upper tier as stated by GPT.

 

[TDAA]

1. As Boonie stated, if the walls are within 2H, the global will likely govern. This should be modeled for internal stability by making one wall the height of the combined piers. Once the grid lengths are determined there, check the global with the proposed profile. I agree with the above that settlement could be an issue, but if your walls are not of great height, it will likely be minimal, and within the tollerance of a stable wall. Remember good CQC/CQA is essential to construction of a good wall, and proper compaction will help to reduce settlement.

2. I thought I had seen someone referring to sloped grids on here and saying it was OK. Might be wrong on that. If anything, they should not slope up from behind the wall face. As for a basis to reject it: Ask to see the calcs performed for internal stability using a slope on the grids. Most, if not all, computer programs for MSE walls model the grids horizontally, as they are typically installed. Unless they can show numerically that it works, reject it.

 

[MSEMan]

To be preicise, the offset distance for wall tiers to be designed independently (i.e. no influence) is tan (90-phi)which for a fill of 32 degrees is 1.6X. Where there is influence but the walls are offset more than H/20, then for preliminary purposes the lower tier should be detailed with a reinforcement length of 0.6H.

If there are a lot of tiers, and none are of great height, the design can be analysed as if it were a steep slope with a face angle represented by the tier height and offset distance.

I would very much disagree with the idea of modelling the structure as single height vertical wall for global stability purposes. First of all the locus of maximum tension will be in the wrong place and you may well overstimate resistance to pull out. A better method is to model the vertical stresses as you would for any embankment structure with sigma zero falling along the tan phi line and sigma max falling along the 45 degrees + phi/2 line. Once you know the vertical stress distribution working out tmax layer by layer is the same as a normal MSE retaining wall.

Finally, with respect to not allowing reinforcements to be downwards sloping - providing the down slope is modest (say 5-10%) and L is a section length rather than a true length,there is no reason to reject it. Actually it is quite a good idea to provide a slight bacakfall to the filling operation since it helps drain rainwater away from the facing during construction. And - by the way - if you wouldn't reject it for soil nails why would you reject it for grid soil reinforcement?

 

[TDAA]

MSEMAN,

I agree: "I would very much disagree with the idea of modelling the structure as single height vertical wall for global stability purposes."

I said: ". . . internal stability by making one wall the height of the combined piers. Once the grid lengths are determined there, check the global with the proposed profile."

 

[wallguy]

The advice on design with respect to height and distance of tiers are correct. The success of the structure then will be on achieving the compaction on the lower walls. I might highly suggest if this is a residential structure with your stamp on it that you request compaction testing be done. Too many "backyard" projects even by contractors miss meeting the compaction requirement because no one is looking! This will be especially critical (Gobal mentioned above) if the tiers are sitting on a slope when completed.