Is it wise to use Geogrid and Soil-Cement?
Is it wise to use Geogrid and Soil-Cement?
(OP)
I have a project where the owner is proposing to use a soil-cement mix in the reinforced zone of a bridge abutment. The reinforcement options are HDPE, PET or hot dip galvanized strips. I have been advised by a local distributor of the PET geogrids that the cement alkalinity will damage the PET geogrid, but what can you say about the HDPE and the soil-cement? The proposed soil is a granular type A-1-a or A-1-b and the bridge is for a 6 lane road. It has three spans of 40 ft. The reinforced walls will receive the load of the abutment, with no piles or structural footings.





RE: Is it wise to use Geogrid and Soil-Cement?
RE: Is it wise to use Geogrid and Soil-Cement?
MSEDesigns
RE: Is it wise to use Geogrid and Soil-Cement?
Why do they want to do cement mixed soil behind a reinforced wall? Seems sort of redundant without knowing the details.
RE: Is it wise to use Geogrid and Soil-Cement?
RE: Is it wise to use Geogrid and Soil-Cement?
RE: Is it wise to use Geogrid and Soil-Cement?
RE: Is it wise to use Geogrid and Soil-Cement?
Answering some questions, I believe the reasoning behind the use of soil-cement was to increase the friction angle and reduce the need of grid. Other than this I dont see any reason.
About the concentration of stress in the connection, I have some doubts. Why we talk about soil-cement as if it is not going to move at all. This is soil-cement, not lean concrete as states CVG above. The soil will move less, but it will move. Anyway, if the soil does not move as GARRETTK mentions, thus not developing the grids strength, there should not be that much load on the blocks either, since the soil-cement is not "moving". What do you think about this?
When we began producing segmental blocks in 1993 we built a 23 ft ht. wall with PET reinforcement and A-1-a and used a 3% cement to soil mix by vol and the wall has performed excellent all this years. Blocks were Keystone Standard with Mirafi grids. Reasoning behind this decision for that wall was the geotech was afraid a nearby pool could leak and cause some damage to the wall. With this he reduced the effect the small amount of water would have in the select backfill material.
Again, thank you all for your input. Let me know your thoughts on my statements above.
Thanks,
MSEDesigns
RE: Is it wise to use Geogrid and Soil-Cement?
One reason to use the soil cement next to the pool might might have been to provide a fill that would be relatively impermeable, therefor would not become saturated if the pool leaks.
RE: Is it wise to use Geogrid and Soil-Cement?
But lets say it was cast into a massive block of concrete. Would it need to move? No. It would develop its strength very early on. There may be some movement right at the interface, but the strength of the material it is in would reduce the development length to get there, concentrating the load near the face of the concrete. The material would be able to resist up to its maximum capacity, and then break, just like at the connection to the block (assuming the connection str is high enough). So what would the downside be? Well, if the grid was locked in so tightly, then it is likely that the length of “loose” grid between the block and the treated soil could vary, and lead to the wall face being able to deflect differing amounts. This could result on more concentrated loads on some of the grids. As MSEdesigns said though, this is not concrete, it would be soil cement. It will still act like soil . . . an improved soil at that.
I think that is the cement is sufficient to hold the soil, the block is merely there as a facing, and the grids are shortened to just hold the facing in place, then good. If the cement does not improve the soil to that degree, and just improves the internal stability, and the grid length is determined by the global stability, then you need to look at if you are actually saving anything (i.e. can you remove some grid, go with a cheaper grid, etc.). And in the second case, is the cost of adding the cement (and all that goes with it) worth changing up the crews normal patterns during construction? Probably not. I think it gives more variables to monitor, and would slow construction, resulting in a less reliable system and more costly construction.
RE: Is it wise to use Geogrid and Soil-Cement?
RE: Is it wise to use Geogrid and Soil-Cement?
The issue regarding mobilisation of reinforcement strength is strain related. Because the ultimate strength of a grid is only mobilised at 10% strain or more, at very low strains very little strength is mobilised. If using cement stabilised fill, you may want to adjust your "at failure" analysis for a lower soil strain (i.e. if you consider that soil failure occurs at 1.5-2% strain consider the tensile strength mobilised at that strain level.) Obviously steel reinforcement (either strips or grids) is fully mobilised at very low strain so no adjustment to the analysis is necessary (although alkalinity is more agressive towards the galvanising its less agressive towards the steel.)
Hope this helps.