lateral deformation of reinforced soil wall

[AK92]

I'm designing a near-vertical temporary reinforced soil wall, to be designed to settle by using prefabricated vertical drains. The temporary FOS against slip surface failure is designed to be around 1.3. Using finite element analysis, the lateral displacements at the wall toe are about 40% of my expected vertical settlement. Is that considered reasonable? If that is too much, what options do I have to minimize the lateral displacements?

 

[RFreund]

Just curious, what software are you using?

EIT

http://www.HowToEngineer.com

 

[AK92]

@RFreund

I'm currently using Plaxis for the modeling. I modeled the soil within the settling area (where drains are installed) as "drained" using the Soft Soil Model and the soil outside the area as undrained, using the Hardening Soil Model.

However, modeling is one thing, and what happens in reality is another thing. I am trying to find literature for justification of the predicted lateral movements but it is quite scarce. Many of those have experience for lateral displacements for slopes, but not for temporary reinforced soil walls. Also most of them don't have data for the impact beyond the slope.

I'm quite sure many people around these forums have done similar projects and would like to know what's their experience.

 

[EireChch]

What is reinforced soil wall made up of? Is it cohesive or non cohesive. What soils parameters are you using?

I assume it is a sandy tye soil, if wick drains are being used?

To answer your question, or try to, maybe you could look at including additional layers of tri axial geogrid or maybe cement stabilising the outer 3-4m when compacting.

Also how much displacement are we talking? Vertical and horizontal.

 

[BigH]

I've been involved in RE wall construction which, using wick drains, was done in staged loading - some 1100 mm of settlement during construction but don't remember seeing any notable horizontal deflection of the wall. Not sure it was measured, but it sure wasn't noticeable.

 

[Doctormo]

AK92 - It would be helpful to know what conditions you are modeling. Height, reinforcement length, soil properties, toe or back slopes, embedment, etc.

Lateral deformation can be a function of the base to height ratio and various external stability considerations. Most MSE walls I have observed that experience intended settlement do not displace laterally very much. However, I have seen lateral displacement happen with unintended settlement (the design did not expect a lot of settlement) where the wall will move laterally in conjunction with vertical movement. A wall built on a marginal fill slope comes to mind where there is less confining pressure and density on the slope side. Not sure how one would predict this ahead of time if you did not expect it.

There is a resultant force on the foundation from a reinforced wall with shorter reinforcement that has a X and Y component to it which could predict some lateral movement. Once a wall starts moving, the forces have to reconcile themselves in some manner. However, when major settlement is expected, reinforcement lengths tend to be longer to satisfy stability considerations thus that concern is minimized.

 

[AK92]

The reason why I'm concerned with lateral displacements is that there are adjacent properties to the site and I need to be able to roughly assess the impact on these properties (to avoid cracking other people's houses). The temporary wall heights will range from 4m-10m and we might need to offset the temporary wall and ground improvement some distance from those houses.

@EireChch and DoctorMo
The fill is compacted low plasticity cohesive fill, however we will combine nonwoven geotextile (to provide drainage and filtration) with geogrids (for strength) to build the temporary retaining wall. I'm using c'= 1 kPa and phi = 27 degrees for the compacted cohesive fill. Reinforcement lengths are about 0.8-0.9 times H. There will be 4 long geogrids (<10m) to provide sufficient FOS so that the staged construction can happen faster.

The RE wall is founded on level, soft clays and it is designed to settle with wick drains installed with surcharging to remove residual settlements. I will be expecting reasonably large settlements (0.5m to 1.0m). From the FEM analysis using Plaxis I'm expecting lateral deformations anywhere from 100mm to 400mm (depending on what parameters I use for the soft soils) for a temporary RE wall height of about 7-8m.

@BigH
Thanks for the input. Would like to know if there was any substantial heaving and lateral deformations beyond the edges of the wall?

 

[BigH]

AK92 - can't tell you . . . nothing was measured - this was India, remember - but I would imagine there was some. However, our walls remained vertical. We had 6 m of very soft clay overlying firm clay. PVDs were installed until N=8 (as observed). We used sand for the fill. Are you planning to have any measures in your fill, since it is clayey silt, to remove any induced porewater pressures during as it is raised? . . . and don't forget you will need a layer of sand at ground surface - before filling so that the water (porewater pressures) that is draining due to the PVD will have a way to escape . . .

 

[AK92]

@BigH

We will have non woven geotextiles for drainage of induced porewater pressures. We also have a sand blanket at the ground surface to drain out all the water.

At least in the FEM analysis, the walls remained vertical however there was still quite some horizontal deformation of the adjacent soils (along with some heaving)