Retaining Wall Reinforcement

[bhiggins]

Hi everybody,

I frequently get asked to evaluate older retaining walls, and I'm trying to develop some typical cost-effective reinforcement details for walls deemed inadequate. For context, I'm currently assessing a 4-foot-tall reinforced concrete cantilever retaining wall that's approximately 30-40 years old. Although I haven't yet inspected the footing condition or confirmed the presence of a keyway, let's assume for discussion that the existing wall is undersized for sliding or overturning stability.

Normally, I'd specify concrete repairs or enlarging the footing, but this could become invasive and expensive. I'm therefore exploring simpler alternatives—specifically, reinforcing the backslope with layers of geosynthetic fabric or something similar to reduce lateral earth pressures on the wall.

While I'm experienced with conventional retaining wall design, mechanically stabilized earth (MSE) systems are new to me. Here's a sketch of my initial concept—could anyone comment on this general approach? Specifically, if the backfill layers behind the wall are fully wrapped in geosynthetic fabric, would this effectively reduce lateral pressure on the wall to near zero, assuming adequate frictional resistance and wrapping integrity?

My understanding is this approach should significantly address sliding and overturning concerns, provided global stability is acceptable. Could someone confirm or share insights on this assumption? Additionally, are there recommended resources or references you'd suggest for calculating loads and verifying this configuration?

Thanks in advance for any input or guidance!

 

[Greenalleycat]

I have no experience in that myself, but here is a paper I came across once that may be relevant

https://www.sciencedirect.com/science/article/abs/pii/S026611440400024X

 

[bhiggins]

After further consideration, I expect that fabric-wrapped backfill would be analyzed similarly to a stacked gravity retaining wall (such as a gabion wall or stacked limestone block wall), where sliding and overturning stability need to be evaluated for each individual unit. However, I'm uncertain about the appropriate analytical approach for a "C"-shaped wrapped backfill configuration.

I've actually read that paper Greenalleycat when I had to design a retaining wall adjacent to a vertical limestone excavation. I don't think it's applicable here because there are no physical restrictions behind the wall.

 

[LOTE]

The one thing I want to point out is an MSE wall is a flexible system, and the parameters for the geogrid are based on 3/4" deflection in the face before the geogrid is fully engaged. You may consider a compressible material between the concrete wall and MSE to combat this, or find the geogrid test data and use the values at 0 deflection.

For references, I recommend reviewing the NCMA manual and/or FHWA GEC-011. The FHWA document has information on wrapped face walls. There are several programs for MSE walls. I use Vespa by CTiware, and you can pay for a month at a time if these are infrequent. It has wrapped face as an option.

 

[jhnblgr]

What about backfilling with lightweight flow able fill?

 

[XR250]

What about backfilling with lightweight flow able fill?

The wall would likely fail from the hydrostatic pressure unless it could be temporarily braced.

 

[Harbringer80]

AASHTO has a very thorough section on the design of MSE walls that will take you through all the necessary checks. Curious why are you evaluating 40 year old retaining walls that have presumably not failed?