Retaining Wall Lateral Load from Soils

[hardyworld]

I have limited experience/knowledge with geotechnical engineering, but here is a problem that I do not know how to approach to find the answer. I hope your collective wisdom can help:

I have a 10'-0" deep retaining wall designed in a natural clay soil with a K0=1. To reduce lateral loads applied to the retaining wall (reducing rebar requirements) I proposed that the area adjacent to the structure be filled with granular fill (CA-6 by Illinois DOT standards, I'm assuming K0=.27). The water table is far below the wall, so we are just assuming saturated soils for worst case conditions (gamma = 120 pcf for both materials). My question is how do you calculate the lateral soil loads applied on the wall? The lateral pressure applied by the clay soil onto the granular fill is easy to calculate. The lateral pressure applied by the granular fill on the wall is easy to calculate. But how the lateral load applied by the clay soil is then applied through the granular fill onto the wall is stumping me. The influence of the clay lateral loads will be dependent on the proximity to the wall (how wide the area of granular fill is), but I do not know how to account for this factor.

The way I am viewing the influence factor for the lateral clay loads applied to the wall, it has 2 major components acting through the granular fill between them:
1) Influence Stress Factor based on the proximity of the clay soils to the retaining wall (how wide the layer of granular fill is between them).
2) Unconfined factor due to the top of the granular fill being free to heave upward due to the lateral pressures applied by the clay.

I'm not sure how to account for both of these factors and it will have a large impact on the quantity of granular fill recommended for use on the project (5' wide x 10' tall granular fill section all the way around the structure VS. a 20' wide x 10' tall granular fill section all the way around the structure).

Thank you for any help you can provide!

 

[Doctormo]

What does the retaining wall look like in cross section? A typical CIP concrete cantilever wall? Usually you check the stem for the granular backfill pressure and the heel for the clay pressure if a simple cantilever wall. The granular fill will not heave upwards as passive resistance is many times active pressure.

 

[fattdad]

if you are considering the clay to be frictionless (i.e., just cohesion), you'd have an active wedge at 45 degrees. If the wall is 10-ft high, that'd mean you'd want the grandular fill to be 10 ft back from the wall. Some may add a couple of feet for good measure.

The at-rest earth pressure for a granular material would be 1-sin(phi) and I'd bet the friction angle is about 36 degrees. If it's a crushed rock, maybe even 42 degrees. Let's use 36, that'd return a Ko of 0.41.

Your problem statement is a bit confusion, so, I'll just take it for what you said. . . For saturated granduar soil with a saturated density of 120 pcf, you'd then subtract out the unit weight of water (64.4) and get a bouyant unit weight of 57.6 pcf. That's the value that you'd relate to at-rest earth pressure. That'd return a horizontal soil force, based on an equlvalent fluid density of 23.6. Add back in the unit weight of water and you'd have 86 pcf.

Now if there is never going to be a phreatic surface above the toe of the wall, you'd just take your moist unit weight (i.e., 120 pcf) and multiply that by the 0.41 and have 49.2 pcf.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

I assume you are making a face cut in the clayey material and backfilling the space between the new wall and the existing soil with granular material. If this is the case, the thickness of the granular material is too small to be of significance, so I would neglect the granular material and consider direct lateral load transfer from the clayey layers.

 

[hardyworld]

Thank you for your assistance, Geotechnical experts. It has helped push me in the right direction.

I was conservatively considering the existing clay soil as cohesionless, to find my lateral stresses applied through the granular backfill onto my concrete retaining wall. After consulting some stress tables based on triangular loading condition and looking at how the backfilling around the structure will be done, we believe that taking into account the cohesion of the existing soil (700 psf) is probably the correct application, especially if the granular fill shall be backfilled at a 1:1 slope against the structure. When considering the cohesion, the loads from the clay soil largely are eliminated, leaving only the granular backfill's lateral loads onto the wall. So the resultant active fluid pressure is approximately 50 pcf.

Thank you!