Lateral Pressure Coefficient k0 for compacted soils

[mchen96]

Hi,

What is the recommended method to estimate the lateral pressure coefficient k₀ for compacted soils (for a culvert design)?

I am aware of the k₀=1-sin(ϕ)×OCR^sin(ϕ) equation, but I'm not really sure how one would estimate the over consolidation ratio for a compacted soil. Is there a specified proctor compaction percentage to OCR relationship? Alternatively, are there any good rules of thumb (preferably with a reference).

So far, the best I've found is the commentary on AASHTO LRFD 3.11.5.2, which states that the lateral pressure coefficient for lightly overconsolidated sands is somewhere between 0.4-0.6 and, for heavily overconsolidated soils, the k₀ may be around 1.0. No idea where a compacted soil would fit on that scale.

For the record, for my particular application a higher k₀ is preferable, so I'm trying to identify what's the maximum k₀ I can reliably obtain if the fill is compacted essentially against the wall.

 

[oldestguy]

When you see the leaning wall that supports a fill which was compacted by a roller running alongside it, the K value is pretty darn high, certainly not 1.0. Successive passes on the fills sequential layers add to the pressure on those below. I measured increases in wall pressure 8 feet below as a roller passed along above.

 

[oldestguy]

Here is a copy of the data for a clean sand backfill and fill height was up to 30 feet. By this you can see the equivalent fluid for compactors at the wall was 75 pcf. That's less than 1.0. Also shown is the effect of moving the compactors out at last 3 feet from the wall. Wall was part of a parking garage and assumed to stay and not move.

That pressure scale is equivalent fluid unit weight.

 

[oldestguy]

As to culverts and lateral pressure that value (such as 1.0) hasn't come up in my experience. However, as to vertical load and pressure, we usually call that the critical load situation. Allowing for effect of trench side effects and the spreading of that vertical load to the sides of trench, the vertical pressure usually is taken as not more than about 3 times the trench width as vertical fill height. It all depends on the details of the job. Of course for shallow fill over the pipe, surface loads also need to be considered. Test loads on pipe are vertical usually.
Edit add: I should add another situation, rather than a tench. That is an area fill, as a highway embankment. In that case there could be loads from the embankment to the surface. Then the lateral pressure might be simiar to the vertical under some circumstances. Should this situation somehow be providing too high a load on the pipe, consider the "imperfect trench" method of keeping that load tolerable. A Google search might not bring up the method, since it was developed in Iowa maybe 80 years or more back. Spangler was the engineer.

 

[ATSE]

See two references for effective lateral pressures on existing (newly placed) walls on backfill operations:
1. Terzaghi, Peck & Mesri. Page 344.
2. NAVFAC 7.2-77.
The wall can "feel" near passive pressures.
You can definitely lean a wall over during backfilling. As well as crack a new concrete wall (I've seen it).
Use thinner lifts and smaller equipment near the wall face.