Lateral eath pressure in compacted soil.

[Espen]

Hi. Looking for some information on how to calculate the added horizontal earth pressure from compacting the soil. All literature suggest an increase in stress, but non suggest how to calculate it. Installing a load-cell on a buried wall and experimenting with different level of compaction, soil types and water content would give a useful chart.

 

[LRJ]

The below is the approach I'd take. However, you should check the various equations, assumptions, etc. are appropriate.

There is a relationship between K₀ and OCR:

K_0,OC = K_0,NC.OCR^sinφ' (can't remember the reference - please check)

Where:
K_0,OC = K₀ corrected for overconsolidation
K_0,NC = K₀ for normally consolidated soil = 1 - sinφ' (Jaky, 1944?)
OCR = overconsolidation ratio = (σ'_v0 + Δσ'_v)/σ'_v0
σ'_v0 = initial vertical effective stress
Δσ'_v = additional vertical stress

With an increase in OCR (due to additional vertical stress) you get your change in K₀. Using K0 = σ'_h/σ'_v you'll be able to get your new σ'_h (which is the radial effective stress).

 

[oldestguy]

Hey, one has to be very careful doing any compaction behind a wall. I usually specify that no com0pactior should be within 2 feet of the wall and probably it should even be farther for large compactors. Then the earth pressure used is at rest. I've seen many a wall tipped severely due to compacting alongside it. If in doubt install earth pressure cells on the wall before the job goes and monitor the results as the work goes on. If pressures increase more than the design figure back off the compactors for the rest of the job. Be aware that even though you think you are compacting a layer, the effect goes way down. My measurements show an effect 8 feet below the compactor in one case.

 

[ATSE]

Both Terzaghi, Peck, Mesri text (chapter 8, article 45) and NAVFAC 7.2 show lateral pressures during backfill vs "rigid" walls.

 

[MotorCity]

Here are two things to consider. First, when material is compacted, the unit weight will naturally increase. Use this increased unit weight in your calculation of the lateral soil pressure. Second, we always design retaining walls for a construction load of 2' of additional fill or roughly 300 psf surcharge. This load will easily cover the surcharge of most equipment used for compaction.

 

[BigH]

From somewhere in the grey cell memory, the "at rest" coefficient for compacted fill is about 0.7 and not 0.5 . . .

Oldestguy is correct in compactors against retaining walls. We always specified no closer than 5 ft for a 10 to 15 vibratory compactor. Use of small 1-tonne tag-alongs from this point towards the wall.

I've seen mortared masonry (stone) walls crack along the horizontal joints when the Chinese contractor didn't respect the distance caveat.

 

[Espen]

In some cases, we want as high as possible lateral soil pressure. Exemplified with a pile supported raft foundation with no basement. Lateral capasity is increased with the use of «structural skirts» along the outer edge.

The discussion at office has been whether the effect of compaction with give during the design lifespan.

 

[oldestguy]

My "excuse for not compacting against the wall is that the uncompacted material gets a little effect from the compaction work beyond, but the column of uncompacted does not settle due to the "silo effect", the fact material in silos tends to hang up on the sides. To give a little cheating on this, I say you can do a little compacting of the surface layer, but don't over do it. Results show no problem in my experience.

 

[oldestguy]

I dug out a plot of wall pressure versus height of fill on a job where there was 28 feet or so placed against a rigid wall. Soil type is fine to medium sand with compacted density perhaps around 105 pcf. All work until changed was compactors stayed 2 feet from wall. However note on the plot the equivalent fluid pressures of 36 to 75. The plots are height above the measuring cells. What is of significance is the fact these pressure cells could detect increases in pressure with filling going far above them and could detect the change of compacting distance away from wall (increased to 3 feet) at about 17 feet height for at least two of them. As I recall the 36 #/cf equivalent fluid pressure was active.