Lateral Pressure on Retaining Walls from Gravel Backfill 1

[cap4000]

I have a great book by M.and A. Reimbert titled "Retaining Walls" that indicates the active pressure on a wall from a purely a granular or gravelly material is based on its angle of repose. Essentially the lateral pressures are considerably reduced when using a crushed stone for backfill in lieu of a mixed soil. The book is filled with nice experiments and totally disputes Rankines active pressure formula. Does anyone have any tips on this gravel issue. Thanks.

 

[motorbiketocrank]

Sure the active, passive and at rest pressures for a granular material are based upon the friction angle. The higher the friction angle, the lower the active earth pressure coefficient. But this doesn't dispute rankine theory, it is rankine theory.

 

[singingk]

If fill (also gravel) is compacted behind a retaining wall, additional earth pressure resulting from the compaction shall be considered in geotechnical analysis. Have a look for the respective approaches.

 

[oldestguy]

Here is a tip.

Having measured pressure against high structure (not a wall capable of movement), I found that compacting earth (sand) near the wall will impose much higher pressures than those which the Rankine theory would predict.

In order to get those "at rest" or "active" pressures there has to develop some shearing effect in the backfill, with the wall moving some. In my case the wall didn't move.

The situation was resolved by requiring compactors to stay at about 2 feet away from the wall leaving a loose zone next to the wall. That was, in effect, a cushion that would give some and allow the active condition to develop in the compacted fill nearby. That loose zone did not settle, since it then hangs up on the wall on one side and the compacted fill on the other side.

 

[msucog]

we recommend using small mechanical hand tamps behind walls for a few feet unless the wall is designed to withstand heavy compaction equipment near the front of it. we have seen issues with certain wall types where the backfill material near the wall face were left relatively loose and settled over time...and eventually caused additional issues that snowballed in to signficant problems. i personally don't suggest that any loosely placed material be left in place. i have also seen instances where the area right behind the wall (that was not well compacted) became saturated...this would seem to me to be a much bigger issue than having to carefully compact the material or design for the necessary compaction effort. if using free draining aggregate then the area is kept from becoming saturated but densification is still necessary to avoid long term settlement related issues.

 

[BigH]

I would never run a heavy roller against the back of a retaining wall - we always recommend to stay 5 ft (1.5m) away with smaller equipment up agasint the wall. Basically the lateral earth pressures are higher with higher compaction (and more so with heavier equipment) is due to residual horizontal stresses in the material. The compaction is leaving the soil "overconsolidated" and the lateral stresses are left from the "removed" vertical pressures. Suggest you read Section 6.5 of Fang's Foundation Engineering Handbook (2nd Edition) (original was Winterkorn and Fang). This has a nice explanation of effects on earth pressures due to compaction and shows ways to compute. Also, Terzaghi Peck and Mesri discusses it to a degree in their book. The residual stresses are very important for non-yielding walls. T&PM indicate that with only a very little movement of the wall, the residual stresses will dissipate and approach the active values.

 

[MSEMan]

The engineering issue with compaction was developed in a paper by Ingold, very often referenced. When you run compaction plant against a wall, you lock in a portion of the compaction stress since the wall is stiffer in the partially backfilled state then it is in the theoretical completed state. Great paper google it.