Lateral pressure

[COEngineeer]

I am just wondering, what test do you guys use to get the active and at rest lateral pressure? Everytime we get a soil report from this company, we always get high numbers. We are starting to think it is the company, not the soil. We just want to do a little research before we question her numbers. Everytime we get a soil report from them, we get 60-70 pcf lateral pressure. I've done soil tests before when I was an intern but I dont remember having to calculate lateral pressue.

 

[JAE]

Depends on whether its active or at rest pressure. The 60-70 range sounds about right for at rest. Active is usually in the 40 to 50 pcf range

 

[COEngineeer]

JAE, it is at rest. 70 is a bit much dont you think? We need to start designing houses like a dam? I am just curious what test geotechs do to get this number.

 

[SlideRuleEra]

"Soil Mechanics - Basic Concepts & Engineering Applications 2001"
Complete Text Online (Free)
See Chapter 8

http://books.google.com/books?id=Qf...juTQB&sig=iXrHOXq57-AWjvWm2OPTJF5N6Yw#PPP1,M1

http://www.SlideRuleEra.net

 

[VAD]

I am not sure what your communication with the geotech was regarding the requirements of the soils report. Very often if situations are not properly defined you will get some value which the geotech feels covers uncertainties.

It is therefore important to discuss with the geotech about the project and design concepts to get a more refined value.

Very often referrals are "drill some holes and provide a soils report".

I would consider the numbers given to be preliminary which could be refined when details of project re structural design concepts are provided. This assumes that such has not been done before.

SlideRuleEra has provided info on how the pressures are determined generally. However, note that other factors that may be dictated by construction, site conditions after construction etc may have an influence on the numbers.

I would recommend speaking to the Geotech about the information provided after reading the info re SideRule Era.

Testing to determine phi values would require knowledge of material to be used as backfill and is normally not done for routine projects. Codes tend to provide lateral pressures that are applicable for certain structures etc, but here again be sure that these do not have to be modified for site conditions.

 

[COEngineeer]

I usually get 45 active and 55 at rest but everything I get soil report from this companya we get 65+.

 

[jike]

I have called the geotech before on high values. I have told them that the other geotechs aren't giving me values that high. I also told him to put any qualifications on the values that he feels are appropriate.

 

[oldestguy]

It all depends on how the backfill is placed. What do you spec for backfill placement? Do they just dump it in? Are you specing a degree of compaction? Where do they compact with respect to the wall. If you build the fill up in compacted layers, say specing 90 percent of Modified Proctor, and the compactors run next to the wall, you may get pressures even well above those that the "higher" guy gives you.

I had the opportunity once to measure wall pressures on a wall 40 feet tall at least, backfilled with bank run sand. The wall was part of a building with no opportunity to deflect under load. We asked the designer to use an equivalent fluid of 35 p/cf. As compactors worked on this sand, in layers, the pressure sensors showed pressures going up much higher than this 35.

I also noted that when a heavy duty hand guided plate compactor was working next to the wall, the sensor some 12 feet lower showed a slight increase with each pass of that compactor.

So, what should we do?

It was obious the compaction effect was causing a wedging of that soil next to the wall.

First we changed the compaction location to outside an 18 inch limit, then going to 24 inches as the closest compaction work. Then,the design pressure came out on subsequent wall backfills. Keep the compactors away from the wall face by a distance sufficient to allow some looser zones next to the wall.

No problem with settlement, since that vertical uncompacted zone acts as a cushion and soil hangs up on both the wall and the compacted earth, the "silo effect".

Maybe your "high numbered guy" did some measuring as I did and the backfill was compacted right next to walls and that is his reasoning. Remember, to achieve active pressure, that wall has to move or the soil has to compress some.

The final result depends on YOUR SPECIFICATION.

Many a wall has deflected excessively due to heavy backfill compaction.

 

[GeoEngineerOK]

Usually the backfill is granular material and is well draining to eliminate any hydrostatic pressure. But for native soils usually I have performed unconsolidated-undrained triaxial tests to obtain cohesion and a friction angle for cohesive soils and a direct shear test (consolidated-drained) for cohesionless sandy soils to obtain a friction angle since all coefficients are dependent upon friction angle. At rest ko=1-sin(phi) and active is ka=tan^2[45-(phi)/2] and passive kp=1/ka=tan^2[45+(phi)/2]. The equivalent fluid pressure is calculated by using the cofficient, ko, ka, or kp, multiplied by the unit weight of the soil in dry conditions which is most typical here in Oklahoma.

 

[COEngineeer]

What is a good way to predict phi w/o test? Just say we know the gradation and the unit weight.

 

[GeoEngineerOK]

The only way to estimate a phi anlge is from experience in the area in question. Most geotechnical text books give ranges of phi angles based on the material and the kind of deposit it is (i.e., terrace, residual, or alluvial).