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at rest lateral earth pressure with sloped backfillHelpful Member!(3) 

jrfroe (Structural)
16 Feb 07 16:39
I'm designing an addition to a concrete wastewater tank that was built in 1999 and I have the original soils report from 99, where it reccomends using equivalent fluid pressures of 65 pcf above the water table and 95 pcf below (at rest).  The tank is 17' deep of which 16' is buried.  Problem is, the addition will be built next to a 1:3 slope.  I can find all kinds of formulas for the effect of a backslope on the active and passive pressures, but every reference I find is silent on the at-rest pressures with a backslope.  The soil on site is mostly silty clay, and the wall will be backfilled with well graded granular soil.  What effect does a backslope have on at-rest lateral earth pressure?

Thanks,
Jason
jdonville (Geotechnical)
16 Feb 07 17:14
jrfroe,

From NAVFAC DM-7.2 (as background):
-----
If a wall is prevented from even slight movement, then the earth remains at or near the value of at-rest conditions. The coefficient of earth pressure at-rest, Ko, for normally consolidated cohesive or granular soils is approximately:

Ko, = 1-sin [theta]'

where: [theta]' = effective friction angle

Thus for [theta]' = 30 degrees, Ko, = 0.5.

For over-consolidated soils and compacted soils the range of Ko, may be on the order of 1.0. In cohesionless soils, full at-rest pressure will occur only with the most rigidly supported wall. In highly plastic clays, soil may creep, and if wall movement is prevented, at-rest conditions may redevelop even after active pressures are established.

Basement and Other Below Grade Walls. Pressure on walls below grade may be computed based on restraining conditions that prevail, type of backfill, and the amount of compaction.

EFFECT OF CONSTRUCTION PROCEDURES.

Staged Construction. As earth pressures are influenced by wall movement, it is important to consider each stage of construction, especially with regard to brace placement and its effects.

Compaction. Compaction of backfill in a confined wedge behind the wall tends to increase horizontal pressures beyond those represented by active or at-rest values. For guidance on horizontal pressure computations associated with the compaction of granular soil, see Figure 13 (after
Reference 7, Retaining Wall Performance During Backfilling, by Ingold).

Clays and other fine-grained soils, as well as granular soils, with considerable amount of clay and silt (>/=15%) are not normally used as backfill material. Where they must be used, the earth pressure should be calculated on the basis of "at-rest" conditions or higher pressure with due
consideration to potential poor drainage conditions, swelling, and frost action.

-----

You should contact the geotech of record for the original soils report and ask them to provide specific guidance based on your specific situation. You may end up paying for their review, but the peace of mind could be worth it.

Jeff
Helpful Member!  fattdad (Geotechnical)
17 Feb 07 17:30
Calculate the active earth pressure for the sloping condition that you have and multiply it by 1.5 to equate to the at-rest stresses.  This was an assignment that I had in graduate school (Virginia Tech, Professor Duncan) and that's how I handled it.  As I recall, it was accepted by my professor.  That's not to say there are not other methods.

f-d

¡papá gordo ain’t no madre flaca!

Helpful Member!  BigH (Geotechnical)
18 Feb 07 9:08
Try doing a search on the subject on the forum in this site. As a starter see thread255-169478
cheers
dirtsqueezer (Geotechnical)
18 Feb 07 18:33

Aside from the numbers, it might also be a good idea to look for some surface clues in the area- drunken trees, soil over the roadway, cracked retaining walls, ect.  It might give you just a diagnostic idea of how things hold up.  
Helpful Member!  Panars (Geotechnical)
21 Feb 07 10:49
In the US Army Corps Manual on Retaining and Flood Walls
EM 1110-2-2502
http://www.usace.army.mil/publications/eng-manuals/em1110-2-2502/toc.htm

you can find two methods to calculate the at-rest earth pressure for sloping backfill.  The first is
Ko=(1-sin phi)(1-sin beta)  where beta is the backfill slope angle.

The second method is to use Coulomb's active earth pressure equation, but with a reduced friction angle.  The reduction (called the Strength Mobilization Factor, SMF) is
SMF=(tan phi')/(tan phi)=2/3
where phi is the actual soil friction angle and phi' is the reduced soil friction angle.

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