Coefficient of active earth pressure for clay vs sandy clay
Coefficient of active earth pressure for clay vs sandy clay
(OP)
Hi,
I'm trying to calculate active earth pressure on a retaining wall. One thing is bugging me; it makes sense to me that a more cohesive soil will 'hold itself together' better than a less cohesive soil (i.e. a clay vs a sandy clay) and will therefore apply a lower lateral load to a retaining structure. Using a table for approximate soil properties I am getting a higher angle of effective internal shear resistance for a sandy clay, which in turn leads to a lower coefficient of active earth pressure and lower lateral force. Could someone explain why this is the case?
Thanks,
Bradley
I'm trying to calculate active earth pressure on a retaining wall. One thing is bugging me; it makes sense to me that a more cohesive soil will 'hold itself together' better than a less cohesive soil (i.e. a clay vs a sandy clay) and will therefore apply a lower lateral load to a retaining structure. Using a table for approximate soil properties I am getting a higher angle of effective internal shear resistance for a sandy clay, which in turn leads to a lower coefficient of active earth pressure and lower lateral force. Could someone explain why this is the case?
Thanks,
Bradley





RE: Coefficient of active earth pressure for clay vs sandy clay
You comment about "hold itself together" actually refers to undrained strength (short term).
Mike Lambert
RE: Coefficient of active earth pressure for clay vs sandy clay
As an exercise, plot the Rankine values for Ka against tan(phi)....an approximation of cohesion. You'll see that the lower the phi angle, the higher the cohesion and thus the greater reduction in Ka under that premise. That premise is only valid for Ka-tan(phi)> 0, so you'll see a break point at phi=23.5 degrees......which implies the break point between cohesive and non-cohesive soils with respect to their effect on Ka.
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
From Bowles, the earth pressure at a depth:
σh = Ɣ•Z•Ka - 2•c*sqrt(Ka)
The first part is the typical triangular distribution from the phi angle and the second part is a fixed amount of negative pressure that the cohesion provides. The frictional part has to overcome the cohesive part at some depth to provide a net driving force thus there is no simple Ka relationship for cohesive soil earth pressure.
However, as Mike noted above, there is short term and long term strength properties to be concerned about. Cohesion decreases over time and may eventually go to zero at some effective frictional strength. Soil also cracks when in tension thus the cohesion benefit is relieved thus increasing pressure, The crack can also fill with water which increases lateral pressure.
Most people use low phi angles to approximate cohesive pressures so a simple triangular pressure diagram can be used at a high enough magnitude to be safe. A Ka = 0.50 (60 psf/ft) might be a reasonable value if one was to guess a number for a decent clay.
As oldestguy notes, clays are quite difficult with different issues depending on where one is at. Many use granular backfill material to avoid the difficulties of predicting the lateral earth pressure from clay.
RE: Coefficient of active earth pressure for clay vs sandy clay
For long-term stress conditions, we have long term strength conditions. Such long-term conditions relate to the effective stresses and the drained strength. We measure the actual drained strength via the triaxial or drained direct shear test. We also correlate it to soil classification and for granular soils relate the drained strength to SPT N-value.
In both direct shear and triaxial strength tests, we will detect a drained cohesion intercept. Some folks like to determine the drained earth pressures allowing for some measure of cohesion, where you will realize some benefit in fine-grained soils or course-grained soils with some activity in the fines.
I ignore drained cohesion, but use the actual friction angle after allowing some consideration for strain compatibility. I mean I would not assemble a failure envelope on two points where failure was at 4 percent strain and one point where failure was at 15 percent strain.
I do not rely on the laboratory certificate's interpretation of friction angle. It is professionally negligent to let a lab provide such professional service! (Bear in mind the ASTM Specification excludes data interpretation for phi and c.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Coefficient of active earth pressure for clay vs sandy clay
Also, do you think that UU triaxial tests for compacted clays can provide acceptable/reasonable values for phi'? A simple reasoning (but not sure if it is true) for this is because compacted clays are not saturated so if we do drained triaxial tests "there will be not too much to drain" so will be some similitude with UU tests. Because sometimes we use clays to backfill against retaining walls, I was trying to check the phi' of compacted clays...
RE: Coefficient of active earth pressure for clay vs sandy clay
I have looked at thousands of geotechnical reports from around the US and almost every one provides an equivalent fluid pressure table or text for retaining wall pressures that are free to rotate. Granular backfill is typically 35-40 pcf/ft and site soils are 50-70 pcf/ft and many recommendations just mimic the IBC values. Thus I concluded that "most people" do this.
That being said, I agree with your technical approach assuming one has enough information to make such an assessment of the retained or backfill soils. The same reports I refer to rarely have enough testing done to determine the parameters necessary and the contractors seems to find the worst fill on a site to backfill walls when granular is not required.
RE: Coefficient of active earth pressure for clay vs sandy clay
UU triaxials do not measure effective stress (they are a total stress test) so it is not possible to derive effective stress parameters such as phi.
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
I'm not an expert in unsaturated soil testing but I would've thought if the soil was not fully saturated then the pore space would be occupied by (compressible) air, thereby undermining the requirement for constant volume in an undrained test.
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay
The phi=0 condition is very typical for saturated clays in UU loading. Heck, we get one or two lectures on this topic in graduate school! Reality is though, many soils below the water table remain unsaturated. So, what's the soil strength for design?
In my practice, I require all UU tests be performed at 3 different cell pressures. That way, I know to what extent the phi=0 condition is realistic, and to what extent the soil is actually saturated.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Coefficient of active earth pressure for clay vs sandy clay
RE: Coefficient of active earth pressure for clay vs sandy clay