Passive Pressure Log Spiral DM 7.2 Updates

[bostonblue]

Hello All,

I've been working with the updated log-spiral charts from the latest version of DM 7.2 (2025). I noticed using Equation 4-12, I am not able to replicate the results provided in the far-right column of Table 4-2. I understand that the equation is an approximation but some the values are a ways off, particularly as you increase in friction angle / delta/phi ratio. Does anyone have an idea of where the values in the table came from?

For example, using the equation with a friction angle of 35 and a delta/phi of 0.5 I get a Kp value of 6.3. However, the table provides a value of 8.0.

Even checking against Figure 4-13, for a friction angle of 35, the equation provides a Kp value of 7.6 while the chart shows ~8.0.

What level of accuracy is expected when using these equations/charts/tables?

Thank you!

 

[CarlB]

I don't have access to DM 7.2.
But other source https://www.sciencedirect.com/science/article/pii/S1674775517303955) indicates a Kp should for those conditions should be about 6.5. But it notes other studies using log spiral finds variations, some higher.
So there doesn't appear to be close agreement between methods, especially with higher wall friction. But you'd expect the equation and graphs to match better, for a single theory.

 

[bostonblue]

Should be able to access DM 7.2 for free here:
https://www.wbdg.org/dod/ufc/ufc-3-220-20

Thank you for the other reference, I will take a look. Understandable that there will be variations.

 

[CarlB]

Thanks for the link. I had only seen the hardcopy version for purchase.

I agree with your calculated value of 6.3 for those phi and delta values. After checking other values in Table 4-2, it appears the table has been mislabeled; the delta/phi values presented are for 0.66 rather than 0.50 (For Coulomb as well as Log Spiral). This d/p value would be in line with Figures 4-12 and 4-13, which are also for delta/phi of 0.66.

The Kp value presented in Table 2 for 35 degrees, d/p=0.50 is 8.0, but is actually for d/p=0.66. This matches the values in Figs. 4-12 and 4-13. Kp calculated from the provided equation (for d/p=0.66) is 7.53. This is still not a great match with the value from the charts. I assume the discrepancy is that the equation provided is a simplification of the methods used to develop the charts, which are reported to be from Kerisel and Absi, 1990.

Since the Kp from the equation or the charts are more conservative than Coulomb (for wall friction), and given other uncertainties of soil properties, and the factor of safety applied, I’d conclude either the charts or equation could be safely used.

 

[bostonblue]

Great catch.

Seeing as the equation is serving as a "catch-all", it makes sense that the discrepancy could be ~0.1 to 0.5 off. And yes, coupled with the multitude of other variables this discrepancy should have a relatively small impact.

Thanks again!