Negative Cohesion Component Value for Active Pressure

[mqr7]

Hello, I am writing seeking some understanding. My background is structural so apologies if this is rudimentary.

At the site under question, there are primarily cohesive soils. The geotech gave us the formulas and parameters to calculate the passive and active pressures. However, at the depth that I am looking at, I am getting a negative value for the active pressure, due to a high negative component for cohesion in the equation with the parameters provided.

Sigma_a = yHKa - 2Su*sqrt(Ka)

I’ve checked my math and it is correct. Is this negative value normal? What does this mean in terms of how the soil is behaving at this depth? Any insight would be appreciated, thanks.

 

[EireChch]

In cohesive soils, the near surface soils are in tension so your math is correct. Have you ever walked in an area of CLAY and noticed desiccation and tension cracks. The tension cracks form due to the negative earth pressure. It essentially means that they have some stand up time.

horizontal force (Pa) = 0.5 x gamm x H² -2SuH

When you let Pa equal zero you get H = 4Su/gamma. That H is the height that your clay can theoretically stand unsupported.

 

[MTSOE]

Yes, it is fairly common to get calculate negative active earth pressure for cohesive soils (at least in my area). A mathematically negative value would seem to imply that the soil is pulling away from your wall (whatever that might be). Instinctively, we know this isn't happening. Rather the soil in theory has some tensile capacity. I've never encountered a geostructural engineer considering tensile capacity of soil. For practical purposes on normal projects, this should always be zero. Instead, think of it as the soil's ability to hold itself together to a certain point (think cohesion as stickiness of soil), to use a crude oversimplification).

Also, I know you stated soils aren't your specialty, but be careful with the terminology of cohesion vs undrained shear strength (Su). If you're using a true Su value, Ka should reduce to 1. Some reviewers might get hung up on stuff like this. Just a heads up.

 

[MTSOE]

Also, in my experience/local area of practice, when I get a theoretically negative pressure, I use a K value of 0.2 (this is just a made up, safe value). This is specific to my area. Ask around to see what is common practice where yoy are.

 

[mqr7]

I appreciate these responses. Yes, @MTSOE i meant undrained shear strength Su. I thought that indicated the cohesive component (vs. the frictional component in the first part of the equation) but thank you for the clarification. My Ka value from the geotech report is 1.

Now I understand from shallow foundation design that the soil isn’t in “tension” (for example when the load is outside the kern), we just disregard that portion of the foundation as being effective in the calculation of bearing pressure. In this case I have large concrete block buried to support piping (an anchor block if you’re familiar), and I’m trying to determine the active pressure on the back side. If I’m obtaining a negative value, do I just disregard the active pressure?

 

[HTURKAK]

.....My Ka value from the geotech report is 1.

Ask to geotechie . If Ka= 1.0 , the soil is neither clay nor gravel. That is , cohesion C= 0 and friction angle Φ =0 . Literally this means the soil is liquid.

The following figure explains the active earth pressure and tension cracks developing.




For the design of thrust blocks ; total resistance of the block is calculated which are base friction and passive thrust.
Provide a sketch and soil properties to get better responds.

 

[MTSOE]

Ask to geotechie . If Ka= 1.0 , the soil is neither clay nor gravel. That is , cohesion C= 0 and friction angle Φ =0 . Literally this means the soil is liquid.

The following figure explains the active earth pressure and tension cracks developing.


View attachment 10052

For the design of thrust blocks ; total resistance of the block is calculated which are base friction and passive thrust.
Provide a sketch and soil properties to get better responds.

I disagree that Ka=1 implies that is neither clay nor granular. To me, Ka=1 implies that the friction angle equals zero. For me, using a friction angle of zero and an undrained shear strength puts bounds to the problem. Undrained shear strength is the cohesion intercept which corresponds to the zero friction angle assumption. It is true that a zero friction and soil is very rare (impossible?) in nature but it is a useful simplification. Most of my work is for temporary support of excavation, so assuming undrained properties for clays is often justified given the project lifespans.

 

[MTSOE]

I appreciate these responses. Yes, @MTSOE i meant undrained shear strength Su. I thought that indicated the cohesive component (vs. the frictional component in the first part of the equation) but thank you for the clarification. My Ka value from the geotech report is 1.

Now I understand from shallow foundation design that the soil isn’t in “tension” (for example when the load is outside the kern), we just disregard that portion of the foundation as being effective in the calculation of bearing pressure. In this case I have large concrete block buried to support piping (an anchor block if you’re familiar), and I’m trying to determine the active pressure on the back side. If I’m obtaining a negative value, do I just disregard the active pressure?

I personally would never assume zero earth pressure unless I'm building against a very competent rock. I would ask others in your area what a safe assumption but not overly conservative equivalent K value would be. Unfortunately, geotechnical engineers can often be reluctant to give realistic values unless you have a demonstrated history with them on similar projects.

 

[mqr7]

This has been very helpful, I greatly appreciate it. Thank you all so much. I’ll be reaching out to my geotech to get more information.

 

[HTURKAK]

I disagree that Ka=1 implies that is neither clay nor granular. To me, Ka=1 implies that the friction angle equals zero.

I said the same ..Active pressure constant for granular soils Ka=(1-SIN Φ)/((1-SIN Φ) if Ka= 1 for granular soil , Φ=0 and for pure cohesive soil literally means cohesion also zero and , Pa can be calculated with the formula i have attached in my first post. You may look any soil mechanics books.

The title of this thread ' Negative Cohesion Component Value for Active Pressure' .Cohesion could be zero but not negative.

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