Active or At-Rest soil pressure - underground concrete utility vault

[LearnerN]

Would the soil pressure on an underground utility concrete vault be active pressure or at-rest pressure? How do you typically distinguish between the two?

(Edit: I originally stated "passive" instead of "at-rest")

 

[Ron]

"Active" soil pressure is the pressure that is "acting" on the structure...meaning the soil that is pushing against the structure. Note that until some movement occurs, active pressure is not mobilized. "Passive" soil pressure is that pressure that resists the movement of the structure against the soil. Again, movement has to occur before passive pressure is mobilized.

If you assume that the soil acts as a viscous fluid, then movement is inherent. If you consider that the soil has some cohesive capability, movement is not inherent and you have to use your engineering judgement as to when movement occurs. The easier approach is to compute active, passive and at rest conditions and take the worst case.

 

[LearnerN]

Ron, I apologize - I meant to ask about active versus AT-REST soil pressure. Sorry for the confusion. I know at-rest soil pressure is usually used for things like basement walls and other situations where the soil is treated as completely confined and immobile. So, then, what are your thoughts on the revised question above?

 

[LearnerN]

To further clarify, ASTM C857 "Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures" refers to using ACTIVE soil pressure for the lateral earth loads on the sides of the utility vault. However, I'm trying to conceptually understand why it wouldn't be better to treat the lateral soil loads on the vault as the AT-REST soil pressure since the utility vault is essentially a box, the walls of which would not hardly be moving. If basement walls generally use at-rest soil pressure for design, then I would think that surely utility vaults would use at-rest soil pressure for designing the walls. But at the end of the day, ASTM is an accepted standard, so that's what I'll follow. I'm just curious.

 

[mtu1972]

Personally I would use At-Rest as it is more conservative than Active. Also, with something like a vault there is pressure on both outside walls that would oppose each other, so the movement that develops Active pressure is not likely to occur.

gjc

 

[dcarr82775]

At-rest for your situation. Most text books related to soil will give you a range of movement in the wall necessary to achieve the active state. Essentially the soil needs to move enough to mobilize its internal strength. The buried box culvert doesn't allow this mobilization.

 

[Ron]

The soil around a utility vault is typically fill and compacted, thus consideration could be given to the active state; however, the active and passive states are more meaningful to a "flexible" structure such as a retaining wall whereby the active and passive states can be readily mobilized. Outside of settlement, I would not anticipate any movement of the vault, particularly in the lateral directions, so the at rest state is reasonable and justifiable.

All of this exercise is an example of using engineering judgment and meeting the established standard of care for this realm of engineering in your area. As you can see, experienced engineers can have differing, yet logically correct opinions on both sides of such a subject. For compliance with an appropriate standard of care, it is important that you go through a process of evaluation prior to making your decision. In fact, the decision you make from an engineering perspective may actually be less important than the process you used to get there!

Further.....document, document, document whatever you do!

 

[dcarr82775]

This might be a first, but I have to disagree with a small part of what you said Ron. I do not believe that you can get active due to placing and compacting the fill. The compactive effort alone produces greater loading than active alone, and the nature of the structure prevents future relaxation in the soil.

 

[TehMightyEngineer]

I've always used at-rest for buried structures that are confined by the soil on all sides.

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[IDS]

Using "at rest" pressures is not necessarily conservative. Sagging at mid-span of the roof slab is often a critical design action, and this is reduced by increased pressure on the walls. Presumably this is why some codes require at rest pressures to be used in design. Other codes specify a minimum and maximum pressure coefficient to be considered, and designs to deal with the worst combination of the two.

Calculation of the actual pressure distribution requires analysis of the interaction between the structure and the soil, including the effect of soil loads being applied in layers and compaction loads. This analysis will show that parts of the structure will have active pressures and parts greater than at rest. For a flexible arch structure a soil-structure interaction analysis is required to get even reasonably close to the pressure distribution that will develop[ in the actual structure. For a rectangular buried structure it depends on the size and flexibility of the structure, but in my opinion the more detailed analysis would often be worthwhile.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

At rest pressure is the pressure which exists in an undisturbed soil mass. Cut a vertical slice, and the pressure on each side is equal. That is usually close to what you have in a buried structure as described. You also have to consider hydrostatic pressure if it exists.

 

[Ron]

dcarr....I don't think we disagree as my point was more toward rationalizing the use of either to comply with a reasonable standard of care.