Bioretention U-Structure (At-rest or Active Pressures)

[ahardy89]

I have a situation where we are designing a cast-in-place reinforced concrete bioretention structure. The structure is a U-Shape structure with one side higher than the other (so it is acting as a retaining structure of sort). The water table is relatively high (the reasoning for the included base slab for the bioretention). The structure is in a parking lot. The high side has a 12-ft sidewalk before reaching the pavement but I have included a 250psf live load per IBC (incase trucks were to drive up on the sidewalk). The low side abuts against the parking lot asphalt. Sketch Provided Below.

The tricky components:
1. There is a differential in height so one side has a net driving force.
2. The slab cannot extend past the exterior face of the wall on both sides to take advantage of additional dead load.
3. The water table is above the base slab (so sliding resistance through friction is reduced due to reduce net weight).

Originally I checked the stability of the wall using at-rest pressures applied on both sides. The live load was only applied on high side (worst case) and the interior of the bioretention was assumed to be drained. The uplift pressures reduce the effective weight which is causing the sliding factor to not meet the minimum 1.5 even with the weight of soil inside.

I feel like visually this should work because the differential in height is only 2-3 feet. The numbers are showing it is not (mainly because of the high groundwater and high live load).

My question to the group - Is it more reasonable to use active and passive pressures (conservatively reducing passive pressures and ignoring 2 feet of the soil) instead of at-rest pressures? I do not think the movement needed to activate active and passive is detrimental to the structure.

 

[r13]

Your approach is correct for structural design of the reinforced concrete walls and slab, but in the case of stability, the structure will go nowhere. Upon immanent movement (from high side to low side), the earth pressure on the pushing side will reduce to active state, and passive earth pressure will develop on the resisting side. Make sense?! Watch out for uplift though.

 

[ahardy89]

r13- That makes total sense and was my thought process was for stability (after the little bit of movement it would stabilize with active pressures on one side and passive on the other). I will still use the at-rest pressures to design the walls and slab structurally as you mentioned.

Uplift is definitely under consideration!

 

[r13]

I would suggest to investigate an extreme condition - the complete removal of low side soil. In such case, the pushing side will have active pressure and water pressure (slow to drain), however, the base is dry because of dewatering, the safety factor shall be ideally set to 1. This practice is very conservative, talk to your boss for inclusion.

 

[STrctPono]

I have designed U-shaped bioswales before, albeit a bit smaller than yours. You certainly do not need to utilize at-rest earth pressures. Active earth pressures is correct. Additionally, I absolutely would use the passive earth pressures on the short wall side to resist the sliding forces on the high side. Honestly, think about it, where is that box going to slide to? Technically, since the parking lot is paved, you don't have to ignore the top 2 feet but I would do the same thing. I can't believe that sliding would not work out if you analyzed it that way. You just need to sharpen your pencil.

I disagree with r13 on the analysis of the structure in that extreme scenario. The design will never work out if you do it that way and is completely unnecessary. I would place on the plans that the both walls need to be backfilled simultaneously with the differential never being greater than 2ft difference. You're covered!

 

[ahardy89]

Thanks for input as well STrctPono!

That was what I was struggling with in my head (how can this thing actually slide other than the minimum movement to activate active and passive pressure). Visually in my head it did not make sense that this box would not work from a stability standpoint.