Cantilevered Retaining Wall Allowable Stress - Using Avg. Stress

[Blooongeey]

Neglecting seismic, my retaining wall footing sizes become unwieldy for retained heights around 8' and up. Using 30pcf EFP, it's not unusual to see these footings grow to a width of 6'++ for toe under type walls. I spoke with a well seasoned soils engineer recently who is encouraging me to use "average calculated stress," derived from the typical triangular/trapezoidal stress profile obtained from analysis against the recommended allowable value provided (2.0ksf). This would bring the footing widths way closer to what I believe is practical/realistic/traditional. Anyone else doing this?

Michel
Redondo Beach

 

[BridgeSmith]

Can you clarify the calculation of R & e for a case with 0:1 backfill slope (flat)?

R and e are determined as what is needed to produce equilibrium, based on the loads applied. In the case of a flat backslope beta goes to zero, which means Ft (the force of the retained soil) is applied horizontally, rather than having vertical and horizontal components. I'll post the equation that goes with the figure tomorrow when I'm in the office.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Blooongeey]

The pool is approximately 5’ from the wall. With reference to the below calc and figure, the geotech seems to be treating the pool as being lighter than the soil it displaces, and accounting for the surcharge by treating it as a “failure wedge” for a 1:1 sloped backfill condition, resolved into the 48psf/f EFP.

 

[BridgeSmith]

Given those conditions, if it was me designing it, I'd use at-rest pressure, since any movement of the wall could be detrimental to the structural integrity of the pool. I'd also neglect the passive resistance of the soil in front of the wall, for the same reason (it takes a significant amount of movement to mobilize passive resistance), and also because there's no guarantee that the soil will always be there.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

Rod Smith, P.E., The artist formerly known as HotRod10

 

[le99]

The geotech tells me that 48psf/f accounts for the pool. The EFP in locations away from the pool is 30psf/f.

I think you have reversed the order - 30 pcf at the pool, the lower earth pressure is due to the limited fill depth between the pool and the wall; 48 pcf everywhere else. To further reduce the earth pressure, I suggest looking into using a lean concrete mix as the backfill.

Since the wall is on a slope, you shouldn't utilize the passive pressure in front of the toe, it can be easily lost - washed away by rainstorms, soil corrosion...

I would tilt the wall towards the pool in anticipation of excessive lateral displacement. Also, it may be best to place the wall on piles.

 

[BridgeSmith]

I suggest looking into using a lean concrete mix as the backfill.

That's a really good idea. With that, you'd really only have sliding and global stability deal with.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Boiler106]

A word of caution: using enercalc/retainpro, its my understanding that the bar needs to be extended above the stem segment height to meet either the bar development length or the lap length for the bars above the segment. It would be inappropriate to cut your #6@8" bars off at 2'-6" above the base.