Controlled Density Fill (CDF) Shear Strength 1

[oregonaut]

Hi,
I am having a hard time finding the shear strength of Controlled Density Fill (CDF). I am designing an underground utility vault that is buried deeper than usual. Due to the excess soil pressures and thin-walled precast vault, the contractor must install CDF around the outside of the vault for added shear strength. I am tasked with providing engineering calcs that prove that the CDF added will resist the shear forces. The Shear capacity of my vault wall is about 4 kips (using the long form from ACI) and I need to provide 7.1 kips of total shear strength with CDF and vault wall combined. The CDF being used is 250-350 PSI. Does anyone know of a shear strength equation for either CDF or unreinforced concrete? Any tips will help thanks.

 

[civilperson]

Use flowable fill rather than compacted soil. 100 psi compressive strength and 20 psi shear strength easy.

 

[twopie56]

Hi,

I am trying to comprehend your situation so i can offer some
assistance, You said you are designing a utility vault,
I designed a lot of Sewer Chambers,Barrel,wingwalls,manholes and other uderground structures but i did not know that back fill can be used to resist shear stresses.

I would recommend for you to use your states Culvert Manual
There is a Gude line on how to Structurally design Underground utilities like yours From Load Recommendations to Stress Determination all formulas you need and sample design are there......and its complete.

Could you kindly let me now if you found that CDF is capable
of resisting Shear Stress. Thanks in advance.

 

[oregonaut]

I found out that the company I work for reccomends flowable fill. We just call it CDF on our drawings for some reason.

Flowable fill is concrete with a high water/cement ratio and fine aggregates. Hence the 300 psi compressive strength. After it hydrates, it is solid and therefore may resist shear forces. CDF is not solid and therefore you wouldn't gain very much shear strength at all.

As for my calculation, I found 4/3*sqrt(f'c)*b*d for plain (unreinforced) concrete in the ACI manual. It has the limitation of 2500 psi as a minimum compressive strength tho. So I still think I am going about this wrong.

 

[dicksewerrat]

Ask your precast supplier to make a thicker wall for you. you are going to dig the hole for the precast and 'CDF' anyway. It may be cheaper in the long run.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[SETast]

ACI did a report ACI229R-94, titled "Controlled Low Strength Materials (CLSM)". I haven't looked to see if there has been an update although I'm sure it has.

CLSM with compressive strengths less than 300 psi are geared toward future excavation into the material. If you do not anticipate future excavation then you can increase the compressive strength to the upper limit of 1,200 psi. But, the use of CLSM or flowable fill is for bearing capacity only. Nowhere in the article does it state that CLSM can or should be used for shear capacity. The intent of CLSM is to be used as a structural fill or backfill in lieu of compacted soil in tight or restricted access excavations where compacting soil would be difficult.

If you want to try to use ACI, UBC or other code provisions for "Plain Concrete" beware that ACI states that the minimum compressive strength must be 2,500 psi. And you may have additional problems if you are in seismic risk Zone 2 or higher.

So your options are to:
1) Design the structure as cast-in-place,
2) Dictate the size of the precast wall section be the same size as the designed cast-in-place wall section,
3) Use 2,500 psi plain concrete as backfill,
4) Calculate for soil interaction similar to pipes or culverts as mentioned previously, or
4) Wait for a differing opinion.