SNAILZ

[dcarr82775]

I am having a debate with another engineer on using SNAILZ as part of a soil nail wall design. If I use the pre-factored strengths and bond reduction of 0.5 I get a factor of safety of 1.0. To me this is too low. The wall is permanent. The other engineer says 1.0 is ok because the strengths are already reduced thus F.S.=1.0 is the target, not the 1.5 I think it should be.

I am looking for thoughts on which one of us is right (or more right than the other anyway)

 

[mmedirtguy]

This answer is based on ASD analysis. Its my understanding of FHWA Geotechnical Circular 7 that a 0.5 bond factor corresponds to a FOS=2.0 applied to the ultimate bond capacity. If you are using prefactored strengths then you have gone through the exercise of calculating nominal facing and tendon strengths and reduced them accordingly. So based on allowable grout to soil bond values and appropriate reductions in the structural elements you need to shoot for 1.50 (static) for equillibrium of the entire soil mass (1.1 seismic if applicable). Your colleague's position seems to imply that becuase you reduce strengths of concrete or steel by ACI or AISC, you can have a factor of safety of 1.0 for sliding and overturning on a retaining wall. It doesn't seem correct. I would refer to FHWA Circular 7 (you can download it off FHWA) ,Section 5. All the factors of safety are listed in Table 5.3.

 

[PEinc]

If you use pre-factored values, you have the safety factors that you need for each strength parameter and the final safety factor should be 1.5 for the permanent wall.

If you use ultimate values, they will all have a safety factor equal to the final safety factor that is calculated by the program. For instance, if you enter an ultimate bond, yield stress, cohesion, and punching shear and the program calculates a final safety factor of 1.5, then you have only a 1.5 safety factor on the input strength parameters. I really do not see a reason to use ultimate values in SNAILZ.

For bond stress, I enter a prefactored bond stress that already has the safety factor I want (usually FS = 2). Then, if I want to modify input for different nail spacings or hole diameters in different tiers, I use the bond reduction factor for that particular tier of nails. If I want to double the horizontal nail spacing, I enter a reduction factor of 0.5. If I want to reduce the drill hole diameter from say 8" to 4", I would doo the same to account for the smaler hole perimeter.

http://www.PeirceEngineering.com

 

[dcarr82775]

Thanks guys. That is what I thought.

 

[PEinc]

Remember that SNAILZ is a slope stability program. The components (nail, bond, punching of face, etc.) need their individually required safety factors for internal stability. However, the program is calculating a global stability safety factor which still needs to be greater than 1.35 (temporary) or 1.5 (permanent). This will cause the soil to grout bond to have a double safety factor. In addition to the input safety factor of 2, there will still be a safety factor of 1.35 to 1.5.

http://www.PeirceEngineering.com

 

[dcarr82775]

Understand. Thanks