Soil Nail Wall 2

[CLMLSP]

I have two questions regarding the use of SNails:

1) Should the Ultimate Bond Strength or allowable (Qu/2) be used?

2) What determines the Bond Stress Factor?

thx

 

[PEinc]

1. Depends on whether you are doing an allowable stress (or service load) design, ultimate stress design, or load and resistance factor design (LRFD). For allowable stress design, you use service loads and ultimate strengths divided by the appropriate safety factors. Then, your safety factor should be at least 1.0. For ultimate stress design, you use ultimate strengths and then check that you have the appropriate safety factor. For LRFD design, you use factored loads with ultimate strengths multiplied by the appropriate reduction factors. The reduced strengths should be greater than the factored loads.

2. Bond stress factor? The bond between the soil and the nail grout depends on the type of soil and the grouting pressure.

 

[CLMLSP]

Thank you.

I don't know how to apply LRFD in Soil Nail Wall Design. I'm using ASD. In Caltrans SNail program, both bond strength (under Soil Parameters) and Bond Stress Factor (under Varying Reinforcement) need to be specified. My question was, for pull out factor of safety of 2.0 (As specified by FHWA), should the ultimate bond strength and and bond stress factor of 0.5 be used?

 

[Panars]

The bond strength factor is used in SNAIL to account for different horizontal spacing or drillhole diameter in some of the rows. If your horizontal spacing was 5 feet, and you had one row of nails that was spaced horizontally at 7.5 feet (not that this is good practice), you would use a bond strength factor of 0.67 (0.5/7.5) for that row. Normally the bond strength factor should be 1.0 where the nails in all the rows are identical.

 

[CLMLSP]

Thank you Panars, this is the clearest answer I ever had.

What about the ultimate bond strength vs. allowable? For an instance, for stiff clay, if I decided that the ultimate bond strength is 60 kPa. The required Factor of Safety for pullout failure mode is 2, and the required factor of safety for global stability is 1.5.

Does this mean that I should use 30 kPa in SNail program under Soil Parameter, and also the nails should be long enough (or a larger hole) so that the global stability factor of safety is 1.5?

thanks

 

[Panars]

Yes, when using the bond strength factor as I described above, you would divide your ultimate bond strength by the safety factor (usually 2.0) to calculate your allowable bond strength and enter that in the soil parameters tab.

Since my last posting, I have found other people do things a little different. They put the ultimate bond strength in as the bond strength soil parameter and use a default value of 0.5 for the bond strength factor. If they had different horizontal spacings or drill hole diameters, they adjust the bond strength factor accordingly. I think either method works.

 

[CLMLSP]

Thank you. This is really helpful to me.

 

[sbw]

I had the very same question regarding the bond strength values input into SNAILZ and resolved it as follows:
under the Reinforcement Strength/Cond. tab,

1. When using Ultimate input parameters the bond strength is input as an ultimate value (and BSF is kept at 1.0).
2. When using Pre-factored input parameters the bond strength is input as 0.5xultimate (FS = 2.0) with BSF = 1.0. You can also input the ultimate value and us a BSF = 0.5 as indicated by Panars above.

Based on a conversation with (I believe it was) Shawn Wei at Caltrans and a quick verification test in SNAILZ, the program applies a factor of safety to input values of bond strength when using Ultimate input parameters. Here's a possible check of this:

height: 20' (vert wall, w/ no up-slope)
rows: 4
nail length: 15.5'
dist to first row: 2.5'
vert spacing: 5'
horiz spacing: 5'
bar diameter: 0.875"
hole diameter: 6"
soil unit wt.: 125 pcf
phi: 33
c: 0
bond stress: 14 psi
BSF = 1.0
Ultimate values: PS=55kips; FY=75ksi

When run I get a global FS of 1.51 with pullout controlling. Change nothing and under the Reinforcement Strength/Cond. tab click in one of the Pre-factored boxes (but don't change anything...this should carry over the same values, which should fine since we're checking bond strength). Pullout still controls but the FS is 2.03.

 

[CLMLSP]

Hi, sbw,

thx for the discussion. I ran you example and got your F.S (nail angle 10 degree).

The Snail says under ultimate "inputed values (and bond stress under soil parameters) automatically divided by indicated factor of safety". I used pre-factored and input bond stress of 14/1.51=9.27 psi, then got the same FS of 1.51. Seems to me that if ultimate value of bond stress is used, the calculated F.S (in this case is 1.51) applies to both pullout and global failure mechanism.

I think the key question now becomes: what is the appropriate procedure if I want to have 1) F.S for pullout failure is 2.0 and 2)at the same time F.S for global failure is 1.5, without overdesign the soil nail wall?

 

[sbw]

The example I provided seems to indicate that using the Ultimate values input and designing for a FS of 1.5 also yields a FS for pullout of 2.0 (confirmed by the FS yielded by the pre-factored results)...both criteria are met. I would request that other members verify this since I cannot point you to published text to confirm this. You should also consider using the charts provided in the FHWA manual as a means of verification. As with any program, your results will only be as good as the input data, so make sure that the design criteria makes sense for the material on hand, verify with testing, and never assume that the person providing the criteria knows more than you (for the sake of safety/reliability, not ego that is).

Hope this helps.

 

[CLMLSP]

thank you