Soil Nail Wall
Soil Nail Wall
(OP)
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
1) Should the Ultimate Bond Strength or allowable (Qu/2) be used?
2) What determines the Bond Stress Factor?
thx





RE: Soil Nail Wall
2. Bond stress factor? The bond between the soil and the nail grout depends on the type of soil and the grouting pressure.
RE: Soil Nail Wall
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?
RE: Soil Nail Wall
RE: Soil Nail Wall
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
RE: Soil Nail Wall
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.
RE: Soil Nail Wall
RE: Soil Nail Wall
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.
RE: Soil Nail Wall
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?
RE: Soil Nail Wall
Hope this helps.
RE: Soil Nail Wall