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Soil nail design question

Soil nail design question

Soil nail design question

(OP)
Hi all,
I'm supervising a soil nail wall construction. I need to carry out some design calculation for rearranging the soil nail layout to accomodate the site. I'm using the FHWA manual as reference.

How to assign the Sv & Sh if I were to use staggered layout. Assume the initial layout was having a 2m spacing in both ways.

I noticed the manual stated that the grout has to be more than 21Mpa; however, I can't seemed to find any correlation between the bond strength stated in table 3.10 and grout compressive strength. I'm assuming that the bond strength data in table is based on 21Mpa grout strength. What would happened if the grout strength is excessively low? By any chance I can find any litreture that addresses
this matter?

P/S: We've conducted the proof & pull out tests and the results were satisfactory.

Thanks in advance  

RE: Soil nail design question

Grout strength does not enter into the design of nails or tieback anchors.  If a nail or tieback tests OK, the grout was strong enough.  Grout strength is used only to help determine or rule out a cause for a failed nail or tieback test.

It does not matter if the nails are staggered between rows or if the nails are innstalled directly bolow upper nails.  None of the design programs ask if the nails are staggered.  Only the horizontal and vertical spacings are considered.

If you are supervising a soil nail test, make sure the test nail is drilled to the proper bond length and diameter or else the test is meaningless if the test nail does not fail.  I also call for an unbonbed length for my sacrificial nail tests.

RE: Soil nail design question

I recently dealt with a soil nail wall where the grout strength was less than 3000 psi (21MPa).  I found, as PEinc states, that the grout strength does not control the pullout of the nail.  However, I was told that a low strength grout will tend to crack more and this may lead to increased risk of corrosion of the soil nails.  Our soil nails were encapsulated in a plastic liner, so this was not the case for this project.

RE: Soil nail design question

(OP)
hi,
Thanks for the input.
The proof tests were carried out accordinjg to the procedure stipulated in the FHWA manual. The bond length was 4m and the creep was less than 1mm in the first ten-minute reading and less than 2mm in the 6 to 60 minutes reading.

Having read both posts and given a thought, 21MPa is the absolute minimum concrete strength for any structure, let alone soil nail. Therefore, it's only common sense to reject the nail if the grout strength is less than 21MPa instead of wasting my time crunching numbers to salvage it. I stand corrected but this might also mean there is no substantial increase in bond strength to warrant a higher grout strength say, 30Mpa, which is achivable on site with proper QC on site.

In reponse to PEinc's comment on the spacing, for arguement's
sake, if I were to have a layout with Sh=2m and staggered vertically at 1m offset to the adjacent row, I'm bound to have a row with 1 nail less & as a rule of thumb, we engineers tend to adopt the model with the row with fewer nails. It wouldn't be economical since I need to either increase the nail rebar size or nail length.    

RE: Soil nail design question

longisland,

Not sure that I agree with your last regarding the spacing. The design of the wall using the horizontal and vertical spacing merely assumes that there will be a nail per each X distance along the wall at each level. Where you put them relative to the upper and lower levels of nails (if applicable) is up to you, and should not affect the stability of the wall.

Jeff

RE: Soil nail design question

Longisland,

I agree with jdonville, you are thinking of the soil nails as individual structural elements.  Soil nailing was intended to be considered as a system where the nails and facing combine to give you your stability.  Therefore, staggering the nail rows will not affect your stability when the system is complete.  Nail staggering will actually increase the systems resistance to block failures.

RE: Soil nail design question

(OP)
Hi,

I think MSEW may be more economical compared to soil nail. The rule of thumb for soil nail length is 70% of the wall height. In your case, the nail length is only 3m.  

RE: Soil nail design question

longisland,

I don't understand your point about MSE walls maybe being more economical because soil nail lengths are usually about 70% of the wall height.  Don't MSE walls have the same usual requirement for strip  or grid lengths?  Both designs consider the length of nail, strip, or grid that extends beyond the failure plane.  Also, both MSE and soil nail walls need a similar overall size and mass to resist sliding and overturning.  Therefore, the lengths of nails, grids, and strips should be similar.

Also, with respect to every other row having one less nail, I make sure that I have a nail near the end of the wall at every row.  I wouldn't want my first nail in a row to be too far away from the end of the wall.

RE: Soil nail design question

(OP)
Hi,

I should be more specific when I mentioned MSEW. Actually, I'm refering to geogrid reinforced soil. In terms of cost both systems may be similar. However, I believed constucting a MSEW or geogred reinforced wall may be faster compared to soil nail. I'm drilling 2 - 5 holes per day. It's pretty time consuming to construct the scafolds if the rig is unable to reach the designated position. There are two rigs on my site, the portable one as a backup and it ain't free. Furthermore, you need sufficient quantity to get a driller in; with geogrid wall, all you need are the plants to place the aggregates but that shouldn't be a problem for an earthwork contractor.

RE: Soil nail design question

An MSE wall should not be compared with a soil nail wall.  MSE walls, whether using metal strips,  metal grids, or synthetic grids, are most economical in a fill situation while soil nail walls are for a cut situation where, usually, you do not have to room or permission to make a sloped open cut in order to build a wall.  As long as the soils are appropriate for a soil nail wall, it does not make much sense to install sheeting in order to build an MSE wall in a cut situation when you can build a soil nail wall.  Why build two walls when you can build one wall?

Soil walls and other anchored walls should be built in cuts from the top down.  I don't understand why you need scaffolds.  Sounds to me like you may be building the wrong type of wall.  One problem that may affect the type of wall to be used is when the wall is partially in a cut and partially in a fill.  Is that your situation?

RE: Soil nail design question

The spacing of nails matters. The spacing has an effect on the width of the slice analyzed and the vertical position determines the effect length beyond the failure planes analyzed.  If you want to mess with spacing on site, I would be loathe to do so unless intimately familiar with the design and analysis and knew just how sensitive the design is to spacing parameters.  The designer realy should have given you tolerances on nail position.

As to the best solution for cut walls, I've been in many situations where its been more economical to cut out, slope back and put in an MSE wall, than to nail.  For, say a 25' high wall in a cut you could be looking at about $1000 a nail.

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