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I attached a new SCDOT Standard Drawing. I would appreciate your review. Would it be unreasonable to ask for comments by June 14, 2018? Thanks!

805-520-00 CURRENT GUARDRAIL POST INSTALLATION IN PARTIALLY WEATHERED ROCK (PWR) 2019, January


Design:
Update to drawing 805-520-00
No change to pay items.

Construction:
Smaller diameter hole changed from 8” to 10” diameter
Holes are deeper – to accommodate full length posts. This is intended to facilitate replacement so that no special posts are needed at these hard to identify locations.
Coarse aggregate fill is shown to finished grade – this is to aid in locating the cored hole when future repair/replacement is needed at damaged post.
21” Dia Hole required at Timber Posts (new note 5)

I reviewed your post in rock details.

To date, the MGS has not been tested in the post in rock configuration that we developed previously. That said, we would expect that the MGS would perform similarly or slightly better than the original post in rock research we did.

Additionally, the original post in rock work and leave out studies with W-beam guardrail were NCHRP 350 not MASH. MASH evaluation of those types of systems have yet to be completed. This is likely needed.

That said, your current detail is consistent with previous research we did regarding posts in rock with W-beam guardrail and with a later FHWA memo that reinforced the guidelines we developed. I have attached that memo and provided a link to several related research reports as well. Note that our original work used more of an oval shaped hole, but that has been extended to a constant diameter hole for easy installation. This is the best guidance for the MGS we currently have until we have a chance to evaluate post-in-rock installations under MASH.

https://unl.box.com/s/yn8spztfk6whv3qke2acenz5kyn37rw4

One item of note. Your detail shows the course aggregate backfill all the way along the post embedment. We have previously only required that course aggregate backfill in the hole drilled in the rock. The soil above the rock can remain the native soil. See below.

However, what you have shown, backfilling the entire depths is acceptable as well.

Thanks for the comments.  We did find some research that indicated the possibility of cutting posts (the formulas in Roadside Design Guide imply lengths shorter than a standard post.) 

For the detail we adopted, we felt that maintenance was more critical than the initial installation, so

• We extended the holes to reach the expected post depth.  (as we believe custom post lengths would be a large problem for repair work.)
• Ran the stone backfill to the surface (to hopefully aid the installer in finding the actual hole when repairs are done.)

The installation in weathered rock is quite uncommon in our state.

If solid rock is encountered.  Could a base plated post be attached with adhesive rock anchors?

Ideally, we would like to be able to grout lifts at each post so the installer can achieve a somewhat consistent post length (assuming that the top of rock elevation will vary.)

Is there any guidance (MASH or earlier) on how to handle posts anchored to solid rock?

Under MASH, are there any base plated posts that would allow variable post lengths?

There is some potential for a post mounted to rock with a base plate, but to the best of my knowledge, that type of design has never been specifically developed.

There are several related designs. We have developed both a strong post and weak post MGS system that can be attached to culvert slabs. The strong post design was developed specifically for low-fill culverts. The system was first developed under NCHRP 350 and then retested recently to MASH TL-3. The report for the MASH evaluation is in progress.

https://mwrsf.unl.edu/researchhub/files/Report144/TRP-03-114-02.pdf

We also developed a system for attaching the MGS with weak posts (s3x5.7 posts) to the top of culverts with fill depths varying from 1’ to 3’. The report for that work is in progress as well. Options for a steel socket and a concrete socket were developed and tested through dynamic component testing.  

Both of these systems have similarities to the post in rock issue, but there are some differences. First, anchoring to the rock may require different anchorage that what was used for the post attachments noted above. Also, only the weak post attachment was designed for variable fill depths. However, there is potential that the strong post could work at fill depths over 9” as well. Finally, as you noted, there are issues with the natural unevenness of the rock below grade that would make effective attachment difficult.

Thus, there may be potential to have a post anchored to rock, but some issues would likely need to be ironed out before we would be able to consider the system MASH compliant. Is this an issue that South Carolina would like to look into further?

Thanks