Rock anchor recommendation without cores 2

[haynewp]

I have been asked to get involved in a project for a 125 ft chimney. The geotech report recommends rock anchors for uplift under overturning but no rock cores were taken. I haven't seen this before.

The report indicates rock was hit at about 5 ft down and the borings were stopped. The report also recommends using 200 psi for the bonded anchor design and states that rock coring will be needed to validate rock anchors as described.

I am trying to foresee what issues may come up if the foundations and anchors are designed prior to the rock cores
being taken at construction time. I would assume a change order for an increase in depth if the rock is not capable of developing the 200 psi, but is it possible that rock anchors could be found to not even be feasible?

 

[PEinc]

You did not indicate if the 200 psi was the ultimate or allowable grout to rock bond. Also, you may not need that much bond. The rock anchors could be made longer or with a larger bond zone diameter to take advantage of a much lower quality rock. Drilling 5 or 10 feet deeper to allow a lower bond stress is not that expensive, depending on the contractor's drilling method.

http://www.PeirceEngineering.com

 

[haynewp]

The 200 psi is given as ultimate.

 

[oldestguy]

Don't forget to consider full saturation submergence in case of future high ground water table.














.

 

[Mad Mike]

This happens all the time where I practice- the rock probably wasn't cored because a client somewhere refused to pay for coring! Exactly as PEinc said- you assume the geotech's recommended parameters and if the rock is found to differ vastly, you add some length or diameter to the anchors.

If in doubt, insist that the geotech. be paid to supervise the first hole or two- it is still much cheaper than coring a borehole and will achieve the same result!

I assume you will be testing the anchors...

Best,
Mike

 

[EDB9]

I had a similar project recently. They made the assumption of where the rock was to be encountered (no deeper than 50’) and based the drilled pier design on competent rock to socket into. They had no information about the rock but the design progressed. I challenged the design and ordered coring. I’m very glad I did because we wound up finding only PWR and a 1/3 of the holes went 80’-110’ and never found any rock. This completely blew the design but thankfully it was done during demolition and not during pier construction. That would have been disastrous and extremely costly. I would push for coring. It’s not THAT expensive if it is truly at the depth you’re staying it’s at.

Keep in mind this was for a fairly large structure, 7 story reinforced concrete structure. However, you may be able to get away with load testing the anchors... how wide is the chimney?

 

[oldestguy]

The least one should do is advance a hole by what ever is handy, such as rock drilling hole and not taking cores. Drilling progress and difficulty of drilling can be logged and recommendations made on that basis.

 

[PEinc]

What is the unfactored design load for an anchor? Was a minumum diameter specified for the anchors? Was a minimum bonded length specified for the anchors?

http://www.PeirceEngineering.com

 

[r13]

EDB8,

How many bore holes hitting rock on the project? People can be fooled for hitting a boulder.

 

[EDB9]

We did 9 holes, we found rock in 6. We originally only had 6 prescribed but we missed 2 holes in the first 6, so I added 3 more to spot check and 1 missed there too. The oddest part was the depths we encountered them. When you looked in our core boxes, the rock didn’t even look similar. It was all very weathered to partially weathered granite from light gray to dark gray. Some was so weak it crumbled with little force. We were able to break some of the samples to get end bearing capacities but they were not consistent either. It’s not atypical here in this area, the Piedmont soil is weird here. Cobbles are very common. It’s another reason why there aren’t many structures over 3 or 4 stories and they are all wood frame construction, not reinforced concrete 7 stories tall with 30’ spans between columns. I should also mention when we did the initial CPT testing at the site, the upper 20’ to 25’ or so of soil was absolute junk soil. That’s why we went to a deep foundation design.

 

[r13]

Was the project near lake or waterway, historically speaking?

 

[johnie134]

yes, possibly.

 

[r13]

Site history is a focus point on site selection and geotechnical exploration. Also, when hitting rocks, its formation, property and integrity should be addressed in the report. I think EDB8 has provided a very interesting and important lesson.

 

[EDB9]

I practice both structural and geotech, so I tend to understand a little more about how to blend the two together. Usually it’s one or the other but I couldn’t make up mind so I chose both.

It is absolutely critical to know what your bearing surface is. Your chimney is a tall structure, it’s not low rise and is also subject to other factors because as you go over the 60’ mark on a structure, you start getting a lot higher loads and your foundation becomes much larger, and typically deeper unless you happen to be in an area with suitable bearing materials. But since you don’t even have RQDs for your bearing surface, I would highly recommend at least checking the depth of the rock strata. Cobbles form in my area and can range in depth from a foot to dozens of feet thick. PWR verses competent rock also completely changes the deep foundation design. You need soils data if you want to use skin friction too as well as additional info is also needed for determining how deep to socket into the rock. If it were me doing the design, I would not put a stamp on a drawing without a full subsurface investigation because that’s too many assumptions and soils are fickle enough as it is. That’s just my personal opinion, but I’ve had several jobs that the foundation design had to be changed because I pushed for the investigations to be performed. For the record, a competent driller should be able to take core samples for you for a reasonable rate. I had 9 holes drilled with 6 cores sampled at various depths taken from 50’-100’+ and it averaged a little over $1K per core. It doesn’t sound like you need to go nearly that deep so you would have a much lower price point. We did the lab testing in house so there was no cost for us to test the cores.

Drillers typically charge by the foot when drilling. If your rock layer is truly only 5’ down, the cost would be remarkably less. Perhaps you should reach out to some drillers and get quotes and push for the info so you can have the necessary data to actually design the foundation without having to make a ton of assumptions. Then you will have a design that will work. Otherwise, you’re in a litigious space and you do not want that kind of liability, it’s just not worth it.