Drilling caissons on pinnacled rock
Drilling caissons on pinnacled rock
(OP)
South Central PA is laden with pinnacled limestone and sinkholes which many times require the use of drilled caissons. The caissons typically have bottom sockets drilled into rock from 36" to 72" in diameter. The rock bearing area must encompass 100% of the cross-sectional area of the caisson hole. Unfortunately, this may require hours, days and weeks of grinding on a pinnacle to get to the 100% requirement. During budgeting, alternative designs were not selected as they were more expensive.
Once the Project is committed to caissons and has commenced construction, is there an alternative to socketing 100% into rock at the caisson bottom?
Once the Project is committed to caissons and has commenced construction, is there an alternative to socketing 100% into rock at the caisson bottom?





RE: Drilling caissons on pinnacled rock
In a few cases we had to use a lean concrete and redrill a hole for voids.
It just might work out for you too.
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Drilling caissons on pinnacled rock
I submit that if it actually takes weeks to grind down the pinnacle, then the appropriate foundation solution was not arrived at.
In reality, I would expect that the foundation contractor would need to extend the foundation depth and remove as much rock as neccessary to acheive the design intent. As long as this is addressed in the contract documents, there should be relatively little problem as Qshake suggests.
Jeff
RE: Drilling caissons on pinnacled rock
One of the first things taught to young engineers working in deep foundations in this part of the country is NOT to design caissons in karstic terrain (PA, TN, FL). The reason for this is the pinnacled rock and voids that are encountered that have been mentioned above. These problems can be dealt with using small hole techniques at competitive pricing to the final cost of the caissons. Having just visited a deep foundation project (small diameter) in South Central PA the only advice I have is good luck-some of the most difficult drilling encountered by the contractor in a long time.
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
Within the same job site, which was a large interchange, a very small bridge was constructed with h-pile. Driving had only just began when the usual "we can't get bearing" came up. So I considered the shafts to be good solution.
Incidentally we did use micro-piles in some locations and those went without problems too. However, by using fewer shafts in main foundations we were able to achieve good economy as micropiles are very expensive.
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Drilling caissons on pinnacled rock
Caisson bids including a 2' rock socket ranged from $225,000 to $250,000. The micropile bid was $352,000.
It is difficult to justify to an Owner whose project is already over budget to use micropiles. The Central PA karst region has had many caissons installed in the past, successfully. However, a project that a contractor got stuck spending hundreds of thousands of dollars drilling rock for free, could have been his last. Also, the project schedule was compressed causing acceleration costs for all trades, liquidated damages, etc.
Once the decision is made to use caissons and there is a problem, what can be done? Once started,what is an alternative to drilling rock day after day to get a caisson bottom with 100% bearing in rock? Can the bottom be modified to 75% of the cross-sectional area with some other modifications?
RE: Drilling caissons on pinnacled rock
Drill deeper, get a bigger caisson rig and drill deeper, or switch to micropiles.
To accurately compare bid prices, you need to look at the scope to see who is actually taking the risk. A cheaper price may have a tighter or narrower scope that puts more risk to the owner or GC. Or, a higher price may include the assumption of risk. You can't just look at the price. Most times, you get only what you pay for. Or, you don't get what you didn't pay for.
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
RE: Drilling caissons on pinnacled rock
A couple times you indicate grinding for extended periods to get the full diameter of the pier into rock. If this is the case, then the wrong equipment is being used.
Several times you discuss the relative cost of foundation alternatives. The truth in karst is that you really won’t know the cost until you are done. You can do decent cost comparisons if you have very good subsurface data and if you reasonably account for anticipated extent of voids, related foundation lowering, etc. But you should expect the unexpected and the ability of the foundation type to deal with increased lengths, etc is therefore a significant consideration (which is often one of the prime benefits of the micropiles). While owners often don’t want to hear it, the cost of extending foundations beyond the basis of the cost agreement is not the contractor’s responsibility. So expect to pay additional for pier extension, etc.
You ask what can be done after a decision to use drilled piers has been made. Depends upon where you are in your construction process. I wan’t clear on whether your contractor was actually performing the work yet or whether you’re just anticipating what might happen. If you aren’t too far into the work then you really might want to reconsider the design to ensure the best alternative is being used before you plow ahead. I’ve seen jobs where the foundation type has to just completely change in the middle of the work because the potential impacts of the karst were not fully considered. Not a fun situation. Usually these problems all arise because owners/non-geotech AE’s, etc don’t want to hear the reality of the karst implications from the start. A straight shooter who tells them of the large cost implications, intensive investigation required, etc is often portrayed as “overconservative”, etc. Pressure is often applied to try and force the geotech into giving an answer that is more “convenient” for the short term rather than one that’s based on sound engineering. And in the end, the ground really doesn’t care what they think and everyone ultimately pays more (often involving lawyers) than if proper decisions were made at the start.
While I don’t know the particulars of your project I noticed you mentioned a small footprint. Was a mat foundation considered as an alternative? You’d have to evaluate sinkhole spanning issues and possibly require certain monitoring, etc but might be an alternative to at least consider.
RE: Drilling caissons on pinnacled rock
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