Drilling caissons on pinnacled rock 1

[GrumpyG]

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?

 

[Qshake]

I just did a project in southwestern MO where there is much the same type of geology and the designed called for drilled shafts of 4.5' diameter. Despite, the potential problems all went pretty well but in some cases rock sockets were drilled out deeper than originally thought.

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,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[jdonville]

GrumpyG,

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

 

[rsully75]

GrumpyG

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.

 

[Geocontractor]

Being a geotechnical contractor, we see this situation quite often. A better solution to drilled shafts is micropiles. They are faster, and work very well in karst geology. Capacities up to 175 tons at costs of $75-$125 per lineal foot, including rock drilling.

 

[PEinc]

I don't believe that there is any best foundation for karst areas. Many types of foundations and grouting have been tried. All have their own problems. Micropiles are one of the better choices as Geocontractor said. You will probably need to use a permanent, left-in-place casing pipe to contain the grout if (when?) you encounter voids. Voids can also lead to large unbraced lengths and buckling of the micropiles. Make sure the driller logs the drilled materials and voids.

 

[msucog]

i haven't done an analysis in the karst formations in nw georgia, but it seems that you could use auger cast piles or caissons as long as you performed some stabilization pregrouting adjacent to the pile locations (sort of like tunnel pregrouting through sections that the soil profile drops down in to the tunnel section). i suppose it would depend the degree of void spaces encountered since the pregrouting might end up costing more than the actual pile/caisson installation. is this about a correct statement? thanks.

 

[PEinc]

Rock elevations in karst can and usually do vary greatly. One driven pile may go 50 feet or less; the adjacent may go 100 feet or much more. This variability can be a big problem for drill rigs with limited drill depth, as you would have with auger cast pile rigs or caissons drills. Micropiles can go much deeper as long as you have enough casing pipe and spliceable core steel. Voids are another great unknown. Unless you use a permanent casing pipe, you may place vast amounts of concrete or grout. A recent project I was involved with spent about $20 million to compaction grout voids prior to drivine HP10x57 bearing piles. The DOT probably will not make that mistake again.

 

[Qshake]

A typical method to deal with voids in karst topography is to use permanent casing at the void. Where seams are encountered it is not uncommon to grout the area and come back and drill out the hole. Both were used on the job I mentioned previously and we were able to get in and get out relatively quickly.

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,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[GrumpyG]

As a follow up, for a small building footprint, I received 3 caisson bids and a Micropile bid for 38 locations.
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?

 

[PEinc]

"Once the decision is made to use caissons and there is a problem, what can be done?"

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.

 

[DRC1]

Safts devlop thier support in bearing and transfer the load as a column. Since you will get little movement out of the bottom of the cassion, positve friction may not come into play. So if the cassion was sized to accomidate the bearing, the question becomes can the harder than anticipated rock support the additional stress of a reuced area. If it was sized as a column, can it be reconfigured, by adding more steel or an H-pile core. One caution is that hard pinicles are often underlain by soft material. We would use a compressed air rock drill to drill 5 feet into competent rock. A large number of holes can be drilled in a day, and you can get a good quantitive feel for the rock. Thus if you do have hard or soft spots, the cassions can be adjusted accordingly.

 

[PEinc]

Yes, but only if the caisson rig can drill deep enough.

 

[motorbiketocrank]

The caisson should not bear on dissimilar materials. It should definitely be fully socketed into rock and you should also have appropriate core/probe holes below the base to ensure that voids do not exist at shallow depth below your bearing elevation. You should likely also have pier entry for visual inspection of the base and sidewalls of the shaft.

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.

 

[BigH]

No mention of this yet, but there is a publication by the put out by ADSC "Construction and Design of Drilled Shafts in Hard Pinnacle Limestone." You might want to check it out - $14 for non-members. I've never seen it so can't vouch for any conclusions/comments regarding it.

http://www.adsc-iafd.com