Percussion vs Rotary drilling 1

[voi]

Hi
We have a situation where a contractor is to install concrete bored piles (to support a multi-storey structure (30 floors)) to a depth of 20 m in weathered rock(4 m) underlain by clay(10 m) and then good rock. The Geotechnical Engineers have designed for 600 mm size piles but the contractor wants to use 1000 mm sizes at his own cost. He wants to drill using cable and tool method but the Geotechnical Engineer is adamant that he must use rotary method of drilling mm because the site is in a city and he feels that the nearby structures will be affected by this method of drilling and also it will take too long and very noisy. He also feels that the large size piles will lead to overlapping of the bulbs of pressure and also lead to cracks in the bedrock.He is insisting that the contractor must use the 600 mm designed sizes and use rotary drilling. Is the Geotechnical Engineer right or he is unduly worried.

Voi

 

[hokie66]

Interesting. I imagine there is more to this than you have described. The contractor obviously is not doing this for no reason. Are you saying that the contractor is proposing to provide the same number and depth of 1000 mm piles instead of 600 mm piles? How would the spacing be affected? Do the pile caps require redesign?

I don't have experience with percussion methods of large diameter bored piles, so although I sense that the geotechnical engineer's concerns are justified, I don't know. In addition to the issues which you noted, I would wonder about the procedure involved in rock socketing. How would the bottom of the holes be cleaned?

 

[voi]

hokie66,
yes the contractor is not changing the number of piles. He says he does not have 600 mm casings but a lot of 1000 mm casings and his procedure for socketing is not in dispute. The cleaning and removing of cuttings is with use of bentonite. The Geotechnical Engineer feels that 1000 mm piles will bring the point of influence much closer and he has refused to redesign the piles to take into account the larger diameters so that the spacing will have been increased. If he did that the Geotechnical is of the opinion that the pile cap cost would go up.
Voi

 

[Ron]

While I agree with the Geotech on the vibration issue, I think he should look harder at the pile size change and apply a bit of common sense to the problem rather than relying on sometimes loose geotechnical theory.....they ain't buildin' a watch.

If the piles are socketed in competent rock, there really is no pressure "bulb" concern as in soil, just overlapping shear in the rock.

 

[hokie66]

The casing availability sounds like a good reason for the change. And yes, the pile caps will cost a bit more, as will the concrete in the piles, but the contractor would be in the best position to judge this. If he is bearing those costs, the building owner is getting more for his money, and the bearing on the rock will be much larger.

The noise, and the time, need to be discussed, but these are construction issues rather than structural.

 

[GeoPaveTraffic]

Drilling method is likely to cause noice and dust problems. However, if those can be overcome; then there should be no problem.

The geotechnical engineer appears to be off base. If the diameter goes up, then the pressure will go down, and the area of influence will go up. So while the pressure bulbs will be closer and may overlap, the acutal pressure is lower.

Sounds like the geot. engineer doesn't want to make the change for some reason and is trying to come up with reasons to support his opinion.

Mike Lambert

 

[voi]

Hi
Most of the answers have concentrated on the design aspect. What about the issue of percussion vs rotary drilling as the heading on the post mentions. Percussion using the method the contractor wants use is basically hammering into the ground and smashing the rock. This method will have effect on the adjoining bedrock and structures and also the issue of speed. Has anybody used this method of cable and tool method for piling work. The Geotechnical Engineer feels the method is outdated is used in drilling for water and also interfere with the skin friction since he is not sure that the annular space between the pile and the bedrock will be effective.
Voi

 

[hokie66]

As I said before, I have no experience in using percussion drilling for installation of large diameter bored piles. But I assume the piling contractor does, or he would not be proposing this method. Not all geotechnical engineers are piling experts, and not all piling experts are well versed in all methods. What is your role in the project? If you are the owner or construction manager, you are within your rights to require further information and justification of the proposed system. I doubt that noise or vibration is a valid objection, but time of construction could be.

I don't understand the concern about skin friction without a better explanation. But with more than twice as much bearing area on the rock, that would probably not be a valid concern.

 

[voi]

Hokie 66,

I also do not have experience with any kind of Piling. I work with the local authorities in the geotechnical section and this issue has been put in front of my Boss by the consultants who have objected to the contractors method. So I wanted to hear what others on this forum would say. I am waiting to hear how my boss would decide.
Voi

 

[dik]

Percussion generally provides a rougher interface... maybe that is his issue...

Dik

 

[BigHarvey]

Vibrations for this type of technique is generally not a concern. There are not lot of data, but we published results from chiselling in a diaphragm wall job. Th results are attached.

More generally speaking, it would be more logical to select your contractor in accordance with what he is able to do. Now you have a contract with him, it might prove difficult to finish the job imposing him techniques he doesn't master. It is true that his method wil be slow, noisy and oblige the geotechnical engineer to check his design again but it might be the only alternative left at this stage !

 

[BigHarvey]

Sorry the attachment was missing

 

[oldestguy]

Then comes the question of who gets sued if the neighboring owner gets upset or has a claim? I'd get that settled before the job starts.

 

[voi]

Hi All,
Thanks for all the comments and suggestions. It was quite educative.I would like to thank particularly BigHarvey for the paper on vibrations.
voi

 

[fattdad]

While the diameter may have been specified, was the drilling method also specified? Were there vibration standards specified?

how the hole is made sounds like a "means and methods" issue. Just not sure if the contract is so tight that it actually speaks to means and methods.

How is "rotary" defined in the contract? 'Cause after all there is air-rotary and mud-rotary drilling methods. "Rotary" drilling is not so much that the rods "rotate" but that there is a circulating fluid (air or water) that runs down the rods lubricates the bit and conveys cuttings to the ground surface. For example, I'd never consider "auger" borings to be "rotary" drilling. So, if "rotary" drilling is specified, does that exclude augering?

On the matter of diameter, I'd think the contractor could submit some shop drawing for consideration on how his/her approach would meet the engineering intent of the design. No doubt, the engineer would have to bless that design submittal though. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

typically when a contractor wants to make a value engineering proposal such as this, he is required to submit sufficient data so the owner can make an informed decision. the data should include advantages and disadvantages, schedule impacts, other project impacts as well as cost. the cost savings should be shared by both. Any cost increase should be born by the contractor. cost and schedule impacts should also include any required engineering analysis.

if the plans or specs did not preclude cable tool drilling methods, than it is not a contractual requirement.