Percussion vs Rotary drilling
Percussion vs Rotary drilling
(OP)
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
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





RE: Percussion vs Rotary drilling
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?
RE: Percussion vs Rotary drilling
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
RE: Percussion vs Rotary drilling
If the piles are socketed in competent rock, there really is no pressure "bulb" concern as in soil, just overlapping shear in the rock.
RE: Percussion vs Rotary drilling
The noise, and the time, need to be discussed, but these are construction issues rather than structural.
RE: Percussion vs Rotary drilling
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
RE: Percussion vs Rotary drilling
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
RE: Percussion vs Rotary drilling
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.
RE: Percussion vs Rotary drilling
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
RE: Percussion vs Rotary drilling
Dik
RE: Percussion vs Rotary drilling
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 !
RE: Percussion vs Rotary drilling
RE: Percussion vs Rotary drilling
RE: Percussion vs Rotary drilling
Thanks for all the comments and suggestions. It was quite educative.I would like to thank particularly BigHarvey for the paper on vibrations.
voi
RE: Percussion vs Rotary drilling
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!
RE: Percussion vs Rotary drilling
if the plans or specs did not preclude cable tool drilling methods, than it is not a contractual requirement.