Pile Hammer, How Does it Work?

The weight(cylinder) never strikes the pile cap.The ignition of the fuel drives the cylinder back up. The force created by the ignition is what drives the pile.

Isn't the force of the hammer coming down exactly the same as the force that sent it up in the first place ie the previous explosion?

Metering the fuel injected would adjust how high the hammer is propelled and thus how fast it is going when it comes back down.

Does anyone here have first-hand knowledge of whether the piston actually bottoms out in the cylinder under normal working conditions? I would tend to agree with ozzb about the cylinder never striking the piston...

Wouldn't the compression of the air fuel mixture ignite before it struck. The forces of combustion would have to be greater then it just striking. Those forces have to over come the downward momentum of the cylinder and apply more to send it back up. You get more force that way and have more lead way for force control by changing air fuel mixture. In that once it strikes you have no increase in compression. If the air fuel mixture isn't right at that point it bottoms out it would not fire.

Aw gee, I thought that John Henry would just swing a BFH at the top of the piling.

(Or maybe a crane would raise a big weight and drop it repeatedly on the piling.)

I'm pretty sure I've seen one of those... in a Bugs Bunny cartoon.

Brand name "Acme"; as Wile E. Coyote can affirm. (Some people around here don't "get" humor.)

And then there is the 8000-ton "Orion" nuclear pile driver, which can drive a pile clear to China in one blast.....

Wikipedia article.

Re: If we have to drive the piles within the rocky soil, how we can drive them into this soil? If it is impossible, is there any other alternative method?

One thing that hasn't been mentioned so far (I don't think) is that there is usually some criteria for deciding when a pile has been driven sufficiently. I was never really involved in the pile driving for any of the projects I worked on, but I sometimes saw the specs.

The specs were usually set by a civil engineer (on our side, we were the customer), after review of soil conditions in the area (determined in various ways including by samples obtained by core drilling). Then a spec might have been something like a minimum insertion distance (often, iirc, distances like 60' or more) and then some kind of criteria like: pile could not be driven more than x inches with y strokes of the hammer with z pounds of force.

My bad memory is not the point--the real point is that the civil engineer involved with setting those original specifications should get involved again if soil conditions turn out to be different than expected such that the original specs for pile driving cannot be met. Substantial changes in approach may be required (or not), but it takes somebody with appropriate knowledge of the design to figure out what might be a suitable alternative.

There are many alternatives to driven piles, such as cast-in place piles, CIP displacement piles, vibratory installations, floating slab foundations. Some of the considerations for piles at least have to do with end bearing or friction bearing, the subsurface stratigraphy, intended bearing loads and lateral loads, allowable pile density, and other environmental factors. You really should discuss this with a geotechnical engineer specializing in deep foundations.

Hmm, Friction piles and end bearing piles both log same data. Friction piles are sensitve to meeting minimum bearing, end bearing piles are sensitive to attempts to over drive them into a end bearing strata, potentially damaging the pile. Neither requires more finesse, as a critical failure in the pile during installation can be as much more of a problem than a reduced load capacity. Actually having overseen the installations of both types in the field, we were always far more concerned about the end bearing on very stiff strata because of the chance of critical failure in the piles than the friction piles in very soft material, after all it is way easier to drive a pile into a material where the weight of the pile carries it down on its own almost (unless someone buried concrete slabs for fill in bay mud 100 years ago and you find it 30 feet down (that can be a shock and break a few piles).

Yes, driving friction piles and end-bearing piles does involve logging the same data, but that doesn't make them equal in driving procedures. By the way, piles don't break but they may collapse. If that is the case, then either the wall thickness is too light for the site conditions, or the driving crew is not on its toes. If one is driving end-bearing pipe piling, the driving criteria is very easy to follow - the piling is simply driven to the number of blows per inch or per foot as defined in the geotechnical/structural engineers' report that is included in and is a part of the project documents. If there are failures, that risk generally falls upon the contractor because the project documents generally do not define means and methods, meaning that the contractor must determine the appropriate pile material details and driving equipment to fit his method of driving and to obtain the results required in the project documents.

Driving friction pile into soft soils is more complicated in that the movement of the pile creates a 'quick' condition in the soil immediately surrounding the skin of the pile, temporarily lowering the skin friction that is the basis of a friction pile's ultimate bearing capacity, thus resulting in artificially lower blow/unit increment counts. Once driving is complete, the 'quick' condition relaxes and the true value of the at-rest skin friction becomes evident. This is the reason that follow-up re-tapping is required to verify the bearing value when that relaxing has taken place. On some projects, load tests are required to verify that the design bearing capacities have been obtained. These tests can be on one pile or on a cluster as prescribed in the project documents, and the results can have risk for the engineer and/or the contractor, depending on the content of the project documents.

One project that I worked on required driving about 10,000' of 12" x .188 wall, closed end pipe pile for an indoor track and football field at a state university. The site was literally a swamp and the subsurface soils report indicated that the soft soils extended to several hundred feet below the surface but there was a thin zone at about 40' deep that the geotechnical/structural engineer believed would permit the piling to develop its ultimate design bearing capacity, if it could be terminated in that zone. Typically, pile driving contracts have a base amount that is the result of the proposed design pile length being used for estimating purposes. Additionally, there are unit prices included for 'add' and 'deduct' quantities for each pile. For example, if the design length is 40' and the contractor drives a pile that goes to 43', he is paid for the additional 3'. Usually the add unit price is much higher than the deduct because there are fixed costs involved. If the blow/unit increment does not near the design criteria when the design length of the pile is fully driven, an additional length must be added and the driving continued. Depending on the time involved for the crew to splice the additional length and the soil conditions, the set-up or relaxing of the 'quick' condition may occur enough that the resumption of driving may immediately reach the design blow/unit increment value. If this happens, the additional length of pipe must be removed, all for no additional compensation for the 5-man crew and equipment costs incurred.

On the subject project, we found it virtually impossible to get the piles to 'take-up' in the desired zone, resulting in driving a total of nearly 30,000' of piling. This obviously created a significant change order as well as complicating the logistics as additional pipe had to be ordered in increments, as we did not know at any point in time just how much might be required. It also created a very uncomfortable position for the engineer as well as the owner. There was even talk of abandoning the project or relocating it but much of the funding had been provided by an alumni group which was very desirous of having this facility built in its designed location. There was a great deal of pressure applied to the crew members to make every attempt to mitigate the additional pile lengths. The PDA consultant was engaged to assist in that process but there was so much variability in the soil conditions that a simple formula was out of reach. Many re-taps were performed without conclusive results, as some would indicate much improvement in bearing capacity while others would simply continue advancing as they had on the first driving. The piles were in clusters and the rate of driving required that the re-taps had to be performed on a cluster once or twice removed from the one in process, resulting in moving all of the equipment without knowing what the result would be.

Finally, and with all due respect, there is a significant difference between overseeing a particular operation and being at financial risk and responsible for its actual performance.

Steel pile casings collapse and fold. Wood and concrete piles fracture and break. As one of those engineers who wrote those geotechnical reports and condcted the field inspections, I can state there is mroe involved then just logging blow counts. Typically on critical pile supported structures we would actually measure the acceleration and distance a pile travels when we get into a the proper blow count range or strata, and in some cases we would even require a load test on the piles before the caps and beams were constructed. I would actually be a bit concerns it the pile bearing strength was solely based on blow counts in the field. In most reports they specify the general type of piles to be used and limitations on the piles, though the contractor must select the specific piles that correspond with the general requirements. This however is standard for all civil projects to specify a general materials and the limitations/requirements and allow the contractor to provide submittals for the specific materials they propsoe to use.

I am still of the belief as are most engineers I know that piles driven into a material such that they get increased frictional resistance as they set is not more complicated than ones that may break or fold. Moving a pile location effects the design of the foundation system due to eccentricities and can reduce the performance of the foundation. Driving extra piles to achieve load capacity is extra time and money. Eccentricities in the foundation system however represent structural problems that may never be fully mitigated to the strength of the original design. I would definitely prefer to have a few piles that had to be driven deeper to achieve strength than a few piles broken.