Comparison of Diesel and Hydraulic Hammers for Pile Driving

Insight into the resurgence of the diesel pile hammer By Michael D. Justason, M.Eng., P.Eng., Product Manager, Bermingham Foundation Solutions

INTRODUCTION This paper compares and contrasts diesel and hydraulic pile-driving hammers. The body of the paper examines the following criteria for comparing and evaluating the two types of hammers: operational criteria, controllability, verifiability, efficiency, effectiveness, cost, and environmental concerns. The future development of ‘intelligent’ pile-driving is also addressed briefly. It is assumed that the reader has a general understanding of the theory and operation of both diesel and hydraulic impact hammers.

Although the basic principles of the diesel hammer have not changed in 65-years, this paper underlines the reasoning behind pursuing, modernizing, and perfecting this simple and efficient technology.

A FRAMEWORK FOR EVALUATION Contractors, owners, and equipment manufactures evaluate several criteria in the selection of pile driving equipment. The following sections will examine the merits of diesel hammers with respect to several evaluation criteria. The merits of diesel pile hammers will be compared and contrasted with hydraulic impact hammers – the main competing technology. The purpose of this section will be to rationalize the further development and modernization of diesel hammer technology. Evaluation criteria will include the following: • Operational criteria such as safety, reliability (minimized

‘down-time’), familiarity, and operating weight • Controllability (can the impact velocity/force of the

equipment be easily controlled?) • Verifiability (is there some means of assurance that the

equipment is performing as intended?) • Efficiency, Effectiveness, and COST (does the hammer

perform the required ‘work’ in a reasonable time at a reasonable cost?)

• Environmental concerns (noise, vibration, and emissions) • Optimization (this is an ‘advanced’ evaluation criteria that

may only apply to high-volume specialty pile-driving contractors)

1. Operational Criteria

A. Safety

Equipment manufacturers of all types of hammers have addressed the issue of safety. It is difficult to give an advantage to either diesel hammers or hydraulic hammers in the area of

safety, as the safety of a pile-driving operation depends more on the activities surrounding the process of pile-driving than the operation of the pile hammer itself.

B. Reliability

The simplicity of operation of a diesel hammer gives it an advantage over hydraulic hammers in the area of reliability. During operation, a single-acting diesel pile hammer can have as few as five (5) moving parts, most of which can be serviced or replaced in a matter of minutes. As a self-contained, self-powered unit, a diesel hammer requires no external power source, and thus avoids potential problems related to a hydraulic power unit engine, pump, and hoses. While problems with hydraulic hammers generally necessitate repairs by a qualified service technician or manufacturer representative, problems with diesel hammers can most often be diagnosed and solved by the contractor’s own site personnel.

C. Familiarity

It would be very difficult to find a piling contractor in the world that does not have experience with diesel pile hammers. This evaluation criterion, while it may seem trivial, can be a major factor in a contractor’s selection of pile driving equipment. Contractors in the United States seem particularly loyal to diesel hammers.

D. Operating weight

The operating weight of a pile hammer can influence the selection of the crane or piling-rig, which can be the single largest equipment cost for a contractor. Typically, a diesel hammer will deliver higher impact energy than a hydraulic hammer at an equivalent or lesser operating weight. A more thorough comparison of two competing technologies is shown later in this paper.

It should be noted that the weight of the hydraulic power unit associated with the operation of a hydraulic hammer is often forgotten. Hydraulic power units are generally large, and while these units can sometimes be mounted on the back of the piling rig, many times they cannot, creating an added complication to the mobility of the piling operation. Dedicated hydraulic piling ‘rigs’ can overcome this problem by incorporating the necessary hydraulic flow to run the hammer into the design of the base-unit itself. However, these dedicated rigs are expensive and as such have very little penetration into the North America.

In the area of operating weight diesels have the advantage.

2. Controllability

One of the main misconceptions about diesel hammers is that they are not controllable. This is partly due to the fact that the performance (impact energy) of a diesel hammer depends on the soil resistance, and the mass and stiffness of the pile foundation being driven. Because the capacity of a pile foundation depends on the performance of the pile-driving hammer, governments and consulting engineers have shown a recent preference for hydraulic hammers, since the impact energy or ‘stroke’ of a hydraulic hammer is relatively independent of the pile resistance. The interdependency of the pile capacity and the hammer performance can add an extra level of complexity and uncertainty to the pile installation process.

Traditional diesel hammers are equipped with discrete ‘energy settings’. These settings control the amount of fuel delivered to the hammer, and thus the resulting ‘stroke’ and impact energy. More modern diesel hammers are equipped with an infinitely adjustable fuel delivery or throttle mechanism, which allows the operator better control over the hammer performance, up to the maximum performance ‘permitted’ by the pile resistance. Many governments and consultants are not yet familiar with these features. While these newer developments have increased the controllability of diesel hammers, the advantage of controllability must still be given to a properly functioning hydraulic hammer.

3. Verifiability

In pile driving, ‘QA’ and ‘QC’ typically consist of pile installation records that log the number of blows per unit of penetration, and the final tip elevation for each pile. Very often, however, the performance of the pile hammer is not logged on the pile installation record, or more disturbing, the pile hammer may appear to be operating at the desired impact energy, while in fact it is not. This can lead to a potentially dangerous situation, whereby piles are believed to have more capacity than they actually do. Historically, the most common problem with diesel hammers has been the phenomenon of ‘pre-combustion’ or ‘pre-ignition’. This problem still exists for diesel hammers that operate using a fuel delivery system known as impact-atomization. Unfortunately, a diesel hammer that is experiencing pre-combustion may not show any visible signs of a problem. The hammer may still ‘run’ with the desired stroke, yet the impact velocity may be reduced, causing the blow-count to increase and creating the artificial impression of pile capacity. A similar problem can occur with an improperly adjusted hydraulic hammer. If the hydraulic cylinders that lift the ram are activated prematurely, then the actual impact velocity of the ram can be reduced. Again, a dangerous over-estimation of pile capacity will result. Diesel hammers that operate using a more modernized fuel injection system do not experience pre-ignition. Verification of the performance of a diesel pile hammer is possible through the use of an instrumentation port that allows for monitoring of the impact velocity of the ram using magnetic proximity switches. Testing has shown that fuel injected hammers do not experience a loss in impact velocity as the hammer temperature increases. This innovation has lead to further development of the velocity

monitoring system to include other features for greater QA and QC in pile driving. In the early 1990’s, Berminghammer began offering a Pile Driving Monitor (PDM) with the ability to record; pile depth, blow count, measured impact energy, and various other pertinent information. A similar innovation by Pile Dynamics Inc. has been the SaximeterTM. This device uses sound to detect the blow-rate of a particular hammer, and using a basic formula, the device can estimate the stroke, or the potential energy of a hammer. It also offers the ability to do blow counts, and to store other pertinent QA and QC information. While the standard SaximeterTM offers no ability to detect impact velocity, there is a version of the device called the E-SaximeterTM that uses proximity switches, similar to the Berminghammer PDM system that allows it to monitor impact velocity for either diesel or hydraulic hammers.

Combined with either of the above monitoring systems, the true impact (sometimes called kinetic) energy of a hammer can be verified.

Some hydraulic hammers have built-in hammer monitoring systems, usually on the hydraulic power unit, which can provide impact velocity and/or energy information. Currently, these features are only available on the more sophisticated hydraulic hammers.

In general, the ‘verifiability’ of both diesel and hydraulic hammers depends more on the hammer manufacturer than it does the ‘type’ of hammer. In other words, some manufacturers of both diesel and hydraulic hammers have addressed QA and QC issues, while others have not. Some consultants and

Figure 2 – photo of Berminghammer’s PDM

Figure 1 – photo of PDI’s E-SaximeterTM

government agencies are familiar with the energy monitoring systems available from leading hydraulic hammer manufacturers (typically European), however, the idea of monitoring the impact energy of a diesel hammer is less well known. Use of energy monitoring systems on diesel hammers has allowed diesel hammers to replace hydraulic hammers on jobs where the owner/consultant had specified a hydraulic hammer with an energy monitoring system.

4. Efficiency, Effectiveness and COST

A. Efficiency

Since the introduction of PDA testing, the concept of ‘efficiency’ has become popular when discussing pile driving hammers. The ‘efficiency’ (as it has been discussed in the ‘popular’ sense) can be defined as the percentage of a hammer’s rated (or ‘potential’) energy that is delivered to a pile (as measured by a PDA testing system). Use of the word efficiency in this manner is incorrect and not what manufacturers of PDA equipment intended.

It is the nature of diesel hammers that some portion of the hammer’s potential energy (ram mass x actual stroke) is used to compress air used for combustion. This results in an impact energy (or kinetic energy) that is less than that of a free-falling mass. Hydraulic hammers, by contrast, operate using a remote power source (power pack), and do not need to use any of the ram’s potential energy for the operation of the hammer. Although frictional and other losses still occur, the ram in a hydraulic hammer acts more like a free-falling mass (in fact some hydraulic hammers even have an ‘accelerated’ ram). Unfortunately, the industry ‘rates’ all hammers using the maximum ‘potential energy’ of the hammer, which is clearly a flawed rating system when comparing diesel and hydraulic hammers. When using this flawed system of hammer energy rating combined with the misuse of the word ‘efficiency’, diesel hammers are at a distinct disadvantage. Surprisingly-educated engineers often refer to diesel hammers as being typically 20-30% ‘efficient’, while hydraulic hammers may get an approving nod accompanied by comments referring to an ‘efficiency’ of 60-70%. Some of the more clever hydraulic hammer manufacturers ‘rate’ their hammers by their actual impact energy – creating a magical class of 90% ‘efficient’ hammers.

More recently, consultants, PDA testing companies, and academics have begun correcting their terminology. What was incorrectly referred to as ‘efficiency’ is now referred to more correctly as the ‘energy transfer ratio’. This ratio is of interest when performing drivability studies and when evaluating the entire hammer-pile-soil system, but should never be used to assess the ‘efficiency’ of a particular hammer. This terminology problem reached a critical point when, on several occasions, project specifications explicitly excluded diesel hammers. One such specification actually declared that; “Diesel hammers will not be allowed due to their inefficiency.” These types of comments are cause for great concern, and illustrate the importance of proper use of language in engineering. The misuse of the word efficiency has undoubtedly hurt the market for diesel pile hammers, and there are a large number of contractors, consultants, and state engineers that still have a false impression of the ‘inefficiency’ of diesel hammers.

The true energy efficiency of a pile-driving system must consider the amount of ‘work’ performed in a given period of time, and the amount of diesel fuel consumed to perform that work.

Data on fuel consumption for diesel hammers, and hydraulic power packs, suggests that a fuel-injected diesel hammer can be as much as ten-times more efficient than an equivalent hydraulic hammer. This fact alone is justification for the further development and pursuit of diesel hammer technology, and a testament to the true efficiency of diesel hammers.

B. Effectiveness

Before exploring the concept of energy efficiency and driving ‘effectiveness’ in greater depth, it must be realized that the ‘nature’ of the energy delivered by a diesel hammer and a hydraulic hammer is fundamentally different. A diesel hammer uses a small mass with a high impact velocity to produce impact energy, while the converse is true of a hydraulic hammer. The higher impact velocity of a diesel hammer is commonly believed to be better suited to steel piles, capable of withstanding high driving stress, while the lower impact velocity of the hydraulic hammer is traditionally deemed more suitable for concrete piles. These different characteristics of the two hammer types make them more ‘effective’ for different types of jobs, with different types of piles and different types of soils. The complications surrounding the whole hammer-pile-soil system makes the evaluation of the effectiveness of a particular hammer very difficult without actually driving a test pile. Even when test piles are driven, it is rare that different hammer types are compared.

Table 1 shows some typical specifications for a mid-sized hydraulic and diesel hammer for comparison. Notice that the rated energy of the diesel hammer is about twice that of the hydraulic hammer, yet at 40-BPM (blows per minute) the diesel and hydraulic hammers produce the same impact energy. This suggests that the two hammers are roughly equivalent. However, remember that the ‘nature’ of the two energies is different. Although both hammers operate at 40-BPM with the same impact energy, because the diesel hammer has a less massive ram then it must have a correspondingly higher impact velocity (E=½mv2). Also, because in pile driving, higher impact velocity means higher impact force (F=vZ, Force equals velocity×impedance), the diesel hammer can overcome a higher soil resistance during driving and produce a pile of higher load carrying capacity. The only disadvantage in producing a high impact force is that the resulting pile stresses are higher; hence the long-held belief that diesel hammers are better for driving steel and hydraulic hammers are better for driving concrete. It should be noted here that certain types of ‘accelerated-ram’ hydraulic hammers can also produce a high impact velocity and a resulting high impact force. It should also be noted that a controllable-energy ‘over-sized’ diesel hammer operating at a low stroke (high blow-rate) can prove as effective on concrete piles as a hydraulic hammer.

Table 1 also shows that the diesel hammer is capable of operating at a 36% higher energy than that shown at the comparison level of 40-BPM, while its weight is about 21% less than the hydraulic hammer. This suggests that the diesel

hammer produces more impact energy per unit of operating weight. This statistic is quantified in the last row of Table 1.

Table 1 – Comparison of operating weights for comparable ‘impact energy’ diesel and hydraulic pile hammers

Hydraulic

Hammer Diesel

Hammer “Rated” Energy (ft-lbs) 26,000 53,000 Impact Energy (ft-lbs) @ 40-BPM 25,000 25,000

Ram mass (lbs) 6,600 4,630 Impact velocity @ 40-BPM (ft/s) 15.6 18.6 Impact Energy at Rated (max) stroke (ft-lbs) 25,000 34,000

Impact velocity at Rated stroke (ft/s) 15.6 21.7

Operating Weight with drive system (lbs) 14,000 11,000

Max. Impact Energy per unit operating weight (ft-lbs/lb) 1.8 3.1

The overall ‘effectiveness’ of a pile driving hammer is difficult to quantify as the above discussion demonstrates. Cutting through the technical jargon related to velocities and energies, leads to a single simple measure of the effectiveness of a pile-driving hammer – time. The time required to drive a given pile to a given penetration and/or capacity is the best measure of effectiveness…keeping in mind…

C. …COST

Cost is probably the single most important variable in a contractor’s evaluation of pile-driving hammers. In this respect, the American Contractor’s loyalty to the diesel hammer is explained. For a given impact energy, efficiency, effectiveness, etc., the cost of a diesel hammer can be estimated at 50-75% of an equivalent hydraulic hammer or even less in the case of Chinese made diesel hammers. Capital cost of the equipment combined with much lower fuel costs make diesel hammers the clear-cut winner on the basis of economy. As diesel hammers continue to catch-up to hydraulic hammers in the areas of controllability and verifiability, their lower cost will ensure their continued presence in the world pile-driving market.

5. Environmental Concerns

Recently, diesel hammers have fallen out of favor in some countries and some urban areas due to their environmental impact – mostly, noise, vibration, and black smoke. In the United Kingdom, for instance, it has been nearly 10-years since diesel hammers have been used. While no formal legislation prohibits the use of diesel hammers in the UK, contractors have avoided their use in an effort to avoid complaints, and likely to appear more modernized. Many observers of the global foundation equipment market would classify the diesel pile hammer as nearing the end of its product life-cycle.

Air pollution from diesel pile hammers is currently being addressed by manufacturers. It is entirely possible to improve diesel hammer fuel combustion so there is virtually no visible exhaust. Fundamentally, the diesel hammer should have an advantage over hydraulic hammers with respect to exhaust emissions given the higher fuel-efficiency of the diesels as described previously. Of course, conventional emission controls for diesel engines can be applied to the hydraulic power units used to run hydraulic hammers.

Currently, the lesser cost Chinese diesel hammers have not addressed the environmental concerns of air pollution and noise, but they have been successful using so-called bio-fuels, as have the manufacturers of more sophisticated diesel hammers. Hydraulic hammer manufacturers also provide the option to run their hydraulic power units on bio-fuels.

Noise continues to be a challenge for manufacturers of both diesel and hydraulic hammers. Hydraulic hammer manufacturers have been more successful in this area due in part to their lower impact velocity, but also due to the fact that a hydraulic hammer can be completely shrouded. It is more difficult to shroud a diesel hammer given its demand for clean air and air circulation for cooling.

Environmental concerns are an area of continued development for both diesel and hydraulic hammers.

6. Optimal Pile-Driving

This is an area of pile driving that is currently under development. By combining the data from an on-board hammer energy monitor and the data from a conventional PDA testing system, together with a system for controlling the energy of a hammer (such as the remote throttle), it is theoretically possible to construct a ‘control-system’ that is designed to operate a hammer at the most effective energy setting, or perhaps a target energy. The potential for an ‘intelligent’ pile-driving control system is very real. So far the only hindrance has been the general lack of sophistication among hammer manufacturers, combined with a ‘separation’ between the manufacturers and design engineers and owners. The recent increased demand for QA and QC for pile driving, has paved the way for these types of ‘intelligent’ systems. These systems are coming, and with them the resurgence of the diesel hammer; even in European and other markets where they have fallen out of favor.

SUMMARY AND CONCLUSIONS Through the discussions presented in this paper, it was concluded that diesel hammers have an advantage over hydraulic hammers in the following areas: operational criteria; reliability, familiarity, operating weight; efficiency, effectiveness, and COST.

The newer, more modernized diesel hammers (with energy monitoring and variable throttle) were generally considered equal to hydraulic hammers in their verifiability.

Hydraulic hammers were slightly more advanced in controllability and environmental concerns.

The general conclusion of this paper is that diesel pile hammer technology is worth pursuing and modernizing. The main advantage diesel hammers have over their hydraulic counterparts is their high ‘efficiency’ and low cost.