pile-soil combination, whereas others may require several hammers to cope with the varying conditions.

One major advantage of an impact hammer is that the blow count record during pile installation is a direct measure of the pile

resistance. The vertical advance of a pile under a given hammer blow is used as a measure of the pile’s bearing capacity. The

hammer’s interaction with the pile-soil system is both modeled before driving (wave equation analysis) and monitored during pile

installation (Pile Driving Analyzer).

Vibratory hammers are widely used to drive and extract sheet piles, but they are less commonly used to drive bearing piles. Where

bearing capacity is required, the use of impact hammers is the predominant installation technique employed.

Impact hammers are also essential to drive sheet piles when soil density increases. SPT ‘N’ values approaching 40 generally indicate

the limit of vibratory hammer eciency. Here, impact hammers come into their own by being able to shear through dense soils to

reach the design penetration depth.

driving system and pile. The ram must have a mass and impact velocity that is suciently large to move the pile.

A properly functioning hammer strikes the pile in quick succession. It transfers a large portion of the kinetic energy of the ram into

the pile. The stroke of a pile driving hammer is usually between three and ten feet (900 to 3,000 mm).

How does an impact hammer work?

The most common forms of impact hammers in use today are hydraulic drop hammers and diesel hammers. While they operate

dierently, they are both used to drive sheet piles, pipe piles, H-piles and specialty wide flange piles by allowing a ram weight to fall

onto the top of the pile.

Hydraulic impact hammers

Hydraulic fluid is applied to the piston to move the ram. A hydraulic power pack provides the pressurized fluid to operate the

Diesel hammers

An open-end diesel hammer consists of a long slender piston (the ram), which moves inside a cylinder. The cylinder is open at its

upper end, thus allowing the ram to partially emerge from the cylinder. The ram falls under gravity to the pile cap. Upon impact, the

ram pushes the pile cap and pile head rapidly downward. The impact block separates from the ram within a very short time and the

pressure of the combusting air-fuel mixture will cause further separation as the ram is forced upward.

A closed-end diesel hammer cylinder is closed at its upper end, thus causing the ram to compress the air trapped between ram and

cylinder top. When the ram falls, it is subject to both gravity and the pressure in the bounce chamber, hence called double acting.

Pile cap

To ensure that as little energy as possible is lost in the transfer to the pile, the driving energy is transferred to the pile via a driving

cap or spreader plate. The driving cap also ensures the hammer blows act centrally on the pile. The pile cap is matched to the shape

of the sheet pile being driven.

The form of driving cap must be matched to the sheet pile that is to be driven. It is attached to the underside of the hammer by a

loose attachment and is guided by the leader where used.

Sizing the impact hammer

Impact hammers are of the size needed to develop the energy required to drive the piles at a blow count that does not exceed 10

blows per inch at the required ultimate pile capacity. The intent is to select the size of hammer at normal operating condition to be

sucient. Occasionally, it may be required to drive to a higher blow count to penetrate an unforeseen thin, dense layer or minor

obstruction. Jetting or drilling may be a preferred means to penetrate a particularly dense layer. Overdriving often will damage the

pile and/or hammer.

In its simplest form, the impact energy delivered per drop hammer blow is simply the weight of the ram times the fall distance to the

pile cap.

penetration underway. Drop height would increase as needed to ensure a minimum penetration rate.

A more scientific and accurate approach is to use wave equation analysis. The industry has largely adopted wave equation analysis

and it has become a well-used tool for pile driving evaluation. Contractors will often use the wave equation to optimize equipment

selection and hammer makers often make equipment recommendations based on the wave equation analysis.

Wave equation analysis is a numerical method of analysis for the behavior of driven foundation piles. It predicts the pile capacity

versus blow count relationship (bearing graph) and pile driving stress – for example, when a soft or hard layer causes excessive

stresses or unacceptable blow counts. While popular, it is best carried out by an engineer familiar with the software to ensure

appropriate results.

strike the pile. Vibratory hammers are relatively quiet and have many advantages, such as fast installation. They can also extract

sheet piles, can be used underwater, are lightweight, protect the environment (especially animal life) and can be used in close

The vibratory hammer’s ability to drive a pile is a combination of driving force, frequency, amplitude and free-hanging weight. The

driving force of a hammer is determined by its eccentric moment and steady-state frequency.

The size of the eccentric moment aects the driving force, attainable amplitude, operating frequency and power requirements for

the hammer.

• Eccentric moment equals the distance from the center line of gravity to the center line of rotation, times the total number of

• Amplitude is the vertical movement of the total vibrating system, and the direct result of the applied force generated by the

Amplitude = 2 x eccentric moment

(11,990) × 2 = (.216) × 2 = .432 amplitude, or amplitude = 7/16"

For eective driving, the hammer must have amplitude of equal to or greater than a quarter of an inch.

Generally speaking, the higher the amplitude, the more eective the hammer will be at driving piles in soils considered marginal to

vibratory driving. Higher amplitudes may also increase risk of damage to adjacent structures.

• The eccentrics of a vibratory hammer are attached to a shaft, and are mounted in pairs opposite one another, on a horizontal plane

What is the dierence between an electric and a hydraulic vibratory hammer?

In the market today, there are two main types of vibratory hammers – electric and hydraulic. Electric hammers and hydraulic

hammers have many dierences but have similar traits.

Hydraulic hammers are much more powerful than electric hammers and are half the weight. The other main advantage is

that they can spin at a much faster speed. The higher the vibration speed, the less vibration will travel through the soil to

surrounding buildings.

These systems are generally used for sti cohesive soil, solidified sand/silt, gravels, cobbles, boulders and relatively soft rock/rock

layers, etc., where the SPT N value is [25 ≤ N < 300]. The drilling that takes place just below the toe of the sheet pile pair while the

sheet pile pair is being pressed in at the same time prevents a pressure bulb from building up at the pile toe. Dierent auger head

diameter sizes can be utilized depending on the soil conditions and project parameters.

Rotary cutting systems for pipe piles, like what is shown in Figure 7, are designed for pipe piles to core through similar soil and

ground conditions as the simultaneous augering system for pressed in sheet piles. This variation of press-in piling for steel pipe piles

is designed to rotate and simultaneously press pipe piles into the ground. Sacrificial cutting shoes are welded onto the toes of each

pipe pile for faster pile installation into hard soil, rock and even existing concrete (Takuma et al., 2013). The bottom image within

piles for watertightness.

Capability to install piles with very limited access

In addition to the aforementioned basic press-in piling components, a method known as the non-staging method allows for press-

in machines and the equipment needed to carry out pile installation to walk or advance on top of the sheet or pipe piles being

installed, which enables the equipment to operate in limited access areas where conventional pile driving equipment cannot reach.

Examples of limited access areas include slope embankments or water. Figure 8 shows the equipment designed to advance atop the

installed piles to complete the operation which includes the press-in machine itself, a clamp crane, a power unit and a pile runner.

Capability to install piles in low headroom

While there is conventional pile driving equipment that is able to drive piles within limited vertical clearances, press-in machines are

also designed for pile installation where vertical access is limited. Figure 9 displays how press-in piling machines are able to install

sheet piles within low headroom conditions. Both sheet pile and pipe pile press-in machines can install within 13 feet of headroom,

although there are limitations for sheet pile installation within low headroom conditions depending on the density of the soil that the

sheet piles will be pressed into.

Press-in monitoring system

Another notable advantage of press-in machines is that with each pile pressed into the ground with an electronically controlled

static load by using a series of hydraulics, real time conditions, skin friction, toe resistance, penetration depth and operation time of

the press-in force can be monitored. These readings can also help determine axial load capacities during press-in pile installation,

hence their advantageous use for the installation of vertical load-bearing piles. This monitoring is described through the illustration

in Figure 10 below.

References

Takuma, T., Nishimura, H., (2017), “Deep Pipe Pile Cell Foundations Built in Rivers for Expressway Viaduct Widening,” Proceedings of