Installation equipment

The principle and effectiveness of vibratory pile driving

The principle of vibratory pile driving is the reduction of friction between the pile and the surrounding soil. The vibrations cause a temporary localised liquefaction, which results in a noticeable decrease of resistance between soil and pile. This enables to drive the pile into the ground with a light equipment. The vibratory driver generates oscillations inside a vibration case by rotating eccentric weights. Most vibratory hammers are nowadays powered by hydraulic pressure, whereas in the past they had also electric engines.

Vibratory hammers are quite effective in granular soils, such as sands and gravel. In clays and very compact granular soils, powerful vibratory hammers with a high amplitude can also be used, but impact hammers may be required to drive the sheet piles to final depth.

Modern vibrators are equipped with variable eccentric weights that allow resonance-free start-up and stop-down, which is especially important if sheet pile works have to be done near existing sensitive structures and buildings.

Diesel Hammers for Efficient Pile Driving

A diesel hammer consists principally of a cylinder, a piston (ram) and an impact block at the bottom of the cylinder.

To start the single acting diesel hammer, the piston is lifted to a preset height and automatically released. The falling piston compresses the air in the compression chamber and activates the fuel pump to spray fuel on top of the impact block. The impact of the piston on the impact block atomises the diesel fuel, which ignites in the highly compressed air. The explosive energy throws the piston upwards thus driving the pile downward and restarting the hammer cycle.

Diesel hammers perform especially well in cohesive or very dense granular soil layers. Under normal site conditions it is usual to select a ratio of ram weight to weight of pile plus cap of 1:2 to 1.5:1

Driving caps are necessary to protect the pile heads during driving.

Battered pile driving is possible within a limited range of angles.

Hydraulic impact hammers: single vs double acting systems

There are two types of hydraulic impact hammers (hydrohammers) : single and double acting.

An hydraulic hammer consists of an enclosed ram which is lifted by hydraulic pressure. On the downward stroke either the ram is driven by gravity, or additional energy is delivered, producing an acceleration of up to 2g for the double acting system. The maximum stroke of 1 meter thus corresponds to a free fall drop of 2 meters.

The hammers range from a minimum energy/blow of 35 kNm to 3000 kNm with a blow rate of 50/60 blows per minute. The electronic control system ensures optimum control of the piling process. The net energy applied to the pile which is measured during every blow and shown on the control panel can be continuously regulated from maximum to less than 5%.

Hydrohammers can operate at any angle, above or below water. They are suitable for driving and extracting piles. Under normal site conditions it is usual to select a ram weight that is in the ratio 1:1 to 1:2 with the weight of the pile plus driving cap. A heavy hammer with short drop is always preferable to minimise pile head damage and noise level emissions.

For standard steel sheet pile driving hydrohammers from 35 kNm to 90 kNm energy per blow are best suited. Larger hammers can be used to install the king piles of a combined wall.

Understanding rapid blow hammers and their operation

In rapid blow hammers the striking ram (piston) is driven by compressed air when rising and falling. Steam hammers are no longer in use.

The air arrives under pressure in a valve box containing a slide valve which sends it alternately to each side of the piston, while the opposite side is connected to the exhaust ports.

When falling, the striking mass hits a flat anvil fixed to the cylinder resting on top of the pile being driven. Then the pressure lifts the piston up and allows it to be forced down again on to the anvil.

In comparison with drop hammers of the same capacity, the ram weight of a rapid blow hammer is much lower: it is only 10-20% of the overall hammer weight but is effectively increased by the higher pressure (5-8 bar) on the upper end of the piston.

The hammers are designed to operate at maximum efficiency when used with standard compressors.

Pressing machines for sheet pile installation in sensitive areas

If vibration and noise are a critical issue for sheet pile installation, for example near sensitive buildings in urban areas, or close to railway lines, the use of a pressing machine should be considered.

The self-walking pressing machines demand only limited space. They are clamped onto already driven piles and are able to install single and double Z- or U-piles, also in corners and circles. To start the operation, an auxiliary frame, ballasted with additional weight (normally sheet piles) can be used.

The machines are especially well suited for use in slightly cohesive soils, are hydraulically operated and take their reaction force from the friction of the previously driven piles. Additional pre-drilling or water jetting can be applied.