For the successful driving of sheet piles, it is essential that a good knowledge of the site conditions is available to enable an accurate assessment to be made of the topographical and geological conditions. After determination of the adequate sheet pile section the choice of a suitable driving system is of fundamental importance to ensure successful pile installation.
Modern installation systems may be divided into three categories: impact driving, vibratory driving and pressing.
Impact driving comprises drop hammers, Diesel hammers, double acting hydraulic hammers and double acting rapid blow hammers.
This hammer is easily adopted to drive any of the pile sections for all ground conditions. A ratio of ram weight to weight of pile plus cap of 1:2 to 2:1 is recommended. The hammer controls are precise and used correctly this hammer can achieve 75-80% of rated output energy. The weight of the hammer is lifted by cable operation, by steam or by hydraulic pressure.
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 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 atomizes 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 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 of flat anvil blocks are necessary to protect the pile heads during driving.
The correct denomination should be double acting hydraulic hammer.
This type of hammer consists of an enclosed ram which is lifted by hydraulic pressure. On the downward stroke, additional energy is delivered to the ram, producing an acceleration of 2 g. The maximum stroke of 1 meter thus correspond to a free fall drop of 2 meters.
These hammers range from a maximum 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 and the design enables a range of safety, monitoring and indicating devices to be incorporated. 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 %.
The hydrohammer can operate at any angles, above and below waterlevel and is suitable both 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.
Up to the present time only hydrohammers from 35 kNm to 90 kNm energy per blow have been known to be used for sheet piling; hammers larger in capacity than these are considered to be too heavy.
Rapid blow hammers
With double-acting hammers the striking ram (piston) is driven by compressed air or steam when rising and falling.
The air or steam 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 sheet pile being driven. Then the pressure lifts the piston and allows it to be forced down again on to the anvil.
In comparison with drop hammers of the same overall weight the ram of the double-acting hammers is much less than that of the drop hammer. It is only 10-20 % of the overall hammer weight but is effectively increased by the pressure (5-8 bar) on the upper end of the piston.
The hammers are designed to operate at maximum efficiency when used with standard sizes of compressors normally available. For such hammers, 90 % of the available energy blow is derived from the action of the air or stream upon the piston.
Vibratory pile drivers
Vibratory pile drivers apply vibrations to the piles to enable them to penetrate certain soil strata.
The principle of vibratory driving is the reduction of friction between the pile and the soil. The vibrations will temporarily disturb the soil around the pile, causing minor liquefaction, which results in a noticeable decrease in resistance between soil and pile. This enables the pile to be driven into the ground with very little added load, i.e. its own weight plus the weight of the driver. The vibratory driver generates oscillations inside a vibration case in which eccentric weights are gear-driven by one or more motors. The weights turn at the same frequency but in the opposite direction, thus eliminating the horizontal components of the forces, leaving only the vertical components operational. The vibratory drivers can be powered by electric or hydraulic motors or a combination of both.
Hydraulically operated clamps mounted under the vibration case ensure a secure attachment and transmit the oscillating movements to the pile. The crane suspending the vibratory driver must be isolated from the vibration case by rubber cushions or spring elements. The variable speed features of hydraulic vibrators enable the frequency of the system to be matched to varying soil conditions.
The frequency for the standard range of vibratory drivers varies from 800 to 1800 RPM and the centrifugal forces go up to 5000 kN. The higher frequency drivers give a range up to 3000 RPM. The high vibrations developed attenuate very rapidly, thus causing very few problems to adjacent properties. The penetration performance depends mainly on the soil conditions.
In the latest development in this type of engine the weights are only moved into eccentric position after reaching work frequency. This means passing the soil frequency at zero amplitude and avoiding the generation of vibrations in the soil in the start and stop phase.
Sheet pile presses
The elimination of the noise of sheet pile driving, which had for years been accepted as nuisance to be tolerated, was the original purpose of the development of sheet pile presses as an alternative to the classical methods.
Originally developed to drive piles silently, the machines are also widely recognised for their vibration-free operation.
The machines, which are especially suited for use in cohesive soils, are hydraulically operated and take most of their reaction force from the friction of the previously driven piles. In the standard system, the engine consist of a crosshead containing hydraulic rams and the hydraulic power pack mounted on the crosshead.
Sheet piles are installed in a panel and the machine is set on the panel by means of a crane.
The rams (hydraulic cylinders) are connected to the piles and by pressurizing two rams, whilst the others are locked, enables the piles to be pushed into the ground, two at a time, to the full extent of the rams. When all the rams have been extended, they are all retracted simultaneously causing the crosshead and power pack to be lowered and the cycle is then repeated to completion.
These presses can develop forces of up to 300 tones.
|Other kinds of presses jack one pile after another to the complete depth while walking on the previously set piles. These machines work completely independently from a crane, and also use the reaction force of the piles already set to operate. These machines can accommodate limited circular construction if required.|
Special driving systems
Besides the common driving systems there are many special types designed for particular jobs or applications, these include:
- Impact hammers incorporating a special cushion system formed out of steel springs or inert gas to transmit the driving energy to the pile more smoothly and over a longer period of time.
- A driving system which vibrates and presses on sheet piles simultaneously.
- An impulse hammer operated hydraulically but with an extremely rapid sequence of strokes.
- A driving system which impacts and vibrates the sheet piles simultaneously.