How to choose the installation equipments of combi walls? The selection of piling equipment should consider the entire system: sheet piling, soil, piling formwork, and, if required, piling aids, disturbance (noise and vibration). Furthermore, piling can only be successful if it is based on extensive soil surveys. Sheet piles can be driven into any type of soil (except rock), but the installation of sheet piles is a very complex subject that requires a lot of experience from the installation team.
Standard equipments of combi wall installation
Other aids of combi wall installation
Advantages of suitable equipments
The standard combi wall installation equipments used today mainly includes vibration hammers, impact hammers and hydraulic presses. Impact hammer includes diesel hammer, hydraulic hammer, fast hammer and other types.
The principle of vibrating pile driving is to reduce the friction between the pile and the surrounding soil. The vibration will lead to temporary local liquefaction, which will result in a significant reduction in the resistance between the soil and the pile. This allows for the use of lighting to drive the pile underground. The vibration driver vibrates inside the vibration box by rotating an eccentric hammer. Today, most vibration hammers are hydraulically driven, whereas in the past they also had electric engines. Modern vibrators are equipped with variable off-center hammers for non-resonant start and stop, which is especially important if sheet piling must be done near existing sensitive structures and buildings.
The diesel hammer is mainly composed of a cylinder, a piston and an impact block located at the bottom of the cylinder. To start the single-acting diesel hammer, raise the piston to the preset height and release automatically. The descending piston compresses the air in the chamber and starts the fuel pump to spray fuel onto the top of the impact block. The impact of the piston on the impact block atomizes the diesel fuel, which ignites in highly compressed air. The explosive energy throws the piston up, driving the pile down and restarting the hammer cycle. Diesel hammers do particularly well in viscous or very dense granular soil.
The compressed air drives the piston as the fast hammer rises and falls. The air under pressure reaches the box with the slide valve, which alternately sends it to each side of the piston, while the other side is connected to the vent. In the fall, the impactor hits the flat anvil seat fixed to the cylinder at the top of the driven pile. The pressure then lifts the piston and pushes it down again on the anvil seat. The weight of a rammer with a fast blow hammer is much lower than that of a drop hammer of the same capacity: only 10-20% of the weight of the hammer, but it can be increased effectively through higher pressure (5-8 bar). The upper end of the piston greatly improves the operating efficiency.
Use of a press should be considered if vibration and noise are critical to the installation of combi walls, for example near sensitive buildings in urban areas or near railway lines. Self-propelled press, as one of the most commonly used, only needs limited space for operation. They are clamped to the already drilled piles and are capable of mounting single and double z-shaped or u-shaped piles on corners and circles. To begin the operation, a ballast assist frame with an additional weight (usually sheet piling) can be used. The machine is particularly suitable for use in slightly viscous soils, is hydraulically operated, and absorbs the reaction force from the friction of previously driven piles. Other pre-drilling or water injection may be performed.
In specific circumstances, such as using heavy equipments or reducing vibration and noise, construction teams should consider providing driving assistance: spraying, pre-drilling or blasting. These measures can not only improve the construction efficiency, but also reduce the impact of construction on the surrounding environment, which are widely used in practical work.
Vibratory hammers are very effective in granular soils such as sand and gravel. In clay and very dense granular soils, strong vibrating hammers with high amplitudes may also be used, but impact hammers may be required to drive the sheet pile to the final depth. Hydraulic hammer is suitable for pile driving and pile pulling, can greatly reduce the damage and noise of pile head. Hydraulic press is especially suitable for use in slightly viscous soil. The construction team decides the use of the equipment according to the specific construction conditions and combi wall requirements.
The introducyion of combi wall installation is as described above. The installation of the combi wall is a complicated project. The type of piling equipment determines the installation speed and its impact on the surrounding environment. The selection of the right equipment not only improves the installation efficiency, but also greatly reduces the contradiction that the project may bring with the nearby residents.