Driving of sheet piles
Sheet piles are installed in the ground by one or a combination of the following methods :
Vibrating is in many circumstances the most efficient method. In combination with leader guiding it is also a very accurate method of driving sheet piles to the required depth. However, if very dense sands and gravel above groundwater level or stiff clay layers have to be driven through, vibrating may be ineffective. In these cases either driving assistance or impact driving may be required. When obstacles are present and cannot be removed, either predrilling or careful impact driving are the best methods to be used.
Generally driving with a vibrator causes a higher level of vibration in the surrounding ground than impact driving. High frequency vibrators, where the eccentricity of the rotating mass can be varied during the start up and stop phases of the driving process, can considerably reduce the adverse vibrations of the process on the surrounding ground.
Vibratory driving generally reduces the tendency for piles to lean during driving when compared to impact driving. The main cause of longitudinal leaning is friction in the interlock between the pile being driven and the adjoining pile. This friction causes an eccentricity of the force acting on the sheet pile which cannot normally be adequately corrected by moving the centre of impact of the hammer. Reducing the friction in the free leading interlock is normally a better alternative. Friction reduction can be obtained by various means such as lubricants or by keeping the space in the leading interlock filled with bentonite or cement-bentonite during the driving process (see also 8.5.7 and 8.5.8). Also soil can be prevented from entering the interlock by closing off the interlock at the pile toe.
Vibrations from impact hammers and vibratory drivers are normally considerable and can travel over relatively long distances. Foundations which are subjected to vibration will pick up part of these vibrations and transfer them to the various elements of the supported structure. As a result damage can be caused to sensitive buildings near to the source of the vibrations. Special care is necessary if such structures are founded on loose sand, especially if it is saturated, because it is subject to sudden settlement resulting from vibrations in the ground.
Where vibration or noise is considered a problem, pressing the sheet piles into the ground may be a solution. Normally pressing is effective in cohesive soils. In difficult soil conditions pre-boring and sometimes water jetting can be effective in assisting the sheet pile to reach the required depth (see Annex D for more information).
Different types of pile driving equipment suitable for the installation of the sheet piles are available. The most common types are :
- free falling hammers;
- diesel hammers;
- hydro hammers;
- air hammers;
- high and low frequency vibrators;
- high frequency vibrators with a variable eccentricity of the rotating mass;
- high frequency vibrator with continuously variable excentricity and resonance free start and stop phases;
- pressing systems.
Specifications for these devices can be found in handbooks and producers manuals.
Theoretical analysis (e.g. wave equations) of the driving conditions can help with the selection of the pile driving equipment.