The best known setup for pile driving equipment is a crane and a set of full-length (of the pile and hammer) leaders, attached to the crane in a variety of ways. But another alternative is to use a “stub” leader, i.e., one that is very short, and a template to align, position and guide the pile. This is traditionally associated with steel piling, so we’ll look at this first.
For these hammers the platform itself is the template, the piles are driven from the top through the legs. Most conventional platforms had angled legs so the hammers almost invariably drove on a batter, which gave rise to the “stick-up” problem, more about that below.
But using stub leaders and a template isn’t restricted to steel piles; it has also been done on concrete piles, as can be seen below.
From a contractor’s standpoint, handing hammers in stub leaders requires a considerable level of skill from the crane operator, but the weight savings and ability to handle the hammer in difficult situations makes the use of stub leaders, when possible, a very attractive option.
Engineering Aspects of Stub Leaders
From the photos above, you can see that piles can be driven with stub leaders either plumb or on a batter. Plumb piles are not much different with stub leaders than with conventional leaders: the key is to have the hammer straight and square on the pile, which means that the leader setup should be balanced to hang straight and side forces on the hammer be avoided.
With batter piles, since the offshore industry used them (and still does) intensively, the most complete specification for such piles is the American Petroleum Institute’s RP2A specification. With stub leaders the pile basically supports the hammer during driving, and the hammer in turn loads the pile with both the impact loads and the static load of the hammer assembly, which in turn acts both parallel and perpendicular to the axis of the pile. Basically there are two important engineering aspects to configuring driving batter piles with stub leaders on a template:
- Column buckling due to the weight of the hammer acting on the axis of the pile.
- Beam loading of the hammer due to the component of the weight which acts perpendicular to the axis of the pile. This creates a cantilever beam with a maximum bending moment at the template. Obviously the weight of the hammer assembly (along with the weight of the pile) will induce bending stresses. These stresses are both tensile and compressive, and both are important to the structural integrity of the pile during driving. The template must also be designed to handle the loads and moments on its structure.
With steel piling, the combined weights of hammer assembly and pile limit the permissible length of the “stick-up” of the pile. Steel piles are easily spliced and added on to, so piles which are much longer than the stick-up can be drive. (Piles which are much longer than practical lengths of conventional leaders can be driven as well.) With concrete piles, these can be splices but there is less flexibility and less resistance to bending moment with splices, which limit the possibilities of driving these with stub leads on a batter. (The ability or lack thereof of concrete to withstand bending stresses also complicates the situation.)
One more important point: the weight of the hammer assembly cannot generally be assumed to be at the pile head, but above it. That’s why the center of gravity information is so important for offshore driving, which led to Vulcan tips such as this.
Stub leaders combined with templates is an attractive option for driving piles, but proper engineering and construction procedures must be followed for successful results.