A good connection between micropile and pile cap

Original post in Spanish written by Juan José Rosas (known in the blogosphere as Geojuanjo).

In this post, Geojuanjo will show his idea of a good type of connector between the micropile and the pile cap. Before showing the connector, we will pose the general problem.  



The foundations based on micropiles are often subjected to important compressive stresses which are resisted by a small concrete area. This is the reason why the micropiles are strengthened with large amounts of metal structural elements such us tubes.

For example, a typical micropile could be designed to bear efforts of 700 kN with a cross section of 180 cm2 (this is approximately the section of a 150 mm diameter micropile). This load implies an average stress in the section of the order of 40 MPa. This compression does not generate major problems given that the micropile has been reinforced generously. However, it poses the problem of passing the high compressions to the pile cap, an element which is usually made of simple reinforced concrete. At this point the need for the connectors of the micropiles arises. The connectors have the function of making compatible the important compression that micropiles withstand and the pile cap, avoiding areas of high concentrations of stress that cannot be resisted by the concrete locally.
 

Type 1 - Rebar handles welded to the tube
Type 2 - Segments of rebars welded to the tube
Type 3 - Intermediate welded plate
Type 4 - Segments of rebars welded as braces
Type 5 - Welded plate on the top of the tube.

Some common typologies of connectors are shown above, but when it comes to micropiles that are reinforced with smooth tubes, most typologies have a major disadvantage: the welding becomes critical. The vast majority of the typologies are based on welding the tube of the micropile and the rebars that anchor the micropile to the concrete pile cap. This is, normally the works of micropiles are done with very little available space and acting on precarious elements of resistance and/or equilibrium. This makes the welding one of the most delicate stages of the work. What is more, the welding needs to be done when what must prevail is both good work and fast execution.

The adequate procedure will be: the head of the micropiles will be visible, an officially approved welder will set and attach the connectors, then the rebars will be put in place and,finally, the concreting to build the pile cap, all without regard that some connectors could be necessary between the pile cap and other existing structural elements.

In this operation, the most problematic part is welding. The other operations are normally done by the same worker, because they are not particularly difficult and they do not require excessive productivity as the tempo of these parts often imposed by external constraints.

To the operational difficulty of welding, some other aspects should be considered such as:

- Welding involves a specific security protocol that is hardly compatible with the constraints of a typical work of rehabilitation, repair or reinforcement.

- Quality control can surprise us with the most disproportionate requirements.

- The welding work will be fully overlapped with other activities, so the occupancy level of the welder is small, with the consequent extra cost.

As a result of these problems, in the vast majority of works there is a temptation to use some brave worker without specialisation to weld the connectors, with a protective mask as the sole safety measure and, of course, without any quality control tests.

What is the main alternative proposed by Geojuanjo's connector?

Well, to remove the welding. Geojuanjo proposes that when the micropile is completed but without the injected grout, a threaded rod (or an element similar to a Dywidag bar) will be introduced inside the tube. The rod will protrude 250 to 500 mm from the head of the tube. Subsequently, in the phase of completion of pile cap, the rod will serve to place a steel plate and a nut to press the plate against the head of the tube: simple, fast, clean and safe.

A scketch of Geojuanjo’s connector is presented below. You are welcome to copy and use this fantastic idea.


1- lean concrete
2- threaded rod
3- reinforcement steel tube
4- micropile
5- nut
6- reinforcement of the pile cap
7- steel plate

N.B.: The threaded rod is inserted into the micropile when the injection is freshly performed. The rod shall be supported by wires so it could not be lost inside the micropile and this is not obvious for some laborers.