Why Helical Piers?

A comparison of alternative foundation supports

Gary Collins, P.E.

The simple answer is price and performance.  In many cases helical piers are the easiest to install and this leads to lower cost.  They also have the most predicable load carrying capacity.  However, this is not always the case.  Discussing these exceptions is the purpose of this article. 

This article is aimed at installers and designers who are unfamiliar with helical piers and are trying to educate themselves to this increasingly popular form of foundation support.  To do this, I will discuss the strength and weaknesses of all the varieties of foundation support.  Then I will summarize them in a table for ease of comparison.  The methods can roughly be divided into light and heavy structure supports.

A.  Light to moderate structure supports

Helical Piers

Helical piers can be used almost anywhere traditional deep foundations can be used according to Don Bobbitt PE, an experienced helical pier engineer.[1]  Typically, they are better suited to the lower capacity applications that make it less economical to use the larger install equipment required by the more conventional deep foundations.  They also tend to be more economical in limited access sites or for vibration or noise free applications.  However, the economics of each case generally controls the foundation selection. 

Helical piers are installed where one has a torque driver machine that can screw them into the ground.  Usually this is a hydraulic torque head mounted on anything from a portable torque frame that can fit into small spaces up to large backhoe mounted devices. 

Helical piers screw themselves through the many layers and finally into bedrock.  The layers are usually revealed by the varying driver torque.  This is monitored by a torque pressure gauge read and recorded by the machine operator.  The pier bearing capacity is roughly 10 times the “kips” indicated on the gauge.  The operator is looking for a significant increase in torque indicating he has hit dense, firm load-bearing strata.  For many locations, this is on average 20 to 30 feet below grade. 

Sections are added as the pier is screwed into the ground.  The final section is cut off at a level even with the other piers and capped with a load-bearing plate.  It is immediately ready to receive a load.  There is no cleanup.  This process is quick if everything goes as planned and is a matter of hours for a multiple pier job.  If difficult soil is encountered and pre-drilling is necessary to break into hard rock, it can take a matter of several days.  A very good comparison with traditional drilled piers can be found at the Helical Pier World’s site: http://helicalpierworld.com/articles/taleof2part2.aspx.  This running account of two side-by-side jobs speaks volumes about many factors in pier installation.

An advantage of helical piers in expansive soil is that they resist upward forces.  The helix is anchored in competent load-bearing soil or bedrock, and the frictional forces along the shaft are negligible compared to the end loading force.  This means the helical pier is versatile with either upward or downward loads.  This is not the case with other types of support without secondary operations or modifications such as filling them with grout.

There are different manufacturers of helical piers, and they are not all equal.  Connection stiffness is an issue.  It needs to be paid attention to since a weak joint under compressive forces will buckle.  There are post installation techniques to stiffen the connections and shafts (see section on Helical Pull-Down Micropiles), but it is better to use piers that have stiff connections to begin with. 

One advantage of helical piers is that if a rock is encountered that stops forward progress, the pier can be withdrawn and drilled several feet away.  Don Bobbit has also written a very comprehensive paper on the many difficulties and non-technical factors involved in successful installation and use of helical piers. [2]