The other day I was reading what I could find on steel piers vs. concrete piers for foundation repair and ran across the term “skin friction.” With my heart beating a little faster, I read on.
It turns out that “skin friction” is an engineering term that describes the drag soil has on a pier or piling as foundation repair companies shove it through the ground on its merry way toward bedrock or a hard stable layer of earth.
Pre-cast concrete pilings, due to their diameters, have a lot of “skin friction.” Because steel piers are much thinner they have less drag on the surrounding soil and can be driven a whole lot deeper. The issue is moot regarding poured or drilled piers because you are going to drill a shaft anyway.
Steel piers may be recommended when you have a large heavy house or one that sits more than 15-20 feet above the stable soil zone or bedrock. Your house may have been built over fill dirt or peat beds and it could be a long way down to the type of earth the pier would need to rest on.
Steel piers are routinely sent 20-50 feet down, sometimes up to 70. One steel pier manufacturer claims theirs can be go as deep as 130 feet.
Steel piers can be used for interior under-pinning with less muss and fuss and are sometimes installed within the space of interior walls or anywhere outside where space is a problem. The piers are attached to the slab with brackets.
Helical steel piers have “flights” that grab the soil as they are twisted into the ground. They can be used at angles and are sometimes recommended to work in conjunction with vertical steel piers to support foundations on hill sides. 
Poured concrete piers are often used by foundation repair companies in areas with deep sand like along seacoasts. Shafts are drilled into which steel rebar is placed and fresh concrete is poured.
The shafts are designed to broaden out at the bottom to form a foot which is why they are sometimes called “bell bottom piers.”
The piers must be allowed to cure after which the foundation is attached and leveled with shims.
My neighborhood sits on old farmland that used to be black land prairie and is not that far above a layer of bedrock. The expansive clay soils damage the foundations with the shrinking and swelling caused by wet then dry weather patterns.
The houses are mostly one story and 1500 to 2,000 square feet. Pressed concrete pilings are what I see being used the most.
The foundation is raised ever so slightly with hydraulic jacks and set down on the piers. Shims are added during the final leveling process.
A neighbor down the street had his foundation done last month and you can read about it here.
Remember that each problem foundation is a different case that may call for different remedies. The prudent homeowner will have a professional structural engineer inspect the slab and recommend the method of foundation repair best suited for your situation if he thinks it’s needed. And be sure to tell him you’ve heard about “skin friction.” He’ll be impressed.