|Home | About Us | Contact Us|
| Call Monday - Friday 8.00 am - 5.00 pm
Dallas 972.412.2171 Fort Worth 817.870.1381
Prepared by Jim Oursler, Certified Foundation Repair Specialist
Granite Foundation Repair, Inc.
This paper addresses the differences between steel pier systems and pre-cast, pressed concrete systems. These are two of the more commonly utilized foundation repair methods in the Dallas/Fort Worth area. There are benefits and drawbacks to each foundation repair system depending on soil conditions, structural requirements and cost. The reason for outlining the differences in the two systems is to provide the consumer with information to make a well-informed decision regarding residential foundation repair and house leveling.
Pressed or pushed piers are often used to underpin, stabilize and level concrete slab foundations. The two types of pressed pier systems used in residential foundation repair in the Dallas / Fort Worth area are the steel pier and the concrete pier/piling. Both types of foundation underpinning are pressed into the ground until "refusal" is achieved. Refusal is defined as that point at which the pier system ceases downward movement, and the hydraulic system forces the foundation upward.
Pier size and strength: For residential foundation repair most foundation repair contractors utilize a steel pier that is about 3 inches in diameter, or a pressed concrete pier that is 6 inches in diameter. The pressed concrete pier/piling is 10"-12" inches in length, while the steel pier segments are typically either 3' or 5' in length. The steel will typically withstand 50000 PSI (pounds per square inch) of end pressure, while the concrete is rated to 5000 PSI. The driving force need to install either system is in the range of 4000 to 8000 PSI.
All pressed or pushed pier systems utilize the weight of the house to achieve maximum pier depth. Regardless of the hydraulic system type, the maximum force available to drive the pier is the weight of the house in the immediate vicinity of the pier being pushed. There is no other force available. What does this mean regarding the selection of a pier system to level a house? The pier system should be selected that can achieve maximum support depth using the available weight of the house. In new construction, drilled piers are installed using large truck mounted rigs that drive to as much as 30 feet.
Resistance to driving a pier system comes from side frictional resistance, commonly called skin friction, and the blunt surface area or end bearing resistance of the pier system. Both of these factors play an important role in residential foundation repair.
Let's look first at skin friction: Steel piers are smooth, and thus have the lowest skin friction. A pressed concrete piling is somewhat porous, and slightly rough. A conscientious foundation repair contractor overcomes the skin friction of concrete by lubricating the soil with water. The normally cohesive clay soil of the Dallas / Fort Worth area loses adhesion, and skin friction is minimal as long as the pier system is installed with relatively continuous pressure. When the drive is completed, the clay soil rapidly (in a matter of hours) reverts to its normal sticky, adhesive properties and impedes further driving of the concrete pier. Clay soil adhesion to a steel pier is minimal in comparison, as there are fewer voids and a smaller surface area over which the skin friction can exert its force.
End bearing resistance is the more significant differentiator when installing steel piers versus concrete piers. Let's look at how this end bearing resistance during the installation of foundation underpinning is effected by pier diameter.
Given a fixed available pier installation force, which is the resistance caused by the weight of the foundation and house, the narrow diameter of the steel pier enables it to be pressed into the ground to a greater depth than that of a concrete pier. Why is this? This is derived using the equation A = πr2, where A is the surface area of the pier to be pressed downward, and r is the radius of the pier. The equation yields the result that a steel pier has a surface area of 7 square inches, while a pressed concrete pier has a surface area of 28 square inches. This means that the steel pier is driven downward below the foundation with a force per unit area that is 4 times as great as that of the pre-cast concrete pier system. Ignoring surface or skin friction, a concrete pressed pier requires 4 (four) times as much force to achieve a comparable depth to that of steel! When skin friction is taken into account, the pressed concrete pier or piling requires even more force to drive than that of smooth surfaced steel pier. It is likened to trying to the differenced in forces required to drive a ball point pin versus a hammer handle into yard.
The Pressed/Precast concrete pier system really shines in sandy soil areas such as occur in many regions of Houston, but not in the Dallas / Fort Worth region of Texas. In Houston the depth to a hard surface like rock or shale may be several hundred feet. The pressed concrete pier system will achieve refusal at a depth of ten to twenty feet before side surface friction and end bearing surface area stop the drive. The underpinning system has been driven to a reasonable region of soil stability. In sandy soils, this stability is resistant to changes in moisture, as the water simply passes by the relatively large sand particles.
The clay soils of the Dallas Fort Worth area pose a problem for pressed concrete pier systems as clay extends to a depth of as much as eighty feet before a hard surface (rock or shale) is encountered. With a reasonably constructed concrete slab foundation house, there is sufficient force available to drive a steel pier all the way to rock or shale. The concrete pressed piling system will again stop after ten to twenty feet. When driving pressed pre-cast concrete piers in the Dallas / Fort Worth area, a professional foundation repair contractor will add moisture to the soil, and utilize a relatively slow drive to achieve maximum depth.
It has been said in foundation repair advertisements that the pressed concrete piling system is "self testing", because the house will begin to rise once sufficient resistant has been encountered from the soil. Should a foundation repair contractor improperly drive the concrete system fast, or into dry soil, the concrete piling depth and support will be adversely impacted. The underpinning strength of a pressed concrete pier system is enhanced by a clay soil phenomenon known as thixotropy. This is the property of a remolded clay soil that enables it to stiffen or gain strength in a relatively short time. During the hydraulic drive of the foundation underpinning, with sufficient soil moisture, a slip stream effect occurs which enables the concrete to push clay out of its path. A few hours after the hydraulic drive has stopped, thixotropy begins to occur. The stiffened soil adds additional resistance against any further downward movement of the underpinning system. The soil presents sufficient resistance to enable the foundation to be stabilized and the house lifted, even if the piling system has not been driven to its maximum attainable depth. The objective of having the pier system driven as deep as possible is to extend downward beyond the zone of seasonal moisture variation. If this has not occurred, a pier system resting on clay will be influenced by water and may drop over time. That means that the foundation will drop over time. Having a poorly installed foundation underpinning system drop undermines the original purpose of the foundation repair.
The steel pier installation system is absolutely self testing. Should a contractor fail to drive the steel pier all the way to a hard surface, he will not be able to "level" or stabilize the foundation. As soon as leveling is begun, the pier will continue its downward movement until it hits a hard surface. This is the reality of the true self testing nature of a steel pier foundation repair as compared with that of a precast pier foundation repair.
Why will a precast concrete piling system fail in clay soil? The pressed concrete pier underpinning system often rests upon a water sensitive layer of clay. In the presence of water, clay particles expand, slip, and slide. The most common problem with the reliance upon stiff soil in the pressed concrete pier system occurs when the soil moisture content increases, such as during a rainy season, with a concrete pier driven shallow. Stiff clay becomes soft. The clay loses the ability to support the underpinning. Furthermore, surface friction drops as moisture increases. The weight of the house pushes down on the pressed concrete pier, and it sinks. To correct this problem, the foundation repair contractor may add steel shims between the concrete piling and the house. But that does not guarantee against further sinking of the pressed concrete pier.
Why will a steel pier system not fail in the same clay soils as that of a pressed concrete foundation repair system?
Quite simply, the steel pier system usually rests upon a solid, water insensitive surface of rock or shale. Changes in soil moisture have no impact on the underpinning capacity of the pushed steel pier system.
But what about the steel rusting?
Mild steel shims used on pressed concrete piling systems are much more sensitive to rusting than the high carbon steel used in a steel pier system.
Concrete pier systems are fundamentally cheaper to install than steel. Assuming that both systems are driven to a depth of 15 feet, the material cost of steel will be three times that of concrete piers. Because steel always goes to rock or shale, the actual depth of steel may be much greater, and thus more material cost is incurred during foundation leveling with steel piers. There is some trade-off in labor. Labor cost to install a precast concrete pier is higher because of the need to dig about 5 times the volume of dirt to facilitate installation. This added digging for concrete causes substantial disruption to plants in the area.
Foundation Problem Signs
Granite's Lifetime Warranty
All of Granite's foundation repairs are guaranteed by a fully transferable warranty. Our goal is to fix it right the first time and we stand behind our work. Learn more about our lifetime transferable warranty...
Granite Foundation Blog
GRANITE FOUNDATION REPAIR |
IS A PROUD MEMBER OF:
|Do I Have a Problem?||Foundation Problem Signs||Financing/MC/Visa||Maintenance Tips|
|2012 Granite Foundation Repair, Inc. | All rights reserved||Dallas: 972-412-2171 - Fort Worth: 817-870-1381|
|All rights reserved. Any and all reproductions in whole or in part without the prior express written consent of Granite Foundation Repair Inc. is strictly prohibited.|
|Designed by UEI
|Webmaster||Cree par UEI