Coil Springs Vs Thrust Springs: What’s the Difference?
There really is a difference. This category of pillars has a dozen names, each with a specific and very narrow design, but it is also used in the broadest sense for general foundation support and repair.
Let’s be a little closer and define the differences and applications.
A coil spring is a square bar or pipe that has propellers (it looks a lot like a posthole digger). A hydraulic motor rotates the pipe making it bolt to the ground. The direction can be vertical or angled (degree of mass) as required by the design. The stiffer the ground, the greater the load. It has the unique characteristic of having the same capacity in compression (vertical load) as in tension (tie to a wall).
It can be installed at capacity with the installation team, excluding any building, foundation or other mass.
It is ideal for new construction, light loads, fractured footings, tie downs, tension loads, temporary applications (easy to remove) and generally has a maximum actual capacity of 156 Kips, (1 kip = 1,000 pounds) although there are some sources with greater capacities.
What about thrust pillars (also called resistance pillars)? These are open ended tubes that are pushed vertically into the ground until they end in ground that is determined to be strong enough to hold the load they are intended to hold. A hydraulic ram is used to insert the pillars and then they are anchored to the structure. The weight of the structure becomes a mass of resistance to push against, hence the name, thrust pillars. Capacity is limited to the direct load provided by the building. It is a one-to-one capacity with no safety factor (FS). FS comes with multiple thrust pillars which, when properly designed, share the load. Full capacity can only be achieved if the reaction load is heavy enough to withstand it.
A thrust spring is a strong spring, albeit a very simple one. It is stable and ideal for heavy loads. It can be installed in a very small area and can shore up an existing foundation that could be undermined by an adjacent excavation. Typical maximum capacity is 90 Kips.
There are overlaps in some applications, such as nominal to fairly heavy shoring and remediation projects. A thrust post or tie cannot be removed because its sections are only compression fit. It also can’t be a newly built dock because it needs the reaction mass in place to push. Not to be used with light load (due to minimal weight) or with cracked or broken shoes.
Helical piers can perform almost any remediation job because it creates its own unique weight capacity of the structure and not “load-loose-load-release-load” the footing like thrust piers. It is not as good as a thrust spring for propping because it cannot be wrapped to decrease sag. Helical pillars are more expensive but go in faster. Most of the time, they use the same foot supports.
Both are vibration-free, cost-effective, durable, approved by construction engineers and inspectors, and have 30 years or more of design, field testing, and actual field use. If installed correctly, the failure rate is so small that it is practically non-existent. Most failures are related to other factors. All pillars can be easily reloaded for proper performance.
Whether your installer chooses a coil or thrust spring, rest assured that it will be a rewarding decision and a great solution to your problem.