What are TerraPiers?

 

Terrapiers are columns of compacted stone.  The most common way of installing TerraPiers is by first drilling a hole to the desired depth, then backfilling the hole with stone, compacting the stone in layers from the bottom up.

I thought those were Geopiers®.

 

Both Geopiers^® and TerraPiers are a form of compacted stone column, but each system uses slightly different equipment, different design procedures, and different evaluation procedures.  The end product is essentially the same.

_{Geopier® is a registered trademark of the Geopier Foundation Company.}

What is the design capacity of each TerraPier?

 

TerraPiers are not designed to have a specific individual capacity.  Instead, they function by stiffening the overall soil mass, reducing the amount of settlement expected under a specific foundation.  The number, diameter, and depth of the TerraPiers under an individual footing can be varied to determine the most cost-effective solution to limit settlement to the desired amount.

So when you use TerraPiers, do you design the footing like a pile cap?

 

No, foundations are designed as typical spread footings.  Allowable bearing capacities of 5 to 8 ksf are common when TerraPiers are used.

Why do you use an open-graded stone? Won't the surrounding soils ravel into the voids?

 

Open-graded stone compacts quickly, and can be used even where some groundwater infiltrates into the drilled hole.  For movement of fines to occur, there has to be groundwater flow, and unless you somehow pump water from the bottom of the TerraPier, there is simply no significant flow.  As an indication, traditional stone columns have used open-graded stone for decades, and migration of fines has never been an issue.

Don't you need a special tamper foot to compact the surrounding soils properly?

 

Not at all.  Every geotechnical engineer knows that as the preconsolidation pressure of a soil is increased, which is just what compaction does, the lateral earth pressure also increases.  Increasing the lateral pressure compacts the surrounding soils. 

How do you test the TerraPiers?

 

Our testing is intended to confirm that the design assumptions are satisfied, primarily that the stone in the TerraPier has been properly compacted, and that the modulus has been achieved.  Consequently, we use a plate load test, and back-calculate an in-place modulus from the results of the test.

How much does each pier cost?

 

The installation cost varies with a number of parameters, including both subsurface conditions and structural requirements.  We price each project only after evaluating in detail the most efficient combination of TerraPier depth and diameter, considering the foundation sizes and loads.

From what I hear, rammed aggregate columns like TerraPiers are the best way to solve any soft soil problem.

 

TerraPiers are an excellent tool to decrease settlements under foundations.  However, certain subsurface conditions, such as shallow groundwater or the presence of obstructions, can make installation of TerraPiers difficult and costly.  On the other hand, other procedures may be equally suitable and more economical, even where TerraPiers are a technically feasible option.  TerraSystems, with our experience in a variety of ground improvement techniques, can assist in the selection of a technically-effective AND cost-effective foundation solution.