Is an “Aggregate Pier” by Any Name, The Same?

 In Feature Articles

Ground improvement techniques have become commonplace in Ontario over the last decade. Owners, developers, contractors, engineers, and architects have all taken notice of the many advantages offered by these innovative systems on their challenging soil projects. They have found opportunities to save themselves and their clients significant amounts of time, cost, and risk versus dig-replace or deep foundations.

Of the various ground improvement techniques used in Ontario, none have been used on more projects than the Geopier Rammed Aggregate Pier® (RAP) system. Since 2005, thousands of linear kilometers of Geopier RAP elements have been installed to support industrial, commercial, and institutional developments, mid/high-rise residentials, train/bus facilities, wind towers, storage tanks, slopes, retaining walls, transportation infrastructure and other applications. The Geopier RAP system reinforces the widest variety of soil types including undocumented fill, loose sands, soft/sensitive clays, silts, and trapped organics to provide increased bearing capacity and engineered settlement control.

Rammed Aggregate Pier Systems have been used worldwide since the early 1990s. The use of stone based ground improvement is nothing new. Stone columns, now branded as Aggregate Piers, or Vibro Stone Columns, were first developed in Germany to densify loose sand soils using a horizontally vibrating probe with surface fed aggregate. Stone columns remain an effective technique for improving sand soils.

The precision RAP System called “Geopier®” was developed by the Geopier Foundation Company in the early 1990s to provide an efficient and cost-effective Intermediate Foundation solution for the support of settlement sensitive structures in more difficult soil conditions. These systems were originally developed to help avoid massive over-excavation and replacement in more cohesive soils. Due to its unique construction method, Geopier RAP elements provide greater capacity than traditional stone columns – particularly in non-sand soil conditions (clay, silt, organic containing and fill soils).

Both Geopier RAP elements and traditional stone columns are constructed with sequential lifts of aggregate. However, traditional stone columns are installed using suspended vibratory probes that only deliver horizontal vibration, while Geopier RAP elements are constructed using the patented Geopier beveled tamper to deliver a high magnitude of vertical compaction energy and 15-30 tonnes of direct crown pressure during the vertical compaction of each lift. This results in a stiffer pier as well as significantly increased horizontal stresses in the matrix soil. The unique Geopier direct ramming process provides unsurpassed strength and stiffness of the RAP element and matrix soil.

How can you do more with the Rammed Aggregate Pier system compared to the traditional stone column process? Here are some things that an RAP can do that leaves a stone column in the dust:

Vertical Ramming

Thin lifts of aggregate are rammed to form a pier with undulating sides. Thin lifts allow for better compaction of the aggregate and results in high shear strength and stiffness. The tamper forces aggregate into the RAP element sidewalls – providing excellent pier-soil coupling.

Direct Crowd Force

Rather than being lowered off a crane, the vertical ramming energy is augmented by direct machine downward crowd pressure of 15 to 30 tonnes on each lift. Direct crowd combined with the vertical ramming energy (not horizontal) massively densifies the stone and drives up lateral stress in the enclosing soil – crowd vertical ramming results in higher performance.

Alternative Aggregates

The Rammed Aggregate Pier® (RAP) construction method is easily adaptable to use recycled concrete or cement treated aggregate. Alternative aggregates provide flexibility to achieve more in various soil types.

Soil Conditions

Geopier RAP elements may be installed in almost all soil conditions. The crowd vertical ramming process contributes to increasing the lateral stresses in the matrix soils, which is especially important for soils with silt or clay content that do not respond well to horizontal vibrations.

Performance on over 8,485 projects, as well as a myriad of independent testing and research papers provide the assurance that the RAP systems delivers the maximum value for your challenging soil projects by delivering:

Greater Capacity

The ramming process produces the superior strength of a Geopier Rammed Aggregate Pier (RAP) and develops constructed aggregate friction angles greater than 48 degrees – almost 40% greater than those reported for stone columns. Higher friction angles provide greater pier capacity, which means fewer Geopier elements versus stone columns.

Higher Bearing Pressure

Higher friction angles allow for higher bearing capacities, which results in smaller footings and lower foundation costs. Be aware that you may find your stone column or generic “aggregate pier” supplier looking to dial back bearing capacity as a result of a lower capacity pier on your next project. Insist on a Rammed Aggregate Pier for maximum bearing performance.

Better settlement control

The unique ramming process creates piers that are two to nine times stiffer than stone columns. Versatile Geopier RAPs can be used for light to heavily loaded structures. Lateral stress build-up in the enclosing soil that happens in vertically crowd-rammed RAP elements results in higher performance.

Proven advantages

Side by side comparisons demonstrate significant performance differences between Rammed Aggregate Piers and traditional stone columns.

The orientation of installation forces is an important distinction when considering ground improvement methods in Ontario. While some geologic regions in the province are known for their deposits of sand, non-sand soil such as silt and clay are more commonly encountered in Ontario and project teams must carefully consider the performance of stone column (aggregate pier) elements in these soils. Fill is also a common occurrence – particularly in the Greater Toronto Area. No ground improvement system performs better than Geopier in undocumented fill.

To the layman, the construction approach seems to be generally the same – aggregate into a columnar shape to improve the ground. It’s important to recognize that maximizing performance requires the right installation methodology. When you are working with soils other than straight sand, an aggregate pier is just stone in the ground. A Rammed Aggregate Pier provides the performance and reinforcement you need for those tougher soil conditions.

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