Predict the Settlement of Foundations Supported on Geopier Rammed Aggregate Pier™ Elements

Geopier Rammed Aggregate Pier elements have been used for over 20 years for support of conventional spread footings.  During that time it has proven to save time and money versus over excavation of fill soils and to be an economical substitute for deep foundations.  In order to be able to provide accurate predictions of settlement and to provide economical designs, the actual footing loads and load distribution are required as this is one of the key input variables.

Over the last several years it seems that in the pre-construction phase, more and more project documents are only providing loading that consists of the footing bearing pressure times the footing area as an estimate of the "apparent footing loads".  In some cases, this has proven to increase the cost of ground improvement by as much as 40% or deemed it unfeasible versus designs that are ultimately based on the actual loading conditions.  

The content below sets out what is needed for an accurate ground improvement design and why it is important.

Settlement Analyses

The key variables required for a settlement analysis to accurately predict total and differential settlement of a structure are:

Soil modulus and stress history for each strata

Consolidation characteristics - time rate of settlement 

Modulus of the Geopier improved soil zone 

Locations of the water table

When ground improvement is recommended by a geotechnical engineer, the ground improvement contractor needs to calculate the settlement of every footing as part of the pre-construction process to assess the feasibility and costs for ground improvement. Because of this, many footing design and loading issues come to light during the pre-construction phase of a project.

From a technical perspective, this is a good thing for the project, since the settlement of each individual footing is being checked early and the design performance that the structural engineer requires will be achieved with the ultimate ground improvement design.

However, often times the structural engineer does not readily have the information needed by the ground improvement contractor - actual column loads and load distributions to accurately determine settlement of each footing because:

1. They are not used to providing it.
2. It was not in their original scope to provide actual loads on the plans.

When the above information is not available, it becomes a bit of a problem for ground improvement contractors to provide accurate settlement calculations and cost estimates.

What Structural Data Information is Needed? 

What geotechnical engineers and ground improvement contractors need to accurately determine settlement of each footing are:

Column Footings 

• Dead Load in kips 
• Sustained live load to consider 

Strip footings

• Dead Load in kips per lineal foot
• Sustained live load to consider 

Retaining Wall footings

• Dead Load in kips per lineal foot
• Sustained live load to consider 
• Load distribution diagram at toe and heel for each wall height

When asked for this data, the structural engineer may say:

"All I need is for you to provide an allowable bearing capacity per the plans of 6000 psf for all footings supported by RAP's (Rammed Aggregate Pier elements)."

While this is true for what the structural engineer needs to have an accurate design for the concrete footings, more is needed by the geotechnical engineer and ground improvement contractor to accurately calculate total and differential settlement between adjacent footings.

Why are Actual Loads Needed?
 

The actual structural loads are needed to be able to accurately predict settlement and optimize the design of ground improvement used.

Although it is important on every job, the difference between designs for ground improvement for a 6 ksf bearing pressure load versus the design using the actual column loads is most evident on parking garage projects. These projects often have shear wall footings and retaining wall footings which are over sized based on the moment applied versus the size required for a uniform bearing pressure of 6 ksf.   The following example illustrates the difference.

For a retaining wall footing with a 6000 psf bearing pressure that has actual loading of 6000 psf at the toe and 500 psf at the heel of the footing, the number of Geopier elements can be readily calculated assuming a Geopier capacity of 105 kips per element.  (Note that pier capacity is not the controlling factor in a design but is used to approximate the initial number of piers required.) 

The design for this single footing using the "apparent loading" using the bearing pressure times the footing area, has twice as many piers as the design using the actual loads.  

This is not an uncommon situation especially on projects with large shear walls and retaining wall footings.  Similar examples can be shown for shear walls in buildings, elevator mats and column footings. 

When bearing pressure times footing area is used for estimating loads for building column loads with a uniform bearing pressure, the pier count can also be over estimated by a factor on the order of 15%. For building column loads, the footing size selected is always larger than the column load times the footing area.  So designing just based on the bearing pressure times the footing area will always result in a larger "apparent column load" than that for the actual column load.

Example:

Geopier Capacity = 105 kips

Actual Column Load = 630 kips                                                   requires 6 Geopier Elements

Bearing Pressure = 6 kips per square foot

Footing Size 11' x11' = 121 square feet

"Apparent Column Load" using footing size = 726 kips          requires 7 Geopier Elements

16.6% increase over amount needed

Bottom Line

1.  The use of actual column loads versus the use of "apparent column loads" (footing bearing pressure times footing area) in designing Geopier Rammed Aggregate Pier support for foundations benefits the structural engineer and the Owner by providing:

• Accurate total and differential settlement predictions
• Lower overall Ground Improvement costs by as much as 15% to 40%

2.  A benefit of using Geopier Rammed Aggregate Pier support through ground improvement contractors is that during the pre-construction phase of the project:

All Footings on the structural drawings are checked for: 

• Total Settlement
• Differential Settlement
• Adjacent Footing Stress
• Utility Conflicts

3.  Soil conditions represent the biggest risk on a construction project. Construction of the structure is pretty straight forward and predictable once you are "out of the ground."

So regardless, if Geopier Rammed Aggregate Pier elements are being used to support footings or footings are being supported on natural ground or piles, it is beneficial to have the Geotechnical Engineer of Record involved during the design process and review the final drawings to protect the design team from the risks associated with soil conditions on a site.

If you have a project that could benefit from the use of Geopier ground improvement visit our website to submit a Project Assessment OR CALL:  410-458-0289.

Grounded Solutions May newsletter will discuss "General Contractor's and Owner's Aggregate Pier Scope Check List."

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