Designing and Specifying Geopier® Rigid Inclusions
Part 3


Rigid inclusions consist of cement treated aggregate, grouted aggregate or concrete columns that are used to transfer the stress from foundation or embankment loads through soft soils down to a stiffer soil or rock layer.  While Geopier rigid inclusions can be specified as the singular solution for a project, they can also be used in combination with conventional Geopier® Rammed Aggregate Pier ground improvement methods which optimizes performance and costs for your project.
When rigid inclusions are specified using a performance based specification you can allow for more innovation and cost savings versus restricting the use of a single method of construction.

Design of the Rigid Inclusion Element

The selection of the sizes and types of rigid inclusions to use on a project depends on the magnitude of loads being supported and the subsurface conditions. In general, the steps needed to design a rigid inclusion are as follows:

Structural Performance – The structural design capacity of a rigid inclusion is controlled by the unconfined compressive strength for the unreinforced Cement Treated Aggregate (CTA), grouted aggregate or concrete inclusion being subjected to compressive loads. The design capacity of a rigid inclusion can be calculated using either a Load and Resistance Factor Design (LRFD) or an Allowable Stress Design (ASD) approach depending on the type of structure to be supported and the type of element to be used on the project. The capacity is typically about 30% of the unconfined compressive strength of the rigid inclusion material. The unconfined compressive strength of the element material can vary from 500 pounds per square inch (psi) for CTA or grouted aggregate columns to over 4,000 psi for precision grouted and concrete columns.  Steel reinforcements are typically not used and the design capacity is rarely controlled by the unconfined compressive strength of the rigid inclusion.
Geotechnical Performance – The geotechnical capacity of a rigid inclusion is governed by the bearing capacity of the soil in which the rigid inclusion is founded.  For example, for rigid inclusions in sand the capacity, q, could be calculated as follows:
q’ N ≤ (Meyerhoff 1976)
q’ = effective vertical stress at the tip
N  = Bearing capacity factor of the soil for driven displacement piles near the tip of the rigid inclusion.
For rigid inclusions in clay the geotechnical capacity, qp, is governed by the undrained shear strength of the clay
qp = su Nc

su = Undrained shear strength
Nc= 9 = Bearing capacity factor for deep circular footings
Shaft resistance may be considered when a rigid inclusion pier extends a minimum of 5 ft into a competent soil stratum. For this case, or one where piers extend through multiple soil strata below an unsuitable layer, a unit friction value can be computed for each layer, and the total shaft resistance be taken as the summation of the individual layers. Skin friction should not be considered in fill materials.  To provide relatively uniform footing support, a compacted crushed stone layer is normally placed over the tops of the rigid inclusions beneath each footing.
Note for projects where new area fills are being placed the inclusions need to be designed for the new fill loading if installed before fill is placed, or settlement of the fill needs to occur prior to rigid inclusion installation.  Otherwise the rigid inclusions need to be designed for negative down drag loading.

Settlement Performance – Once the length, diameter and capacity of the rigid inclusion is defined, then the settlement of the system can be determined. Settlement is governed by compression of the crushed stone placed between the footing and the top of the rigid inclusion, the individual rigid inclusion elastic compression, and the compression of the soil below the rigid inclusions under the full footing load.  Compression of the soil below the rigid inclusion is calculated using typical geotechnical settlement analyses.
Capacity and compression of the load transfer layer and the individual rigid inclusion is verified with a full scale load test generally following the procedures outlined in either ASTM D 1143 (static) or D 7383 (dynamic)(Statnamic).  

Performance Specifications vs. Prescriptive Specifications

For projects where the foundation support is provided using a design-build delivery system it is always better to provide a specification that defines performance requirements versus a specification that defines how a system is to be installed.  By using a performance specification you allow for more innovation and cost savings versus restricting the use a single method of construction.  The key items that a performance specification should include are:
Pre-Qualified Bidders
            Bonding Capacity
            General and Professional Liability
Project Experience
            Previous Experience on Similar Size Projects
            In House Design and Construction Experience
Technical Performance Criteria
Example Geopier Rigid Inclusion Specification 


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