Copy

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)
 
Where:
 
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
 
Where:

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
            Structural
            Geotechnical
            Settlement
   
Example Geopier Rigid Inclusion Specification 

 

Have a project that needs Geopier rigid inclusions?

Click the link below for a project assessment!

Project Assessement

OR CALL:  703-771-9844
 

News & Events


Northern Virginia Transportation Alliance (NVTA)
10th Annual What You Need to Know about Transportation Seminar
September 30, 2014
Capital One Auditorium - McLean, VA
Click here for more information


Upcoming GeoStructures Workshops:

Baltimore Workshop
October 29, 2014 
BWI Marriott - Linthicum, MD

Newark Workshop
November 5, 2014 
Newark Liberty International Airport Marriott - Newark, NJ

Pittsburgh Workshop
November 11, 2014
Marriott Courtyard Pittsburgh Airport - Corapolis, PA

* Registration Information available soon through the above links.


You can now connect with us on various social media platforms for up-to-date news:

Like us on Facebook
Follow us on Twitter
Subscribe to our YouTube Channel
Follow us on LinkedIn
 

Did You Know?

10 Green Structural Engineering Marvels

Architects design buildings. Civil engineers build bridges. Structural engineers keep it all from crumpling and shaking apart. What happens when you mix all three and throw in a green twist?
Read More
 

Featured Projects

In Progress
Potomac Yard North Pond Walls

Design and construction of foundation support and retaining walls for the Potomac Yard Park North Pond area. 
 
Completed
FDA SE Quadrant Laboratories and Office Buildings 71 and 75

FDA & GSA will be hosting the dedication ceremony on September 26th.

Interested in scheduling a site visit?
Click here
Share
Tweet
Forward
Share
Copyright © 2014 GeoStructures, Inc, All rights reserved.


Our mailing address is:
413 Browning Court
Purcellville, VA  20132
703-771-9844

unsubscribe from this list
 
Twitter
Twitter
Facebook
Facebook
LinkedIn
LinkedIn
YouTube
YouTube
Google Plus
Google Plus
Website
Website
Email
Email