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Introduction
   
 
In areas of poor soil, generally recognized as soil having Standard Penetration Blow Counts less than “N” = 5, or on projects where the pier pipe is exposed, or may become exposed; the stiffness (axial moment of inertia) of the pier and the strength of the joints are of concern. There are several ways to reinforce the pier pipe in such cases. One of the simplest is to grout the pier pipe after installation. Many designers also require that the contractor install a reinforcing bar with the grouting to improve joint strength.

Earth Contact Products offers a patented product called the ECP Inertia Sleeve. This unique product is shown in Figure 9, and is the most economical way to quickly improve the axial moment of inertia (stiffness) of the pier system. The Inertia Sleeve is easy to install when driving the pier pipe. One simply allows the Inertia Sleeve to drop by gravity into the current section of pier pipe prior to coupling together another section of pier pipe. The low cost Inertia Sleeve takes nearly no labor to install and instantly increases the stiffness of the pier shaft. The unique design of the Inertia Sleeve also strengthens the pier pipe couplings. The coupling on the Inertia Sleeve extends nine inches. This allows the Inertia Sleeve coupling to fully pass through the pier pipe coupling and to engage inside a previously installed section of Inertia Sleeve. The couplings are therefore doubled and staggered, providing a strengthened coupled joint.

Another means of increasing the axial moment of inertia of the pier shaft is to install an external pier sleeve.
Many designers like this method and it provides a significant increase in pier stiffness because the sleeve has a larger diameter than the pier shaft. When installed, the external sleeves must be positioned such

 

that the joints in the sleeving are staggered with the pier pipe couplings. This method is not as economical as an Inertia Sleeve, because the external sleeving must be installed after the pier has been driven and prior to field load testing.

The external sleeving installation requires the use of the hydraulic drive cylinder. The installation time for the external sleeve is generally equivalent to the time required for installing pier pipe, but only to the required depth for the sleeving.

Presented in Figure 8 is a graphic representation of the shaft stiffness related to differing pier pipe configurations. It is interesting to note that the combination of the Model 350 pier pipe plus the inertia sleeve provides 92% of the axial stiffness of the more costly Model 400 system when there is a lack of sufficient lateral support for a distance of 8 feet. If the designer provides 4” diameter exterior sleeving and grouts the Model 350 pier pipe, the allowable load on the system will be 160% that of the grouted Model 400 pier system. One must keep in mind that the pier sleeving is required only in the areas of weak soil or exposed pier pipe. The cost savings should be very evident especially on projects that require extra strength only in the upper several feet of soil.

When specifying sleeving, the designer must extend the sleeving at least three feet into competent material. For example, if a site has 6 feet of peat with blow counts from 0 to 2 blows
per foot followed by sand having suitable density as indicated by an SPT reading of 5 blows per foot or greater; the designer should specify sleeving to a depth of 9 feet in order to provide adequate protection through 6 feet of weak soil.

 
 

 
 

 
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