FHWA Resource Center
Geotech and Hydraulics Reading Room
Grouting Tips: An Outlook on Post-grouted Drilled Shafts
Benjamin S. Rivers, PE
FHWA Resource Center
Interest in post-grouted drilled shafts has been growing within the United States transportation community as word spreads about the apparent benefits post-grouting can provide. Interest in the transportation community began in earnest with a two-phased research effort conducted by the University of South Florida for the Florida Department of Transportation. Results of the research efforts were published in 2001 and 2004, respectively. As part of the research, a design rationale and methodology was developed, primarily for granular soils, which had not before been established in the US. Most recently, SynchroPile, Inc., a division of AH Beck Foundation Company that holds a US patent on post grouting drilled shafts, has been working on further developments. At this point, however, questions still remain with certain aspects of the grouting process and design, especially in soils containing significant fines. With more and more states interested in the potential benefits, the Federal Highway Administration is looking closely at design and grouting process developments that will assure reliable and consistent practice.
Post-grouting drilled shaft tips involves the installation of a grout pipe and a connected base grout-distribution system during shaft construction. After the shaft is constructed, the grout system flushed and the drilled shaft concrete has gained adequate strength, neat Portland-cement grout is pumped through the grout system until grout is returned. After the grout is returned, the return valves are closed and grout is pumped under pressure until a design pressure is sustained.
The two-phased research project conducted by the University of South Florida has practically defined current design and construction practices for post-grouting drilled shaft tips utilized for recent highway projects. The research was originally intended to evaluate the potential benefits in sands, where the most improvement was anticipated to be practicably obtainable. The original and subsequent phase of the research showed that post-grouting drilled shafts tips can provide substantial end-bearing capacity improvement in loose to medium-dense sands, and that capacity improvement diminishes as the bearing material consistency increases and as the grain-size of the bearing material decreases. In all cases, however, some improvement was observed. Intuitively, post-grouting drilled shaft tips should at the very least provide some minimal amount of improvement in end-bearing capacity for clays by pre-mobilizing the soil so that more of the soil’s unimproved end-bearing can be utilized for a given tolerable downward displacement. In addition to end-bearing improvement, the process provides a measurable indication of available side-shear and end-bearing capacity with certain assumptions. As an added benefit, post-grouting would also minimize concerns associated with soft-bottom conditions, although it should never preclude good drilled shaft construction practices for assuring performance.
Post-grouting is ideally suited for drilled shafts tipped in loose to medium dense sands with minimal fine contents. Currently, the most well defined design methodology is intended for sands. When the current design methodology and post-grouting process is applied to drilled shafts tipped in finer-grained soils, the effects of grout introduction relative to pore-pressure dissipation is lost from consideration.
The Federal Highway Administration (FHWA) has identified several areas where additional study and development is needed in current practice to assure quality and consistency in grouting procedures and the end result. These uncertainties are predominately related to the rate at which grout is introduced relative to pore-water pressure dissipation, soil volume change and the formation of the grout mass, as well as the associated zone of improved soil, especially in fine grained soils. Of course, there are practical limits to capacity improvements in fine-grained soils, which will need to be addressed but would not exclude the other potential benefits of post-grouting.
SynchroPile and the FHWA met in December 2005 to discuss technical issues, technical developments with which SynchroPile is currently involved, and the eventual necessary standard procedures and protocol for design and construction to ensure consistency in practice. SynchroPile is working with the FHWA to address the issues identified as they continue in their new developments. The FHWA looks forward to seeing advances with post-grouted drilled shafts. The outlook is promising.