Even the most conscientious and careful drilled shaft installation program can be subject to incidents during construction. Bad weather, unexpected delays in concrete delivery, equipment breakdowns or unanticipated ground conditions can produce imperfections or defects in the completed drilled shaft foundation.
In most cases where a problem exists with a drilled shaft, the cause of the condition is known because of some incident that occurred during construction. For instance, a traffic accident or power outage at the plant might interrupt concrete delivery for some period of time, leading to a cold joint under slurry or difficulty with the tremie operations. It is important that responsible inspection and construction personnel describe and record the nature and observations of the incident immediately so as to help define the nature and possible extent of any imperfection that might result. Whenever practical, measures should be taken during installation of the drilled shaft to avoid forming a defect in the shaft. However, such corrective measures are not always possible or fully effective. Post-construction integrity testing methods are useful to help define the magnitude and extent of some types of imperfection. It is important to consider that perfection in drilled shaft construction is an unattainable ideal. Some small imperfections are to be expected, and a robust design should be tolerant of small and unavoidable flaws that may or may not be detected.
After evaluation of any defect and determination of a deficiency in geotechnical or structural strength, a plan for remediation is required. The plan should provide a means to restore the required performance characteristics of the drilled shaft in a way that is constructible and reliable, and that includes a program of monitoring or verification that will provide assurance that the remediation is successful.
Ground ImprovementGround improvement techniques can be used to restore geotechnical strength for situations where disturbance of existing materials may have occurred during construction or due to environmental factors. Ground improvement techniques might include:
- Penetration grouting to fill voids around a shaft, such as
might occur with a casing left in place in an oversized
- Compaction grouting to restore density of granular soils that may
have been loosened below or around a drilled shaft.
- Vibro-compacted stone columns to improve density and reduce
liquefaction susceptibility around an existing drilled
- Grouting also might be employed to treat loose materials below the base of a shaft by drilling through the shaft to access the bearing soils below the base.
Supplemental FoundationsIn some cases where the strength or stiffness of a drilled shaft is less than required, the most effective remediation strategy might be to add additional deep foundation elements. These might be designed to supplement or even completely replace the existing shaft. Additional shafts or micropiles generally are the most suitable type of deep foundation support element for use near an existing shaft.
It should be noted that the design of a foundation incorporating additional deep foundation elements into a common cap with the existing drilled shaft must address the issue of strain compatibility; the shear distribution in the cap will be affected by the relative stiffness of the various elements supporting the cap. For instance, if additional deep foundations extend into rock and are very stiff in comparison to an existing shaft, the cap may be subject to high shear forces. Additional deep foundations composed of relatively flexible micropiles may be relatively less stiff in comparison to a large-diameter existing drilled shaft foundation. Pre-loading of the new foundation elements sometimes can be used to address strain compatibility.
Additional ShaftsUnderpinning with “straddle shafts,” sometimes called “sister shafts,” is a method of supplementing or replacing a defective drilled shaft. The existing shaft may be chipped away and eliminated from inclusion in the new foundation, but this approach usually is not necessary unless the existing shaft is so badly damaged that it is considered a liability and of no benefit. An example of the use of straddle shafts is shown in photo 1 (on p.18), where existing pile foundations for a bridge in Arizona were completely undermined by scour. These foundations represented a complete replacement of the existing foundation.
Photo 2 (on p.18) illustrates another example of the use of sister shafts to provide supplemental support for an existing shaft that was structurally sound but deficient in axial resistance. An additional 4-foot-diameter shaft was constructed on each side of the existing column, which itself was supported on a single 6-foot-diameter shaft. In order to keep the size of the cap as small as possible, the sister shafts were constructed as closely as possible, and the composite foundation was analyzed as an elliptically shaped barrette foundation with end-bearing contributions only from the two new, deeper shafts. The column was roughened and tied to the new post-tensioned cap using dowels.
MicropilesMicropiles may be used as additional deep foundation elements to supplement or replace an existing shaft. Micropiles typically are 12 inches or smaller in diameter, and are constructed as a steel member (pipe or high strength bar), which is grouted into the bearing material. One advantage of micropiles for underpinning work is the small size of the equipment used to install these foundations, which often allows their use in locations with restricted access or severe height limitations. Sometimes micropiles can even be installed by drilling through an existing drilled shaft in order to anchor the shaft into an underlying formation. For this application, the top of the shaft must be accessible. In some cases, it even is possible to install these by drilling through existing tubes used for integrity testing. High-strength bars or even conventional reinforcing bars, depending upon the application and loads, may be used to form the micropile. The micropile steel must have sufficient development length to bond to the existing shaft. When installed through the existing drilled shaft, micropiles can provide additional structural strength to the shaft.
Where micropiles are used around an existing shaft, as illustrated in photo 3 (on p.20), it frequently is possible to construct the piles very close to the existing shaft, and thereby minimize the size of the cap. These micropiles are constructed using 10.75-inch O.D. steel pipe casing installed to rock, with a 75-ksi grouted bar installed within the casing to form a socket into bedrock.
Excavation and ReplacementExcavation and replacement is a feasible repair method for structural defects in shafts that are relatively shallow, and therefore accessible from the surface. Poor-quality concrete in the top of a large-diameter shaft may be removed by hand with impact tools, and it may be possible to use a drilled shaft rig to excavate concrete within the reinforcing cage with rotary tools. If the upper portion of the soil around the shaft can be excavated and a dry working environment secured, hand methods may be used to chip away defective concrete, and the upper portion of the shaft re-cast within a form. Photos 4 (on p.20) and 5 (above) depict this type of repair work.
Structural enhancement can be performed to increase the structural strength of a defective shaft without complete removal of the defect. For instance, if the center of the shaft is drilled out as shown in photo 4, it may be possible to install a structural steel or pipe section cast into the central portion of the shaft with high-strength concrete. This additional member may be designed to restore the structural strength of the shaft to meet the project requirements. It also may be possible to extend a central drilled section into the bearing formation below the shaft in order to increase the geotechnical strength of the shaft.
Structural enhancement also can be accomplished by drilling holes in the shaft and grouting in additional rebar, high strength bars or strands.
In part 2 of this article, we’ll look at using grouting techniques to treat drilled shaft defects.
This article is provided through the courtesy of the National Highway Institute.