Grouting may provide a means of directly treating the defective area to improve or restore the structural and/or geotechnical strength of a drilled shaft. Grouting may be performed at the base of the shaft to address a problem of loose material below the shaft base, within the shaft to address structural defects, or around the shaft to provide bond to the soil and/or cover for corrosion protection.
Base of the Shaft
A defect at the shaft base, caused by trapped material or washed aggregate, sometimes can be treated by grouting the base of the drilled shaft. The grouting of the base of a drilled shaft usually will involve the following steps:- At least two holes are drilled through the full length of
the shaft so that fluid can be circulated to and through the soft zone at the
base of the drilled shaft. These conduits normally are available as access
tubes if access tubes have been installed for integrity testing or as drill or
core holes that were placed during an investigation of the quality of the
base.
- Wash or flush the defective zone by pumping water or an air-water
mixture down one tube, and returning it with suspended debris through the
other(s).
- After a clear return is obtained and debris has been removed to the
extent possible, inject the grout into one hole until grout returns through the
other(s). After grout has flushed all of the wash water through all of the
tubes, seal all of the return tubes, and apply pressure
grouting.
- Monitor the shaft for upward movement, and log the volume and
pressure of grout taken as a function of time, as would be performed with
planned base grouting for enhancement.
- With at least two holes through the drilled shaft and into the weak base material, the weak material at the base can be washed away by forcing fluid down one of the holes and having it return through the other. An air-water mixture under high pressure can be an effective technique, except when the drilled shaft is founded in cohesionless material. In that case, no air and low pressures must be used, so as not to undermine nearby drilled shafts. The solids in the returning fluid should be monitored during the process as a means of evaluating the efficiency of the washing operation. As the cleaning of the base of the drilled shaft progresses, it may be possible or desirable to inspect using a television camera or a concreteoscope if the base is within a rock formation.
Within the Shaft
Grouting may be used to repair zones of defective concrete within the shaft. In this application, the most challenging part of the operation may be the removal of low-strength material. Hydroblasting, or erosion of weak concrete using high-pressure water jets, has been used with success. Access holes are used to introduce water jets and observe communication between holes and return up through these holes. These high-pressure water jets are capable of nozzle pressures in excess of 20,000 psi, and can cut concrete at close range (within a foot or so) if the jet can be directed and is not shadowed by steel reinforcing. It normally is not feasible to remove large quantities of concrete in this manner.After completion of the hydrodemolition and water jetting to flush the cuttings and debris from the hole, the access hole typically should be pumped dry or vacuumed to provide the most effective means for grout to fill voids effectively. The photo on p. 36 shows a core taken through the grouted zone within a repaired shaft, with the grout stringers clearly visible within the concrete matrix.
This technique can be used to remediate and improve concrete that has inclusions of soil or low-strength concrete, and often is sufficient to restore the structural strength needed to meet the project requirements. Grouting cannot be expected to restore the shaft to a perfect condition. Post-treatment cores or cross-hole sonic logs should show improvement, but will not be free of anomalies.
Grouting within the shaft may not be effective if the defects to be treated include zones on the outside of the reinforcing cage in granular soils below the ground water. In such a case, attempts to hydroblast outside the shaft would erode unstable soils, which might be expected to cave. Jet grouting around the perimeter of the shaft is a technique that might be considered.
Around the Shaft
If the shaft is structurally sufficient except for concerns regarding the concrete cover on the reinforcement, or if a void exists between the outside of the shaft and the soil, then grouting around the perimeter may be considered. In the simple case of a stuck casing with a void left around the casing, tremie grout into the void using small-diameter tubes may be sufficient to fill the gap and restore the lateral soil resistance. If there are voids or defects in the concrete around the perimeter of the shaft below the ground water, jet grouting may be considered as a means to erode soil or weak material, and provide a cementitious encasement of the shaft.Jet grouting has a history of use for ground improvement – often for stabilization of weak soils around excavations or walls. A column of grout is formed by a drill equipped with sideward-directed nozzles, which cut the soil and flush most of it back to the surface, replacing it with grout. The jets can include combination of grout, water and air to enhance the cutting ability.
The nozzle pressure is much lower than that used for hydrodemolition, and typically around 1,000 psi. This pressure will cut soil or relatively weakly cemented inclusions in the shaft, but will not erode good concrete. Therefore, the technique is most useful for encapsulating a drilled shaft as illustrated in the figure {location}. Note that the jet grout columns can be started and stopped at predetermined elevations corresponding to the zone needing treatment, and it is not necessary that the column extend the full height of the shaft. It also is possible that jet grouting could be used to treat an area around a shaft in order to stabilize the shaft prior to internal hydrodemolition and grouting.
ND
This article is provided through the courtesy of the U.S. Department of Transportation’s Federal Highway Administration. It is excerpted from the publication, “Drilled Shafts: Construction Procedures and LFRD Design Methods,” authored by Dan Brown (Dan Brown and Associates, Sequatchie, Tenn.), John Turner (University of Wyoming, Laramie, Wyo.) and Raymond Castelli (Parsons Brinckerhoff, Columbus, Ohio).
