Underwater Drilled Shafts
August 1, 2007
Because they can be installed in virtually any soil type, can support large axial and lateral loads on a single member, and provide the most economical foundation type for most situations, drilled shafts are the most commonly utilized bridge foundation.
On some projects, excavation of drilled shafts encounters no soft soils or ground water. In these shafts, concrete is placed “in the dry” – that is, without the need to displace water or drilling slurry. Placing concrete in this type of shaft is similar to placing concrete in a column form, and it does not require specialized methods or equipment.
However, a considerable number of drilled shafts require either drilling slurry to stabilize the excavation or encounter enough ground water that the shaft excavation cannot be dewatered. For these shafts, the concrete is placed using slurry displacement or underwater placement. These shafts do require specific methods and equipment.
Stabilizng SlurryDrilling slurry stabilizes the drilled shaft excavation in soft soils or when ground water is present. The most common drilling slurry is mineral slurry consisting of commercial bentonite clay mixed with water. Mineral slurry offers a combination of higher-than-water specific gravity to offset hydrostatic pressure, and it deposits clay particles on the excavation walls (filter cake) to help stabilize them. During excavation, the main function of the slurry is to maintain hole stability. During placement of the concrete, however, the slurry must displace cleanly from the hole and not leave the reinforcing steel or sides of the hole coated. Before placement of the concrete, slurry should be sampled from the bottom of the hole with a device known as a “thief.” Then the slurry is tested for specific gravity, viscosity and sand content. These three tests show the amount of suspended soil particles in the slurry, and how easily the slurry will be displaced during the concrete pour. Determining the amount of suspended solids is important, as these materials will thicken the slurry and actually may settle out of it before and during the concrete pour. Excessive amounts of particles, especially sand, can leave deposits against the sides and bottom of the shaft and along the reinforcing steel.
If slurry sampled from the bottom of the hole does not meet specific requirements, pump slurry from the bottom of the hole, replacing it with fresh slurry introduced at the top, until the slurry meets the necessary requirements.
Polymer SlurryThe use of polymer slurry has increased in recent years. Although it offers advantages over mineral slurry in specific situations, it does not offer the same stabilizing properties in many others. For instance, polymer slurry has a slightly lower specific gravity than water and, therefore, does not have the same ability to offset hydrostatic pressure as mineral slurry. After allowing polymer slurry for several years, the Texas Department of Transportation reported examples of settlement of several bridge structures founded on drilled shafts that used polymer slurry. On these projects, polymer slurry was used in conditions incompatible with the slurry, leading to severe soil disturbance and defects within the shafts.
Concrete PlacementThree issues are critical to the placement of concrete under water or slurry:
- A delivery method must be used that keeps the concrete separated from the water or slurry and prevents intermixing.
- The concrete must maintain a high slump to completely fill the shaft and flow easily around the reinforcing steel.
- The top of the shaft must be thoroughly flushed at completion of the pour.
Pumped concrete is placed in a similar manner. The concrete and water must be prevented from mixing at the start of the pour, and a foam rubber plug often is used in the pump line as a separator. Additionally, embedment of the pump line into the concrete is critical throughout the pour.
Concrete Slump – Using a high-slump concrete and maintaining slump during the pour are keys to a quality drilled shaft. For underwater placement of drilled shafts, the slump should be too high rather than too low. Low-slump concrete will not flow out through the tremie and around reinforcing steel in the manner necessary to achieve a quality shaft. Many exposed shafts poured with low-slump concrete have honeycombing outside the reinforcing steel cage, or, in some cases, they have no concrete at all. Drilled shaft concrete should have a slump of 8 inches, with a maximum of 9 inches allowed.
Retention of slump throughout the concrete pour also is important. Adequate retarder must be included in the concrete to retain workable slump throughout the duration of the pour. Inadequate retardation can cause the upper portion of the concrete to stiffen as it begins to set during the pour. If this occurs, the tremie must be raised very near the surface (or out) of the concrete to restore flow. When this happens, the concrete actually flows up alongside the tremie and over the top of the stiffened concrete, including cuttings, sediment and diluted concrete. This may happen several times during a lengthy pour if slump is not maintained. Concrete used for underwater pours should maintain a 4-inch slump for the duration of concrete placement, including extraction of casing if it used.
Completing the Pour – During an underwater pour, cuttings, sediments and diluted concrete are carried to the top of the concrete column as it is being placed. As the pour is completed, all loose material and contaminated concrete should be flushed off the top of the shaft. In some cases a cubic yard or more of concrete will be wasted to assure that all contaminated material is removed. The top of the shaft should be strongly flushed while concrete is being placed through the tremie or pump at the same rate as the rest of the pour. Attempting to dip or shovel this material off the shaft often leaves behind inclusions and low-quality concrete.
This article is provided through the courtesy of the Texas Department of Transportation.