Drilled shafts are reinforced concrete columns, which, for the most part, are built below the surface of the ground. They are designed to provide a foundation for structures and carry the entire load of the structure. They are sometimes referred to in the field as caissons.
Prior to installing drilled shafts, the Contractor is required to submit a written installation plan to the Engineer. This plan should be closely reviewed for conformance with the specifications. Among other things, the plan should describe how the Contractor proposes to excavate the hole, clean out the hole, and place the concrete. The plan needs to include procedures for maintaining correct horizontal and vertical alignment of the excavation. The plan should also include procedures and the proposed equipment required to deal with the possible presence of and subsequent removal of underground obstructions within the hole excavations.
If a permanent casing is specified, the casing should be installed to the prescribed depth before excavation begins. In some cases, the Contractor may not have the required equipment to completely install the casing prior to excavation. If the Contractor is not able to completely install the casing prior to excavation, he is allowed to either excavate the material within the casing, or excavate a pilot hole ahead of the casing. If the Contractor proposes to excavate the material within the casing to aid in the installation, it is important that the excavation does not proceed beyond the casing.
If the Contractor proposes to either pump or tremie the concrete under water while utilizing a temporary casing, his plan should describe how he proposes to remove the casing while not disconnecting or breaking apart the tremie or pump hose. In order to ensure that the end of the pump or tremie hose is always embedded into the concrete, his plan should detail how he proposes to monitor the level of the top of the concrete and the bottom of the pump or tremie hose. If the Contractor does not include these provisions in his plan, and encounters water in the field, he should be required to stop and resubmit a plan containing the necessary information. He should not proceed with verbal approval as it is too difficult to document what was said versus what may have been intended.
Drilled shaft installation can be very complicated since a large amount of work is performed in an area with very little access. The plan should be very detailed and site specific. A generic or “canned” plan should not be accepted.
There are two types of drilled shafts:
1. End bearing.
End bearing drilled shafts derive most of their capacity through end bearing on a hard substrate, such as bedrock.
Friction type drilled shafts derive most of their capacity through a combination skin friction with the soil along the perimeter of the drilled shaft and end bearing on the substrate immediately below the drilled shaft. To obtain the required skin friction, it is important that the integrity of the soil be maintained during the drilling operation and prior to placing the concrete.
There are several different methods used to stabilize the sides of the excavation during the construction of the drilled shaft. Factors that impact the method chosen are types of soil, the elevation of the ground water, types of drilled shafts, plan requirements, and equipment utilized by the Contractor.
The dry construction method is accomplished by excavating the hole without the use of steel casing. The sides and bottom of the excavation should remain stable and should not experience any caving, sloughing, or swelling. It should be possible to visually inspect the excavation prior to the placement of concrete.
The excavation should be done in a relatively dry condition with very little ground water present. The flow rate of any water that might enter the excavation should be such that the elevation does not change by more than 12 inches (300 mm) per hour. At the time of concrete placement, there should be no more than 3 inches (75 mm) of water in the bottom of the excavation. Both the flow rate test and the amount water in the bottom of the hole should be documented.
The wet construction method should be used at sites with or without casing and where a dry excavation cannot be maintained. This method consists of using either water or slurry to contain or prevent the seepage of ground water into the drilled shaft. With the use of slurry, this method may be used in place of a temporary casing to maintain the stability of the perimeter of the hole while advancing the hole to its final elevation.
If this method is used to excavate a hole for a friction-type drilled shaft, it is important to not compromise the integrity of the soil along the perimeter of the drilled shaft through the seepage of ground water. It is not only important to prevent the seepage of ground water into the excavation after it is complete, but it is important to prevent ground water from seeping into the excavation during the drilling process. To prevent this, it will be necessary to continually pump either water or slurry into the hole during the drilling operation to maintain an elevation slightly higher than the elevation of the static water table.
Either a tremie or a concrete pump will be used to place the concrete when the wet construction method is used.
Unless waived by the Engineer, it is required for the Contractor to use a temporary surface casing to prevent soil at the top of the casing from sloughing and falling into the excavation. This casing should never be shorter than 10 feet (3.0 m) long. The temporary casing aids in the proper alignment and positioning of the drilled shaft.
Temporary casing may be used at sites where dry excavation cannot be maintained and the Contractor elects not to use slurry.
It is important that the Contractor begins removal of the temporary casing while the concrete remains workable. Failure to remove the casing could result in a drilled shaft that is not capable of supporting the design load.
When the casing is withdrawn, there is a possibility that fluid trapped behind the casing will contaminate the concrete. To prevent this, it is important to maintain a head of concrete at least 5 feet (1.5 meter) in the casing. This minimum head may need to be increased to counteract any ground head that might be in the casing at the time it is withdrawn. Casing should be removed by pulling at a slow uniform rate. However, if the casing gets stuck, the Contractor may rotate, vibrate, or tap the casing to facilitate extraction. Rotating the casing may twist the reinforcing cage, so only rotate the casing enough to get it unstuck.
Friction-type drilled shafts derive much of their capacity through the adhesion of concrete with the surrounding soil. If the Contractor elects to use a temporary steel casing and fails to remove it, or he fails to protect the integrity of the soil adjacent to the drilled shaft, much of the capacity of the drilled shaft could be lost.
When drilled shafts extend below the top of the water table, it is important that the water or slurry fluid inside the shaft excavation be maintained higher than the top elevation of the water table at all times. To accomplish this, it is not only important for the Contractor to add water or slurry fluid after the excavation is complete, but it is also important for him to add water or slurry fluid during the drilling operation. If this is not done, the surrounding ground water will begin to enter the excavation and erode the soil. This will result in the reduced capacity of the drilled shaft.
The dry construction method can be used in construction of friction-type drilled shafts. It should be used when the bottom of the drilled shaft is above the water table and the excavation can be made without the sides or bottom of the excavation experiencing any caving, sloughing, or swelling. If the dry construction method results in the sidewall becoming softened or swelling, the Contractor shall over ream the sidewall to sound material.
If the Contractor elects to use slurry, a delay in placing the concrete could result in the sidewalls degrading due to slurry cake buildup. Any slurry cake buildup shall be corrected by reaming the sidewalls to sound material.
If a temporary casing is not used, and concrete is not placed the same day that the excavation is complete, the excavation shall be re-drilled 6 inches (150 mm) larger in diameter immediately prior to the placement of the concrete.
If a temporary casing is used, it should be smooth and free of dried concrete and other foreign materials that might contaminate the fresh concrete. While the strength and thickness of the steel casing is not specified, it should be strong enough to withstand handling, installation, and extraction stresses as well as the pressures exerted on it by the fresh concrete and surrounding earth.
The outside diameter of the casing should be at least equal to the plan diameter of the drilled shaft. Many times the Contractor will elect to use a casing larger than the specified casing. Oversized casings are acceptable; however, all additional costs associated with the oversized casings should be borne by the Contractor.
Typically, the diameter of the bedrock socket will be less than the diameter of the remainder of the drilled shaft. When the diameter of the bedrock socket is the same as the remainder of the drilled shaft, the diameter of the drilled shaft may need to be increased to permit the excavation of the bedrock socket. Again, increasing the diameter of the drilled shaft should be done at no additional cost to the state.
One potential method of excavating a hole through unstable or caving soils is through the use of slurry. The slurry should be added to the excavation during the drilling process and replace the material that is being removed. This is accomplished by mixing the slurry with the material to be removed. The combination of slurry and soil is then pumped from the hole while clean slurry is added. The slurry that was pumped from the hole is then cleaned of foreign material and placed back into the hole. This process is continued until the original soil has been removed.
There are two different types of materials used to produce slurries. One type of material produces mineral slurry and the other type of material produces polymer slurry.
If the Contractor elects to use polymer slurry, he must first demonstrate the slurry's ability to prevent caving of the hole. If the slurry is not capable of stabilizing the perimeter of the hole while the hole is being excavated, it should not be allowed. This should be accomplished by the use of a separate trial hole. This trial hole should not be one of the production shafts and no separate payment should be made for the trial hole. The trial hole should be the same size and diameter as the largest production drilled shaft except the depth of the hole need not be more than 40 feet (12 meters). The slurry used in the trial hole should be the same as that used in the production shafts.
An important factor in the performance of the drilled shaft is the cleanliness of the hole excavation prior to the placement of the reinforcing steel and the concrete. The Contractor must provide equipment to check the dimensions, alignment and cleanliness of the hole excavation.
Reinforcing should be placed just prior to concrete placement. It should be placed as one continuous cage. If a casing is not used, care should be taken when lowering the reinforcing steel cage into the shaft so it does not drag down the face of the shaft and compromise the integrity of the exposed soil surface.
Spacing devices, commonly referred to as “donuts,” need to be installed at quarter points around the shaft to ensure that the required concrete cover is obtained. On the bottom of the shaft, the Contractor can use plastic “shoes” to keep the reinforcing cage at the proper elevation. These shoes are normally 6 inches (152 mm) to 8 inches (203 mm) tall and about as big around as a soda can. In the past, mortar blocks were wired to the end of longitudinal steel to accomplish this task, but were unstable and the cage often fell off the blocks.
The concrete used in the drilled shaft is Class QC 2. In order to aid the consolidation of the concrete without vibration, it is necessary to increase the slump to 6 inches (150 mm) ± 1 inch (25 mm). If the concrete is placed using a tremie, the slump should be increased to 8 inches (200 mm) ± 1 inch (25 mm). The accepted JMF’s maximum water-cementitious ratio shall not be exceeded. It may be necessary to achieve the additional slump through the use of a super-plasticizer.
If the Contractor uses the wet method or places concrete under water or slurry, increase the cement content by 10 percent and place the concrete by either tremie or concrete pump.
If a temporary casing is used, it should be removed slowly and carefully. As the casing is removed, concrete that has been previously placed will fill the void left by the casing thus causing the top level of the concrete in the excavation to lower. As the level of the concrete drops, the concrete will tend to pull down on the reinforcing steel. If the casing is removed too quickly, the downward force of the concrete on the reinforcing steel will cause the reinforcing steel to be displaced.
A tremie may be used to place concrete in a wet hole. If concrete is placed in a wet hole, it is important that the concrete not be placed into moving water. If concrete is placed into moving water, the water will have a tendency to wash the cement off of the sand and aggregate. To prevent moving water in the excavation, the level of water or slurry in the excavation must be equal to or higher than the level of the ground water.
The tremie must not contain aluminum parts that will come into contact with the concrete. In order for the concrete to pass freely through the tremie, the minimum diameter of the tremie shall be at least 10 inches (250 mm). It is important that the tremie be clean, smooth, and free of built-up concrete and other foreign material.
Prior to placing the tremie tube into the water, it is important to plug the end of the tremie to prevent the intrusion of water into the tremie. The tremie can be placed into the excavation after the plug is in place. After the tremie is filled with concrete, it should be raised up no more than one diameter of the tube. This allows the plug to be displaced and the concrete to begin flowing into the excavation. If the tremie is not plugged, the tube will fill with water. When the concrete is dropped through the tube, it would drop through the water which separates the cement from the sand and gravel.
During the placement of the concrete, the end of the tremie should always be at least 10 feet (3 meters) below the surface of the concrete to prevent the water from contaminating the fresh concrete. It is important to devise a method to determine elevation of the top of the concrete and the bottom of the tremie since the concrete will be under water and not visible. This method should be determined and agreed upon with the Contractor prior to the concrete’s delivery.
In order to prevent air voids in the concrete when a tremie or pump is used, place the concrete in one continuous operation. If the Contractor is allowed to break apart the tremie tube or pump hose to facilitate the removal of temporary casing, the tremie tube or pump hose could get air voids in them that will be forced down into the drilled shaft concrete. If the end of the tremie is pulled out of the concrete prior to completely placing all the concrete, the drilled shaft will contain concrete that will be contaminated by water. As a result, the drilled shaft may not have the required strength and should be considered defective.
After the concrete placement has been completed, there will be a layer of concrete at the top of the drilled shaft called, “laitance,” that has been contaminated with water. This concrete should be removed either by overfilling the drilled shaft and causing the contaminated concrete to flow out of the drilled shaft or by shoveling off the concrete. If the contaminated concrete is shoveled off, the Contractor must place additional concrete to replace the concrete that was shoveled off.
A pump may be used to place concrete in a wet hole. If concrete is placed in a wet hole, it is important that the concrete not be placed into moving water. If concrete is placed into moving water, the water will have a tendency to wash the cement off of the sand and aggregate. To prevent moving water in the excavation, the level of water or slurry in the excavation shall be equal to, or higher than, the level of the ground water.
Due to the adverse reaction of concrete with aluminum, the pump must not contain aluminum parts that will come into contact with the concrete.
In order to allow the concrete to pass freely through the pump, the minimum diameter of the pump pipe must be at least 4 inches (100 mm).
During the pumping operation, the pipe used to convey the concrete to the bottom of the drilled shaft must be anchored to the steel casing or other suitable stationary object to prevent the pipe from undulating. Otherwise, the tendency of the pipe to undulate could cause it to pull out of the concrete that was previously placed.
In order to lubricate the pump equipment, grout should be first pumped through the hose prior to pumping the concrete. The grout should not be placed in the drilled shaft. This process does not need to be repeated as long as the process is continuous.
Prior to placing the pump pipe into the water, it is important to plug the end of the pipe to prevent the intrusion of water into the pipe. After the plug is in place, the pipe can be placed into the excavation. When the pipe is filled with concrete, the pressure of the concrete will dislodge the plug. If the pipe is not plugged, and the concrete drops through the water, the water would separate the cement from the sand and aggregate.
During the placement of the concrete, the end of the pump pipe should always be at least 10 feet (3 meters) below the surface of the concrete to prevent the water from contaminating the fresh concrete. It is important to devise a method to determine elevation of the top of the concrete and the bottom of the pipe since the concrete will be under water and not visible. This method should be determined and agreed upon with the Contractor prior to the concrete’s delivery to the project.
In order to prevent air voids in the concrete when a tremie or pump is used, place the concrete in one continuous operation. If the Contractor is allowed to break apart the tremie tube or pump hose to facilitate the removal of temporary casing, the tremie tube or pump hose could get air voids in them that will be forced down into the drilled shaft concrete.
If the end of the pipe is pulled out of the concrete prior to completely placing all the concrete, the drilled shaft will contain concrete that will be contaminated by water. As a result, the drilled shaft may not have the required strength, and should be considered defective.
After the concrete placement has been completed, there will be a layer of concrete at the top of the drilled shaft called, “laitance,” that has been contaminated with water. This concrete should be removed either by overfilling the drilled shaft and causing the contaminated concrete to flow out of the drilled shaft or by shoveling off the concrete. If the contaminated concrete is shoveled off, the Contractor must place additional concrete to replace the concrete that was removed.
It is the Contractor's responsibility to provide the Engineer with all the necessary labor and equipment to obtain measurements of the drilled shaft. Since it is not possible to obtain these measurements after the concrete is placed, it is necessary to obtain these measurements prior to placing concrete.
Due to the risks involved, at no time should the Engineer ever go down into a drilled shaft for inspection or any other purpose.
The pay length of the drilled shaft is the required, accepted length measured along the axis of the shaft. It should be measured from the required bottom of the shaft to the proposed top plan elevation. Any over excavation below the required bottom of the shaft should not be measured for payment.
Drilled shafts that extend into bedrock should be divided into two sections. The lower section is the length of the drilled shaft that extends into the bedrock or the bedrock socket. The upper section is the length of drilled shaft above the bedrock. If the top elevation of the bedrock is lower than indicated on the plans, the additional upper section or length of drilled shaft above bedrock should be measurement for payment. The Contractor should not be paid for any over excavation of the bedrock unless he is ordered to do so by the Engineer.
1. Review Contractor installation plan.
2. Holes accurately located to line and spacing.
3. Documentation of flow rate of ground water into shaft to validate Dry Construction Method.
4. Fill out form CA-S-1.
5. Document drilling method: dry, wet, temporary casing, or permanent casing.
6. Slurry use per 524.07.
7. Shaft excavation clean on bottom.
8. Reinforcing steel cleaned.
9. Placement of reinforcing steel, center alignment with spacers, clearances, plumbness, etc.
11. Notify Engineer when unexpected obstructions are encountered.