Auger Pile Frequently Asked Questions

Question

Answers About Auger Piles

Auger Cast Pile (CFA Pile, Continuous Flight Auger Pile, Auger Pile, Auger Grouted Pile, Auger Grouted Cast in Place Pile)

How are Auger Cast Piles Installed?

Auger Cast in Place Grouted Piles are installed by rotating a continuous flight hollow shaft auger into the soil to a specified depth or to refusal. High strength sand cement grout is pumped through the hollow shaft as the auger is slowly withdrawn while slowly turning in a clockwise direction. The pile top is cleared of drill spoil, an 18 inch deep galvanized metal “can” is placed around the top of the pile and the pile is screened to remove contaminants. The pile is then dipped (see Below) if necessary and the reinforcing steel, if required, is then inserted into the pile to the correct elevation. The resulting grout column hardens and forms an Auger Cast in Place Grouted Pile.

How do you build up or cut off Auger Cast in Place Grouted Piles if the grade at the time of drilling is above or below design top of pile elevation?

If the design top of pile (pile cut off) elevation is below the ground surface at the time of drilling and the soil conditions allow it, the grout can be dipped from the top of the pile to lower the grout surface. If settlement of the pile grout is consistent and soil conditions allow, piles can be accurately dipped three to four feet. As the depth to pile cut off elevation increases the accuracy of dipping decreases. If a large amount of dipping is necessary the dipping can be performed using an air lift. Note that dipping must be performed immediately after the pile is cast. If the sides of the hole begin to cave dipping should stop. If the soil conditions do not allow dipping to the final top of pile elevation the pile can be cut off using chipping guns, concrete saws and/or hydraulic cutters after the pile cap is excavated. If the pile cut off elevation is above working grade at the time of pile installation the pile can be built up by using a round form. If the buildup is 8 inches or less it is typically accomplished using the standard 18 inch tall galvanized steel cans commonly used in the industry. If the buildup is greater it can be accomplished by using longer “cans” or by using sonotube. Once the form is firmly in place it is filled with fluid grout to the proper elevation.

What if an Auger Cast in Place Grouted Pile settles or the grout level drops?

Grout settlement is a normal part of an Auger Cast in Place Grouted Pile installation in some soil conditions. Once an Auger Cast in Place Grouted Pile is constructed, while the grout is still fluid, the pressure of the grout column can cause the soil surrounding the pile to compress. This process is observed by the lowering of the grout level at the top of the pile. Fresh grout can be added to the top of the pile (topping off the pile) as long as the grout in the pile is still fluid and no contamination is occurring at the top of the pile.

How is reinforcing installed in Auger Cast in Place Grouted Piles?

Reinforcing, when required, can be installed while the cement grout is still fluid or if necessary (in the case of full length single reinforcing bars), through the hollow shaft of the auger prior to the auger withdrawal and grouting process. If the reinforcing is installed after the pile is grouted centralizers are used to maintain the required clearances.

What reinforcing is typically installed in Auger Cast in Place Grouted Piles?

Per the 2009 IBC 1810.3.9.2 No reinforcing is required except where subject to uplift or where required due to the moment strength determined using the load combinations of Section 1810.3.9.1.

Cage reinforcing is used to carry lateral or seismic loads in the upper portion of the pile typically the top 15 to 20 feet. Rather than using the cage to carry uplift loads it is typically more economical to use a center bar in the pile to carry the uplift loads. The longer the pile, the more difficult and expensive the installation of a full length cage becomes.

What reinforcing centralizers are used in Auger Cast in Place Grouted Piles?

The centralizers most commonly used are made of a plastic or pvc material. They are designed to maintain the minimum clearance (minimum coverage) between the reinforcing steel and the surrounding soils and between the bars.

The spacer wheel for cages is six inches in diameter and has a center groove which snaps on the #3 ties most commonly seen in cage design. This groove places the wheel so that 3 inches of the wheel is to the inside of the cage and 3 inches outside to maintain clearance. The wheels are attached such that they may rotate around the #3 tie as the cage is lowered into the pile.

For center reinforcing bars the best centralizer is a PVC “football” or plastic collar and strap centralizer that slides over the bar and is tied in place. The centralizer is a small diameter at each end with the diameter at the center large enough to ensure the proper clearance.

What is the minimum concrete cover required in uncased cast in place piles?

IBC Table 1808.8.2 requires a minimum cover of 2.5 inches; however a cover of 3 inches is standard in the industry and improves the ease of installing the cage.

How close can you drill to a newly completed adjacent Auger Cast in Place Grouted Pile?

IBC Table 1810.3.3 requires a minimum spacing of at least 6 pile diameters center to center for piles installed within 12 hours of each other, unless approved by the building official.

What are the uses of Auger Cast in Place Grouted Piles? Are Auger Cast in Place Grouted Piles friction piles or are the end bearing piles?

There are many ways Auger Cast in Place Grouted Piles can be used:

• Friction Piles - the load is transferred to the soil through friction between the pile surface and the soil;

• End Bearing Piles - the load is transferred through the pile tip into a dense stratum of soil or rock;

• Combination Friction and End Bearing Piles – the load is transferred as a combination of the two descriptions above;

• Uplift Anchor (Soil Anchor) - hydrostatic or other uplift forces are resisted by the grout to soil contact along the entire surface of the pile and are transferred though a full length tendon embedded in the center of grout column;

• A series of adjacent Auger Cast in Place Grouted Piles forming a continuous auger cast wall in temporary or permanent shoring systems and below grade walls.

• A temporary or permanent diagonal tie-back in auger cast curtain walls, beam and lagging walls, and sheet pile walls.

What are the advantages of Auger Cast in Place Grouted Piles?

Less noise - Auger Cast in Place Grouted Piles are a drilled and pumped pile, not a driven pile. This eliminates the hammer impact noise created by driving piles.

No objectionable vibration - The elimination of a pile-driving hammer allows the installation of Auger Cast in Place Grouted Piles adjacent to existing structures without the danger of settlement or damage to existing footings, walls, other structural components, or nearby equipment caused by vibrations.

Auger Cast in Place Grouted Piles in many situations can offer a considerable cost savings to an alternate pile type. The total cost of the grout and reinforcing steel per linear foot can be considerably less than material for an alternate pile with the same load bearing capacities.

The length of Auger Cast in Place Grouted Piles is totally variable up to the maximum length that can be installed with the pile rig that is mobilized. On projects with variable pile bearing lengths, the material waste can be reduced because the material placement stops at the drilling grade. The pile contractor doesn’t have to try to select the appropriate material length for each pile location.

The size of the Auger Cast in Place Grouted Piles can be quickly changed using the same pile rig by simply changing the auger flighting and guide devices, allowing the installation of various sizes for load testing to determine the most efficient size or the installation of different sizes for different capacities on the same project.

An Auger Cast in Place Grouted Pile has a very irregular shape and rough surface in most soil conditions, maximizing the friction capacity of the pile.

Since an Auger Cast in Place Grouted Pile is cast in place there are no concerns with the shipment of over length pile material or handling or moving piles around the jobsite.

No casing required - During the installation of an Auger Cast in Place Grouted Pile, the earth filled auger maintains the shape of the pile hole during the drilling phase. The pressures produced by the head of grout in the auger during the withdrawal and grouting phase exceed the lateral pressures exerted by the soil, and while the gout is still fluid, the lateral pressures per foot of depth of the grout exceed those pressures per foot of depth of the soil.

An Auger Cast in Place Grouted Pile can be installed in limited headroom conditions - Auger sections can be manufactured to accommodate installation of Auger Cast in Place Grouted Piles inside existing buildings with a minimum clearance of 7 to 10 feet.

Soil at each pile hole can be inspected - The grout that is pumped into the pile displaces the existing soil for the entire length of the pile. This displacing action allows the soil along the length to be available for visual inspection and testing.

In varying soil conditions, prior to test pile installation probe holes can be drilled at different locations with the pile rig around the site. This allows the inspecting engineer to evaluate the soil conditions and determine the most effective test pile location at a very low cost since no material is wasted.

What are Typical Pile Loads for Auger Cast Piles?

Typical pile loads:

  • Diameter (Inches)
  • Load Range (Tons)
  • 12”
  • 10-75
  • 14”
  • 50-90
  • 16”
  • 75-150
  • 18”
  • 100-175
  • 24”
  • 125-200

*Pile capacity is dependent on soil conditions. Other pile diameters available but less common are 20”, 36” and 42”. Thirty six and forty two inch auger cast piles are very dependent on soil conditions.

What Strength Grout is used in Auger Cast Piles?

The most typical grout strengths for Auger Cast in Place Grouted Piles range from 3000 psi to 5000 psi. 6000 psi grout is becoming more common and has been used consistently in several locations. Higher grout strengths such as 7000 psi can be used in some locations, however special attention must be paid to curing of grout specimens especially prior to them being delivered to the controlled environment at the test laboratory.

How is an Auger Cast in Place Grouted Pile Tested in Compression?

Typically the Auger Cast in Place Grouted Pile to be tested is installed centered in a rectangle of four Auger Cast in Place Grouted Piles to be used as reaction piles to hold down the test beams. If four reaction piles are not sufficient to counteract compression test load additional reaction piles can be installed or reaction piles could be replaced by rock anchors. ASTM D1143 Standard Test Methods for Deep Foundations Under Static Axial Compressive Load applies to Auger Cast in Place Grouted Pile compression load tests and requires that there be a clear distance of five times the maximum pile diameter between the test pile and any reaction pile with a minimum of eight feet.

Two reaction beams (H Beam or wide flange) are connected to two reaction piles each on opposite sides of the test pile, typically by threaded reinforcing imbedded in the piles. These reaction beams hold down each end of a test beam (Usually a wide flange strengthened with gussets) which is centered over the test pile.

A hydraulic jack is placed between the test pile and the test beam in a position to put a known compression load on the test pile. The load is transferred through the test beam to the reaction beams and the reaction piles or rock anchors.

The compression load being applied to the test pile is measured by a load cell that is placed between the test pile and the test beam as required by ASTM D1143 -07 if the test load is 100 tons or more. If the test load is less than 100 ton the load may be measured by a load cell or by a calibrated jack.

The movement of the test pile must be measured by two methods. The primary measurement is typically by dial gages placed on at least two sides of the pile between a connection to the test pile and a reference beam that is supported independently of the load test frame with its supports not less than five times the largest pile diameter or 8 feet whichever is greater from test or reaction piles. The secondary method typically is a scale attached to the test pile that is read, either by a surveyor’s level or a wire that is independent of the load test frame and is supported not less than five times the largest pile diameter or 8 feet whichever is greater from test or reaction piles.

Additional methods to test Auger Cast in Place Grouted Piles in compression are statnamic pile testing and PDA pile testing. Since both of these methods require the mobilization of additional equipment they are not commonly used for Auger Cast in Place Grouted Piles. They are an option, however.

How is an Auger Cast in Place Grouted Pile Tested in Tension?

If a pile is to be tested in compression and the desired tension test load is not more than the load that will be applied to each of the reaction piles during the compression load test the most economical tension test is to simply monitor any movement of one or more of the reaction piles during the compression pile load test. In some cases it may be practical to put more load on the compression pile in order to increase the test load on reaction piles and achieve the desired tension test load. To eliminate the risk of failing the compression test pile prior to completing the compression test it may be prudent to run increase the load on the pile after the compression test has been completed.

If a separate tension test is needed it should be run in accordance with ASTM D 3689 Standard Test Method for Individual Piles Under Static Axial Tensile Load.

Reaction piles may or may not be needed for a tension test, depending on the required test load and the soil conditions at the test location. If reaction piles are not required the test beam would typically rest on timber matts located on opposite sides of the test pile otherwise reaction piles would be installed on opposite sides of the test pile and the test beam would rest on the reaction piles. ASTM D 3689 requires that there be a clear distance of five times the maximum pile diameter between the test pile and any reaction pile or cribbing with a minimum of eight feet.

A hydraulic jack is placed on the test beam directly above the test pile. Typically the test beam is actually two wide flange beams running parallel with a space between. A threaded reinforcing bar is placed in the tension test pile to the depth of the anticipated reinforcing in the production piles. This bar is then extended between the two beams and through the jack, which is manufactured a hole through the center. A plate and nut is assembled on the bar above the jack to transmit the load from the jack into the test pile.

The tension load being applied to the test pile is typically measured by a calibrated jack.

The movement of the test pile must be measured by two methods. The primary measurement is typically by dial gages placed on at least two opposite sides of the pile between a connection to the test pile and a reference beam that is supported independently of the load test frame with its supports not less than five times the largest pile diameter or 8 feet whichever is greater from test or reaction piles. The secondary method typically is a scale attached to the test pile that is read, either by a surveyor’s level or a wire that is independent of the load test frame and is supported not less than five times the largest pile diameter or 8 feet whichever is greater from test or reaction piles

When Should I not use Auger Cast in Place Grouted Piles? When might I have problems with Auger Cast in Place Grouted Piles?

If there are rock layers that will not support the required pile load and that are too close to the ground surface to allow a friction pile you should not use Auger Cast in Place Grouted Piles unless you are able to predrill the rock. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.

If there is an extensive amount of material with n values of 3 or below you may not want to use Auger Cast in Place Grouted Piles. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.

If you are installing piles against or close to an existing structure that is not pile supported and the soil is granular or weak. Installation of Auger Cast in Place Grouted Piles could remove material and undermine the existing structure. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.

If you require only a few piles it may be less expensive to install driven piles that the load can be determined by the blow count of the hammer, eliminating the need for a load test. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.

If the water at the ground surface and you are not able to lower it or fill above it, it could be impractical to install Auger Cast in Place Grouted Piles. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.

If the soil is contaminated and you do not want to bring that contamination to the surface you don’t want to use Auger Cast in Place Grouted Piles. ADF will be glad to review your soil information and give our opinion of the best and most economical pile type since we install nearly every type of pile.


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