DUCKBILL ANCHORS

FREQUENTLY ASKED QUESTIONS

1) Will weather conditions affect the integrity of the anchors over time?
2) Does the depth of the anchor affect the long term holding capacity?
3) How far apart do the anchors need to be from each other?
4) It takes 15 minutes to drive the anchor to 1m (3ft.) depth, what can I do to speed up the installation? This is too long.
5) Should the pilot hole be drilled for the entire depth of the anchor, or should the anchors be driven for the last 18" (.5m)?
6) What should we do if the anchor does not tip and lock in place and slides back up the hole?
7) Can anchors be installed in Permafrost or frozen soil?
8) What is Moderate soil?
9) What is Aggressive soil?
10) What is the design life expectancy of a Duckbill anchor?
11) What happens to the Duckbill anchor head in the ground over time?

CORROSION

1) What is the design life expectancy of Standard Duckbill anchors?
2) What can be done to improve the life expectancy?
3) What if I have aggressive soils?
4) What if I want more than 5 years life and I have aggressive soils?
5) What happens to the Duckbill anchor head in the ground over time?
6) What if I want more than 10 years life and I have aggressive soils?
7) What if I want more detailed information?

1) Will weather conditions affect the integrity of the anchors over time?
Yes, weather conditions can affect the moisture content of the soil. If variation is unavoidable, then the anchor must be sized and proof tested for the worst case (wettest) soil condition. Anchors should be placed into soil that will not be subject to freeze thaw cycles.

2) Does the depth of the anchor affect the long term holding capacity?
Not unless the anchor is placed into soil that is unstable or seasonally variable. In general, greater depth usually results in greater capacity, but not always. In some cases loose wet soil can exist below stable competent soil, so increased depth can be a detriment.

3) How far apart do the anchors need to be from each other?
A conservative rule of thumb is to place the anchors no closer than the depth to which they are installed.

4) It takes 15 minutes to drive the anchor to 1m (3ft.) depth, what can I do to speed up the installation? This is too long.
Consider drilling a pilot hole with an earth auger. This will speed up the total installation time. In many cases drilling a pilot hole to about 21m (3ft.) depth can be done in 3 minutes or less. The anchor is then driven quickly down the pilot hole. By drilling the pilot hole the installer has reduced his installation time significantly. If the soil is hard enough that this is required there is no capacity reduction from the pilot hole. Drilling a pilot hole in hard soils can result in a faster installation and significantly increases drive steel life. Recommended pilot hole sizes are:

5) Should the pilot hole be drilled for the entire depth of the anchor, or should the anchors be driven for the last 18" (.5m)?
We have seen installations with both methods, depending upon the soil. In some very hard, slippery soils it was most effective to drive the anchors a bit beyond the end of the pilot hole if possible. But sometimes it is not possible, and in these cases it is best to do a good job of back filling and compacting prior to load locking.

6) What should we do if the anchor does not tip and lock in place and slides back up the hole?
This does happen occasionally in some hard slippery soils where a pilot hole has been drilled. We have not had a report of this happening where the anchors were driven without a pilot hole. If a pilot hole is used it is wise to back fill the hole with the drill cuttings after the anchor is placed, but before load locking. It is wise to back fill and compact the soil (usually done with the drive steel) prior to load locking because it will help the anchor to tip and lock. We have seen some installations where the drive steel was placed backwards down the hole to hit the "duckbill" of the anchor to start it to tip and lock. We have had some installers sharpen the "duckbill" with a grinder to help it grab into hard soil. It is generally accepted by most installers that the anchors lock better if the initial pull is done at as fast a speed as possible.

7) Can anchors be installed in Permafrost or frozen soil?
Yes, but if the soil is frozen for the entire installation depth then a pilot hole will be required. Beware, frozen soil is very tough and may require heavy duty mechanized drilling and driving equipment. Anchor capacities in frozen soil are very high as long as the soil remains frozen, but if the soil around the anchor thaws the soil will revert to its thawed, lower capacity state.

8) What is Moderate soil?
Soil with resistivity greater than 3000 ohm-cm, pH between 5 an 9, Chlorides less than 1000 ppm, Sulfates less than 500ppm, and fair aeration. The above soil parameters are obtained from chemical tests on soil samples

9) What is Aggressive soil?
Soil with resistivity less than 3000 ohm-cm, pH less than 5, Chlorides greater than 1000 ppm, Sulfates greater than 500ppm, and poor aeration. The above soil parameters are obtained from chemical tests on soil samples

10) What is the design life expectancy of a Duckbill anchor?
Standard Duckbill anchors (68,88,138-DB1) should be considered only for temporary applications. The weak link of the Duckbill anchors is the stranded wire rope tendon. The anchor heads also corrode, but at a slower rate than the steel cable, and they have more material "to loose" to corrosion before becoming critical. In Moderate soils they should be limited to two years or less of service.

The next step up in corrosion resistance is to replace the galvanized steel cable with AISI 304 Series Stainless cable with aluminum swage sleeves. These are designated as 68-DB1-SSA, 88-DB1-SSHDA, 138-DB1-SSHDA. In Moderate soils this can provide up to 5 Years of service life. If aggressive soils are known to be present, it is best to take the next step up and equip the Duckbill with AISI 304 Stainless cable and copper swage sleeves.

The copper swage sleeves are more resistant to general and pitting corrosion than the standard aluminum swage sleeves. These are designated as 68-DB1-SSC, 88-DB1-SSC, 138-DB1-SSC. Because of the relative size of the swage sleeves compared to the anchor head the aluminum anchor head does not become the critical component until about 5 years of service in aggressive soils. If more than 5 years of service are desired, then it is best to take yet the next step up and equip the Duckbill with AISI 316 Stainless cable and copper swage sleeves.

This provides the greatest corrosion resistance available for Duckbill anchors, but still should be limited to approximately 10 years of service. After about 10 years of service the aluminum anchor head becomes the weak link if 316 stainless cable is used. If more than 10 years of service is desired, then a Manta Ray anchor should be used. The comprehensive corrosion analysis of Duckbill, Manta Ray and Stingray anchors (from which the above results are obtained) is available from Foresight products upon request. Contact your Duckbill, Manta Ray or Stingray supplier or Foresight.

11) What happens to the Duckbill anchor head in the ground over time?
The aluminum alloy of the anchor head does not rust, but it does corrode. Eventually the Duckbill corrodes away but this is usually after the anchor has served its useful life.

Corrosion

The information provided here is based upon a comprehensive analysis of the components of Duckbill anchors. The analysis is based upon underground corrosion research done by the National Bureau of Standards. It is believed to be conservative, but it must not be construed as any guarantee of corrosion life. It is based upon broadly and conservatively categorizing soils as Moderate or Aggressive as defined below.

Moderate soil: Soil with resistivity greater than 3000 ohm-cm, pH between 5 an 9, Chlorides less than 1000 ppm, Sulfates less than 500ppm, and fair aeration.

Aggressive soil: Soil with resistivity less than 3000 ohm-cm, pH less than 5, Chlorides greater than 1000 ppm, Sulfates greater than 500ppm, and poor aeration.

1) What is the design life expectancy of Standard Duckbill anchors?
Standard Duckbill anchors (68,88,138-DB1) should be considered only for temporary applications. The weak link of the Duckbill anchors is the stranded steel wire rope tendon. The aluminum anchor heads and swage sleeves also corrode, but at a slower rate than the steel cable, and they have more material "to loose" to corrosion before becoming critical. In Moderate soils they should be limited to two years or less of service.

2) What can be done to improve the life expectancy?
The next step up in corrosion resistance is to equip the Duckbill anchor with AISI 304 Series Stainless cable. These anchors are designated as 68-DB1-SSA, 88-DB1-SSHDA, 138-DB1-SSHDA. In Moderate soils this can provide up to 5 Years of service life.

3) What if I have aggressive soils?
If aggressive soils are known to be present, it is best to take the next step up and equip the Duckbill with AISI 304 Stainless cable and copper swage sleeves. The copper swage sleeves are more resistant to general and pitting corrosion than the standard aluminum swage sleeves. These anchors are designated as 68-DB1-SSC, 88-DB1-SSC, 138-DB1-SSC. Because of the relative size of the swage sleeves compared to the anchor head the aluminum anchor head does not become the critical component until about 5 years of service in aggressive soils.

4) What if I want more than 5 years life and I have aggressive soils?
If more than 5 years of service are desired, then it is best to take yet the next step up and equip the Duckbill with AISI 316 Stainless cable and copper swage sleeves. This provides the greatest corrosion resistance available for Duckbill anchors, but still should be limited to approximately 10 years of service. After about 10 years of service the aluminum anchor head becomes the weak link if 316 stainless cable is used.

5) What happens to the Duckbill anchor head in the ground over time?
The aluminum alloy of the anchor head does not rust, but it does corrode. Eventually the Duckbill corrodes away but this is usually after the anchor has served its useful life.

6) What if I want more than 10 years life and I have aggressive soils?
If more than 10 years of service is desired, then a Manta Ray anchor should be used.

7) What if I want more detailed information?
The comprehensive corrosion analysis of Duckbill, Manta Ray and Stingray anchors (from which the above results are obtained) is available from Foresight products upon request. Contact your Duckbill, Manta Ray or Stingray supplier or Foresight.