HULK Earth Anchor Frequently Asked Questions
What is an Earth Anchor?
A Hulk Earth Anchor is a geotechnical anchor designed for holding tensile loads. Hulk Earth Anchors can also be referred to as ground anchors, mechanical anchors, or more specifically, percussion anchors, driven anchors, or tipping plate anchors. If you have not read it already read, please see How Hulk Earth Anchors Work
Will weather conditions affect the integrity of the anchors over time?
Only if the anchors are placed in soil which is affected by the weather. A responsible anchor design attempts to place anchors to a depth where the soil is unaffected by weather and seasonal variations. Or if soil variation is unavoidable, then the anchor must be sized and proof tested for the worst case soil condition.
Does the depth of the anchor affect the long term holding capacity?
Not unless the anchor depth is one that places the anchors 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. Anchors should be placed into soil that will not be subject to freeze thaw cycles.
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, but in many retaining wall installations anchors are placed as close as 1.5m x 1.5m rectangular pattern. Account for a 25% load loss factor which represents a reasonable maximum interference factor as the anchors cannot practically be placed any closer together. If interference of anchors is a concern then this is a reasonable way to account for this effect.
What is the best way to link multiple anchors together to one guy line or other structure?
A steel spreader structure with multiple anchor attachment points and one guy line attachment point. The HULK anchor rods (CTB or All thread) pass through the steel spreader structure and are terminated with steel plates, beveled washers, and hex nuts.
What is the Proof test based upon?
The working load, the application, and a responsibly chosen safety factor, limited by the structural capacity of the anchor assembly. For example: Working load = 44kN, Safety factor = 2.00, Proof Test = 89kN with less than 1.27cm movement over 1 minute should provide a very safe anchor for 45kN working load.
What about Creep?
If a HULK anchor application is properly designed and installed creep should not be an issue. By choosing a reasonable safety factor between the long term working load and the proof test, Creep can be virtually eliminated. Example: Working load = 44kN, Safety factor = 2.00, Proof Test = 89kN with less than 1.27cm movement over 1 minute should virtually eliminate creep.
What about Surge Loads?
Surge loads should be accounted for in the basic design of the anchorage. It may be reasonable to use one safety factor for long term constant design loads, and a different one for surge loads which may happen in frequently. For example: Normal Basic Working load = 44kN Safety factor = 2.00, and Onetime per year Surge load = 66.8kN pounds Safety factor = 1.33. Then a Proof Test = 89kN with less than 1.27cm movement over 1 minute will satisfy both Normal and Surge load requirements. In some conservative designs the 2.00 safety factor is applied to the Surge load. This is the safest approach, but sometimes may be over conservative.
It takes 30 minutes to drive the anchor to 2.4m depth, what can I do to speed up the installation. This is too long?
Consider drilling or auguring a pilot hole. This will speed up the total installation time. In many cases drilling a pilot hole to about 2.4m depth can be done in 10 minutes or less. The anchor is then driven quickly down the pilot hole (usually in 10 minutes of less). By drilling the pilot hole the installer has reduced his installation time by 10 minutes. If the soil is hard enough that this is required there is no capacity reduction from the pilot hole. If the anchor cannot be driven quickly down the pilot hole then a smaller anchor with the same structural capacity should be used. Drilling a pilot hole in hard soils can result in a faster installation and significantly increases drive steel life. Speak to your Hulk distributor about pilot hole sizes for different sizes of anchors.
We drilled a 100mm pilot hole and the HG-100 fell down the hole. Is the 75mm auger a better choice for HG-100 Anchors?
It depends upon the soil, and the required depth. In very hard soils, the 100mm auger is a good choice for the HG-100. The anchor will easily pass down the hole, yet will still lock in place against the sides of the hole. For medium soils a 75mm auger may be a better choice, or it may not be necessary for an efficient installation. In either case, if a pilot hole is used it is important 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. Virtually every anchor installation is different, but the 100mm diameter pilot hole is a good general choice for pilot holes for all HULK HG anchors. Because every installation is unique, there may be better choices for different soil conditions, drive depths, etc.
Should the pilot hole be drilled for the entire depth of the anchor, or should the anchors be driven for the last 0.5m?
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. We have seen some retaining wall installations where the Specifying Engineer did not allow the pilot holes to be drilled beyond the failure plane, and required the pilot holes to be filled with grout after installation.
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 scoop of the anchor to start it to tip and lock. We have had some installers sharpen the scoop 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.
Can anchors be installed in Permafrost or frozen soil?
Yes, HULK anchors work very well in frozen soil, but a pilot hole is usually a requirement. If there is only 1m or less of frozen soil, then the anchor can be driven through the frozen soil into the underlying non frozen soil. 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. If the soil is of good high capacity prior to freezing, then this is not as large a concern.
Why does the Proof Tester register 18kn when there is obviously no load on the anchor and the valve handle is in the neutral position?
This is a characteristic of the hydraulic valve on the load locker called trapped pressure. Hydraulic pressure is trapped on both sides of the cylinder piston and registers on the gage. When the valve handle is pulled to retract the piston, the gage reads zero. This does not affect load locking of the anchors if the proper procedure is used. Refer to the Operation Manual for more details, and the proper load locking procedure.
What if I want to hold a constant load for a period of time as a proof test?
This is our recommended proof test method. We suggest that you set the bypass valve on the load locker to that load. This allows the installer to shift the valve on the proof tester wide open while the bypass valve limits the pull force to the bypass setting. Then the installer can monitor movement of the anchor without worrying about the load level. Instructions for setting the bypass valve are included with each load locker.
What is the origin of the Ultimate Holding Capacity Charts?
The published holding capacity charts have been developed over the years from a combination of sources. The primary source has been projects with geotechnical soil data and proof test results. Secondary sources are analytical prediction methods and mathematical interpolation and extrapolation based upon the analytical parameters of anchor bearing area, depth, angle of internal friction, and blow count.
What is The Blow Count (N) of the soil?
The blow count is a byproduct of geotechnical soil sampling, and is an indicator of the hardness and capacity of the soil. The higher the blow count the harder and more competent the soil is. Higher blow counts mean it will be harder to drive the anchors into the soil, but capacities will be high. Lower blow counts mean it will be easier to drive the anchors into the soil, but capacities will be lower. HULK anchors work well in soils with blow counts from about 7 to 50. The best performance is obtained in soils with blow counts from about 15 to 30.
What is Ductile Iron?
Ductile Cast Iron, also known as nodular iron, or spheroidal graphite (SG) cast iron is a form of cast iron in which the graphite is present as tiny spheres or nodules. This is accomplished by proper chemical control and the addition of alloying elements just before casting. These alloying elements cause the spheroidal shape of the graphite which tremendously increases the ductility and impact resistance of the iron. HULK anchors are further enhanced by a post casting heat treat process called annealing. This annealing refines the micro structure of the cast iron and further increases the impact resistance.
What is Moderate soil?
Soil with resistivity greater than 3000 ohm-cm, pH between 5 and 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.
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.
What is the design life expectancy of a HULK anchor?
HULK anchors with galvanized cables should be considered only for temporary applications. The weak link of the HULK anchor is the stranded wire rope tendon. 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 Grade 316 Stainless cable with copper ferrules. This provides the greatest corrosion resistance available for cabled HULK anchors, but still should be limited to approximately 10 years of service. If more than 10 years of service is desired, then the clevis type HULK anchor which accommodates a rod, should be used. Hulk with hot dip galvanized steel bar tendons above 20mm can be considered for up to 50 years life span and 75 years when coated with HULK hard armor.
What is the design life expectancy clevis type Hulk anchors?
HULK anchors with galvanized anchor rods retain 94% of their original ultimate strength after 50 year exposure to moderate soils. The critical component is the shackle. HULK anchors with Continuous threaded anchor bar retain 89% of their original strength after 50 year exposure to moderate soils.
What about corrosion of the internal threads on the HULK anchor shackles and anchor rod coupling nuts?
The internal threads are actually protected by the galvanizing from the rod that is threaded into them. This was graphically proven on a utility guy anchor that was retrieved from an extremely corrosive environment complete with stray currents and aggressive soils. The anchor rod and shackle of the anchor were corroded to approximately 50 % of their original dimensions (the residual strength would have been about 25 percent of original). Disassembly of the shackle and the rod showed that the internal threads of the shackle and the external threads of the anchor rod had retained their original dimensions.
What about grout for corrosion protection?
Grout is commonly used with HULK anchors to enhance holding capacity and to provide corrosion resistance at the expense of a more complicated installation. Typically a grout tube is attached to the anchor rod and grout is pumped through the grout tube to the bottom of the hole. This has historically provided good coverage of the anchor, shackle, and anchor rod, but there are many variables and care must be taken to ensure good complete coverage. A more detailed treatment of grouting anchors is available from Percussion Earth Anchors upon request.
What about double corrosion protection (DCP)?
Percussion Earth Anchors can provide a system of double corrosion protection by supplying galvanized anchor components that are painted. Installation methods are a bit more involved in order to minimize damage to the paint, but it has been specified by some Engineers and can be supplied. Contact your HULK distributor or Percussion Earth Anchors .