DUCKBILL ANCHORS

HOLDING CAPACITY

Drawings

Duckbill anchoring details GTL-0000-10

The holding capacity of duckbill anchors varies depending on soil conditions. Please refer to attached "Duckbill Holding Capacity Chart in Kilograms". In most soil types, an 88 Duckbill should achieve a load of at least 900kg.

With this in mind, the use of 8 anchors per tank results in approximately 7,200kg of hold down capacity. In conditions where the soil is very poor and 900 kg per anchor is not achieved, it is recommended that 3 Duckbills be ganged together at each tie down point on the tank.

Ganged anchors should be installed at an included angle of 60 degrees. This angle will result in a deration of the anchors. Two derated anchors installed in a ganged configuration at 60 degrees will equal 1.5 times the capacity of a single Duckbill.

An example is as follows:

If a load of only 600kg is achieved on a single anchor, the ganging of two anchors would achieve 600 x 1.5 = 900kg per tie down point. 8 x tie down points per tank would result in 7,200kg of hold down capacity.

The use of the 88 model Duckbills, either singular per tie down point ie. 8 anchors per tank or ganged in two’s at 60 degrees ie. 16 anchors per tank, will result in sufficient hold down capacity for tanks in all but the poorest of soils (based on a maximum hold down capacity of 5000kg). The capacity required per anchor or per pair of anchors is only 625kg.

If a situation, with very poor soils is encountered, then longer wire ropes are recommended. The above figures are based on the standard length wire rope on an 88 model Duckbill which is 1.1mt (usual embedment depth of 1mt).

The use of longer wire ropes will enable anchors to be installed into the harder ground usually found at a greater depth. A depth of up to 3mt is practical for the purpose of driving but special drive steels may need to be looked at. The drive steels may need to couple together or the person driving the anchors may need to stand on something to gain sufficient elevation to enable driving to occur.

An important factor for the installer to consider is that there is an obvious correlation between ease of driving and the ultimate holding capacity that will be obtained in a certain soil condition. If an anchor is driving into the ground very easily, then even with limited experience, the installer should feel that the holding capacity will not be as great as that of an anchor which is harder to drive ie. the ground is harder thus higher capacity will be obtained.

Longer wire ropes attached to the anchor allows the installer to keep driving the anchors to a greater depth until it is felt that the ground has become harder and thus the anchor will achieve a greater hold.

Three different Duckbill kits are offered depending on the soil conditions.
Kit 1 – 8 x 88 Model Duckbills with 1mt of wire rope.
Kit 2 – 16 x 88 Model Duckbills with 1mt of wire rope.
Kit 3 – 16 x 88 Model Duckbills with 2mt of wire rope.

If ground conditions are known before installation then the appropriate anchoring kit can be chosen to suit those conditions. Kit 1 would suit hard to medium soils, Kit 2 would suit poor soils and Kit 3 would suite very poor soils. If ground conditions are not known before installation, then Kit 3 will cover the absolute worst case scenario.