Strand Anchors

 

Strand anchors offer numerous advantages over other anchorange systems.

The most important advantages include:

  • Full flexibility in anchor working loads and lengths by selection of appropriate number of strands and grade of steel. 
  • Splice-free fabrication even for large anchor lengths.
  • Relatively low self-weight.
  • Favourable installation characteristics and small space requirements, ensured through high flexibility of strands.
  • Good transport characteristics and small space requirements during transport and storage (strands can be coiled).

Available steel grades:
 

ST 1570 / 1770 0,60 " (140 mm²)

ST 1570 / 1770 0,62" (150 mm²)

ST 1660 / 1860 0,60" (140 mm²)

ST 1660 / 1860 0,62" (150 mm²)

The following types of strand anchors are available:

Temporary Strand Bars

This type of anchor is generally designed for a lifespan of up to two years.

A temporary strand anchor consists of the following components:

Anchor head  Customized for different applications andsite conditions.

 

Bond length Also called "fixed anchor length", this represents the section of the strand anchor which transfers the tensile load to the ground

 

free Anchor length  This represents the elastic section of the strand anchor between bonded length and anchor head that does not transfer any load to the surrounding soil.

Semipermanent Strand Anchors

This type of anchor is generally designed for a lifespan of 2 to 4 years. A semi-permanent anchor is similar to a temporary anchor with regard to the construction of the anchor head, free anchor length and bond length. The only difference is that the strands are greased in the free anchor length for corrosion protection. The anchor head assembly can be customized as required. As a standard feature, a bearing plate with steel stub tube and anchor seal is recommended.

In addition, the upper section (approx. 0.5 - 1 m) of the free anchor length is encapsulated by a plastic tube to ensure full continuity of corrosion protection in the transition zone between anchor and anchor head assembly. It is also possible to provide a re-stressable anchor head assembly to fit a steel protection cap instead of the standard PE cap. Anchor head assemblies should generally be protected by layers of Denso tape.

Permanent Strand Anchors

This type of anchor is designed for permanent application.

To ensure the durability of  permanent strand anchors, special attention must be directed to the quality of corrosion protection. (details can be found in the relevant Approval Certificates under under Downloads).


Permanent strand anchors are available with two types of bond length:

Pre-grouted bond length: this means that the bond length is the bond length is factory pre-grouted with cement grout. The advantage of this type is the provision of continuous quality control in the SPANTEC factory (internal and external quality control)

Non-grouted bond length: the advantage of this type is its low transport weight and the ability to coil the anchor. 

Removable Strand Anchors

Type 1

Removable strand anchors where the bond length remains in the ground                   

This type of anchor features strands that have been weakened at the transition between bond length and free length. During anchor removal, individual strands are stressed to failure at the pre-determined rupture point.

Attention to site safety is paramount throughout this process. When the strand is destressed abruptly after rupturing, it is possible to be ejected from the borehole in an uncontrolled manner. It is, therefore, imperative that the area be sealed off or other precautionary measures be taken. In general, the installation of anchors, which are to be removed after usage, requires increased care and attention to detail. Even minor carelessness can result in the inability to remove an anchor. For example, greater care should be exercised with regard to strand projections or when backfilling a borehole with grout.

 

Removable Strand Anchors

Type 2

Fully removable strand anchors

This type is the so-called "blast anchor". The strand anchor is equipped with an additional PE tube.  To facilitate the removal of the anchor, the tube is charged with explosives by an explosives expert. When detonated, the grout body of the strand anchor is ruptured and thus destroyed. The bond between the strand and the grout body is eliminated. The strand can then be completely removed from the borehole by jacking or winching. This removal method is, however, only possible, if the grout body is able to expand during the explosion and break up into smaller fragments. If this is not possible, e.g. in rock.