A ground anchor is a system capable of transmitting a tensile load into a competent stratum, drawing support from the structure to be anchored.

Ground anchors are commonly used to prevent deformation of a retaining wall after excavation. In such case, anchors are installed through the retaining structure during the excavation phase. Earthworks are resumed once these anchors have been tensioned.

 

A ground anchor is therefore always carefully treated, as it contributes to structural stability.

 

An anchor comprises three parts:

• The head, transmitting the anchor force to the structure via a bearing plate
• a free length of tendon, from the head to the near extremity of the bond length,
• a bond length, which is the length of tendon through which the tensile force is transmitted to the surrounding ground through a bond grout.

 

An anchor is called “temporary” if it is to be used for a limited period of time, generally during construction of the structure. A “permanent” anchor is a ground anchor lasting more than two years or, more generally, a component required for the entire lifetime of the structure.

 

 

There are two families of anchors – active and passive:

• Passive: This type of anchor does not usually have a free length. Generally speaking, the anchor consists of a steel bar or is sometimes made of composite material.  A passive anchor is only tensioned by the structure itself applying load to it.
• Active: This type of anchor has a free length and a bond length.  The armature is usually made of steel cables, pre-tensioned to reduce the displacement of the structure.

Each anchor is protected against corrosion to prevent any impact on the system. This is defined based on two main criteria: the lifespan of the anchor and the aggressiveness of the environment.

 

Ground anchors have a wide range of applications

Temporary and permanent retaining walls

Diaphragm walls – Sheet piling – Retaining walls – Underpinning curtain – Berlin-type walls and similar.

 

Transmission of tensile loads

Anchoring of slabs below the water table – Pre-stressing of piles working in traction – Anchoring of narrow structures (pylons, tower buildings, chimneys, etc.) – Transmission of tension for rigging lines (suspension bridges, pylons, etc.).

 

Anchorage – Nailing

Stabilisation of slopes and landslides (cracked rocks, cliffs, scree areas) – Consolidation of tunnels – Blocks of penstocks.

 

Miscellaneous

Transmission of pressure on vaults – Post-stressing of structures – Dam stability improvement.

What are the advantages of this technique?

Implementation

 

• The sequence of installing a ground anchor is as follows: Boring of 100mm to 200mm diameter depending on the dimensions of the anchor body, at the appropriate angle, using a drilling rig a fluid to suit the ground conditions

 

• Clean the borehole, replace fluid by a high cement content grout
• Insert the ground anchor by crane, from drum or even by manhandling
• Once the grout has set, the bond length is pressure-grouted.
• Various grouting systems can be used to suit ground conditions and degree of bonding enhancement required (see @grouting chapter). The European Standard EN1537 describes two main methods:
  • A single hit overall grouting
  • Selective Repetitive Grouting, generally by TAM Grouting.
• Depending on ground conditions and the bonding product used, a curing time of two to five days is required between the last grouting phase and the tensioning of the anchor.
• The anchor head protection is added once tensioning has been checked.

 

Did you know?

 

• Soletanche Bachy created the Repetitive Selective Grouting which increase significantly the capacity of ground anchors. This innovation was inspired from the Father of “Tube a Manchettes” device, Ernest Ischy.

 

• The capacity of an active anchor may vary from a few dozen kiloNewtons to more than 15,000 kN

 

• Very high-capacity ground anchors are often used to improve the stability of dams. Our record was established in 2014 at Wanapum Dam in the United States (Washington State) with the installation of 35 pre-stressed ground anchors. Each of these comprising 61 metallic wires at a depth of 80m.