A ground anchor transmits the tensile forces applied to it to a competent stratum.

It consists of three parts:

- The head, transmitting the anchor force to the structure via the bearing plate.

- The free length of tendon, from the head to the near end of the anchorage.

- The grouted anchorage, which is the length of tendon by which the tensile force is transmitted to the surrounding ground through the intermediary of the grout.

There are "active" and "passive" soil anchors:

A passive soil anchor is tensioned as the structure itself applies load to it. It does not usually have a free length of tendon (it is grouted over its whole length).

An active soil anchor is pretensioned before it takes up the load, which is a means of limiting deformation.

Anchor capacity ranges from a few tonnes to 1,500 tonnes. The usual range is 20-200 tonnes.

Overall length is specific to each project, ranging from 10 to more than 60 metres. They are commonly 15-25 metres long.

Tendons are usually of the type used in structural prestressing, or similar.

Exemple :
Toron T15,7 seven-wire strand, 1860 Mpa class
0,1 % yield point 248 KN or ó_e = 1653 Mpa.
Breaking strength 279 KN
Cross section : 150 mm²

Technology

The sequence of operations for installing a soil anchor is as follows:

- Drill boreholde, diameter 100-200mm, at any angle, using a drill rig and drilling fluid to suit soil conditions.

- Clean borehole, replace drilling fluid with grout, usually a high-cement-content mix.

- Insert tendon (which may be a bar, strand, etc.) by crane, from drum or even by manhandling.

- Once the grout has set, the anchorage may be pressure-grouted with a cement mix. Various grouting systems are used by different contractors to suit ground conditions. The most common method is the tube à manchettes sleeved grout pipe.

- After 1 to 7 days, depending on ground and grout type, the anchor is pretensioned by jacking, and the head cover set in place.

Corrosion protection

The TA.95 design rules state that a permanent soil anchor must have a lifespan of more than eighteen months; in French Standard NF EN 1537, the figure specified is more than two years.

The tendon material in permanent soil anchors must be protected against corrosion of the steel. Corrosion potential is aggravated by high service stresses. Preventive measures cover all three parts of the anchor; protection must be continuous over the whole soil anchor.

According to TA.95, protection must be level P2.

- Anchorage zone: the tendon is sealed inside a plastic sheath or metal tube.

- Free length: plastic sheath or metal tube filled with non-rigid material (grease, wax, etc.).

- Head: Plastic or metal cover filled with usually the same material as used for the free length.

Soletanche Bachy permanent soil anchor

Design

Design parameters are steel cross section, length or anchorage, and length of free length.

Steel cross section:

TA.95 requires: section > 1,67 x service tension / steel elastic stress
NF EN 1537 requires: section > 1,54 x service tension / stell breaking strength

Anchorage length:

The object is to mobilise friction in the particular type of soil. In rock, the load is transmitted from the tendon to the rock by compression. In a (o, C) soil, the skin friction available increases with the normal stress at the grout/soil interface.

Normal stress is governed by:
- the initial stress,
- soil compaction through dilatancy.

As a general guide, typical figures are:
Losse sand and gravel: 2-4 t/m
Dense sand and gravel: 6-12 t/m
Stiff clay and silt: 2-6 t/m
Hard clay and silt: 4-9 t/m
Weathered chalk: 6-10 t/m
Sound chalk: 10-15 t/m
Rock: 15-30 t/m

Anchorage length for an 80t anchor is commonly between 4m and 15m.

Special care is needed with ground subject to creep flow (plastic index in excess of 20) such as clay and marl. Moderately hard to hard rock, and clean dense alluvium, are good anchorage strata. TA.95 offers a preliminary design method for anchorage length. Anchorage capacity is always checked by pull-out tests.

Free length :

Selected on three main criteria:

• Position of anchorage stratum,
• Minimum lentgh for the tendon to be wedged at the design tension, allowing for mechanical losses,
• General stability of the mass of ground taking the load (e.g. : Kranz method or TA.95).