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

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.

TA.95 states that a temporary anchor has a lifespan of not than eighteen months; the figure in French Standard NF EN 1537 is less than two years.

The most common processes are the tube à manchettes sleeved pipe and singleshot grouting hoses arranged along the anchorage length.

TA.95 permits two months extension under certain conditions; the standard allows no extension but accepts that an anchor may be temporary for more than two years, if so provided for in the design.
Temporary anchor tendons do not have to be protected against corrosion of the steel except under certain conditions, concerning lifespan and ambiant aggressivity. The requirements concern all three parts of the ground anchor and protection, if any, must be continuous over the whole length of the ground anchor.

Example: Protection P0

- Anchorage zone: the cement grout provides the protection.

- Free length: single or individual or individual plastic sheaths around tendons.

- Head: no special protection.

Corrosion Protection
TA.95			NF EN.1537
Relié au temps et à l'agressivité du milieu
Lifespan/ambient conditions	Less than 9 months	9 months to 18 months	Basic protection is similar to P0 and increases with aggressivity, lifespan may be more than 2 years if so provided for in the design
Non-agressive	P0	P1
Moderately agressive	P1	P2
Agressive	P2	P2

Tirant provisoire ECO de Solétanche Bachy

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 :

• 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).
  

1. Design: pul-out test to failure to determine anchorage capacity of ground
2. Fit-for-purpose: pull-out test to failure to check anchorage capacity
3. Acceptance test (+ inspection): all anchors are tested to 1.15 times their service tension.