Design of ground anchors
The recommendations of this annex apply to structures which are supported by ground anchors, defined in clause 1, transmitting a tensile force to a load-bearing formation of soil or rock.
Anchored structures can consist of the following types of structures:
- retaining structures;
- embankment and slope stabilizing structures;
- underground openings;
- underground structures and basements subjected to uplift forces due to ground water;
- structures transferring tensile loads generated by the superstructure or by actions on the superstructure, to the ground.
This annex deals with the design of ground anchors associated with structures taking into account the behaviour and the requirements of the structures. For the design of the structure as a whole reference should be made to ENV 1991-1-1: Eurocode 1: Part 1-1 and ENV 1997-1: Eurocode 7 - Part 1-1.
D.2 Limit states
A list of limit states which are to be considered should be compiled.
The minimum number of limit states to be considered for the design of an anchored structure is given in Eurocodes 1, 2, 7.
In addition the following limit states should be considered for anchored structures:
- failure of the ground anchor by tension;
- structural failure of the ground anchor due to shear forces, distortion at the anchor head or corrosion;
- loss of anchor load due to excessive displacements of the anchor head or by creep and relaxation;
- failure or excessive deformation of parts of the structure due to the applied anchor force.
For all types of anchored structures consideration should be given to combinations of the limit states above as these apply to the entire structure itself.
D.3 Actions, ground properties, geometrical data and design situations
In the selection of actions for the calculation of limit states, consideration should be given to the actions listed according to clause 2.4.2 of ENV 1997-1:1994.
NOTE 1 An analysis of the interaction between the structure, anchor and ground may be required in order to determine the actions from the structure to be adopted in the design of the ground anchor.
Anchor loads are usually considered as actions.
NOTE 2 Anchor loads usually act in a favourable manner. The anchor load is, in this case, taken as a lower value, i.e. less than that which is calculated to occur based on the analysis undertaken in accordance with D.4. Where, however, the anchor load is unfavourable, an upper value is adopted in the design.
Appropriate design situations should be selected in accordance with the principles stated in clause 2.2 of ENV 1997-1:1994
The design situations should consider the environmental conditions in accordance with clause 2.3 of ENV 1997-1:1994
Transient design situations associated with the construction should be considered.
NOTE 3 When selecting the design situations for anchored structures it is important to assess the level of the groundwater table and the water pressures in confined aquifers.
D.4 Design methods
The design of anchored structures should be undertaken in accordance with the requirements of Eurocodes 1, 2, 4, 7 depending of the type of structure under consideration. Design verifications which take into account the effect of anchor loads on the structure should be undertaken in accordance with this annex.
For the design of an individual anchor the following verifications and design calculations are necessary:
- verification of the internal anchor resistance;
- verification of the external anchor resistance;
- verification of the serviceability and durability of the anchor;
- calculation of the necessary free anchor length;
- determination of the anchor lock-off load.
Steel strands and bars used for ground anchors should be designed according to the principles contained in ENV 1992-1-1.
The external anchor resistance should be determined on the basis of the results of investigation or suitability tests (see clause 9), the results of site investigation or from experience in similar ground conditions.
The minimum free anchor length and the anchor lock-off load are derived from the design of the anchored structure.
The determination of the anchor lock-off load P0 should be such that the anchor load P during the design life of the structure remains below the limit:
P ≤ 0,65Ptk
Ptk is the characteristic load capacity of the tendon.
The anchor lock-off load should be:
P0 ≤ 0,60Ptk