9 Retaining structures
(1)P The provisions of this Section shall apply to structures, which retain ground comprising soil, rock or backfill and water. Material is retained if it is kept at a slope steeper than it would eventually adopt if no structure were present. Retaining structures include all types of wall and support systems in which structural elements have forces imposed by the retained material.
(2)P Pressure from granular material stored in silos shall be calculated using EN 1991-4.
(1) In considering the design of retaining structures the following three main types should be distinguished:
walls of stone or plain or reinforced concrete having a base footing with or without a heel, ledge or buttress. The weight of the wall itself, sometimes including stabilising masses of soil, rock or backfill, plays a significant role in the support of the retained material. Examples of such walls include concrete gravity walls having constant or variable thickness, spread footing reinforced concrete walls and buttress walls.
relatively thin walls of steel, reinforced concrete or timber, supported by anchorages, struts and/or passive earth pressure. The bending capacity of such walls plays a significant role in the support of the retained material while the role of the weight of the wall is insignificant. Examples of such walls include cantilever steel sheet pile walls, anchored or strutted steel or concrete sheet pile walls and diaphragm walls.
composite retaining structures
walls composed of elements from the above two types of wall. A large variety of such walls exists and examples include double sheet pile wall cofferdams, earth structures reinforced by tendons, geotextiles or grouting and structures with multiple rows of ground anchorages or soil nails.
9.2 Limit states
(1)P A list shall be compiled of limit states to be considered. As a minimum the following limit states shall be considered for all types of retaining structure:
- loss of overall stability;
- failure of a structural element such as a wall, anchorage, wale or strut or failure of the connection between such elements;
- combined failure in the ground and in the structural element;
- failure by hydraulic heave and piping;
- movement of the retaining structure, which may cause collapse or affect the appearance or efficient use of the structure or nearby structures or services, which rely on it;
- unacceptable leakage through or beneath the wall;
- unacceptable transport of soil particles through or beneath the wall;
- unacceptable change in the ground-water regime.
(2)P In addition, the following limit states shall be considered for gravity walls and for composite retaining structures:
- bearing resistance failure of the soil below the base;
- failure by sliding at the base;
- failure by toppling;
and for embedded walls:
- failure by rotation or translation of the wall or parts thereof;
- failure by lack of vertical equilibrium.
(3)P For all types of retaining structure, combinations of the above mentioned limit states shall be taken into account, if relevant.
(4) Design of gravity walls often requires solution of the same types of problem encountered in the design of spread foundations and embankments and slopes. When considering the limit states, the principles of Section 6 should therefore be applied, as appropriate. Special care should be taken to account for bearing resistance failure of the ground below the base of the wall under loads with large eccentricities and inclinations (see 6.5.4).