8.6 Construction of reinforced embankments

8.6.1 General The construction procedure of all embankments with basal reinforcement and embankments with reinforcement against frost heave in the upper part shall conform to the requirements that are common to all types of reinforced embankments, as itemized in this section. In addition, the construction procedure shall conform to the recommendations that are specific to the relevant type of reinforced embankment, as set out in the instructions provided by the supplier of the reinforcement.

8.6.2 Embankments over weak ground Reinforced embankments with basal reinforcement may include embankments constructed over weak ground. Weak ground may take the form of naturally occurring deposits of predominantly cohesive soil or ground which has been weakened by man made voids, such as mine workings, or naturally occurring subterranean voids such as sink holes. Several different reinforcing techniques can be used to suit the construction of embankments over weak ground. The precise construction method may depend upon the particular technique employed.

8.6.3 Placement of reinforcement Reinforcement in the form of geosynthetic grids or sheets, or metal grids, meshes or sheets, which may have different tensile strengths and tensile stiffness in different directions, shall be installed in the specified orientation. Reinforcement should be selected to provide the required degree of survivability to construction induced damage at design stage, and should not be exposed to direct trafficking by construction plant. It shall be ensured that the non-rigid reinforcement is tight and that any slack has been removed, in order to minimize any deformation during the mobilization of tensile forces in the reinforcement. This may be achieved by pulling the reinforcement tight and holding in this position while it is covered with fill. Geosynthetic reinforcement may be prone to degradation when exposed to sunlight and therefore should be covered with fill within a specified time of laying. Where no such time is specified, exposed reinforcement should be covered within 24 hours of placement. Geosynthetic reinforcing products are supplied in strips or rolls of limited width and length. Where the dimensions of the area to be reinforced exceed the roll dimensions then specified joints or overlaps will be required. Where joints have been specified, these may be formed in-situ or prefabricated elsewhere prior to final placement. In addition to attaining specified design strengths, joint strengths shall be sufficient to resist loads induced by strenuous handling techniques such as rope hauling. Placement of reinforcement through shallow water will require the weighting of polymeric material with a specific gravity less than unity. Placement through deeper water may additionally require the use of shallow draft vessels or craft using rope hauling technique. In general, the placement of sheet material may be disrupted by wind uplift. Where this is likely, the material should be locally ballasted.

8.6.4 Placement and compaction of fill Placement and compaction of the fill should generally follow the procedures adopted for earth retaining structures and reinforced slopes, except where the fill is placed over deep deposits of soft or ultra soft ground. The sequence of fill placement over soft or ultra soft ground, should be specified within the design. If not, care should be taken to ensure that the sequence of filling, including any trafficking by construction plant, at no time exceeds the bearing capacity of the underlying ground. Where fill is deposited by end tipping extreme care should be taken to ensure that any bow waves formed in the underlying ground do not displace or rupture the reinforcement. Where placement of fill is advanced in the direction of the longitudinal axis of an embankment, development of unwanted bow waves may be reduced by advancing the central section of infilling ahead of infilling along the toes of the embankment. The development of bow waves may be further reduced by limiting the depth of the first layers of fill to the minimum required for trafficking by light low bearing pressure construction plant. For wide embankments, consideration may be given to first placing fill along the two toes of the embankment to form dykes from which subsequent infilling might proceed towards the centre of the embankment.

EN 14475:2006 Execution of special geotechnical works – Reinforced fill