ENGINEERING GEOLOGY/Made Ground 539 embankments can be used to retain canals An embankment dam will usually be composed of several different types of fill: a low-permeability fill to form the watertight element, stronger fill to support the watertight element, and fills to act as filters, drains, and transition materials Possible hazards affecting the dam include internal erosion, slope instability, and overtopping during floods Embankment dams are also used to retain lagoons of sedimented waste material from mining and industrial activities If the waste is not toxic, these embankments may be designed so as to allow water to drain through The embankment may be built in stages as waste disposal progresses Another use for a special type of embankment is to protect harbours and the shoreline from the sea Such embankments are often built as rubble-mound breakwaters and require high-quality quarried rock for the fill Placement to the required profile presents obvious difficulties Embankments to carry roads and railways are usually built to reduce gradients The engineered fills use material excavated from adjacent cuttings during the cut-and-fill operations, so there is limited scope for material selection and whatever is excavated has to be placed in the adjacent embankment unless it is clearly unsuitable In England, during the construction of the M6 motorway between Lancaster and Penrith in the 1960s, much of the soil excavated along the line of the road was very wet, and a geotechnical design was developed that involved the use of drainage layers built into the embankment during construction Buildings may be founded on made ground Old excavations are infilled, and sometimes embankments are built above the level of the surrounding ground The objective is to support buildings safely while minimizing the risk of damaging settlement, and, where structures sensitive to settlement are to be built on made ground, a high-quality fill that is not vulnerable to large post-construction movement is required Opencast-mining sites have often been restored for agricultural use, but where the sites are close to urban areas they may subsequently be used for housing and commercial developments Loose backfill usually has considerable settlement potential, and some existing areas of loose fill have been improved by preloading with a temporary heavy surcharge of fill Because of the free-draining nature of the loose fill, it soon consolidates, and this type of treatment has made some sites quite suitable for normal housing Where building is foreseen prior to backfilling an opencast mining site, the fill material should be placed in thin layers and heavily compacted as an engineered fill Fill Placement There are two basic elements in the quality management of engineered fills: placement of a fill with the required quality; and evidence that the fill has the required quality Both an appropriate specification and rigorous quality-control procedures are required It is not possible to prevent some variability in the made ground, as there will be a degree of heterogeneity in the source material and some segregation during placement It is necessary to determine how the required properties can be achieved with an acceptable degree of uniformity Placement in thin layers with heavy compaction at an appropriate water content is the method usually adopted to obtain the required performance from an engineered fill used in dam, road, or foundation applications Figure shows the compaction of a clay fill, and Figure shows a rock fill being watered during placement There are three basic approaches to the specification of engineered fills Figure Compacting clay fill Figure Watering rock fill during placement