7.6.9 Filling into water

7.6.9.1 Standing water

NOTE Standing water is the term applied to ponds, lakes, canals and water-filled mineral workings.

Where it is impracticable or uneconomical to drain standing water, particular attention in the design of the embankment should be given to the maximum and minimum water levels and to the characteristics of the soil underlying the water. Where practicable, any soft silt, clay or peat should be removed before placing fill, as it is difficult to compact the fill material under water. Fill should be selected from material which remains stable when inundated or when within the zone of a fluctuating water table, particularly in saline tidal water. Broken concrete, broken brick or granular material should be used to reduce settlement and maintain stability. Where it is impracticable or uneconomical to remove soft materials displacement by end tipping of bulk fill may be adopted. Measures should be taken to equalize water levels on each side of the embankment by means of pipes or pervious blanket drains.

For large areas of standing water, it may be practicable and economical to adopt hydraulic filling using a suitable type of granular material.

The slopes of an embankment in standing water should be flatter than those required above water level and they should be protected against wash or wave action.

7.6.9.2 Tidal, river and flood waters

In tidal and flood waters the effects of the rise and fall of the water level and of wave action on the embankment should be given special consideration and techniques such as are necessary in the design of maritime structures should be considered. Where a sudden rise or fall in the level of the water can occur, precautions should be taken to avoid external erosion and to mitigate the effects of sudden drawdown.

NOTE 1 This condition can occur where an embankment crosses the flood plain of a river where the embankment is, for most of the time, on dry ground but where, under flood conditions, erosion of the slopes of the embankment in the vicinity of a bridge or culvert is possible owing to the increase in velocity of the flood water passing through the opening.

Where flowing water against the earthworks face can be expected then measures should be include to prevent erosion of the earthworks.

The earthworks engineer will need to consider the risk of erosion and options available for protection, but is likely to require input from a specialist with experience of design of erosion protection to ensure that site conditions are properly understood and that design, installation and maintenance factors are properly allowed for.

NOTE 2 When the risk of erosion in port, coastal and river engineering is judged as sufficient to require the use of rock fill for erosion protection then reference can be made to CIRIA C683 [45]. References are available for river engineering, such as Escaramia and Wallingford [46] and Hemphill and Bramley [47]. For less severe erosion cases then options of green engineering can prove very effective to protect the face of the embankment, examples are given by River Restoration Centre [48].

7.6.10 Filling adjacent to structures

Earthworks operations adjacent to structures are frequently carried out separately from the main earthworks operations and may be considered in the following categories:

  • a) filling over large pipes and culverts; in these cases it is important that fill is brought up equally on each side of the structure to prevent unbalanced loading and that great care is taken with the first layers of fill over the top of the structure;
  • b) against abutment and wing walls of bridges and retaining walls of all kinds;
  • c) around and between skeleton abutments, buried piers and bank seats.

Because satisfactory compaction of fill adjacent to structures is often more difficult to achieve owing to the restricted nature of the operation, it is usual practice to specify particular types of fill, such as selected granular materials (including specialist fill such as pulverized fuel ash), in the immediate area of the structure. Satisfactory compaction to reduce to a minimum differential settlement between backfill and structure is important enough to warrant the use of more expensive materials. Both the type of compaction plant and the method of compaction may be modified from those used in general embankment construction to prevent the development of excessive horizontal forces on foundations, retaining walls or piles.

NOTE Transition zones are commonly utilized to manage the settlement difference between embankments and structures, good practice guidance is provided with UIC 719 [49]. The problem of design of remedial works due to inadequate transition zones at existing structures is a complex issue upon which there is little standard guidance.

7.6.11 Filling over compressible ground

There are various circumstances where earthworks will be required over soils liable to significant settlement, such as soft ground (e.g. alluvium), compressible (e.g. loose Made Ground), collapsing ground (e.g. loess and karst geology), and unstable areas (e.g. land prone to mining subsidence); the designer should assess the magnitude of the risk and give consideration to the acceptable level of deformation for the proposed earthwork and determine an appropriate design logic to suit the site conditions. Guidance is provided in various references, e.g. CIRIA SP32 [50] (currently under revision) and Charles and Watts [42].

NOTE Methods of constructing an embankment over compressible ground include:

  • excavation and replacement of the poor material;
  • grouting;
  • consolidation of the soft material by surcharge;
  • staged construction or controlled rate of filling;
  • improvement of the engineering properties of the soft material by ground improvement techniques;
  • modification of the engineering properties of the soft material by the use of additives such as lime or cement;
  • use of lightweight fill;
  • drainage of the soft material by the installation of horizontal or vertical drains;
  • reduction in the gradient of the side slopes and / or the provision of berms;
  • use of synthetic reinforcement; and
  • use of piles.

The selection of the method of construction proposed should give particular attention to the potential implications on the environment or adjacent structures and earthworks.

7.6.12 Embankments on sloping ground

The inherent stability of the natural ground forming a slope should be investigated carefully, particularly in regions known to be prone to landslips; in some cases evidence of existing instability can be seen on the site in the form of undulations, hummocks, lobes and water seepages. Investigations should be made of the geological stability of the slope, including long-term monitoring, and the likely re-activation of the existing slips under the loading conditions arising from the embankment construction.

Where an embankment is to be constructed on sloping ground and there may be a danger of a slip developing at the interface, benches or steps should be cut into the existing ground surface to key-in the new construction. Preferably, the bottom of the bench should be graded away from the surface of the slope, with provision for positive drainage measures to deal with any subsoil water which might collect at low points of the benching.

In order to deal with instability problems connected with the existing ground, the cross sections of the embankment may be designed to ensure a safe distribution of loading on the ground. The method of building up the embankment may also be specified to prevent unbalanced loading. Drainage of the interface between the slope and the embankment and of any potential slip planes is most important and adequate cut-off and subsoil drains should be provided.

BS 6031:2009 Code of practice for earthworks