11.7 Maintenance issues
11.7.1 General
During the original construction, designers should have considered potential maintenance issues, and aimed for solutions that minimize the maintenance requirements and result in easy to maintain earthworks. When this approach has not been taken there is likely to be an increased risk of earthworks failures; however, in practice all earthworks should undergo routine maintenance in order to achieve the intended design life. Aspects of maintenance should include:
- drainage – surface water and earthworks drainage;
- vegetation (leaves, tree falls, slope stability issues, seasonal movement on clay soils), see Scott et al [64], CIRIA C591 [29] and CIRIA C592 [63];
- animal damage (rabbits, badgers, etc.); those responsible for maintenance should be aware of the relevant legal obligations; see The Management of Problems involving Badgers [65];
- prevention of further erosion;
- scaling of rock slopes.
Good records of maintenance works undertaken as part of the asset management process should be maintained.
11.7.2 Drainage
NOTE 1 A description of drainage systems for slopes is given in 7.5.3.
Regular inspection should be undertaken to ensure that the drains are working effectively and that they are not becoming silted up or blocked as a result of pipe fractures or slope deformations. Where access to a drainage system is available through manholes, the manhole covers should be lifted at regular intervals, silt traps cleaned out and pipes examined for blockage and rodded and flushed as necessary.
A watch should be kept for infiltration of soil into open or closed joint piped drains, and remedial measures taken if there are signs of appreciable internal erosion of soil into the pipes, or indeed if they are not carrying any water at all; in this latter case it could be that water is being discharged into the slope at some point where the drain is broken and this might present a threat to its stability. In addition, water seeping out of the ground might also indicate a pipe fracture; the fracture should be located and repaired with as little delay as possible.
Special inspection should be made at times of heavy rainfall to check whether or not any of the drains are surcharged or are carrying eroded soil.
Outfalls of drainage systems should be checked to ensure that pollution or damaging erosion of water courses is not occurring, and to check that the water courses have adequate discharge capacity for the run-off of the drainage system at times of storm. They should also be checked in freezing weather in case water is impounded in the system by icing.
NOTE 2 Coarse backfill to drains can become clogged in time and require replacement. When this is done the insertion of a filter fabric to surround the backfill can keep clay and coarser particles out of the collector system.
11.7.3 Vegetation
Vegetation should be managed to prevent trees encroaching onto the earthworks or obscuring signage, lighting or signalling equipment.
COMMENTARY ON 11.7.3
Trees also require management because they can give rise to seasonal movements of clay earthworks that can adversely effect the stability and performance of structures founded upon them including road pavements, rail track and services. There is also evidence that successive cycles of movement can, in the long term, give rise to a reduction in the mass strength of the earth structure possibly leading to failure [69, 53, 55].
Trees also require management because they can give rise to seasonal movements of clay earthworks that can adversely effect the stability and performance of structures founded upon them including road pavements, rail track and services. There is also evidence that successive cycles of movement can, in the long term, give rise to a reduction in the mass strength of the earth structure possibly leading to failure [69, 53, 55].
11.7.4 Animal damage
Burrowing animals should be controlled as damage from burrowing activity can undermine the stability of an earthworks and where severe enough can lead to collapse particularly where loading is applies close to the burrows or water retaining structures are involved.
Certain animals such as badgers are legally protected and appropriate procedures should be followed in dealing with them [68].
11.7.5 Erosion
Attention should be given to the potential for erosion due to action of water (drainage, heavy rainfall, water courses and floods, etc.) on the earthworks [70]. In particular, drainage should be maintained and where appropriate, upgraded to accommodate changes in adjacent land use.
When water stands against earthworks, soft or loose material can in time be eroded by wave action and this action should be considered separately from general flood conditions.
COMMENTARY ON 11.7.5
The most common source of erosion is that resulting from water action. Rain, which constitutes the most significant eroding agent, affects the slopes of earthworks and can be a serious threat to stability. Heavy downpours of rain initially loosen the surface material and can thereby allow the earthworks to absorb the water into the surface, producing saturated conditions. Generally, the action only affects the outer surfaces of the earthworks and usually, if these are shaped correctly, the water will run off into either permanent or temporary drains. In so doing however, and depending upon the type of material in the earthworks, large washouts may occur and the arisings can cause serious hazards to adjacent property. Absorption of heavy rainfall within the body of the earthworks can increase the moisture content of the material to unacceptable limits and the subsequent seepage of this excess water from the earthworks in the long term can cause surface erosion and slips. In most cases, these slips are fairly shallow, usually being confined to the outer surfaces only, and although not structurally damaging in themselves, they are unacceptable both aesthetically and in general to the maintenance of the slope.
Streams or other water courses running along the foot of a slope can erode the toe and in times of flood can immerse part of the slope, lessening its stability. Increased runoff from the new works themselves can affect the behaviour of existing streams.
Existing surface protection can be loosened by suction action of the waves. In tidal waters the constant raising and lowering of ground water levels can cause migration of soil particles from slopes.
Closely related to water as an eroding agent is frost. Alternate freezing and thawing loosens the surface of rock cuttings and opens cracks and fissures that could have been caused by the construction processes or occur naturally.
