11.8 Renewal/remedial works

Reference should be made to CIRIA C591 [29] and CIRIA C592 [63] for guidance on the selection of options for renewal works. Reference should also be made to 7.7.2 of this standard, thus re-initiating the earthworks design, construct, adopt and manage cycle.

11.9 Systems integration

Earthworks can carry a number of different infrastructure elements such as services, drainage or signage and integration should be implemented to ensure that all required duties are met. It is important to co-ordinate all maintenance activities so that best use is made of time and the works are integrated to prevent damage by piecemeal working.

Asset managers should be kept informed of and approve all activities to be undertaken involving the earthwork, as it is possible for overall performance to be adversely affected by the activities of others, e.g., an excavation for a service at the toe of the slope could trigger slope movement.

Damage to assets, either reported directly or identified through inspection, should be logged and tracked so that common causes can be identified and recurrences can be prevented.

11.10 Climatic factors

11.10.1 General

COMMENTARY ON 11.10.1

The stability and performance of earthworks, particularly those in a poor condition, is often related to climatic events. The principal driver is rainfall either in the form of short-lived storm events, generating rapid flow failures, or longer lasting events which will modify ground water profiles. Another important factor is evapotranspiration which removes moisture through either direct evaporation or uptake and transpiration from vegetation. Evapotranspiration will have a positive effect on stability by reducing pore water pressures but, in the case of embankments formed of high plasticity clay, can lead to unacceptable seasonal deformations of the supported infrastructure.

Understanding the connection between climatic events and the performance of earthworks enables trigger levels to be selected in order to provide a warning of impending performance reduction; appropriate mitigation measures such as increased inspection, monitoring, maintenance or service restriction should then be implemented.

Climate change should also be considered as future climatic events might be different from those that earthworks have typically experienced to date. Therefore, it is important to understand the current climatic trigger levels that impact earthworks performance so that the significance of likely future changes may be determined.

11.10.2 Rainfall

Rainfall can be measured as a total quantity within a set period of time, either annually to enable comparison between different climatic patterns, or over the short term, say mm/hour, to indicate the intensity of a particular event. The latter approach can be linked to statistical analyses to assist with the selection of return periods for design storm events. However, in order to understand the overall impact of rainfall on earthworks a cumulative approach should be adopted. This entails comparing rainfall over a particular time period, such as a specific week or a month, with the long term statistical average for that same time period.

11.10.3 Soil moisture deficit

COMMENTARY ON 11.10.3

The soil moisture deficit (SMD) is a means of quantifying the combined effects of rainfall and evapotranspiration. It represents the cumulative reduction in soil moisture content below field capacity as evapotranspiration exceeds rainfall. In simple terms it represents the amount of rainfall (in mm) required to bring the soil to its full capacity. Any further precipitation would either pass through the profile, run off or pond. Thus the SMD is higher in the summer as evapotranspiration exceeds rainfall and low or even zero during the winter periods as rainfall increases and moisture uptake from vegetation decreases. As the SMD takes into account the effects of rainfall, evaporation and vegetation it represents a useful indicator for linking climatic events to the performance of earthworks.

A comparison of SMD values for the London area with the occurrence of slope failures over the period January 1988 to January 2001 was undertaken by Ridley et al [68]. It demonstrated that slope failures occurred more frequently during the winter period when SMD values were low and that the calculated values for deciduous trees appear to correlate better with the failures than those for grassed sites. Seasonal movements on clay soils are more problematic in the summer months and that not all summers are equal in this respect. Therefore, SMD values should be a good indicator of the likelihood of seasonal movement adversely affecting earthworks.

The Meteorological Office can provide SMD data on a regional basis through MORECS (Met Office Rainfall and Evaporation Calculation System). The information is provided on a weekly basis for a grid of 40 km by 40 km MORECS squares for both grass and tree covered sites. Obviously a certain amount of averaging takes place over each square but it is nevertheless a useful index that is readily available and can also be used to investigate historic events.

11.10.4 Climatic indicators

From the information in 11.10.1, 11.10.2 and 11.10.3, climatic indicators may be based on both rainfall and SMD that should be used to predict the performance of earthworks both in terms of slope failure and seasonal movement.

However, care should be taken in adopting trigger levels developed in one region for another as local variations can occur.

In order to properly manage risks a schedule should be developed of earthworks at risk from climatic extremes so that appropriate mitigation can be put in place as trigger levels are exceeded.

12 Decommissioning and disposal of earthworks assets

12.1 General

The removal of earthworks should not normally present significant health and safety risks.

12.2 Decommissioning

Major infrastructure owners are likely to have their own established procedures for the decommissioning of assets, however, in the absence of such guidance the following aspects should be considered in the design and implementation of decommissioning.

• a) Stability. Shallow surface movements of decommissioned slopes are likely to be acceptable provide that there are no safety implications. Deep seated movements should be prevented if there are any safety implications or obligations to third parties.
• b) Maintenance. The need for any ongoing maintenance operations should be identified.
• c) Inspection. The need for any ongoing inspection requirements should be identified.
• d) Access. It should be decided if the asset is to remain accessible after decommissioning and what measures are to be taken to prevent unauthorized access.
• e) Systems integration. Consideration should be given to the identification and protection of services or interfacing structures, etc., that might pass through or about the decommissioned asset.

12.3 Disposal

This is likely to be driven by property disposal considerations but all information relating to the earthworks should be made available to the new owners.

12.4 Partial removal

Where the partial removal of existing earthworks is a part of a new earthworks project, a comprehensive search should be made for records relating to the existing earthworks. If available these should form part of the information on which the design of the new earthworks is based.

Where technically possible, surplus material from the partial removal operations should be incorporated into the new earthworks.