Long term behaviour of engineering solutions

Many opportunities exist to reduce long term stewardship costs, reduce environmental impacts and enhance the longevity of engineered features.

Consideration of long term stewardship in engineering at the design stage, with periodic updating if and when required, is one of the critical areas to achieve this integration. A mentality of the minimally acceptable with the least short

term cost could cloud good decision making over the whole life cycle of the site.

Likewise the notion to remediate to background levels everywhere can also limit good decision making by spending too much without gaining adequate benefit in performance or protection, while having an impact on the environment and potentially on worker safety.

While the ‘useful service’ or ‘design’ life of engineering solutions are certainly concepts that all design engineers are familiar with, the timescales are generally orders of magnitude shorter than those of interest in the present context. For most civil engineering structures, continuous or periodic maintenance is also implicitly assumed. Methods and concepts to predict the long term behaviour of near surface structures are still in their infancy, while the problem itself has been explicitly recognized in the context of the performance assessment for radioactive waste repositories [48].

Thus, the erosion resistance features can be modelled on the basis of short term data, but methods to assess the long term performance need to be developed on the basis of insight into geomorphological processes. Basin scale, statistical studies, rather than discrete mechanistic studies, might provide the necessary insight.

The long term stability of engineering structures has also to be assessed in view of the probability of major accidents such as seismic events. Over the last few decades, highly engineered capping designs have been developed, which are also commonly required by regulators with the intention of reducing radon emanations and external exposures to gamma radiation, as well as minimizing water infiltration. However, these designs are likely to retain their high sealing performance for only a limited period of time. Signs of deterioration in performance (an increase of permeability in the sealing layer) are usually already observable 5–10 years after emplacement. A good way forward to ensure long term stability of the capping appears to be an emulation of the natural soil structure as found in the vicinity of the remediated site. Although such ‘natural’ capping designs (with the use of long lasting natural materials and structures mimicking as far as possible the natural soil profile) are likely to have a lower immediate sealing performance than plastic liners, for instance, this will be outweighed by their long term stability.

Recent flooding events in various parts of the world often seem to indicate, inter alia, that the design base, in particular with respect to the magnitude of infrequent events, is insufficient. Precise flood water level records only go back some 100 years, while anecdotal evidence may extend this to a few hundred years. Thus, a design base may not capture an event that occurs, on average, every 1000 years. Similar effects may occur in areas other than flood defences.

8.5.2. Design for long term stability

In order to select and implement the most efficient design from the point of view of self-sustainability over the long term, learning from natural processes and environmental behaviour may be a valuable strategy. The paradigm is engineering with nature and not against it.

The natural evolution of soils and diagenesis also give valuable insights into the development of long term management plans. The contaminated material will not remain unchanged in the long term, and assessment of its evolution will give confidence in the project if diagenesis improves the retention of contaminants.

Limiting infiltration will reduce the need for seepage control downstream. Long term management of the quality of drainage or seepage from the site is best provided for by some form of passive water treatment.

Active water treatment plants are labour and maintenance intensive, and there are no guarantees that the resources will be available over the longer term.

Passive forms of treatment may include, for instance, either a limestone layer to prevent the formation of acid drainage or a wetland to polish seepage water before release to surface water courses [12].

Cappings and similar features are also intended to prevent biointrusion.

The structure of the cover, as heavily engineered as it may be, may not be able to prevent root intrusion in the long term if it has not been designed to be compatible with the natural vegetation cover and plant succession typical of the surrounding environment.

The ecosystem around a remediated site is the result of a process lasting for centuries or millennia and is shaped by a wide variety of initial conditions and contributing factors, such as the initial rock type, climatic evolution, and surrounding flora and fauna. The result is a (dynamic) equilibrium between soil type, vegetation cover and climatic conditions. Any attempts to reconstitute an ecosystem at the site, such as revegetation, need to be as compatible as possible with the surrounding ecosystem(s).

The final use of the site needs to be compatible with the ecosystem in order to minimize pressure on the site due to human use.

Any environmental impact study is intended to assess the potential of a site to be integrated into the surrounding environment. Indeed, the best shape for a remediated site is achieved when it is compatible with the surrounding geomorphology. This concerns in particular slope stability. From a geome-chanical point of view, gentle slopes contribute to achieving low relief energy.

Natural geological processes achieve this over millennia, and engineered structures may benefit from observation of the evolving geomorphology and slopes around the environment of a site.

While completing engineering for remediation, consideration of the stewardship requirements on a site by site basis is recommended. In general, when considering stewardship the following points should be kept in mind:

(a) Designs with low inherent (potential) energy are preferred to designs with higher energies. This applies in particular to geomorphological relief energy: all above ground structures are subject to the forces of erosion and will eventually disappear, starting, of course, with any engineered capping. In addition, the surrounding environment may have a high relief energy, although the actual engineered structure may be below the surface (Fig. 9).

(b) Designs with a low likelihood of failure and limited effect if failure occurs are preferred to those that are less reliable: for example, self-sustaining systems and approaches such as waste rock or tailings cemented by geochemically stable secondary minerals or vegetated slopes similar to

Potential energy relative to surface

Potential energy relative to surface

Potential energy relative to surface = 0

Impoundment below surface, but high relief energy

FIG. 9. Diagrams illustrating the concept of inherent potential energy in the design of impoundments.

naturally sustainable slopes in the area would probably have a good chance of surviving the long durations required for long term stewardship.

(c) Designs that mimic diagenetic processes are preferred.

(d) Designs that maximize natural systems in the area and are compatible with the surrounding area are preferred. Experience with existing disposal cells and similar structures indicates that nature soon attempts to encroach on cells. This experience favours designs with an ecosystem type of approach rather than a barrier control one.

(e) Designs that are based on natural attenuation and retention are preferred [81].

(f) Designs that include redundancies in protection are preferred.

A technical issue related to intergenerational communication is the longevity of permanent markers to warn future generations of previous land use and possible residual hazards, for example gravestones and other forms of visual sign. As this form of communication may be the final layer of defence for warning future populations, markers and signs must be developed with great care to ensure physical longevity. The problem of coding the information is discussed in Section 9.11.

8.6. REGULATORY REQUIREMENTS VERSUS TECHNOLOGICAL