Sustainable and resilient

Good engineering is no longer enough. Geotechnical engineering needs to focus more on sustainability and resilience for better engineering, says the Institute of Civil Engineers (ICE) in the UK.

Sustainability is a balanced approach which strives for harmony among the three Es – environment, economy and equity – so that the quality of life of future generations is not compromised. While resilience is defined as being the ability to withstand and recover from disruptions, such as sudden shocks.

In order to help geotechnical engineers further understand the two concepts and how they can be balanced, ICE recently commissioned two special issues of its Engineering Sustainability journal to address sustainability and resilience in geotechnical engineering.

As Jeffrey R Keaton, Principal Engineering Geologist at Amec Foster Wheeler in the US, explains: “These two concepts are related because resilient systems support sustainable communities by remaining functional or having redundancy. This is particularly important for geotechnical engineering because facilities and geo structure like tunnels, dams and retaining structures that may not be able to return to functionality after disruptions, such as earthquakes or bomb blasts, would not support sustainable communities even if they were constructed with a balance among the three Es.”

Research

In an attempt to present an approach that balances both sustainability and resilience aspects, Jasawee T Das and his colleagues presented a research paper called Sustainability and Resilience Analysis in Slope Stabilisation.

The authors say that traditional civil engineering design and construction processes have focused on the safety and serviceability of infrastructure but in recent years, more infrastructure has been designed to withstand external and internal disturbances, without considerable loss of functionality, while enhancing sustainability elements in every phase of the design.

“Geotechnical engineering is an inherent component of almost every constructed system or major infrastructure and improving the sustainability of geotechnical processes is paramount to ensuring overall sustainable development,” Das et al state. “Geotechnical engineering, being positioned at the incipient stages of a project, provides immense opportunities for sustainable development practices. The adoption of sustainable geotechnical alternatives at the initial stages can contribute towards the sustainability of the project at later stages.”

However, the authors warn that in advocating a rigid sustainable approach, “prudence must be exercised such that the safety and reliability of the design is not compromised”; a particularly important consideration for critical lifeline infrastructure such as dams and levees. From a geotechnical perspective, it has also been suggested that sustainability may be interpreted as a dynamic equilibrium between four Es – engineering design, economy, environment and equity.

The primary focus of Das et al’s paper was to present a comprehensive framework to evaluate the sustainability and resilience of a given infrastructure project. A research study performed at the University of Texas involved using several treatment methods to stabilise dam embankment slopes and prevent surficial failure.

As the authors explained, surficial failures on slopes of earth dams are common in North Texas, particularly after heavy, sustained rainfall following a long dry spell when the predominant cause of failures is attributed to shrinkage-induced cracking in soils. Repair costs can be exorbitant, ranging from US$10 -100,000 for repairs.

“Many dams are critical infrastructures,” the authors say, “and substantial loss of their functional integrity due to surficial slope failures might be catastrophic in the long run. It is therefore important to address issues concerning surficial stability and come up with suitable ground improvement and/or other remediation measures to arrest these failures.”

Case study

The study was supported by the Fort Worth District of the US Army Corps of Engineers and focused on Grapevine Dam which was constructed for flood control purposes and is located 32km northwest of Dallas. Research addressed the efficacy of several combinations of stabilising agents including lime, fibres and compost, towards inhibiting surficial slope failures at the dam. Based on laboratory studies, four different combinations were considered: 20% compost, 8% lime, 4% lime with 0·30% polypropylene (PP) fibres, and 8% lime with 0·15% PP fibres.

Laboratory studies were performed on soils stabilised with the treatments and were subjected to swell, shrinkage and soil strength tests. A field implementation programme was also carried out by building test sections on Grapevine dam where moisture probes, temperature sensors and inclinometers for monitoring lateral movements along the slope were installed.

Both sustainability and resilience elements were quantified for each treatment method and attached with appropriate weights based on their relative importance and at the discretion of the engineer. The sustainability index was determined based on resource consumption (embodied energy), environmental impact (global warming potential, acidification potential and eutrophication potential) and socio-economic impact (cost). The resilience index was quantified using the probability of failure and maximum lateral movement observed in the field. These weighted indicators were subjected to a multi-criteria analysis to designate a cumulative sustainability and resilience index that parametrised the degree of sustainability and resilience in the system. Based on these evaluations it was found that the 8% lime treatment had the least cumulative resilience and sustainability index and was judged to be the most appropriate stabilisation method, and recommended for final field implementation.

Das et all go on to add that the proposed flexible framework allows the operator to attach weights to the different sustainability and resilience elements based on their discretion. In this example a higher weight was attached to resilience compared with sustainability. Resilience would be paramount for the safety and functionality of the dam as surficial slope failures are a recurring phenomenon during both dry and wet seasons in Texas, and involve high annual repair costs. Furthermore, the propensity of geo-professionals towards higher resilience in design might lead to a lesser weight being assigned to sustainability elements.

The authors explain that there are certain limitations in the present framework and efforts are being made to address these in future studies. For example non-quantifiable metrics such as noise, vibrations, downtime and policy constraints could not be incorporated into the analysis. While the adopted methodology does not illustrate the true interdependence between resilience and sustainability as the two aspects were analysed as separate entities and then combined to yield the cumulative sustainability and resilience index. It is well known, they add, that some overlap exists, and future research should be directed towards a combined probabilistic treatment of sustainability and resilience that accurately captures the intersecting attributes.

Sustainability and resilience analyses in slope stabilisation by Jasawee T Das, Anansd J Puppala, Tejo V Bheemasetti, Lucas A Walshire and Maureen K Corcoran. Engineering Sustainability Volume 171 February 2018. Issue ES1. Pages 25-36. The full article can be seen at https://doi.org/10.1680/jensu.16.00054

All information courtesy of www.ice.org.uk/