Technical and economical aspects related to the geotechnical design of resilient levee systems

TECHNICAL AND ECONOMICAL ASPECTS RELATED TO THE GEOTECHNICAL DESIGN OF RESILIENT LEVEE SYSTEMS

Lo Presti D.C.F.

University of Pisa, Pisa Italy, d.lopresti[AT]ing.unipi.it Typically, river embankments are earth - works that have been constructed several centuries ago, using poor materials and poor compaction methods. Their function is to protect the territory and the built environment against floods (i.e. events of limited duration, from few hours to few days).

In order to improve their capability to resist the hydro - mechanical actions during floods, it appears necessary to:

define an expeditious method for the “continuous” assessment/control of their physical/mechanical characterization;

define a methodology for a “realistic” assessment of the hydraulic actions during floods;

develop cost - effective methods to improve their capability to resist possible failure mechanisms.

Hydraulic actions are intended as “seepage forces” which mainly depend on the hydraulic boundary conditions. It is worthwhile to remember that between 1998 and 2009, Europe suffered over 213 major damaging floods, including the catastrophic floods along the Danube and Elbe rivers in summer 2002. Severe floods in 2005 further reinforced the need for concerted action. Between 1998 and 2009, floods in Europe have caused some 1126 deaths, the displacement of about half a million people and at least €52 billion in insured economic losses [1].

The design of resilient river embankments (levees) is essentially aimed at reducing flood risk. This is a multidisciplinary task that can be regarded from different point of views. What is described in this paper deals with the initiation of levee failures (breaches) and more specifically considers the mechanical characterization of existing or new embankments and possible retrofitting methods (i.e. geotechnical design of levee systems and remedial measures).

On the other hand the following aspects/approaches also could be considered:

the breach evolution (i.e. the impact on the territory that a failure can have) that is essentially conditioned by the hydraulic actions of the river stream;

measures of flood risk mitigation like early warning systems, evacuation planning and flood control schemes.

In the case of existing embankments, it is necessary to identify screening methodologies, to plan the retrofitting interventions in consideration of the available budget and to design the retrofitting measures. In the case of new embankments, it would be necessary to define construction materials, compaction techniques and control - methodologies. As for the control it would be preferable to define performance criteria. In any case (old or new embankments), it is necessary to define the criteria for a cost-effective investigation taking into account the total length of the embankments (could be several km), the requested level of detail and analyses and the capability of indirect methods (CPTu, 2D geo-electric tomography) to infer/control the soil stratigraphy. A similar approach should also be kept in mind when defining the retrofitting measures.

An empirical approach has been proposed to calibrate CPTu results against the information gathered from boreholes [1]. Figure 1 and Table 1 respectively show the proposed correction of the “classification charts” and the beneficial effects of such a correction.

3195955244475000The poor knowledge of the physical/mechanical characteristics of existing river embankments is generally recognized (Cosanti, 2014; Cosanti et al. 2014; 2014 (a); Simonini et al. 2014). On the other hand, a detailed and accurate geotechnical campaign may be very costly and time consuming. (Cosanti et al. 2014 b) have developed a cost/effective and expeditious method to assess the degree of compaction of existing and new embankments and therefore to evaluate their mechanical resistance. The method is based on the in situ cone penetration test (CPTU - electric tip) and on appropriate calibration in the laboratory by means of a mini - cone. Figure 2 clearly shows the evident correlation between tip resistance and dry density of compacted soils, as obtained from such a calibration.

Figure 1. Ic from CPTu and Boreholes and Д Ic correction

Table 1

Effect of SBT correction

Figure 2. Dry density vs. tip resistance for three different materials

However, the repetition of the CPTU even every 200 m, along the embankments, does not guarantee a continuous assessment of the earth work characteristics. On the other hand geophysical testing has been explored as potential tool for obtaining an expeditious and continuous characterization of the embankments and foundation soil (Cosanti, 2014; Cosanti et al. 2014; 2014 (a);). Geo - electric tomography (2D) seems one of the more promising methods. Unfortunately accurate and reliable geo - electric tomography it is not applicable for routine works because appropriate calibration is still missing. The development of a reliable, expeditious and cost/effective methodology for continuous embankment characterization is necessary to evaluate the risk of embankment failure against various Ultimate Limit States (ULS): external erosion because overtopping, internal erosion (in a general sense) and embankment collapse under hydro - mechanical loading. Such prediction capability will help in reducing the flood risks with limited economical resources. Indeed, a reliable characterization of existing embankments will help in defining riskiest areas and therefore priorities in the refurbishment/retrofitting works. This is in agreement with the Directive 2007/60 issued in October 2007 by the European Parliament and the Council of the European Union. The directive not only recommends to consider the risk of failure but also the consequences of failures on the territory and plans to manage the disaster (prediction and early warning systems)

The main causes of embankment failures are:

external erosion because of overtopping;

internal erosion (piping, suffusion, etc.)

mechanical collapse of the bank under gravity and hydraulic loading;

uplift of the foundation soil on the country side (undrained condition);

hydraulic failure of the foundation soil on the country side (drained condition).

As for the occurrence of embankment overtopping, the only possible countermeasure is a general increase of the bank - height, considering a given return period. This is very costly and time consuming. As an alternative it is very important to improve the embankment resistance to the external erosion. Indeed this countermeasure enable the embankment to resist external erosion and therefore to reduce the impact on the flooded area and the size of the flooded area. Post - disaster emergency can also be managed in an easier way in case the embankment survives to overtopping.

Hydraulic barriers or increase of the embankment horizontal size are the necessary countermeasures for facing the other ULS other than overtopping. Some case histories on possible countermeasures and related costs are also shown.

References

DIRECTIVE 2007/60/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of23 October 2007 on the assessment and management of flood risks

Lo Presti D.C.F., Cosanti B. and Squeglia N. Uso delle carte di classificazione per la definizione del SBT da prove CPTu (terreni particolari). IARG Chieti/Pescara 2014

Cosanti, B. Guidelines for the geotechnical design upgrading and rehabilitation of river embankments. PhD Thesis, University of Pisa. 2014

Cosanti, B.; Lo Presti, D.C.F.; Squeglia, N. Adeguamento degli argini del fiume Serchio: aspetti geotecnici. XXV Convegno Nazionale di Geotecnica: La geotecnica nella difesa del territorio e delle infrastrutture dai rischi naturali. Baveno, (VB). 4-6 Giugno 2014. Edizioni AGI - Roma - ISBN 978 88 97517 03 0. 2014

Cosanti, B.; Squeglia, N.; Lo Presti, D.C.F. Analysis of existing levee systems: the Serchio river case. Rivista Italiana di Geotecnica. AGI. 2014a

Cosanti, B.; Lo Presti, D. CF.; Squeglia, N. An innovative method to evaluate degree of compaction of river embankments. XII IAEG Congress. Torino, 15-19 September 2014b

Simonini et al. Atti XXV Convegno Nazionale di Geotecnica "La geotecnica nella difesa del territorio e delle infrastrutture dalle calamita naturali", Baveno, 4-6 Giugno, Vol. 2, 823-830. ISBN 978-88-9751705-4. Edizioni AGI, Roma. 2014

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