Assessing and Improving the Resilience of infrastructure Earthworks subjected to extreme rainfall: an advanced Unsaturated Soil mechanics perspective (AIREUS)

Overview

Earthworks such as dams and embankments, are crucial for the functioning of infrastructure networks and play a fundamental role in the socio-economic and environmental wellbeing of our society [1]. However, the stability of earthworks can be compromised by the climatic change with the alternation between longer droughts (drying) and extreme rainfall (wetting) events [2]. These meteorological conditions combined with the lack of proper adaptation/maintenance measures are the major causes of earthwork failures, involving costs above £110M each year in the United Kingdom [3]. A deeper understanding of the mechanisms through which earthworks deteriorate their mechanical properties and eventually fail under the action of consecutive drying-wetting cycles has become of the utmost importance to improve the resilience of our infrastructure systems.

Earthworks are constituted by soils in an unsaturated state (i.e. soil pores filled by both air and water). Over the past decades, geotechnical researchers have produced many incremental hydromechanical models for unsaturated soils to assess the stability of infrastructure earthworks [4-6]. However, these models are not adopted in mainstream geotechnical practices because of the benefits of their use remain unclear to practitioners. Additionally, the accuracy of these models strongly depends on the size of the increments used to integrate the incremental constitutive relations, which lead to large computational costs. The aim of this project is therefore to devise a simple and robust closed-form model for unsaturated soils to assess the stability and resilience of earthworks subjected to extreme rainfall.

From an hydromechanical perspective, soils shrinks during droughts due to water evaporation and this generates networks of large pores. During extreme rainfalls, these pores constitute preferential paths for water infiltration that eventually saturates the soil. In these conditions, three main phenomena can affect the stability of earthworks: a) the cyclic variation of dryings and wettings progressively deteriorates the mechanical properties of the soil [7], b) the loss of stability provided by water capillary actions during the transition to saturated states and c) the coupling of water flow and soil deformation. These three phenomena are not well predicted by existing models and still constitute matter of discussion within the scientific community. More worryingly, these phenomena are overlooked by the geotechnical industry, whose design practices still rely on saturated soil mechanics [8].

To address these limitations, the present project proposes an advanced model for predicting a) the deteriorating effect of wetting and drying cycles on the hydromechanical behaviour of unsaturated soils, b) the transition from unsaturated to saturated soil states and c) soil deformation (i.e. shrinkage/swelling) due to changes in water capillary actions. The model will be defined in a closed-form formulation with solutions that are independent on the chosen increments of the applied stresses. This will drastically reduce the computational demand when the model will be implemented into numerical codes, thus easing its adoption by geotechnical companies in their every-day design practices. The outcomes of this project will ultimately deliver a fundamental understanding of the soil saturation mechanisms and their impact on the stability of earthworks as well as enhancing our ability to assess the resilience of infrastructure networks subjected to extreme rainfall.

Aim
This project will devise, via advanced modelling, practical engineering solutions to enhance the resilience of infrastructure earthworks subjected to extreme rainfalls.

Objectives
The main objectives of this project are:
O1: to formulate a closed-form model to predict the hydromechanical behaviour of unsaturated soils
O2: to validate the model against different sets of laboratory data and to compare its numerical outputs with other well-known incremental models for unsaturated soils
O3: to implement the model into a Finite Element code and simulate rainfall-induced instability of infrastructure earthworks
O4: to devise engineering solutions in view of preserving the stabilising capillary suction in soils and postpone the transition to more unstable saturated states.

Methodology

The project methodology is built around the four objectives and it provides a well-structured workplan for the research activities. The methods employed include:

Constitutive modelling: a coupled hydromechanical model will be developed to predict deformation, degree of saturation and water flow in unsaturated soils (O1). Model predictions will be validated against experimental data published in literature and compared against other existing models (O2). The model will be employed to study the key mechanisms that govern the transition from unsaturated to saturated soil states and the mechanical deterioration of soil when subjected to strong wetting-drying reversals.

Numerical modelling: the proposed constitutive model will be implemented into a Finite Element code to study the impact of different extreme rainfall events on the stability of infrastructure earthworks (O3). Numerical simulations will be developed to assess the efficiency of various engineering solutions in preserving the stabilising capillary actions and delaying/preventing the transition to the saturated state (O4).

Project Timeline

Year 1

Literature review and attendance of training workshops. The supervisory team has already developed a) a soil-water retention law for predicting the degree of saturation [9], b) an isotropic mechanical law for predicting soil deformation [10], and c) a permeability function for the analysis of water flows in soils [11]. The PhD candidate will couple the retention law and the permeability function with the mechanical law to study coupled flow-deformation problems. The coupled hydromechanical model will then be validated against experimental data published in literature (O1).

Year 2

The PhD candidate will extend the isotropic hydromechanical model to non-isotropic stresses by means of a closed-form formulation, validate model predictions against experimental data from literature and compare them against other existing models (O1, O2). Introduction to FEM numerical modelling (O3).

Year 3

The PhD candidate will implement the constitutive model into an in-house Finite Element code to investigate the stability of an infrastructure earthwork subjected to extreme rainfall (O3). Suitable engineering solutions (e.g. drainages, water runoff interceptors, etc.) will be proposed to preserve soil suction and postpone transition to saturated states (O4).

Year 3.5

Writing-up of the PhD thesis and a journal paper (Targeted journals: Geotechnique, Acta Geotechnica, Computers and Geotechnics).

Training
& Skills

The PhD candidate will collect at least 60 PGRDP credits via attendance of in-school workshops and taught modules. Training is provided through: (i) a programme of taught modules; (ii) internal workshops on key research skills and techniques; (iii) input from supervisors; (iv) School and research group seminars delivered by visiting and internal speakers and presentations by other PGR students; and (v) external workshops.

References & further reading

[1] Power, C., Mian, J., Spink, T., Abbott, S., & Edwards, M. (2016). Procedia engineering, 143, 726-733.[2] Martinovic, K., Gavin, K., Reale, C., & Mangan, C. (2018). Geomorphology, 306, 40-50.[3] Briggs, K. M., Loveridge, F. A., & Glendinning, S. (2017). Engineering Geology, 219, 107-117.[4] Alonso, E. E., Gens, A., & Josa, A. (1990). Geotechnique, 40(3), 405-430.[5] Wheeler, S. J., Sharma, R. S., & Buisson, M. S. R. (2003). Geotechnique, 53(1), 41-54.[6] d’Onza, F., Gallipoli, … & Pereira, J. M. (2011). Geotechnique, 61(4), 283-302.[7] Crozier, M. J. (2010). Geomorphology, 124(3-4), 260-267.[8] Lloret-Cabot, M., Wheeler, S. J., Pineda, J. A., Romero, E., & Sheng, D. (2018). Acta Geotechnica, 13(1), 15-37.[9] Gallipoli, D., Bruno, A. W., D’onza, F., & Mancuso, C. (2015). Geotechnique, 65(10), 793-804.[12] Gallipoli, D., & Bruno, A. W. (2017). Geotechnique, 67(8), 703-712.[13] Scarfone, R., Wheeler, S. J., & Lloret-Cabot, M. (2020). Journal of Geotechnical and Geoenvironmental Engineering, 146(10), 04020106.

Further Information

Dr Agostino Walter Bruno: agostino.bruno@newcastle.ac.uk

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