Investigation of a pore pressure driven slope failure using a coupled hydro-mechanical model O. Davies a , M. Rouainia a, ⁎, S. Glendinning a , M. Cash b , V. Trento b,1 a School of Civil Engineering and Geosciences, Newcastle University, NE1 7RU, UK b ATKINS, 71 Old Channel Road, Belfast BT3 9DE, Northern Ireland, UK abstract article info Article history: Received 2 July 2013 Received in revised form 11 December 2013 Accepted 21 May 2014 Available online xxxx Keywords: Slope stability Pore water pressures Rainfall Limit equilibrium Hydro-mechanical Numerical modelling The Belvoir Park Forest landslip is situated in Northern Ireland. It was monitored for more than five years with inclinometers, standpipe piezometers, and slip indicators providing detailed information on the long term behav- iour and rate of movement of a large low angle failure in Glacial Till. A detailed geological model was constructed on the basis of the geomorphology and geology of the area and ground investigation data. A detailed geotechnical model was prepared with estimates of stiffness, permeability, shear strength and unit weight data. The geometry of the failure plane, residual strengths, movement rates, pore pressure fluctuations and rainfall data was used to complete a model of the failure with limit equilibrium methods. This model was able to demonstrate that pore water pressure was the most influential factor when considering the stability of the slope, but could not deter- mine the effect of rainfall fluctuations on the observed rate of movement of the slip. A more sophisticated coupled hydro-mechanical model based on Shetran and Flac-tp flow was used to determine the effect of fluctuations of pore pressures, rainfall and mechanical strains on slip movement. This paper illustrates some of the advantages of using more sophisticated slope modelling approaches alongside more ‘traditional’ modelling of residual slope failures to understand their future evolution. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 1. Introduction Infrastructure slopes and cuttings constructed within a clay material are susceptible to seasonal changes in pore water pressures, which have been monitored by a number of authors (Ridley et al., 2004; Smethurst et al., 2006) amongst others. Authors have used these observations to numerically model seasonal affects of current-day climate on infrastruc- ture (Kovacevic et al., 2001; Nyambayo et al., 2004; O'Brien et al., 2004; Vaughan et al., 2004; Take and Bolton, 2011) whereby an average sum- mer suction has been applied to a geotechnical model boundary and then an average winter suction applied. Rouainia et al. (2009) undertook modelling work using the hydrological code Shetran (Ewan et al., 2000) coupled with the geomechanics code Flac-tp flow (Itasca, 2009). Shetran allows long term climate data to be applied to an embankment model and the fluctuations in pore water pressures to be modelled. These pore water pressures were then transferred into Flac-tp to model the me- chanical response of the embankment. This improved on previous work as the use of daily meteorological data allowed the modelling of the grad- ual transition between seasonal pore pressure conditions. The authors of the present paper highlighted how this approach is particularly important if the impact of climate change on seasonal fluctuations in embankment pore water pressures is to be investigated (Rouainia et al., 2009). In this paper, a slope situated on the banks of the River Lagan, in the out- skirts of Belfast, Northern Ireland is modelled. The slope was subject to se- vere instability problems over a number of years and was threatening significant sewer infrastructure. Analysis of the site using limit equilibrium methods revealed that an influential factor in the future move- ment of the slip was the elevated pore water pressures observed along the failure plane. However, detailed analysis of the effect of rainfall patterns on the magnitude of pore pressures acting along the failure surface was not possible using these methods. The aim of this paper is to describe how coupled hydro-mechanical modelling of the site, using local rainfall records, was able to shed light on the movement trends of this landslip. 2. The site 2.1. Location The site is situated in an area of Belvoir Park approximately 4 km south of Belfast city centre as shown in Fig. 1. The area is used for recre- ational purposes and hosts important sewer infrastructure, which was installed along the lower section of the river bank slopes. The area of Engineering Geology xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +44 191 222 3608; fax: +44 191 222 3522. E-mail address: M.Rouainia@ncl.ac.uk (M. Rouainia). 1 Formerly at Atkins. ENGEO-03793; No of Pages 12 http://dx.doi.org/10.1016/j.enggeo.2014.05.012 0013-7952/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo Please cite this article as: Davies, O., et al., Investigation of a pore pressure driven slope failure using a coupled hydro-mechanical model, Eng. Geol. (2014), http://dx.doi.org/10.1016/j.enggeo.2014.05.012