Sagging and filtration behaviour of nonwoven geotextiles overlying different bedding materials Ennio M. Palmeira * , Janaina Tatto 1 , Gregorio L.S. Araujo 2 University of Brasilia, Department of Civil and Environmental Engineering, Faculty of Technology, 70910-900 Brasilia, DF, Brazil article info Article history: Received 30 April 2011 Received in revised form 13 August 2011 Accepted 18 September 2011 Available online 12 October 2011 Keywords: Nonwoven Geotextile Filter Sagging Bedding Material Geotextile Strains abstract This paper presents and discusses the results obtained in laboratory tests to evaluate sagging and filtration behaviour of nonwoven geotextiles overlying particles of different bedding materials. Nonwoven geotextiles with masses per unit area of 200 g/m 2 , 400 g/m 2 and 600 g/m 2 were used in the tests. Stainless steel spheres and gravels were employed as granular materials underlying the geotextile filter. Specific tests to evaluate the intensity of sagging of the geotextile filter in the voids of the underlying material were carried out. Average strains mobilised in the geotextile were assessed in the tests. The grain sizes of the particles that piped through the geotextile filter for different arrangements of spheres as bedding layer for the filter were measured. The results obtained showed that bedding conditions and geotextile structural characteristics can influence its retention capacity. Good compari- sons between predicted and measured average geotextile strains were observed. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Geotextiles have been extensively used as filters in drainage systems in geotechnical and geoenvironmental engineering works for the last decades. To function properly as a filter these materials must fulfil some requirements and several studies on geotextile filters can be found in the literature (Calhoun, 1972; Giroud, 1982, 1996, Carroll, 1983; Fannin et al. 1994; Lawson, 1986; Fischer et al. 1990; Bhatia and Huang, 1995; Lafleur, 1999; Palmeira and Fannin, 2002; Gardoni and Palmeira, 2002, for instance). Most of the filter criteria available were based on results from extensive laboratory testing programmes with different types of apparatus and with soil-geotextile filter systems being tested without the application of vertical stresses. Therefore, except under very low stresses, the current filter criteria do not reproduce accurately the conditions that a geotextile filter may find in the field. Besides other factors, stress level can influence the retention capacity and clog- ging conditions of the filter (Heerten, 1982; Faure et al. 1999; Gardoni and Palmeira, 2002; Palmeira and Fannin, 2002; Palmeira et al. 2005, 2010). Another aspect yet to be properly investigated related to geo- textile filter behaviour is the influence of the bedding conditions of the geotextile. In drains with gravel drainage material, the geo- textile filter can sag in the pores of the gravel and the intensity of sagging is likely to influence its filter behaviour. For such drains, poor construction conditions or higher stress levels (Fig. 1a) increase the complexity of the problem, because of compression of the filter and more favourable conditions for mechanical damage, as shown in Fig. 1(b). For instance, Faure et al. (1999) reported small punctures in specimens of a geotextile filter exhumed from an embankment dam. Sagging of the geotextile filter in the voids of the underlying coarse drainage material will also mobilise tensile stresses in the filter which, in addition to possible mechanical damage, may influence flow rate and filter retention capacities. Fourie and Kuchena (1995) investigated the behaviour of geotextiles speci- mens subjected to in-plane tensile stresses and verified that these stresses can have a marked effect on the flow rate capacity of the geotextile. Reductions of flow rates up to 80% were observed for woven and nonwoven geotextile specimens subjected to tensile loads smaller than 2.2% of the ultimate geotextile tensile strength. Fourie and Addis (1999) examined the variation of filtration opening sizes of two woven slit-film polypropylene geotextiles subjected to in-plane uni-axial and bi-axial stresses using * Corresponding author. Tel.: þ55 61 3107 0969; fax: þ55 61 3273 4644. E-mail addresses: palmeira@unb.br (E.M. Palmeira), janatatto@brturbo.com.br (J. Tatto), gregorio@unb.br (G.L.S. Araujo). 1 Tel.: þ55 61 3107 0958. 2 Tel.: þ55 61 3107 1272. Contents lists available at SciVerse ScienceDirect Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem 0266-1144/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2011.09.002 Geotextiles and Geomembranes 31 (2012) 1e14