Behavioural features of fissured clays: experimental evidence and modelling

Federica COTECCHIAa and Claudia VITONE a,1

a Technical University of Bari, ITALY

Abstract. Fissured clays are widespread all over the world and they are often catalysts of several and unexpected in-situ problems. In the last decade, a research started at the Technical University of Bari aiming to interpret the influence of fissuring on the clay behaviour. According to the approach adopted, a Fissuring IDentity (F-ID) has been associated to each clay being studied, as extracted from a general fissuring characterisation chart. Hence, the analysis of the fissured clay behaviour has benefited from systematically coupling the micro- to meso- features (i.e., F-ID charts) and processes, and the observations of the macro-response. The comprehensive experimental programme carried out on several Italian fissured clays has allowed to recognise that they can be still modelled as single geotechnical class which follows an extended sensitivity framework of behaviour. Moreover, for clays of specific F-IDs, most recent developments of the research have put in evidence discrepant behavioural facets induced by scale effects.

Keywords. fissured clay behaviour, sensitivity framework, scale effect, REV.

1. Introduction

In the last century, intensive experimental geotechnical studies were addressed to characterise the decayed strength of fissured clays. They prompted the development of two main streams of research: the behaviour very highly fissured clays on one side [e.g. 1-3] and that of stiff clays and weak rock masses of low fissuring intensity on the other [e.g. 4-8]. Nonetheless, no definite general assessment of the mechanical behaviour of fissured clays has been pursued to date, probably for the apparent necessity of using different methodological approaches to the interpretation and modelling of soils depending on their fissuring features [8-10]. The present paper represents a further step of an on-going experimental research that aims to interpret and model the general influence of fissuring on the mechanical behaviour of clays [11]. According to the approach adopted, a Fissuring Identity (F-ID) has been associated to each clay being studied, as extracted from a general fissuring characterisation chart. The analysis of the influence of the F-IDs on the clay behaviour has been carried out through element testing in the laboratory, analysis of the results in the framework of Critical State Soil Mechanics (CSSM, hereafter) and modelling according to traditional elastic-plastic theory. In this paper, the behaviour of four fissured clays outcropping within the Apennine chain in the South of Italy (Figure 1a) is analysed in the light of the framework of behaviour of unfissured clays, either natural or reconstituted [12-13]. The experimental results show that, despite their different F-IDs, fissured clays can be still modelled as single geotechnical class through an extended sensitivity framework of behaviour. However, the most recent studies have allowed to both recognise discrepant behavioural facets induced by size effects and to identify the triggering F-IDs.

1 Corresponding author.

2. Origin,

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This chart allows to convert the soil meso-fabric in a set of parameters, i.e. the F-ID of the material, to which specific behavioural features can be related. It accounts for both the lithology and consistency of the soil matrix (categories A and B) and the discontinuity features: fissuring nature (C, D, E), orientation (F) and geometry (G-H-I). Fissuring intensity can be characterised by either the volume of the inter-fissure clayey elements (i.e. the inter-fissure volume, VI), or by their specific surface. The research entailed the investigation of clays of different fissuring features whose F-ID envelope is represented in grey in Figure 1f. Moreover, in the Figure, the F-IDs of the four clays under study have been distinguished by specific keys. Experimental results showed that the categories that influence most the fissured clay behaviour are both the fissuring orientation (category F) and the intensity (category I; Figure 1f). Both SEN and SCM clays (i.e. the most intensely fissured clays being studied) are I6 clays and the fissures split the clay into millimetre-sized lens-shaped elements. They are known as scales, which is the origin of their being referred to as scaly clays in the literature. Differently, BENT and the SGP clays are of I5 and I4 intensity respectively, and fissuring confines elements of centimetre size and prismatic shape (Figures 1d-e). In addition, reference is made solely to either random, F3 (SGP, SEN and BENT clays), or single, F1 (SCM scaly clay), fissuring orientation. The mechanical characterisation of the SCM scaly clay, discussed in the following, has been carried out with fissures oriented normally to the direction of the maximum principal stress.

3. Mechanical characterisation

Figure 2a shows the results of the restrained-swelling oedometer tests carried out on the natural specimens of the four clays up to medium-high vertical effective pressures. The Figure also shows the oedometer tests on the corresponding samples when reconstituted in the laboratory following Burland [6]. Despite their different F-ID charts, all the clays being studied follow a single framework of behaviour in compression. In particular, the Intrinsic Compression Lines (ICLs; [6]) of the reconstituted clay specimens, are on the right of the compression curves of the natural clay up to high pressures. In Figure 2a an arrow indicates the gross yield state of the fissured clays under study. It has been identified by making reference to the highest gradient in compression index (Cc) increase. Figure 2b shows the variation in specific volume, and mean effective stress, p’, experienced by 38 mm diameter specimens of BENT clay during consolidated isotropically undrained shearing (CIU) triaxial tests. The data show that, during isotropic compression, BENT clay still follows the same pattern of behaviour recorded in one-dimensional compression. The contractive nature of the shear response of the specimens isotropically consolidated to -p’ states to the left of the isotropic normal consolidation line of the reconstituted clay, INCL*, confirms that wet shearing behaviour and gross yielding occur on the left of INCL*. It follows that, as in oedometer compression, the INCL of the natural clay is located on the left of the INCL* up to medium-high pressures also in the -p’ plot. The undrained shear state paths of the BENT clay are shown in the q-p’ plane in Figure 3, where q is the deviator stress and both q and p’ are normalised for by means of the equivalent pressure pe* taken on the INCL* (see Figure 2b). The data confirm that, according to CSSM, dry behaviour is exhibited by specimens swelled to overconsolidation ratios R (= p’y/p’) higher than 2. Moreover, the q-p’ states possible for the BENT clay are

located inside the state boundary surface of the reconstituted clay, SBS*, and the size of the SBS of the natural clay is smaller than SBS*. The normalised stress paths of the specimens consolidated beyond gross yield before shearing (e.g. specimens BENT-B and BENT-C in Figures 2b and 3), confirm that the wet side of the SBS of the fissured clay increases in size with compression (i.e. the size of SBC1 is smaller than SBC2 and SBC3).

3.1 The extended sensitivity framework

The results in Figures 2a-b and 3 have been found to be common not only to the four fissured clays here of reference, but also to the clays of F-IDs that cover the grey envelope in Figure 1f. This means that the general effect of fissuring on the clay behaviour can be derived as the effect of a new internal state variable. To this purpose, Figures 4a-b compare the sketches of the behavioural trends recognized in compression and shear for the fissured clays, with those applying to intact clays, either natural or reconstituted. Figure 4a shows that fissured clays reach either one-dimensional or isotropic compression states on the left of the normal compression line of the reconstituted clay (i.e. ICL or INCL*, respectively) up to high pressures, differently from sensitive unfissured soils. This reveals that fissuring bars out the space on the right of the ICL, that is the so-called structure permitted space [12], regularly stated as attainable by all structured soils. From what above, this space should be renamed as unfissured structure permitted space. Cotecchia & Chandler [13] quantified the influence of microstructure on the clay behaviour through the stress sensitivity ratio, S= ’y/*e (where *e is the equivalent pressure on the ICL). S is equal to one by definition for the reconstituted clay. The Authors found that S values higher than one characterise unfissured sensitive clays. The results here presented suggest that fissuring generates the common geotechnical effect of making the clay under-sensitive, that is characterised by S values below one, i.e. even lower than those of the same clay when reconstituted.

The sketch in Figure 4b shows that fissuring is also detrimental to strength, even with respect to the reconstituted clay. Consistently with S<1 the SBS of the fissured clays is smaller than SBS*, irrespective of any other F-ID feature. The increase in size of the SBS of the natural fissured clay, reveals that structure strengthening is on-going with post-gross yield compression, giving rise to a positive hardening contribution. Furthermore, for fissured clays, S St and, as such, the sensitivity framework can be extended to represent the influence of the fissured clay structure on the gross yield surface (or SBS) of the clay.

3.2 Discrepancies from the framework: REV and size effect

New recent experimental investigations have been carried out in order to focus on the effect of the specimen size on the clay behaviour, in the light of its F-ID. Figures 5a shows the same data reported in Figure 2b together with new data of CIU triaxial tests carried out on three specimens of larger size, i.e. 50 mm diameter - 100 mm high (BENT-F50) and 70 mm diameter- 140 mm high specimens (BENT-G70 and BENT-H70). Figure 5a clearly shows the overlapping up to high pressures of the compression curves of standard triaxial specimens (BENT-B and BENT-C) and of the larger specimen BENT-H70.

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specimens isotropically consolidated up to the same p’ values. The figure also shows the pictures of the specimens at the end of the tests. Contrarily to what observed in compression, the specimen size affects the peak strength of the BENT clay. The larger specimens, either sheared pre-gross yield (BENT-F50, BENT-G70) or post-gross yield compression (BENT-H70), exhibit higher strength than expected if compared to the corresponding data from standard size specimens (Figure 5b). Recording these discrepancies suggests that the standard specimen size, 38 diameter by 76 mm high, is not the Representative Element Volume (REV, hereafter) of the BENT clay. However, although the bigger is the specimen size, the higher are the peak strength values of the natural clay, the behaviour in undrained shear of larger specimens is similar to that of the standard ones having the same R (=p’y/p’). Therefore, the specimen size seems to influence just the value of the strength parameters, because no modifications are recorded on the pattern of the fissured clay behaviour. Moreover, the fissure surfaces in the photographs became visible to the naked eye on the natural clay specimens since shear strains, s, about 3.0 – 6.0%, i.e. near q/p’ peak ratio for almost all the clay specimens. This allowed the BENT clay specimens to exhibit predictable behavioural patterns that are consistent with their overconsolidation ratios although an effect of the specimen size on the clay peak strength is recorded. Similar results have been found for the other F-IDs (I5 to I4 and F1 to F3) on 38 mm diameter specimens, whereas no size effect even on the peak strength has been recorded on I6 clay specimens.

If the REV is the minimum specimen size above which no variation of mechanical behaviour is measured for increasing specimen size, it can reproduce the in-situ soil behaviour both quantitatively and qualitatively. It follows that 38 mm diameter specimen is the REV only for I6 clay specimens. For I5 to I4 and F1 to F3, the experimental results provide indication that this specimen size, although smaller than the REV, is still useful to qualitatively analyse the main patterns of mechanical behaviour of the fissured clay. One might conclude that for these F-IDs, 38 mm diameter specimens represent a Qualitative REV (Q-REV hereafter). It is worth noting that Q-REV still complies with the definition of representative sample provided by [15], as homogeneously heterogeneous material that should contain a sufficient number of the patterned cells that the variations within the cells are smeared out. Figure 3 also shows that the normalised effective stress path of the standard specimen BENT-D does not join the boundary curve SBC1 of the I5 clay pre-gross yield. Moreover, the friction angle at peak stress of specimen BENT-D equals the residual one, that is ’r = 5° (Bromhead ring shear measurement [16]). Only for this specimen a single failure surface inclined 40° to the horizontal was already visible sinces much lower than 3%. This behaviour has to be imputed to the presence of a pre-existing single fissure which crossed the specimen, making change its F-ID. In particular, the maximum inter-fissure volume (VI) increases, so that the specimen becomes representative of a I3 clay. It follows that, in this case, the specimen size becomes too small to be even considered a Q-REV. Similar results have been found by Marsland [5] during triaxial tests on 38 mm diameter specimens of London clay (i.e. I5-I3 clay). In particular, similarly to what observed for BENT-D specimen (Figures 2b and 3), lowest strengths are measured on specimens containing a single fissure (i.e. I4-I3 specimens) which made the specimen far smaller than REV. Also Ward et al. [4], when testing London clay specimens at Ashford Common, found that, in general, weaker specimens failed at smaller strains, suggesting that the lower strengths are associated with planes of weakness, probably fissures too fine to be seen (and that are making vary the F-IDs of these clay specimens). The Authors consider that the scatter in the strength results is much more

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4 Conclusions

Despite their different geological histories and F-IDs, fissured clays represent a single geotechnical class of materials whose behaviour at the macro-scale still follows the sensitivity framework in compression and shear. However, if for unfissured clays the natural (micro-)structure adds bonding to the reconstituted clay as evolving internal factor and provides the natural clay of a temporary additional strength, the natural (micro- to meso-)structure of fissured clays makes their stress sensitivity become even lower than that of the reconstituted. Moreover, new developments of the research have demonstrated that this framework of behaviour holds not only when the REV is tested, but also for specimens smaller than the REV, but large enough to qualitatively represent the soil behaviour (i.e. the Q-REV). Within the Q-REV, strain localisation processes are still similar to those of the REV up to the onset of the sliding mechanism, i.e. they are diffuse pre-peak and become more confined about peak. It follows that the specimen still maintains its load-carrying capacity and significant softening occurs only after one of the shear bands takes control. It has been observed that 38 mm diameter specimens are REVs for I6 clays and Q-REVs for I5-I4 clays. They are too small to be even the Q-REV for I4-I3 specimens, so that, in these cases, early sliding occurs the more frequently the lower is their specimen/inter-fissure volume ratio (VS/VI).

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