Physical and mechanical characterization of weak schistose rocks T. Le Cor INSA Rennes, Groupe Dacquin, France
D. Rangeard INSA Rennes, France
V. Merrien-Soukatchoff Université de Lorraine, CNRS, CREGU, GeoRessources UMR 7359, France
J. Simon Groupe Dacquin, France
ABSTRACT: The anisotropic behaviour of schistose rocks is rarely considered in the conception of retaining structures (diaphragm wall, soldier piles wall…) in this type of rocks. Brioverian shales are often encountered underground the city of Rennes (Brittany France) and its surrounding area. They are fractured, altered and folded along a vertical plane which lead to lateral variations of facies and alteration changes on short distances. Consequently of the numerous construction projects, including the excavation of the second subway line, the physical and mechanical knowledge of this material has to be improved to help structure design. This paper presents the results of physical and mechanical characterization on schistose rocks extracted from several locations in the Rennes area. Some of the methods employed were classical geotechnical tests (X-ray diffraction, thin sections observations, uniaxial compression tests and shear tests along open joints) whereas others were modified tests usually used in the road construction fields (grindability test based on LCPC abrasiveness test and Micro Deval test used on prismatic samples). This characterization showed an important heterogeneity of the mechanical properties of the Brioverian shales despite homogeneity in their mineralogical contain. The important sensitivity to water of the material was revealed. 1 INTRODUCTION Brioverian shales can be encountered undeground a large part of Brittany (West of France, Jegouzo 1973, Dabard 1990). The unpredictable anisotropic behaviour of these rocks during construction projects can be a real problem. Indeed, those shistose rocks present a high level of fracturation and alteration that leads to important lateral variations on short distances. The most common problems encountered are : deviation during drilling (piles used as foundations or retaining structures, diaphragm wall), collapsing between vertical elements of a retaining structure during excavation, higher ground pressures than expected in calculations leading to unacceptable displacements or moments of the structure... Numerous underground construction projects are under view in the Rennes Metropole area and the construction of a second subway line will begin in 2014 through these schistose formations. Considering those expected projects in this type of ground, the physical and mechanical knowledge of this material have to be improved to help structure design. Rock samples, from different sites in the city of Rennes, were extracted during excavation steps of different construction sites. The physical characterization consisted in X-ray diffraction analyses on shale powders, thin section observations,
grindability tests on aggregates and a modified Micro Deval test on prismatic samples. The mechanical characterization used uniaxial compression tests and direct shear tests along open joints. An overview of the results obtained so far is presented in the next pages. 2 PHYSICAL CHARACTERIZATION 2.1 Localization The extraction sites of the samples were spread over Rennes (Fig. 1) on different construction sites.
Figure 1. Localization map of the sampling sites spread over Rennes (Source: Google maps).
2.2 X-ray diffraction analysis and thin sections observations Thin section observation is a usual technique used to study anisotropic rocks (Behrestaghi 1996, Wenk & al., 2010). Thin sections parallel and perpendicular to the schistosity were made for each sample. Observations through microscopes revealed that the granulometry of all the samples tested was small (lower than 60 µm) but some were smaller than others: B, D and G with a granulometry lower than 30 µm. Schistosity direction can be, generally, easily identified, yet stratigraphy cannot always be recognized or differentiate from schistosity. Oxides (goethite and leucoxene) were identified in important quantities in some samples (A, B and C3) which reveal their important alteration degrees. The main minerals presented in all the samples are quartz and phyllosilicates. Samples D to G also contained feldspars (plagioclase and orthose). Examples of thin sections perpendicular to schistosity are shown in Figures 2a.b.
According to figure 3b, samples can be divided in two categories. The first one includes samples D, E, F, H, I, J3 to K3 which contain the lowest quantity of clays relative to the quartz. The second category includes samples A, B, C, G, J2 and L with clay quantities in the same range or larger than quartz. Complementary treatment (glycerol saturation) was conducted on samples B, D and L that confirmed the suspected presence of expansive clay (smectite) revealed by the observed fractures of the samples after water immersion. 2500 intensity (u.a.)
For each site, samples were taken at different depths varying from 2 to 8 metres. Sample sites are named from A to L. Subcategories, for the same site, representing variations of visual facies are numbered (examples: L1 and L2) in the results presented in this paper.
Quartz
2000 1500 1000
Chlorite Illite Kaolinite
500 0 2
7
12
17
22
27
32
2θ (°) 250 RI Kaolinite (%) 200 150
RI Illite (%) RI Chlorite/Smectite (%)
100 50 0 A B C1C2C3 D E F G H I J2 J3 J4 K1K2K3L1 L2
Figure 3. Intensity peaks of clays and quartz for each sample. Top (a) : X ray diffraction results for one sample (I). Bottom (b): Relative intensity of the clays compared to quartz.
2.3 Modified tests: Grindability and Micro Deval tests Simple tests were elaborated in an attempt to evaluate the alteration degree of the Brioverian shales. These tests, relevance to estimate mechanical strength of rocks, are discussed hereinafter. 2.3.1 Description of the tests
Figure 2. Thin sections of two different samples. Top(a) : B sample zoom × 25. Bottom (b): F sample zoom × 25
X-ray diffraction analyses (Fig. 3.a), regularly used in the rock mechanics field (Gy & al., 1995, Nasseri, 2003), were lead on powder of schistose rocks in order to evaluate the quantities of quartz and clayey minerals.
The first test performed was the grindability test on aggregates based on an existing test (AFNOR, 1990). It consisted in subjecting the aggregates to the action of a rotating steel plate (Fig 4a). Initially, the only measure of the test was the quantity of fine elements (< 1.6 mm) produced by the crushing action of the plate after 15 minutes. As our material was completely crushed after a shorter duration, the test was shortened to 5 minutes and fractionated to realize a measure every 15 seconds during the first 90 seconds.
differences between samples are more significant in the first 90 seconds of the test and are reduced at the end of the test. Most of the samples reached a grindability index between 75 and 90% after 300 seconds. The most pertinent indicative parameters for this test is the grindability index after 30 seconds (differences between samples more important than after 300 seconds). 100 90 80 70 60 50 40 30 20 10 0
GI ( %)
The second test developed is based on the Micro- Deval test usually used to characterize wear resistance of aggregates. The modified version of the test is based on the adaptation proposed by Takarli (2007) to test granite aggregates. It consists in exposing plane-parallel samples (instead of aggregates) to the abrasive action of steel balls (Ø10mm). Sample and steel balls are placed in a rotating container at a constant speed (100 rev/min) during 120 minutes (Fig. 4b). The weight loss of the sample is measured (and divided by the surface of the sample exposed to the abrasive load) during the test and the evolution of the loss is used to calculate two parameters: final wear corresponding to the weight loss after 120 minutes of testing and the wear factor which is the slope of the linear part of the plot.
0
50
100 150 200 Duration in seconds
250
300
Figure 5. Evolution of the grindability index (GI) for all the samples
2.3.2 Results of the tests The grindability index (GI) for each sample is calculated according Equation 1. ( ) =
,
(
)
× 100
(1)
where mfines
