1. INTRODUCTION

The Shale-Limestone Chaotic Complex (SLCC) is a mélange formation of the Northern Apennines mountain belt, which, from a geomechanical point of view, represents a typical block-in-matrix rock (bimrock) [1, 2, 3, 4].

The SLCC forms a wide mine slope (with a maximum height of 200 m and a planar extension of about 1300 m x 400 m) in the disused Santa Barbara lignite open pit mine (Tuscany, Italy) (Fig. 1). Since the beginning of its excavation, the slope suffered from a diffuse instability phenomena characterized by rotational landslides and toppling countercracks. Gravitational movements must be considered as still active: rotational landslides show evidence of reactivation due to intense precipitation while toppling countercracks has displayed a net slip up to a meter, over the last two years.

In the past, during mine activity, several stability analyses were carried out, assuming the SLCC as an homogeneous material governed by the only mechanical properties of the clayey matrix, without taking into account the influence of calcareous blocks on the mechanical behaviour of the bimrock [5, 6, 7]. On the other hand, recent advances in the understanding of

bimrock’s mechanical properties have pointed towards a strong influence of the blocks on the strength of the bimrock [8, 9, 10, 11, 12].

For this reason, a research study on the geomechanical characterization of the SLCC according to modern bimrock theories is being carried out.

Fig. 1. A panoramic view of the Santa Barbara mine slope cut in the SLCC bimrock.

ARMA 09-184

In situ large size non conventional shear tests for the mechanical characterization of a bimrock in The Santa Barbara open pit mine (Italy)

Coli, N., Berry, P., Boldini, D. and Bruno, R. Department of Chemical, Mining and Environmental Engineering (DICMA), University of Bologna, Bologna, Italy

Copyright 2008, ARMA, American Rock Mechanics Association This paper was prepared for presentation at Asheville 2009, the 43rd US Rock Mechanics Symposium and 4th U.S.-Canada Rock Mechanics Symposium, held in Asheville, NC June 28th – July 1, 2009.

This paper was selected for presentation by an ARMA Technical Program Committee following review of information contained in an abstract submitted earlier by the author(s). Contents of the paper, as presented, have not been reviewed by ARMA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.

ABSTRACT: A high mine slope in the disused Santa Barbara open pit mine (Tuscany, Italy) is cut in the Shale-Limestone Chaotic Complex (SLCC), which is a typical bimrock made up of a scaly-fabric clayey matrix including heterometric calcareous blocks. The slope shows evidence of instability phenomena: mainly rotational landslides and toppling countercracks. In order to characterize the mechanical behaviour of the SLCC bimrock, in situ large size non conventional shear tests were performed. The aim of in situ tests is to overcome the size limitation of laboratory specimens and namely to take into account the influence of calcareous blocks on the strength of the bimrock. Bimrock’s strength parameters under natural conditions can be evaluated by means of a limit-equilibrium analysis taking into account shear test data and the geometry of the sliding surface.

The research is conducted by the Department of Chemical, Mining and Environmental Engineering (DICMA), University of Bologna. In the first step of the research program the geometrical properties and spatial variability of calcareous blocks were investigated by means of advanced image and geostatistical analyses [13, 14].

The activity is now focusing on defining the mechanical behaviour of the SLCC. In order to investigate the actual strength of the bimrock, non conventional in situ shear tests are being carried out on large-sized material specimens (80 cm x 80 cm x 50 cm). The tests are performed with the financial and logistical support of ENEL Santa Barbara Department.

2. GEOTECHNICAL CHARACTERIZATION OF THE CLAYEY MATRIX

In order to characterize the SLCC clayey matrix from a geotechnical point of view, laboratory tests were carried out during the years of mining activity on undisturbed specimens taken with a triple core barrel sampler (Mazier) at a depth ranging from 5 m to 75 m in the slope [5, 15].

The matrix characterization indicated an average saturation rate (Sr) of 0.9, an average void index (e) of 0.28 and a unit weight of volume (γ) of 23.5 kN/m3.

A prolonged mechanical disgregation of the material during specimen preparation caused a marked increase of the finer size of particles as well as of clay fraction (CF), liquid limit (WL) and plasticity index (IP), compared to the values obtained with the standard A.S.T.M. specimen preparation (Table 1).

Table 1. Geotechnical index properties of the clayey matrix (from [15])

CF (%) γ (kN/m3) WL IP e Sr Standard A.S.T.M. specimen preparation

17 23.5 28 9 0.28 0.9

Prolonged mechanical disgregation

37 23.5 38 15 0.28 0.9

Laboratory triaxial and direct-shear tests were also carried out. In the stress range of 0.05 < σn’ < 1.2 MPa, values of peak friction angle and cohesion are φp’ = 20° ÷ 25°, cp’ = 50 ÷ 100 kPa. Residual friction angle φr’ ranges from 13° to 16°.

3. BLOCK GEOMETRICAL PROPERTIES

The size distribution, shape, orientation and volumetric proportion of blocks have a large influence on the strength parameters of a bimrock [8, 9, 10, 11, 12, 16, 17]

Fig. 2. Log-log histograms of the size distribution of block major axes.

As a consequence, a detailed indirect non-destructive characterisation of geometrical properties of the SLCC calcareous blocks was carried out on photographic images by means of advanced image and geostatistical analyses [13, 14].

Block size distribution in the SLCC Bimrock resulted in being self-similar following a power law relationship with a 2D fractal dimension (D) ranging from 1.2 to 1.9 (Fig. 2). This result is in agreement with previous studies conducted on the Franciscan Melange, California [1, 2, 18, 19].

The ratio between major and minor axes of blocks (block shape ratio) was also investigated. The shape ratio varies between values of 0.40 and 0.65, with a modal value of 0.55, indicating a prevalent oblate ellipsoid shape of blocks.

Geostatistical experimental semivariograms and Variographic Surfaces (Fig. 3) were calculated for each of the analyzed images in order to also identify the possible directions of anisotropy of block shape and block spatial distribution.

At the scale of investigated samples (4 m2 area covered by each image) the structural analysis of semivariograms always showed a nested structure and often some anisotropies. However, when the semivariograms indicated a presence of anisotropy they did not show a common principal direction of anisotropy among the images that were taken all over the slope.

In-situ sieving tests on 1 m3 bulk samples were also performed to evaluate the ratio between blocks and the clayey matrix, a large size sieving cage was specifically designed for this purpose (Fig. 4). Test results gave an average volumetric percentage of blocks within the matrix of 33%.

Fig. 3. Variographic Surface of one of the analyzed images, showing an evident anisotropy of block spatial distribution.

Fig. 4. The iron cage used for in situ sieving tests.

4. IN SITU SHEAR TESTS

Large size in situ non conventional shear tests are being carried out in order to investigate the mechanical behaviour of the SLCC bimrock by taking into account the influence of blocks and to overcome the size limitation of laboratory specimens.

The in situ shear test procedure performed by Xu et al. [20] and Li et al. [21] was improved in terms of testing apparatus and data acquisition system. The testing procedure differs from the ISRM Suggested Methods for in Situ Determination of Direct Shear Strength [22] because the failure surface is not forced to develop along a horizontal plane, but it is free to grow inside the specimen without any conditioning.

This kind of non-conventional shear test is preferable respect to the ISRM Suggested Method when performed on a slope cut in a bimrock formation. Moreover, these non conventional tests are cheaper than the ISRM ones, thus allowing for a large number of tests to be carried out, which is very useful to characterize a heterogeneous material as a bimrock.

4.1. Data acquisition system A modern digital portable acquisition system (named STB) for the in situ recording of force and displacement data was designed and assembled by the DICMA.

The apparatus is made up of two main components: the transducers and the digital acquisition (Fig. 5). Force and displacement transducers are constituted by: HBM 50 mm LVDT displacement transducer HBM 200 kN “C2” compressive force load cell

Analogue signals from the transducers are converted to a digital output by a N.I. DAQ Card and recorded by a notebook PC.

A specific algorithm for data processing was implemented in the LabVIEW code, allowing for real time recording and visualization of data, with a sampling rate of 1 Hz.

Fig. 5. Overview of the STB data acquisition system

4.2. Preparation of test specimen All tested specimens are 80 cm long by 80 cm wide by 50 cm high. A standard operating procedure was developed in order to achieve and maintain a high quality for all the tests (Fig. 6, Fig. 7):

(i) Taking particular care to obtain an undisturbed specimen, two parallel trenches are excavated at a distance of about 1.5 m each other for a depth ranging from about 1.5 m to 1.8 m. Afterwards, the front and top material is removed until a block of 1.5 m wide by 1.5 m long by 1 m high is obtained. The block is then manually refined to achieve the final testing dimension. The specimen is characterized by four free faces (two side faces, one frontal face and one top face), while the bottom and the back sides are in continuity with the rock mass.

(ii) A 1.5 cm thick steel plate and a 3 cm thick Plexiglas plate are forced along the two lateral faces of the specimen in order to avoid any lateral displacement. The lateral confinement is obtained by blocking the plates with iron bars deeply driven into the foundation soil (Fig. 7). One of the two lateral plates is made of Plexiglas in order to visualize the deformations and the development of failure during the test.

(iii) A 1.5 cm thick steel plate, with two horizontal bottom guides, is placed against the frontal face of the specimen (Fig. 7). The bottom guides are placed over two perfectly horizontal iron rails. The rails’ function is to keep the frontal plate from sticking onto the bottom material during the test.

(iv) The hydraulic cylinder (Fig. 8), with a maximum available loading force of 180 kN, is placed at about one third the height of the frontal steel plate. The cylinder is supported by four small hand-jacks, which allow for an easy and fast horizontal positioning. The hydraulic cylinder and the four hand-jacks are placed and locked over a “L” shaped iron plate (Fig. 8), with the back side of the cylinder placed against the vertical side of the “L” shaped plate, in order to guarantee its horizontality during the application of the loading force. A Caterpillar D10 (Fig. 6), of 70 t in weight, is placed behind the “L” shaped support plate to act as back-support ensuring that the displacement will take place only in the frontal direction of the cylinder (Fig. 6).

(v) The load cell transducer of the STB data acquisition system is fixed in front of the cylinder’s stem through a specific docking (Fig. 7 and Fig. 8). The LVDT displacement transducer is placed coaxial with the hydraulic cylinder by means of an aluminium trestle, which keeps it isolated from the testing apparatus.

Fig. 6. An overview of the in situ shear test: 1) specimen, 2) hydraulic cylinder and transducers, 3) oil pump, 4) STB data acquisition system, 5) Caterpillar D10 ripper.

Fig. 7. In situ shear test equipment: 1) specimen, 2) frontal steel plate, 3) lateral Plexiglas plate, 4) hydraulic cylinder, 5) iron bars, 6) Caterpillar D10 ripper.

Fig. 8. The hydraulic cylinder and its support system.

4.3. Testing procedures During the test the hydraulic cylinder applies a horizontal force on the frontal steel plate; as a consequence shear deformations develop inside the specimen leading to the formation of a sliding surface (Fig. 9).

In order to perform a strain-controlled test, a displacement rate of 1/20 mm/s is set for the hydraulic cylinder and keep constant by a 3-way compensated oil flow regulation valve.

The specimen’s upper sliding body and the frontal steel plate show minor vertical displacements (max 4 cm), because they slip following the geometry of the sliding surface (Fig. 9B).

This induced vertical movement tends to skew the hydraulic cylinder upward, which would lead to a non-horizontal application of the loading force and a wrong functioning of the load cell.

In order to avoid the inclination of the hydraulic cylinder as much as possible, the cylinder is anchored to the bottom face of the “L” shaped plate support plate by a nylon strap and petroleum jelly is used to decrease friction between the frontal steel plate and the hydraulic cylinder.

No vertical load is applied on the specimen, which is only subjected to its own body force.

Fig. 9. Schematic representation of the in-situ shear test. A) Initial configuration. B) Configuration during the test. (1. original slope profile, 2. frontal steel plate, 3. specimen, 4. LVDT transducer, 5. hydraulic cylinder, 6. “L” shaped plate, 7. hand jack, 8. anchor nylon strap, 9. load cell, 10. sliding surface)

Fig. 10. Force-displacement curve recorded by STB relative to one of the performed tests (P3). Continuous curve: first loading step, dashed curve: second loading step. Pmax = 74.4 kN, Pmin = 60 kN .

During testing the STB data acquisition system performs a real-time continuous recording of the applied horizontal force and displacements, which are visualized on the laptop computer monitor.

Each specimen is subjected to two loading steps. During the first step the horizontal force is applied until the peak value is reached, then the pressure is released and the maximum force (Pmax) of the load-displacement curve recorded (Fig. 10). The force is then re-applied until reaching a new constant value, which is recorded as the minimum force value (Pmin) (Fig. 10).

After the test, all the testing equipment is removed and the material above the sliding surface collected for sieving.

4.4. Investigation of sliding surface Once the material above the sliding surface is removed, the sliding surface is photographed and scanned by a laser total station in order to reproduce its actual 3D shape.

Sliding surfaces show an irregular geometry (Fig. 11), characterized by the presence of protruded calcareous blocks and by a gradient variation from a low inclination to a steeper inclination and again to a more gentle one from the bottom to the top of the sliding surface.

Fig. 11. Sliding surface after testing.

4.5. Test results Three profiles (A, B, C) are performed across each sliding surface. Profiles are subdivided into slices (Fig. 12) in order to apply a limit-equilibrium criterion [20, 23] to calculate the Mohr-Coulomb strength parameters (φ, c) (Table 2) (Fig. 13):

n

iiil

PPc

1

minmax

cos

n

i

n

iiiii

n

i

n

i

n

iiiiii

ggGP

lcggGP

1 1max

1 1 1max

cossin)/(

sincos)/(

tan

where c is the cohesion (kPa), φ is the internal friction angle (°), Pmax is the maximum registered force (kN), Pmin is the minimum registered force (kN), G is the weight of the upper sliding body (kN), gi is the weight of the ith column (kN), αi is the angle between the sliding surface and the horizontal plane for the ith column (°), li is the length of the sliding line of the ith column (m).

Fig. 12. Profile of a sliding surface. The specimen area above the sliding surface is subdivided into slices.

Table 2. Strength parameters relative to shear tests P2 and P3.

P2 P3 Profile A B C A B C Pmax (kN) 75 75 75 74.4 74.4 74.4 Pmin (kN) 70 70 70 60.0 60.0 60.0 c (kPa) 11.8 9.8 9.8 28.8 29.5 28.8 φ (°) 44.2 42.7 39.6 41.5 42.2 38.3

All tests were carried out under natural moisture conditions of the specimen. During the test it was not possible to control the drainage of the specimen, therefore we can only assume partially drained conditions inside the specimen. Hence, the evaluated strength parameters, cannot be considered as effective, but rather as operative parameters under natural conditions.

Fig. 13. Schematic representation of a slice of the upper sliding body, and acting forces.

5. FINAL REMARKS

At the time this paper was written only three shear tests were performed (P1, P2, P3). From one of these tests (P1) it was not possible to calculate the strength parameters (φ, c) through the aforementioned method because the specimen broke irregularly along sub-horizontal pre-existing discontinuities rather than along a newly-formed sliding surface. This behaviour was related to an overall marly-limestone lithology of the tested specimen, and represents a case where, at the scale of investigation, the percentage by volume of blocks exceeds 75% of the total volume. In this context, in fact, the bimrock seems to behave as a very fractured rock mass [8, 9].

The values of strength parameters obtained from in situ tests P2 and P3 are different from those obtained with laboratory tests on the clayey matrix [15] (Table 3). In fact the friction angle of the bimrock is greater than the one of the clayey matrix while the cohesion is lower.

Table 3. Comparison between the strength parameters of the clayey matrix (from [15]) and of the SLCC bimrock (from the in situ shear tests P2 and P3).

Clayey matrix SLCC bimrock φ’ (°)

c’ (kPa)

φ (°)

c (kPa)

20 - 25 50 - 100 38 - 44 10 - 29

These results seem to be in agreement with the results of experimental laboratory tests on artificial bimrock specimens [8, 9] and numerical analysis [12, 21, 24], which show an increase in friction angle and a decrease in cohesion with the increase of the volumetric block proportion within the matrix.

The obtained c-φ in situ values are also comparable with the ones obtained with the same kind of in situ non-conventional shear tests performed on bimrocks outcropping in the Hutiao Gorge and Three Gorges (China) [20, 21].

The next step will be to perform more tests and to evaluate the actual volumetric block proportion within each test specimen in order to correlate it with the strength parameters of the relative shear test.

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