Shear behaviour of cemented pastefill-rock interfaces

Shear behaviour of cemented pastefill-rock interfaces

Engineering Geology 101 (2008) 146–153 Contents lists available at ScienceDirect Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev...

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Engineering Geology 101 (2008) 146–153

Contents lists available at ScienceDirect

Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o

Shear behaviour of cemented pastefill-rock interfaces O. Nasir, M. Fall ⁎ Department of Civil Engineering, University of Ottawa, Ottawa, Ontario, Canada

A R T I C L E

I N F O

Article history: Received 22 January 2008 Received in revised form 4 April 2008 Accepted 22 April 2008 Available online 8 May 2008 Keywords: Cemented paste backfill Interface Shear strength Stress–strain Rock Mine

A B S T R A C T The shear stress–strain behaviour and shear strength parameters of the interface between cemented paste backfill (CPB) and rock are of practical importance in the optimal and safe design of CPB structures. An understanding of the shear behaviour and properties at this interface is also required to develop comprehensive interface models for CPB-rock analyses, interface design methods for the static and dynamic stability analysis of CPB structures, and building high performance CPB structures. In this study, direct shear tests were conducted to investigate the interface shear strength behaviour between CPB and rock. All tests were carried out in a standard direct shear test apparatus for a range of curing ages of 1 to 28 days for the CPB. The procedures of the laboratory tests will be described. Results will be presented for interface shear behaviour, including stress–strain curves, vertical deformation and shear strength parameters. The test results show that the shear strength parameters and behaviour of the CPB-rock interface are time-dependent and significantly influenced by the normal load. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The removal of economically important minerals from the earth's crust is one of the main activities of mining operations. As a result, very large voids are created due to the extraction of underground ore. At the same time, large dams of tailing waste material are created at the surface. In Canada, there is approximately 500 million tonnes of tailings and waste rock that are produced annually (Amaratunga and Yaschyshyn, 1997), while in Australia, 10 million cubic meters of underground voids are generated annually as a result of mining (Grice, 2001). Therefore, mine backfill has become a common practice in mining operations around the world and can play a significant role in overall mining operations (Fall et al., 2005; Sivakugan et al., 2006). Backfilling processes return much of the waste material to the underground mine. At the same time, it can be used as a structural element that participates in ground support, as well as improving local and regional stability. This allows the mine operator to extract a greater amount of underground ore in a safer manner. There are three main types of cemented backfill: hydraulic, rock and paste fill. Each has certain advantages and disadvantages and its usage is dependent on certain aspects of the present mining methods and surrounding ground conditions (Amaratunga and Yaschyshyn, 1997). During the last decade, cemented paste backfill (CPB) or cemented paste fill has become a popular mine backfilling method around the world due to several operational and environmental benefits (Landriault, 1992; Brackebusch, 1994; Kesimal et al., 2003; Fall et al., 2007a). ⁎ Corresponding author. E-mail address: [email protected] (M. Fall). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.04.010

CPB is an engineered mixture of dewatered tailings (generally composed of fine silt-size particles between 70% and 85% solid by weight) from the milling or processing operations of the mine, water and hydraulic binders (usually 3% to 7% by weight) (Hassani and Archibald, 1998). In addition to the environmental benefit by reducing the surface tailings by up to 60% (Fall and Samb, in press), CPB helps with the mechanical stabilization of stopes and maximization of ore recovery. The stability of CPB structures is a function of many factors, such as the mechanical properties of the CPB as well as the interface shear strength between the CPB and rock mass surrounding or adjacent to the CPB structure. Fig. 1 shows an example of interactions between the CPB and rock. Several studies have been performed to characterize and better understand the mechanical properties of CPB (Williams et al., 2001; Yilmaz et al., 2004; Fall and Benzaazoua, 2005; Helinski et al., 2006; Fall et al., 2007b). However, an area that has been almost overlooked for many years is the mechanical behaviour at the interface between the CPB and rock. There is a need to increase our understanding of the shear behaviour or properties of the CPB-rock interfaces as explained below. Interfacial properties between the CPB and rock mass play a significant role in the transmission of loads from the rock to CPB. Thus, they are important components in a reliable evaluation of the static and dynamic stability of CPB structures. Furthermore, regardless of its strength, the CPB represents a soft material in comparison to the mechanical properties of the adjacent or surrounding rock. Hence, in narrow stopes, the transfer of CPB's vertical stress to the much stiffer and stronger rock walls leads to an arching effect (Janssen, 1895; Marston, 1930). When arching occurs, the vertical stress at the bottom

O. Nasir, M. Fall / Engineering Geology 101 (2008) 146–153

Fig. 1. Example of CPB-rock interactions.

of the filled stope is significantly less than that from the self-weight pressure (Marston, 1930; Terzaghi, 1943; Pirapakaran and Sivakugan, 2007) as schematically shown in Fig. 2. In other words, arching helps reduce the vertical stress in a CPB structure. For example, because of an arching effect, it is possible that a CPB with a strength (unconfined compressive strength) of 500 kPa can withstand an exposure of over 100 m in height. If arching is not present, such conditions would need 2 MPa strength, which in turn, require higher cement addition and thus, the cost of CPB operations will be high (Kuganathan, 2005). In this sense, a thorough analysis of the effects of arching during backfill operations is essential for a safe and economical design of CPB structures. The shear behaviour of the CPB-rock interface, particularly its frictional angle, is an important parameter in the evaluation and prediction of arching effects in CPB (Janssen, 1895; Marston, 1930). However, due to the lack of information about the shear strength parameters of the CPB-rock interface and technical difficulties in performing field shear tests on the CPB-rock interface, the friction angle between the CPB and rock wall is often taken as the friction angle of the CPB materials (Terzaghi, 1943; Li et al., 2003). The reason

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for this is based on the following assumptions: (i) due the frequent high roughness of the rock wall, the frictional angle of the CPB-rock interface is often higher than that of CPB material; (ii) in many cases, the large-scale roughness of the stope walls will be such that shear failure at a CPB-rock interface will involve shearing through the backfill rather along the CPB-rock interface itself (Pierce, 2001). Thus, the friction angle of CPB-rock interface is often underestimated. This underestimation will result in underestimation of the arching effect in the CPB and hence, the overall stability of the CPB (Mitchell et al., 1982), i.e. in a conservative design of CPB structures. However, in cases of smooth rock interfaces (e.g., foliated rock) the shear failure can occur along CPB-rock interfaces. Consequently, the aforementioned assumption would lead to an overestimation of the arching and shear properties of CPB-rock interface. In turn, this will result in an overestimation of the stability of CPB structures, whose failure can have serious technical, economical and human consequences. The facts that are mentioned above suggest that the analysis of the impact of the arching effect on the stability of CPB structures should take into account, the shear properties at the CPB-rock interfaces between the backfill pouring stages. In addition, in order to accurately model this phenomenon, a thorough understanding of the interface shear properties of CPB/rock from early to late age is essential. Finally, due to progressive depletion of ore available at shallow depths, mining is now being conducted at greater depths. This is naturally associated with geomechanical conditions that are more severe than those met at shallow depths. This will result in a higher significance of the impact of the interactions between the CPB and rock on the static and dynamic stability of CPB's structures. To the best of the authors' knowledge, there are no studies published on the interface shear properties of CPB-rock. A safe and economical design of CPB structures should not only consider the mechanical properties of the CPB materials, but also those of the interfaces between the CPB and the surrounding or adjacent rock. In consideration of the facts that are mentioned above, a research program has been conducted at the University of Ottawa to study the shear characteristics of the interface between the CPB and rock. One part of the obtained results will be presented in this paper. The main objectives of this paper are: - to present the results of the experimental evaluation of the shear behaviour of CPB-rock interface at various ages;

Fig. 2. Idealised presentation of the effect of arching on stress distribution in backfill mass: a) arching and schematic stress distribution in backfill mass; b) arching and qualitative vertical stress distribution in CPB.

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Fig. 3. Grain size distribution of the tailings used (SI) and the average grain size distribution of tailings from 9 Canadian mines.

- to present the results of the experimental studies of the effect of curing time on the shear strength parameters of CPB-rock interface; - to develop a basic understanding of the shear behaviour of CPBrock interface. 2. Experimental program 2.1. Test materials In order to study the required interface shear properties, two materials were used, which were CPB and rock. To achieve the objectives of this study, many samples for these two materials were prepared as follows. 2.1.1. Cemented paste backfill samples CPB samples were prepared by mixing tailings, Portland cement and water in a concrete mixer until a homogeneous paste was obtained. The water-cement ratio and the slump of the CPB mixtures were held constant at 7.6 and 18 cm, respectively. Portland cement type I (PCI) in weight proportion of 4.5% was used. The tailings used were ground silica. The silica contained 99.8% SiO2 and showed a grain size distribution close to the average of 9 Canadian mine tailings. The particle size distribution of the ground silica used is shown in Fig. 3. Tables 1 and 2 give the main physical and chemical characteristics of the silica. It is observed that the silica, with about 45 wt.% fine particles (b20 µm) could be classified as medium tailings (Fig. 3). They were well-graded with a coefficient of uniformity CU of approximately 16.2 (Table 1) and free of sulphide minerals (Table 2). The use of ground silica as tailings enables the avoiding of any potential impact of the chemical components of tailings on the shear properties of CPB-rock interface. Indeed, sulphide-rich tailings can be oxided during CPB's preparation and produce sulphate in the prepared

CPB. In turn, this sulphate can significantly affect the shear strength of CPB-rock interface. In addition, natural tailings contain various chemical species that can interact with the cement and thus, significantly affect the results of the study. The engineering properties of the CPB materials used were obtained from laboratory uniaxial compressive, triaxial, and direct shear tests. Fig. 4 shows the development of the uniaxial compressive strength of the used CPB materials with time. The strengths developed by the CPB materials were comparable to those measured in dense soil and “soft” rock. The curing time-dependent nature of the CPB may significantly affect the interface shear behaviour between the CPB and rock. 2.1.2. Rock samples Rock samples of 60 × 60 × 8 mm dimension, with a compressive strength of 27 MPa were prepared by cutting a piece of big limestone rock into the required size by using a special rock cutter. Prior to interface shear testing, a series of surface roughness measurements (using high precision LVDT) were performed on selected rock samples. The surface roughness of the rock samples was evaluated as smooth according to the profile roughness parameter Rl (Gokhal and Underwood, 1990). This smooth roughness is graphically reflected in Fig. 5, in which profiles of selected rock material is plotted to the same scale and to a larger vertical scale. 2.2. Specimens preparation All specimens were cast in specially designed molds. The molds were made of water proof cardboard. Fig. 6 shows a cross section diagram of the mold. After casting, the molds were covered with a plastic sheet (to avoid any water evaporation) and cured in a controlled temperature room at 20 °C for 1, 3, 7 and 28 days. Just before the test, samples were removed from the molds for testing (Fig. 7).

Table 2 Main chemical elements in the tailings used

Table 1 Physical properties of the tailings used Element unit

Gs

D10

D30

D50

D60

D90

Cu

Cc



µm

µm

µm

µm

µm





Silica

2.7

1.9

9.0

22.5

31.5

88.9

16.2

1.3

Element unit

Silica

Al

Ca

Si

Fe

Na

Pb

S

K

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

0.1

b 0.01

99.8

b 0.01

b0.01

0.0

0.0

0.0

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Fig. 6. Mold of preparing the specimens.

Fig. 4. Impact of curing time on the strength of the CPB used.

2.3. Testing technique 2.3.1. Testing apparatus Despite some inherent problems (e.g., principal stress rotation, stress non-uniformity, failure plane definition), the direct stress shear apparatus is a commonly used device for interface testing because of its simplicity and suitability for interface testing (Miller and Hamid, 2004). Therefore, a standard direct shear test machine was used for conducting the shear strength test on the CPB-rock interface. The schematic diagram of the direct shear machine used is shown in Fig. 8. 2.3.2. Testing procedures The prepared specimen was installed in the shear box in such a way that the bottom halve contained the rock material, while the top halve contained the CPB (Fig. 9). The interface between CPB and rock was located exactly between the two halves of the shear box as shown in Fig. 9. A normal load (constant normal load, CNL) was applied to the sample and then the shear force applied to it. The normal load acting on the interface remained constant during the shearing process. Each test was conducted by using a rate of shear deformation of 0.5 mm/min. All data regarding the test (shear force, shear and normal displacement) were collected by using a computerized data logging system. The results were monitored and saved by using the computer software LabView. 2.3.3. Testing plan Interface shear tests were conducted on CPB-rock specimen cured at 1, 3, 7 and 28 days. For every CPB-rock sample, three tests were performed by applying normal stresses of 50, 100, and 200 kPa. Similarly, direct shear tests were conducted on CPB samples for the objective of comparison (Table 3).

results are presented by using three types of graphs: the shear stress– shear strain curves, the shear strain–vertical deformation curves and the shear strength envelopes, which give the interface shear strength parameters (cohesion or adhesion, c, friction angle, φ). Fig. 10 shows typical shear stress versus strain relationships for the interaction between rock and CPB cured at one day. The stress–strain curves from direct shear tests performed on the CPB cured at one day are also presented in Fig. 11 for comparison. The shear stress–strain curves of the CPB-rock interface shear tests show a relative similar trend and shape to those for the CPB direct shear tests (Figs. 10 and 11). It can be observed that the interface shear stress gradually increased with shear strain until it reached the maximum shear stress, after which it became relatively constant with respect to shear displacement. The stress–strain curve showed slight to no post-failure strain softening. The residual state was not reached in any of these tests (residual shear condition could not be achieved in these tests). The reason is the rearrangement of the tailings particles in the interface shear surface was restricted at a shear deformation of 12%. It can be seen that as the normal stress increased, the maximum interface shear stress and the corresponding shear strain increased. This is due to the fact that an increase in normal stress on the interface shears plane resulted in increased contact area between the rock surface and the tailings particles and hence, increased frictional resistance between the tailings particles and rock surface. This led to a higher maximum shear stress. It can also be noted that the CPB–rock interface offered a slight increase in initial shear stiffness with normal stress. Fig. 12 illustrates typical results of shear deformation versus vertical deformation of the interface between rock and CPB cured at one day. The results clearly indicate that the vertical deformation of the interface could be globally classified into two stages during the course of shear displacement for given normal stress. For low normal stress (100 kPa), at the first stage, the CPB material underwent a

3. Results and discussion Typical direct shear test results for different CPB-rock interfaces are presented and discussed in this section. In general, shear test

Fig. 5. Roughness profile of the rock used (L: profile length; Lo: straight length).

Fig. 7. Specimen of the CPB and rock sample.

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Sample

1 day

3 days

7 days

28 days









⁎Three individual tests with three normal stresses 100, 150, and 200 kPa.

Fig. 8. Schematic diagram of the direct shear machine.

vertical contraction associated with a corresponding increase in shear stress. This contraction could be attributed to the fact that at very early ages (one day curing) the cementation of the tailings particles was weak. Consequently, at the beginning of the normal and shear loadings, more contacts between the tailings particles themselves as well as between the tailings particles and the rock were created, and the local voids became smaller. A crushing of large tailings particles might have contributed to contraction (Hoteit, 1990; Jensen et al., 2001). A larger vertical deformation was observed for the specimens under a higher applied normal stress (150, 200 kPa) due to higher reduction of local voids associated with higher applied normal stress. At the second stage, for low applied normal stress (100 kPa), as loading progressed, the specimens exhibited expansion due to dilatancy. This dilatancy could be mainly attributed to the sliding of the tailings particles over the top of the rock asperity. During this dilation period, the contact area between the tailings particles and rock surface reduced progressively with increasing shear displacement, resulting in a further increase in shear stress. The shear stress continued to increase until failure of the rock asperities through shearing (Plesha, 1987; Kamari and Stead, in press). However, from Fig. 12, it can be noted that normal dilation at the ultimate depended on the normal stress level. For normal applied stresses of 150 and 200 kPa, no dilatation was observed after the contraction phase (in the studied range of shear strain or displacement). The possible reason for this is that the relative high applied normal stresses degraded the rock asperities and thus, altered the dilation of the interface. Fig. 13 shows typical shear strength envelopes for one day CPBrock interface. The shear strength envelope for one day CPB is also presented for comparison. These envelopes were obtained by fitting linear regression lines through each set of interface shear stress vs. normal stress data. For all regression lines, R2 was approximately 0.99. The shear failure envelope for the one day aged interface obeyed the

Fig. 9. Specimen of CPB and brick sample installed in the bottom halve direct shear box.

Mohr–Coulomb failure criterion: shear stress increased linearly with applied normal stresses. Thus, the following Mohr–Coulomb type equation was used to obtain values of interface friction angle (φ) and adhesion or cohesion (c) for the range of normal stress: τ ¼ c þ σ n tanðuÞ;

ð1Þ

where τ is the interface shear strength and σn is the total normal stress. Peak friction angle at the interface of the one day aged samples was only 19.6°, while the adhesion was 28.7 kPa. A comparison of the interface shear strength parameters (φ = 19.6°, c = 28.7 kPa) with those of one day CPB (φ = 28.1°, c = 27.0 kPa) material underlines that the interface frictional angle is about 35% lower than the frictional angle of the CPB material. This finding is significant since it will mean that in the case of smooth rock interface, the shear strength parameters of the CPB materials should not be used as those of the CPB-rock interface. Figs. 14–16 summarize the mechanical behaviour of the interface between rock and CPB cured at more at 28 days. From these figures, it can be noted that the mechanical behaviour of 28 days CPB-rock interface was relatively different from that of one day. Fig. 14 shows typical results of shear stress, which is shear strain curves of the interface between the rock and the 28 days CPB at different normal stresses (100, 150 and 200 kPa). It can be seen from this figure that the interface shear stress first increases with shear displacement until it reaches an initial peak stress which appeared with a range of 0.5–1.5% strain (part A). This behaviour may be explained by the fact that as the shear stress increases from zero to the first peak shear stress, major asperities on the rock and CPB sides has a tendency to come more into contact, thereby changing the “at rest” contact positions. Furthermore, some small elastic deformations are likely experienced by the asperities. However, there was no significant relative movement between the CPB and the rock. Similar observations were made on rock-concrete interface shear tests (Kodikara and Johnston, 1994). The first peak shear stress observed at the end of part A may be due to the failure of the cementation bounding force between the CPB and the rock surfaces. After this first peak, the interface shear stress remains relatively constant or slightly decreases as the shear displacement increases (part B). The reason can be that the mobilization of the full frictional resistance of the CPB-rock interface occurs, thereby resulting

Fig. 10. Shear stress versus horizontal deformation of one day CPB-rock interface.

O. Nasir, M. Fall / Engineering Geology 101 (2008) 146–153

Fig. 11. Shear stress versus horizontal deformation of one day CPB material.

in relative sliding between the CPB and rock surfaces. The slight decrease of shear resistance observed in part B might result from the remoulding of micro-asperities and some tailings particles from the CPB surface. Part B is followed by an increase of the stresses (part C) until mobilization of the peak interface shear stress at shear displacement of about 7–11%. After that, the shear stress remains relatively constant (for normal stress = 200 kPa) or slightly decreases (for normal stress ≤ 150 kPa) as the shear strain increased for the studied shear displacement (part D). The behaviour observed in part C can be attributed to the fact that the increase of shear stress causes the interface to dilate as discussed in more detail below and shown in Fig. 15. During this period, the asperities contact area or the contact area between the CPB and the rock surfaces reduces progressively with increasing shear displacement. This reduction results in an increase in the contact normal stress. The interface continues sliding until a critical displacement where the local stress (critical stresses) of the asperity exceeds the strength of asperity on the CPB surface. At this critical local stress, the major CPB's asperities can no longer carry the loading and a failure of the interface occurs. The asperities on the CPB surface will be probably the most affected or altered due to the relative low strength of the CPB in comparison to the rock. This assumption is supported by post-shear visual observations of the rock surfaces. These surfaces were covered by several tailings particles that mainly resulted from the crushing of the asperities on the CPB surface. This suggests that in part D, the shear behaviour of the CPB/rock interface is likely mainly controlled by the sliding or rolling of the tailings particles over the rock surface and friction between the tailings particles themselves and/or the rock surface. This slight decrease of post failure shear stress observed for low normal stress may suggest

Fig. 12. Horizontal deformation versus vertical deformation of one day CPB-rock interface.

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Fig. 13. Shear stress vs normal stress for CPB-rock interface and CPB material (one day curing time).

that bonding between tailings particles was still preserved at an axial strain of 7%. Typical results of vertical deformation versus shear displacement of the interface between rock and the CPB cured at 28 days are shown in Fig. 15. It can be observed that for high normal stress (200 kPa), the CPB-rock interface exhibited a contracting behaviour. The reason for this behaviour is mainly the fact that the high normal load (200 kPa) applied to the interface led to significant alteration of the asperities and also to the reduction of the local voids due to the compression of the CPB materials adjacent to the interface. However, from this figure, it can be observed that for lower normal stresses (100, 150 kPa), the evolution of vertical strain shows a transition between contracting and dilating behaviour. The CPB-rock interface first exhibits a short contracting behaviour followed by dilatation. The contracting behaviour can be attributed to the fact that as soon as a normal load was applied, the surfaces of the CPB and rock moved closer together. The reason is that at the beginning of the normal and shear loadings, more contact between the asperities on both surfaces of the CPB and rock are created, and the local voids become smaller. The compressibility of CPB could be an additional reason for the observed contractive behaviour. The observed dilating behaviour is the result of the relative movement of the asperities as observed. The possible reason of the slight decrease of dilatancy observed in the 150 kPa curve from 2% to 5% shear strain values may be a partial damage of the weaker asperities on the CPB surface. The observed increasing dilatancy at failure can be viewed as the remaining cementation bonding between the tailings particles. Fig. 16 shows typical interface shear strength envelopes for 28 days CPB-rock interfaces. These envelopes were obtained by fitting a linear

Fig. 14. Shear stress versus horizontal deformation of 28 days CPB-rock interface.

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Fig. 15. Horizontal deformation versus vertical deformation of 28 days rock-CPB interface.

regression line through the set of interface shear stress vs. normal stress data. The peak strength envelope provides excellent straight line fits with R values of 0.998. For each regression line, values of interface friction angle and adhesion were obtained by using Eq. (1). The results are consistent with expected trends in behaviour in that the shear strength increases with increasing net normal stress. The comparison of the results of interface shear tests presented above suggest that the curing time of CPB has a significant impact on the mechanical behaviour at the interface between CPB and rock. This influence is illustrated by Figs. 17–19. Shear stress versus horizontal displacement curve for the CPB-rock interface for different curing is shown in Fig. 17. It can be seen from this figure that more curing time gives greater peak, as well as higher modulus of elasticity for the interface shear stress. The strain at peak stress is lower for higher curing time. In addition to that the shape of the curve for higher curing time (28 days) starts to behave less plastic with an obvious peak stress. The reason for this behaviour is the increasing in the degree of hydration with time. Indeed, when the age of the CPB increases, CPB tends to harden due to the progression of binder hydration. This hardening will result in stronger asperities on the surface of the CPB, thereby increasing the value of the critical stress needed to break these asperities. This will consequently lead to the increase of the internal friction angle of the CPB-rock interface with the CPB's age as shown in Figs. 18 and 19. Figs. 18 and 19 show the impact of CPB's curing time on the shear envelopes of the CPB-rock interface, as well as on its friction angle and cohesion, respectively. It can be observed from Fig. 18, that regardless of the curing time, the strength data can be well described by a Mohr– Coulomb failure criterion. It can also be seen from this figure that at

Fig. 16. Shear stress vs normal stress for CPB-rock interface and CPB material (one day curing time).

Fig. 17. Shear stress–shear strain curves of CPB-rock interface under a normal stress of 150 kPa for different curing time.

Fig. 18. Shear envelopes of the CPB-rock interface vs curing time.

early ages (≤7 days) the CPB-rock interface showed relatively similar cohesion (the slight decrease of the value of cohesion at 3 days can be attributed to shear test uncertainties and experimental variations from sample to sample). From the ages of 7 days to 28 days (more advanced age), this cohesion increased by about 20%. This can be particularly attributed to a higher cementation binding force between the CPB and the rock surface due to higher degree of binder hydration. Results also show that there is a general trend of increase of internal friction angle of CPB-rock interface (Fig. 19) with increasing curing

Fig. 19. Effect of curing time on the friction angle and cohesion of the interface CPB-rock interface.

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time. This can be explained by the fact that a higher binder hydration with time will lead to the formation of more hydration product and thus, to stronger asperities on the CPB surfaces, thereby increasing the shear strength via an increase in dilatancy at failure. 4. Summary and conclusions The purpose of this study is to evaluate the interface shear behaviour and strength between the CPB and rock by using a direct shear test apparatus. Comparisons were made between the interface shear test results and those from direct shear tests on the CPB materials. The shear failure envelope for the studied CPB-rock interfaces obeys the Mohr–Coulomb failure criterion. For the studied interfaces, the shear strength of the CPB materials is greater than that of the CPB-rock interface for similar stress conditions. This finding means that in cases of smooth rock interfaces assuming the shear strength parameters of the CPB materials as those of the CPB-rock interface will lead to unsafe design of CPB structures. It was also shown that the shear behaviour of CPB-rock is highly influenced by the level of the normal stress. For example, for high normal stresses (≥200) the interface shows a contracting behaviour regardless of the curing time. The magnitude of the dilation is highly dependent on the magnitude of the applied normal stress. Under low applied normal stresses, as expected, dilation is the greatest. The interface shear behaviour was also found to be strongly dependent on the age of the CPB material. The shear strength parameters (friction angle, cohesion) of the CPB-rock interface generally increase with curing time. Results that are presented in the paper will enhance the understanding of the shear behaviour of the CPB-rock interface that is crucially important in stability analysis of underground CPB structures. However, this study is performed on a limited range of interface roughness. It is believed that the interface roughness has a great influence on CPB-rock shear behaviour. Further research is needed to provide a better understanding of the behaviour of the CPB-rock interface for a wider range of surface roughness. The research in this area was not implemented in the project for this paper. However, it is currently being performed in a new project. Acknowledgments The authors would like to acknowledge the department of Earth Science at the University of Ottawa and IRND. References Amaratunga, L.M., Yaschyshyn, D.N., 1997. Development of a high modulus paste fill using fine gold mill tailings. Geotechnical and Geological Engineering 15, 205–219. Brackebusch, F.W., 1994. Basics of paste backfill systems. Mining Engineering 46, 1175–1178. Fall, M., Benzaazoua, M., 2005. Modeling the effect of sulphate on strength development of paste backfill and binder mixture optimization. Journal of Cement and Concrete Research 35 (2), 301–314.

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