Creep Damage Characteristics of Soft Rock under Disturbance Loads

INTRODUCTION

Surrounding rock of underground chambers is subjected to geo-stress or static stress, as well as under the dynamic loading in engineering practice.Due to excavation, stress of surrounding rock is quite great in deep mining; the surrounding rock is in high stress state.Within a certain range of chamber, rock is in or close to limit damage state.Dynamic loads were superimposed on them, and will greatly influence the stability of the surrounding rock.

Disturbance load originates from blasting, excavation and other engineering activities (shaft station, roadways haulage) vibration caused by these disturbances, and its effect of mode is short time stress wave.Dynamic loads are random, such as uncertain frequency and intensity, which compared with static loads are small.

Mechanical experiment is an efficient means of exploring the evolution law of internal damage during rock compression.The crack distribution on a rock surface in a certain stress state can be observed by optical or electronic microscope (Fu, 2007; Ren, 2001; Kawakata et al., 1999).Furthermore, the crack distribution on an internal cross-section under a certain stress state can be established using the computerized tomography (CT) technique (Feng et al., 2004; Li et al., 2004; Kawakata et al., 1999).Yang et al. (1996) and Li et al. (2007) described obtaining CT images by scanning the damaged rock specimen at a single given stage.This procedure cannot obtain real-time CT images at different stress stages during the full process of rock damage evolution.Analysis of CT value distribution and mathematical model of rock damage were made in detail, and relationship of CT value and rock damage variables was established (Chen et al., 2004; Ren, 2004; Ge et al., 1999; Helman, 1985).

EXPERIMENT

The rock was the Tertiary gray green mudstone (Fig. 1) collected from the Beizao colliery under the sea, 5 km away from Longkou City in Shandong Province, China.

Figure  1.  Photographs showing the grey green mudstone from the cores.

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The bulk density is 26.36 kN/m³, moisture content is 0.25%.The rock samples are processed into a cylinder with a diameter of 50 mm and height of 100mm (Fig. 1).Impact rock samples along spindle by disturbance loop with masses of 2.5 kg, 5 kg, 7.5 kg and 10 kg, respectively.Load impulse dropping from different heights was produced by different mass disturbance loops.Impact velocity is 4.75–4.87 m/s, dropping height is 10–50 cm.Rock samples had been impacted from the same dropping height until they were damaged.

At present, about study on effects of disturbance to creep by using different mass disturbance loops, there is still no theory and means to measure object collision time accurately, and so using disturbance load energy ∆W to express impact load is more practically significant.Mass of disturbance impacting loop is M, and dropping height is h.According to impulse theorem and energy theorem, ignoring energy consumption, gravitational potential energy of disturbance loop impact can be seen as converting into impulse, ∆W is disturbance load energy of unit area, which is expressed as

where A is sectional area of samples (m²); g is gravity acceleration, g=9.8 m/s².Substituting data into equation (1), we get equation (2), the parameters are shown in Table 1.

Table  1.  Parameters of impact disturbance energy

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The tests have been conducted at the CT Laboratory in Shandong University of Science and Technology and Shandong Coal Sanatorium.The X-ray Spiral CT scanner, SIEMENS SOMATOM plus made in German and triaxial loading systems developed were used.

The space resolution of the CT machine is 0.35mm×0.35 mm, identifiable minimum volume is 0.12mm³, and density contrast resolution is 0.3%.The basic performance parameters of spiral CT are voltage is 137 kV, electric current is 220 m A scanning thickness is 0.1 mm, and the amplification coefficient is 6.0.

Before testing, the rock samples are put into the triaxial pressure cell, which is in turn put on the CT bed horizontally (the CT bed can be moved vertically or horizontally).Initial positioning and CT scanning of the rock samples are needed under no confining pressure.There are 5–10 scanning layers.The confining pressure is gradually increased to 5 MPa and then fixed.Then, the axial stress is applied with the given loading rate (the average loading rate is0.1×10^-3 mm/s) by step loading method until the specimen breaks.Real-time CT scanning for selected cross sections has been conducted under different disturbed loading states.The scanning thickness is10–20 mm.

CT SCANNING IMAGES AND CREEP DEFORMATION OF GREY GREEN MUDSTONE

Micro-characteristics of gray green mudstone are shown in Fig. 2.The crystal structure is dense, mudstone-like structure.

Figure  2.  Microphotographs of EMS showing the micro-characteristics of grey green mudstone.(a) Micro pore developed (×300); (b) micro porosity of interlayer is 5-20µm and with high porosity and permeability (×1 320); (c) flake clay is mainly composed of smectite, interlayer quartz, few leaf-shaped feldspars crystal (×3 300); (d) intergranular, albite granules and granular chlorite (×3 860).

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CT images can be divided into four evolution stages, that is, uneven change stage, local crack evolution stage, penetrating crack evolution stage, and fail stage.

Uneven change stage (σ₁= (0–2.85) MPa, disturbance load energy is in the range 0–2 500 J) (Fig. 3a, 3b).Five scanning cross sections have no apparent changes in this stage; the evolution law of CT number shows that variance is similar in every point.

Figure  3.  CT images showing scan sections (scanning layers from (a) to (h)) under different disturbing energy conditions of grey green mudstone; disturbance load energies are 2 500 J, 5 000 J, 7 500 J, 10 000 J12 500 J, 15 000 J, 17 500 J and 20 000 J, respectively

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Figure  4.  Diagrams of axial creep curve showing scan sections (a–h) under different disturbing energy conditions of grey green mudstone.

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Local crack evolution stage (σ₁= (2.85–4.59) MPa, disturbance load energy is 7 500 J) (Fig. 3c).Rock samples enter into a state of crack evolution on the CTscale.Firstly, low-density anomalous points appear in the upper right of the third scanning layer (Fig. 3c); low-density points occur under the first scanning layer; two points develop slowly and there is a zigzag ribbon distribution, which connects on the first scanning layer and the second scanning layer; the crack surface is parallel to the orientation of pressure stress.

Penetrating crack evolution stage (σ₁= (4.59–5.03) MPa, disturbance load energy is over 10 000 J) (Fig. 3d).When σ₁ comes to 6.59 MPa and disturbance loads energy is 15 000 J (Fig. 3f), branching cracks occur, which is perpendicular to the original cracks with two cracks vertically penetrating the rock sample quickly thereby branching the crack parallel to the pressure, and the stress-strain curves fluctuate.Subsequently, the penetrating crack evolutes slowly, becoming wide along the transverse direction, and some irregular cracks emerge on the fourth layer and the fifth layer cross sections, and extend toward the second layer and the third layer.When the stress reaches the limit strength, and if disturbance loads are added continually, then the rock suddenly fails.According to failure modes, there is strength loss because failure sliding occurs along the main crack plane.

As far as the microstructure is concerned, CT values of the first and second layers increase and the pore decreases, but transverse swelling failure is more obvious in the third scanning layer.At the same time, CT variances of every layer increase.With the increase of disturbance loads, particles'density reduces, and pores increase in every layer; the arrangement of rock particles and pore is from initial disorganization to orientation.It is close to instability and failure stage of rock (the fifth and sixth scanning layers).Rock fails fully after a transient pause phase (the seventh and eighth scanning layers).

THE DAMAGING EFFECTS OF SOFT ROCK UNDER DISTURBANCE LOADS

Rock damage variable equation was represented using CT values, according to the reference published report; the computational method is shown as follows

where D_creep is creep damage variable rock under disturbance load; m₀ is the space resolution of CT machine; the CT value of the initial rock density is H₀; H_d is the CT value of the damage rock density during the disturbance load process of rock material.

Damage variables of different damage development stages under triaxial compression were calculated by equation (3).Stress, strain and damage variables at different damage development stages are given in Table 2.

Table  2.  Stress, creep and damage for grey green mudstone under triaxial loading condition

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Axial strain increases with stress increases under triaxial compression for every sample.With the extension of damage, the strain and the stress increase and the damage variables increase accordingly.

According to Tables 2–4, when CT mean values decrease with disturbance loads increasing, creep damage level increases.It shows that the rock sample is compact, and new micro fissures and holes occur.Disturbance load is applied, and the micro-cracks increase continuously, and they extend from the center to circumference explosively; finally, rocks fail.From the rock appearance, it is found that uneven deformation and local swelling occur during this stage.Damage occurs on the local swell.

Table  3.  Results of CT value and creep of rock specimen

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Table  4.  Calculation results of creep damages for rock under the disturbance impact load

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The relationship of cross-section density damage, disturbance load and axial strain are shown in Fig. 5 by curves.

Figure  5.  Diagram showing relationship between density damage increment, axial creep and disturbance load energy of scanning cross sections of grey green mudstone.

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According to Fig. 5, the density damage increment D in four scanning layers increased or decreased as axial stress and disturbance load energy increased, corresponding to the rock compacting and dilating, respectively.When the first scanning cross section comes into the dilation stage, other scanning cross sections are still compact; it means that there is partial dilation in the entire compacting stage.The inflection points of curve D are transition points of densification and dilation, and corresponds to images of D.Images indicate that macro-cracks are generated, and extend from up to down.Rock dilation is the essence of crack strengthening causing local volume to swell.CT dimension crack initiates on dilation location.Densification quantity is large at the stages where confining pressure is exerted and there is initial compacting; rock pore is big and the rock properties are soft.It is the transient dilation stage.In the early evolution phase of CT dimension crack, curve is straight, and the swelling effect caused by longitudinal compression and transverse expansion is cancelled.At the late evolution phase of CT dimension crack, density damage increment reduces due to the cracks extending obviously.

CONCLUSIONS

Testing results have indicated that when axial load comes into limit strength neighborhood, rock micro cracks link into larger cracks, and the creep rate increases in a short time; larger plastic deformation occurs and this is called as disturbance accelerating creep stage.When the rock is within limit strength neighborhood, the rock begins with creep micro-damage under smaller disturbance load (disturbance energy is 2 500–5 000 J).If disturbance load is larger, they directly enter into disturbance accelerating creep stage, with failure occurring instantaneously.