Effects of low temperatures on strength and power input into rock failure

Evgenii V.Zakharov ,Alexander S.Kurilko

Federal State Academic Institution "Chersky Mining Institute of the North",Siberian Branch of the Russian Academy of Sciences,Lenin Avenue 43,Yakutsk 677980,Russia

1 Introduction

Most researchers do not consider mechanical properties of igneous and metamorphic rocks to be associated with their temperature.The ultimate strength of sedimentary solid rocks increases with freezing (Burstein and Kurochkin,1965;Daydkin,1968;Elchaninovet al.,1970;Skuba,1974;Senuk,1983;Kozeevet al.,1995).

In loose water-saturated rocks,strength increases with a temperature decrease from 0 °C to-30 °C,and the rocks transfer to the category of hard rocks (Tsitovich,1973;Shusherinaet al.,1974;Rzhevskaya,1989).Rock strength is associated with cementing effects of ice and is dependent on temperature,humidity,and quantity of unfrozen water.

According to experimental results of enclosing solid rocks (Burstein and Kurochkin,1965;Dyadkin,1968) from coal deposits of "Sangarskaya" and "Arkagalinskaya",the ultimate strength of compressed rocks is not temperature dependent,and increases by no more than 25%–56% at a freezing point equal to-20 °C.Also,the ultimate tension strength increases to 2–3 times as compared with that at the melting point.

As evidenced by previous studies (Inada and Kinoshita,2003),the compression strength of air-dry and saturated granite samples increases by 20%,35%,and 45% at-10,-60,and-160 °C,respectively as compared with the strength at positive temperatures.Andesite and sandstone strength increases by 15%–30%at-160 °C as compared with the indoor temperature strength.

Dolomite limestone strength tests taken from flanks of the open-pit mine of "Mir",Russia (Kozeevet al.,1995),demonstrates that a dramatic strength increase of 40%–100% occurs when the freezing point of interstitial moisture is within the range of 0 °C to-5 °C.However,Rosenbaum (1981) noted that either an increase or decrease of solid rock compression strength can occur with freezing.At-5 °C,independently of effective porosity,the strength of medium sandstone frozen samples increased by 17%,of slate coal by 34%,and that of coal by 45%.A-5 °C temperature decrease of water saturated samples of marble,limestone,diabase,slate,medium sandstone,and lime-sand brick results in a strength decrease of 40%,33%,14%,20%,27%,and 25%,respectively.On average,a 30% decrease of sample strength occurs with lowering of the indoor temperature to-5 °C.

Goldaevet al.(1977) found that with cooling of hard rocks from the indoor temperature to-50 °C and to-90 °C,a 30%–50% decrease of the ultimate tension strength is observed.With a further temperature decrease,the tension strength increases,and at-150 °C is equal to the ultimate strength at the indoor temperature.The ultimate tension strength of hard rocks remains practically unchanged or decreases insignificantly with a temperature decrease of-50 °C to-80 °C.A further temperature decrease leads to an increase of the ultimate tension strength,and at-150 °C it increases by 50%–70% than at the indoor temperature.

Some investigations show that a temperature decrease can bring about changes in the power input into rock failure,alongside strength alterations (Shekhurdin,1999).Rzhevsky and Novik (1984) noted that at temperatures lower than-150 °C,the specific power of gabbro-diabase and various sandstones is 4–6 times lower than the failure energy at positive temperatures,although the ultimate strength of these rocks increases by 10%–70% at low temperatures.It has been shown experimentally (Moskalevet al.,1987) that an indoor temperature decrease to-60 °C and to-80 °C leads to a granite strength decrease of 15% and lowering of its failure energy by 50%.Thus,according to the majority of researchers,at temperatures below 0 °C,rock compression strength is independent of temperature,or increases by 5%–50%,while rock tension strength increases to 2–3 times as compared with that of melted rocks.

The objective of this work is to study in detail and to define the pattern of alterations of strength properties of carbonate rocks and kimberlites from diamond deposits of Yakutia at low temperatures.We also describe the impact of low temperatures on the compression strength (0 °C to-50 °C) of rocks and the specific power input into rock failure (0 °C to-20 °C).

2 Rock strength

Argillaceous dolomite and limestone,bituminized argillaceous limestone,marly dolomite,and bituminized limestone samples were examined from mine dumps of broken muck at a 125 m horizon of an open-cast mine of "Udachnyi" (the joint-stock company "ALROSA").

650 cube-shaped samples with 30 mm edges have been manufactured from large rocks.Some of the cubes are homogeneous,some bear evident layers.Rock specific density is 2,786 kg/m3,and open porosity of the examined samples comprises 1%–7%.The rocks are saline,while the content of soluble substances defined by the solid residue of fine-grain components of broken rock equals 2.7%.

The diamond pipe "Udachnaya" kimberlite and host rock samples,both of equal size,were taken from two cores stored at the Institute of Diamond and Precious Stones (Siberian Branch,Russian Academy of Sciences).The volume density of the kimberlite samples is 2,530–2,650 kg/m3,while their open porosity comprises 4%–6%.Also,the moisture content of the samples in an air-dry state is 0.5%–1.1%,while in water-saturated samples it is 1.7%–2.1%.

One set of limestone and kimberlite samples were tested in an air-dry state,while another set in a water-saturated state.The test was conducted at 20,-5,-20,and-50 °C.The samples were placed in a freezing chamber and were kept at-5,-20 and-50 °C for five hours,and each test was repeated 5–10 times.

Uniaxial compression strength of rocks was determined in the installation UTS-250 (UTS test system,Zwick upgrade 2006,Germany).The samples were loaded in a position parallel to stratification,with 15 mm thick steel layers inserted between the samples and shackles,all cooled to the same temperature within in a freezing chamber.The speed of the testing machine was 2.5 mm/min,with results presented in table 1 and figure 1.

Table 1 Temperature-dependent average compression strength of rocks (MPa)

Figure 1 shows the correspondence of rock strength alterations at the test temperature of +20 °C(σТ/σ20).In constructing the diagram of strength dependence with temperature,it was assumed that at positive temperatures the rock strength corresponds with the value obtained at the standard test temperature of +20 °C.

With a temperature decrease to-5 °C,sample strength dropped by 4%–66%,with an average decrease of 35%.With a temperature decrease to-20 °C,rock strength continued to lower,dropping by 35%–75%.At-20 °C,marly dolomite samples had the lowest relative strength of 25% at the indoor temperature,with an average decrease of 53%.With a further temperature decrease to-50 °C,rock sample strength increased,with an average value reaching close to the sample strength at +20 °C.The samples of marly dolomite had strengths of 64% higher than that at the indoor temperature.

Figure 1 Temperature-dependent relative compression strength of air-dry rocks.1:argillaceous dolomite;2:argillaceous limestone;3:bituminized argillaceous limestone;4:marly dolomite;5:bituminized limestone

Sample strength tested in a water-saturated state is 23%–73% lower (average of 44%) at-5 °C,and by 36%–66% (average of 38%) at-20 °C,than the strength at +20 °C.According to our results (Table 1),rock strength (melted or frozen) decreases with water saturation (100%).

Figure 2 presents the correspondence of average kimberlite strength and temperature.Similar to host rocks,average kimberlite strength is reduced by 45%with an indoor temperature change of-10 °C to-20 °C.A further temperature decrease results in increased kimberlite strength.Figure 2 shows that average kimberlite strength in a water-saturated state at any temperature decreases by 25%–50% than the strength in an air-dry state.

Experimental results show that an indoor temperature decrease of-5 °C to-15 °C results in a host rock strength reduction of more than 50% (of kimberlite –by 45%).Further temperature decrease leads to increased rock strength,and on average,reaches rock strength values at indoor temperatures,i.e.,within the range of-5 °C to-15 °C there is a local minimum of rock strength.This strength decrease is significant enough to be incorporated into engineering calculations (Kurilko,2004;Kurilko and Novopashin,2005).

Figure 2 Kimberlite strength dependence on temperature

3 Power input into rock failure

The impact of low temperatures on rocks can change their strength characteristics,with the resultant power change applied to rock failure.

This study examined the physical properties of limestone from Udachnyi and Mokhsogollokh open cast mines,and kimberites from Internatsionalnaya and Udachnaya diamond pipe (Table 2).

To determine impacts of low temperatures on the specific power input into rock failure,a special method(Zakharov and Kurilko,2009;Zakharov,2012) has been elaborated based on the usage of an impact testing machine (Baronet al.,1963).

The specific power input of rocks has been determined by the correspondence of power applied to mechanical grinding with the area of a newly formed surface.The grinding equipment consists of an impact tension machine and volumenometer.Weighed amounts of the samples from each group are placed in turn into the charging bucket of the impact tension machine and are ground by discharge of the 2.4 kg load of 0.6 m height.

Table 2 Major physical properties of the samples

The number of load discharges was selected so that the output of a fraction identified with the help of the volumenometer of less than 0.5 mm did not exceed 15%–20% of the volume of material to be ground.In our tests,a load to the weighed amount of the samples was discharged five times.

Argillaceous inclusions were washed from the original rock samples and dried at +20 °C.The dried samples were grinded to a coarseness of 10–20 mm and then were quartered.The weight of each sample was 50 g.The samples were tested air-dry,water-saturated,and saturated with 5% NaCl solution.The samples were kept in an aqueous brine medium for two days to ensure pore saturation.After saturation,the samples were taken from the medium,wiped and placed into a freezing chamber together with air-dried samples.In the chamber,the samples were kept for six hours at-5,-10,-15,and-20 °C.Attainment of the preseted temperature was controlled with a thermal sensor placed in the rock being frozen.

The frozen samples were withdrawn from the freezer and immediately subjected to grinding in the impact testing machine.Grinding was performed in a non-heated building,at close to freezing temperatures.The ground material was subject to the size test to determine surface area resulting from grinding.Using data on grinding power and formed surface area,the specific power input into material failure was calculated.The tests were repeated five times.

Originally,at +20 °C,the specific power input into rock failure was 4,060 J/m2for Udachnyi and 3,500 J/m2for Mokhsogollokh open cast mine limestones,and 2,100 J/m2for Internatsionalnaya and 3,200 J/m2for Udachnaya diamond pipe kimberlites.The relative error in reference to the rock samples in an original state atα= 0.95 was no more than 4%.

Test results of the specific power input into rock failure are presented in figures 3–6.Here,the power input obtained at +20 °C was 100%.Data analysis suggest that at-5 °C and-15 °C the specific power input into carbonate rock failure at Udachnyi and Mokhsogollokh open cast mines,and kimberlites at Internatsionalnaya diamond pipe are reduced by 10%–40% than at positive temperatures.

Figure 3 presents the relative power intensity of limestone failure at Udachnyi in correspondence with temperature fluctuations.A temperature decrease from+20 °C to-5 °C results in a 25%–40% decrease of the specific power input of the samples in correspondence with preparation conditions of the test samples.A further temperature decrease leads to an increase of the failure power input,and on average remains lower by 13% than the power input at +20 °C.

The degree of the temperature effect on the specific power input into rock failure is associated with rock porosity.On average,in reference to the Udachnyi open-cast mine limestone with 12% porosity,the specific power input failure is 35% lower at-5 °C than at positive temperatures (Figure 3).The specific power intensity for the Mokhsogollokh limestone (Figure 4)with 1% porosity is at its minimal at-10 °C and on average,is 15% lower than that at a low temperature.

Figure 5 presents the relative specific power intensity failure kimberlite at Internatsionalnaya diamond pipe,showing that a temperature change from +20 °C to-20 °C results in a 15% decrease of the relative power input failure of air-dry kimberlite samples.

Testing of kimberlite frozen samples saturated in an aqueous brine medium suggests a significant decrease of power intensity failure.The maximum decrease of power consumption is observed at-5 °C,and the specific power consumption is 35%–40% lower than that at a positive temperature.A further temperature decrease to-20 °C is followed by a power consumption increase that remains at 20%–25% less than that at +20 °C.

In the view of its limited quantities,kimberlite from "Udachnaya" diamond pipe (Figure 6) was tested only in an air-dry and water-saturated state.Freezing of water-saturated kimberlite samples at-5 °C results in a 17% decrease of the specific power intensity failure.A further temperature decrease to-20 °C leads to an increase of power intensity failure.

Figure 3 Relative specific power input into limestone failure of the Udachnyi open cast mine

Figure 4 Relative specific power input into limestone failure of the Mokhsogollokh open cast mine

Figure 5 Relative specific power input of kimberlite failure at Internatsionalnayae diamond pipe

Figure 6 Relative specific power intensity of kimberlite failure at Udachnaya diamond pipe

Testing of air-dry samples suggests that the specific power intensity of kimberlite failure gradually increases with a temperature decrease.A similar effect was found in Internatsionalnaya diamond pipe kimberlites (Figure 5).Most researchers have mentioned that a temperature decrease results in an increase of rock strength (Burstein and Kurochkin,1965;Daydkin,1968;Elchaninovet al.,1970;Tsitovich,1973;Skuba,1974;Senuk,1983;Rzhevskaya,1989;Kozeevet al.,1995;Inada and Kinoshita,2003).

The local minimum of rock strength and specific power input failure of examined rocks within the range of-5 °C to-15 °C is explained,first and foremost,by the change of the aggregate state of water as one of the rock components.Moreover,a change of rock temperature presupposes a tension predetermined by differences in elastic properties and the thermal extension coefficient of individual mineral grains forming a rock (Rzhevskyi and Novik,1984).

The transition of free water into ice results in a volume gain of 9% due to which a disjoining tension occurs.Internal tensions grow while intensive freezing of rock pore moisture is underway.With the termination of ice crystal growth,internal tensions ceases.In rocks saturated with fresh water,an intensive process of water freezing occurs at 0 °C to-3 °C.Intensive phase transitions in brine saturated rocks shift to lower temperatures (-20 °C and lower) depending on solution concentration and content.Our experiments on quantities of unfrozen water in host rocks at low temperatures show that intensive phase transitions terminate at-10 °C.

With a further temperature decrease,the pore ice cements and strengthens the rock.This results in an increase of strength and rock power intensity when the temperature drops below the temperature of intensive phase transition and a quantity of unfrozen water decreases.

Our experiments suggest that inner tension occurring at a temperature decrease from +20 °C to-10 °C are likely to result in significant weakening of rock strength.Tensions at 40%–70% of rock strength lead to rapid accumulation of inner defects with cyclically alternating impacts of the temperature that cause weak freeze-resistance of the rocks under study.

4 Conclusions

The local minimum of tension strength and power input into failure of carbonate rocks and kimberlites from diamond deposits of Yakutia has been identified for the first time.Experimental results show that a temperature decrease from +20 °C to-5 °C to-15 °C results in a loss of host rock strength of more than 50%,and that of kimberlite by 45%.The power input failure of examined rocks decreased by 10%–40%within the same temperature range of-5 °C to-15 °C.The decrease degree of power input failure and strength of rocks is associated with their porosity,dampness,and mineralization of pore solution.This decrease is due to inner material tension occurring at low temperatures and predetermined,first of all,by alteration of the aggregate state of water (volume increase by 9%),as well as by variations in elastic properties and thermal expansion coefficients of individual grains of rocks.

The determined regularities of rock strength index alterations should be incorporated into calculations of underground and open cast mine stability,and in energy-saving technologies of mineral resource extraction and processing.

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