Chen Cui, Ivan Gratchev

School of Engineering, Griffith University, Gold Coast, Australia

Keywords:Pre-existing crack Rock strength Infill material Laboratory test

A B S T R A C T This study aims to examine the influence of pre-existing discontinuities on the strengths of four natural rocks of different origins. A series of unconfined compression tests was performed on specimens of two types of sandstones, argillite and basalt that contain open and filled cracks. It was found that the presence of cracks tends to decrease the overall strength for all studied rocks;however,the magnitude of strength reduction is related to the property of rock. The larger strength decrease was observed for the relatively harder argillite and basalt, compared to the softer sandstone. It was also found that the infill material could increase the strength of rock specimens, while the obtained strength depended on the characteristics of the fill material.

1. Introduction

In geotechnical engineering, there are several types of discontinuities associated with shear zones, bedding planes and faults that can affect the strength and stiffness of rock mass(Shen,1995;Eberhardt et al.,1998).Some studies were conducted to analyze the process of initiation, propagation and coalescence of pre-existing cracks in rocks or rock-like materials (e.g. Ramamurthy and Arora,1994; Shen et al., 1995; Liao et al., 1997; Eberhardt et al., 1998).Their effects on the strengths of rock and rock-like materials were also investigated by unconfined compression or shear and tension tests (e.g. Yang et al., 2012; Xu et al., 2013; Nikolic et al., 2015;Cheng et al., 2016; Shang et al., 2016; Guha Roy et al., 2017). Subsequently, this subject was widely studied based on uniaxial tests(Sagong and Bobet,2002;Xu et al.,2013;Lisjak and Grasselli,2014;Wong and Zhang,2014;Yin et al.,2014;Zhou et al.,2014,2018;Cao et al., 2015; Nikolic and Ibrahimbegovic, 2015; Gratchev et al.,2016), biaxial tests (Bobet and Einstein, 1998; Shang, 2020), and shear and tension tests (Gehle and Kutter, 2003; Indraratna et al.,2010, 2014; Shang et al., 2016, 2018) conducted on rock-like material with multiple pre-existing either closed or open cracks.

These studies showed that crack propagation mostly initiates from the pre-existing cracks and grows into a rock bridge, eventually producing new discontinuities that would connect the preexisting ones (Gehle and Kutter, 2003; Yin et al., 2014; Cao et al.,2015). Although such studies have significantly advanced our knowledge in this area,the rock-like material is still rather different from natural rocks as it cannot represent the inhomogeneity(such as voids and macro-cracks), which is common for natural rocks(Zhang,2010).In addition,only limited research was conducted to investigate the effect of infill on rock strength even though the rock discontinuities are often filled with either soft soil material(sand or clay) or crystalline material such as quartz in the field. Compared with soft soil material, crystalline material such as quartz can increase the strength of rocks (Shang, 2020). Therefore, this paper discussed more about the soft filling material in the relatively larger open crack, in which, under that condition, minerals can be considered as one type of soils with different components. This study aims to investigate the effect of pre-existing cracks on the strengths of four natural rocks and clarify the effect of infill material on crack development.

2. Experimental program

2.1. Rocks used

Core specimens of fresh sandstone,argillite and basalt sampled from the region of Southeast Queensland,Australia were provided for this study by local geotechnical companies. Two types of sandstones, i.e. S1 and S2, were used. Sandstone S1 was grey,relatively weak with high porosity(n=29.1%)and a relatively low unconfined compressive strength (UCS) of 11.5 MPa. Sandstone S2 had an average porosity of 8.4%,and its UCS value was 47.5 MPa.The argillite and basalt specimens were of low porosity,about 4.9%and 1.4%,respectively.The properties of each rock as well as its mineral composition are presented in Table 1.

2.2. Testing program

All rock specimens tested in unconfined compression had a height of 100 mm and diameter of 50 mm (Fig. 1). Pre-existing discontinuities (cracks) were made on top and bottom of the specimen by means of electric grinder. For the two types of sandstones (S1 and S2), cracks with the depth of either 1 cm or 2 cm were made. However, for the argillite and basalt, it was rather difficult to create pre-existing cracks deeper than 1 cm without fracturing the whole specimen. For this reason, both argillite and basalt contained pre-existing cracks with the depth of only 1 cm.The width of cracks for each type of rock was 3 mm (Fig.1).

The infill material for all four rock types was moist sand (classified as poorly-graded sand according to unified soil classification system (USCS)) with a friction angle of 35°and water content of 15%.In addition,moist clay(classified as CL according to USCS)was used as infill material for two types of sandstones as there were sufficient number of sandstone specimens available to complete this series of tests.The clay had a water content of 25%and UCS of 98 kPa. For each type of rock, the infill was placed in the preexisting crack in two layers and each layer was gently tamped toachieve the overall densities of 1.9 g/cm3(sand infill)and 2.2 g/cm3(clay infill).

Table 1 Rock properties.

Fig.1. Size of specimens with (a) 1 cm and (b) 2 cm deep cracks.

For each specimen, a compressive load was applied with an increment of 0.5 kN/s until the specimen failed (AS 1141.51-1996,1996). A high-speed camera was used to record the behavior of the specimen throughout the test.For each experimental setup,the test was repeated at least 3 times and the average value was used for analysis. The summary of the test results is given in Table 2.

Table 2 Summary of test conditions and laboratory data.

Fig. 2. Experimental stress-strain curves for argillite specimens.

3. Results and discussion

3.1. Test results obtained for argillite and basalt

Typical results from unconfined compression tests are given in Fig. 2 for argillite and Fig. 3 for basalt specimens. For the argillite,the maximum UCS of about 40 MPa was observed for the intact specimen,while the lowest strength of 30 MPa was recorded for the specimen with open cracks. The basalt was the hardest rock with UCS of 76 MPa for the intact specimen (Fig. 3). However, the maximum strength was observed for the specimen with 1 cm filled cracks (Fig. 3).

As can be seen in Figs. 2 and 3, the 1 cm open cracks tend to decrease the overall strength of argillite and basalt by 23%and 17%,respectively,while the sand infill results in increase in strength.For the argillite (Fig. 2), the strength increase due to the infill was observed to be rather small (about 6%). However, the basalt specimens (Fig. 3) with the filled pre-existing cracks exhibited a significant strength increase from an average of 69.3 MPa(open crack)to as high as 104.3 MPa(filled crack with sand),even exceeding the strength of intact rock (an average of 83.7 MPa). Such changes in strength can be attributed to the stress-strain behavior of the basalt specimens (Fig. 4). The specimens with open cracks exhibited a distinct brittle behavior with sudden failure under high loads,while the specimens with sand infill were able to undertake larger deformations(greater strain),leading to greater stresses at failure.The summary of the stress-strain data at failure obtained for the basalt specimens with open and filled cracks is given in Fig.4.It can be inferred from this figure that the failure of intact specimens and the specimens with open cracks occurred at smaller strain values,while the specimens with filled cracks failed at relatively greater strains and stresses.On the contrary,for the softer argillite,greater stresses and strains were measured for the intact specimens(Fig. 6).

Fig. 3. Experimental stress-strain curves for basalt specimens.

Fig. 4. Summary of stress-strain data at failure for basalt.

3.2. Test results obtained for sandstone

The test results obtained for two types of sandstones with preexisting open cracks of 1 cm and 2 cm in depth are given in Figs.6 and 7,respectively.The laboratory data obtained for sandstone S1(Fig. 6) indicated that the specimens with 2 cm crack (no infill)produced lower strength (UCS = 8.8 MPa) than both the intact specimens (UCS = 11.5 MPa) and the specimens with 1 cm crack(UCS = 9.4 MPa). The same tendency was observed for S2. The maximum strength was obtained for the intact specimens(average stress value of 47.5 MPa), while the lowest strength(34.5 MPa)was measured for the specimens with the longer open crack of 2 cm.

Fig. 5. Summary of stress-strain data at failure for argillite.

Fig.6. Summary of stress-strain data of sandstone S1 with(a)1 cm and(b)2 cm deep cracks.

Compared to the specimens with open cracks,the infill material(either sand or clay) generally increased the rock strength, even exceeding the strength of intact rock in some cases. The effect of infill on UCS was more pronounced for sandstone S1,which can be attributed to the fact that the infill formed‘inclusions’which were slightly denser than the rock(density of 1.78 g/cm3).The inclusion of sand(density of 1.9 g/cm3)or clay(density of 2.2 g/cm3)seemed to add some additional strength of about 35%and 41%,respectively.Compared to the sand infill,the denser clay infill appears to be the main reason for the slightly larger values of UCS obtained for both S1 and S2 specimens in which cracks were filled with clay.It is also noted that the effect of infill on UCS becomes less significant for the denser sandstone S2(average density of 2.57 g/cm3),which can be clearly seen in Fig. 7.

It is observed that for sandstone S1, regardless of the crack length and infill type,greater strength correlates with larger strain at failure.For sandstone S2,this correlation is not very clear as the specimens with larger UCS do not necessarily fail at greater strain.

3.3. Typical failure patterns

Visual observations during testing and analysis of high-speed camera footage revealed similarities in the failure pattern of different rock types with pre-existing cracks. Figs. 8 and 9 summarize the typical failure patterns with crack initiation and propagation (from left to right) observed for specimens with different crack characteristics.

Fig.7. Summary of stress-strain data of sandstone S2 with(a)1 cm and(b)2 cm deep cracks.

For most intact specimens, regardless of the rock type, newlydeveloped cracks propagated through the whole specimen resulting in failure, as shown in Fig. 8a. Unlike the other rocks, the hard basalt exhibited a distinct brittle behavior under high stresses.

The specimens with pre-existing cracks failed in different fashions. It was found that for the both sandstones and argillite,new cracks typically formed near the pre-existing discontinuities and then propagated through the specimen, leading to failure shown in Fig. 8b.For the basalt specimen with 1 cm open crack, it was rather difficult to analyze the crack propagation pattern as most of specimens failed in a sudden and brittle manner.It was also noted that there were a few cases where newly-developed cracks in sandstones S1 and S2 did not initiate from the existing discontinuities.Compared to the available literature,the failure patterns of natural rocks appear to be similar to that reported for rock-like specimens (Gratchev et al., 2016).

As for the specimens with filled cracks, the typical failure patterns are shown in Fig.9a and b.For 1 cm cracks,newly-developed discontinuities originated from one of the existing cracks(not from both)and then propagated through the specimen,thereby causing failure. A special case was observed for sandstone S1 with 1 cm depth cracks filled with clay, as presented in Fig. 9c, where new crack developed at the edges of specimen and coalesced,suggesting failure of specimen.

3.4. Effect of infill on rock behavior

Comparing with argillite and basalt, a difference between the deformations of these two types of rock specimens is found. In comparison with the basalt specimen, multi-layer combined form can be observed on argillite specimen when the unconfined compression stress is loaded, which relatively limits the particle movement. Due to this, the argillite specimens are easier to fail.However, the basalt specimen with defects can deform more than intact specimens, thus the basalt with infilled pre-existing crack has a larger strength.

Fig. 8. Typical failure patterns of (a) intact specimen and (b) specimen with open cracks.

Fig. 9. Typical failure patterns of specimen with (a) 1 cm and (b) 2 cm filled cracks, and (c) S1 specimen with 1 cm crack filled with clay.

For low-density sandstone S1, the infill has a significant influence on the strength compared with the high-density sandstone S2.It is also found that the effect of infill is also related to the rock type.

It is also noted that even though the sand infill is cohesionless,it still affects not only the strength of the rock specimen,but also the failure process.It can be found from the UCS results that except the argillite specimen,the UCSs of sandstone and basalt with sand infill have a marked difference compared with the specimen with open crack and intact specimen. As for the failure procedure, despite there is no cohesion in sand infill, the bearing capacity of the sandstone is still enhanced, and the failure patterns are different from that of the specimen with open crack.

4. Conclusions

In this work,the effects of pre-existing open and filled cracks on two types of sandstones,argillite and basalt were studied through a series of unconfined compression tests. Based on the obtained results, the following conclusions can be drawn:

(1) For all the rock types,the pre-existing open cracks decreased the UCS. The larger strength reduction was observed for the relatively harder rocks such as argillite and basalt.

(2) The filled pre-existing cracks were found to increase the UCSs of all studied rocks.Furthermore,it was found that the denser infill produced greater increases in UCS.

(3) The failure process was dominated by newly-developed discontinuities that initiated from the pre-existing cracks.This finding seems to agree with the previous data obtained from rock-like specimens.

Declaration of competing interest

The authors confirm that there are no known conflicts of interest associated with this publication, and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

The authors would like to acknowledge Mr.Beau McDonald for his invaluable help with laboratory testing.