Layered rock mass of significant strength changes for adjacent layers is frequently observed in underground excavation, and dynamic loading is a prevalent scenario generated during excavation. In order to improve the driving efficiency and reduce engineering accidents, dynamic compression characteristics of this kind of rock mass should be understood. The dynamic properties of a layered composite rock mass are investigated through a series of rock tests and numerical simulations. The rock mass is artificially made of various proportions of sand, cement and water to control the distinct strength variations at various composite layers separated by parallel bedding planes. All rock specimens are prefabricated in a specially designed mould and then cut into 50 mm in diameter and 50 mm in height for split Hopkinson pressure bar (SHPB) dynamic compression testing. The test results reveal that increasing strain rate causes the increases of peak strength, σp, and the corresponding failure strain, ep, while the dynamic elastic modulus, Ed, remains almost unchanged. Interestingly, under the same strain rates, Ed of the composite rock specimen is found to decline first and then increase as the dip angle of bedding plane increases. The obtained rock failure patterns due to various dip angles lead to failure modes that could be classified into four categories from our dynamic tests. Also, a series of counterpart numerical simulations has been undertaken, showing that dynamic responses are in good agreement with those obtained from the SHPB tests. The numerical analysis enables us to look into the dynamic characteristics of the composite rock mass subjected to a broader range of strain rates and dip angles than these being tested.

Dynamic compression characteristics of layered rock mass of significant strength changes in adjacent layers

Dynamic mechanical properties and cracking behaviours of persistent fractured granite under impact loading with various loading rates

Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions. Four types of samples, i.e. intact, jointed with no grouting, jointed and grouted with cement, and jointed and grouted with epoxy, were fabricated using model material. A series of dynamic split Hopkinson pressure bar (SHPB) tests was conducted on prepared samples with strain rates varying between 53–130 s −1 along with static uniaxial compression tests (10 −4 s −1 ). Progression of fracture/failure along samples was monitored using high-speed imaging and digital image correlation (DIC). Strength was observed to be significantly lower for jointed samples as compared to intact samples. However, the increasing trend of strength with strain rates remained similar for all types of samples. Epoxy was observed to be a better grout due to greater improvement in the strength of epoxy grouted jointed samples than cement grouted samples under both static and dynamic conditions. Significant changes were observed in fracture behavior (initiation, pattern and mechanism) with strain rate for intact and jointed unfilled/grouted samples. Fracturing was dominated by shear and tensile cracks at high strain rates compared to tensile cracks at low strain rates in all samples. Unlike static loading conditions, the location of cracks initiation shifts away from joint tips with increasing strain rate and depending upon existing infill conditions (unfilled/grouted). • The effect of an impersistent joint on the rate-dependent strength and fracturing behaviour of jointed rocks is investigated and compared with that of intact rocks. • The effect of joint grouting on the rate-dependent behaviour of jointed rocks is investigated and compared with that of jointed rocks with clean joints. • The efficiency of different grout materials in the improvement of the dynamic response of jointed rocks is investigated. • Suggestive guidelines are provided for the selection of grout materials for jointed rocks under static and dynamic conditions. 

Rate-dependent mechanical behavior of jointed rock with an impersistent joint under different infill conditions

Bending failure is a common failure mode of layered rock mass. Making clear the mechanical behaviors and energy evolution characteristics of layered rock mass, it is beneficial to prevent geological disasters caused by the bending deformation of layered rock mass. In this study, the mechanical behaviors and energy evolution characteristics of hard and soft rocks by conducting the three-point-bending (TPB) test with different loading rates were investigated. The results show that as the loading rate increases, both the peak load of hard and soft rock increases, the peak displacement of hard rock decreases, while the peak displacement of soft rock increases. The horizontal crack width at the bottom of the sample of hard rock is greater than that of soft rock, but the instantaneous crack widths show opposite results. Both the failure pattern of hard rock and soft rock are the typical tensile fracture, yet the fracture surface of hard rock is denser and smoother than that of soft rock. For hard rock, the total input energy, elastic energy, and dissipated energy increase with the increase of loading rate. For soft rock, however, the total input energy and elastic energy increases, while the dissipated energy decreases. Under the TPB test, the peak load, displacement, instantaneous crack width, total input energy, elastic energy, and dissipated energy of both hard rock and soft rock present linear relationships with the common logarithm of the loading rate.

Experimental study on three-point-bending characteristics of hard and soft rock-like materials under different loading rates

A Study of the Dynamic Damage Characteristics of Rock-Like Materials with Different Connectivity of Concealed Structural Surfaces

Intact rock-like specimens and specimens that include a single, smooth planar joint at various angles are prepared for split Hopkinson pressure bar (SHPB) testing. A buffer pad between the striker bar and the incident bar of an SHPB apparatus is used to absorb some of the shock energy. This can generate loading rates of 20.2–4627.3 GPa/s, enabling dynamic peak stresses/strengths and associated failure patterns of the specimens to be investigated. The effects of the loading rate and angle of load applied on the dynamic peak stresses/strengths of the specimens are examined. Relevant experimental results demonstrate that the failure pattern of each specimen can be classified as four types: Type A, integrated with or without tiny flake-off; Type B, slide failure; Type C, fracture failure; and Type D, crushing failure. The dynamic peak stresses/strengths of the specimens that have similar failure patterns increase linearly with the loading rate, yielding high correlations that are evident on semi-logarithmic plots. The slope of the failure envelope is the smallest for slide failure, followed by crushing failure, and that of fracture failure is the largest. The magnitude of the plot slope of the dynamic peak stress against the loading rate for the specimens that are still integrated after testing is between that of slide failure and crushing failure. The angle of application has a limited effect on the dynamic peak stresses/strengths of the specimens regardless of the failure pattern, but it affects the bounds of the loading rates that yield each failure pattern, and thus influences the dynamic responses of the single jointed specimen. Slide failure occurs at the lowest loading rate of any failure, but can only occur in single jointed specimen that allows sliding. Crushing failure is typically associated with the largest loading rate, and fracture failure may occur when the loading rate is between the boundaries for slide failure and crushing failure.

Dynamic strength of rock with single planar joint under various loading rates at various angles of loads applied

Intact rock-like specimens and specimens that include a single planar joint or triangular sawteeth joint at various angles are prepared for split Hopkinson pressure bar (SHPB) testing at loading rates of 303.1–5233.6 GPa/s. Only results that are associated with an error (ee) of less than 20.0% are utilized in subsequent analyses. The effects of the loading rate and angle of the load applied to various joint patterns on the failure type and dynamic peak stresses/strength of the specimens are investigated. Experimental results demonstrate that failure of each specimen can be classified into the following four types, Type A: integrated with or without tiny flake-off, Type B: slide failure, Type C: fracture failure, and Type D: crushing failure. The results of statistical analysis of variance (ANOVA) indicate that the loading rate, the angles of the base plane (β), and the asperity (α) of the sawteeth joint of the specimen all affect its dynamic peak stress when fracture failure occurs. The loading rate and β are important when the slide failure occurs, and the loading rate is the sole factor that significantly influences its dynamic peak stress when the specimen is crushed to failure. The dynamic peak stress of the specimen increases with the loading rate, while the rate of increase gradually decreases. The β and α of a jointed specimen affect the location of stress concentration during loading, further influencing the dynamic peak stress of such a specimen under slide and fracture failure. When the loading rate is high and the specimen is crushed to failure, the influences of β and α disappear, and the increase of loading rate reduces the efficiency of raising the dynamic peak stress.

Hung-Hui Li

Papers

Dynamic strength of rock with single planar joint under various loading rates at various angles of loads applied

Dynamic Response of Rock Containing Regular Sawteeth Joints under Various Loading Rates and Angles of Application