Prediction of Uniaxial Compression Strength of Limestone Based on the Point Load Strength and SVM Model

Uniaxial compression strength (UCS) is a fundamental parameter to carry out geotechnical engineering design and construction. It is simple and efficient to predict UCS using point load strength (PLS) at engineering sites. However, the high dispersion of rock strength limits the accuracy of traditional fitting prediction methods. In order to improve the UCS prediction accuracy, 30 sets of regular cylindrical specimen tests between PLS and UCS are conducted on limestone mines. The correlation relationship between PLS and UCS is found by using four basic fitting functions. Then, a prediction model is established by using SVM algorithm. Multiple training test data are used to achieve high-precision prediction of UCS and the results show it is less different from the actual values. Especially, the R2 coefficient reached 0.98. The SVM model prediction performance is significantly better than the traditional fitting function. The constructed SVM model in this study can accurately predict the UCS using the PLS obtained in the field, which has a great significance to the rock stability judgment in the actual construction environment.

Investigation of Engineering Properties of Cement-Stabilized Calcareous Sand Foundation

Calcareous sand is widespread around Nansha Islands, South China Sea. In oceanic and coastal engineering, calcareous sand is usually used as a building foundation and backfill material for airport runway embankments. The engineering characteristics of calcareous sand is different from terrigenous sand because of its irregular grain shape, lower particle strength, and internal voids, which have caused many engineering problems in the last decades. Cement-stabilized soil, as a common foundation reinforcement method, can solve these engineering problems and improve the foundation strength effectively. Therefore, it is very important to estimate the engineering characteristics of cement-stabilized calcareous sand foundations. In this paper, the basic engineering characteristics, bearing capacity, and deformational behavior of calcareous sand were studied by carrying out a series of tests on cement-stabilized calcareous sand. It is found that: (1) the uniaxial compression strength of calcareous sand is higher than that of Guangzhou soft soil but lower than that of filter medium quartz sand; (2) the deformation of the calcareous sand under compression is mainly plastic, and the elastic deformation gradually increases with increasing cement content; (3) the apparent cohesion of calcareous sand increases, while internal friction angle decreases with increasing cement content; (4) cement-stabilized method can significantly improve the bearing capacity of calcareous sand foundation, especially for the saturated state. A cement content equal to or more than 15% and a thickness of 1/8 of the foundation can effectively improve the bearing capacity of the foundation; and (5) the ultimate bearing capacity of the foundation by numerical calculation is higher than that by experiments, while the settlement by calculating is lower.

Mechanical Behavior of Frozen Porous Sandstone under Uniaxial Compression

The influence of low temperature on longitudinal wave velocity, uniaxial compression strength, tensile strength, peak strain, secant modulus, and acoustic emission characteristics of yellow sandstones was studied. The results show that the secant modulus increases with decreasing temperature when the axial strain is less than 0.6%, and a contrary influence performs for the subsequent stage due to the fracture of the pore ice. With the decrease in temperature, the uniaxial compression strength first increases and then remains at a relatively constant value of 34.44 MPa at about -40°C while the temperature ranges from -40°C to -70°C. The tensile strength shows an approximate linear increment as the temperature. The peak strain gradually increases with temperature in a three-stage piecewise linear form, and the increasing rate gradually decreases with the decreasing temperature. The phase transformation from liquid water at a temperature of 20°C to solid ice at a temperature of -3°C significantly increases the longitudinal wave velocity from 1.55 km/s to 3.36 km/s. When the temperature is lower than -10°C, the longitudinal wave velocity approximately increases linearly at a rate of 2.67 × 10 − 3 km / s · ° C − 1 with decreasing temperature.

Analysis of different interface treatments between masonry of AAC blocks and reinforced concrete structure after uniaxial compression strength test

Autoclaved aerated concrete (AAC) masonry is widely used in civil construction but requires further investigation. Hence, this experimental study evaluated three types of interface treatment between the reinforced concrete structure and AAC masonry, in scale, after a uniaxial compression resistance test. The types of interface treatment considered are reinforcement with steel bars, with rough polymeric cementitious mortar, and without treatment. The maximum load capacity, displacements, and occurrence of cracks were analysed. The results showed that the maximum individual load capacity did not significantly differ among the examined groups. However, the analysis of the displacements and cracks showed that the group with steel reinforcement had the smallest displacements and largest cracks. This behaviour is owing to the greater solidarity of forces conferred by steel reinforcement.

Carbonate Soil Cryogenesis in South Yakutia (Russia)

The present study investigates changes occurring in the material composition and properties of the South Yakutian carbonate soils during cryogenesis. The nature of the transformations of certain limestone varieties composing the surfaces of rock massifs was determined using scanning electron microscopy, 3D X-ray tomography, as well as lithological–mineralogical and optical–petrographic studies, over a 10-year period. The areas in carbonate rock massifs with increased clay content, pyritisation, dolomitisation, and baritisation, as well as zones of calcite and dolomite junction, were found to be least resistant to the effects of processes associated with water phase transitions, i.e., freezing and thawing. The mineral proportion of limestone on the surface of soil massifs chemically processed over a 10-year period reached 5–7% of the volume of the weathered rocks. In the process of transformation, not only the composition of the rocks changed, but also the nature of the structural bonds that significantly influence their mechanical strength properties. The number of cracks for weathered soil samples increased by 9–16%; their opening increased by 13–18%. For rocks initially having uniaxial compression strength in the range of 33–46 MPa, this strength was reduced by 19–27%. Laboratory experiments on 1000-fold cyclic freezing and thawing of carbonate rock samples (which corresponds to an 8–10-year period of weathering on the surface of a mountain outcrop under the natural conditions of South Yakutia) demonstrate the similarity of these changes with those observed in samples taken from the sides of open pits 10 years ago. In general, soils are influenced by a wide range of environmental factors under natural conditions. The significant influence of alternating temperatures on the changes in the composition and structure of limestones in South Yakutia is characterised in detail.

Deterioration Characteristics and Energy Mechanism of Red-Bed Rocks Subjected to Drying-Wetting Cycles

The red-bed rocks were chosen and studied by using uniaxial compressive experiment and scanning electron microscopy to investigate the effect of drying-wetting (D-W) cycles on the mechanical properties and microstructural characteristics of red-bed rock. Additionally, the energy mechanism of specimens subjected to drying-wetting cycles was also explained. Experimental results showed that, the stress-strain could be divided into four characteristic stages in the compression failure process. After subjecting to cycles of D-W, the stress-strain curve gradually changed from softening to hardening. At the same time, uniaxial compression strength (UCS) and elastic modulus dropped obviously, while Poisson’s ratio gradually raised. Microstructural analysis results indicated that the microstructure of the specimen surface was no longer dense and uniform, and the porosity of tested specimens significantly increased with D-W cycles increasing. As the porosity grew, UCS and elastic modulus gradually declined. According to the first law of thermodynamics, the process of rock failure was an event of energy transfer and conversion. As the number of D-W cycles increased, the energy density of specimens all present linear fell. From the perspective of the theory of energy dissipation, the dissipated energy was essential for rock failure, and closely related to the strength of the specimen. With D-W cycles increasing, the specimens were more prone to failure, and the dissipated energy required for failure decreased gradually.

LAB-SCALE TESTS OF INCOHERENT ROCK REINFORCEMENT USING TWO-COMPONENT POLYMER BLENDS

The study addresses improvement of physical and mechanical properties of incoherent sediments by means of their chemical reinforcement using two-component resins. The lab-scale testing data on reinforcement of fine-grained quartz sand using two-component activated mineral and polyurethane resin blends are presented. Resins were injected by two ways on the tests. The first method was sequential injection of the polymer blend components in rock samples. The second method was injection of finished polymer blends. The two-component activated mineral and polyurethane resin blends ensure more effective reinforcement of incoherent sediment rocks as against foamed polyurethane blends. The uniaxial compression strength of reinforced fine-grained sand is 2.5-3 times higher in case of sequential injection of the components than in injection of finished mixtures. The elasticity modulus of the reinforced samples is 5.5-6 times higher in sequential injection than in injection of finished blends. The test results are useful for selection and optimization of injection method for two-component polymer blends in stabilization of broken rocks and in water-proofing of underground excavations.

Investigation on Dynamical Mechanics, Energy Dissipation, and Microstructural Characteristics of Cemented Tailings Backfill under SHPB Tests

As mining depth increases, the backfill mining method is more and more widely used in underground mines. The dynamic load generated by the blasting can affect the stability of the cemented tailings backfill (CTB). The CTB samples were prepared to conduct a test of the split Hopkinson pressure bar (SHPB) to investigate the dynamic disturbance of CTB. The present paper discusses dynamical mechanics, energy dissipation, and microstructure analysis of CTB. Micro-computer tomography (micro-CT) scanning of CTB samples after the SHPB test was performed to analyze the evolution of internal cracks. The experimental results showed that when the average strain rate (ASR) increased from 30 to 98 s−1, the dynamic uniaxial compression strength (DUCS) of the CTB showed a trend of first increasing and decreasing with the increase in ASR. The dynamic stress–strain pre-peak curve of CTB directly enters the linear elastic stage. As ASR increases, the absorbed energy of the CTB shows a trend of first increasing and then decreasing. Moreover, according to the micro-CT scanning results, the crack area of CTB accounts for about 16% of the sample near the incident bar and about 1% near the transmitted bar. The crack area ratio is exponentially related to the specimen height. These findings can provide reasonable dynamical CTB strength data selection for underground pillar mining.

Thermal Effects on the Drilling Performance of a Limestone: Relationships with Physical and Mechanical Properties

This work evaluates the effect of high temperatures and cooling methods on the drillability of Prada limestone. Samples from boreholes drilled during the design stage of the Tres Ponts Tunnel in the Catalan south Pyrenean zone (Spain) were subjected to temperatures of 105, 200, 300, 400, and 600 °C, and then cooled at a slow rate or by quenching. Sievers’ J-value (SJ) and brittleness (S20) were determined on thermally treated samples, and the drilling rate index (DRI) was calculated for each temperature. The results show that thermal treatment implied a sustained increase in the drillability of the rock of up to 40% at 600 °C and a change in the drillability category (from medium to high). At 600 °C, SJ and S20 tripled and doubled, respectively, the initial values obtained for the intact rock. The results were inconclusive about the influence of the cooling method on the drilling performance of Prada limestone for the tested range of temperatures. The substantial improvement observed in the drillability of Prada limestone when heated, measured in terms of DRI, could help in the development of novel thermally-assisted mechanical excavation methods. Additionally, strong correlations between drillability variables (i.e., SJ and S20) and physical and mechanical variables of Prada limestone (i.e., P- and S-wave velocities, uniaxial compression strength, elastic modulus, and Poisson’s ratio) are proposed. Correlations will help make preliminary predictions of drillability based on properties such as uniaxial compression strength and ultrasound wave velocities.