Research On Soft Rock Simulation And Its Large-diameter Drilling Technology

Soft rock, the most widely distributed type of rock in the Earth's surface, a complex rock mechanics media that become significantly deformed in a specific environment, has brought a series of special issues to the construction, structural design and construction technology. based on the geological features and technical difficulties in a construction project, this study selected a sample of broadly representative raw soft rocks to, for the first time, explore the combination formula and testing methods of similar materials of soft rocks, to seek optimal combination formula for the production of indoor simulated drilling rock samples, and finally to improve the large-diameter soft-rock drilling technology and optimize the process parameters. The application results have important engineering significance.The simulation methods adopted for selecting lithologic materials identify three types of prototype soft rock:shale (including shale, and silty mudstone), siltstone (including muddy siltstone) and fine-grained sandstone (including silty fine-grained sandstone). By analyses and comparisons, the optimal formula of simulation of soft-rock were obtained, which are:mudstone (A2B2C3, No.621), siltstone (A1B2C2, No. 437) and fine-grained sandstone (A2B3C1, No.355), with their corresponding compressive strengths of the original rocks well simulated. An opposite trend has been found in each of the three types of rocks between its cohesion and internal friction angle, without significant differences. This, taking into consideration their shear strengths that fall within the intensity scope of the original rocks, has no significant impact on the simulation tests. The density of the samples was kept as close to that of the original rocks as possible, through similarity ratio calculations. The experimental values of the elastic modulus of the models fell within the scope of that for the original rocks, although they could not show a trend as constrained with experimental conditions. Therefore, in outdoor bulk sample production process, taking into account the pressure bearing capacity of the sample and the production capacity of the laboratory, the simulation of large sample was determined to be a cylinder with a diameter of 300 (mm) and a height of 400 (mm), in order to ensure that a triaxial stress test machine can smoothly produce bulk sample test block and the stripping and maintenance of 28 days to meet the drilling simulation test requirements.The paper discusses the role of mechanical parameters on the drilling speed.1) According to indoor conditions and the role of mechanical parameters, it experimented on the impact of scraper blade's cutting angle on the drilling speed by changing the blade angle, so that the blade could cut and break the modules from different perspectives. The test results showed that, in terms of rock breaking effect and drilling speed, the blade had the best performance with an angle of 10°for mudstones, and 5°for sandstones.2) The impact of drilling pressure on the drilling speed:Based on laboratory experiments and on-site field test data, using Microcal Origin software, the experimental data are weighted residuals Variation Analysis for the first time fitting a large-diameter drilling operations drilling pressure and drilling speed of change of the:y = (A-B)/(1+(x/x0) p)+B. Field test showed that when WOB controlled between 100 and 200kN, the drilling can achieve the better results.3) The impact of speed on the penetration rate:Based on laboratory experiments and on-site field test data and regression analysis, the first drilling fitting out the speed and speed of change of the:y= a (1-ebx), field tests show that speed controlled within 6-8rpm is appropriate and can get better ergonomics.Based on the drill's rock-breaking mechanisms, the paper has explored on how to improve the function of drills. Studies suggest that soft rocks can be categorized as plastic soft rock formations and-elastic-plastic rocks, with their characteristics mainly reflected in the rock-breaking modes such as collision, crushing and small shear and large shear. In the elastic-plastic rocks, in each cutting cycle and between the various cycles, the horizontal force leaps and changes regularly, whereas in plastic rocks, the horizontal force, without significant change, basically can be considered constant. This paper therefore analyzed and argued for an improved structure of the drill from five aspects including: the shape and material selection for the cutting blade, drill-oriented and smoke residue nozzle structural optimization design, the choice of blade angle drill, cutting flange design and drill cloth tooth design. Through on-site testing, the improved drill bit achieved the expected results.1) For the improved drill, the central angle for the guiding drill bit is smaller than that of the blade-wings, a decrease from the original 100°to 80°, which increases the inclined length of the guiding cone in order to improve the stability of orientation in high-speed drilling; 2) The improved scraper blade is sized 200mm long,80mm wide and 60mm thick, made of hard alloy YG11Cin a flat diamond structure, and based on the simulated testing results of penetration rates against cutting angles, the inclination angle of the flange is not greater than 10°; 3) In the effect evaluation of the engineering applications, the new blade can half the time for hole forming, and the number of holes formed using a single set of the new tools is about three times as many as that using common tools, i.e., converted into a unit cost per hole, the new blade is far more cost-effective than common ones. In the same conditions of WOB, rotational speed and other hydraulic parameters, statistics of the net drilling time and progress of two projects has proven that the efficacy of the new scraper has more than doubled, and therefore the new tool has been widely used in the operating areas; 4) The objectives and content of the optimization of drilling parameters are multiple dimensional, including many indicators such as minimum cost, quality, speed, footage, duration and others. These indicators are again related to some physical conditions such as mechanical equipment and geological conditions, as well as various measures taken and parameters selected in the construction projects.Trough risk assessment on construction organization and management, the paper promotes the optimization of drilling process parameters and management decision-making.1) Applicable to a variety of formation conditions and different pile diameters and lengths and capable of meeting different bearing capacity requirements, as well as many other advantages, large-diameter bored piles are widely used in many large foundation engineering constructions, especially in deep water and long-span bridge foundation engineering constructions, and these foundations continue to grow towards an extra-ultra-deep direction. The risks associated with construction are growing as engineering and geological conditions become increasingly complex. To address the complexity and unpredictability of engineering and geological conditions, this paper discussed risk-based decision-making methods and risks of decision-making program.2) In view of the outstanding issues in the construction organization and management of large-diameter bore piles, using mathematical statistics and calculation methods, and based on systems science and decision-making theory, this paper has proposed a mathematical model for risk-based decision-making, which could be used to guide similar projects.