Pilot tests on the usage of vacuum preloading combined with short prefabricated vertical drains (PVDs) to form a working platform for future soil improvement work were conducted at a land reclamation site in Tianjin, China. The short PVDs were connected using three methods – the conventional vacuum preloading method, fish-bone connectors and embedded horizontal pipes. To investigate the influence of PVD spacing, short PVDs were installed in square grids with spacings of 0·4 m and 0·6 m. The ground surface settlement, pore water pressures, water content and undrained shear strength of the soil were measured during the specific period of vacuum preloading. The pilot tests indicated that short PVDs connected using embedded vacuum pipes installed at 0·4 m spacing were the most suitable method to form a working platform on the surface of the dredged marine clay. This method yields a substantial saving on construction cost and time while exhibiting similar efficiency to the conventional vacuum preloading method. The undrained shear strength and degree of consolidation of the soil after 60 d of vacuum preloading were 23·6 kPa and 85·1%, respectively, which met the requirements for the working platform.
Coal gangue is a common solid waste in coal mining area, which causes great environmental pollution. Using coal gangue as filling material of highway subgrade can turn waste into treasure. In order to study the greening situation of the road section with coal gangue instead of soil as subgrade filling material, the pH and electrical conductivity (EC) were studied, The electrical conductivity (EC) and pH value of coal gangue used in a highway subgrade test section under different soaking time and initial pH solution were measured, and their change characteristics were analyzed. Both of them increased with the extension of time, and the change trend of EC was more severe. The results of correlation analysis showed that pH and EC were highly positively correlated, and the correlation coefficients were 0.7941, 0.8198 and 0.8639 at initial ph4.1, 5.2 and 6.5, respectively. On the whole, pH is proportional to EC. By mastering the change characteristics and correlation between pH and conductivity of coal gangue under different pH conditions, it can provide certain data and theoretical support for the greening research of coal gangue as subgrade filler.
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