Laboratory Researches On Mechanical Properties And Permeability Of Composite Cement Soil

The development of underground space inevitably involves a large number of geotechnical engineering problems such as foundation pit excavation and foundation treatment.To solve these problems,the development of soil mechanics theory,testing technology,calculation technology,construction equipment,construction technology and geotechnical materials must be promoted constantly.Cement-soil is one of the traditional geotechnical materials and it has been difficult for the pure cement-soil material to meet the high-performance requirements of the modern construction in terms of strength,permeability resistance and micro-disturbance control.In this paper,cement-based admixtures with unique properties were mixed into different soils to produce composite cement-soil.Mechanical properties and permeability properties of the cement-soil are tested and an empirical formula of cement-soil strength was proposed based on field tests and laboratory tests.The main research contents and results include:(1)Unconfined compressive strength laboratory tests of cement-soil show that the type of original soil has a great influence on the strength of cement-soil.The strength of cement-silt and cement-sand with"granular·skeleton”structure is relatively high,while that of cement-clay is relatively low as it is unlikely to form a stable spatial grid structure.The strength of cement-soil is positively correlated with cement content and curing age.(2)The unconfined compressive strength of composite cement-soil laboratory tests show that the modification effect of gypsum on the strength of the cement-clay is most obvious with 221%maximum increase,and also improves the early strength,while the modification effect on the cement-silt is relatively poor,and the gypsum even lower the strength growth of the cement-silt.GS soil hardener can increase the strength of cement-clay by up to 581%.Replacing the cement with equal amounts of GS soil hardener can increase the strength of cement-clay by up to 2.33 times.It also improves the strength of cement-silt and cement-sand to a certain extent.The addition of zeolite powder has almost no effect on the strength of cement-clay,and will reduce the strength of cement-silt.However,it can improve the strength of cement-sand by 78%at the appropriate proportion.S composite additive can increase the strength of cement-clay by 32%at most,and the strength of cement-sand by 15%at most,while has no effect on the strength of cement-silt.(3)Laboratory tests of permeability of cement-soil show that the permeability coefficient of cement-clay decreases with the increase of cement content.When the cement content is greater than 7%,the permeability coefficient of cement-clay is less than Gl 10-8 cm/s.For the cement-silt,when the cement content is 4%,the permeability coefficient is the lowest,which is about 2×10-6 cm/s.The permeability coefficient of cement-silt increases with the increase of cement content due to the high water-cement ratio.The permeability coefficient of cement-sand decreases with the increase of cement content.When the cement content is greater than 9%,the permeability coefficient of cement-sand is less than 2×10-7 cm/s.(4)Laboratory tests of permeability of composite cement-soil show that gypsum can obviously improve the permeability resistance of different types of cement-soil comparing to the other admixtures.GS soil hardener can improve the permeability resistance of cement-clay and cement-silt to some extent.Zeolite powder can improve the permeability of cement-sand,while reduce the permeability resistance of cement-clay and cement-silt.S composite additive has little effects on the permeability of cement-soil.(5)Based on the strength test data,an empirical formula is obtained by the regression analysis of the relationship of the unconfined compressive strength,the cement content,the clay content,the silt content and the sand content using the statistical analysis method.Based on field tests of cement-soil of SMW method and TRD method and comparing the measured value of strength with the calculated value,a correction coefficient? for different methods is introduced in this paper(0.6?0.7 for SMW method and 0.4?0.5 for TRD method),the modified empirical formula can be expressed as:(?)...