Experimental Study And Durability Assessment Of The Inorganic Compound Corrosive Action On Cemented Soil And Silty Soil Under

Study of engineering material durability in the corrosive environment is one of international highly attention objects. It is hard to find the report relevant to engineering property study of cemented soil column composite foundation working in a corrosive environment and the design method is also rare to the environment pollution effect on the foundation durability. Based on collecting, checking and summarizing domestic and foreign literatures, this thesis studies the physical, chemical and mechanical properties and working mechanism of silty soil and cemented soil under various inorganic compound environmental conditions. This work has positive meaning for establishing composite foundation durability design theory and safety evaluation standard considering environment action, insuring the long-term engineering safety, lowering the engineering cost, promoting our country economy and society development.The main achievements are as followed:(1) Through the analogous experiment in lab, the physical and mechanical indexes are taken for the silty soil samples soaked in the alkaline solutions (NaOH and NH₃.H₂O) and saline solutions ( NaCl and Na₂SO₄) with various concentrations. The test results show that density, liquid limit, plastic limit, plasticity index, compression coefficient are increased and void ratio is decreased for the samples soaked in alkaline solutions with the increment of the concentration; compared with the test data, the effect of NaOH solution on the sample is stronger than that of NH₃.H₂O solution. The density and specific gravity of soil particle are increased, and void ratio, liquid limit, plastic limit, plasticity index, compression coefficient are decreased for the samples soaked in saline solutions with the increment of the concentration; compared with test data, the effect of Na₂SO₄ solution on the sample is stronger than NaCl, which may be the result of SO₄^2- stonger than Cl^-. Based on the test data, the statistical functions are set up, such as the linear function of liquid limit and plasticity index, exponential function of compression coefficient and solution concentration, which could be used in actual engineering.(2) According to the migration regulation of contamination in subsoil, primarily migration types are permeating and diffusibility. The migration model of pollutant ingredients in the foundation soil is established, in which the chemical reaction in the migratory process is taken into account.(3) In order to simulate and study the erosion effect process, the photos of the cemented soil samples are taken, when they are soaked in various inorganic compound solutions with different concentrations and times. The out-looks of the samples are obviously changed, mainly represent the surface soften, the size reduced or crack, when they are soaked in H₂SO₄, MgSO₄, Na₂SO₄ solutions. However, the out-looks of the samples soaked in MgCl₂ and CaSO₄ solutions are changed but not seriously, mainly represent crystallization adhering to the surface.(4) At the meantime, the unconfined compression tests are conducted on the cemented soil samples which are soaked in the inorganic compound solutions with various concentrations and times. The cemented soil strength modified coefficient under each corrosive environment is obtained, and the cemented soil strength modified coefficient are great than 1 for the CaSO₄, MgSO₄ (1.5 g/L), Na₂SO₄ (1.5g/L) and MgCl₂(1.5g/L) solution environments, from which it may be drawn out that these environments are beneficial to the strength increasing of cemented soil. All of the strength modified coefficient for the other solution environments are less than 1, from which it may be drawn out that these environments propose the corrosion to the cemented soil and reduce the strength. Therefore the working environmental condition of the composite foundation should be taken into account when designing and calculating the foundation bearing capacity and settlement in order to optimize design, reduce the cost, adopt suitable measures, and assure the safety. Additional, full stress-strain curve of cemented soil is obtained too. Based on the existing damage model and test data, a modified stress-strain equation is set up for inorganic compound environment considering the cemented soil strength lost caused by environment affect.(5) The test results show that some inorganic compound solutions give cemented soil higher influence than others. In this experiment, inorganic compound solutions can be arranged in a series for the influence as follows: Na₂SO₄ > MgSO₄> H₂SO₄> MgCl₂> CaSO₄, When the solutions exsits SO₄^2- ion, the influence degrees of cations Na⁺, Mg²⁺, H+, Ca²⁺ are Na⁺> Mg²⁺> H⁺> Ca²⁺; When the solutions exsits Mg²⁺ cation, the influence degrees of SO₄^2- anion is greater than that of Cl^- anion.(6) The concentration of the main chemical ingredients in the solutions are measured when the cemented soil samples are soaked in the solution for different times. For the H₂SO₄ solutions, the pH value is increasing, and the SO₄^2- concentration is decreased with the increment of erosion time; for the MgSO₄ solutions, Mg²⁺ cation concentration is decreased with the increment of erosion time, but SO₄^2- anion concentration is increasing when the sample soaked time is less than 14 days, then decreasing when the soaked time greater than 14 days; for the Na₂SO₄ solutions, Na⁺ concentration and SO₄^2- concentration are decreased with the increment of erosion time; for the CaSO₄ solutions, Ca²⁺ concentration and SO₄^2- concentration are decreased with the erosion time increasing; for the MgCl₂ solutions, Mg²⁺ concentration is increasing with the erosion time increasing, Cl^- concentration is increasing before first 7 days, then decreased after 7 days.(7) By calculating main chemical ingredients and analyzing the mechanism, the corrosive type of H₂SO₄, MgSO₄ and MgCl₂ solutions to cemented soil is a composite type of resolving and crystallizing combination; that of Na₂SO₄ solution to cemented soil is a composite type of dissolving and crystallizing combination; CaSO₄ solution shows no corrosive action to the cemented soil. Based on the relationship of the main ingredient concentration of the solution and cemented soil strength, the cemented soil strength model is established for each corrosive type, the limit for each model is also described.(8) The research contents of cemented soil column composite foundation contain durability mechanism, durability valuation, estimate of its surplus life, durability design and technique measure to increase durability. Based on the environment condition surrounding the cemented soil column, the design formula of the bearing capacity and composite modulus for this composite foundation are proposed in this thesis, in considering corrosive time action. Finally, the durability of the cemented soil column composite foundation are classified into A～D four grades, and composite foundation durability assessment model is established with considering the column durability and surrounding soil durability based on the fuzzy assessment theory.