碳化搅拌桩加固软弱地基的理论与方法研究

Soil-cement deep mixing method is one of the most widely used soft ground improvement methods in China,in which Portland cement (PC) is the most commonly employed binder. However, there are significant environmental impacts associated with the production of PC in terms of high energy and non-renewable resources consumption as well as CO2 emissions. Hence, the aim of this research project is to develop an alternative low carbon footprint, environment friendly and efficient ground improvement method to meet the global environment protection and modern engineering requirements. The principle of this new method is to use reactive magnesia (MgO) instead of PC as the soil stabilization binder ,in which the MgO stabilized soil carbonated by gaseous CO2 to form stronger deep mixed column for soft ground improvement application. The efficacy, mechanism, optimization and applicability of this new method will be investigated through laboratory test, microstructure analysis, model test and theoretical analysis. The main research topics are as following: (1) the soil stabilization mechanism of carbonated soil-MgO deep mixing method; (2) the strength inceasing and durability behavior of carbonated MgO stabilized soil; (3) the optimization of carbonation method and construction technique of carbonated soil-MgO deep mixed column. The physical, chemical and microstructure stabilization mechanisms of the carbonated MgO soil stabilization method are expected to be revealed after this research project, the strength development and durability behavior of carbonated MgO stabilized soil will be understood, the strength design method of carbonated deep mixed column will be presented and the optimization of the construction technique of carbonated deep mixed column will be proposed. The results of this research project will lay a firm foundation for the engineering application of this new low carbon footprint, efficient and 'green' ground improvement method, which will significantly improve the ground treatment technique and benefit the energy conservation, pollution reduction sustainable development of civil, hydraulic engineering in China.

针对全球环境保护和现代工程建设的需要，本项目以低碳、环保、高效的绿色地基加固技术为目标，研究以活性氧化镁固化剂代替硅酸盐水泥加固软弱地基的碳化搅拌桩技术机理和应用方法。拟采用室内试验、微观分析、模型试验和理论分析相结合的技术路线，着重研究下列关键内容：（1）碳化搅拌法加固软弱土机理；（2）碳化搅拌加固后的碳化土基本工程性质；（3）碳化土强度增长规律与耐久性；（4）合理喷气碳化方式与碳化搅拌桩工艺模拟研究等。通过研究，旨在揭示活性氧化镁碳化加固软弱土的物理化学和微观机理，阐明碳化土的强度变化规律和耐久性规律，提出碳化搅拌桩的强度设计方法，总结给出碳化搅拌桩最优碳化方式和工艺技术参数。研究成果将为低碳型高效绿色搅拌桩加固软弱地基技术的工程应用奠定基础，提升我国软弱土地基处理技术水平，对我国土木水利工程行业的节能减排和可持续发展具有重要的现实意义。

针对全球环境保护和现代工程建设需要，本项目以江苏典型粉土、粉质粘土和软土为对象，以活性MgO和CO2为固化剂代替传统硅酸盐水泥固化稳定软弱土为目标，采用室内试验、微观分析、模型试验与现场试验相结合的技术路线，重点研究了下列内容：（1）碳化搅拌法加固软弱土机理；（2）碳化土基本工程性质；（3）碳化土强度增长规律与耐久性；（4）碳化固化工艺等。完成了预计研究内容，取得了系列研究成果，为MgO碳化固化技术在地基处理中的应用奠定了坚实基础，对土木工程行业的节能减排和可持续发展具有重要的科学意义和应用前景。主要成果包括：.（1）碳化土化学成分、微观结构和孔隙特征分析表明：碳化产物主要为棒状三水碳镁石、薄片或细丝状球碳镁石和水菱镁石等；碳化时间、MgO活性指数和CO2通气压力的增加以及似水灰比和天然土液限的减小，均有利于碳化土孔隙体积的减小和CO2吸收量的增加。碳化土加固机理包括水化反应、离子交换吸附、碳化反应和填充作用，提出了粉土和粉质黏土碳化加固的微观结构模型。.（2）MgO固化土碳化放出大量热，MgO碳化土质量、体积密度和电阻率较碳化前有不同程度增加，而含水率、比重、孔隙率、饱和度和pH值则明显减小，且pH值远低于石灰和水泥固化土的pH值。这些指标均可与似水灰比进行幂函数拟合，且MgO固化土的碳化程度可通过电阻率法来评价。.（3）MgO碳化固化土强度随碳化时间增长而提高，一般碳化3～6小时可以达到水泥土28天的强度，提高CO2通气压力可加速碳化；建立了强度预测模型，总结出MgO碳化效果排序：粉土>粉质黏土>黏土>淤泥质粉质黏土>淤泥质黏土。碳化土的干湿循环、冻融循环和硫酸盐侵蚀等耐久性试验结果表明：与水泥土相比，碳化土具有较差的抗干湿循环性能、良好的抗冻融循环性能和更强的抗硫酸盐侵蚀能力。.（4）采用管注-覆盖的搅拌桩或浅层整体碳化方法进行CO2通气碳化可有效控制通气压力和时间、提高碳化效果，距气源越近，效果越好；粉土的碳化效果优于粉质黏土；提出了现场碳化固化土的施工工艺流程。