Study On Mechanical Mechanism Of Soft Soil Foundation Reinforced By Geopolymer Mixing Piles

Cement is a traditional binder that has been extensively used in ground improvement at present.However,its shortcomings of high energy consumption,high CO₂emission and insufficient durability have driven engineers to find alternative cementitious binder with low energy consumption,environmental friendly and excellent properties in place of cement.Geopolymer（GP）is a new type of cementitious material with low carbon emission,excellent mechanical properties and good durability,which has received research attention worldwide.The research on geopolymer mainly focus on using GP to make building materials and immobilize heavy metals,while the study on GP stabilization of soft soil is less explored.Recent years have witnessed the fast growing research interests in GP stabilizing soft soil.However,the underlying stabilizing effect of GP is still unclear.Therefore,in this study GP was used as a binder instead of cement to strengthen soft ground by deep mixing technique.The mechanical mechanisms of GP stabilized soft ground were investigated.The main research contents and conclusions are summarized as follows:1.The mechanical properties of soils stabilized with 5%and 10%GP were investigated by liquid and plastic limit test,undrained shear strength test and unconfined compressive strength（UCS）test.The results show that the liquid and plastic limits,the undrained shear strength and the UCS of GP stabilized soil samples all increase obviously with increasing GP mixing ratios.2.Soil samples mixed with 10%GP were prepared to cured in varied humidity conditions and water contents with different ages,so as to study the influences of curing age,curing humidity and moisture content on the unconfined compressive strength of GP stabilized soil.The UCS of GP stabilized soils increases with increasing of curing age,but decreases with increasing curing humidity and moisture content.The UCS of GP stabilized soil at 90d age can reach 2.1 MPa.Based on test results,the formula for predicting the long-term strength of GP stabilized soil was proposed,and it is found that UCS₉₀/UCS₂₈of GP stabilized soil is close to that of cement stabilized soil.3.Compressibility and permeability tests were conducted on soil samples stabilized with10%GP.It is found that inclusion of 10%GP can obviously decrease compressibility and permeability coefficient of soft soil.The filling and cementing effects of GP lower the compressibility of original soil from medium compressibility to low compressibility,while decreases its permeability coefficient about 10 times.4.Micro characterizations using XRD,FTIR,TG and SEM-EDS tests were conducted on GP samples to analyze the micromorphology,hydration products and their evolution with curing age.When GP was added to soil,the hydrated cations Si⁴⁺,Al³⁺and Ca²⁺can exchange with the surface cations of soil particles and reduce the distance between soil particles,leading to agglomeration of soil particles and hence increasing the undrained shear strength of soil.Meanwhile,the alkali environment makes covalent bonds such as Si-O-Si and Al-O-Al break,leading to dissociation of silicon and aluminum monomers.With preceding of geopolymerization process,increasing amount of these monomers are liberated which eventually reaches saturation and agglomeration reaction occurs,which results in chain polymers.After dehydration and condensation,three-dimensional network gel-like polymers are formed.These hydrated gels coat the soil particles and fill the void between them,leading to a dense micro structure.The XRD results indicate that the hydration products of GP are calcium aluminosilicate hydrate（CASH）.And the amount of its formation increases with increasing of curing age.The CASH gels can produce cementing and filling effects,and thus reduce the soil porosity and improve the strength of soft soil.The mechanism of GP stabilized soft soil can be generalized as follows:ion hydration,ion exchange,flocculation and agglomeration,pozzolanic reaction and carbonation reaction.5.Composite foundation model tests were conducted to study the reinforcing effect of GP mixing pile on soft ground.The settlements,load sharing ratio between piles and foundation soil and bearing capacity of composite foundation under step loading with different area replacement rates were investigated.The results indicate that the settlements,pile top stresses,soil stresses and pile-soil stress ratios decrease with increasing area replacement rate.And the bearing capacity of GP mixing piles composite foundation increases obviously.Under similar area replacement rate,the bearing capacity of GP mixing pile composite foundation can close to that of cement.6.Finite element analysis using ABAQUS software was conducted to study the GP mixing piles composite foundation under different area replacement rates and the results were compared with those of model tests.Mohr-Coulomb model and Modified Cambridge model were used respectively to model the mechanical response of GP mixing piles and foundation soil.It is found that the numerical settlements,pile top stresses,soil stresses between piles and pile-soil stress ratios obtained from numerical simulations are consistent with the measured data of model tests.