Research On Mechanical Characteristics Of Soft Soil Under Unloading And Stability Of Foundation On Weak Underground

Available land in cities decreases rapidly with the development of cities,which forces the cities to develop toward three-dimensional spaces.The development of underground space becomes more and more important.The exploiture of railway traffic and underground space of big cities have been carried out.The big cities of our nation mainly distributed at coastland, middle or downward position of rivers and lake land,where soft soil is the main foundation subsoil.Therefore,the research of soft soil unloading behavior has a significant meaning to the exploiture of subterranean space in big cities.Besides,foundation ditch project is a key tool in underground space exploiture,and further research to the stability of foundation ditch project in weak underground becomes urgent.Therefore,it is important both in theoretical and practical meaning that we take experiments and numerical analysis to soft soil,discuss the deformation, safety and stability of foundation ditch project.This thesis carries out a systematic study on the typical grayer silt-like soft soil at triangle area and soft soil excavation.The main results are listed as follows:1.In order to analyze the lateral unloading behavior of soil excavating,triaxial-shear tests were taken to the intact sample.The results indicated that:(1) Soil that unloading a lateral load obviously had a lower strength than that undertaken a axial load,that is to say,the shear stress of soil decreased with the decrease of lateral restriction;(2) The effective stress strength was lower than the total stress strength because minus pore water pressure were produced under lateral unloading,which was opposite to the stress law under axial-loading condition;(3) Under the condition of lateral unloading,yielding condition of soil at p-q plane meet the yielding condition of Drucker-Prager model;(4) Calculation expressions of unloading deformation modulus under lateral unloading was established.2.In order to analyze the influence of construction process or intermission to the stability of foundation,triaxial unloading rheological tests under different unloading paths were carried out. The results indicated that:(1) The unloading rheological deformation of the soil was shown to be compress deformation if the soil undergone a lateral unloading while the axial loading was kept as a constant;The total deformation of the soil was slight under a small deviator stress,but the creep could not be ignored;The deformation developed rapidly when further increase the deviator stress,and drum-like destroy occurred;(2) Obvious hysteresis rebound deformation was observed,which increased obviously with the increase of average consolidation pressure;Shear deformation of the soil was observed if the soil was undergone both axial and lateral unloading; Without obvious rheological characters,minus pore pressure appeared which increased with the increase of unloading pressure;(3) There was a linear relationship between deformation ratio and logarithm of time whatever the unloading path was.A rheology model to the unloading of soft soil was built to be(?)=bt~m,where b and m are rheological parameters in relationship with unloading path and stress.3.In order to analyze the rebound behavior in excavating project,tests under different unloading paths and various pre-loading conditions were carried out.The results indicated that: (1) Water content and unloading path had little influence to the rebound modulus of soft soil; Rebound was obvious with the increase of unloading ratio and pre-loading;(2) Experiential relation was found between void ratio,pre-loading and loading;(3) There was a critical unloading ratio R=0.8 in soil rebound deformation.When R＜0.8,the soil rebound ratio was low; (4) Secondary rebound coefficient increased with the increase of unloading ratio;(5) The soil rebound modulus was time dependent which varied with unloading ratio,time and pre-loading; Rebound modulus decreased with the increase of time,reaching a stable value at about 10 min.4.In order to analyze the permeability character of soil in excavating project,permeability test under various unloading conditions were carried out.The results indicated that:(1)When soil was unloaded at different pre-solidified pressure,permeability coefficient varied with the change of unloading ratio;When unloading ratio R＜0.8,permeability coefficient varied slightly;when R＞0.8,permeability coefficient increased rapidly with the increase of unloading ratio;(2) The unloading path had little influence to the permeability character of saturated soft clay.The permeability character was greatly influenced by initial consolidation pressure.The greater the pre-solidified pressure,the smaller the permeability coefficient under the same solidified pressure.5.In order to analyze the destroy character in excavating project,centrifugal model tests in the whole simulative excavation and support process were carried out.Factors such as support mode and soft soil strength were considered.Intuitionistic description of deformation and destruction under various conditions and basic conditions in the stability of foundation project in deep soft soil clay were obtained.The results indicated that:(1) With the increased depth of excavation,retaining wall and lateral soils deformation of the foundation ditch were too much, so the foundation ditch had overturn failure;At this time,and also cause a huge crack of the soil around the excavation,which would influence the surroundings of the foundation ditch;The retaining wall inclined toward the foundation ditch and soil around the foundation ditch had a lateral unloading were the main reasons for the settlement occurs on lateral soils;The basal heave deformation of soil under the foundation ditch were main caused by the soil unloading rebound deformation;(2) In the retaining wall supporting with bracing,strength of soft soil on the foundation ditch of a great influence on the stability;With the increased depth of excavation, basal heave had excessive deformation,retaining wall to the excavation outside the tilt,No.1 platoon support lost its effect,so the foundation ditch was damage caused by the entire supporting structure failure;the main factor that caused the settlement of the foundation ditch lateral soils was the flow of soil under the retaining wall toward the excavation,which was also one of the reasons that cause the basal heave deformation under the excavation;Another reason which caused the basal heave deformation under the excavation was the unloading rebound deformation of the soil after the excavating of the soil above the excavation.6.The deformation caused by stress deflection and strength problems were discussed in order to analyze the deflection of surrounding soil caused by foundation excavating unloading: (1) Theoretical analysis was carried out to the surrounding soil after foundation excavating unloading;Through the introduction of stress vector ratio,stress vector constitutive relationship was built considering primary stress axial deflection;(2) Through the shear tests of soft soil under different primary stress azimuth angles,we could get the conclusion that after the primary stress deflection,the relationship between stress and strain was hyperbola,which was unitary; The strength and the deformation modulus of the soil decreased,which indicated that the security of the project was decreased if the stress deflection of the foundation project was neglected;(3) The reduction formula on the relation between soil initial deformation modulus and the primary stress azimuth angles was built.7.Numeric simulation was carried out to archetypal foundation in centrifugal model tests based to the test results and centrifugal model results obtain above.The results indicated that:(1) Without the influence of stress deflection to soil deformation and strength,the deformation law calculated was similar to the results measured,but the deformation was small;The biggest displacement happen in the middle of the whole foundation in numeric simulation,while in centrifugal tests the biggest displacement happen in the middle of the foundation measured; Rheology effect of the soil on the side of the foundation was more obvious than the soil under the foundation;The surrounding soil of the foundation had various deformation deflection after foundation excavating unloading;(2) The reduction of deformation modulus was carried out aider every calculation in order to consider the influence of stress deflection to soil deformation; The results indicated that when the stress deflection effect was considered,the soil deformation around foundation increased greatly and rheology effect became obvious.