Stability Ofattenuated Strength Subgrade And Mechanics Response Of Its Pavement Structure

The strength attenuation of silt subgrade was discussed by the way of the on-site investigation, laboratory testing, field testing. The stability of the attenuated strength subgrade and its mechanical response of pavement structure were analyzed by the numerical simulation. The mechanical and deformation characteristics of subgrade were tested by the model experiment of the new subgrade, the attenuated strength subgrade, and the strengthed subgrade with the grouting mini-pile technology. The effective reinforcement technology was proposed and the evaluation methed of the pavement structure based on the index of failure approach was proposed. The main contents and conclusions are as follows:(1)The moisture content of subgrade was described on the base of field testing. The systematic experimental study on penetration and capillary rise was conducted to reveal the strength law of the silt subgrade under the different water content and compactedness. The results show that the silt has the bigger permeability coefficient and the stronger capillary water action. The increase of water content results in the strength attenuation of the subgrade. The tests of the resilient modulus and the triaxial test (UU) results under the different soil moisture on the compacted silt showed that the resilient modulus and the internal friction angle changed a little and the cohesion and modulus of deformation gradually increased when the water content was less than the best moisture content. The resilient modulus, cohesion and deformation modulus of the subgrade soil decreased significantly with the increase of the water content when the water content was above and beyond the best moisture content. But the internal friction angle reduced drastically when the water content reached to the saturation state.These revealed that the sucking water silt had a large deformation, low strength.(2)The on-site investigations had been finished about the failure modes of the slope stability, and the numerical simulations were conducted to analyze the stability of the attenuated strength subgrade and the subgrade strengthened with grouting micro-pile by ABAQUS / Standard. Field surveys show that the failure modes of the actual silt embankment had shallow landslide, slope collapse, embankment slipslip. The computed results showed the slope stability decreased with the increase of moisture content. But the slope stability of the embankment strengthened with the grouting mini-pile increased observably.(3)Based on the attenuation of subgrade strength, the mechanical response of pavement structure on the typical highway was computed and analyzed under the action of the Standard load and overload. And based on the elasto-plastic analysis of the pavement structue, the failure approach index of the pavement structure was computed on the new subgrade and the attenuated strength subgrade to judge and analyze the force balance of the pavement structural layer. The results showed that the stress and the surface deflection increased nonlinearly with the attenuation of subgrade strength and increased linearly with the loads. Under the action of the wheel loads, the bottom of the semi-rigid basement layer was tensed and was in the most unfavorable stress state.The computed results also showed that the fatigue life of the semi-rigid basement layer reduced exponentially with the increase load or the decay of subgrade strength. The elastic-plastic analysis of the grade road showed that the stress of pavement structure layer increased significantly after the attenuation of subgrade strength. The computed results of the failure approach index of the pavement structure showed that the pavement structural layer was in danger of the unbalance state, resulting in the plastic failure of the semi-rigid basement layer, FAI> 2.(4)The model experiment study was conducted on the new subgrade, the attenuated strength subgrade and the subgrade strengthened with grouting micro-pile. The test results showed that the capillary action was very observable. The moisture content of subgrade strengthened with the grouting micro-pile decreased 3.7% to one of the attenuated strength subgrade, and the compactedness and the elastic modulus of subgrade increased 1.35% and 9.46MPa to the original subgrade. When the subgrade strength decreased, the redistribution of its vertical stress happened so that the stress at the top of the subgrade increased about 16% than the new embankment. The vertical stress of subgrade strengthened with the grouting micro-pile stress (especially in roadbed) reduced approximately 50%. The lateral force of subgrade increased significantly with the attenuation of subgrade strength under the action of the same loads. While the lateral force of subgrade strengthened with the grouting micro-pile lowered significantly, and the ratio of lateral force of attenuated strength subgrade and subgrade strengthened with the grouting micro-pile is about 2:1. The differential settlement increased rapidly with the attenuation of subgrade strength. But the differential settlement of subgrade strengthened with the grouting micro-pile reduced observably to reach the state of the original subgrade. The deformations of original subgrade and strengthened subgrade were almost elasticity, and the plastic deformation is very small. The plastic deformation of attenuated strength subgrade increased rapidly with the attenuation of subgrade strength. The whole stiffness decreased with the attenuation of subgrade strength and increased observably with the subgrade strengthened. With the attenuation of subgrade strength, the lateral horizontal displacement of subgrade increased. But after the reinforcement of subgrade with grouting micro-pile, only minimal horizontal displacement occurred in subgrade top. Therefore, the grouting mini-pile technology is an effective method to strengthen the attenuated strength subgrade. It can not only improve the shear strength of soil between piles, but also increase its degree of compaction and compressive strength, resulting in a remarkable increase of the overall bearing capacity and the slope stability.(5) The work mechanism of the grouting mini-pile technology was analyzed on the basis of the model test. The subgrade that had been compacted according to layers would be easily split between layers when the subgrade was strengthened with the grouting mini-pile technology, resulting in the horizontal hardening layer of cement paste between the compacted soil layers. The horizontal hardening layer of cement paste would become the horizontal curtain to block capillary water up and avoid the continuing increase of the moisture content, which could prevent the subgrade strength from continuing the reduction. The cement paste column surrounded with steel pipe played the role of the bolt connection and the skeleton to the soil of subgrade. The model load test indicated that the work process of subgrade strengthened with grouting micro-pile could be divided into three phases under the action of loads. That is the acceleration loadcarrying of grouting mini-pile, the coordination work of pile-soil together, and the fracture of grouting solidification body resulting in the entire failure of subrade. The practical application of the grouting mini-pile technology showed that it is a very effective to improve the vertical bearing capacity of subgrade, prevent uneven settlement, and improve the stability of slope.